• Author / Uploaded
• Sonu Singh

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June 2014

PRESSURE RELIEF SYSTEMS Acknowledgment

DEDICATED TO:

API STD - RP 520/ 521/ 526/ 537 Various Client/ Project Standards/ Specifications Pictures from many sources, suppliers, internet

My friend Winston Yeo, KBR, Singapore/ Chevron, Thailand

Topics
Introduction
Relief Devices
Codes & Standards
Relieving Scenarios (Demands) & Loads
Sizing
Installation
Isolation
Design Features

PSV

Introduction

PAHH PIC/PAH NOP


Control system maintains stable operation
Trip / shutdown system provides primary protection,

when control system fails
Relief system provides secondary protection, when control and trip systems fail – ultimate protection or last line of defence PIC

T0 Flare PAHH PALL

T0 Compressor SDV

Well Fluids SDV

Production Separator SDV SDV

RV lifting: a serious incident

Oil/ Condensate Produced Water

Code Vs Recommended Practice
Relief devices – key part of plant Layer of Protections to protect plant and personnel. Prevent production loss
Relief devices are required by national codes and standards, mandated under law

Community Emergency Response Emergency, Evacuation Plant Emergency Response Containment/ Evacuation Procedure Mitigation Mechanical mitigation, Relief System Operator Action Prevention SIS Trips Operator Response Controls & Monitoring Controls, Alarms Operator Supervision

Process

ASME is a Code. Compliance is mandatory. API is a recommended practice. API is also getting adopted as a National / International Standard

Where Pressure Relief is not possible The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.


Fast chemical reactions:  Pressure propagation rate is very high and loss of

containment may occurs before RV pops.
“hot spots,” decompositions & internal detonation/fires  Relieving rate requires large relief areas


Plugging, polymerization or deposition that may

partially or completely block RV  Relieved chemicals may polymerize and plug. PSV useless


Multi-phase relief: where rate is difficult to predict
Relief may create additional hazards due to stack

location or very large vent/ flare system
Use HIPPS

Relief Scenarios

Tank drained. Pulled vacuum The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

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Air freshener can in a closed car - Thermal The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Vacuum column fire

Column internals - Pyrophoric fire The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Semi sub – what was left

RELIEF DEVICES

Relief

Devices The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

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Relief Valves
Rupture Disks The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.


Rupture Pins
Buckling Pins
PVRV
Blow-off Hatches
Explosion Doors

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Relief Devices The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

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Relief Valves
Conventional
Balanced
Pilot

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Conventional RV





Set Press 

Most common Simple, cheap and reliable Backpressure reduces capacity Variable back pressure limited to 10% of set pressure
Large spring required limits set pressure of bigger PSV
Constant or superimposed backpressure increases set Why? point on a 1 for 1 basis 150# RV Set Press, psig at 100°F

100

0 Back Press 

50

D–P

Q

R

T

285

165

100

65

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Balanced Bellows RV










Not allowed per ASME section I Back pressure max 30% on all except smaller sizes. Up to 50% with capacity correction Fragile bellows. Mechanical limit imposed by bellows Bellows can plug; movement restricted In plugging and polymerizing service Bellows sealed in hydrate, solid, foaming and coking services to keep foreign matter out of bonnet Bellows prone to fatigue and pin-hole leaks. [Leaks take away ability to handle backpressure; hence bonnet is vented. As long vent is bigger than “holes” OK.] Bonnet vent must be routed to safe location in toxic service Why?

Bellows original purpose was to protect the spindle & guide from corrosive fluids. Beyond 30% back pressure, lift and hence capacity affected

Balanced Disk RV This image cannot currently be display ed.


Backpressure acting on top and

bottom of disk cancels each other
Backpressure ha no effect on RV opening or closing pressure

Set Press 

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100 RV Opening Pressure RV Reseating Pressure 0 Back Press 

50

Pilot RV
Process pressure on a differential area piston keeps the seat closed
Pilot: A small PSV that pops and removes piston top pressure,

allowing the main valve to open

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Pilot

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Pilot Dome Dome

Piston

Pilot Tube Pilot Tube Note: Piston top area > bottom area. For the same pressure, force on top > force on bottom, keeping the seat closed

More on Pilot RV
Process pressure on the larger piston (top) area opposes






pressure on the smaller seat, keeping the valve shut Higher the process pressure, greater the downward force, keeping the seat tightly closed. c.f spring loaded RV A small auxiliary relief valve (pilot) controls the main RV. It pops open relieving top pressure, opening main RV Larger RVs can have higher set pressures; no longer limited by spring force. c.f spring loaded RV Full lift and capacity achieved near set pressure as there is no heavy spring load to overcome With pop action, full lift at set pressure; with modulating pilot, full lift at relieving pressure; modulating pilot relieves only what is required

More on Pilot The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Pilot is a small RV! 1. As process pressure reaches set pressure, the spring is compressed; lower feeding seat closes, isolating process gas 2. Upper seat opens, venting gas and pressure in dome; and opening RV
Process gas isolated during a relief – no flow pilot
Flowing pilot, discharges process gas before, during and after a relief. Not recommended

2 1


Flowing design may lead to freezing or particulates into the pilot
Based on one-shot venting or gradual venting “pop” or “modulating” action
“Pop” or fast action is for rapid relief of gas. Recommended. Spring loaded RV

Spindle travel - decides blowdown. 3% blowdown possible


“Modulating” allows RV opening with a small pressure rise; fast response. Relieves what is reqd. Diaphragm RV

More on Pilot RV
Most have soft seats; remote sensing capability – pilot







tube intake need not be at RV inlet Polymerizing, plugging service, sensing line can plug. Use non-flow type or filtered sensing line in dirty service Backpressure: Set pressure not affected unless pilot is vented to header Caution: Higher backpressure can lead to reverse flow and product contamination, during start-up and shutdown. Use check valve Usual to have no-flow, pop action elastomer seat/ seal type. Less commonly used. May require prior approval Liquid filled systems: Blowdown may change c.f gas service. Operating time too rapid producing water hammer or too slow. Pilot affected by particulate contamination or corrosion

Capacity - Back Pressure Impact
RV: ‘Nozzle’ or ‘orifice’ flow decided by:

Transition Point Sonic to Sun-sonic

 upstream pressure, as long

as it is ‘critical’ or ‘sonic’  ∆P, Pressure drop (P1-P2), if sub-sonic


Back-pressure adds to spring force, reduces lift and flow (‘capacity’ of RV)

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Back Pressure - Conventional RV The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.


Backpressure affects lift; impacts capacity severely At 15% valve fully closed The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Back Pressure - Balanced Bellows
Bellows nullify backpressure effect to an extent The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

At 30% capacity reduced The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Difficult to have small bellows. Size D & E, may be a ‘modified’ F !! Bellows fixed at at upper end. High back pressure lengthens the bellows at the lower end, restricting seat lift

Back Pressure - Balanced Disk RV
Disk nullify backpressure effect to an extent At 30% capacity reduced

At 20% capacity reduced The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

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Balanced Spindle type can withstand higher backpressure; Sizes to 2J3 only

Back Pressure - Pilot RV Flow
Flow follows closely nozzle flow
For k = 1.3 & BP = 70%. Flow:

Nozzle = 92% Pilot = 78%
Back flow Preventer:

Backpressure may open the main valve when process pressure is low as at start-up. May contaminate products. C3 Refrigeration

Back Pressure Impact - Comparison Set Pressure Vs Lift The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

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Conventional This image cannot currently be display ed.

Pilot

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Backpressure Impact - Tests
University of Milan Test on 5 Balanced RV, 2 J 3
Supposed to be good to 50% BP viz k = 1
Capacity Lost, %

at BP, %

Remarks

A

10

50

B

30

50

C

40

30

D

20

30

0% at 32%

E

60

18

bellow ruptured

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Terminology - Refresher
Relief Valve: Valve opens in proportion to

overpressure. Liquid (incompressible fluids) service
Safety Valve: Valve opens rapidly with pop action. Vapour (compressible fluids) service
Safety Relief Valve: Either a safety or relief valve
Pressure Relief Valve: Generic term for all of above

Terminology - Refresher
MAWP: Max Allowable Working Pressure on top of vessel







based on wall thickness provided at coincident temperature. ≈> Design Pressure Design Pressure: Equipment/ system design pressure at design temperature Set Pressure: Pressure at which RV is set to open. May be same or less than Design Pressure Overpressure: Pressure increase over set pressure Relieving Pressure: Set pressure + Overpressure Accumulation: Pressure increase over MAWP Back pressure: Pressure at the outlet flange/ pressure in discharge system Where RV is set below MAWP, overpressure can be higher to match MAWP + Accumulation

Terminology - Refresher V-002


Superimposed back pressure - Affects set pressure.

Pressure at outlet flange before RV opens

E-001

 Constant superimposed BP: Always the same pressure. When RV

discharges to a closed system. Can be high ~ 50% of set pressure.  Variable superimposed BP: Varies based on flow from other sources. When multiple sources discharge to a common header


Built-up back pressure - Does not affect set pressure but

affects capacity Pressure that develops in the discharge header as a result of flow thru RV


Total back-pressure = Superimposed + Built-up BP
Spring differential: Difference between set pressure and

Why?

superimposed constant BP. It is not wise to give a superimposed constant BP in a data sheet unless one exists.

Back Pressure - Example Flare Stack

Normal Operating Press = 0.3 U

Flare Header

∆P = 10 U due to flow from this + other PSVs SP = 100 Units Relief valve







∆P = 5 U

Care needed while specifying constant BP

Superimposed Constant Back Press = 0.3 U Built-up Back Pressure = 15 U Total Back Pressure = 15.3 U Spring Differential (Set Pressure – Constant BP)
Spring set at: = 99.7 U

Flare Knockout Drum

Back Pressure - More Info
Backpressure adds to spring load, prevents full

Size

lift  

Flow and backpressure reduced; Valve opens again Close  Open. Rapid cycling or chattering


P1, backpressure at valve outlet flange is

known and NOT PB inside the valve at nozzle outlet
Bigger the RV, smaller is Outlet: PSV area (Ao/A) ratio; Higher is PB
PB, controls flow in sub-sonic cases
Vendors have come up with a correction factor to Nozzle Coefft, to account for this – based on valve body / nozzle geometry The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Design Tip: Backpressure mechanical limit on RV is decided by bellows. Bigger the RV lower is allowable backpressure. Affects non-flowing RVs too; forgotten by Process Engineers. See RP 526

P2 PB

Outlet: PSV Area Ratio Area, in2 (Ao/A)

1½ D 2

0.110 (31)

1½ E 2

0.196 (17)

1½ F 2

0.307 (11)

2G3

0.503 (15)

2H3

0.785 (9)

3J4

1.287 (10)

3K4

1.838 (7)

3L4

2.853 (5)

4M6

3.600 (8)

4N6

4.340 (7)

4P6

6.380 (5)

6Q8

11.050 (5)

6R8

16.000 (3)

8 T 10

26.000 (3)

Terminology - Refresher
Blowdown: Difference between set pressure and reseating

pressure, % of set pressure. Usually 3%
Cold differential test pressure: Set pressure with correction for backpressure and/or temperature service condition
Simmer: Audible or visual release of fluid across the RV just prior to opening at set pressure. Excessive simmering is detrimental to valve seating surfaces
Chattering: Rapid opening and closing of RV in quick succession. Wear and tear on seating surfaces leading to leak in normal operation. Caused by:    

Oversized RV Inlet loss > 3% Excessive back-pressure Broken or leaking balanced bellows


Lift: Rise of the disc to open the RV

RV Operation - Refresher

Boiler Board Formula with Lift


As the seat lifts,







S Seat Disk Curtain Lift, L




flow is thru (i) nozzle at full lift or (ii) curtain for partial lift Nozzle Area = πD²/4 Curtain Area = πDL; L = D/4 Usual lift is about 35 to 40% At PSV opening point, press * area = spring load To reach full lift, additional overpressure required, say 10% to compress the spring. Not enough. Solution? Add a skirt to seat, to add ‘area’ and redirect flow to add to lift Blowdown Ring, controls blowdown Top: Short Simmer; long blowdown

P

Skirt

Increases Decrease Blowdown

Nozzle Diameter, D Blowdown Ring

RV Operation - Refresher This image cannot currently be display ed.

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Rupture Disks
Non-reclosing

Non-reclosing: Unlike a PSV that closes once the pressure < set pressure, RDs remain open and discharge the contents. It has to be replaced after an event

 Good for large relief; instantaneous and unrestricted relief  For valuable/ toxic fluids (no leak) and viscous, high melting

point fluids  For corrosive and slurry (no exposed seat/ spring)  Used upstream and downstream of RV in corrosive services  Upstream of RV


Protects RV internals from corrosion – save $$ using standard MOC; Prevents leakage thru RV; Prevents plugging and gumming of RV; Allows in-situ calibration testing of RV  Downstream of RV Note: Max distance between RD and PSV = 5D
Protects RV internals from corrosion – save $$ using standard MOC; Check leakage thru RV; Prevents fouling and gumming of RV; Cushions impact of variable backpressure  In parallel to or in series with RV Design Tip: RD + RV requires Combined Capacity Factor ≈ 0.9 factor on RV area; combined inlet ∆P E-001 tube side design pressure.  Relief rate: Based on pump head at PSV-001 relieving pressure and max suction pressure of pump P- 001  Good design to have pump outlet designed for shut-off head

V-001

Capacity

Relief flow is less than Operating flow

Blocked Outlet
To satisfy ASME, a PSV

Relieving

Operating Head

is required on equipment at pump outlet, even if its design pressure > pump shutoff pressure
Relief rate may be nominal or Nil.
If a PSV is provided for some other reason, say fire, then it will do

Capacity

Pump suction valve & piping downstream of it to suit discharge conditions

Control Valve Failure
Causes     

Instrument air failure; Signal (wiring) failure; DCS hardware/software failure Improper manual operation by operator Mechanical malfunction of control valve Hand wheel left engaged on control valve Plugging


Evaluate both Open and Closed position of control valve
No credit: for interlocks / Emergency Shutdown System in RV size;

Credit may be taken for total load to flare header
Credit may be taken for normally open flow paths and not affected
Simultaneous failure control valve and bypass: Owner preference 

Options: No bypass; RO in bypass; bypass valve Cv same as control valve; parallel but not-connected control valve; parallel control valve on its own

Control Valve Failure PSV - 002


LCV - 1 fails open

PSV - 001

 PSV - 002 Size:

Max flow thru LCV-1 minus V-2 normal flow  Max flow thru LCV-1: Max Cv + downstream PSV - 002 relieving pressure + ∆P between LCV and PSV


LCV – 1 fails closed

SDV-2 V-2 V-001 SDV-1 SDV-3 LCV-1 V-002

L-2

 PSV - 001 Size: Blocked

outlet

Note: Several approaches to gas blowby load estimation: All gas; gas volume equivalent volume of liquid; both gas and liquid limited to max inflow etc. Dynamic Simulation helps get realistic results If it overloads or is the largest LP Flare load, consider same design pressure for the d/s vessel to eliminate gas blowby case

Heat Exchanger Tube Failure

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Shell & Tube heat exchangers tubes

may fail due to thermal shock, vibration, corrosion etc
No PSV, if high pressure side design/ operating pressure is ≤130% of design (= hydrotest) pressure of low pressure side * temperature correction
130% or ‘0.77 rule’ does not mean tubes don’t rupture – a common mistake  Evaluate potential overpressure of connected equipment  Evaluate potential chemical reactions when two sides mix


2 Options - tube failure at mid tube viz 2 orifices and

failure at tube sheet viz 1 nozzle + 1 orifice
For PCHEs, one full channel failure
No PSV for tube failure in double pipe exchangers

Heat Exchanger Tube Failure The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.


Credit: Flow thru normally open path if LP fluid is gas or vapour  If LP side is liquid, pressure build-up to push and

accelerate large liquid mass. It is as good as blocked


Some consider tube rupture only when HP to LP differential pressure > 65 bar (1,000 psi)
On tube rupture, pressure spike is rather quick. Usually rupture disks are provided as spring loaded RVs take time to react  Opening time: Rupture pin: 2ms; Rupture disk: 5ms; RV

25ms  Recommended to have 2 RDs at either end of LP side  Dynamic Simulation studies help, select location

Check Valve Failure
All valves leak or pass. In early designs check valve leak was

NOT considered

 Check valves stop “bulk” flow but can’t avoid leak past them  Some considered check valve leak only in high pressure or dirty or

surging service. Some considered specially designed, power assisted check valve can stop reverse flow. No longer valid


No credit to single check valve. Reduced flow area for 2

dissimilar check valves

 Standard calculation methods available to estimate leak past a

check valve  Note: Along with leak, pressure is transmitted. That is HP side can pressurize LP side shut-in This point out is missed out by Process Engineers in a Hazop review

When a compressor trips, discharge from other running compressors can back flow into the tripped one, pressurizing its Suction Drum

Check Valve Failure
At a common manifold, when one of the stream stops

flowing or a pump/ compressor feeding it stops, fluids from other streams may back flow thru the non-flow pipe

Header A

Xmas Tree

Well

3 Workers Killed Header B

Test Header

Wellhead inlet manifolds: A common check valve or one per header

Thermal Expansion
Liquid filled equipment / piping that is blocked-in and heated  Solar radiation; Hot side of exchanger; Heat tracing


Heat Exchangers: Cold side vapor pressure > design pressure  At ambient temperature; At hot side fluid inlet temperature; Heat

tracing


OSBL: Yard piping
10% overpressure for vessels and 33% for piping
CSO or LO valves can eliminate thermal PSV, provided Owner

agrees to administrative control

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Air freshener can in a closed car - Thermal

Thermal Expansion
Thermal Expansion  Massive Force
Liquid Ammonia Tank in a closed garage, exploded and

propelled the truck 40m

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External Fire Vacuum column fire


Pool fire under equipment, even if contents are not flammable
Radiant + direct heat boils liquid / expands vapour  increasing

pressure
Equipment assumed blocked in and isolated when fire occurs and inflow stopped 

There can be exemptions for this rule, example, heat exchangers


ASME stamped equipment must be protected unless fire can be

ruled out or equipment/ system cannot be blocked-in
Piping and piping components do not require protection. 

Interconnecting piping included in adjacent equipment


Equipment grouping: 8.6m (28.2’) radius (2,500 sq.ft area) and 7.6m

(25’) high from grade are grouped in a single fire zone 

Liquid at NLL or HLL


Evaluate: Effects of chemical reaction, fluid decomposition and fluid

behaviour (foaming, frothing, etc.)

Do NOT design for Jet Fire load as some do. As API RP 521 says, jet fires are handled by blowdown viz. removing fuel

Design Tip: API indicates max fire zone size. Use it wisely to reduce Blowdown load + Flare size

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External Fire

Caution: Corrosion under insulation can bring a vessel down before fire does!. Need Inspection windows


Fire NOT considered if:  Sloping or proper drainage eliminates pool fire possibility  No flammable hydrocarbon exists in the area  Air Coolers/ equipment located 7.6m (25’) above grade OR

over open grating  If fire load is relieved thru any passage that can’t be shut  If Owner instructs: “Equipment will be vented and drained when taken out of service”. e.g. Pig launcher/ receiver


Credit for fireproof insulation as allowed by API; it

should withstand firewater jet impact
Gas vessel: Fire PSV not effective as vessel metal temperature > Creep temperature PETRONAS: No fire PSV for gas vessels Design Tip: If fire relief temperature > equipment design temperature, use design temp for RV material and flange selection. Say so in data sheet. Vessel metal temperature will be 200-300°C > RV relief temperature. Vessel will fail/ rupture/ deform first before RV lifts

External Fire

RP 521 Figures • Heat up rate • Time to Rupture


Vessel under fire

will deform/ rupture before PSV lifts, as metal wall rapidly loses strength as its temperature rises.
Blowdown Valves are provided to depressurize the vessel within 15 minutes
Against fire:  Blowdown  FW spray  Fireproofing

Temp °F

%

400

100

800

80

1,100

36

RVs do NOT protect against structural failure when the vessel is exposed to extremely high temperatures during a fire The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

1/3 Tensile

2/3 Yield

Max Temp

°C °F

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Pressure Surges
Transient Analysis required for

Water hammer. 24 t piping flew off 800m. Sheared off telephone poles

 water, liquid filled or rundown OSBL lines  oil/condensate export pipelines


Transient Analysis is NOT required for  ISBL piping. Short runs and generally do NOT have quick

closing valves


Code allowed - short term - margins may be used to avoid a PSV The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Design Tip: It is common for GRE Fire Water/ Sea Water piping to burst during start-up, fill the Flare KOD and bring the plant down. Have a good surge study; leave design and construction/start-up to a single source. Take exception from Owner, giving him the risk.

Seawater flooding a column sank semi-sub

Column Cases

Excess heat may not pressurize the exchanger but will over pressure the column


Reflux failure is usually the controlling case:  Reflux Pump / Power Failure  Reflux Control Valve Fails Closed  Overhead Condenser Failure or Flooded on Draw-off Control Valve

Fails Closed  Non-condensable Accumulates in the Condenser  Operator Error: Block Valves Closed


Loss of Cold Feed  Feed Control Valve Fails Closed or Feed Pump Fails - Transient Surge

in Vapour Rate


Excess Heat to Reboilers  Steam or Heating Control Valve Fails Open  Excessive Fuel to Fired Reboiler Safety Alerts Column blows off top – leakage reacts  Additional Vapours generated Column overflows – 15 killed; 150 injured


Reboiler Tube Rupture

Credit: Reduced vaporization in reboiler at relieving pressure. Reduced ∆T relieving pressure reduces relief rate

Column Cases
Column load calculations is complicated  3 approaches – flash, gross overhead vapour, unbalanced heat; last

one gives the best estimate

Column

Overhead Vapor, kg/h

DC Steam Stripper

Unbalanced Heat, kg/h

60,000

168,000

DC Fractionator

296,000

448,000

HC Debutanizer

69,000

171,000

 Dynamic simulation can reduce

Dyn Sim load 60%. Case 2 PSV does not pop The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

column and reactor loads

DC Fractionator

Conventional, kg/h

Dyn Simulation, kg/h

.. Total power fail

448,000

259,000

.. Single power fail

85,000

0

258,000

172,000

.. Blocked outlet

From: “Optimize relief loads with dynamic simulation”, CL Xie, ZG Wang and YF Qin, HP, Dec 2013

Accidental Mixing of Fluids
Runaway reaction - Polymerization:  Some chemicals, when mixed in wrong

ratio or sequence may lead to run-away reaction  Inadvertent mixing of reactive streams  Decomposition or polymerization due to abnormal heat input or loss of cooling  See Safety Alert

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Runaway Reaction Relief Rate  Determination is complex. Inputs from

Owner, Catalyst Manufacturer, Process Licensor.  Owner/ Licensor to provide the relief load. Pass them the responsibility

Bhopal. 4,000 to 10,000 dead; 500,000 injured

Liquid Overfill of Storage Tanks The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.


Inflow exceeds outflow
Overfilling from an offsite

pump during start-up or LAH/LAHH failure The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

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Design Tip: 1. Let level transmitters for Control and Trip track each other. 2. While filling large tanks, let DCS put a time lock based on pumping rate and ullage

Vacuum Relief

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Equipment may come under vacuum:  Fluid withdrawn without matching inflow  Excessive condensation in Column   

Overhead Condenser Condensation or cooling of vapours upon atmospheric temperature drop Compressor suction side blocked Condensing side of exchanger blocked in while cooling continues Draining with vent closed Cool down and condensing after steaming a vessel This image cannot currently be display ed.

 

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Vacuum Relief
Equipment that could come under vacuum is designed to withstand full vacuum  Note: For large diameter columns and storage tanks,

cost of designing to full vacuum is prohibitive


Check the consequences of air mixing with vessel inventory before providing vacuum relief. Usual to provide Nitrogen padding/ blanketing
No RV required if Owner instructs that his administrative procedures can prevent vacuum  Draining test water; Steam condensing after a steam-out

Atmospheric Tank Protection
Inbreathing (vacuum relief) is required for  Maximum outflow without matching inflow  Vapour shrinkage due to atmosphere cooling –

showers etc  Blanket gas supply valve fails/ closed


Out breathing (pressure relief) required for    

Maximum inflow without matching outflow Vapour expansion due to atmosphere warming Blanket gas supply valve fails open Vapour outlet valve fails/ closed


Fire relief required unless tank has frangible

roof

 Fire generally does not engulf the entire tank

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Atmospheric Tank Protection
Refer API 2000, for calculation of relief load  N2/ gas padding for thermal inbreathing/ outbreathing  PVRV for thermal inbreathing/ outbreathing  Gauge hatch / manway vents for fire relief  Tanks have a low design pressure, mmWC. PVRV/

manway are weight loaded; sizing by vendor  PVRVs installed directly on roof nozzle

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Blanket Gas Regulators

Emergency Vent Gauge Hatch and Manhole Cover

PVRV

Fired Heater
Blocked outlet and thermal  No PSV required for process coils, unless mandated

or underrated  PSV required for BFW and Steam coils – Code/ IBR


Thermal PSV in Hot Oil WHRU  Residual heat in refractory/ insulation. Not effective

against oil coking inside the tube

Pumps & Compressors
Centrifugal Pump 

Usually designed for shut-off or highest head at zero flow


Reciprocating/ Positive Displacement Pump/ Compressor 

RV for blocked outlet. Due to pulsation in discharge pressure keep good margin between operating and set pressure


Centrifugal Compressor Suction side for settle-out pressure. 2-3 stages in a common casing may settle-out together + Check valve leak  Refrigeration or low temp service ~ vapour pressure at ambient temperature  Discharge/ Casing: Design for surge pressure at 105% speed with maximum [suction pressure; molecular weight] and minimum suction temperature [Oil & Gas Industry practice] or RV provided at 120% of Normal Operating Press  Inter-stage: Usually fire case 

When a compressor trips, discharge from other running compressors can back flow into the tripped one, pressurizing its Suction Drum

Typical Relief Cases – Oil & Gas











Flowline - Blocked Outlet/ Thermal Inlet Sep - Blocked Outlet LP Sep - Blocked Outlet/ Gas Blowby Compressor - Fire/ Check Valve Leakage Compressor Last Stage - Blocked Outlet ? Glycol Contactor – Fire Fuel Gas KOD - PCV Failure Glycol Pump - Blocked Outlet Filters - Fire Air Vessels - Fire

RVs may not be the right solution…
For a few cases, RV is impractical. Instrumented safeguards is needed
Hot Oil WHRU. Hot Oil Boiling Pt at Relief Press >≈ Incoming Flue Gas Temp  Heat to boiling fluid ≈ zero  Hot oil will decompose and coke before it boils  Instrumented protection to remove source of heat + minimum flow at all times + thermal PSV against residual heat in WHRU




Export pipeline of 1,000 MMSCFD A huge flare. HIPPS contains the HP fluid avoiding a release  Instrumented Protection to isolate the HP source such as compressor and/or HIPPS (2 independent SDV) from 2 “independent” trip systems. 


READ ASME CODE CASE 2211-1, now part of RP 521, Annex E

Design Tip: SDVs leak, Ha Ha

Design Tip: HIPPS/ IPF requires Documented User Approval. Only User may specify pressure protection by system design.

SIZING

Design Tip: Analyzing Relief Scenarios and Estimating Relief Loads is the important part. Sizing is a matter of routine.

RV Sizing
3 Equations  Vapour - Critical  Steam - Critical (ASME Div VIII)  Liquid K = Sizing constant W = Relief flow rate C = Coefficient P1 = Upstream relieving pressure Kd = Coefficient of discharge Kb /Kw = Back pressure correction factor Z = Compressibility M = Molecular weight T = Upstream relieving temperature A = Required orifice area

KW TZ A= CP1KdKb M KW A= P1KdKbKnKsh KQ G A= 38KdKwKv P1-P2

Kn = Correction factor for Napier Equation Ksh = Correction factor for steam superheat Kv = Correction factor for viscosity P2 = Total back pressure SG = Specific gravity of liquid Liquid sizing: trial & error step required. Start with an assumed size to determine Re and hence Kv. Repeat to match See API for sizing 2 Phase Flow. Older method of vapor + liquid area is no longer valid

RV Sizing
Subcritical Flow – Vapour, pilot and conventional  Valid for RVs that have their cold spring setting

adjusted to compensate for the constant superimposed BP  Built-up back pressure 10% back pressure

RV Sizing
Coefficient of Discharge Kd  Depends on relief valve design  National Boiler Board certifies capacities of all RVs  Manufacturer back calculates Kd from certified capacity

and test conditions  If unknown, assume 0.975 for vapour and 0.65 for liquid  RV capacity must be checked based on vendor Kd  For all vapour and liquid RVs, manufacturer should supply sizing calculation based on his Kd

Design Tip: Kd varies from manufacturer to manufacturer. Our calculations should not be passed to clients. Final calcs from supplier should be the deliverable.

RV Standard Sizes

Standard RV Sizes

API 526 Orifice Designation


RVs made in standard sizes
Each standard orifice given a

letter designation
Select a standard size larger than the calculated one
If calculated size, marginally exceeds a standard size, it may be OK, as the actual orifice area for most RVs are higher than the standard API area. Actual areas are listed in National Boiler Board Book

Area, in2

Size D

0.110

E

0.196

F

0.307

G

0.503

H

0.785

J

1.287

K

1.838

L

2.853

M

3.600

N

4.340

P

6.380

Q

11.050

R

16.000

T

26.000

RV Standard Sizes

Standard RV Sizes

API 526 Orifice Designation

RV Inlet x Outlet Sizes

Area, in2

Size D

0.110

1 x2

E

0.196

1½ x 2

F

0.307

1½ x 3

G

0.503

H

0.785

J

1.287

K

1.838

L

2.853

6 x 10

M

3.600

8 x 10

N

4.340

P

6.380

Q

11.050

R

16.000

T

26.000

Size

2 x3 3 x4 3 x6 4 x6 6 x8

D

E

F

G

H

J

K

L

M

N

P

Q

R

T

Standard RV Sizes Air/Gas/Steam Service This image cannot currently be display ed.


API area is not actual RV area 

Actual area and nozzle coefft vary from manufacturer to manufacturer


2J3 API Area = 1.287 in2  

Actual area = 1.427 to 1.635 in2 Coeffts = 0.788 to 0.975


National Board certified capacity - based

on nozzle coefft and orifice area - varies
Why the difference? 

In 1962 ASME Sec VIII derated certified capacities by 10%. Manufacturers did not derate their advertised capacity or nozzle coefft, but increased nozzle area by 10%. But API orifice areas as advertised remain same.

Board RV Area is based on • Nozzle bore for full lift valves • Lift for restricted lift valve

KA is more comparable. Explanation AG/Crosby Advertised KA = 0.975*1.287 = 1.255 National Board = 0.788*1.635 = 1.288 API (K= 0.9) = 0.9*1.287 = 1.158 Actual capacity may be 10-16% more So don’t jump from P to R (73%) when calculated size marginally exceeds standard size

Thermal Expansion
Relief Rate, q =

αv.φ K.d.c αv = cubic expansion coefft of liquid at expected temp φ = Heat Transfer Rate Exchangers: use max Heat Duty Solar Radiation: use as per Project Design Basis K = Sizing constant d = Relative Density c = Specific heat of trapped liquid


For thermal protection of piping, generally ¾” D 1” threaded

or 1” D 2” flanged RVs are provided; No calculations done

Fire Relief


Liquid:   




Latent heat for multi-component is tricky, but rules of thumb help. λ = 50 to 100 units Assumed that entire heat goes to boil-off. With large liquid inventory, only a small part goes to vaporization; rest heats the liquid

Gas: 

Temperature, T2, calculated under fire may exceed base equipment design temperature. RV with required inlet flange rating is usually not available. In RV data sheet, specify design temperature and indicate that T2 is for area calculation only. The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Fire Relief - Blowdown
Without BDV, internal pressure (Hoop’s stress) rises over time;

Stress 

metal’s ability to hold pressure (yield strength) falls with increasing temperature. The vessel will fail when internal stress exceeds ability
Blowdown brings down internal pressure and stress. As long as internal stress is below allowable stress, vessel will not rupture
Judiciously use to extend blowdown time when blowdown rate is higher than design inflow capacity to reduce flare size

Ka Boom

Time, minutes 


API RP 521: Thinner plates (LP service)

heat up faster; higher the temperature, faster it ruptures
If BDV initial pressure (PAHH/PSV) >> operating pressure, zoning can cut peak rate

Flow + Pressure

Fire Relief - Blowdown

RP 521 Figures • Heat up rate • Time to Rupture

Groups 1 +2 + 3

and adjacent equipment within a fire zone (8.6m radius x 7.6m high) are taken at BDV initial pressure; rest at operating pressure

Group 1, then Group 2 and last Group 3 50% reduction Flow

capacity

Pressure = Poe-θt Time 

 Take one source at a time. Loads from this


Staggered blowdown can reduce flare

Flow = Foe-θt

Time  Staggered Blowdown • Each BDV with secured air vessel, sized for 3 valve strokes; PAL; 2 check valves at inlet; no bleeding devices like regulators

From: “Design staggered depressurization sequence for flare systems”, R Dole, S Bhatt and S Sridhar, HP, Dec 2013

INSTALLATION & ISOLATION

Design Tip: Improper installation restricts capacity. Next time you visit a plant, walk around and cringe in horror!!.

Inlet Line

Resonant Chatter in a pilot can self-destruct it


Size on RV rated flow - not on relief load
Inlet loss 3% with pilots, use actual inlet pressure to size RV


Upstream of demister. from vapour space;

below Normal Liquid Level for PRV
10d min from Control Valve
Free draining to source; Bleed/drain @inlet
Nozzle Entrance Loss  

Friction Loss

Entrance Loss 1 Velocity Head

Friction Loss

1 VH if RV is off vessel ½ VH if RV is off outlet pipe


RV mounted upright
Inlet line/ Vessel Nozzle ≥ RV inlet

Bleed Hard T

Entrance Loss ½ Velocity Head

Design Tip: Common Error: Ignoring ∆P in common piping, specially in a group of vessels protected by a single RV.

Inlet 3%
It is difficult to meet 3% From: “Address inlet pressure loss concerns with restricted lift relief devices”, Smith D, Yoram S, HP, Mar 2014

Outlet

Like high inlet loss, high back-pressure can make a RV chatter. As soon as RV closes, flow stops, back-pressure falls, making the RV to open


Atmospheric Discharge

Safe Location

 To Safe Location - for steam, air and N2; not HC  Weep or drain hole in outlet low point


To Closed Drain

¼“ drain hole

 Thermal etc RV  Avoid, if cross contamination is possible  Avoid if water in drain will freeze


To Flare     

Line should free drain to flare header; Top entry No liquid accumulation Backpressure limitation Outlet line size ≥ RV outlet < 70% sonic and ρV² criteria

Free Drain

Inlet/ Outlet Isolation


Inlet & outlet Isolation valves  




If a spare RV is required by Owner,    




install with FB inlet & outlet valves Inlet valve of one RV is LO and the other LC Some Owners require interlocked valve to ensure that one RV is always in service Why? Both outlet valves should be LO.

A (globe) vent valve across RV to depressurize before draining 





Not permitted by ASME Section I; not recommended by ASME Section VIII If required by Owner, then both should be FB locked open (“LO” or “CSO”)

LO

LO 600 mm gap

LO LO LC

A 2nd ball isolation valve located 600mm upstream of vent valve in HP service, if globe valve is stuck on icing ~ JT cooling

A bleed valve u/s of RV inlet block valve ~ in-situ testing If Owner agrees, a single common LO outlet valve for all RVs in a system, say compressor train or Fuel Gas System

LO

Inlet & Outlet Piping Inlet Lead

Size, in Eq L, ft

Fittings No off

2

3

4

6

8

10

12

14

16

18

20

open system

25

25

25

25

25

25

25

25

25

25

25

Closed system

75

75

75

75

75

75

75

75

75

75

75

Eq L, ft each 3

Elbows

4

4

5

8

9

12

14

16

18

20

23

1

Hard T

10

14

19

28

37

47

55

62

72

82

90

1

Reducer

1

2

3

4

5

7

8

9

10

11

13

0

Gate Valve

2

2

3

4

6

7

9

10

11

12

14

Eq L, ft - Open system

48

53

62

81

94

115

130

144

161

178

197

Eq L, ft - Closed system

98

103

112

131

144

165

180

194

211

228

247

2

3

4

6

8

10

12

14

16

18

20

open system

25

25

25

25

25

25

25

25

25

25

25

Closed system

50

50

50

50

50

50

50

50

50

50

50

Outlet Lead

Size, in Eq L, ft

Fittings No off

Eq L, ft each 3

Elbows

4

4

5

8

9

12

14

16

18

20

23

1

Hard T

10

14

19

28

37

47

55

62

72

82

90

1

Reducer

1

2

3

4

5

7

8

9

10

11

13

0

Gate Valve

2

2

3

4

6

7

9

10

11

12

14

Eq L, ft - Open system

48

53

62

81

94

115

130

144

161

178

197

Eq L, ft - Closed system

73

78

87

106

119

140

155

169

186

203

222

Poor Piping

Design Tip: It is a pain; but MUST review RV inlet and outlet piping and pump suction piping in 3D model. Easier to do than “wish I could bury myself in sand” feeling at site


Pipers locate RVs at “convenient” locations – viz

access, ignoring inlet ∆P. Need to check piping 3D model
Horizontal dead legs collect trash / liquid in service
RVs in turbulent zone can chatter and get damaged  Downstream of a Pressure Reduction Station – Fuel Gas?  Downstream of orifice plates/ flow nozzles  Downstream of pulsating compressor / pump discharge. Pilot

RVs may be better because of high seat loading This image cannot currently be display ed.

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Difficult to achieve but

recommended by suppliers This image cannot currently be display ed.

Common Errors

Prod Sep


Ignoring static head

LC

Hydrocyclone

 Between upstream & downstream

equipment as in the case of Prod Sep and d/s vessel or Hydrocyclone  Between PSV and piping at a lower deck

Pump at Upper Deck


Ignoring ∆P in common piping, specially in a group of vessels protected by a single RV
Ignoring mechanical limit on backpressure. Bigger the RV lower is backpressure allowed.

Piping at Lower Deck

Friction Loss

150# RV Outlet Press Limit, psig at 100°F

Design Tip: Important to check mechanical limit on backpressure on flowing and non-flowing RVs. Forgotten by Process Engineers. See RP 526

Size

D–J

M

R

T

Convn

285

285

60

30

Bellow

230

80

60

30

PSV - 001

HIPPS Errors

HP | LP

PAHH PALL Riser SDV

PAHH PALL


HIPPS used

SDV-3 HP Section

SDV-1 SDV-2 HP | LP

 Against PCV / choke failure  Blocked outlet/ stuck pig/ hydrate blockage

Subsea Flowline


In  subsea section to derate flowline/riser,

instead of designing for shut-in pressure  Topside piping

LP Section

Subsea Wells

HP | LP Fortified Section

This image cannot currently be display ed.


Check  Pressure build-up in trapped LP section by

the time PAHHs detect and fully closes SDVs  Provide minimum length of fortified or HP section d/s of HIPPS SDVs - pig/ hydrate blockage, SDV leakage


Need to provide a PSV in LP SDVs leak!

This image cannot currently be display ed.

RV Discharge Velocity & Noise Force


Tail pipes may operate at high velocities
Based on Process input, piping to calculate the reaction forces
RV may need supports to counter momentum and velocity effects of the flowing fluid  Dual outlet PSV can help mitigate – resultant force


Noise levels should be calculated per RP 520  Provide noise insulation or relocate RV away

Flow

Flow Induced Vibration
Flow induced vibrations may result in fatigue

failure.  May require piping supports, increased wall

thickness, etc The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

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Failure to update Relief Studies
Codes and standards; methods/ assumptions

keep changing … additional insights,, Flarenet  RVs in old plants should be revisited every 10 years  If control valves/ equipment have been replaced


In Oil & Gas plants, GORs, liquid profiles change

RELIEF SYSTEM DESIGN

Relief System Design
Analyze Relief Scenarios

Analysis
Calculate Relief Loads and RV Size for each Scenario to get Governing Case Sizing
Summarize results for each RV for each contingency to determine Peak Load to Flare Info Required 1. Heat and Material Balance 2. Process Flow Diagrams (PFDs) 3. Piping and Instrumentation Drawings (P&IDs) 4. Instrument Data (Control Valve, Bypass, RO sizes, etc.) 5. Mechanical and Rotating Equipment Data

Total Load Reduction via Dyn Sim
On total plant failure cases, viz Power, Cooling Water, Air, it is unlikely all the PSVs will pop at the same instant + maintain initial rate  Columns may take time build to relief pressure  Dynamic simulation can help find realistic load  Note: Compressor interstage drum pops in total system

study but does not impact total load

Total Power Failure Fractionator

Conventional, kg/h

Dyn Sim - Individual, kg/h

Dyn Sim – System, kg/h

448,000

259,000

140,000

0

0

160,000

Stripper Feed Drum

45,000

45,000

50,000

Debutanizer

72,000

72,000

5,000

565,000

376,000

355,000

Comp Interstage Drum

Total

From: “Optimize relief loads with dynamic simulation”, CL Xie, ZG Wang and YF Qin, HP, Dec 2013

Total Load Reduction via Dyn Sim The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Dyn Sim total load is 275,000. But design taken as 355,000 kg/h

Total Power Failure Fractionator

Conventional, kg/h

Dyn Sim - Individual, kg/h

Dyn Sim – System, kg/h

448,000

259,000

140,000

0

0

160,000

Stripper Feed Drum

45,000

45,000

50,000

Debutanizer

72,000

72,000

5,000

565,000

376,000

355,000

Comp Interstage Drum

Total

From: “Optimize relief loads with dynamic simulation”, CL Xie, ZG Wang and YF Qin, HP, Dec 2013

Still Accidents Happen Boat hits platform The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

PSV not bolted right The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Boiler started without purging The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Hydrotest done with cold water The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Some of these accidents are not preventable by RVs but by • Common Sense • Good Operating Practice • Good Instrumentation & Controls

CS bend used instead of AS in H2 plant The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Piping Support Not Fire Proofed The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Bend d/s of water Injection or LCV = Erosion-corrosion The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Missing Check at UC The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Internal in Flare KOD The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Pig “Launched” The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

No gas detectors in onshore plant The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

Water Hammer The link ed image cannot be display ed. The file may hav e been mov ed, renamed, or deleted. Verify that the link points to the correct file and location.

THANK YOU