(27a) Pressure Relief Valves - Restricting Capacity to Mitigate Oversizing

INTRODUCTION

Pressure relief valves are frequently oversized for small required relief rates. Although oversizing is conservative, it can lead to other difficulties.  Therefore it may be advantageous to limit the capacity to only that required. Physically restricting the lift is an inexpensive and simple option.

RESTRICTING THE LIFT

Oversizing of pressure relief valves is widespread and frequently exacerbated because relief valves are normally available in standard sizes, with an increase of 21% to 78% from one orifice area size to the next standard size. 

Oversizing results in excessive flow through the valve, and may incur high inlet pressure, high backpressure and choking in the body bowl.  The resulting high velocities increase erosion and intensify pipe vibration induced by flow or acoustic pressure waves.  The relief valve operation may become unstable. Personnel may be affected by noise levels.  Piping and engineering costs can increase. 

Many solutions are not appealing, including revising the process and equipment or utilizing safety instrumented systems to reduce the required relief load or relief probability.  Purchasing a custom relief valve may be expensive.  Studies to predict stability may be undertaken. 

However, one successful method is to mechanically restrict the opening - or the lift of the disk - to flow only the required capacity. The option adheres to applicable ASME Boiler Codes and Code Cases, plus API-520 and API-526.  Most vendors provide the option for both spring operated and pilot operated pressure relief valves.  The additional cost is minimal per relief valve with no delay in schedule.

WHAT YOU WILL LEARN

· This paper will explain the advantages and disadvantages of restricting the lift. · Different methods to restrict the lift will be illustrated. · The paper will outline the testing and certifying of the restricted lift relief valve design and discuss the regulatory Codes.  · The effect on flow issues such as blowdown and overpressure will be provided. · National Board Certification, proper name plate marking, vendor calculations to determine the lift and capacity will be presented.   · The process for obtaining a restricted lift will be described.