40628248 Pressure Relief Safety Valves

7/30/2019 40628248 Pressure Relief Safety Valves

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Pressure Relief

Grace under pressure

Ernest Hemingway

Harry J. Toups LSU Department of Chemical Engineering with

significant material from SACHE 2003 Workshop presentationby Scott Ostrowski (ExxonMobil)

and Professor Emeritus Art Sterling


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What is the Hazard?

Despite safety precautions

Equipment failures

Human error, and

External events, can sometimes lead to

Increases in process pressures beyond safe

levels, potentially resulting in

OVERPRESSURE due to a RELIEF EVENT


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What are Relief Events?

External fire

Flow from high pressure source

Heat input from associated equipment Pumps and compressors

Ambient heat transfer

Liquid expansion in pipes and surge


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Potential Lines of Defense

Inherently Safe Design

Passive Control

Active Control

Low pressure processes

Install Relief Systems

Overdesign of process equipment


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What is a Relief System?

A relief device, and

Associated lines and processequipment to safely handle the material

ejected


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Why Use a Relief System?

Inherently Safe Designsimply cant

eliminate every pressure hazard

Passive designs can be exceedingly

expensive and cumbersome

Relief systems work!


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Pressure Terminology

MAWP Design pressure

Operating

pressure Set pressure

Overpressure

Accumulation

Blowdown


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Code Requirements

General Code requirements include:

ASME Boiler & Pressure Vessel Codes

ASME B31.3 / Petroleum Refinery PipingASME B16.5 / Flanges & Flanged Fittings


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Code Requirements

Relieving pressure shall not exceed

MAWP (accumulation) by more than:

3% for fired and unfired steam boilers

10% for vessels equipped with a single

pressure relief device

16% for vessels equipped with multiple

pressure relief devices 21% for fire contingency


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Relief Design Methodology

LOCATERELIEFS

CHOOSE

TYPE

DEVELOPSCENARIOS

SIZE RELIEFS

(1 or 2 Phase)

CHOOSE

WORST CASE

DESIGN RELIEF

SYSTEM


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Locating Reliefs Where?

All vessels Blocked in sections of cool liquid lines

that are exposed to heat

Discharge sides of positivedisplacement pumps, compressors,and turbines

Vessel steam jackets

Where PHA indicates the need

LOCATE

RELIEFS


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Choosing Relief Types

Spring-Operated Valves

Rupture Devices

CHOOSE

TYPE


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Conventional Type

CHOOSE

TYPE


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Picture: Conventional Relief

Valve

Conventional

Relief Valve

CHOOSE

TYPE


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Superimposed Back

Pressure

Pressure in

discharge header

before valve opens

Can be constant or

variable

CHOOSE

TYPE


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Built-up Back Pressure

Pressure indischarge headerdue to frictionallosses after valveopens

Total =

Superimposed +Built-up

CHOOSE

TYPE


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Balanced Bellows Type

CHOOSE

TYPE


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Picture: Bellows Relief

ValveBellowsRelief Valve

CHOOSE

TYPE


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Pros & Cons:

Conventional Valve Advantages

+ Most reliable type if properly sized and operated

+ Versatile -- can be used in many services

Disadvantages

Relieving pressure affected by back pressure

Susceptible to chatter if built-up back pressure istoo high

CHOOSE

TYPE


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Pros & Cons:

Balanced Bellows Valve Advantages

+ Relieving pressure not affected by back pressure

+ Can handle higher built-up back pressure

+ Protects spring from corrosion

Disadvantages

Bellows susceptible to fatigue/rupture

May release flammables/toxics to atmosphere

Requires separate venting system

CHOOSE

TYPE


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Rupture Devices

Rupture Disc

Rupture Pin

CHOOSE

TYPE


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Conventional

Metal Rupture Disc

CHOOSE

TYPE


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Conventional

Rupture Pin Device

CHOOSE

TYPE


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When to Use a Spring-

Operated Valve

Losing entire contents is unacceptable

Fluids above normal boiling point

Toxic fluids Need to avoid failing low

Return to normal operations quickly

Withstand process pressure changes,including vacuum

CHOOSE

TYPE


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When to Use a Rupture

Disc/Pin

Capital and maintenance savings

Losing the contents is not an issue

Benign service (nontoxic, non-hazardous)

Need for fast-acting device

Potential for relief valve plugging High viscosity liquids

CHOOSE

TYPE


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When to Use Both Types

Need a positive seal (toxic material,

material balance requirements)

Protect safety valve from corrosion

System contains solids

CHOOSE

TYPE


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Relief Event Scenarios

A description of one specific relief event

Usually each relief has more than one reliefevent, more than one scenario

Examples include:

Overfilling/overpressuring

Fire

Runaway reaction

Blocked lines with subsequent expansion

Developed through Process Hazard Analysis(PHA)

DEVELOP

SCENARIOS


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An Example: Batch Reactor

Control valve onnitric acid feed linestuck open, vesseloverfills

Steam regulator tojacket fails, vesseloverpressures

Coolant system

fails, runawayreaction

DEVELOP

SCENARIOS

Product

Raw

Material

Feeds

Organic substrateCatalyst

Nitric Acid

Reactor ~ 100 gallons


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Sizing Reliefs

Determining relief rates

Determine relief vent area

SIZE RELIEFS

(Single Phase)


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Scenarios Drive Relief Rates

Overfill (e.g., control valve failure)

Fire

Blocked discharge

SIZE RELIEFS

(Single Phase)

Maximum flow rate thru valve into vessel

Vaporization rate due to heat-up

Design pump flow rate


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Overfill Scenario Calcs

Determined maximum flow thru valve

(i.e., blowthrough)

Liquids:

Gases:

SIZE RELIEFS

(Single Phase)

PgACQ cvm 2

)1/()1(

1

2

og

c

ovchokedm

TR

MgAPCQ


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Fire Scenario Calcs

API 520 gives all equations for

calculating fire relief rate, step-by-step

1. Determine the total wetted surface area

2. Determine the total heat absorption

3. Determine the rate of vapor or gas

vaporized from the liquid

SIZE RELIEFS

(Single Phase)


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Determine Wetted Area

SIZE RELIEFS

(Single Phase)

180/wet

BDLEDA

DEB 21cos 1


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Determine Heat Absorption

Prompt fire-fighting & adequatedrainage:

Otherwise:

where

SIZE RELIEFS

(Single Phase)

82.0

wet000,21

Btu/hr

AFQ

82.0

wet

500,34

Btu/hr

AFQ

Q is the heat absorption (Btu/hr)

F is the environmental factor

1.0 for a bare vessel Smaller values for insulated vessels

Awet is the wetted surface area (ft2)


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Determine Vaporization

Rate

vap/HQW

where

W = Mass flow, lbs/hr

Q = Total heat absorption tothe wetted surface, Btu/hr

Hvap = Latent heat ofvaporization, Btu/lb

SIZE RELIEFS

(Single Phase)


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Determine Relief Vent Area

LiquidService

where

bs25.1

)ref(

bpvovQ

gpm38.02/1)psi(2in

PPKKKCA

A is the computed relief area (in2)

Qv is the volumetric flow thru the relief (gpm) Co is the discharge coefficient

Kv is the viscosity correction

Kp is the overpressure correction

Kb is the backpressure correction

(/ref) is the specific gravity of liquid Ps is the gauge set pressure (lbf/in

2)

Pb is the gauge backpressure (lbf/in2)

SIZE RELIEFS

(Single Phase)


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GasService

where

MTzPKCAbo

mQ

A is the computed relief area (in2)

Qm is the discharge flow thru the relief (lbm/hr) Co is the discharge coefficient

Kb is the backpressure correction

T is the absolute temperature of the discharge (R)

z is the compressibility factor

M is average molecular weight of gas (lbm/lb-mol) P is maximum absolute discharge pressure (lbf/in

2)

is an isentropic expansion function

SIZE RELIEFS

(Single Phase)

valverelieffor thepressuresettheiss

pipingfors33.1max

firetoexposedsfor vessels2.1maxvesselspressureunfiredfors1.1max

7.14max

P

PP

PPPP

PP


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GasService

where

)1/()1(

125.519

is an isentropic expansionfunction

is heat capacity ratio for the gas

Units are as described in previous

slide

SIZE RELIEFS

(Single Phase)


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A Special Issue: Chatter

Spring relief devices require 25-30%of maximum flow capacity to maintainthe valve seat in the open position

Lower flows result in chattering,caused by rapid opening and closingof the valve disc

This can lead to destruction of thedevice and a dangerous situation

SIZE RELIEFS

(Single Phase)


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Chatter - Principal Causes

Valve Issues

Oversized valve

Valve handling widely differing rates

Relief System Issues

Excessive inlet pressure drop

Excessive built-up back pressure

SIZE RELIEFS

(Single Phase)


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Worst Case Event Scenario

Worst case for each relief is the event

requiring the largest relief vent area

Worst cases are a subset of the overallset of scenarios for each relief

The identification of the worst-case

scenario frequently affects relief size

more than the accuracy of sizing calcs

CHOOSE

WORST CASE


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Design Relief System

Relief System is more than a safetyrelief valve or rupture disc, it includes:

DESIGN RELIEF

SYSTEM

Backup relief device(s)

Line leading to relief device(s)

Environmental conditioning of relief device

Discharge piping/headers

Blowdown drum

Condenser, flare stack, or scrubber

I t ll ti I ti d


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Installation, Inspection, and

Maintenance

To undermine all the good efforts of a

design crew, simply

1. Improperly install relief devices2. Fail to regularly inspect relief devices,

or

3. Fail to perform needed/requiredmaintenance on relief devices


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?? Reduced Inlet Piping

Anything wrong

here?

Reduced

Inlet Piping

?? Pl d B ll F il d


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?? Plugged Bellows, Failed

Inspection, Maintenance

Bellows plugged

in spite of sign

Anything wrong

here?

Failed

Inspection

Program

Signs of

Maintenance

Issues

?? Di h P i ti


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?? Discharges Pointing

DownAnything wronghere?

Anything wrong

here?

Discharges

Pointing Down


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?? Long Moment Arm

Anything wrong

here?

Long

Moment Arm

?? Will th b lt h ld i


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?? Will these bolts hold in a

relief event

Anything wrong

here?

Will these

bolts hold

in a

relief event?


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Mexico City Disaster

Major Contributing Cause:Missing Safety Valve


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Summary

Pressure Relief

Very Important ACTIVE safety element

Connected intimately with Process Hazard

Analysis Requires diligence in design, equipment

selection, installation, inspection andmaintenance

Look forward to Two-phase flow methodology/exercise


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References

Crowl and Louvar Chemical Process

Safety, Chapters 8 and 9

Ostrowski Fundamentals of PressureRelief Devices

Sterling Safety Valves: Practical

Design, Practices for Relief, and Valve

Sizing


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END OF

PRESENTATION

Documents

40628248 Pressure Relief Safety Valves