PSV Inlet Line 3 Percent Rule

When I was young process engineer, I learnt sizing of pressure relief valve, include inlet line sizing rule:
API RP 520 Part II (Ed 2003), section 4.2 recommends that the total non-recoverable pressure loss between the protected equipment and the pressure relief valve should not exceed 3 percent.

WHY?
In API RP 520, section 4.2 "PRESSURE-DROP LIMITATIONS AND PIPING CONFIGURATIONS"
"Excessive pressure loss at the inlet of a pressure-relief
valve can cause rapid opening and closing of the valve, or
chattering. Chattering will result in lowered capacity and
damage to the seating surfaces."

"When a pressure-relief valve is installed on a line directly
connected to a vessel, the total non-recoverable pressure loss
between the protected equipment and the pressure-relief
valve should not exceed 3 percent of the set pressure of the
valve except as permitted in 4.2.3 for pilot-operated pressure relief
valves."

"Keeping the pressure loss below 3 percent becomes progressively
more difficult at low pressures as the orifice size of a
pressure-relief valve increases. An engineering analysis of the
valve performance at higher inlet losses may permit increasing
the allowable pressure loss above 3 percent."

Clearly, the API guideline is to avoid the PSV chatter.
In 2007, API is responded that the 3% rule is under investigation:[1]

In March 2010 Ballot outlines: [2]
- 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

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%)

In Spring 2011Metting [3]

In November 2011, Hydrocarbon Processing featured a Special Report [5] Title : "Relief Device Inlet Piping: Beyond the 3 Percent Rule"


Current status, (3/7/2013), API RP520 Part 2, 6th Ed Committee Draft [4]
PSV Inlet Pressure Loss Criteria:
The total non-recoverable
pressure loss between the protected equipment and the pressure-relief valve should not exceed 3 percent of
the pressure relief valve set pressure except as noted below: 

• Thermal relief valves
• Remotely sensed pilot operated relief valves
• keeping the pressure loss below 3 percent becomes progressively more difficult at low pressures as the orifice size of a pressure relief valve increases
• An engineering analysis is performed for the specific installation

Reference link
1. API replied
2. Spring 2010 API CRE Meeting
3. Spring 2011 API Meeting Minutes
4. API RP520 Part 2, 6 Ed, Committee Draft
5. Relief Device Inlet Piping: Beyond the 3 Percent Rule, Hydrocarbon Processing, Novmber 2011 issue