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Pressure Safety ValvesPressure Safety ValvesSome considerations on their use & sizingSome considerations on their use & sizing
the Dynaflow Research Group lectures9 July ‘09
Jean-Paul Boyer
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Pressure Safety Valves
Useless valves...No need for processNever used (hope!)High up, forgotten
But big impactCan limit productionCan have high life-cycle costCan 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, KAflow coefficient ‘K’ times flow area ‘A’
ZTMKPAKCW1
b1=Gases :
( )21VW PPKKAKW −ρ=
Liquids :
( need some factors depending on units used)
KAKA
KAKA
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The KA Factor
Area or Flow Coefficient alone not enoughCapacity is what matters
Manufacturer cannot claim capacity higher than the certified capacityBut he can claim a lower one (so called ‘safe’)
It all relates to KxAOther factors Kb, Kw, Kv… will depend on service conditionsKA 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 capacityAPI Std 526 reflected actual valve values
In 1962, ASME VIII revised ‘K’ to include10% safety factor: certified K = 0.9 x KactualNational 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 certificationA is measured
Smallest section in the flow path (throat)
Actual flow is compared to theoretical flow through perfect nozzle (K=1)
Wactual/Wtheor= Kactual
K=0.9 x Kactual
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KA Factor
Since 1962, most manufacturers haveOverstated KUnderstated A
Example on ‘Q’ orifice on gasAPI 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%11.470.62718.29AG POSV
+0.2%10.790.86512.47Crosby JOS
DiffKACertified KCertified A
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Safety Valve Sizing
Preliminary sizingAPI 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 TEffective coefficients API RP 520
Gases, K=0.975Liquids, K=0.6502-phase, K=0.850
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Safety Valve Sizing
Final, using manufacturer’s dataAPI 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 of discharge… 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 downloadedAll data on each and every valve (& RD) certified per ASME codes (I, III & VIII)
K=Kd x 0.90
ActualActualAreasAreas
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Sizing Safety Valves
Sizing ‘per API’ does not imply using API standard orificesAPI RP 520 (contains sizing) doe not list the orifices (listed in API Std 526)API RP 520 recommends to use manufacturer’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 an opposite 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 SV
Safety valve full capacity (actual = rated/0.9)Instantaneous flow rate on opening is the maximum capable flow of the valve
Modulating action POSVRequired capacity
Rupture Discs and Buckling Pin ValvesRequired 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 between equipment and safety valve
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Tailpipe to Atmosphere
Minimises bending moment at base of inlet riser
FlowFlow
FORCEFORCE
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Do and Don’t?
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Reaction Force
Dual Outlet valvesno 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 beindependently supportedfree from misalignmenttaking care of expansion/contraction, thermal loading
Pipe strains can causemisalignment of valve internals:
leakageseizure
stress on valve bodycracks 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 of dissipated energy, a lot of it into noise
SV are noisySV must have no pressure recovery to do their jobDue 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
InletInletPipingPiping NozzleNozzle BodyBody
BowlBowl OutletOutletPipingPiping
P1
PC
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Noise through Safety Valves
Basic formula for stack tip (ISO23251/API521)
30 m of tipOpen outlet
( ) ( ) ( )221
fig30 cWLog10LL +=
MTk2.91c =
( ) ( ) ( )30d
30d Log20LL −=
20
60
50
40
30
1.5 2 3 4 5 6 7 8 9
P2/P1
L, dBAcoustic Efficiency of Choked Jet
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Noise through Safety Valves
Control valves standards (IEC 534-8-3, ISA 75-07, VDMA 24-422…)
Accuracy for 0.3 to 0.8 Ma outletSafety valves: outlet Ma >> 1
Safety valves normally designed to withstand stress
Potential problems for outlet pipingExpander, reduce speedAvoid accident close to the valve outlet (new choke points)
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A Dual Answer
Exhaust to atmosphere: angle-cut tail pipe
Reduced noiseReduced reaction forceAexit ⇒ 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 Valveis Open and
Flowing
In
Built-Up Back Pressure
BPBUBPBUBuilt-Up BP iscaused bypressure dropin dischargepiping
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ConventionalSpring 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 Ful
l Rat
ed L
ift
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ConventionalSpring Valve
BalancedSpring Valve
Lift vs Built-Up Back Pressure
60%
70%
80%
90%
100%
0% 20% 40% 60% 80%
Correction factor (Kb, Kw)Reduced Capacity
Built-Up BP in % of Set Pressure
% o
f Ful
l Rat
ed L
ift
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ConventionalSpring Valve
BalancedSpring Valve
Pilot OperatedSafety 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 Ful
l Rat
ed L
ift
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Protected Protected SystemSystem
PRV (Closed)
DischargeHeader System
To Flare, Recovery System, 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 pressurepump 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 closedflare 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 valveThis valve may open at a pressure higher than the design pressure of the equipment
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Conventional Valve
LLDownwards springforce (constant)
Fd = K / L
AA
PP
Upwards fluid force (variable)
Fu = P x A
Set : Fd = Fu
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Conventional Valve
LLSuperimposed BP adds itself to the spring force:Fd = K / L + BPxA
AA
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 barg
Superimposed variable BP = 0.5 to 2.0 bargCold 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 Bellowsisolates top side of disc for Back-Pisolates spring and guide from outlet environment
Typically, this type of valve can be used:
Any backpressure up to ≈50% of set
absolute value of BPReduced 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 > PSback-flow may occur
Flare system:start-up of installationvalves with different sets
vacuum in processBP
PS
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Back-Flow Preventer
If BP > PSback-flow may occur
Flare system:start-up of installationvalves with different sets
vacuum in processBP
PS
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BackflowPreventer
PP11
P2P2
PP2 2 > P> P11
Back-Flow Preventer (POSV)
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