In a previous post, we identified six factors that determine the sizing and selection of pressure relief valves:

  • Connection size and type
  • Set pressure (psig)
  • Temperature
  • Back pressure
  • Service
  • Rated capacity

In this post, we’ll explore six more factors you need to consider.

Applicable code compliance

ASME code compliance is a requirement for many pressure relief valves. This designation ensures that valves are designed, constructed, and installed according to the highest standards of safety.

  • The ASME V Stamp applies to all Section I valves.
  • The ASME UV Stamp applies to all Section VIII valves.

Any valve that you purchase or have repaired needs to bear all required stamps.

View Allied Valve’s ASME stamps.

Physical properties of the media to be relieved

Valves are rated for different service applications. For example, the Consolidated 1700 Series Safety Valve is rated for steam service, while the Kunkle Relief Valve Model 91 is rated for liquid. Some valves, like the Kunkle Safety Relief Valve Model 140, are rated for air, gas, and liquid.

But service alone doesn’t tell the whole story. Liquids, especially, vary greatly in their physical properties, and these properties are crucial for determining the correct valve for your application. Here are the physical properties to consider for steam, gas vapor, and liquid.

 

Steam and Gas Vapor Liquid
Molecular weight Density
Ratio of specific heats Specific gravity
Compressibility factor Viscosity

Maximum allowable seat leakage

All valves leak. Even valves labeled as “zero leakage” or “bubble tight” aren’t completely leak-free. The maximum allowable leakage rates for pressure relief valves is laid out in the American Petroleum Institute standard API 527. For valve selection, the maximum allowable leakage rate determines whether a metal- or soft-seated valve is appropriate.

Fire sizing

The ASME code specifies that pressure devices must be protected against overpressure due to external fire. Fire can change the media in the device (for example, a liquid may transform into a gas) and the valve must be equipped to handle it. Fire sizing determines the capacity of the valve in a fire situation by allowing for greater overpressure.  

Installation area and requirements

The available installation area may constrain the valve size and configuration.

Extreme environmental conditions

Environmental conditions such as extreme heat and extreme cold can affect the operation of pressure relief valves. Make sure the valves you select are capable of withstanding the conditions in which they will operate. (Learn how to winterize valves and actuators.)

Our valve experts are on hand to answer any questions you may have about sizing and selecting pressure relief valves. Contact us today.

Source: Kunkle Safety and Relief Products Technical Reference

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