VALVE MAGAZINE Summer 2023

the amount of flow at operating con ditions (Figure 16). This is particularly useful in throttling applications where the system is dependent on flow con trol to optimize performance. Scales may be affixed to the valve topworks to measure the valve stroke, which can be correlated to flow curves to accurately control the actual flow through the valve. Where tighter con trol such as modulating is required — and where consistent throttling at less than 20% open is expected — control valves, which incorporate valve operating systems designed for the application, are recommended. It is common to include an equalizing pipe to the non-pressure side to help balance the valve, increase lift, and control turbulence. Pressure seal globe valves are not subject to CCOP, pressure locking, or thermal binding; however, the effects of high temperature (e.g., thermal expansion) must be evaluated on component parts (stem, seats, etc.), especially when the valve is to be actuated. PRESSURE SEAL CHECK VALVES The primary responsibility of pressure seal check valves is to seal against a system flow reversal, thus protect ing piping and components (pumps, instruments, etc.) not designed to handle that condition. They are sup plied in the same materials, pressure classes, and orientations (vertical and inclined) as pressure seal globe valves. Selection of check valves is typically based on a number of vari ables, including system flow charac teristics (e.g., Cv, velocity), media (e.g., type and size/concentration of particulate), and plant operating characteristics. PRESSURE SEAL CHECK VALVES MAY BE SUPPLIED IN CONFIGURATIONS, AS FOLLOWS: 1. Swing Check (Figure 17). Pressure seal swing check valves are commonly used in combination with gate-type isolation valves for reverse flow pro tection. Their relatively higher Cv (vs. piston checks), simple operation, and relative ease of maintenance make

n Bypass with one or more bypass valves and an equalizing pipe joining the center cavity of the valve with the bypass pipe. Depending on the number and configuration of the bypass valves, bi-directional sealing may be maintained (Figure 6). n Proprietary bypass valves that change sealing direction as does system pressure. Bi-directional sealing is maintained (Figure 13).

Figure 15

isolation, etc.). Pressure seal globes may be supplied in the same material types, actuation varieties (manual, gear operated, motor operated, pneu matic, electrohydraulic, etc.), trim combinations and materials, and ASME pressure classes as their gate valve counterparts. They can be supplied in a stem vertical (Figure 14) or inclined (Y-pattern) orientation (Figure 15), as a function of required flow (Cv). Pressure seal globes may be supplied with the disc mechanically affixed to the stem so that, when in the open position, flow is free to occur. However, they may also be supplied with the stem freely floating in the disc pocket

PRESSURE SEAL GLOBE VALVES

B

A

C

Figure 13

Pressure seal globe valves are utilized in applications where some degree of flow control (or throttling) may be required (e.g., in plant startup or shut down modes). They are well suited for isolation applications in power plants (e.g., main steam isolation, feedwater heater isolation, boiler/economizer

SUMMER 2023 VALVE MAGAZINE

Figure 16 (see Figure 16). In this orientation (stop/check), when the valve is in the open position, the valve disc will close when a reversal of flow occurs, thus providing a check valve function in addition to the basic stop (or isolation) function. The profile of the disc in globe valves may be modified to more finely control

Figure 14

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