Transcript Liquid PRV Chatter Recommendations Rev2

LIQUID PRV INSTABILITY
TEAM
RECOMMENDATIONS
API-520 Subcommittee
November 14, 2011
Brad Otis
Principal HSE Consultant
Shell Global Solutions (US) Inc.
©2011 Shell Global Solutions (US) Inc. All rights reserved.
10/25/2011
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Objective
1. Share team’s recommendations for API-520 Parts I &
II
2. Explain technical basis and potential issues
3. Layout a plan that provides effective and practical
guidance to avoid liquid PRV chatter
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Agenda
• Liquid PRV Instability Team
• Findings 1-4
• Acoustics
• The Acoustic Effect
• Proposed Acoustic Criteria
o PRV Inlet Line Length
o Maximum Acoustic Length
o Speed of Sound
o PRV Opening Time
• Proposed Design Criteria
• Significance
• Homework
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Liquid PRV Instability Team
API Team Members
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Liquid PRV Instability Team
Mission
o Get clarity on the relative susceptibility of PRV
instability with liquid reliefs
o Give user, as a minimum, qualitative guidance for
designing/installing PRVs intended for liquid relief
service.
o Give user guidance to avoid liquid relief related PRV
stability issues
The team recommendations need to be vetted with the
API-520 Subcommittee
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Finding #1
Finding #1: Vapor certified PRVs relieving liquid are
significantly more susceptible to PRV chatter than liquid
certified PRVs relieving liquid
o See “PRV Chatter Incidents” presentation by Brad
Otis Spring 2011 API-520 meeting
o This has been observed on PRV test stands

Valves of up to 2” size have chattered on the test stand (larger
valves are more difficult to test).
o Liquid chatter damaged PRVs have been received
from the field
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Finding #1
o Vapor certified PRVs are not very modulating when
relieving liquid
PRV performance when
relieving vapor
Modulating
PRV performance when
relieving liquid
Modulating?
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Finding #1
o Figure 38 shows typical valve performance after 10%
overpressure. What’s happening between 0-10%?
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Finding #1
o Figure 38 shows typical valve performance after 10%
overpressure. What’s happening between 0-10%?
?
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Finding #1
o One manufacturer describes performance as:
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Finding #1
o With liquid, these valves can momentarily pop wide
open
o
o
Recommend NOT using required relief for inlet pressure drop
calculations when relieving liquid
Consider using maximum liquid flow for inlet pressure drop
calculation (P=1.1*set and Kp =1.0 instead of 0.6)
o Valves are very sensitive to oversizing
o
o
Higher instability if relief load is well below PRV capacity
Can be stable with restricted lift
o Modern vapor certified valves relieving liquid do not
need 25% overpressure to achieve full lift.
o
Use of Figure 38 with modern valves will increase valve over-sizing
and their propensity to chatter
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Finding #2
Finding #2: There may be opportunities with API 520
Parts 1 and 2 to clearly describe this issue and provide
clearer guidance
o Neither have a section on PRV selection
considerations
o Liquid relief chatter is described in API-520 Part I*
but this comes across as a description of history, not
valve selection guidance.
“Where liquid PRVs were required to operate within the
accumulation limit of 10% a conservative factor was applied to
the valve capacity when sizing the valves. Consequently,
many installations were oversized and instability often
resulted.”
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*4.2.1.2.6 (section describes how liquid PRVs function)
Finding #2
o Liquid relief chatter is described in API-520 Part I in
Figure 38, but only in a note:
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Finding #3
Finding #3: Adjustments and add-ons not likely to
improve PRV stability
o Blowdown adjustments may not reduce liquid PRV
chatter
o
o
Conflicting guidance from different manufacturers on raising or
lowering blowdown rings
Some operating companies have not observed any positive effect
o Dampeners not likely to help with most PRVs
o
o
o
One PRV mfg offers these for smaller valves
Unclear if this would be effective for larger valves
Dampeners currently are not permitted by ASME Section VIII Code
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Finding #4
Finding #4 PRVs have an acoustic interaction with their
inlet line
o PRVs are dynamic devices
•
Mass and spring rates
•
Response time (time to open)
•
Sensitive to inlet pressure
o PRV inlet pressure is dynamic
•
Pressure at the PRV depends how quickly the PRV opens
•
How quickly the pressure signal can be transmitted down the
PRV inlet line
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Acoustics
The acoustic interaction
o PRV meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV meeting acoustic length criteria
o PRV remains open
o Relief flow is now stable
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o PRV NOT meeting acoustic length criteria
o PRV was closed when pressure wave returned
o PRV inlet repressurized
o PRV/inlet line acoustic cycle then repeats
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
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Acoustics
The acoustic interaction
o Another PRV NOT meeting acoustic length criteria
o PRV was closed well before the pressure wave
returned
o PRV inlet repressurized
o PRV/inlet line acoustic cycle then repeats
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The Acoustic Effect
What is the magnitude of the rarefaction wave?
o Could be estimated using the Joukowski equation
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The Acoustic Effect
o Joukowski equation is used for assessing water
hammer and pressure surge
o Premise that sudden surge pressure reduction with
initial PRV lift is equal but opposite to pressure surge
when valve closes
o This surge pressure reduction is a “recoverable” loss
o Surge pressure reduction can be well below the PRV
reseat pressure for liquids
o
This is OK provided that the pressure wave comes back in time!
o See worksheet for magnitude of pressure surge
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The Acoustic Effect
The surge pressure with valve closure assumes an initial
velocity. Can the inlet pipe actually achieve relief flow
velocity during the PRV lift?
o Yes, but it depends on PRV orifice size, PRV opening
time, and line volume
o Can be calculated using basic principles
o Team did not explore what it would mean if premised
relief velocities cannot be achieved during valve lift.
o See worksheet for examples
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The Acoustic Effect
What is the magnitude of the acoustic effect with other media?
Conclude that this is not significant for vapor, gases, two-phase, or
fluids that behave like gas
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Proposed Acoustic Criteria
The applicable physical length of the PRV inlet should
not exceed the maximum acoustic length
This criteria would be applicable to:
o Spring loaded vapor certified valves and pop-action
pilot operated PRVs (with local sensing) relieving
liquid or a super-critical fluids that behave similar to
liquid (the change in density as a function of pressure
is low).
Note, two-phase relief shall be considered liquid in this application if
the fluid in the PRV inlet line remains liquid (any flashing only
occurs downstream of PRV inlet)
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Proposed Acoustic Criteria
This criteria would NOT be applicable to:
o Any pilot operated PRV with a remote sense located
on the protected equipment
o Any modulating pilot operated relief valve
o Vapor certified, liquid certified, or dual certified spring
loaded PRVs flowing two-phase, vapor, or supercritical fluids that behave similar to vapor
o
Although installations could exceed the calculated acoustic line
lengths limit, the effect on the PRV will be low. The acoustic
pressure wave magnitude is relatively small and the presence of a
highly compressible fluid in the PRV’s huddling chamber will tend to
keep the PRV open.
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Proposed Acoustic Criteria
This criteria would NOT be applicable to:
o Liquid certified (or dual certified) PRVs relieving
liquid
o
These have longer opening times and exhibit very good modulating
behavior during initial valve lift
o Thermal relief valves
o
Since the relief load is very small and transient
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PRV Inlet Line Length
PRV inlet line length is physical length (not hydraulic
length)
o Measured
from
PRV inletfrom
flange
toinlet
the protected
Alternatively,
measured
PRV
system
flange to first acoustic reflection point.
o An acoustic reflection point in the
piping must be abrupt and have
sufficient capacitance to absorb the
rarefaction wave.*
o
An elbow is not an acoustic reflection point.
o
A series of reducers is not abrupt enough to
cause a reflection.
*Fundamental of Acoustics, Kinsler et all, 4th edition.
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PRV Inlet Line Length
o
Example reflection point:

An abrupt cross sectional area change where the
upstream piping area is at least 10 times larger than the
downstream piping diameter, AND

Length of the upstream piping is more than 20 times the
diameter of the downstream piping

e.g. 4” diameter pipe connected to a 12” diameter pipe
that is greater than 80 inches long

Calculations show that this results in about 98% of the
rarefaction wave being absorbed.
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Maximum Acoustic Length
The maximum acoustic length is calculated as:
Example: a PRV with 20 ms opening time in a process
that has a speed of sound of 3000 ft/sec (914 m/s) has a
maximum acoustic length of 30 feet (9.1 m)
.
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Speed of Sound
The speed of sound is the square root of the partial
derivative of pressure with respect to density at constant
entropy
Alternatively this may be calculated as:
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Speed of Sound
Speed of sound in fluid is affected by pipe rigidity
o Slower speed of sound REDUCES the maximum
acoustic length!
o Negligible effect (4%) for typical steel process piping
o Consider effect if using less rigid piping materials
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Speed of Sound
Issues
o Speed of sound varies with temperature
o
For LPG speed of sound reduces 7% for every 10C increase
o Values for speed of sound vary depending on data
source… see worksheet
Note: temperature basis is not known for values in this worksheet
o If a simulator is used to estimate the speed of sound,
the method for calculation should be validated
against measured speed of sound values for
common fluids
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PRV Opening Time
Acoustic length is based on PRV opening time, not full
PRV open/close cycle
o Consistent with other approaches
o
EPRI research
o
Exxon/Mobil research
o
Consolidated Dresser’s PI-10 work process
o No credit for valve closing time
o
Concern that the incoming pressure wave might not have enough
force to counteract the valve closure momentum
o
Detailed analysis might prove otherwise on specific cases
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PRV Opening Time
Issues
o Wide variability in published PRV opening times
o Function of
o
Valve type
o
Valve size
o
Set pressure
o
Overpressure speed
o
Fluid
o Users could consult with PRV manufacturer’s to get
representative values
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Proposed Design Criteria
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Significance
Potential Impact?
o See Worksheet for acoustic length calculations
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Summary
1. Vapor certified valves relieving liquid should meet
acoustic line length criteria and inlet loss should be
based on PRV capacity at full lift
2. To apply acoustic line length user needs:
o
Physical length of the PRV inlet line
o
Speed of sound for the liquid at relieving
temperature
o
Expected PRV opening time
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Homework
Recommend subcommittee evaluate the proposal in
detail before Spring 2012 API meeting
1. Any liquid PRV relief experience that suggests
acoustic line length model/criteria is too conservative
or too liberal?
2. Any PRV incident data that suggests that the acoustic
line length criteria should include liquid certified
PRVs?
3. Any PRV incident data that suggests that the criteria
should include vapor relief?
4. Is application of the proposed criteria practical?
5. Other related issues?
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