I have dozens of in-line relief valves fitted with external springs, these are fitted as Mono pump protection devices. Some of these have been installed for years, they are not on a maintenance program but are refurbished as the pumps fail. My concern is that a sizeable number of these valves look near as dammit coil bound. What is, or should be the relationship between valve seat area and the disc lift. I suspect that the area of disc lift should at least equal the seat area.

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.

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