TECH TALK
Pressure Locking Solutions for Gate Valves
Double-seated Gate valves trap fluid in the center cavity when closed . Under specific circumstances this can result in pressure being high enough in the center cavity that prevents the valve from being opened . This “ pressure locking ” condition is a known issue for which several avoidance methods have been developed . Prevention of pressure locking is necessary to ensure the operability of the valve in critical moments .
By Randall Elmore , Order Engineer – Flowserve
The industry has been aware for decades of the possibility of gate valve operability issues caused by pressure locking . Pressure locking can happen in valves with two seats in series , notably but not limited to : flexible-wedge , split wedge , parallel slide , and double-disk gate valves . Pressure locking can occur when the pressure in the bonnet cavity area is higher than line pressures both upstream and downstream of the valve . The differential pressure and increased friction between the seat and disc can be great enough that the valve actuator cannot open the valve . This is because actuators are normally sized based on a maximum differential pressure expected at closing .
When a gate valve is closed with the system full , fluid will be trapped in the cavity between the disc / seat ring seals , including the body neck area beneath the bonnet . This trapped fluid can end up at a higher pressure than the upstream and downstream lines by various methods :
• One method is a pressure transient , such as a decrease in pressure in the upstream line trapping higher pressure in the valve bonnet area , resulting in a higher differential pressure between the center cavity and the upstream line .
• Another common method is a temperature rise in the trapped fluid . This leads to a thermal expansion of the fluid and a resultant increase in pressure . The temperature increase could be due to outside environmental conditions or due to a rise in process fluid temperatures . Heating of a trapped fluid may occur by convection and conduction from a nearby heated system . Even sunlight or an ambient temperature change could generate a small but significant temperature change . An extreme case would be exposure to fire .
Failure to address potential pressure locking in advance can result in the valve being unable to open at a critical moment . Remediation of an unexpectedly pressure locked
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Figure 2 .
valve is awkward ; it may simply require process steps to reduce the differential pressure between the center cavity and the lines , but there will be instances where the more extreme steps of physically relieving the center cavity pressure , or applying an opening force surpassing that which the actuator is sized to produce . Prevention is certainly preferred to remediation .
When conditions arise that could result in pressure locking , there are several approaches available to address the issue .
A ) Thrust Analysis . Using analytical procedures to predict the increased thrust necessary to open the valve under pressure locking conditions is an option . One such approach for analysis is known as the “ Generalized Pressure Locking Methodology ”, developed by Kalsi Engineering . 1 Once the necessary thrust is known , the valve engineer can size the actuator to ensure the availability of the requisite force to open the valve . Simply applying more power is appealing to many of us but , in general , prevention of the problem through operating procedures or valve design features is a preferred approach .
B ) Operating Procedures . Using operating procedures as an administrative control to prevent pressure locking can be quite effective . This permits the valve to maintain bidirectional seating ability without the addition of cost-increasing specific design features . However , this approach requires identifying valves susceptible to pressure locking and then developing and implementing the necessary procedures . Such procedures would ensure valve operation only at times of negligible risk of pres-
30 Valve World Americas | October 2024 • www . valve-world-americas . net