Module 9 : Process Hazards and Safety Measures in Equipment Design

Lecture 3 : Safety measures in equipment design and Pressure Relief Devices

 

6.7 Pressure relief devices

Selection, design and specification of appropriate pressure relieving facilities is the most important safety device used in the process equipment to prevent the failure of equipment due to over pressure. The more common causes of over pressure are external fire, closed outlets, liquid expansion, failure of reflux. The relief devices fall into six categories.

  1. 1.  Safety valve - It is an automatic pressure-relieving device actuated by the static pressure upstream of the valve and characterized by rapid full opening or pop action. It is used for gas or vapor service. (In the petroleum industry it is used normally for steam or air)

    2. 2.  Relief Valve - A relief valve is an automatic pressure-relieving device actuated by the static pressure upstream of the valve, and which opens in proportion to the increase in pressure over the opening pressure. It is used primarily for liquid service.

    3.  Safety Relief Valve - A safety relief valve is an automatic pressure-relieving device suitable for use as either a safety valve or relief valve, depending on application. (In the petroleum industry it is normally used in gas and vapor service or for liquid). These safety relief valves are classified as conventional or balanced, depending on the effect of back pressure on their performance.

    a.  Conventional Safety Relief Valve - A conventional safety relief valve is a closed-bonnet pressure relief valve that has the bonnet vented to the discharge side of the valve and is therefore unbalanced. The performance characteristics, i.e., opening pressure, closing pressure, lift and relieving capacity, are directly affected by changes in the back pressure on the valve
    b.  Balanced Bellows Safety Relief Valve - A balanced safety relief valve incorporates means for minimizing the effect of back pressure variation on the performance characteristics; opening pressure, closing pressure, lift and relieving capacity. This is usually achieved by the installation of a bellows

     

     

     

     

     

     

    4.  Pressure Relief Valve - This is a generic term applying to relief valves, safety valves or safety relief valves and it is commonly abbreviated to "PR Valve".

    5.  Rupture Disc Device - A rupture disc device is actuated by inlet static pressure and is designed to function by the bursting of a pressure-retaining diaphragm or disc. Usually assembled between mounting flanges, the disc may be of metal, plastic, or metal and plastic. It is designed to withstand pressure up to a specified level, at which it will fail and release the pressure from the system being protected

 

References:

 

Mahajani, V V., Umarji, S.B., 2009, Process Equipment Design, Fourth Edition, Mcmillan Publishers India Ltd., India.

Crowl, D.A., Louver, J. F., 1990, Chemical Process Safety Fundamentals with Applications, Prentice-Hall International, Inc., New Jersey.

Sinnott, R. K., 2005, Coulson & Richardson's Chemical Engineering Series, Chemical Engineering Design, Fourth Edition, Butterworth-Heinemann An Imprint of Elsevier, Oxoford.

Perry, R. H., Green, D. W., Maloney, J. O., 1997, Perry's Chemical Engineers' Handbook, Seventh Edition, McGraw-Hill Book Company, USA.

Cheremisinoff, N. P., 2000, Pressure Safety Design Practices for Refinery and Chemical Operations, Noyes Publications, USA.

Sanders, R. E., 1999, Chemical Process Safety: Learning from Case Histories, Butterworth-Heinemann, USA.