Transcript Origin of 3% Rule?

Review of Development of
proposed response to OSHA
Letter to API
PRV Inlet Pressure Drop
Engineering Analysis
Clark Shepard & Phil Henry
Spring 2010 API CRE Meeting
• Background
• History
• Current Status
• Path Forward
1
API 520 Part II - Background
API RP-520 Part 2, 5th Ed (2003) Section 4.2.2 states:
– An engineering analysis of the valve performance at higher inlet losses may permit
increasing the allowable pressure loss above 3 percent.
March 2010 Ballot outlines:
– Typical blowdown set by the manufacturer for PRVs is 7 to 12% of the set pressure
– Original basis of 3% inlet pressure loss was related to blowdown settings in the range
of 4 - 5 %
– A suitable margin relative to the blowdown shall be specified by the user
– When exceeding 3% inlet loss, an engineering analysis shall include but is not limited
to the following:
a. Verification from the manufacturer the minimum blowdown value for the PRV model
based on the manufacturer’s standard setting.
b. Prior to any increase in blowdown to allow for higher inlet pressure drop, the
manufacturer shall be consulted to make sure that an increase in blowdown is
possible.
c. Re-evaluation of the flow capacity of the valve taking into consideration the reduction in
pressure at the inlet to the valve.
d. The user shall conduct a thorough review of the valve’s inspection/maintenance
records and obtain experience from Operations, to identify any indications of chatter.
2
History of Inlet Pressure Drop
•
API RP 520 introduced maximum PRV inlet pressure drop in 1963
•
API sponsored 1940’s work at University of Michigan by Sylvander and Katz “The Design
and Construction of Pressure Relieving Systems”
•
University of Michigan Press (1948) pages 72-73 excerpts:
– “Pressure drop through inlet piping has a two-fold importance in relief system design.
First, flow capacity varies with the pressure drop available. Second, the operating
characteristics of many relief devices indicate that improper pressure drop on the
inlet side may cause intermittent operation.“
•
– “For a relief valve having approximately 4 per cent blow-down (that is, the valve will
snap shut when the pressure has decreased to 4 per cent below the opening or set
pressure), these recommendations are made:
Combined pressure loss of 3% maximum related to PRVs with 4% blowdown (margin of
1%)
3
Current Status
•
•
•
•
Most API members have internal practices that accept inlet pressure drops that exceed 3%
(most at 5%) and have demonstrated successful performance over time
March 2010 Ballot revision to API 520 Part II implies inlet pressure drop could be greater
than 3% when blowdown settings are at 7% to 12% (standard factory setting for PRV).
OSHA sent letter outlining March 2010 Ballot does not have strong technical basis and
experience is not conclusive evidence for exceeding 3%
PERF 99-05 PRV Stability Project evaluating multiple factors that could lead to instability
(chattering) including inlet pressure drop
– Literature search concluded no past work available that conclusively draws conclusion
relating the various factors that may impact PRV stability
– Progressing testing and development of model to evaluate potential stability factors
– Testing involves
• Three different size valves from three vendors
• Two set pressures
• Multiple system pressure ramping rates (at least three rates per valve)
– Completing inlet pressure drop tests runs
– Entering Phases 5 – 7
• Phase Five - Development of Engineering Tool
• Phase Six – Tool Programming
• Phase Seven - Final Report
– Plans to complete during 4Q10
4
Proposed Path Forward
•
Develop draft response to OSHA letter – Discuss current status (5/2010)
•
Develop new inlet loss criteria (5/2010)
− Maintain 3% for new facilities
− Provide criteria for accepting 5% inlet pressure drop for existing equipment
e.g., experience
•
Develop and evaluate engineering analysis procedure when exceeding above
inlet loss criteria based on valve lift, inlet pressure and capacity (6/2010)
•
Vet through API CRE (Review with others – Law SC, Process Safety) (6/2010)
•
Initiate contact with Jim Lay, OSHA’s technical expert on PRVs, and vet
response to letter and potential revisions to API 520 Part II (7/2010)
•
Communicate to API PRS – Task Force (8/2010)
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