Pressure Safety Valve vs Pressure Relief Valve: What’s the Difference?
There appears to be confusion in the industry over the correct terminology of whether it’s a safety valve or a relief valve. Some of this confusion could stem from the actual standard itself or a lack of understanding of what a “fluid” really is.
AS 1210 Pressure Vessels says –
- A safety valve vents to prevent a predetermined safe pressure from being exceeded. Normally used with compressible fluids.
- A relief valve vents to prevent system pressure exceeding a predetermined pressure. Normally used with incompressible fluids.
Air, nitrogen and steam are examples of compressible fluids i.e. gases and some examples of incompressible fluids are water and hydraulic oil i.e. liquids.
Although the term “fluid” includes both the liquid and gas phases, many people struggle with the notion of a fluid being compressible as the term “fluid” is commonly used as a synonym for “liquid” and a liquid is incompressible.
The underlying principle of hydraulic systems is that hydraulic oil is a liquid and therefore incompressible. For example, “brake fluid” is hydraulic oil and will not perform its required function if there is gas in it. This colloquial usage of the term is also common in the fields of medicine and nutrition e.g. take plenty of fluids.
Perhaps the definition of a fluid needs be clarified to eliminate any confusion over the definition in AS1210 of whether it is a safety valve or a relief valve.
Definition of Fluid
A fluid is a substance (as a liquid or gas), whose molecules move freely past one another when acted upon by a force and is capable of “flowing” to assume the shape of its container.
Under this definition a fluid includes three of the four states of matter i.e. liquids, gases and plasma.
So What’s the Difference Between Pressure Relief Valve and Pressure Safety Valve?
In practice it is almost universally called a relief valve, irrespective of the application, although technically if it is installed on a pressure vessel containing gas it is a safety valve and if it installed on pressure vessel containing a liquid it is a relief valve.
Like Pressure Vessels, Pressure Relief and Pressure Safety Valves require inspections at given intervals.
Additionally a Relief Valve must be recalibrated, which typically involves bench testing, at the same frequency as the vessel internal inspection or five years whichever is the lessor.
At the completion of the bench testing a Relief Valve Test Certificate is issued and acts as proof that the valve was correctly set and a test was performed to prove it was in proper working order.
This article is an excerpt from the full report. AME Managing Director Trevor Hughes explored this subject in further detail in his presentations Calculating the Hazard Level of your Pressure Equipment and Sizing and Selecting Pressure Relief Valves at the IDC Dangerous Goods Conference on 23/24 March 2016. Read more here >