VALVE MAGAZINE Summer 2023

When the valve is cycled, these seats slide over one another. The two discs are held apart by spring(s). Parallel slide valves are said to be position seat ed, in that optimal sealing is achieved when the disc and seat ring mating sur faces are most closely positioned (i.e., no amount of additional turning of the hand-wheel will effect better sealing once the seating surfaces are mated). Other designs are available including “two-piece” and “double-disc” wedge types that have proven to be effective. WHICH VALVE TO USE? So why choose one over the other? Parallel slide valves rely almost entire ly on upstream pressure to affect a seal on the downstream seat. Because of this, at low-pressure conditions, leakage through the seat may occur. In addition, the sliding action of the seating surfaces is more prone to wear, and if particulate is trapped between the seating surfaces, damage is more likely to occur. Due to the width and orientation of the seats, parallel slide

valves are more difficult to maintain than their flex-wedge counterparts. But before you replace all your parallel slide valves with flex-wedge gates, read on. Because the flex-wedge gate valve requires torque to energize the sealing surfaces (and then “un-wedge” itself), an actuator capable of providing com paratively higher torques (to parallel slide valves) is required, usually at a higher cost. Then there’s the thermal binding issue. At operating temperatures approxi mating 800°F and higher, wedge-type gate valves have the potential to bind in a variety of modes (e.g. when exposed to high operating tempera tures, closed, then allowed to cool; or as is common in startup, beginning at ambient temperatures, exposed to a rapid thermal transient, then opened). This phenomenon is dependent on a wide variety of design and operating conditions, but can be mitigated by incorporating operating procedures that verify thermal equalization

between the upstream and downstream bores (delta T of 200°F or less). Bypass es (Figure 6) connecting the valve upstream and downstream bores can facilitate this thermal equalization. Care must be taken to verify upstream vs. downstream thermal equalization at the valve itself, not away from the valve on connecting pipe. The new generation combined-cycle plants with their comparatively (with coal-fired plants) fast startup procedures are par ticularly susceptible to thermal bind ing. However, the most effective way to guard against thermal binding is to

Equalizing pipes

Bypass equalizing valve

Figure 6

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SUMMER 2023 VALVE MAGAZINE

Uh, okay . . . but what is it, exactly? Changing out valves on petrochemical storage tanks is messy or expensive, or messy AND expensive. But now comes the Valve Xchanger – a patented, unprecedented tool developed by an oilfield roustabout that enables you to change out the valve of a liquid-filled tank in minutes — without hiring a vac truck crew or incurring a nasty HAZMAT spill. Check out the demo video at www.ValveXchanger.com .

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