Safety Valve, Relief Valve, and Safety Relief Valve

Is this a question on semantics?

OK. What's the difference between these valves, then?

I m nt sure y people come across this forum n post their queries straight away without even bothering to search out the previous threads...!!

As soon as mankind was able to boil water to create steam, the necessity of the safety device became evident. As long as 2000 years ago, the Chinese were using cauldrons with hinged lids to allow (relatively) safer production of steam. At the beginning of the 14th century, chemists used conical plugs and later, compressed springs to act as safety devices on pressurised vessels.

Safety valve, gas or steam blow down no less than 2% ( ie Normally use on vessels, tanks......)

Relief valve Liquids ( ie Normally use on vessels, tanks......)

Safety relief valves are automatic discharge valves type and have the function of manteining the pressure in a circuit below a set limit

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Yes, this is a matter of semantics

The process engineer puts together a P&ID showing a Power or a process system. Included on that P&ID are some valves designed to open when and if the pressure exceeds a specific level. These valves may discharge to atmosphere (if the commodity is steam or other non-hazardous material) or to a closed system (if the commodity is hazardous, toxic or flammable).

These valves may be called by any of a number of names depending on the Process Engineer or the Instrument Engineer.

These designations include:

• PSV (Pressure Safety Valve)
• PRV (Pressure Relief Valve)
• SRV (Safety Relief Valve)

Regardless of the "Name" attached to the device it is and does the same thing. The real difference between any two actual devices is in the construction based on the Maximum Pressure, the temperature and the commodity.

In my opinion they are all just a plain "PSV."

Hi JAMHOUR,

There is a difference in purpose and function ascribed to these names, the first two at least.

Safety valve; as its name implies it is there to prevent a catastrophy!

Relief valve; as its name indicates it provides a method of controlling a pressure rise which whilst not falling in the former category is an undesirable condition.

An example: Every pressure vessel ,boiler ,air receiver etc will have a SV fitted this is mandatory. Gas regulator systems where pressure is progressively reduced say from a boosted supply line to combustion equipment will feature relief valves in the reduced pressure stages. These devices are there to allow over pressure transients (pulses or weepages)to be vented to avoid causing problems further down the line. At no point in this arrangement is there any physical risk to the installation of the maximum pressure being sustained.

You will see that this terminology is appropriate; however I think the combination of both safety and relief is both confusing and inappropriate.

i would suggest you to go through the API 519 & API 521. All the terms are very clearly defined.

I am sure it will help.

what is the API 519 & API 521 ?

API591 is Process Valve Qualification Procedure

Provides recommendations for evaluation of a manufacturer's valve construction and quality assurance program for the purpose of determining a manufacterer's capability to provide new valves manufactured in accordance with the applicable API standards. Qualification of valves under this recommended practice is "manufacturing facility specific" and does not cover valves manufactured by other manufacturing facilities, whether owned by the same manufacturer or a third party. Ffugitive emissions testing is outside the scope of this RP.

API521 is applicable to pressure-relieving and vapor-depressuring systems. Although intended for use primarily in oil refineries, it is also applicable to petrochemical facilities, gas plants, liquefied natural gas (LNG) facilities and oil and gas production facilities. The information provided is designed to aid in the selection of the system that is most appropriate for the risks and circumstances involved in various installations. This standard is intended to supplement the practices set forth in API RP 520-I or ISO 4126 for establishing a basis of design.

API 521 specifies requirements and gives guidelines for examining the principal causes of overpressure; determining individual relieving rates; and selecting and designing disposal systems, including such component parts as piping, vessels, flares, and vent stacks. This Standard does not apply to direct-fired steam boilers.

This edition of API 521 is the identical national adoption of ISO 23251, Petroleum and natural gas industries-Pressure-relieving and depressuring systems

read more at valve and flange connection data