anderson greenwood crosby- pressure safety valves- considerations on their use & sizing _juli 2009
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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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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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Noise through Safety Valves
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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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 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
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 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
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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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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