The difference is generally in capacity and setpoint.  A relief valve is ment to relieve pressure to prevent an over pressure condition.  A relief valve may have an operator on it to assist in opening the valve in response to a control signal.  A safety valve is ment to relieve pressure without operator assistance and a safety valve, or combination of safety valves, must be have a capacity to relieve more than the energy input to the volume being protected.  

For example, a thermal relief valveis used to bleed off pressure in a heat exchanger if the heat exchanger is isolated but the possibility of thermal expansion of the fluid could cause over pressure conditions.  The capacity of thermal reliefs are generally small.

A safey valve on a boiler or other types of fired pressure vessels must be capable of removing more energy that is possible to be put into the vessel.  110 percent of boiler rated capacity may be acceptable.  The ASME Boiler & Pressure Vessel Code would be the source to check for adaquatley sizing safety valves.

What is PIA pressure release valve installed in a petrol tank?

ASME SecVIII defines relief valve as an automatic pressure relieving device used primarily for liquid service.  This would be a liquid that would not necessarily vaporize when it passes through the valve.
A safety valve is an automatic pop type valve used primarily in steam service.
A safety relief valve can be used in either service, provided necessary adjustments are made to have the valve perform correctly (proportional or pop full open)dependant on the service conditions.

A relief valve typically relieves the over-pressure by modulating and opens only enough to based on the spring pressure to relieve.  A typical liquid relief valve.

A safety relief valve has (2) positions closed & full-open.  The valve nozzle/disc assembly contains a "huddling" chamber to assure this snap-action upon opening.

According to ASME PTC 25, A Safety valve is a sub-category of Pressure Relief Valve.  Pressure Relief Valve is a sub-group of Pressure Relief Device.  Pressure Relief Devices include Rupture Discs and other non-reclosing devices.  Pressure Relief Valves may be either Relief Valve primarily for Liquid service, Safety valve primarily for Steam/Air/Gas Service or Safety-Relief Valve for either Liquid or Gas Service depending on application.

J. Alton Cox
President
DeLuca Test Equipment
www.delucatest.com

Safety Valves are automatic pressure relieving devices actuated by the pressure upstream of the Valve and is characterised by means of a pop action. They are used for compressible fluid service eg. Steam, air etc.
Relief Valves are also automatic pressure relieving devices but they are characterised by proportional opening, depending on pressure above set pressure. They are used for incompressible fluid service eg. water.

I think in my opinion, its fair to say that in day to day factory situations when talking to the technicians, the two terms are interchangable.

Regards

Steve

In a Relief Valve (RV) on Liquid Service, Lift (Travel of the Disc away from the Seat)is generally proportional to Overpressure.

In a Safety Valve (SV)on Steam, Air or Gas, the Lift is typically 60 to 70% of Full Lift at initial opening.  Further lift is gradual up to the maximum overpressure.

In a Safety-Relief Valve (SRV), the Valve will act like a Relief Valve on Liquid or a Safety Valve on Steam/Air/Gas.

One of the problems with using the terms interchangeably is the DIfference in Adjustment Ring Settings.  Failure to recognize the differences between RV, SV & SRV is what separates a Mantenance Mechanic from a PRV Technician.

J. Alton Cox
President
DeLuca Test Equipment
www.delucatest.com