lpg safety distance guide

PETRONAS TECHNICAL STANDARDS

DESIGN AND ENGINEERING PRACTICE

MANUAL

GUIDELINE FOR CALCULATING

SAFETY DISTANCES IN LPG

STORAGE AND HANDLING

INSTALLATIONS

PTS 20.162

JANUARY 1988

PREFACE

PETRONAS Technical Standards (PTS) publications reflect the views, at the time of publication,of PETRONAS OPUs/Divisions.

They are based on the experience acquired during the involvement with the design, construction,operation and maintenance of processing units and facilities. Where appropriate they are basedon, or reference is made to, national and international standards and codes of practice.

The objective is to set the recommended standard for good technical practice to be applied byPETRONAS' OPUs in oil and gas production facilities, refineries, gas processing plants, chemicalplants, marketing facilities or any other such facility, and thereby to achieve maximum technicaland economic benefit from standardisation.

The information set forth in these publications is provided to users for their consideration anddecision to implement. This is of particular importance where PTS may not cover everyrequirement or diversity of condition at each locality. The system of PTS is expected to besufficiently flexible to allow individual operating units to adapt the information set forth in PTS totheir own environment and requirements.

When Contractors or Manufacturers/Suppliers use PTS they shall be solely responsible for thequality of work and the attainment of the required design and engineering standards. Inparticular, for those requirements not specifically covered, the Principal will expect them to followthose design and engineering practices which will achieve the same level of integrity as reflectedin the PTS. If in doubt, the Contractor or Manufacturer/Supplier shall, without detracting from hisown responsibility, consult the Principal or its technical advisor.

The right to use PTS rests with three categories of users :

1) PETRONAS and its affiliates.2) Other parties who are authorised to use PTS subject to appropriate contractual

arrangements.3) Contractors/subcontractors and Manufacturers/Suppliers under a contract with

users referred to under 1) and 2) which requires that tenders for projects,materials supplied or - generally - work performed on behalf of the said userscomply with the relevant standards.

Subject to any particular terms and conditions as may be set forth in specific agreements withusers, PETRONAS disclaims any liability of whatsoever nature for any damage (including injuryor death) suffered by any company or person whomsoever as a result of or in connection with theuse, application or implementation of any PTS, combination of PTS or any part thereof. Thebenefit of this disclaimer shall inure in all respects to PETRONAS and/or any company affiliatedto PETRONAS that may issue PTS or require the use of PTS.

Without prejudice to any specific terms in respect of confidentiality under relevant contractualarrangements, PTS shall not, without the prior written consent of PETRONAS, be disclosed byusers to any company or person whomsoever and the PTS shall be used exclusively for thepurpose they have been provided to the user. They shall be returned after use, including anycopies which shall only be made by users with the express prior written consent of PETRONAS.The copyright of PTS vests in PETRONAS. Users shall arrange for PTS to be held in safecustody and PETRONAS may at any time require information satisfactory to PETRONAS in orderto ascertain how users implement this requirement.

GUIDELINES FOR CALCULATING SAFETY DISTANCES IN LPG STORAGE AND HANDLINGINSTALLATIONS

CONTENTS

1. Introduction

2. Assessment of Fire Situations

2.1 Audit of the Facilities

2.2 Selection of Leakage Scenarios and Assessment of their Consequences

2.3 Radiation Criteria for Personnel Protection

2.4 Vapour Cloud Explosion

2.5 Boiling Liquid Expanding Vapour Explosion (BLEVE)

2.6 Selection of Leak Reduction Measures and Methods to Mitigate the Effects

3. Consequence Assessments

3.1 Introduction

3.2 Calculation of Flow Rates

3.3 Vapour Jets - Dispersion and Fires

3.4 Two-phase Jets - Dispersion and Fires

4. Worked Example

4.1 Description of Facility

4.2 Audit of the Facility and Choice of Scenarios

4.3 Consequence Assessment, Analysis and Proposed Action

5. References

TABLES

1. Discharge Areas for 'REGO'. 'FISHER', and other pressure relief valves

2. Relief Valve Fire and Radiation Flux Data for Propane

3. Relief Valve Fire and Radiation Flux Data for Butane

FIGURES

1. Schematic of Model Facilities

2. Example PLUMEPATH Dispersion Profile for Butane

3. Distances to LFL for Propane and Butane Releases

4. Distances to 1.5 kW/m Radiation Flux for Vapour and Liquid Horizontal Butane Jet Fires

5. Distances to 5 kW/m Radiation Flux for Vapour and Liquid Horizontal Butane Jet Fires





10. Flame Lengths for Vapour and Liquid Horizontal Butane Jet Fires

11. Distances to 1.5 kW/m Radiation Flux for Vapour and Liquid Vertical Butane Jet Fires

12. Distances to 5 kW/m Radiation Flux for Vapour and Liquid Vertical Butane Jet Fires





17 . LPG Depot Layout - Worked Example

18. Flow scheme - Worked Example

19. Nozzle details of Propane Sphere Worked Example

20. Nozzle details of Butane Sphere Worked Example

1. INTRODUCTION

One of the main differences between the recently-issued Supply and Marketing (SM) LPGManual Part 2 Section 03 PTS 30.06.10.12. LPG Bulk StorageInstallations (hereafter referred to as the Manual/PTS) and previous issues is the requirementto relate siting of equipment to the radiation flux levels that would be experienced from fires inthe installation. This approach applies to LPG bulk storage installations with individual tanksof 135 m3 and above and is consistent with the Institute of Petroleum Model Code of SafePractice, Part 9, Liquefied Petroleum Gas, Volume 1, dated February 1987.

The covering letter which was sent to all companies with the Manual/PTS suggestedthat all sites should compare the design of their facilities with the new standards and considerwhether any aspects should be modified. Site layout is one aspect that in many cases willbe difficult, if not impossible, to alter. Therefore, it will be important to examine whether a firecould endanger human life, equipment or property, inside or outside the site. If this is recognizedas a possibility, it will be necessary to consider changes to the design to reduce the probability of the incident and/or provide additional protection to people and equipment. Ina completely unacceptable situation it may be necessary to shut down the installationand transfer the activities elsewhere.

Since the Manual/PTS does not indicate how the evaluation should be carried out, or provideinformation on the calculation of thermal radiation levels, an inter-functional team hasproduced this set of guide-lines with the assistance of Thornton Research Centre. It has been written in three parts:

Section 2. Assessment of Fire Situations

Section 3. Consequence Assessments

Section 4. Worked Example

It should be noted that although these guide-lines have been produced as an aid forevaluating the layout requirements for bulk storage installations with individual tanks of 135m3 and above, they are also in general applicable to layout aspects of all other sections ofplants handling pressurized LPG at Marketing and Manufacturing installations (e.g. loading/unloading ships, bulk road vehicles or rail tank wagons and filling cylinders). As stated in theManual/PTS, operating companies may choose to apply these guidelines to installations withindividual tanks of less than 135 m3.

2. ASSESSMENT OF FIRE SITUATIONS

2.1 AUDIT OF THE FACILITIES

Section 03.02.01.02 of the Manual/PTS covers safety distances. It states that possible leaksources should be identified and their rate of leakage and duration assessed. The first step isthe identification of such leak sources. This requires a systematic evaluation of the designagainst the Manual/PTS and a review of the operating/maintenance procedures for theinstallation.

The assessment should be carried out by a team of three or four people, who between themhave a good knowledge of the design, operation and maintenance of the facility. They shouldhave available up-to-date flow schemes and engineering information about the equipmentbefore they start.

The team should study the facility, line by line and piece of equipment by piece of equipment,and consider whether there could be any circumstances which might lead to a leak of LPG.This should be done in an imaginative way with the team considering the usual operatingconditions at the site and also any unusual conditions which they can conceive.

They would consider, for instance:

- possible high or low temperatures

- possible high or low pressures

- overfilling of storage vessels, bulk lorries or rail tank wagons

- the effect of impurities in the LPG (e.g. water)

- the wrong grade of LPG (e.g. C3 instead of C4)

- the effect of incorrect operation or maintenance

- incorrect material selection or equipment fabrication

- malfunctioning equipment

- impact (e.g. vehicles)

The procedure outlined above is a simplified version of the more structured HAZOP study,where the team leader takes the team through a series of guide words for each part of theplant which they are examining.

2.2 SELECTION OF LEAKAGE SCENARIOS AND ASSESSMENT OF THEIRCONSEQUENCES

In section 03.02.01.02 (c) of the Manual/PTS it states that the evaluation of leak sourcesshould take into account failure modes, likelihood and consequences. It is not possible to givedetailed guidance on the likelihood of particular leak scenarios, because the probability thatan event will occur will depend on the design of the specific facility and the quality of itsoperation and maintenance. However, a review of serious incidents that have occurred in theLPG industry shows that they have usually been the result of the following situations:

- Product discharge through a relief valve (including overfill).

The Manual/PTS requires in section 03.02.01.02 (a) that product discharge to atmospherethrough relief valves on LPG storage vessels should be considered as leakage scenarios inall cases. This requirement is consistent with the Institute of Petroleum Model Code of SafePractice, Liquid Petroleum Gas, Volume 1. The rationale behind this requirement is that theserelief valves are the only devices in an LPG facility which are designed to be able to ventlarge quantities of LPG to atmosphere.

- Flange leak or other joint leak (see Section 4 for typical leakage hole sizes)

- Pump seal leak (see Section 4 for typical leakage hole sizes)

- Open drain valve

- Rupture of small bore connections (e.g. breakage of an instrument line)

- Hose leak or rupture (e.g. vehicle pullaway)

It is recommended that these failure modes should always be considered for consequencecalculations, unless the audit team has very good reasons to discount them. Other situationswill be possible on many installations. These could include the leak of a vessel or pipeline dueto internal or external corrosion, or the effect of an external incident such as vehicle impact.Inclusion of these scenarios for consequence calculations will depend on the team'sjudgement of whether their likelihood is greater or less than that of the failure modes listedabove.

Using the information given in Section 3 the team should next assess the flow rate of theleaking liquid or vapour. Factors in the design which may control the flow should be taken intoaccount.

An assessment should also be made of the time that the release will last. This will depend onthe presence of operators, the accessibility of manually-operated isolating valves and theexistence of remotely-operated valves. The likelihood of ignition will depend in part on thetime that a flammable mixture persists.

When a leak or spill occurs the hydrocarbon vapour will disperse, forming a cloud. Thedistances to the lower flammable limit can be obtained from Section 3. Should the cloudextend beyond the site boundary or to another area where ignition sources are not controlled,measures will be necessary to limit the size of the release.

Flash calculations for liquid butane and propane suggest that product leaks may formsignificant liquid pools. However, experimental work and field trial studies by ThorntonResearch Centre have established that the jet formed when these materials are releasedentrains considerable amounts of air. Small droplets of liquid LPG are formed. Theseevaporate rapidly and the result is a cold vapour cloud with no significant pool formation. Theonly situation for which pool formation can be envisaged is with butane in cold climates whenits vapour pressure is low. At the same time the discharge velocity must be low, as in thecase of a leak in the discharge line from a storage vessel, when the driving force is providedlargely by the head in the vessel. Therefore, in nearly all leakage situations ignition of the leakwill result in a jet fire, rather than a pool fire.

Finally the distances to various levels of radiation intensity from ignited leaks can be readfrom the tables associated with Section 3. The results should then be compared with thecriteria given in Figure 03.02.01.02 of the Manual/PTS, which are discussed in Section 2.3.

2.3 RADIATION CRITERIA FOR PERSONNEL PROTECTION

In Figure 03.02.01.02 of the Manual/PTS maximum radiation flux levels are given forpersonnel inside the site boundary and for the situation at the site fence. The notes attachedto this figure only give brief guidance on the choice of type of area. In order to give additionalguidance the following amplified notes have been prepared.

The acceptability of maximum heat flux levels is based, in part, on the person's ability toescape, since injury is a function of heat flux and exposure time. Employees engaged in thelocation's activities should be trained in what to do in an emergency and will be healthy andactive. On the other hand, members of the public at or near a location are unlikely to knowwhat to do and may also include the full community age and health range.

2.3.1 Plant boundary situations

- Remote Area. An area where there is a low likelihood of people. Those likely to bepresent will be fit but may be lightly clothed. There is no shelter available but escape isboth easy and obvious.

- Urban Area. An area where there is a strong possibility of people of full community ageand health range being present. They will be fully clothed. There is no shelter availablebut escape is easy or only slightly hindered (e.g. need to cross a road).

In a situation where there is no site fence, such as an automotive LPG station, it may benecessary to relate the size of the fire to the time needed to get away from the heat. Forinstance, not more than 30 seconds should be required to move from a radiation intensity of 5kilowatts/ metre (kW/sq.m) (second degree burns in ca. 30 secs) to an intensity of 3 kW/sq.m(second degree burns in 60 secs) and a further 90 seconds to get to an intensity of 1.5kW/sq.m.

- Critical Area. This is the same as an urban area, but with hindered means of escape.

2.3.2 Plant areas

- Process Area. Those likely to be present will be healthy and trained in emergencyprocedures. They will be fully clothed and will be able to be clear of the area within oneminute.

- Protected Work Area. This refers to permanent buildings where personnel are obliged toremain in order to operate plant, but may be exposed through glass. It may also provide arefuge for those escaping from the fire. The radiation level refers to the building exposure.

- Work Area. There is minimal shelter from the fire and slightly hindered escape. Thosepresent will be healthy and be fully clothed.

- Critical Area. This is one where an operator may have to be present for short timesoccasionally, e.g. to check the state of equipment. He will be trained in what to do if a firestarts, but escape routes may be hindered because of plant complexity.

2.3.3 Flash fires

When a cloud of hydrocarbon vapour ignites the initial flash fire will be of high intensity, but ofsuch a short duration that only people actually enveloped in it will be seriously burnt.

2.4 VAPOUR CLOUD EXPLOSION

If an LPG installation is designed to the current Manual/PTS and other Group standardsfor handling LPG, or has been adequately updated, the probability of a leak or spill beinglarge enough to result in an explosion will be very low. In addition, trials carried out byresearch organizations have shown that vapour cloud explosions in an open situation are veryunlikely. If LPG vapour enters an area where equipment or pipework are closely spaced, orenters a building, drainage system or another confined space, an explosion may well bepossible.

Vapour cloud explosion is a complicated subject, and is still an active research topic. If acompany considers that it is necessary to review the possibility of such an event, it should consult CHSE for advice.

2.5 BOILING LIQUID EXPANDING VAPOUR EXPLOSION (BLEVE)

This type of explosion occurs when a ductile vessel, containing a liquid whose vapourpressure is well above atmospheric, ruptures. Because the vessel is made of a ductilematerial its shell tears, generating a relatively small number of large fragments. If the liquid inthe vessel is flammable and the rupture has been caused by heat from an external fireweakening the wall of the vessel above liquid level, the BLEVE produces a buoyant fireball.The size of the fireball, its duration and the intensity of its radiation are determined by the totalcontents of the product in the vessel. The pieces of the vessel can travel several hundreds ofmetres. This is the situation usually associated with the occurrence of a BLEVE.

In the past a BLEVE has been considered as an unrealistic event for an installation designedto Group standards. However, it must be recognized that BLEVEs occur somewhere inthe world, either with transport tanks or in fixed installations, at the rate of about one everytwo years. Therefore, the possibility has to be recognized, particularly by PETRONAScompanies with older installations .

If the installation meets the requirements of the Manual/PTS the probability of a BLEVE willbe low enough to be considered unrealistic, because these standards have been developedspecifically to eliminate the possibility of the vessel walls being overheated. However, if someof the requirements are not met and/or if operating procedures are not strictly enforced, theprobability could be much higher. A critical review of the design, operation and maintenanceof the installation should be carried out if any concern is felt by a company. CHSE would be prepared to assist in such a review.

Another possible cause of vessel failure is severe over pressurization, probably associatedwith vessel imperfections due to faults in the material of construction, faults in its fabrication,or possibly due to internal corrosion. Failure for these reasons is considered extremelyunlikely for a vessel installed to the requirements of the Manual/PTS and operated correctly. Ifa source of ignition is also present a fireball similar to that produced by a BLEVE will occur.

2.6 SELECTION OF LEAK REDUCTION MEASURES AND METHODS TO MITIGATE THEEFFECTS

If any of the leakage scenarios that have been examined are found to give unacceptableradiation levels at the site boundary or within the facility it will be necessary to considermeasures that will either reduce the probability of the release, reduce its length of time, orreduce the radiation level. In the first place the requirements of the Manual/PTS should beapplied. However, if these are not practicable other methods may be considered. Thesuggestions given below are not exhaustive; some will be more suitable for Marketinglocations, while others may be preferred by Manufacturing locations. It must be recognizedthat there may be occasions when it is not practicable to improve the installation to anadequate level of safety. In that case it may be, necessary to shut down or relocate thefacilities.

- Installing hydrocarbon gas detectors/alarms, possibly interconnected to emergency shutdown valves.

- Incorporating fusible links in the actuating systems for emergency shut down valves.

- Installing secondary emergency shut down valves with a mode of failure non-common tothe primary valves.

- Welding more of the pipework and valves.

- Connecting all relief valves to a flare or vent system.

- Providing fire protection/tank cooling for small tanks as well as for large tanks.

- Providing fire protection for adjacent equipment.

- Pressurization or sealing of nearby buildings containing sources of ignition (e.g. anelectrical substation).

- Using breakaway couplings or drive-away protection in road and rail tank filling/dischargesystems.

- Installing vehicle impact barriers.

- Using load cells/weigh bridge to reduce the chance of overfilling bulk lorries or rail tankwagons.

- Using computer controlled loading/unloading.

- Replacing hoses with loading arms.

- Using mounded storage at new sites.

3. CONSEQUENCE ASSESSMENTS

3.1 INTRODUCTION

This part of the Guide-lines provides a range of dispersion and fire hazard assessments tocomplement the leakage scenarios described in Section 2. The structure which has beenadopted is intended to mirror the main classes of hazard which can arise, which are vapourand two-phase jet releases. However, a prerequisite for carrying out dispersion and firehazard assessments is the calculation of the relevant mass flow rates. The three resultingsections are headed:

- Calculation of flow rates - vapour leakage flows- liquid leakage flows

- Vapour jets, dispersion and fires

- Two-phase jets, dispersion and fires

3.2 CALCULATION OF FLOW RATES

The following sections provide calculation methods for most leakage situations. For a generaltreatment of the calculation of leakage flows, the reader is referred to the review written byRamskill and entitled "Discharge rate calculation methods for use in plant safetyassessments" (Ref. 1).

The leakage from equipment at LPG installations will be of two types. It may come from thevapour space of vessels or from lines handling LPG vapour. when only vapour flow has to beconsidered. Alternatively, the leak may be from lines or equipment handling liquid LPG. Inthese cases liquid flow or two-phase flow may occur. Vapour leakage flow calculations andliquid/two-phase flow calculations are described in the following subsections.

3.2.1 Vapour leakage flows

Vapour leakage flows may be divided into two categories:

- Flow through relief valves and other cases where choked flow occurs.

- Flow through holes where choked flow does not occur.

Choked flow occurs where the ratio of upstream and down-stream pressure is greater than acritical value. For propane and butane this occurs where:

Pressureinpipeorvessel

Atmosphericpressure 1.8

In practice this will be the likely situation at many installations. For ease of calculation it isrecommended that all vapour leaks are treated as choked flow and handled as describedbelow, although this may be a conservative assumption. If required, calculation methods forunchoked flow can be found in Reference 1.

These flows should be sufficiently accurate for most leakage situations. However, in complexcases, e.g. where there is a large hole in a pipe which is more than, say, 20 m from the vesselwhich is the source of pressure, pipe friction losses will reduce the flow rate. In these casesreference should be made to the review by Ramskill (Ref. 1).

The calculation of choked flow is demonstrated by the operation of a relief valve. The reliefvalve on a partly filled vessel of LPG will open as soon as the pressure in the vapour space ofthe vessel has risen to the relief valve set pressure. This could for example be due to theeffect of a fire close to the vessel. The formula to be used for the flow calculation is thatderived from API RP 520 - "Recommended Practice for the Design and Installation ofPressure Receiving Systems in Refineries" (Ref. 2). The equation is derived from a generalequation for conditions of critical flow and may be written:

WKAP

C

M

T= (1)

where:

W is the flow rate (Kg/s),

C is the gas/vapour constant,

K is the discharge coefficient,

A is the discharge area (m),

P is the vessel design pressure x 1.2 (N/m), (or upstream pressure for holes in equipment)

M is the molecular weight,

T is the relief valve inlet temperature (K).

The value of C is 145 for propane and butane.

The discharge coefficient (K) of a relief valve varies with the inlet and disc shape and also liftcharacteristics. It should be taken as 0.9.

Tables 1(A), (B) and (C) give the discharge area (A) for typical relief valves.

Equation (1) may also be used for vapour flow through holes in equipment. In this case thevalue for the discharge coefficient (K) should be 0.8.

For the case of tanks affected by fire, equation 1 reduces to the following working equations(2) + (3):

For propane tanks:

W = 419

KAP (Kg/s) (2)

For butane tanks:

W = 367

KAP (Kg/s) (3)

In this case the relief valve inlet temperature (T) has been taken as 100C, not the equilibriumtemperature for the pressure at which the relief valve is blowing. The temperature of 100C isbased on experimental fire engulfment trials.

3.2.2 Liquid leakage flows

Simple liquid flows may generally be calculated by the use of Bernoulli's equation. Examplesinclude leaks at temperatures well below the boiling point, or orifice type leaks driven by theproduct head. In general, however, leakage flows will be two phase in nature with varyingvapour to liquid ratios.

Propane and propane/butane mixtures are generally handled as liquids well above theiratmospheric boiling points, so that a large fraction will flash during emission. W ith butane,however, it is more likely that on occasion the temperature may fall below its boiling point atatmospheric pressure. Whilst this condition increases the likelihood that the leakage flow hasa higher liquid percentage, significant vapour emission will still arise. This may be understoodby examining Figure 01.02.08.01 of the Manual/PTS, which shows the absolute vapourpressures of those light hydrocarbons which are the principal components of commercialLPG.

In general terms, the calculation of flashing LPG flows from pressurized systems is complexsince rapid evaporation of the liquid can take place before, during and after emission. Theprecise form of emission, pipe rupture, flange leak, valve leak, hose burst, etc., can play amajor part in conditioning the resultant flow.

For simplicity the equations presented in these Guidelines are adequate for nearly all leakagesituations that an assessment team could meet. More generally, however, the calculation oftwo phase flows is complex so that where greater precision in calculation becomes importantthe matter should be referred to PETRONAS.

Appendix 3 of the Institute of Petroleum Model Code of Safe Practice, Part 9, LiquefiedPetroleum Gas, Volume 1, describes the calculation of release rates based on the applicationof a typical simple equation which assumes a homogeneous equilibrium two-phase flashingliquid release from the orifice. The discharge coefficient has been taken as 1.0. Theseassumptions will obviously produce leakage rates at variance with those calculated using theequations set out herein.

3.2.2.1 Overflows through relief valve vents

This case can arise when the relief valve opens as a result of product being pumped into thevessel after it is full. Maximum liquid flow rate into the vessel should be used for theconsequence calculations.

It is recommended that the capacity of the relief valve is checked for this flow, since changes tothe LPG handling system may have been made since the system was designed. Therecommended method is to be found in section 3.17 of API RP 521. The flow formulae to beused are presented in Appendix C of API RP 520 (Ref. 3).

3.2.2.2 Flows from broken equipment

When pressurized LPG is released from containment the discharge is usually a two-phasemixture of vapour and liquid. The behaviour of such a discharge is difficult to analyse and notfully understood. There is a maximum discharge rate which exists for a two-phase mixture. Thisoccurs at some critical pressure ratio between the upstream pressure and the exit pressure.

Several methods have been proposed to evaluate the critical discharge rate of a two-phaseflow from a pipe. Here the simple equations described in Ramskill's review are used . Theequation to be used is dependent on the length/diameter ratio (L/D) of the leakage path. In allthe equations used here the discharge coefficient is taken as 0.6.

Three leakage path situations may be considered:

For L/D = 0

We have orifice flow and the flow rate is described by:

M = 0.6 A )PP(2 o a-r ( 4 )

where

M is mass flow rate (kg/s)

Po is upstream pressure (N/m2)

Pa is atmospheric pressure (N/m2)

A is area of hole (m2)

r is density (kg/m3)

Examples of this situation are small holes in pipes or equipment, e.g. due to corrosion

For 0 < L/D

(2) The critical temperature Tc ( K ) corresponding to the satured choke pressurePc is then found using vapour pressure data.

(3) Assuming thermodynamic equilibrium, the vapour mass fraction which would flash offfrom the liquid is calculated :

m = 1 exp ( l

-c(T1 Tc )) (8)

where

m = vapour mass fraction

c = liquid specific heat (j/Kg) at Tc

l = latent heat of vaporization of liquid (J/Kg) at Tc

T1 = reservoir storage temperature (K)

Note : If m is negative then liquid flow only will occur and equation (4) should be used to calculated the flow rate.

(4) Assuming homogeneous mixing and so slip between the phases, the mixture density is calculated as :

r r rcmg

m= +

-

-11

1

(9)

where

rg is the vapour density at Tc and Pc (kg/m3)

r1 is the liquid density at Tc and Pc (kg/m3)

(5) The standard discharge formula is then used to calculate the critical flow rate :

M = 0.6 A 2rc o cP P( )- ( 10 )where M is the mass flow rate ( kg/s)

This calculation should be used for large leaks and ruptures of lines and hoses, wheretwo-phases flow will form upstream of the break. Note that if the leak is fed by a pumpthe flow rate could be determine by the pump capacity rather than the flow regime.

In the case of a pipe rupture the escaping LPG will probably be fed from both sides ofthe break, e.g. from the pump as well as from the receiving vessel.

3.3 VAPOUR JETS DISPERSION AND FIRES

3.3.1 Relief valve vapour releases

Single phase (vapour only) emission is assumed from vessel relief valves, as generallyobserved under fire engulfment conditions. The case of vessel overfill and subsequent twophase emission through the pipe is taken as a special case of the two-phase jets and firesdealt with in the next section.

For the discussions of relief valve dispersions and fires which follow ,dispersion and jet fireradiation calculations have carried out for wide range of emitted vapour flow rates, whichcover all sizes of tanks in general use within Group operations. The selected flow ratesare:

Propane : 2.5, 5, 7.5,10,15, 20, 25, 30, 40 ( kg/s)

Butane : 2.5, 5, 7.5, 10,15, 20, 25, 30 ( kg/s)

In deriving the various consequences it is necessary to make various assumption in regard tothe particular facility and equipment. In order to carry out calculation to complete Tables 2 and3,typical tanks sizes, valves sizes, vent stack height and length have been used (see Figure1) . Where the facility under examination differs from this model facility, interpolation betweenthe calculated figures will be necessary.

3.3.1.1 Dispersion

As the source of emission under consideration in this section is vapour only, thePLUMEPATH dispersion package has been used for emissions well above ground level. Anexample is shown in Figure 2 for butane emitted from a vertical relief valve vent pipe.Dispersion plumes have been calculated for the mass flow rates set out above. In all thesecases mixing with air occurs rapidly and it may be assumed that flammable plumes do notreach down to the ground. Therefore no dispersion data is presented here for vapouremission from relief valves.

3.3.1.2 Fires

The radiation fields generated by relief valve fires have been modeled with computerpackages which have been developed by Thornton Research Centre and validated byexperimental studies. Tables 2 and 3 present distances to the critical ground level fluxes of1.5, 5, 8 and 13 kW/m and to the critical tank top level fluxes of 8, 32 and 44 kW/m, asidentified in the Manual/PTS.

A typical wind speed of 5 m/s has been assumed in all calculations. It should be noted that ifwind speeds are considerably higher radiation flux levels close to the stack can besignificantly higher. If this could be critical, further radiation calculations will be required forhigher wind speeds in which case PETRONAS should be consulted.

As may be seen from the Tables the higher flux levels are frequently not achieved. In additionto the critical distance data, the Tables also present flame lengths, flame lift-off and stackoutlet height above grade.

3.3.2 Other vapour releases

For the dispersion of vapor releases from sources which do not behave as relief valvereleases, in that they may be horizontal and near the ground, dispersion calculations will bevery unreliable. This is because the presence of other equipment in the area will causeunpredictable air movement. The distance to lower flammable limit (LFL) will be smaller thanthe conservative estimates which can be obtained by using the data in Figure 3 for liquidreleases.

For the assessment of vapour fires reference should be made to the vapour fire curves shownin Figures 4 to 16 (a description is given in section 3.4.2). As propane is less radiative thanbutane these results may be used for propane as well.

3.4 TWO-PHASE JETS - DISPERSION AND FIRES

Two-phase jets may arise from a wide range of events in a facility and may lead to a variety ofconsequences. As set out in Part One of these Guide-lines, the following situations have beenidentified as having the potential for the creation of a serious incident.

- flange (joint) leak

- pump seal leak

- open drain valve

- small bore pipe rupture

- hose leak or rupture

- tank overfill/relief valve discharge

As may be seen from this list, a more disparate collection of release situations is possible incomparison to the relief valve cases considered in the previous section. Again consequenceinformation is presented calculated for a wide range of mass flow rates at the source. The onlygeometrical factor which has been introduced to help differentiate two phase discharge cases isthe division into horizontal and vertical emissions.

A wide range of flow rates to cover the range and scale of the principal incidents listed abovehave been considered.

3.4.1 Dispersion

Before describing the dispersion calculations adopted and results obtained, some words ofcaution are necessary. The calculation of highly-turbulent flashing two-phase flows is an activeand difficult research field. The guidance provided here is therefore subject to revision as bettermodels are developed and validated.

The principal purpose of the dispersion calculations is to assess the extent of the flammablecloud which will be formed. The distance to the lower flammable limit (LFL) is thus the mainparameter calculated. This then enables a judgement to be made in regard to facilities engulfedby the flammable plume and the proximity to the site boundary. As mentioned in Part One, theinitial consequence of ignition of this cloud will be a transient flash fire which although of highintensity will only seriously affect people within it. The major effect of a flash fire will be toinitiate a jet fire or, much less likely, a pool fire.

Dispersion distances for dense gases, such as LPG vapours, are strongly dependent on thenature of the surface over which the gas disperses. The relevant parameter used in the models,called the surface roughness length, can be estimated. A reasonable conservative valuerelevant to typical LPG facilities is 0.1 m.

The vertical temperature gradient in the atmosphere has a considerable effect on gasdispersion. Strong surface cooling, under clear skies at night, and a low wind produce stableconditions. Weather conditions are denoted by letters A to F (after Pasquill - discussed in Ref.4). The most stable conditions are denoted by the letter F. Neutral conditions, under cloudyskies or in higher winds, are most common, and are given the letter D. Strong sunshine in thedaytime, with low winds, produces unstable temperature gradients, the most extreme beingdenoted by A. In these circumstances, gas will dilute in a shorter distance than for D stability.The largest dispersion distances are found in F stability weather. Calculations are normallyperformed for 5 m/s wind speed and D stability (5D) to give typical average results. 2 m/s windand F stability (2F) represents a typical worst case.

3.4.1.1 Horizontal dispersion

The source for the dispersion calculations was assumed to be a flashing jet of pressurizedliquid propane or butane. No dilution due to the jet has been assumed. For horizontal jetsdirected downwind, this dilution is counteracted by the fact that the jet pushes the gasfurther downwind. For other jet orientations, the results are conservative (predicteddispersion distances greater than actual).

Figure 3 presents distances to LFL for the 5D and 2F atmospheric conditions for a surfaceroughness factor of 0.1 metre.

3.4.1.2 Vertical dispersion

Calculation methods for two-phase vertical releases are still being developed. In themeantime the horizontal dispersion calculations described above may be taken as a worstcase analysis of the vertical dispersion case. Where this failure mode is critical,PETRONAS should be asked to advise.

3.4.2 Fires

The radiation fields generated by two-phase Jets have been calculated with computer modelswhich have now been partially validated by large-scale jet fire trials. A typical worst case windspeed of 5 m/s has been assumed in all calculations.

Results for varying mass flow rates are presented for butane in Figures 4 to 16. As propane isless radiative than butane, these results may be used for propane also.

3.4.2.1 Horizontal jet fires

Curves showing the distance to critical flux levels for a range of mass flow rates arepresented in Figures 4 to 9. These give radiation fluxes at ground level. The flame shapesand lengths are different for liquid and vapour fires. Two limiting curves are shown, theupper for liquid and the lower for vapour fires. Depending on the precise ratio of liquid tovapour, actual cases will lie between the two extremes. In the high momentum jet fireswhich these Figures represent, jet momentum decreases with increasing distance alongthe jet until at a given point buoyancy forces become dominant and the flame lifts off theground. This phenomenon enables an effective jet impingement distance to be defined.Figure 10 shows the horizontally projected flame lengths calculated for the range of flowrates.

3.4.2.2 Vertical jet fires

In general, vertical jet fires may be expected to have a smaller radiative impact than thecorresponding horizontal jet fire. A set of curves showing the downwind distance to criticalflux levels for vertical jet fires with a wind speed of 5 m/s is presented in Figures 11 to 16.These do not give exactly the same figures as Tables 2 and 3 because there is no stackpipe and the emission conditions are different.

4. WORKED EXAMPLE

In order to demonstrate the application of Sections 2 and 3 of the Guidelines, a workedexample of a study at an existing facility has been completed. This part of the Guidelinesfollows the assumptions, considerations and calculations involved in that example.

4.1 DESCRIPTION OF FACILITY

The facility is an LPG depot which is supplied by barge via the local river. It comprises abarge unloading berth, bulk storage for both commercial propane and commercial butane,and bulk road vehicle loading. The unloading berth is separated from the remainder of thefacility by a motorway. Figures 17 and 18 show the depot layout and a simplified flow scheme,respectively.

4.1.1 Barge unloading berth

The unloading berth includes two 100 mm dia loading arms, one for liquid service, theother for vapour. Barge unloading is performed using a shore-based compressor. Theloading arms are hydraulically operated from a control station at the berth. Each armcontains several swivel joints and the connection with the barge is via a flanged joint. Thearms are not equipped with a breakaway coupling or other quick release device.

On the shore side of the loading arms, the line from each arm bifurcates for eitherpropane or butane service. The vapour line is equipped with an hydraulically-operatedball valve before the line bifurcates. This valve is actuated electrically from thecompressor station. The liquid line has no such valve.

As soon as each line bifurcates the four lines (i.e. propane liquid, butane liquid, propanevapour, butane vapour) are each fitted with two flanged manually-operated ball valves.Each liquid line is then fitted with a flanged non-return valve. There are several small borefittings for instruments, draining and hydrostatic pressure relief in this area.

Once the pipelines are clear of the valve locations, the two vapour lines are lagged toprevent vapour condensation.

Firefighting facilities at the barge berth consist of dry chemical fire extinguishers. Ahydrant is located on top of the river bank some 30 metres from the loading arms. Thishydrant is partially obstructed from the berth by a building housing fire pumps which takesuction from the river. A fire alarm is located on the berth.

There is no emergency shutdown or leak detection system in place on the berth apartfrom the remote operated valve on the vapour line.

4.1.2 Bulk storage

The vapour pipelines to the barge berth are routed via a compressor which takes suctioneither from the vapour space on one of four butane spheres (each of 1 025 m3 capacity)or from the vapour space of one propane sphere (1 750 m3 capacity). The liquid filling lineinto the bottom of each butane sphere also doubles as a liquid withdrawal pipe. The liquidfilling line into the propane sphere is directed into the top of the vessel.

The four butane spheres each have two flanged connections directly under the vessel.These connections are liquid inlet/outlet and drain. The liquid inlet/outlet is fitted with aflanged hydraulically-operated ball valve followed by a Shand and Jurs hydraulic valve.The valve nearer the sphere includes a fusible link which is designed to cause the valveto fail closed under fire engulfment conditions. The drain connection is equipped with aflanged manually-operated ball valve followed by a spring-loaded dead-man ball valve.The drain line then extends beyond the periphery of the sphere. This line is lagged as isthe liquid inlet/ outlet line.

The top connections into each butane sphere comprise vapour line, pressure reliefvalves, Whessoe contents gauge and maximum fill level float gauge. All connections areflanged. The vapour line is equipped with an hydraulically-operated ball valve, againactuated electrically from the compressor station. The Whessoe contents gauge is localreadout only. The maximum fill level float gauge is linked to the compressor station suchthat in the event of overfill the compressor will cut out.

The propane sphere has a welded line from the base of the sphere to the primary valvewhich is located at the edge of the bunded area some ten metres from the periphery ofthe tank. The primary valve is flanged and is remotely operated. It is followed again by aShand and Jurs valve. A catchpot is located downstream from these valves and a laggeddrain line connected to the pot. The drain line is fitted with a manual valve followed by aspring loaded valve.

The top nozzles into the propane sphere are as for the butane spheres plus the liquidinlet line. The liquid inlet line is fitted with an hydraulically-operated ball valve withelectrical actuation from the compressor station as above.

All pressure relief valves discharge direct to atmosphere. There is no flare or vent system.

All spheres are located in separate low-bunded areas. All are equipped with a waterspray sprinkler system.

The nozzle details for the propane and the butane spheres are indicated in the attachedFigures 19 and 20.

4.1.3 Bulk road vehicle loading

Product stored at the depot is pumped into bulk road vehicles. The bulk loading facilityincludes a swivel-jointed loading arm for liquid transfer. There is no vapour return line.Connection with the road vehicle is made using an Acme coupling. The loading arms areequipped with a manually-operated valve adjacent to the Acme coupling. There is nobreakaway coupling fitted or other driveaway protection/prevention. A remotely-operatedvalve is installed in the line upstream from the loading am. This valve is operated from anemergency stop located directly adjacent to the loading point. Loading is performed usinga preset turbine meter. The loading area is protected from vehicular damage by highwayguard-railing.

Firefighting facilities at the loading area consist of hand held fire extinguishers plusadjacent fire hydrants. There is no sprinkler system nor are there fire water monitors.

4.1.4 Depot operation

Barge unloading is completed under the control of a shore-based supervisor inconjunction with barge crew. The supervisor does not stay at the barge berth during theoperation; he is mostly at the compressor station which is located over 100 metres awayfrom the berth and not in direct line of sight. The supervisor checks the pipelines and theberth about once per hour during unloading operations. The unloading rates areapproximately 180 m3/hour for propane and 140 m3/hour for butane. At the completion ofunloading, liquid lines are blown clear of liquid as far as is possible, using the plantcompressor.

The compressor station is equipped with a control panel which enables the supervisor toset the remotely operated valves on the storage tanks and on the vapour line at the bargeberth. The supervisor thus controls transfer of product into the spheres. As stated above,there is no remote readout of product level in each tank, only a high level overfill cutout.Filling level is controlled by observing the local readout from the Whessoe gauges.

The bulk road vehicle loading operation is controlled by the vehicle driver. The depotsupervisor is usually not present during loading operations. The driver does not haveaccess to storage tank valves, only to valves in the delivery pipework. Loading rate isapproximately 20 m3/hour.

4.2 AUDIT OF THE FACILITY AND CHOICE OF SCENARIOS

The audit of the facility was completed by a team of four people including the depotoperations superintendent, a company safety adviser and two LPG engineering specialists.The team spent a day on site gathering information and inspecting the layout, operation andmaintenance of the facility. Depot staff were able to advise on operations and maintenanceprocedures and engineering staff provided layout drawings, flow schemes and technical data(i.e. pump curves, pressure relief valve data, etc.).

Having assembled all relevant information the team then studied the facility to determinethose situations which might result in leakage of LPG. The approach adopted was tocommence with the barge berth and then follow the pipe track to the compressor station, intothe storage vessels, and then out to the bulk vehicle loading point. Using the procedure setout in Section 2.1 the team identified a number of leakage scenarios which they considered tobe credible and/or the consequences potentially severe.

4.2.1 Barge berth

4.2.1.1 Scenario 1

There are no protective devices on the loading arms (e.g. breakaway couplings etc.) toprevent damage or rupture in the event of excessive barge movement. It was thereforedecided that loading arm rupture should be considered. There are no emergencyshutdown valves on the barge so full bore continuous flow driven by the pressure in thebarge tank was adopted. There are similarly no emergency shutdown valves on the liquidlines to the storage tanks on shore. Although the non-return valve in these linesdecreases the probability of liquid flow from the pipelines, it cannot be relied upon foremergency shutdown purposes. The pipeline must therefore also be included as anadditional leakage source.

Scenario 1: Loading am rupture. Leak fed by barge storage and lines on shore.

4.2.1.2 Scenario 2

The barge berth area contains many flanged joints, the loading arms contain severalswivel joints, and there are numerous small bore connections in the area. The number ofjoints and connections warrants the consideration of a flange or joint leak.As the liquid line is blown clear after each delivery, there is no need to consider theeffects of barge impact during berthing.

Scenario 2: Flange, swivel joint, or small bore connection leak during barge discharge.

4.2.1.3 Scenario 3

The pipelines from the barge berth to the storage vessels run parallel and adjacent to themotorway for some distance before crossing under the motorway. The pipe track is lowerthan the motorway and it is conceivable that either a vehicle or goods from a vehiclecould leave the motorway and impact upon the pipelines.

Scenario 3: Damage to pipelines due to vehicular or vehicular goods impact.

4.2.2 Pipe track

4.2.2.1 Scenario 4

The pipelines from the barge berth to the storage tanks are flanged along the entirelength. Where the pipelines enter the depot after crossing under the motorway there are anumber of redundant valves and connections which, as above, warrant the considerationof a flange leak in this area. The pipelines enter the depot at the south west corner of theproperty. A concrete block wall in excess of two metres high separates the depot from theneighbouring property.

Scenario 4: Flange leak on pipe track at entry point into depot.

4.2.3 Storage vessels

4.2.3.1 Scenario 5

The pipelines at the barge berth and throughout the depot are not clearly marked toindicate product carried. There is no interlock in the valving system to prevent propanebeing delivered into the butane pipework. Given the above, it is quite conceivable thatpropane could be delivered into the butane spheres. The butane spheres are notdesigned for propane vapour pressure.

Scenario 5: Propane delivered into butane rated spheres.

4.2.3.2 Scenario 6

The butane spheres are fitted with flanges on the sphere side of the primary valve on theliquid inlet/outlet and on the drain line. Given the inability to control a leak from theseflanges and the sphere inventory, leaks from these flanges must be considered.

Scenario 6: Leaks from flanged joints on butane spheres on sphere side of primary valve.

4.2.3.3 Scenario 7

Based on Scenario 6, a fire fed from a flange leak underneath the butane spheres mustbe considered. This fire may in turn lead to overpressurisation of the vessel andconsequent vapour discharge through the vessel relief valve(s).

Scenario 7: Vapour release from butane sphere pressure relief valve due to fire engulfment.

4.2.3.4 Scenario 8

The Whessoe level gauges on the butane and propane spheres are only of the localindicator type. They are not equipped with alarm or emergency shutdown features. Thelack of a remote readout at the control point in the compressor station is likely to lead toan estimation of ullage by the depot supervisor and the possibility of error.

The maximum fill level gauge can only be tested by exceeding the safe filling level. Thegauge is not tested and its reliability is therefore questionable.

The level instrumentation as described above on both the butane and propane spheres isinadequate. Overfill of the spheres must therefore be considered.

In the event of overfill, liquid will flow through the vapour suction line to the compressor.The knockout drum adjacent to the compressor is not equipped with any liquid levelalarms or trips. Liquid could therefore enter the compressor giving a liquid stroke. Thiswould cause a major leak.

Scenario 8: Overfill of propane/butane spheres leading to major leak at compressor.

4.2.3.5 Scenario 9

The propane sphere is fitted with a flanged connection on the liquid outlet line on thesphere side of the primary valve at the edge of the bunded area. As for the butane case aleak from this flange must be considered.

Scenario 9: Leak from flanged joint on propane sphere on sphere side of primary valve.

4.2.3.6 Scenario 10

A fire fed from the flanged joint leak on the propane sphere in Scenario 9 may lead tooverpressurisation of the vessel and consequent vapour discharge through the vesselrelief valve(s).

Scenario 10: Vapour release from propane sphere pressure relief valve due to fire engulfment.

4.2.4 Product transfer

4.2.4.1 Scenario 11

The pumps for transfer of product from the storage spheres to the bulk vehicle loadingpoint are located at the edge of the bunded area approximately 10 metres from the tankshell. Each pump is fitted with a mechanical seal but not with a throttle bush. Total failureof a pump seal is not a common event but neither is it rare enough to discount.

Scenario 11: Leak from pump seal due to total failure of seal.

4.2.4.2 Scenario 12

The compressor station is an area of complicated pipework again with many flangedjoints and small-bore connections.

Scenario 12: Flange or small-bore connection leak at compressor station.

4.2.5 Bulk road vehicle loading

4.2.5.1 Scenario 13

As for the barge berth there are no protective devices on the loading am at the bulk roadvehicle loading point to prevent damage or rupture in the event of a driveaway. Loadingam rupture is therefore included. The only emergency stop button in this area is locatedadjacent to the loading point. It would probably be inaccessible in the event of loading amrupture.

The bulk vehicles are equipped with emergency stop buttons on each corner of the tankframe. However, given that a positive action is required to activate these buttons and thatsuch action is required adjacent to a large leak. the vehicle should still be considered as aleakage source.

Scenario 13: Loading arm rupture. Leak fed by delivery pump and bulk road vehicle storage tank.

4.2.5.2 Scenario 14

The loading area also contains many flanged and swivel joints and small-bore fittings.

Scenario 14: Flange, swivel joint or small bore connection leak at bulk road vehicle loading point.

4.2.5.3 Scenario 15

The coupling between delivery pipework and vehicle is a vulnerable area in that the drivermay not make the coupling correctly or a sealing ring may be damaged or missing. Thecoupling must be made at each delivery. The frequency of connection warrants theinclusion of possible leakage.

Scenario 15: Leak from incorrect or damaged coupling.

4.3 CONSEQUENCE ASSESSMENT, ANALYSIS AND PROPOSED ACTION

In this section each of the leakage scenarios is considered in turn and a consequenceassessment is completed as described in Section 3. The acceptability of the consequences arethen considered and possible actions suggested to reduce the probability of the scenarios,reduce their impact by minimising the leakage rate, and/or provide additional protection topeople, property and equipment.

The possible actions suggested for this example should not be seen as 'correct answers' forthese scenarios. Neither are they exhaustive. They are merely typical of the types of actionwhich might be relevant to a particular situation. Actions should be selected according tospecific site requirements.

W ith the exception of vapour release from pressure relief valves, the consequenceassessments have only been completed for leaks from liquid pipelines since these leaks givemore severe consequences than for the vapour case.

If the proposed actions for offending scenarios in liquid lines are not to be also applied for thevapour cases then these cases must be considered in their own right.

4.3.1 Scenario 1: Barge berth loading arm rupture

Calculations have only been completed for the propane case since the vapour pressure,release rate and consequent hazards are higher than for butane.

Leak = Two phase leak from 100 mm dia hole

From Section 3.2.2.2

Adopt L/D >12

Pc = 0.55 Po

For Po choose equilibrium vapour pressure at 15C =

9 x 105 N/m2 (Ref Fig 01.02.08.01 of Manual /PTS)

Pc = 0.55 x 9 x105

= 4.9 x 105 N/m2

Tc = 268K

m = 1 exp [ l

-c( T1 Tc)]

with c = 2 407 J/kgK (Ref Fig 01.02.11.01 of Manual /PTS)

and l = 383 000 J/Kg (Ref Fig 01.02.10.01 of Manual /PTS)

m = 1 - exp -

-

2407

383000288 268( )

= 0.12

Mixture density rc = m m

gr r+

-

-1

1

1

rc = 01293

088535

1..

.+

-

(Ref Tables 01.02.04.01 and 01.02.0.03. of Manual /PTS.) = 69kg/m3

Critical flow rate

M = 0.6 A )(2 coc PP -r

= 0.6 x p x

x x x014

2 69 9 10 49 102

5 5. ( . )-

= 35.4 kg/sec

From Figure 3 estimate distance to LFL as follows:

5D conditions - 105 m 2F conditions - 190 m

Probability of early ignition is high. Vehicles on motorway most likely ignition source.

From Figures 4 to 16 distances to radiation flux levels from the ignited mixture are as follows:

Horizontal Jet Fires Vertical Jet Fires

1.5 kW/m 140 m 120 m

5 kW/m 110 m 65 m

13 kW/m 90 m 40 m

This scenario is clearly unacceptable, even though the leak considered above is fed by thebarge storage alone. The leakage rate would be much greater if the shore based pipelineswere also to contribute to the leak.

The consequences of this incident are so severe that the leakage rate and duration should bereduced. The probability of the incident should also be reduced.

Possible means of achieving the above are:

Emergency release couplings on loading arms.

An inter-related system of remotely-operated fail-safe emergency shutdown valves atthe termination of the barge pipework and on the berth on the shore side of the loadingarms.

Inclusion of fusible links in emergency shutdown systems.

Supervision during entire unloading operation at the berth.

Improved fire protection/fire fighting and gas detection/ gas dispersion facilities.

Construction of a water curtain adjacent to the motorway.

4.3.2 Scenario 2: Flange., swivel joint, or small-bore connection leak at barge loadingberth

In this scenario, typically consider one segment of a flange gasket blown out. Based on anumber of pipe sizes from 50 mm to 300 mm diameter. the average leakage path areaequates to an effective hole diameter of 12 mm. A small-bore connection leak would give asimilar case. As above, the calculation is only completed for the propane case.

From Section 3.2.2.2

Adopt L/D = 0

M = 0.6A 2r( )P Po a-

= 0.6 x p x

x x x00124

2 510 9 10 1 102

5 5. ( )-

= 2.0 kg/sec


5D conditions - 25 m2F conditions - 35 m.

The elevation of the barge berth is lower than the motorway and the berth is elevated abovethe river. The above distances will therefore be conservative because LPG vapour is heavierthan air and the vapour will tend to fall to the river.

From Figs 4 to 16 distances to radiation flux levels from liquid fires are as follows:

Horizontal Jet Fires Vertical Jet Fires

1.5 kW/m 55 m 45 m5 kW/m 40 m 30 m

8 kW/m 40 m 25 m

13 kW/m 35 m 20 m

The scenario is unacceptable. The radiation flux levels on the motorway particularly areexcessive.

The probability of leakage should be reduced.

The pipework in the barge berth area contains many joints and small bore connections.Possible action to reduce the probability of leakage includes:

Rationalisation of pipework to reduce number of joints/ connections.

Replace flanged joints with welded joints.

In addition, possible action to prevent the vapour cloud reaching the motorway and/or toprotect the motorway from the effects of an ignited leak includes:

Construction of vapour barrier wall.

Installation of water curtain with or without automatic actuation triggered by gas detectorson the berth.

4.3.3 Scenario 3: Damage to pipelines due to vehicular or vehicular goods impact

In this scenario the range of damage caused by impact could range from shearing thepipelines to springing a flange. Typical leakage rates for these two extremes have alreadybeen considered in the above two scenarios.

As both the above scenarios are unacceptable then this scenario is also unacceptable.

Possible actions are:

Install reinforced highway guard-railing adjacent to pipe track.

Bury pipelines in this area.

Install remote operated emergency shutdown valves at either end of vulnerable pipework.

4.3.4 Scenario 4: Flange leak on pipe track

The leakage rate in this scenario will be as for the similar case at the barge berth (i.e. M =2.0 kg/sec). Where the pipelines enter the depot there is significant local confinement,especially in a disused compressor shed. Confinement raises the possibility of vapourcloud explosion in the event of ignition. The probability of ignition is high due to theadjacent motorway and offices. This scenario is unacceptable.

Possible actions:

Reduce probability of leakage as for Scenario 2.

Examine need for disused building and demolish if possible and/or investigate othermeans to reduce confinement.

4.3.5 Scenario 5: Propane delivered into butane rated spheres

In order to calculate the vapour pressure of a mixture created by delivering propane intothe butane spheres, a worst case of assuming 100 per cent propane at a receipttemperature of 25C into an empty sphere has been adopted.

The vapour pressure of propane at 25C is 11.5 x 105 N/m which is higher than thedesign pressure of the butane sphere of 7.3 x 105 N/m . This is also the start-to-discharge pressure of the relief valves. The fully open pressure (accumulated pressure) is8.0 x 105 N/m.

The scenario is unacceptable. If product contamination is a credible scenario, then therelief valves should be sized to relieve sufficient propane vapour to avoid over-pressurization of the vessel.

Possible actions to prevent product contamination are:

Clear and positive identification of pipelines throughout the depot and particularly at thebarge berth.

Introduction of a valve interlock system to ensure only one set of valves is open at thebarge berth.

Installation of in-line densitometers linked to an alarm/ shutdown system to detectincorrect product in pipelines.

4.3.6 Scenario 6: Leaks from flanged Joints on butane spheres on sphere side of primaryvalve

The leakage in this scenario will again be based on leakage from an equivalent holediameter of 12 mm.

From Scenario 2

M = 0.6A 2r( )P Po a-

= 0.6 x p x

x x x0012

42 577 31 10 1 10

25 5. (. )-

= 1.1 kg/sec


5D conditions - 15 m2F conditions - 25 m

From Figs 4, 5, 6, 7, 9 distances to radiation flux levels from liquid fires are:

Horizontal Jet Fires

1.5 kW/m 40 m

5 kW/m 30 m

8 kW/m 25 m

13 kW/m 25 m

44 kW/m 20 m

Vertical jet fires will impinge on the vessel shell in all cases. No radiation distances aretherefore given.

In the event of ignition the radiation effects on the sphere and on adjacent spheres exceed 44kW/m and are therefore unacceptable.

Possible actions to reduce the probability of this scenario are:

Modification to sphere pipework and provision of correct valve to remove flanged jointupstream of primary valve.

4.3.7 Scenario 7: Vapour release from butane sphere pressure relief valve

From Section 3.2.1, for fire-engulfed butane tanks

W = 367

KAP kg/sec

Spheres are equipped with two pressure relief valves although only one valve is lined up atany time. The valves are labelled as 6R10.

From Table 1c for R orifice,

Discharge area = 103.23 x 10-4 m

W = 09 10323 10 73 12 10

367

4 5. . . .x x x x x-

= 22.2 kg/sec.

Section 3.3.1.1 describes that flammable vapour plumes from vertical relief valves may beassumed not to reach ground level. The distance to LFL is therefore not applicable in thisinstance.

From Table 3, distance downwind to radiation flux levels are:

Tank Top Radiation Flux

8 kW/m - 45m44 kW/m - -

Ground Level Radiation Flux

1.5 kW/m - 95 m5 kW/m - 40 m13 kW/m - -

The above flux levels indicate that the 5 kW/m radiation contour crosses the site boundary.Whilst this boundary should probably be described as an urban area, the region adjacent tothe boundary is not developed and a higher radiation intensity could be tolerated. Given thatthe flux level at the plant boundary is not much greater than 5 kW/m, this scenario isconsidered to be acceptable and the existing location of the butane sphere closest to theplant boundary does not warrant any action. Any future development outside the plant shouldbe monitored as construction of a hospital, school or other facility difficult to evacuate at shortnotice would require a reappraisal of the situation.

4.3.8 Scenario 8: Overfill of propane/butane spheres

Overfill of the propane or butane spheres will not result in product leakage from thespheres but will result in liquid flow through the suction line to the compressor.

A liquid stroke in the compressor is unacceptable.


Installation of remote readout level gauge in transfer control area.

Installation of level gauge capable of registering low level, high level and high-high level,with alarms and inlet valve shutdown devices attached. Gauges should permit regulartesting to ensure satisfactory operation.

Regular testing of maximum fill level gauge.

Installation of liquid level emergency alarm on knockout drum with compressor trip.

4.3.9 Scenario 9: Leak from flanged joint on propane sphere on sphere side of primaryvalve

This scenario as per Scenario 2:

M = 2.0 kg/sec.

Distances to LFL:


Distances to radiation flux levels from liquid fires are:

Horizontal Jet FiresVertical Jet Fires

1.5 kW/m

5kW/m

8 kW/m

13 kW/m

44 kW/m

55 m

40 m

40 m

35 m

30 m

45 m

30 m

25 m

20 m

10 m

The radiation flux level on the propane sphere in the event of an ignited leak is excessive,although the location of the primary valve remote from the sphere mitigates the impact on thesphere. Possible actions are as for Scenario 6.

4.3.10 Scenario 10: Vapour release from propane sphere pressure relief valve due to fireengulfment

From Section 3.2.1 for fire engulfed propane tanks:

W = 419

KAP kg/sec

The propane sphere is equipped as per the butane spheres with two pressure relief valvesalthough only one is lined up at any time. The valve is labelled 6R10.

From Table 1C for R orifice:

Discharge area = 103.23 x 10-4 m

W = 09 10323 10 155 12 10

419

4 5. . . .x x x x x-

= 41.2 kg/sec

As for the butane case, the distance to LFL is not applicable as the flammable plume may beassumed not to reach ground level.

From Table 2, distance downwind to radiation flux levels are:

Tank top Radiation flux

8 kW/m - 50 m44 kW/m - -

Ground level radiation flux:

1.5 kW/m - 1105 kW/m - 4013 kW/m - -

As for the butane case, the 5 KW/m2 radiation contour crosses the site boundary, The figuresare similar for both the propone and butane case and the points set out for the butane case aretherefore also applicable to this case.

4.3.11 Scenario 11: Leak from pump seal due to total failure of seal

Adopt L/D = 0

For pumps without throttle bushing, adopt effective diameter hole =17mm

M = 0.6A 2r( )P P )o a-

(a) For Propane

M = 0.6 x p x

x x x00174

2 510 9 10 1 10 )2

5 5. ( )-

= 4.0 kg/sec

From Figure 3 estimate distance to radiation flux levels from liquid fires are :

5D condition 35m2F condition 55m

From Figures 4 to 16 distances to radiation flux levels from liquid fires are:


1.5 kW/m

5 kW/m

8 kW/m

13 kW/m

44 kW/m

80 m

60 m

55 m

50 m

45m

65 m

45 m

40 m

30 m

15 m

(b) For butane

M = 0.6 x p x

x x x00174

2 577 31 10 1 10 2

5 5. (. )-

= 2.2 kg/sec


5D condition 25m2F condition 35m



1.5 kW/m

5 kW/m

8 kW/m

13 kW/m

44 kW/m

60 m

45 m

40 m

35 m

30 m

50 m

30 m

25 m

20 m

10 m

The transfer pumps are positioned at three locations in the depot. The propane pumphas the greatest potential for impact outside the depot whereas the butane pumpshave greater potential for internal impact. The vapour cloud from the propane pumpseal failure extends well beyond the depot boundary. Ignition of this leak givesradiation flux levels which are unacceptable beyond the plant boundary. The fluxlevels from the butane pump seal failure within the depot are also unacceptable forprocess, protected work and work areas.


Installation of throttle bushes. This has the effect of reducing effective holediameter from 17 mm to 10 mm.

Installation of improved integrity seals (e.g. double mechanical seals).

Review of operating and maintenance procedures to ensure rotatingequipment is regularly inspected and serviced.

4.3.12 Scenario 12: Flange, small bore connection leak at compressor station

This consequences of this scenario are similar to those of Scenario 2.

A leak in this area could well see the vapour cloud extend into the residential area wherethe probability of ignition is high. The flux levels from an ignited release impinging on theresidential area are excessive and action is required to reduce the probability of leakageand/or reduce flux levels in the residential area.


Reduce probability of leakage as for Scenario 2.

Construction of fire wall or water curtain at property boundary to limit flux levels beyondthe boundary.

4.3.13 Scenario 13: Bulk road vehicle loading am rupture

As in Scenario 1 this situation has only been calculated for propane.

From typical vehicle loading pump performance chart, the maximum discharge rate againstzero pump differential is approximately 4 kg/sec. This rate is clearly the limiting factor in thisscenario for leakage fed from the depot pipelines. However the leak can also be fed fromthe bulk vehicle and there is no such limiting factor from this source.

Leak = Two-phase leak from 76 mm dia hole

Conditions for this scenario are similar to Scenario 1 thus rc = 69 kg/m3

Critical flow rate

M = 0.6A 2rc( )P Po c-

= 0.6 x p x

x x x00764

2 69 9 10 49 102

5 5. ( . )-

= 20.5 kg/sec

From Figure 3 estimate distance to LFL:


Probability of ignition is high as LFL extends well beyond site boundary and into housing areaon eastern side of depot.

From Figures 4 to 16 distances to radiation flux levels from the ignited mixture are:


1.5 kW/m

5 kW/m

13 kW/m

110 m

85 m

70 m

95 m

55 m

30 m

This scenario is unacceptable. As for Scenario 1, the probability of the incident and also theleakage rate from the incident should be reduced.

Possible means of achieving the above are:

Breakaway couplings.

Driveaway prevention device(s) on the bulk road vehicle or installed as part of fixedfacility (e.g. boom).

Remote-operated emergency shutdown system capable of operating valves on bothdelivery pipeline and bulk road vehicle.

Fusible links in emergency shutdown system.

Increased supervision during loading.

Improved fire-fighting/fire protection and gas detection/ dispersion facilities (e.g.automatic sprinkler system over loading bay).

4.3.14 Scenario 14: Flange, swivel joint, or small bore connection leak at bulk vehicleloading point

This consequences of this scenario are similar to those of Scenario 2.

The distances to LFL for this scenario maintain the flammable vapour cloud from aleakage mostly within the site boundary. The probability of ignition is therefore low.However, in the event of ignition, the radiation flux on the store to the west isunacceptable.

Possible actions include:

Reduce probability of leakage as per Scenario 2.

Install improved fire-fighting/fire protection and gas detection/gas dispersion facilities.

Construct fire wall on store wall adjacent to loading point.

Install water curtain on store wall.

4.3.15 Scenario 15 : Leak from incorrect or damaged coupling at bulk road vehicle loading point

Assume equivalent hole diameter = 25 mm

Adopt L/D = 0

M = 0.6A 2r( )P Po a-

As for Scenario 13 choose Po = 9 x 105 N/m2

M = 0.6 x p x

x x0025

42 510 9 10 1 10

25 5. ( )- -

= 8.4 kg/sec

From Figure 3 estimated distances to LFL are




1.5 kW/m

5 kW/m

8 kW/m

13 KW /m

44 kW/m

110 m

80 m

75 m

70 m

60m

95 m

60 m

50 m

40 m

20 m

The distances to LFL for this scenario extend well beyond the site boundary. The probabilityof ignition of leakage is quite high. The radiation flux levels exceed allowable limits at the siteboundaries and at buildings within the depot.

Possible actions:

Remote-operated emergency shutdown system as per Scenario 13.

Fusible links in emergency shutdown system.

Regular training of drivers to ensure correct operating procedures.

Improved fire-fighting/fire protection and gas detection/ gas dispersion.

5. REFERENCES

(1) Discharge Rate Calculation Methods For Use In Plant Safety Assessments. P.K.Ramskill, UKAEA Safety and ReliabilityDirectorate. Report SRD R352, February 1986.

(2) API Recommended Practice 520. Recommended Practice for the Design of

Pressure-Relieving Systems in Refineries. Part I - Design.

(3) API Recommended Practice 521, Guide For Pressure Relief and DepressuringSystems.

(4) Loss Prevention In The Process Industries, Frank P. Lees Chapter 15. Butterworths1980.

TABLE 1(A) : DISCHARGE AREAS FOR REGO PRESSURE RELIEF VALVES

Relief ValvePart No/Series

Orifice dia(D) (in)

Discharge Area(A) (in2)

Discharge Area (m2)(A) (x 10-4)

A8434 GNA8436 GNA8534 FGN

7534 B7583 G8684 G8685 G7534 G

A3149 L050 3149 L2003127 G3129 G3131 G

W3132 G3132 G

T3132 GMV3132 G3135 G

AA3135 UA 2503133 GA3149 G

AA3135 UA 2653127 K3129 K

1.0151.7661.015

1.8430.7950.9211.2181.843

1.6411.6410.2740.3860.736

0.9371.0321.0321.0321.156

1.1561.2181.6411.1560.2740.386

0.8092.4490.809

2.6680.4960.6661.1652.668

2.1152.1150.0590.1170.425

0.6890.8360.8360.8361.049

1.0491.1652.1151.0490.0590.117

5.2215.805.22

17.213.204.307.5217.21

13.6413.640.380.752.74

4.445.395.395.396.77

6.777.5213.646.770.380.75

TABLE 1(B) : DISCHARGE AREAS FOR FISHER PRESSURE RELIEF VALVES

Relief ValvePart No/Series

Orifice dia(D) (in)

Discharge Area(A) (in2)


H 280 SERIESH 5110 SERIESH 110 SERIESH 135-250 H 160 SERIESH 185 SERIESH 148H 173H 225 SERIESH 250 SERIESH 275 SERIESH 365 SERIESH 385 SERIES

1.8441.8440.2830.3900.3900.7420.3900.3900.7841.0061.2300.5230.581

2.672.670.060.120.120.430.120.120.480.791.190.210.26

17.2317.230.390.780.782.770.780.783.105.107.681.351.68

TABLE 1 (C) : DISCHARGE AREAS FOR OTHER PRESSURE RELIEF VALVES

( As used on Manufacturing sites )

Inlet/Orifice/Outlet Discharge Area (in2) (A)


1D21E21F22H33K44L64P66Q86R108T10

0.1100.1960.3070.7851.8382.8536.38011.04016.00026.000

0.711.261.985.0511.8618.4141.1671.23103.23167.75

TABLE 2 : RELIEF VALVE FIRE & RADIATION FLUX DATA FOR PROPANE

Mass Stack Stack Stack Flame Flame Distance downwindFlow Dia. Output Length Length Lift Tank Top Level Ground Level

Height Radiation Flux Radiation Flux

kg/s ins m m m m 8 32 44 1.5 5 8 13

kW/m2 kW/m2 kW/m2 kW/m2 kW/m2 kW/m2 kW/m2

2.5 1.25 1.65 0.2 10.3 1.8 12 - - 30 15.5 10.5 6

5.0 1.77 4.6 2.0 13.9 2.5 15 - - 40 19 11 -

7.5 3.0 4.6 2.0 17.1 3.1 19.5 - - 50 25.5 16.5 -

10.0 6.0 15.4 2.0 17.1 3.8 26.5 - - 60 10.5 - -

15.0 6.0 9.0 2.0 24.2 4.3 30 - - 73 35.5 21.5 -

20.0 8.0 21.0 2.0 28.0 5.0 36.5 - - 82 23 - -

25.0 8.0 23.0 2.0 30.3 5.4 39.5 - - 89 - - -

30.0 8.0 23.0 2.0 32.4 5.8 42.5 - - 95 31 - -

40.0 8.0 23.0 2.0 36.1 6.4 47.5 - - 109 42 - -

Notes :1. For layout see Figure 12. All calculations for windspeed 5 m/s3. Mass flows of 2.5, 5.0, 7.5 and 15 kg/s refer to propane in bullets.4. Mass flows of 10, 20 kg/s, and greater refer to propane in spheres.

TABLE 3 : RELIEF VALVE FIRE & RADIATION FLUX DATA FOR BUTANE

Mass Stack Stack Stack Flame Flame Distance downwind (m)Flow Dia. Output Length LengthLift

Height Off Tank Top Level Ground levelRadiation Flux Radiation Flux

kg/s ins m m m m 8 32 44 1.5 5 8 13

kW/m2 kW/m2 kW/m2 kW/m2 kW/m2 kW/m2 kW/m2

2.5 1.25 1.65 0.2 10.6 1.9 12.5 - - 30 16 11.5 6.55.0 1.77 4.6 2.0 14.4 2.6 16 - - 42 20 12 -7.5 3.0 4.6 2.0 17.7 3.2 21.5 - - 52 27 18 -10.0 6.0 9.0 2.0 21.9 3.9 28 - - 65 32 19 -15.0 8.0 21.0 2.0 26.7 4.8 35.5 - - 77 23 - -20.0 10.0 23.0 2.0 33.2 3.0 44 6 - 93 38 - -25.0 10.0 23.0 2.0 32.9 5.9 45 - - 98 39 - -30.0 10.0 23.0 2.0 34.8 6.2 48 - - 105 43 - -

Notes :1. For layout see Figure 12. All calculations for windspeed 5 m/s3. Mass flows of up to 10 kg/s refer to butane in bullets.4. Mass flows of 15 kg/s, and greater refer to butane in spheres.

FIGURE 1 : SCHEMATIC OF MODEL FACILITIES (at windspeed 5m/s)

FIGURE 2 : EXAMPLE PLUMEPATH DISPERSION PROFILE FOR BUTANE

FIGURE 3 : DISPERSION DISTANCE TO LFL FOR HORIZONTAL PROPANE & BUTANERELEASES

WEATHER 2F = WIND SPEED 2 m/s. WEATHER STABILITY FWEATHER 5D = WIND SPEED 5 m/s. WEATHER STABILITY DSURFACE ROUGHNESS = 0.1 m

FIGURE 4 : DISTANCE TO 1.5 Kw/M2 RADIATON FLUX FOR VAPOUR AND LIQUID HORIZONTAL BUTANE JET FIRES

FIGURE 5 : DISTANCE TO 5 Kw/m2 RADIATON FLUX FOR VAPOUR AND LIQUID HORIZONTAL BUTANE JET FIRES

FIGURE 10 : FLAME LENGTHS FOR HORIZONTAL BUTANE JET FIRES

FIGURE 11 : DISTANCE TO 1.5 Kw/m2 RADIATON FLUX FOR VAPOUR AND LIQUID VERTICAL BUTANE JET FIRES

FIGURE 12 : DISTANCE TO 5 Kw/m2 RADIATON FLUX FOR VAPOUR AND LIQUID VERTICAL BUTANE JET FIRES

FIGURE 17 : LPG DEPOT LAYOUT WORKED EXAMPLE

FIGURE 18 : FLOWSCHEME WORKED EXAMPLE

FIGURE 19 : NOZZLE DETAILS OF PROPANE SPHERE WORKED EXAMPLE

FIGURE 20 : NOZZLE DETAILS OF BUTANE SPHERE WORKED EXAMPLE

TITLEPREFACECONTENTSINTRODUCTIONASSESSMENT OF FIRE SITUATIONS AUDIT OF THE FACILITIESSELECTION OF LEAKAGE SCENARIOS AND ASSESSMENT OF THEIRRADIATION CRITERIA FOR PERSONNEL PROTECTIONVAPOUR CLOUD EXPLOSIONBOILING LIQUID EXPANDING VAPOUR EXPLOSION (BLEVE)SELECTION OF LEAK REDUCTION MEASURES AND METHODS TO NITIGATE THE

CONSEQUENCE ASSESSMENTSINTRODUCTIONCALCULATION OF FLOW RATESVAPOUR JETS DISPERSION AND FIRESTWO-PHASE JETS - DISPERSION AND FIRES

WORKED EXAMPLEDESCRIPTION OF FACILITYAUDIT OF THE FACILITY AND CHOICE OF SCENARIOSCONSEQUENCE ASSESSMENT, ANALYSIS AND PROPOSED ACTION

REFERENCESTABLES1- DISCHARGE AREAS FOR REGO PRESSURE RELIEF VALVES2 - RELIEF VALVE FIRE & RADIATION FLUX DATA FOR PROPANE3 - RELIEF VALVE FIRE & RADIATION FLUX DATA FOR BUTANE

FIGURES1 - SCHEMATIC OF MODEL FACILITIES (at windspeed 5m/s)2 - EXAMPLE PLUMEPATH DISPERSION PROFILE FOR BUTANE3 - DISPERSION DISTANCE TO LFL FOR HORIZONTAL PROPANE & BUTANE4 - DISTANCE TO 1.5 Kw/M2 RADIATON FLUX FOR VAPOUR AND LIQUID HORIZONTAL BUTANE JET FIRES5 - DISTANCE TO 5 Kw/m2 RADIATON FLUX FOR VAPOUR AND LIQUID HORIZONTAL BUTANE JET FIRES6 - DISTANCE TO 8 Kw/m2 RADIATON FLUX FOR VAPOUR AND LIQUID HORIZONTAL BUTANE JET FIRES 7 - DISTANCE TO 13 Kw/m2 RADIATON FLUX FOR VAPOUR AND LIQUID HORIZONTAL BUTANE JET FIRES8 - DISTANCE TO 32 Kw/m2 RADIATON FLUX FOR VAPOUR AND LIQUID HORIZONTAL BUTANE JET FIRES9 - DISTANCE TO 44 Kw/m2 RADIATON FLUX FOR VAPOUR AND LIQUID HORIZONTAL BUTANE JET FIRES10 - FLAME LENGTHS FOR HORIZONTAL BUTANE JET FIRES11 - DISTANCE TO 1.5 Kw/m2 RADIATON FLUX FOR VAPOUR AND LIQUID VERTICAL BUTANE JET FIRES12 - DISTANCE TO 5 Kw/m2 RADIATON FLUX FOR VAPOUR AND LIQUID VERTICAL BUTANE JET FIRES13 - DISTANCE TO 8 Kw/m2 RADIATON FLUX FOR VAPOUR AND LIQUID VERTICAL BUTANE14 - DISTANCE TO 13 Kw/m2 RADIATON FLUX FOR VAPOUR AND LIQUID VERTICAL BUTANE JET FIRES15 - DISTANCE TO 32 Kw/m2 RADIATON FLUX FOR VAPOUR AND LIQUID VERTICAL BUTANE JET FIRES16 - DISTANCE TO 44 Kw/m2 RADIATON FLUX FOR VAPOUR AND LIQUID VERTICAL BUTANE JET FIRES17 - LPG DEPOT LAYOUT WORKED EXAMPLE18 - FLOWSCHEME WORKED EXAMPLE19 - NOZZLE DETAILS OF PROPANE SPHERE WORKED EXAMPLE20 - NOZZLE DETAILS OF BUTANE SPHERE WORKED EXAMPLE

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