124479095 PSV Calculation Ppt

5/20/2018 124479095 PSV Calculation Ppt

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OIL & GAS

WorleyParsons

OIL&GAS

PSV Sizing Calculation

23-June-05


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CONTENT

INTRODUCTION

PSV TYPES

CHATTERING PROBLEM

SIZING CALCULATION

EXAMPLE


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INTRODUCTION

Relief systems are provided on a platform in order toensure the safe operation of the facilities.

In accordance with API RP 14C, all hydrocarbons handling

equipment and pressure vessels will be provided with two

levels of over protection, high pressure trip (PSHH) withshutdown action, and protection by mechanical devices,

Pressure Safety Valve (PSV) or Rupture Disc.

PSVs are installed at every point identified as potentially

hazardous, that is, at points where upset conditions

create pressure which may exceed the maximum

allowable working pressure.


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INTRODUCTION

How High Pressure Develop

Over heating

High head ( from pumping or compression)

Over Filling

Failure of Regulator / Control valve.External Fire

Runaway Reaction

Combustion of gas/dust

Freezing

Thermal Expansion

Loss of Mixing

Others


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INTRODUCTION

Definitions

Operating pressure: The gauge pressure during normal

service.

Set Pressure: The pressure at which the relief device

begins to activate or open.

Maximum Allowable Working Pressure (MAWP): The

maximum guage pressure permissible at the top of a vessel

for a designated temperature.

Vessel fails at 4 to 5 times of MAWP!!!! . But onlyhydrostatically tested to 1.5 times.

Accumulation: The pressure increase over the maximum

allowable working pressure of a vessel during the relief

process. Expressed as % of MAWP.


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INTRODUCTION

Set Pressure

Relief begin to open

Accumulation

Pressure

Time


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Definitions

Over Pressure: The pressure increase in vessel over the set

pressure during the relieving process. Overpressure is

equivalent to the accumulation when the set pressure is at the

MAWP. Expressed as % of set pressure. Must be specified prior

relief design. Typically 10 % ( or for fire 21%) will be used.

Pressure

Time

Relief begin to open

Set Pressure

Over Pressure

INTRODUCTION


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Definitions

Blow-down: The pressure difference between the relief

set pressure and the relief reseating pressure.

Maximum Allowable Accumulated Pressure: The sum of

the maximum allowable working pressure plus the allowable

accumulation.

INTRODUCTION


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INTRODUCTION

Guideline for relief pressures

(Adapted from API RP 520 : Sizing, Selection, and Installation of Pressure-RelievingDevices in Refineries., page 3)


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Definitions

Back Pressure: The pressure at the outlet of the relief

device during the relief process due to pressure in the

discharge system.

1. Superimposed Back Pressure.

2. Built-up Back Pressure.

Total Back Pressure = Superimposed Back Pressure + Built-up Back Pressure

INTRODUCTION


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INTRODUCTION

Definitions

1. Superimposed Back Pressure is the back pressure which

may exist at the outlet of a particular relief valve when connected

to a closed system. The pressure can be constant or variable. The

Superimposed back pressure always exists even when the relief

valve is closed.

2. Built-up Back Pressure is the pressure at the discharge of a

relief device which develops due to the relief flow through the

device when the relief valve opens. The built-up back pressure

depends on the valve itself but also on the design of the relief

piping. It can reach excessive values in the case of vary highset pressures and/or poorly designed piping with too much pressure

Loss. The built-up back pressure is variable.


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PSV TYPES

General Design of PSV

1. Direct acting type

Oldest and most common.

Kept closed by a spring or weight to oppose

lifting force of process pressure.

2. Balanced Bellows.

3. Pilot operated type

Kept closed by process pressure

4. Modulating Pilot.


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PSV TYPES

The following types of PSV are generally used.

1. Conventional PSV

2. Balanced Type PSV (Balanced Bellow)

3. Pilot Operated PSV

4. Modulating Pilot Operated PSV5. Thermal PSV


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PSV TYPES

Conventional PSV

AdvantagesMost reliable type if properly sized

and operated

Versatile -- can be used in many services

Compatible with fouling or dirty service.

Disadvantages

Relieving pressure affected byback pressure

Susceptible to chatter if built-up

back pressure is too high


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Balanced Bellow PSV

PSV TYPES

AdvantagesRelieving pressure not affected by back

pressure.

Can handle higher built-up back pressure.

Protects spring from corrosion.

Wide range of materials available &

chemical compatibility.

Disadvantages

Bellows susceptible to fatigue/rupture.

May release flammables/toxics to

atmosphere.

High maintenance costs.


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Pilot Operated PSV

Advantages

Relieving pressure not affected by

backpressure

Can operate at up to 98% of set pressure

Less susceptible to chatter (some models)

Disadvantages

Pilot is susceptible to plugging (thereforenot recommended for fouling service,

eg. Wax).

Limited chemical and high temperature

use by O-ring seals

Vapor condensation and liquid

accumulation above the piston may

cause problems

Potential for back flow

PSV TYPES


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CHATTERING PROBLEM

Modulating Pilot Operated

There are two main types of pilot control operation

Pop action Main valve fully open at set pressure

Modulating : Main valve opens according to relief demand.

Modulating pilots have additional advantages:

They only open the main valve enough to keep the system at set

pressure which leads to less wasted product being relieved

through the valve.

Less noise generated by the valve when it is required to relieve.

Recommended for Wellhead platform PSV.


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CHATTERING PROBLEM

Main valve

piston lift

Pressure

Opening

Closing

100 % Lift POP Action


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CHATTERING PROBLEM

Main valve

piston lift

Pressure

Opening

Closing

100 % LiftModulating


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Chattering Problem

Chattering is the rapid, alternating opening and closing

of a PSV.

Resulting vibration may cause misalignment, valve seatdamage and, if prolonged, can cause mechanical failure

of valve internals and associated piping.

Chatter may occur in either liquid or vapor services

CHATTERING PROBLEM


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Cause of Chattering

Excessive inlet pressure drop

Excessive built-up back pressure

Oversized valve

Valve handling widely differing rates

CHATTERING PROBLEM


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Excessive inlet pressure drop

Normal PSV has definite pop

and reseat pressures.

CHATTERING PROBLEM

Reseat orclose pressure

Pop or opening pressure

Blow-down = Different between pop and reseat pressure


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CHATTERING PROBLEM

Excessive inlet pressure drop : Solution

If you cant change the

piping

Install smaller PSV

Install different type of

PSV


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CHATTERING PROBLEM

Excessive Built-up Back Pressure

Excessive outlet pressure will also cause chatter.

Avoid

Long outlet piping runs. Elbows and turns.

Sharp edge reductions.

But if you must

Make outlet piping large!


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CHATTERING PROBLEM

Improper Valve Size

Oversized valve

Must flow at least 25% of capacity to keep valve open.

Especially bad in larger sizes.

Valve handling widely differing rates

Leads to oversized valve case.


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SIZING CALCULATION

Step Of Sizing CalculationDevelop Process Safety Diagram (PSD)

Develop relief scenarios by using general possible cause and PSD

Determine required relief area for each cases (Gas, Liquid, 2-Phases)

Choose the worst case scenario to be a governing case

Select proper orifice and valve body size based on STD

Calculate inlet line size (Line P < 3%of RP)

Perform preliminary estimate of tail pipe

Perform Flare system modeling to indicate total back pressureand most suitable tail pipe size.

Select PSV type (Conventional, Balanced Bellow, Pilot Operated


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SIZING CALCULATION

Develop Process Safety Diagram (PSD)

Process safety diagram is the diagram which show

all installation of all process safety equipments (PSHH,

TSHH, PSV, BDV, Rupture disc, etc.) intend to be used in

evaluation of all possible relieving scenarios in process

facilities. Process safety diagram must be developed beforeperforming any PSV calculation.


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SIZING CALCULATION

Develop Process Safety Diagram (PSD)


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SIZING CALCULATION

Develop relief scenarios by using general possible cause and PSD

EquipmentPossible Scenarios

PumpHEX LineVessel /Column

Compressor

Blocked Outlet

Thermal Expansion

Tube Rupture

Gas Blow-by

Inlet Control Valve Failure

Exterior Fire

Note : Exterior Fire is not applicable for PHEX!!


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SIZING CALCULATION

Blocked Outlet/ Blocked Discharge

CLOSED

CLOSED

CLOSED

CLOSED

CLOSED

Process Vessel

Compressor Pump

This situation arises from the inadvertent closure of a block valve or failure

of a control valve in the closed position on an outlet line. Typical

overpressure application where this form of protection is required includes

production separators, compressor discharge piping or pump discharge

piping. The safety relief valves are sized to handle 100% of the anticipated

upstream flow. If the feed stream(s) is multi-phase, the relief rate is the totalinlet flow (gas plus liquids).


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SIZING CALCULATION

Thermal Expansion/ Thermal Relief

Heat Exchanger

Line

Hot Side

Cold Side

CLOSED CLOSED CLOSED

Thermal expansion relief valves (TERVs) are required in liquid-full systemsif the system can be blocked in and/or subjected to heat input from theatmosphere or process that results in overpressure.


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SIZING CALCULATION

Tube Rupture

Heat Exchanger

Hot Side

Cold SideOR

Heat Exchanger

Hot Side

Cold Side

Tube rupture causing vapour to enter either the tubeside or shellside

of a heat exchanger will cause a pressure spike to travel through the

fluid at sonic velocity. Generally, a PSV will not lift fast enough to protect

the system. Protection against tube rupture is typically by the useof bursting discs.


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SIZING CALCULATION

Suddenly OpeningPressure at PSHH

Liquid at LSLL

Gas blow-by occurs when liquid level in a partially filled vessel drops solow that gas exits via the liquid outlet nozzle due to level control failure.

Loss of liquid level will result in gas from the high pressure vessel passinginto the low pressure system downstream of the liquid level control valve.

Gas Blow-by


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SIZING CALCULATION

Fail to openPressure at normalLiquid at normal

Inlet Control Valve Failure

The relief rate is equal to the difference between the maximum inlet

flow and the flow, at relief conditions, from the outlet valves that

remain open. No credit for outlet valves response will be taken i.e.

they will be assumed to remain at their normal operating point

(% open).


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SIZING CALCULATION

Exterior Fire

Generally, the production & processing facilities will be segregated into fire

areas, by means of plated decks, fire walls, or edge of the platform. During

a fire in one of the fire areas, all equipment within that area is assumed to be

fully exposed to the fire.

It is assumed that during a fire there is no feed to or product from an

affected system, and all normal heat inputs have ceased.

No credit will be taken for the presence of any water spray systems.No credit will be taken for thermal insulation on vessels.


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SIZING CALCULATION

Pool Fire

Jet Fire

Fire Type

Systems with significant liquid

hydrocarbon inventory will be

considered for pool fire case.

Heat flux will be calculated as

per API RP521. Credit for a 40%

reduction in heat flux may be

taken for good drainage and

the presence of prompt fire

fighting efforts as specified inAPI RP 521.

Where potential for jet fire exists, relief

load will be calculated for higher heat flux

from heat fire. The heat flux generated by

jet fire ranges from 50 kW/m2 to 300 kW /m2.

However, the net heat flux into the fluid will

depend on many factors such as fuel type,

vessel temperature, the surface emissivity,

the fire environment, the radiative andconvective components of the fire.


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SIZING CALCULATION

Vessel Type & Fluid relief type

Vessel type could be :

Dry or empty vessel

Vessels containing liquid -> Need to consider effect ofwetted area of vessel

Fluid relief type could be :

Fluid is sub-critical: P 1.1*Pc

Pc = Critical PressureP = Relieving Pressure


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SIZING CALCULATION

Fire case

Jet Fire

Dry Vessel Wetted vessel

Sub Critical FlowNear Critical FlowSuper Critical Flow

Pool Fire2 CASES

2 CASES

3 CASES

Totally = 2x2x3 = 12 cases possibleHence 12 methods of calculation for Fire case


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SIZING CALCULATION

Determine required relief area for each case (Gas, Liquid, 2-Phases)

Target of sizing relief valve

1. Determine the relief rate.

2. Determine the required relief area.

3. Select the standard relief area.

Code and standard required

API RP 520 : Sizing, Selection, and Installation of Pressure-Relieving

Devices in Refineries.

API RP 521 : Guide for Pressure Relieving and Depressuring Systems.


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SIZING CALCULATION

Relief Phase

Single Phase Relief

Two Phase Relief

Vapor Phase Relief

Liquid Phase Relief

Relief Type

Continuous Relief

Transient Relief Thermal Relief

Generally

Gas/Vapor

Steam

Relief fluid category



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SIZING CALCULATION

Determine relief rate, some cases (eg. Gas blow-by, tube rupture,

fire case etc.) are very complicated and contain many steps of calculation

especially fire case.

API RP520/ 521 shows the simple method of calculation in order todetermine the relief rate. This does not govern all relief scenarios. Hence

some companies have developed their own procedure to determine the

relief rate.



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SIZING CALCULATION

Vapor Phase Relief

Relief valves for single phase vapour flow will be sized according

to the methodology presented in API RP 520, Section 3 (reference 4).

The sizing equations fall into 2 categories, depending on whether

flow is critical or subcritical.

The critical pressure must be checked using the equation below:

)1(

1

1

2.

k

k

cf

k

PP

where

Pcf= critical flow throat pressure (psia)

P1= upstream relieving pressure (psia)k= ratio of specific heats

Ref. API 520 section 3.6.1.4


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SIZING CALCULATION

Vapor Phase Relief

If flow is critical (Pcf > downstream pressure P2), the criticalsizing equation is used:

A W

C K P K

TZ

Md b

1


WhereA = required effective discharge area of the valve ( in2).

W = required flow through the valve (lb/hr).

C = coefficient determined from an expression of the ratio of the specific heats of the gas

or vapour at standard conditions. This can be obtained from Figure 26 or Table 9 in

API 520.

Kd = effective coefficient of discharge = 0.975

P1 = upstream relieving pressure, in psia.Kb = capacity correction factor due to back-pressure. This can be obtained from the

manufacturers literature or estimated from Figure 27 in API 520. The back-pressure

correction factor applies to balanced-bellows valves only.

T = relieving temperature of the inlet gas or vapour, in R.

Z = compressibility factor for the deviation of the actual gas from a perfect gas, a ratio

evaluated at inlet conditions.M = molecular weight of the gas or vapour.


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SIZING CALCULATION

Vapor Phase Relief

If flow is subcritical, the following equation will be used:

)(..735 2112 PPPM

ZT

KF

WA

d Ref. API 520 section 4.3.3.1

Where A = required effective discharge area of the valve, in square inches.

W = required flow through the valve, in pounds per hour.F2 = coefficient of subcritical flow

=

r

rr

k

k kk

k

1

1)(

)1(

12



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SIZING CALCULATION

k = ratio of the specific heats.

r = ratio of back pressure to upstream relieving pressure, P2/P1.


Z = compressibility at relieving inlet conditions.

T = relieving temperature of the inlet gas or vapour (R)

M = molecular weight of the gas or vapour.P1& P2 = upstream relieving pressure and back-pressure pressure respectively (psia)

Vapor Phase Relief


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SIZING CALCULATION

Vapor Phase Relief

Relief valves in steam servicewill be sized as follows:

A W

P K K Kd N SH

51 5 1.Ref API 520 Section 3.7.1

Where

A = required effective discharge area of the valve ( in2).

W = required flow through the valve (lb/hr).

P1 = upstream relieving pressure (psia)


KN = correction factor for Napier equation.= 1 where P11515 psia.

= (0.1906P11000)/(0.2292P11061) where P11515 psia and 3215 psia

KSH = superheat steam correction factor. This can be obtained from Table 9 in API RP520.For saturated steam at any pressure, KSH-= 1.0.


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SIZING CALCULATION

Liquid Phase Relief

For single phase liquid flow, the sizing method specified for

certified relief valves will be used.

The following equation only applies to non-flashing liquids.

A Q

K K K

G

P Pd w v

38 1 2Ref. API 520 section 4.5.1

Where

A = required effective discharge area of the valve ( in2).

Q = flow rate, in U.S. gallons per minute.

Kd = effective coefficient of discharge that should be obtained from the valve manufacturer.For a preliminary sizing estimation, a discharge coefficient of 0.65 can be used.

Kw = correction factor due to back-pressure. If the back-pressure is atmospheric, KW= 1.

Balanced-bellows valves in back-pressure service will require the correction factor

determined in Figure 31 in API RP520. Conventional valves require no special correction.

K-V = correction factor due to viscosity as determined from Figure 32 in API RP520.G = specific gravity of the liquid at the flowing temperature referred to water = 1.0 at 70F.

P1 = upstream relieving pressure, set pressure plus allowable overpressure (psia).P2 = back-pressure (psia).


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SIZING CALCULATION

Two Phase Relief

For 2-phase relief, the methodology in the newly released API 520Part 1 Appendix D Sizing for Two-Phase Liquid/Vapour Relief shall

be used (see Appendix 1).

This method has superseded the previous API methods because, in

certain circumstances, it has been found that the previous APImethods for 2-phase flow can undersize PSVs significantly.

This sizing method is based on the Leung Omega method, which

assumes thermal and mechanical equilibrium; these assumptionscorrespond to the Homogeneous Equilibrium Model (HEM).


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SIZING CALCULATION

Required relief area is calculated from relief rate. Hence

relief rate calculation is very important step

The case which giving the maximum relief area will be a

governing case.

Choose the worst case scenario to be a governing case



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SIZING CALCULATION

Selected standard relief area follows API RP526 or

GPSA chapter 5 Figure 5-7

Valve body size ( Inlet diameter x Outlet diameter) follows

API STD 526 Flanged Steel Pressure Relief Valve 4th Ed June 1995

Rated flow = (STD relief area) x Relief rate

(Required relief area)

Select proper orifice and valve body size based on STD


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SIZING CALCULATION

Calculate inlet line size (Line P < 3%of RP)

Inlet line sizing is based on pressure drop less than 3 % of

relieving pressure

Inlet Line design Consideration

Inlet line size must be at least equal to PSV inlet flange size.

Inlet piping should slope continuously upward from vessel

to avoid traps.

Inlet piping should be heat traced if freezing or congealing

of viscous liquids could occur.


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SIZING CALCULATION

Inlet Line design Consideration

A continual clean purge should be provided if coke/polymer

formation or solids deposition could occur

CSO valves should have the stem horizontal or vertically

downward


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SIZING CALCULATION

Perform preliminary estimate of tail pipe

Discharge line sizing is based on Mach No. less than 0.75

Tail Pipe Design Consideration

Discharge line diameter must be at least equal to PSV outletflange size.

Atmospheric risers should discharge at least 10 ft above

platforms within 50 ft horizontally

Radiant heat due to ignition of release should be considered.


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SIZING CALCULATION


No check valves, orifice plates or other restrictions

permitted.

Atmospheric discharge risers should have drain hole.

CSO valves should have the stem oriented horizontally or

vertically.

Piping design must consider thermal expansion due to

hot/cold release.

Autorefrigeration and need for brittle fracture resistant

materials.


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SIZING CALCULATION


Closed discharge piping should slope continuously downward

to header to avoid liquid traps


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SIZING CALCULATION


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SIZING CALCULATION

Perform Flare system modeling to indicate total back pressure and most

suitable tail pipe size.


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SIZING CALCULATION


Type

Value(% if set)

Conventional

Balanced Bellow

Pilot Operated

Effects on Valves : Gas applications

Back Pressure

Constant

VariableSuperimposed

VariableBuilt-up

< 30%

30-50%

>50%

No effect

Lift/Capacity reducedNo effect

Set point increasedby back pressure


Flow become sonic

Generally unstableDo not use

Flow becomesubsonic

Set point varieswith back pressure

10-30%

< 10%

No effect

> 50%

30-50%

UnstableDo not use

Lift/Capacity reduced


No effect

Flow becomesubsonic

< 10%

UnstableDo not use


No effect

No effect

No effect


10-30%

> 50%

Flow becomesubsonic


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SIZING CALCULATION


Type

Value(% if set)

Conventional

Balanced Bellow

Pilot Operated

Effects on Valves : Liquid applications

Back Pressure

Constant

VariableSuperimposed

VariableBuilt-up

< 20%

20-50%

>50%

No effect


No effect



Flow become sonic


Set point varieswith back pressure

10-20%

< 10%

No effect

> 50%

20-50%

UnstableDo not use



No effect

< 10%

UnstableDo not use


No effect

No effect

No effect


10-20%

> 50%


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EXAMPLE

DP = 400 psig

PAH = 350 psig

OP = 275 psig

OT = 80 oF

DP = 250 psig

OT = 130 oF

CV= 433

SP = 250 psig

SP = 400 psig

CV= 433FC

FC

V-1

V-2

F-1

F-2

Process SchematicPSV-1

PSV-2


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EXAMPLE

Assess Relief Scenarios

Scenario Source ofOverpressure

JudementScenario to be

consideredseparately

1 Fire Case An external fire could cause the pressure

in V-6010 to rise to the relief valve setting

through vaporization of liquids. Equipment

layout shown no potential that jet firewill happen on located area thus pool firewill be determined as a basis for calculation.

Yes

2 Tube Rupture This case is not applicable. No

3 Inlet controlvalve failure

CVs are fail to close type therefore this caseis not applicable for sizing PSV.

No


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EXAMPLE




4

Gas Blow-by Gas blow-by can occur by

Liquid in a V-1 drop so low that gas

exits via the liquid outlet nozzle due to level

control failure. Loss of liquid level will result

in gas from V-1 passing into V-2 via

F-1 and/or F-2.

Gas blow-by is based on follow assumption

Control valve upstream fails 100 % open

The upstream pressure is at the high

pressure alarm. The downstream pressure is 110 % of the

set point of the relief valve on the

downstream vessel.

No possibility that the bypass valve around

the control valve is opened.

Yes



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EXAMPLE




5

Blocked Outlet 1. Water Blocked Discharge.

This scenario will relates to two trips; first

LIAHH and secondly LAHH. Credit can be

taken that either of these two will actuate.It is unreasonable to assume both will fail to

response. Hence, this scenario cuts off the

feed to vessel and no relief is needed.

2. Oil Blocked Discharge.

This scenario relates to only oil trip, while

interface level is already healthy. No credit

can be taken that this trip will work. Hence

PSV needs oil relief.

Yes


6 Thermal Relief This case is not applicable. No


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EXAMPLE

Possible Relief Scenarios

There are three different possible relief scenarios :

Fire Case.

Gas Blow-by Case.Blocked Outlet Case.


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EXAMPLE

Fire Case

Scenario

An external fire could cause the pressure in V-2 to rise to the relief valve

setting through vaporization of liquids. Equipment layout shown no potential

that jet fire will happen on located area thus pool fire will be determinedas a basis for calculation.

Relief Condition

PSV-2 set pressure = 250 psig

Allowable Accumulation = 21%

Maximum allowable accumulated pressure = 250x1.21 = 302.5 psig.Relieving temperature = ????


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EXAMPLE

Fire Case

250 psig@61 min ,relief temperature = 191.5 oF

Since relief can beinitiated after 61 min

fired but API RP 520

allow 15 min to handle

fire hence

this case will rarely

happen in real operation.


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EXAMPLE

Fire Case

By input all required parameters (eg. Vessel dimensions, fluid relieving

properties), in to calculation sheet (WS-CA-PR-025, Rev 0)

Required relief area = 0.183 in2

Relief Load = 4,066 lb/hr

Therefore

Select 1xEx2 orifice with discharge area of 0.196 in2


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EXAMPLE

Gas Blow-by

Scenario

Liquid in a V-1 drop so low that gas exits via the liquid outlet nozzle due to

level control failure. Loss of liquid level will result

in gas from V-1 passing into V-2 via F-1 and/or F-2.

Gas blow-by is based on follow assumption

Control valve upstream fails 100 % open.

The upstream pressure is at the high pressure alarm.

The downstream pressure is 110 % of the set point of the relief valve on the

downstream vessel.

No possibility that the bypass valve around the control valve is openedas locked close.

(Note : This may need to be considered in some cases).

Relief Condition


Allowable Accumulation = 10%

Maximum allowable accumulated pressure = 250x1.1 = 275 psig.


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EXAMPLE

Gas Blow-by

Assume

F-1 and F-2 are same maximum Cv of 433.

C1 assumed 26.5 (C1 is normally obtained from valve vendor)

Assume only one control valve is fail.

Therefore Cg = C1 x Cv = 433 x 26.5 = 11,474.5V-1 hold pressure at PAH = 350 psig

V-2 operating pressure = 275 psig ( 250 psig x 1.1 = 275 psig)

Differential pressure = 350-275 = 75 psi

Reliving temperature = 80 deg F

How to find the maximum relief rate via F-1 and/or F-2 ?????


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EXAMPLE

Gas Blow-by

Maximum relief rate = 245,703.5 lb/hr

Maximum relief rate

can be evaluated from

control valve sizing

Programs (eg. Fisher,

Masoneilan etc.) or sizing

equations eg.in GPSA.


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EXAMPLE

Gas Blow-by

By input all required parameters (eg. fluid relieving

properties), in to calculation sheet (WS-CA-PR-025, Rev 0) for generally

vapor relief.

Required relief area = 11.57 in2Relief Load = 245,703.5 lb/hr

Therefore

Select 6xRx8 orifice with discharge area of 16 in2


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OIL & GAS

EXAMPLE

Blocked Outlet

1. Water Blocked Discharge.

This scenario will relates to two trips; first LIAHH and secondly LAHH.

Credit can be taken that either of these two will actuate. It is unreasonable

to assume both will fail to response. Hence, this scenario cuts off the feed to

vessel and no relief is needed.

2. Oil Blocked Discharge.

This scenario relates to only oil trip, while interface level is already healthy.No credit can be taken that this trip will work. Hence PSV needs oil relief.

Relief condition


Allowable Accumulation = 10%Maximum allowable accumulated pressure = 250x1.1 = 275 psig.Relieving temperature = 130 oF


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OIL & GAS

EXAMPLE

Blocked Outlet

From simulation

Oil flow rate = 508,100 lb/hr (25,000 bpd)

Oil density = 55.21 lb/ft3

By input all required parameters (eg. fluid relieving properties), in to

calculation sheet (WS-CA-PR-025, Rev 0) for generally liquid relief.


Therefore select a 3xLx4 orifice with discharge area of 2.853 in2


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OIL & GAS

EXAMPLE

Relief Case

Design Orifice Size

Relief Phase

Fire Case 1xEx2 Vapor

Gas Blow-by 6xRx8 Vapor

Blocked Outlet 3xLx4 Liquid

Selection of Governing Relief Scenarios


75/79

OIL & GAS

EXAMPLE

Therefore the governing case is Gas Blow-by case

Required relief rate = 287,530 lb/hr


Selected relief area = 16 in2

Rated flow = 16 x 287,530 lb/hr = 397,621 lb/hr

11.57

Selection of Governing Relief Scenarios


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OIL & GAS

EXAMPLE

Inlet line sizing calculation

Inlet line sizing calculation is based on pressure drop less

than 3 % of set pressure

PSV set pressure = 250 psig

Allowable pressure drop = 0.03x250 = 7.5 psi

Mass rated flow = 397,621 lb/hr

Based on maximum rated flow and its relief properties

For 8 inlet line, single phase pressure drop is 5.3 psi therefore

suitable for inlet line sizing criteria.


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OIL & GAS

EXAMPLE

Outlet line sizing calculation

Outlet line sizing calculation is based on the maximum Mach No. of 0.75

From preliminary estimation 12 line give Mach No. of 0.71 hence suitable

for outlet line sizing criteria.

Result from FlareNet modeling show 10 give Mach No. of 0.45 ?????

EXAMPLE


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OIL & GAS

Selection of Relief Valve Type

Result from FlareNet simulation give total back pressure of 70 psig

PSV set pressure = 250 psig

PSV type

Conventional

Balanced Bellow

Pilot Operated

% of Maximum Back

Pressure Allowable

10% of SP

30% of SP

N/A

Maximum Back

Pressure Allowable(psig)

25 psig

75 psig

N/A

EXAMPLE


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Summary

6xRx8 Balanced Bellow PSV

Inlet line size is 8

Outlet line size is 10

108x10

8x6

8

Gas Blow-by

Set@250 psigBalanced Bellow

Date post:	09-Oct-2015
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124479095 PSV Calculation Ppt

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