depressuring systems

CONSULTING ENGINEERS LIMITED

Rajesh AAssistant Engineer

Emp Code: 104321

Tata Consulting Engineers Ltd.


OUTLINE Introduction Types of Depressuring When to use Depressuring Design Considerations Calculation Methods References


Introduction Protective arrangement of valves and piping intended

to provide for rapid reduction of pressure inequipment by releasing vapours.

Actuation of the system can be automatic or manual. API 521 states:Provide depressurizing on all equipment that

process light hydrocarbons and set thedepressured rate to achieve 100 psig (690kPag)or 50% of the vessel design pressure, whichever is lower in 15 minutes.

In connection with fire protection, particularly inhigher-pressure services, the designer should considervapour depressuring facilities.

Vapour Depressuring


Controlled depressuring of the vessel reduces internal pressure andstress in the vessel walls.

Depressuring systems are used to reduce the failure potential forscenarios involving overheating (e.g. fire).

Continued


Non-controlled Type Depressuring

Controlled Type Depressuring Controlled Type Depressuring Non-controlled Type Depressuring

Depressurization types


When to use DepressurizationVessel requiring depressurization capability

A vessel operated above 690 kPa (100psi) Contains volatile liquids with vapour pr. above atmospheric Fire condition may occur that weakens a vessel to below safe

strength levels, with in several hours, which may causesignificant exposure loses.

Vessel which may not require depressurization capability

A vessel operated at or less than 690 kPa A vessel containing less than 907 Kgs (2000 lb) of vapours A vessel whose time to rupture from a fire exposure is more

than several hours.Detailed Flow Chart


Examples

The ASME pressure vessel rupture stress formula is applied to calculate avessel stress is:

S = P(R+0.6t)/Et

Where:

S = Rupture Stress

P = Operating Pressure in Psig

R = Shell Inside Radius, Inch

t = Shell Wall Thickness, Inch

E = Weld Joint Efficiency (generally assume 100%)

Example 1

Example 2


Design ConsiderationsThe following should be considered when designing/specifying thedepressurization system:

Rupture time Rupture pressure of pipes & vessels Total release of flammables Instantaneous release rate Loss of production, reputation and rebuild cost Damage to internals of equipments Manual controls near the vessel may be inaccessible during a fire. Failure position Metallurgy of the vessel Safe disposal of vented streams.


Grote Equation

API RP 521 Method

Software Packages HYSYS

BLOWDOWN/BLOWFIRE

LNGDYN

PRO II

Depressurization Calculation Methods


Assumptions : Critical flow throughout entire depressuring process Constant mass flow throughout entire depressuring process System being depressured is maintained as gaseous throughout entire depressuring

process Constant temperature, molecular weight and compressibilityMethodologyFollowing is the derivation of the manual equation.Vapour flow passing an orifice at critical flow condition

where,W = Mass flow (kg/h)CD = Discharge Coeff.Ao= Orifice area (mm2)P = Upstream pressure (KPa abs)MW = Molecular weightT = Upstream temperature (K)Z = Upstream fluid compressibilityM0= Initial mass

TZMWPACW OD (a)

Grote Equation


A vessel with fixed volume (Vo) depressure from P0 to P1, take time t, assumingflow rate is constant (W0=W1=W2=),depressure mass,

Assuming Temperature, Molecular weight and compressibility maintain constantthrough-out the depressuring process,

Integrate above equation with condition P0 @ t0 and P @ t.

WdVdt

dtWdMU0

u

(b)

dPPTZ

MWRAC

Vdt

dPZRTMW

WVdt

D

1

0

0

0

(c)

0

0

00

0

ln

ln

PP

tMW

PP

TZMW

RACVt

D

(d)


API 521 Method* To reduce the internal pressure in equipment involved in a fire, vapour

should be removed at a rate that compensates for the followingoccurrences:

* Vapour generated from liquid by heat input from the fire;* Vapour expansion during pressure reduction;* Liquid flash due to pressure reduction. (This factor applies only when a system

contains liquid at or near its saturation temperature).

* The total vapour load = sum of the individual occurrences for allequipment involved.

qm - vapour mass flow rate, kg/h (lb/h)

m - mass flow rate per unit time

t - depressuring time interval, hr

x

x

iivm

x

iidm

x

iifm tqtqtq

1,

1,

1, ).().().(.tm (1)


Vapour from fire-heat input

Assumption: the vapour generation is a function only of the heat absorbedfrom the fire and the latent heat of the liquid.

The mass, mf, of vapour generated by the fire during the depressuringinterval in a vessel, i, of the system can be determined by Equation (2):

( mf t )I = t (Q / L)i (2)

Q= total heat absorption, W (Btu/h)

L= latent heat of liquid, KJ/Kg (Btu/lb)

This calculation should be repeated for all vessels in the system.

CFAQ 0.82ws


Vapour from density change and liquid flash It is necessary to know the liquid inventory and vapour volume of the

system.

Following equation is used for calculating the vapour load due to densitychange

where

V - volume available for the vapour, m3 (ft3);

p - absolute pressure, expressed in kPa (psi);

M - relative molecular mass of the vapour;

Z - compressibility factor, dimensionless;

T - absolute temperature of the liquid or vapour, K (R);

the subscript a represents the higher-pressure condition and

b represents the lower-pressure condition.

i relates to an individual vessel of the system

iiidm, .

p.M.

p.M0.1205V.t)(q

ba TZTZ(3)


Amount of liquid flashed is given by equation (4)

where

qm - vapour mass flow rate, expressed in kg/h (lb/h);

t - depressuring time interval, expressed in hours (usually assumed to be 0,25 h);

Q - total heat absorption (input) to the wetted surface, expressed in kJ/h (Btu/h);

L - average latent heat of the liquid, expressed in kJ/kg (Btu/lb);

Cp - average specific heat of the liquid, expressed in kJ/kgK (Btu/lbR);

T - absolute temperature of the liquid or vapour, expressed in K (R);

subscripts:

a - original condition at the start of the depressuring time interval,

b - depressurized condition at the end of the depressuring time interval;

i - relates to an individual vessel of the system if more than one vessel is involved

v - relates to liquid flash or vapour generated from pressure reduction;

|

ibaii

ibai

i

i

TTCpLTTCp

LtQ

)(2)(2.

2..t)(q.t)(q iam,ivm, (4)

Continued


Depressuring using HYSYS Open a new case in HYSYS

Add the required components and select Fluid package

Add a stream with the following properties and molar flows:

Stream Name FeedTemperature 108 C (226.4 F)Pressure 1000 kPa (145.04 psia)

Component Molar FlowMethane 30.0 kmol/h (66.138 lbmol/h)Ethane 30.0 kmol/h (66.138 lbmol/h)Propane 30.0 kmol/h (66.138 lbmol/h)i-Butane 30.0 kmol/h (66.138 lbmol/h)n-Butane 30.0 kmol/h (66.138 lbmol/h)i-Pentane 30.0 kmol/h (66.138 lbmol/h)n-Pentane 325.0 kmol/h (716.495 lbmol/h)n-Hexane 30.0 kmol/h (66.138 lbmol/h)


1) Go to "Attachments" "Utilities"

ContinuedTo attach the Dynamic Depressuring utility to the stream,> open the stream property view,> go to "Attachments""Utilities" and press "Create> Select "Dynamic Depressuring> Press the "Add Utility" button

2) Press "Create"

3) Select "Dynamic Depressuring"

4) Press "Add Utility"


Continued

Enter the following vessel information on the "Design" ->"Connections"page of new window opened.

Height 4.50mDiameter 1.25mInitial Liquid Volume 1.45m3


ContinuedHeat Flux Parameters

APIEquation

Q = total absorption to wettedsurface (BTU/h)

(field units) F = environmental factorA = total wetted surface (ft2)

APIEquation

Q = total absorption to wettedsurface (kJ/s

(metricunits)

F = environmental factor

A = total wetted surface (m2)

82.0AF21000Q uu

82.0AF116.43Q uu


ContinuedValve Parameters


Continued On the "Options" page, enter a PV Work Term of 90%.

On the "Operating Conditions" page, select "Calculate Cv" and enter a finalpressure of 500 kPa (50 % of operating pressure).


Continued Once you have submitted the required information, press the "Run" button

to execute the calculations.

Go to the "Performance" "Summary" page to view the results.

Press the "Run" buttonto start the calculations.


Continued Performance Summary


Yes

System & scenarioinformation

Estimate size oforifice

Calculate P(t) for theprocess segment and

T(t) for the steelIncrease orifice

size

Is Flare Capacityutilized?

Will equipment/pipe rupture?

Are theconsequence of therupture acceptable?

Improve design/apply PFP

Ok

Failure criteria

Yes

NoNo

No

Yes

Design of Depressurization


References API RP 521 Guide for pressure-relieving and depressuring

systems, American Petroleum Institute, Fifth Edition, May2008.

Depressurisation: A Practical Guide, Aspentech TechnicalSupport Knowledge Base Article, rev 2004-1.1 Feb 2006

Perry, R. H. Chemical engineering handbook, McGraw Hill,5th edition, 1973.

Gayton, P.W. and Murphy, S.N. Depressurisation SystemsDesign. IChemE Workshop The Safe Disposal of UnwantedHydrocarbons, Aberdeen 1995.

Review of the Response of Pressurised Process Vessels andEquipment to Fire Attack, Offshore Technology Rreport -OTO 2000 051, June 2000


If vapour depressuring is required for both fire and process reasons, thelarger requirement should govern the size of the depressuring facilities.

A vapour-depressuring system should have adequate capacity to permitreduction of the vessel stress to a level at which stress rupture is not ofimmediate concern.

The required depressuring rate depends on the metallurgy of the vessel,the thickness and initial temperature of the vessel wall and the rate ofheat input.

Vessels with thinner walls generally requires faster depressuring rate.

Depressuring is assumed to continue for the duration of the emergency.The valves should remain operable for the duration of the emergency orshould fail in a full-open position.


Example 1 SEPARATOR (Honzontal)

ASSUMPTIONS:

Shell Inside Radius : 60 " -0 "Shell Wall Thickness : 1/2"Liquid Sp. Gr. : 1.0Material of Construction : A515 Gr. 70Operating Pressure : 50 PsigDesign Pressure : 90 PsigNormal Liquid Level : 5'-0" from bottom

S = P (R + 0.6t)/Et ref.: ASME. DIV. Vlll for circumferential stress)S = 50 (60 + 0.6 x 0.5)/1.0x 0.5S = 6,030 psi

From Figure 2 (API 520 7th edition),Time before rupture at 6,030 psi and 1,300 0F isapproximately 5 Hrs.

CONCLUSION: Depressurization system is not required.


Example 2 CRUDE STABILIZER

ASSUMPTIONS:

Shell Inside Radius : 30 " -0 "Shell Wall Thickness : 7/16"Liquid Sp. Gr. : 0.85Material of Construction : A515 Gr. 70Operating Pressure : 150 PsigDesign Pressure : 175 PsigNormal Liquid Level : 5'-0" from bottomVessel is insulated but no credit given for insulation

S = P (R + 0.6t)/Et ref.: ASME. DIV. Vlll for circumferential stress)S = 150 (30 + 0.6 x 0.4375)/1.0 x 0.4375S = 10,374 psi

From Figure 2 (API 520 7th edition),Time before rupture at 10,374 psi and 1,300 0Fis approximately 0.3 Hrs.

CONCLUSION: Depressurization system is required.

depressuring systems

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vessel walls

tata consulting engineers

depressuring systems

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rupture stressp

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