Chatter of Safety
y Valve
November, 2011
Hisao IZUCHI
PLE Technology Center
Chiyoda Advanced Solutions Corporation
Copyright © 2011 Chiyoda Advanced Solutions Corporation. All Rights Reserved.
Contents
1. Purpose
2. Experimental Results / Simulation Results / Stability Analysis
3 Classification
3.
Cl
ifi i off Instability
I
bili
- Dynamic Instability (Acoustic Interaction)
- Static
St ti IInstability
t bilit (P
(Pressure D
Drop D
Development)
l
t)
4. Effect of PRV Inlet Pressure Drop
5 Effect of PRV Outlet to Orifice Area Ratio
5.
6. Conclusion
Reference
(1) H. IZUCHI, “Chatter of Safety Valve”, API Meeting, April 2008
(2) H.
H IZUCHI
IZUCHI, “Stability
Stability Analysis of Safety Valve
Valve”, AIChE Spring Meeting
Meeting, April 2010
(3) V. Dossena, F. Marinoni, B. Paradiso, “Valve Size Influence on the Discharge Capacity
of Spring Loaded Safety Valves”, Paper 722, Valve World Conference 2007
(4) D.
D W
W. Sallet
S ll t and
dD
D. W
W. Somers,
S
“Flow
“Fl
Capacity
C
it and
d Response
R
off Safety
S f t Relief
R li f V
Valves
l
to Saturated Water Flow”, Plant/Operation Progress, 4-4, 1985, 207-216
Copyright © 2011 Chiyoda Advanced Solutions Corporation. All Rights Reserved.
1
Purpose
Safety valve chatter would result in
(1) Mechanical failure of the valve and related piping system
(2) Reliving
R li i flow
fl
rate reduction
d i caused
db
by iinsufficient
ffi i
valve
l
opening due to chatter
Chiyoda had executed to study safety valve chatter for the
following purposes:
(1) Investigate mechanism of chatter
(2) How to prevent chatter
Study Program
(1) Chatter test at a manufacturer experimental facility with air
(2) Dynamic simulation (taking valve motion and pressure wave
propagation throughout inlet/outlet piping into account)
(3) Stability
St bilit analysis
l i (th
(theoretical
ti l iinvestigation)
ti ti )
Copyright © 2011 Chiyoda Advanced Solutions Corporation. All Rights Reserved.
2
Experimental Facility
Displacement
Meter
No Inlet Pipe
Safety
Valve
Inlet Piping (5m)
Inlet Pipe Length is 1m
Vessel
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3
Experimental Results / Effect of Inlet Pipe Length
Chatter occurs
Inlet length < 5m
1E2
1"/0m
-
1.5F2
1-1/2"/0m
74-92Hz
1E2
No Chatter
Inlet Length >= 10m
I l t Pipe
Inlet
Pi
Size
Si / IInlet
l t Pipe
Pi
L
Length
th
Chatter Frequency
1"/1m
1"/3m
1"/5m
1"/10m
1"/15m
55-68Hz
55
68H
71 111H
71-111Hz
79 104H
79-104Hz
1-1/2"/1m
42-59Hz
1-1/2"/3m
-
1"/20m
-
1-1/2"/5m 1-1/2"/10m
-
1-1/2"/1m
1-1/2"/5m
-, 43-52Hz
-
-
value
l
A t l length
Actual
l
th iis fi
figure
iin ttable
bl + 1
1.2m
2 off safety
f t valve
l stand
t d
Chatter occurs
Both cases were observed with chatter and without chatter
Natural frequency of valve disc and spring is 75 Hz
Longer line length means larger pressure drop in piping.
Therefore, chatter could not be caused by excessive pressure drop in pipe
because the safety valve system stabilized as inlet line length increased
increased.
Chatter is caused by acoustic interaction between safety valve and inlet pipe.
Copyright © 2011 Chiyoda Advanced Solutions Corporation. All Rights Reserved.
4
Dynamic Simulation Model
Safety Valve
Equation
E
ti
off Motion
M ti for
f Valve
V l Disc
Di
Orifice Flow Equation at Nozzle
Flow Equation at Outlet
Mass Conservation in Valve Body
IInlet
l t / Outlet
O tl t Piping
Pi i
（divided into several segments)
Equation of Mass Conservation
Equation of Motion for Gas Flow
Equation for Energy Conservation
Equation of State for Gas
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5
Simulation Results
Experiment
Simulation
4.0
3.5
3.0
2.5
2.0
1.5
1.0
05
0.5
0.0
0.0
0.1
0.2
0.3
0.4
0.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
05
0.5
0.0
71msec = Duration pressure wave
propagates from safety valve to vessel
and return back to safety valve
Oscillation is attenuated
0.0
0.1
0.2
0.3
0.4
0.5
Time (s)
Time ((s))
Interaction between valve disc motion and
pressure wave propagation (acoustic
phenomena) could cause instability
instability.
1E2, Inlet : 0m, 1” Reducer at Outlet
Lift
ft (mm)
1E2, Inlet : 1”/10m, No Reducer at Outlet
Lift (mm
L
m)
Lift (mm
L
m)
1E2 Inlet
1E2,
I l t : 1”/1m,
1”/1 N
No R
Reducer
d
att O
Outlet
tl t
4.0
35
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Simulation effectively indentifies safety
valve instability caused by both of inlet
piping
i i andd small
ll outlet
l to orifice
ifi area ratio.
i
0.0
0.1
0.2
0.3
Time (s)
0.4
0.5
Stability theory supports the safety valve
instability caused by inlet pipe acoustics.
acoustics
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6
Classification of Instability
( ) Dynamic
(a)
D
i IInstability
t bilit (Acoustic
(A
ti Interaction)
I t
ti )
Diff. Press.
Valve Lift
Opposite phase between lift and differential
pressure though valve disc
2.5
2.0
1.5
10
1.0
0.5
0 05
0.05
0 10
0.10
Time (s)
0 15
0.15
0.0
0 20
0.20
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0 00
0.00
Stable
0 10
0.10
0 20
0.20
0 30
0.30
2.1
2.0
2.0
1.9
19
1.9
1.8
1.8
1.7
0 40
0.40
Diff. Press. (MP
Pa)
Unstable
Lift (mm)
4.0
3.5
3.0
2.5
2.0
15
1.5
1.0
0.5
0.0
0 00
0.00
1E2, Inlet : 1”/10m, No Reducer at
Outlet(experimental results)
Diff. Press. (MP
Pa)
Lift (mm)
1E2, Inlet : 1”/1m, No Reducer at Outlet
(experimental results)
Time (s)
- Caused by interaction between valve motion and pressure wave propagation at
inlet pipe
- Relatively high frequency (determined by combination effect of acoustic natural
frequency and valve natural frequency)
- No
N relation
l ti to
t inlet
i l t pressure drop
d
- Stable for longer length of safety valve inlet line due to attenuation effect
Copyright © 2011 Chiyoda Advanced Solutions Corporation. All Rights Reserved.
7
Classification of Instability
Diff. Press
Diff
Press.
Valve Lift
3.0
2.5
2.0
1.5
1.0
0.5
00
0.0
0.0
0.5
1.0
1.5
2.0
Time (s)
E cessi e Inlet Press
Excessive
Pressure
re Drop
4.0
3.5
3.0
2.5
20
2.0
1.5
1.0
0.5
00
0.0
0.00
2.5
2.0
1.5
1.0
0.5
0.05
0.10
0.15
00.00
0.20
Diff. Press. (MPa))
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
00
0.0
1E2, Inlet : 1”/0m, 1” Reducer at Outlet
(experimental results)
Lift (m
mm)
Lift (m
mm)
1E2, Inlet : 1”/100m, No Reducer at Outlet
(simulation results)
Diff. Presss. (MPa))
(b) Static Instability (Pressure Drop Effect)
Time (s)
Excessive Outlet Pressure Drop
(Smaller outlet to orifice area ratio)
After safety valve opens, available differential pressure
decreases and stable opening cannot be kept
- Caused by large pressure drop of inlet pipe / outlet pipe (safety valve outlet)
- Relatively low frequency (basically determined by duration time of pressure
accumulation and valve lift/blowdown characteristics)
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8
Effect of PRV Inlet Pressure Drop
1
1E2,
Set 20barg,
0
Blowdown = 10%
50m
100m
(simulation)
(simulation)
4.8%
9.2%
10.2%
Stable
Stable
Unstable
-
Static /
Excessive
Press. Drop
Inlet Length
1m
10m
20m
Inlet
Press. Drop*
2.6%
3.8%
Instability
Unstable
Stable
Cause of
Instability
Dynamic /
Acoustic
(Press. Wave)
-
-
* : average figure at actual PRV lift
- PRV static instability due to excessive pressure drop occurs if inlet pressure drop
exceeds the blowdown of PRV. 3％ rule for inlet pressure drop would be too much
conservative.
- There is another mechanism of PRV instability
instability, dynamic instability caused by interaction
between valve motion and pressure wave propagation at inlet pipe (acoustic effect).
This dynamic instability should be considered separately from inlet pressure drop.
- If inlet
i l t pressure d
drop would
ld b
be llarger th
than 3%
3%, flflow capacity
it should
h ld b
be checked
h k d ttaking
ki
both effects of pressure drop and PRV lift reduce into account.
Copyright © 2011 Chiyoda Advanced Solutions Corporation. All Rights Reserved.
9
Effect of PRV Outlet to Orifice Area Ratio
- Chiyoda study shows PRV instability would occur if outlet to orifice
area ratio would be lower than 6.0.
Pressure att
P
PRV outlet
Q [[ft3/s]
- Sallet and Somers (4) also show that the
flow capacity of PRV would decrease if
the outlet to orifice area ratio is lower
th 6.0.
than
6 0 (Lower
(L
ffrequency di
disc
vibrations, which suggests static
instability, were observed when the PRV
outlet to orifice area ratio was lower
lower.))
Q [m3/s]
- This instability is caused by pressure accumulation in the PRV body.
Thi pressure accumulation
This
l ti iin th
the PRV b
body
d can b
be confirmed
fi
db
by
pressure drop calculation at the PRV outlet as shown in Fig.1.
- Dossena (3) shows that flow reduction
Fig 1
Fig.1
would occur for 8T10 PRV due to “high
backpressure on the valve disc” based
on CFD analysis. Relatively small outlet
t orifice
to
ifi area ratio,
ti 3.04,
3 04 would
ld result
lt iin
lack of valve lift force and insufficient
Lower frequency (5Hz)
valve opening for 8T10 PRV.
vibration were observed
PRV Outlet Area / Orifice Area
Flow Rate vs. Area Ratio (Sallet and Somers (4))
(F Orifice, Saturate and Subcooled water,stitic
0.69MPa, 10degC)
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10
Experimental Results / Effect of Outlet Area Ratio
SV Size
1E2
(1) Orifice
Area
1 82 cm2
1.82
Chatter
1.5F2
2.43 cm2
Chatter
4P6
47 80 cm2
47.80
Outlet
Size
2"
2
1-1/2"
1-1/4"
1"
2"
1-1/2"
1-1/4"
1"
6"
6
(2) Outlet
Area
20 3 cm2
20.3
13.6 cm2
10.0 cm2
6 0 cm2
6.0
2
20.3 cm2
13.6 cm2
10.0 cm2
6.0 cm2
182 4 cm2
182.4
Ratio
(2) / (1)
11 2
11.2
7.5
5.5
33
3.3
8.3
5.6
4.1
2.5
38
3.8
Almost
Equivalent
(similarity
law)
For larger size safety valves such as 4P6, where there is a
relatively small outlet to orifice area ratio, would result in chatter.
Outlet to Orifice Area Ratio < 6.0 Î There is possibility of chatter
Chatter is caused by pressure accumulation in the valve body
body.
Safety valve size including outlet area is specified in API526.
Copyright © 2011 Chiyoda Advanced Solutions Corporation. All Rights Reserved.
11
Conclusion
- Instability of PRV can be classified into dynamic instability and static instability.
- Dynamic
y
instabilityy is caused by
y interaction between valve motion and pressure
p
wave propagation at inlet pipe. Longer inlet pipe length results in stable
condition due to attenuation effect.
-E
Excessive
i iinlet
l t liline pressure d
drop causes static
t ti iinstability
t bilit if iinlet
l t pressure d
drop
exceeds the PRV blowdown. 3％ rule for inlet pressure drop would be too much
conservative to prevent PRV instability.
- Outlet to orifice area ratio lower than 6.0 would result in static instability and
insufficient flow through PRV.
Copyright © 2011 Chiyoda Advanced Solutions Corporation. All Rights Reserved.
12
Thank You
END
Chiyoda Advanced Solutions Corporation
Technowave 100 Bldg.,1-25 Shin-Urashima-Cho 1-chome,
Kanagawa-ku, Yokohama 221-0031, Japan
Hisao IZUCHI
hisao.izuchi@chas.chiyoda.co.jp
@
y
jp
Tel: +81-45-441-1277
Copyright © 2011 Chiyoda Advanced Solutions Corporation. All Rights Reserved.