Seat leakage classification shall be in accordance with FCI 70-2 or MSS-SP-61. The leakage class shall be determined by the service application. Soft-seated valves shall not be applied in services with design temperature conditions over 230°C  (450°F) or in flashing liquid services.

Leakage problems are commonly related to insufficient seat loading. While a control valve with insufficient seat loading may pass a specified leakage test after it is manufactured, the same valve will not meet the specified leakage rate after it has been subjected to thermal distortion, flashing, or erosive service.

The following minimum seat load requirements (after hydraulic loads, spring loads and friction loads are considered) shall apply:

Class I – III Per Seller’s Recommendation
Class IV 5.4 kgf/mm (300 lbf/in) of seat ring circumference
Class V 8.9 kgf/mm (500 lbf/in) of seat ring circumference
Class VI Per Seller’s Recommendation
MSS-SP-61 17.9 kgf/mm (1000 lbf/in) of seat ring circumference

Seat ring circumference is defined as the line of contact between the plug and the seat when the valve is in the fully closed position.

The maximum shut-off differential pressure shall always be calculated and specified for the selected control valve. For all metal-to-metal shut-off applications equal to or exceeding FCI 70-2 Class IV requirements, actuator load calculations shall be provided. Calculations are required to show the minimum load to be applied to the plug-seat arrangement for meeting the required shut-off specifications.