A valve having tight shutoff will have virtually no flow or leakage in its closed position. Generally speaking, only single-seated valves have tight shutoff.
Double-seated valves may be expected to have a leakage of 2 to 5% while in closed position.
Control valve seat leakage shall be designed and constructed in accordance with the requirements to ANSI B 16-104.
Control valve seat leakage
• Class I – N/A
• Class II - 0.5% of maximum valve capacity
• Class III - 0.1% of maximum valve capacity
• Class IV - 0.01% of maximum valve capacity
• Class V – 0.0005 ml/min/inch of port dia./psi differential
Close-off pressure is an important parameter that defines the differential pressure required to seat the valve and stop flow completely and is a function of the hydraulic design of the system and the criteria for deciding the type of valve actuator.
Typically, actuators are either electronic or pneumatic. The significant performance differentiator between the two is speed of operation. While the electric actuators are better known for their high levels of precision; the pneumatic actuators are very popular due to their relatively low cost, high power output, and reliability.
The choice of pneumatic or electric actuators involves an evaluation of performance, component costs, system costs, and productivity gains. Each has inherent advantages and disadvantages.