Valve Magazine - Summer 2009

Figure 5

Figure 6

Figure 7

; A balanced safety relief valve incorporates a means of minimizing the effect of back pressure on the operational characteristics (opening pressure, closing pressure and relieving capacity) of the valve (Figure 5).

; A pilot operated pressure relief valve is a pressure relief valve in which the major relieving device is combined and controlled by a self-actuated auxiliary pressure relief valve (Figure 6).

; A vacuum relief valve is a pressure relief device designed to admit fluid to prevent an excessive internal vacuum; it is designed to reclose and prevent further flow of fluid after normal conditions have been restored (Figure 7).

Common Causes of
Overpressure
Now we are going to look at three
common types of overpressure:

a boiler, or may be unplanned, such as a fire in a tank farm.

3. Thermal expansion is as simple as the sun’s heat on a trapped liquid.

1. Many times the over-pressure condition is caused by a blocked discharge or the overpressure could create an explosion if it was allowed to continue….fortunately the pressure relief valve is calibrated to open at Maximum Allowable Working Pressure.

2. External fire can be planned, such as

In each case, pressure is exerted on a piping system or pressure vessel with potential danger to people, property and production. VM

Education & Training Committee member WILLIAM TRAVIS—training manager, Dresser Consolidated ( www.dresser.com) in Alexandria, LA—is the lead author for the PRV section of Valves 101. Reach him at william.travis@dresser.com.

The Importance of PRVs

Let’s face it: Pressure relief valves don’t have any fancy lights or indicators. Usually they are automatic, and plant personnel normally don’t pay any attention to them (unless they are leaking). However, the National Board [of Boiler and Pressure Vessel Inspectors] thinks they’re important.

Part II of the National Board of Boiler and Pressure Vessel Inspectors’ Inspection Code NBIC states: “The most important appurtenances on any pressurized system are the Pressure Relief Devices.” For the uninitiated, appurtenances can be defined as necessary equipment.

Part II of the NBIC goes on to state: “These devices are not designed or intended to control the pressure (that’s a function of the control room). Instead, they are intended to function when normal operating controls fail.”

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