By: Hans K. Fauske, D.Sc., Regent Advisor, Fauske & Associates, LLC (FAI)
Typical causes of valve chatter (instability and potential valve damage) include:
• Excessive inlet pressure loss (3% rule)
• Excessive back pressure (10% rule)
• Oversized valve
Considering Tempered Reactive Vapor Systems, given uncertainties related to vapor disengagement and two-phase flow regime, valve sizing based upon two-phase flow can lead to significant oversizing and the potential for valve instability.
In order to eliminate oversizing it is recommended to calculate the required vent area based upon all vapor venting evaluated at a practical relief set pressure well below Maximum Allowable Working Pressure (MAWP). Also to be considered, there are finite number of standard valve-nozzle sizes to choose from, and the calculated vent area may not correspond exactly to one of these sizes. The practice to select the standard size nozzle area which is closest to the calculated value on the high side, may lead to potential oversizing of more than 50%. Here it is recommended to change this practice and select the relief set pressure resulting in vent area equal to the standard size nozzle on the low side.
Following the above recommended procedure based upon all vapor vent sizing, the valve is initially undersized and the pressure will continue to rise as the relief valve remains open, due to the occurrence of two-phase flow, but the resulting overpressure will not exceed MAWP.* The allowance of significant overpressure will have the following benefits:
• Assure the smallest valve size
• Eliminate oversizing and valve instability
In summary, to simply assure valve stability select the relief set pressure sufficiently below MAWP. In this regard, while plant people are reluctant to give up the practice of setting relief pressure equal to MAWP, it is time to change. Setting pressure relief activation for all reactive systems (vapor, gassy and hybrid) at practical level below MAWP, is always beneficial and does not violate any standards.
* Hans K. Fauske, "Revisiting DIERS Two-Phase Methodology for Reactive Systems Twenty Years Later," Process Safety Progress (Vol. 25, No. 3) 2006.
Dr. Hans K. Fauske is an original founding partner of Fauske & Associates, LLC and currently serves as Regent Advisor. For more information, please contact Ken Kurko at Kurko@fauske.com, 630-887-5226, www.fauske.com