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Monday, May 19, 2008

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Pressure Relief Device (PRV) inlet line lose shall be limited to 3% of set pressure while the outlet built-up back pressure shall be limited to 10% for a conventional PRV and 30 for balanced PSV (without affecting it flow).

Excessive PRV inlet loss can result rapid opening and closing of the valve which normally called chattering. Chattering will result in lowered capacity and damage to the seating surfaces. The inlet line loss here is referring to non-recoverable entrance losses (turbulent dissipation) and frictional line lose.

The impact of excessive built-up back pressure on discharge piping discussed in "Several Impact of Backpressure on Conventional PRV".

One of the question raised is that should the rated flow or required relieving flow to be considered for inlet line loss and discharge built-up backpressure estimation ? Should the flowing fluid phase affecting this requirement ?

Flowing phase & PRV type
The flowing fluid phase would affect the PRV type. Example, pop action spring loaded PRV is good for compressible vapor or gas service and modulating type PRV is good for incompressible liquid service.

Flowrate to be considered for Discharge line
As per API Std 521, section 7.2.1, table 12, the flowrate to be considered for tail pipe, lateral & main header subject to PSV type (refer table below).

Above is inline with API RP 520 PII - Ed 5 Aug 2003, section 5.3.
"...The rated capacity of a conventional spring loaded, balanced spring loaded or pop action pilot-operated pressure relief valve should typically be used to size the atmospheric vent piping or the discharge line from the pressure-relief valve to the relief header. Common relief header piping in closed discharge systems should be sized using the protected system's required relieving capacity.

For a modulating pilot-operated pressure-relief valve, the discharge piping can be sized using the required relieving capacity of the system that the valve is protecting..."

Flowrate to be considered for Inlet line
Following API Std 521, section 7.2.1, table 12, the inlet line loss for Modulating type PSV may consider required relieving flow. However, API RP 520 PII - Ed 5 Aug 2003, section 4.2.2. stated :
"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. When a pressure-relief valve is installed on a process line, the 3 percent limit should be applied to the sum of the loss in the normally non-fowing pressure-relief valve inlet pipe and the incremental pressure loss in the process line caused by the flow through the pressure-relief valve. The pressure loss should be calculated using the rated capacity of the pressure-relief valve."
API RP 520 PII - Ed 5 Aug 2003 has not clarified if required relieving flow can be used for modulating type PSV.

2-cents opinion
Modulating type PSV is intended to work to modulate the flow passing the PSV. Those theoretically, on the required relieving flow should be passing the PSV. In taking conservative approach and in compliance with API recommendation, rated flow is recommended for modulating type PSV inlet line loss estimation.

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posted by Webworm, 1:46 PM


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