VALVE MAGAZINE Summer 2023

choose the parallel slide design. If you recall, in the parallel slide design (Figure 5), the two discs are held apart in the cage by a spring(s). This spring(s) allows the discs more than enough travel to exceed the effects of thermal expansion. Thermal binding is not endemic to pressure seal valves. However, because pressure seals are routinely used in high-temperature service, take special care when addressing this issue. OTHER OPERATING CONCERNS In addition, users of pressure seal valves must address two related operat ing concerns: center cavity over pres surization (CCOP) and pressure locking. Like thermal binding, these phenome na can result in an inability to stroke the valve. Note that thermal binding, CCOP, and pressure locking are three distinct concerns, the potential of which must be carefully evaluated and addressed in the design/procurement phase of the project. Paragraph 2.3.3 of ASME B16.34 places the responsibility on the owner to determine the poten tial for, and provide a means to protect against, CCOP and pressure locking. The closure element of double-seat ed valves (wedge gates, parallel slide gates, ball valves, etc.) may become locked in place by either a buildup of pressure in the center cavity (CCOP) or an increase in the differential pressure upstream, downstream, or both of the seats in a closed valve as a function of decreased line pressure (pressure locking). In the case of CCOP (Figure 7), fluid trapped in the center cavity at ambient temperatures will expand when heat is introduced (e.g., during

startup). This will cause the fluid to expand and, depending on the fluid type and temperature, could reach a pressure where insufficient torque is available (manually or actuated) to overcome the pressure and open the valve. Pressure locking (Figure 8) occurs in double-seated valves where the line pressure drops (as a function of plant operation or accident) on either the upstream, downstream, or both sides of the valve seats, creating a sufficient differential pressure to preclude open

High pressure side when valve is closed

Equalizing valve

HP

Equalizing pipe

Figure 10

n A pressure equalizing pipe, drilled and tapped from the center cav ity to the valve’s “pressure side” bore (Figure 10). When a bypass valve is included, bi-direction al sealing is maintained. Just remember that when the bypass valve is closed, center cavity pressure is not being relieved. n A pressure relief valve that is con nected to a pipe drilled and tapped into the valve’s center cavity. This method maintains the valve’s bi-directional sealing

P 1

Pla

Plb

P 1 = Pressure of trapped liquid between seats Pla & Plb = Line pressures

Relief valve (self-activating)

Figure 8

ing the valve. As in thermal binding, there are several methods to guard against CCOP and pressure locking. These include the following: n A pressure relief hole drilled through the “pressure side” of the body or wedge/disc half into the valve’s center cavity, thus relieving overpressure to that pressure side. This effectively makes the valve unidirectional in its sealing capability (Figure 9).

Figure 11

capability (Figure 11). n A drain valve that is connected to a pipe drilled and tapped into the valve’s center cavity (Figure 12). Remember that when the drain

valve is closed, center cavity pressure is not being relieved.

P 1

Trapped Liquid

VALVE MAGAZINE SUMMER 2023 30 Figure 7 HEAT

High pressure side when valve is closed Internal hole

HEAT

Drain type valve

P 1 = Pressure of trapped liquid between seats

Figure 9

Figure 12