THIS IS A COMPLICATED ISSUE:

Seat diameter is a old term from the days when Seat Inside Diameter and Bore were equal.  Most newer designs have a seat ID > Bore Dia.

If the Relief Valve is a top guided, flat seated, high lift design, the lift should be 1/4 the bore.  

For example,
a Pressure Relief Valve with a Bore Diameter of 2.000" should have a lift of .500".  Area = pi x radius sqared, therefore, the Area of the Bore = 3.1416 x 1" x 1" = 3.1416 sq in.  When the disc lifts off the seat, the flow area is controlled by the "curtain area" or the circumference of the bore x the lift of the disc.  If the Bore is 2.000 and the lift is 1/4 the bore or .500", then the Curtain Area is 3.1416 x 2.000" x .5 or 3.1416 sq in.  

Therefore, more lift (> .500") is superfluous and less than full lift or 1/4 the bore results in Curtain Area determining Flow Capacity rather than Bore Area.

There are, however, low lift Relief Valves which use Curtain Area as the determining factor for capacity.  There are are also Wing Guided (Bottom Guided) Relief Valves which require a more detailed method of determining flow area and typically have lower lift.  In addition, there are 45 degree Angle Seated Relief Valves which have a .7 derating factor for flow and Ball Seated Designs which have a very low flow characteristic.

In short, the Make, Model, Size (Orifice) & Set Pressure of your Relief Valve are needed to determine the actual required Lift.

I hope this is helpful information.

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

To add to what JAlton has stated, while a lift of 25% of the bore diameter creates a curtain area equal to the bore of the valve for top-guided, flat seated, high lift design, I have observed flow limiting curtain areas up to 40% of the bore diameter.  This is dependant on the trim geometry and the fluid mechanics are complicated.  

Also, ASME Section VIII Division 1 UG-136 Minimum Requirements for Pressure Relief Valves states:

"The spring shall be designed so that the full lift spring compression shall be no greater than 80% of the nominal solid deflection."

JAlton, does this apply in all cases?

Yes.  The Spring Requirement for Max Deflection is also an ASME Sec. I requirement.  A PRV Spring is typically non-linear in the first 15% of deflection and in the final 25%.  Therefore, the 80% rule does not "rob" the designer of Potential Spring Range, but it does provide a safety factor to prevent damage that might result from the spring be collapsed solid.  Some low set pressure PRVs have a lift stop to prevent the Spring from going "solid".

The National Board of Boiler & Pressure Vessel Inspectors website www.nationalboard.org contains a link to their Pressure Relief Device Certification (NB-18) file in PDF format.  It gives the minimum lift for every ASME Certified Valve worldwide.

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

jet1749, I have a question based on your description of the relief valves.  When you say "in-line" are you referring to a straight thru pattern like a globe valve or are these relief valves in the classic 90 degree pattern?

Also, if the Relief Valves are in Liquid Service, i.e. pump protection, there is a good chance they are non-code (No ASME Stamp) relief valves.

I am also curious about the "External Springs".  Tis is typical for Steam Service, but not Water.  An external Spring would need to be isolated from the internal portion of the Relief Valve (Secondary Pressure Zone).  I know of only one such design.  It is a Crane Model 18854 all SS Material with a packing gland on the stem.  These are no longer permitted.  Do you have a Manufacturer or Model Number?

When you refer to the time "years", I should also point out that any Liquid Relief Valve built prior to 1984 will not have an ASME GPM Rating.

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

The valves are 3" "Hindle" in line straight through similar to globe and connect the discharge of the Mono pump back to the suction.The valves handle acrylic dope for fibre production at 400 poise(40000cps)against a press pressure of up to 300psi max, these are set to lift at 320psi. All 316SS construction and have been installed since the mid 1970's. Thanks for your comments, and I will follow up some of your suggested sources

Jet1749,
These are non-code liquid relief valves.  That means the ASME 80% max deflection rule for springs does not apply.  Also, you will not find these valves listed in National Board PRD Certifications.

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