factors impacting atmospheric discharges and selection of...

Copyright © Siemens AG 2010. All rights reserved.

Factors Impacting Atmospheric

Discharges and Selection of

Pressure Protection Disposal

Systems

Patrick Smith and Abdul Aldeeb

MKOPSC 2010 International Symposium

College Station, Texas

October 26, 2010

MKOPSC 2010 International Symposium ● October 26, 2010 ● College Station, TX

Introduction

Direct discharge to atmosphere is a common practice that has long been considered safe, simple, dependable, and economically feasible compared to other overpressure protection mechanisms

Discharging highly hazardous chemicals (HHC) to atmosphere remains a major challenge

Several recent incidents have shown the need to eliminate HHC disposal to atmosphere


Agenda

Current Practices to Design Atmospheric Discharges

Factors Influencing Discharges to Atmosphere

Discharge Location Decision-Making Approach


Current Practices to Design AtmosphericDischarge Systems

The American Petroleum Institute Standard 521 Fifth Edition, 2007 (Addendum, 2008) [API 521] provides design approach for two types of systems:

Pressure relief valves that discharge directly to atmosphere

Vessel 1

RV-1

4×N×6

2'

15'

15'


Current Practices to Design Atmospheric

Discharge Systems

Disposal through a common vent stack that discharges to the atmosphere


Current Practices to Design AtmosphericDischarge Systems – Direct Discharge to Atmosphere

API provides two criteria to determine if directing a relief device to atmosphere is advisable

Both criteria focus on jet-momentum effects of the discharged vapors

High velocities at the discharge of the tail pipe improve the mixing with air and reduce the concentration of the flammable or toxic material below threshold values over a short period of time



The first criterion presented in API 521 indicates that the vapor can be diluted below its lower flammability limit due to mixing with air when the Reynolds number, Re, meets the following criteria:

j

eR 41054.1

Where j is the density of the gas at vent outlet and is the density of air.



The second criterion indicates that for exit velocities that exceed 500 ft/s, the discharged flammable hydrocarbon is expected to be efficiently mixed with air and diluted to concentrations below the lower flammability limits at a distance of 120 diameters (based on tail pipe inner diameter) from the end of the tail pipe.

Release

Plume

Wind


Current Practices to Design AtmosphericDischarge Systems – Atmospheric Vent Stacks

Sections 6.7 and 7.3.4 detail the design requirements associated with atmospheric vent stacks:

Recommends modeling of the release to ensure that the flammable vapor concentration does not exceed 10% to 50% of the lower flammable limit

Approach presented for individual relief devices is not recommended for atmospheric vent stacks



It is evident from the API guidance that atmospheric discharges of vapors are not necessarily inherently unsafe.

For liquid and two-phase discharges, the problem is more challenging.

Experiments have shown that at high discharge velocities, the resultant droplets of liquids are extremely small.



In order to address the atmospheric discharge suitability,several factors will be evaluated:

1. Chemical type and concentration

2. Discharge Phase

3. Discharge Location

4. Scenario Likelihood

5. Disposal Options and Cost


Factors Influencing Discharges to Atmosphere- Chemical Type and Concentration

Primary hazards are typically associated with the flammability and toxicity of the fluid

Corrosive properties and potential formation of explosive mixtures should also be considered.

For each property, the maximum acceptable concentration threshold should be determined.Lower Flammability Limit (LFL) Immediately Dangerous to Life or Health (IDLH)Short Term Exposure Limit (STEL)Permissible Exposure Limits (PEL)

In many cases, dispersion modeling will be required.




2. Discharge Phase





Factors Influencing Discharges to Atmosphere- Discharge Phase

Vapor discharges are generally considered safer compared to liquid and two-phase discharges

Liquid and two-phase discharges should be evaluated for the additional potential flammable, toxic, corrosive, or explosive hazards




2. Discharge Phase





Factors Influencing Discharges to Atmosphere- Discharge Location

Discharging to confined spaces or to areas where personnel or source of ignition may be present will impact the design of atmospheric discharges




2. Discharge Phase





Atm

Stm

Cond

CWS

CWR

LT2 LT1 LC1

HLA

Impacted by the operator overpressure guidelines, training, operating procedures, and credits for available layers of protection

Factors Influencing Discharges to Atmosphere- Scenario Likelihood




2. Discharge Phase





Factors Influencing Discharges to Atmosphere- Disposal Options and Cost

Once all the previous factors are addressed, a comparison of the various disposal options should be implemented.

A decision to accept atmospheric discharge should be impacted the least by the cost factor


Discharge Location Decision Making Approach

Liquid or

two-phase discharge

is possible.

Process analysis resulted in

identifying applicable

overpressure scenario.

Toxic, flammable,

or corrosive fluid is

expected.

Toxic vapors are

expected.NO YES



Toxic, flammable,

or corrosive fluid is

expected.

Is there a

potential for chemical

hazard or

environmental

impact?

Is there an open

drain open?

Atmospheric

discharge

is acceptable.

Discharge

To closed-system is

required.

YES

NO

NO

YES

YES

NO



Toxic vapors are

expected.

Flammable vapors

are expected.

Is concentration

<25xIDLH?

Is the

discharge velocity

>500 ft/s at device

capacity?

Are all points

of interest >120

diameters from

the discharge

point?*

Atmospheric

discharge

is acceptable.

Are jet

effects sufficient

to disperse the material

at 25% of device

capacity?**

Discharge

To closed-system is

required.

Does a rigorous

dispersion modeling

indicate that atmospheric

release will be safe?

NO

YES

YES

YES

NO

YES

YES

NO

NO

NO

YES

YES

NO

NO


Conclusions

Safe disposal system decision-making approach should consider atmospheric discharge impact and possible alternative discharge locations.

Atmospheric pressure relief should only be considered safe when thorough evaluation is completed.

Atmospheric pressure relief valves and common atmospheric vent stack design considerations and risk are not exactly the same.

Most flare headers have limited capacity and forcing atmospheric relief valves into potentially undersized headers could create more problems than are solved.


Thank you for your attention!

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