Valve Magazine - Summer 2009

EDUCATION & TRAINING
A ‘Valve Ed’ Preview: Pressure Relief Valves BYWILLIAMTRAVIS

Based on the many inquiries that VMA has been receiving about our “Valve Ed” program, we know that a lot of you are eagerly anticipating the arrival of these new educational tools. Valves and Actuators 101, which will first be offered in the form of a CD that contains a PowerPoint presentation with lecture notes, is expected to be released for sale late this year. The first public presentation of the program will take place at the Valve Basics Seminar & Exhibition, Oct. 29, in Houston. (Go to www.VMA.org > Events > Valve Basics Seminar to register or for more information.)

Meanwhile, our Education & Training Committee is in the process of developing and refining the content. To give you an idea of what’s to come, we present here a short segment from the pressure relief valves (PRVs) section. In addition to PRVs, Valves 101 also includes a comprehensive overview of the industrial valve industry and sections on quarter-turn valves, multi-turn valves, check valves and control valves. Plus, we will offer a companion program, Actuators 101, to address the different types of actuators.

Figure 1

–Judy Tibbs, Editor in Chief

This portion of the Valves 101 pro- gram deals with a special type of valve; one that does more than turn the flow of media on and off, or regulate its flow through a line. We’re here to learn about pressure relief valves.

The pressure relief valve doesn’t control fluids like a control valve; in fact, it really doesn’t have any part in the process. It doesn’t make production faster nor does it improve flow through piping. What it does is channel over-pressure excursions in the protected vessel in a safe direction. We could look at this valve somewhat like an insurance policy. It sits in a drawer and hopefully we never need it. But… if we do… then we want these valves to open and relieve the overpressure in the vessel and defuse any potential explosions.

Let’s take a look at all the different types of pressure relief valves:

; A safety valve is a pressure relief valve actuated by inlet static pressure and characterized by rapid opening or pop action (Figure 1).

; A relief valve is a pressure relief valve actuated by inlet static pressure that opens in proportion to the increase in pressure over the opening pressure (Figure 2).

; A safety relief valve is a pressure relief valve characterized by rapid opening or pop action, or by opening in proportion to the increase in pressure over the opening pressure, depending on the application (Figure 3).

; A conventional safety relief valve has its spring housing vented to the discharge side of the valve. The operational characteristics (opening pressure, closing pressure and relieving capacity) of the valve are directly affected by changes in the back pressure in the outlet of the valve (Figure 4).