they both relieve pressure, but the valves are designed and sized to different standards.
Case in point are boiler safeties versus over pressure relief of a unfired vessel.
you can start with standards commonly used in your industry or just contact a vendor. most catalogs have a brief technical section to get you started. engineering handbooks (Marks, Perry's, etc) also have sections that deal with over pressure protection.
To follow up on hacksaw's response;
ASME B&PV Code Section I, part UG-67 foot note 1 has excellent definitions regarding the functions of a safety valve, relief valve and safety relief valve. I will try to summarize;
safety valve; an automatic pressure relieving device actuated by static pressure upstream of the valve... used for gas or vapor service.
relief valve; same definition as above but opens futher with an increase in pressure over the opening pressure - provides relieving capacity - used in liquid service.
safety relief valve - can perform either function as above depending on application.
In my experience, rightly or wrongly, the terms tend to get used interchangeably. I would not want to make any assumptions about a particular valve just because it's called a safety valve versus a relief valve.
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I would have to say that I'm guilty of using the terms interchangeably but the definitions as given by Metengr are the same that I've always seen.
In the Jan-2000 edition of API RP-520 Part I, similar definitions are given. Also, as found in the following section
2.2.1.1.2 Spring-loaded pressure relief valves are referred to by a variety of terms, such as safety valves, relief valves and safety relief valves. These terms have been traditionally applied to valves for gas/vapor service, liquid service, or multi-service applications, respectively. The more generic term, pressure relief valve, is used in the text and is applicable to all three.
Recently, I happened to be looking at a copy of API RP-576, Inspection of Pressure-Relieving Devices, Dec-2000, and found they have a broader discussion about the different designs and actually list limitations to their use. Among the limitations listed: Safety valves should not be used in liquid service, Relief valves should not be used in vapor service.
Seems that it would have been more appropriate to include the API RP-576 discussions in API RP-520 but I guess it is fairly obvious, don't expect a valve designed for liquid service only to perform equally well in vapor service.
Anyway, the bottom line is you need to identify all requirements for your relief system design whether a case controls for relief area sizing or not. Then select the appropriate relief valve for all possible cases you identified. If you do have a situation that requires a relief valve to relieve a vapor for some cases or a liquid for others, be sure the valve you select has certified capacities for both vapor and liquid service or gives at least orifice area/nozzle discharge coefficient for both vapors and liquids.
I just learned a safety valve is used mainly for gas and steam and is caracterised by a "pop" action. A relief valve is caracterised by a gradual lift proportional to thje increase in overpressure, mainly for liquid applications. However the vocabulary may have slightly different meanings depending if you are in North America or Europe.
For more info you can check the terminology chapter (chapter 3) at
The two terms are often confused, even in some ASME correspondence. Here is the difference:
Safety valves are for vapors such as steam and 'pop' open. As the vessel pressure approaches the safety valve set pressure it might start to weep slightly but when it goes, it pops open. When the vessel pressure drops a certain set pressure below the pop pressure, the safety valve closes. The pressure it starts to weep, pops and reseats is determined by the ASME code.
Relief valves are for liquid service like hot water. They open gradually and close gradually.
One of the consequences of how the two types of valves are made and the service they see is this. To lift a safety valve by hand, the pressure should be close to the pop pressure. The lever is lifted to cause the safety valve to pop open. The valve should be allowed to close on its own.
To lift a relief valve by hand, lift it slowly. Allow the fluid to flow and then slowly release the lever.
As always, care should be taken when testing safety and relief valves by hand. My advice is follow the valve manufacturer's recommendations.
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