anderson greenwood crosby- pressure safety valves- considerations on their use & sizing _juli 2009

8/20/2019 Anderson Greenwood Crosby- Pressure Safety Valves- Considerations on Their Use & Sizing _Juli 2009

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Copyright © 2009 Tyco Flow Control. All rights reserved

Pressure Safety ValvesPressure Safety ValvesSome considerations on their use & sizingSome considerations on their use & sizing

the Dynaflow Research Group lectures

9 July ‘09

Jean-Paul Boyer


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Copyright © 2009 Tyco Flow Control. All rights reserved 2

Pressure Safety Valves

Useless valves...

No need for process Never used (hope!)

High up, forgotten

But big impact Can limit production

Can have high life-

cycle cost Can cause havoc


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Pressure Safety Valves

Sizing

Reaction Force

Noise

Back-Pressure


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Flow Sizing

The All Important factor, KA

flow coefficient ‘K’ times flow area ‘A’

ZT

M

KP AKCW 1b1=

Gases :

( )21VW PPKK AKW −ρ=

Liquids :

( need some factors depending on units used)

KAKA

KAKA


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The KA Factor

Area or Flow Coefficient alone not enough

Capacity is what matters

Manufacturer cannot claim capacity higher

than the certified capacity

But he can claim a lower one (so called ‘safe’) It all relates to KxA

Other factors Kb, Kw, Kv… will depend on service

conditions KA defines the particular valve, fixes its capacity

Certification (e.g. ASME with National Board)


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KA Factor

Long ago…

Certified capacity = actual capacity API Std 526 reflected actual valve values

In 1962, ASME VIII revised ‘K’ to include

10% safety factor: certified K = 0.9 x Kactual National Board allowed deviations between

certified values and published values as long as:

Published capacities ≤ Certified CapacitiesOr

Published KA ≤ Certified KA


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KA Factor

During certification

A is measured Smallest section in

the flow path (throat)

Actual flow is compared

to theoretical flowthrough perfect nozzle

(K=1)

Wactual/Wtheor = Kactual

K=0.9 x Kactual


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KA Factor

Since 1962, most manufacturers have

Overstated K Understated A

Example on ‘Q’ orifice on gas

API Std 526: A = 11.05 in2 K=0.975 KA=10.77

0%10.760.87812.26‘Y’

+2%10.990.85512.85‘X’

+6.5%11.470.62718.29 AG POSV

+0.2%10.790.86512.47Crosby JOS

Diff KACertified KCertified A


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Safety Valve Sizing

Preliminary sizing

API RP 520 part 1, clause 5.2.1:«PRV’s may be initially sized… [using] effective coef-

ficients of discharge and effective areas which are

independent of any specific valve design. In this way,the designer can determine a preliminary PRV size.»

Effective areas API Std 526, D through T

Effective coefficients API RP 520

Gases, K=0.975

Liquids, K=0.650

2-phase, K=0.850


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Safety Valve Sizing

Final, using manufacturer’s data

API RP 520 part 1, clause 5.2.5:«When a specific valve design is selected… the rated

capacity of that valve can be determined using the

actual orifice area, the rated coefficient ofdischarge… The actual orifice area and the rated

coefficient of discharge shall always be used to verify

the actual capacity of the PRV.»

Actual areas certified (e.g. ASME, NB-18) Rated coefficients certified


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National Board NB-18 ‘Red Book’

www.nationalboard.orgwww.nationalboard.org


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National Board NB-18 ‘Red Book’

NB-18 viewed on line or downloaded

All data on each and every valve (& RD) certified per ASME codes (I, III & VIII)

K=Kd x 0.90

Actual Actual

Areas Areas


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Sizing Safety Valves

Sizing ‘per API’ does not imply using API

standard orifices API RP 520 (contains sizing) doe not list the

orifices (listed in API Std 526)

API RP 520 recommends to usemanufacturer’s data for final sizing

ASME (National Board) does not bother

what area or coefficient is used as long as:

K x A ≤ Actual value - 10%


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Reaction Force

At relief, the jet out of

the valve creates anopposite force, a thrust

Fd=c2.W

Added to static force

from pressure at outlet

flange

Fs=(P2-Patm).A2

FlowForce

1

2


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Reaction Force

W, flow, kg/hr

T1

, inlet temp, ºK

k, ratio specif. heats

M, molar mass

p2, outlet pressure,

barg

A2, outlet section,

cm2

G, relative density

v1, specif volume,

m3/kg

P1, relieving

pressure, barA

P2, outlet pressure,

barA1= Inlet, stagnation conditions 2= Outlet conditions


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Reaction Force

Capacity W to consider:

Pop action POSV and Spring Loaded SVSafety valve full capacity (actual = rated/0.9)

Instantaneous flow rate on opening is the

maximum capable flow of the valve

Modulating action POSV

Required capacity

Rupture Discs and Buckling Pin Valves

Required capacity

Same for noise or back-pressure calculations

(Ref ISO23251/API521, 7.2.1 table12)


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Discharge Bracing


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Tailpipe to Atmosphere

FlowFlow

FORCEFORCE

Bending moment on

riser betweenequipment and safety

valve


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Tailpipe to Atmosphere

Minimises bending

moment at base of inletriser

FlowFlow

FORCEFORCE


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Do and Don’t?


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Reaction Force

Dual Outlet valves

no stress on the valve


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Braced Dual Outlet POSV


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Double exhaust


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Discharge to Piped System


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Pipe Strains

Discharge piping should be

independently supported free from misalignment

taking care of expansion/contraction, thermalloading

Pipe strains can cause misalignment of valve internals:

leakage

seizure stress on valve body

cracks in body casting


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Noise through Safety Valves

Aim of SV is to drop pressure as much as

possible. ΔP very high, high amount ofdissipated energy, a lot of it into noise

SV are noisy

SV must have no pressure recovery to dotheir job

Due to turbulences inside the body bowl,

gas velocity at outlet is most often super-sonic


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Pressure Drop in Safety Valves

Pressure continues to drop after nozzle in the body

bowl, with shock-waves

VCVC

InletInlet

PipingPipingNozzleNozzle BodyBody

BowlBowl OutletOutlet

PipingPiping

P1

PC


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Basic formula for stack

tip (ISO23251/API521) 30 m of tip

Open outlet

( ) ( )

2

2

1

fig30 cWLog10LL +=

M

Tk2.91c =

( ) ( ) 30d

30d Log20LL −=

20

60

50

40

30

1.5 2 3 4 5 6 7 8 9

P2

/P1

L, dB

Acoustic Efficiency of Choked Jet


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Control valves standards (IEC 534-8-3, ISA

75-07, VDMA 24-422…) Accuracy for 0.3 to 0.8 Ma outlet

Safety valves: outlet Ma >> 1

Safety valves normally designed towithstand stress

Potential problems for outlet piping

Expander, reduce speed Avoid accident close to the valve outlet (new

choke points)


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A Dual Answer

Exhaust to

atmosphere: angle-cuttail pipe

Reduced noise

Reduced reaction force

Aexit⇒ cexit

⇒ Fd

and noise…


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InletInlet

(back pressure)(back pressure)

THE PRESSURE AT THE OUTLET

OF A PRESSURE RELIEF DEVICE.

Back Pressure


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Discharge

Safety Valve

is Open andFlowing

In

Built-Up Back Pressure

BPBUBPBU Built-Up BP is

caused by

pressure dropin discharge

piping


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Conventional

Spring Valve

Lift vs Built-Up Back Pressure

60%

70%

80%

90%

100%

0% 20% 40% 60% 80%

Built-Up BP in % of Set Pressure

% o

f F u l l R

a t e d L i f t


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Conventional

Spring Valve

Balanced

Spring Valve


60%

70%

80%

90%

100%

0% 20% 40% 60% 80%

Correction factor (Kb, Kw)

Reduced Capacity


% o

f F u l l R

a t e d L i f t


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Conventional

Spring Valve

Balanced

Spring Valve

Pilot Operated

Safety Valve


60%

70%

80%

90%

100%

0% 20% 40% 60% 80%


% o

f F u l l R

a t e d L i f t


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ProtectedProtectedSystemSystem

PRV(Closed)

DischargeHeader System

To Flare,RecoverySystem, or

AtmospherePossible Pressure Source

Possible Pressure Source

Possible Pressure Source

Constant Purge?

BPS

Superimposed Back Pressure


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Constant Back-Pressure

Constant Back-

Pressure

valve discharge into a

system at a constant

pressure

pump suction, steam

tank, ...

Always superimposed


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Variable Superimposed BP

Variable Super-

Imposed

valve discharge into a

system at a variable

pressure which exists

when valve is closed

flare system, ...

Opened &Flowing

Closed


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Superimposed Backpressure

Exists even when the considered valve is

closed It can affect the set pressure of this valve

This valve may open at a pressure higher

than the design pressure of the equipment


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Conventional Valve

LL Downwards spring

force (constant)Fd = K / L

A A

PP

Upwards fluid force

(variable)

Fu = P x A

Set : Fd = Fu


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Conventional Valve

LL Superimposed BP

adds itself to thespring force:

Fd = K / L + BPxA

A A

PP

Actual Set:Spring set + BP

Set : Fd = Fu(Fu = P x A)


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Superimposed Variable BP

Effects…

Desired set pressure = 10 bargSuperimposed variable BP = 0.5 to 2.0 barg

Cold Set Pressure = ???

The valve will open between 10.5 to 12.0 barg Safety valve shall not open at a pressure higher

than the MAP or PS (≈ design pressure)

If MAP < Cold Set + BP…

Conventional valve cannot be used, even if

superimposed variable BP < 10% set


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Balanced Bellows Safety Valve

The Balanced Bellows

isolates top side of disc for

Back-P

isolates spring and guide

from outlet environment

Typically, this type ofvalve can be used:

Any backpressure up to

≈50% of set

absolute value of BP

Reduced capacity

Check with

manufacturer


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Pilot Operated Safety Valves

Main valve piston

inherently balanced

against backpressure

Theoretically no limit on

backpressure


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Back-Flow Preventer

If BP > PS

back-flow may occur

Flare system:

start-up of

installation

valves with different

sets

vacuum in processBP

PS


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Back-Flow Preventer

If BP > PS

back-flow may occur

Flare system:

start-up of

installation

valves with different

sets

vacuum in processBP

PS


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Backflow

Preventer

PP11

P2P2

PP22 > P> P11

Back-Flow Preventer (POSV)


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anderson greenwood crosby- pressure safety valves- considerations on their use & sizing _juli 2009

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