Valve Selection and Sizing Considerations
A simple approach to valve sizing would be to determine the required valve coefficient (C v ), as defined in ANSI/ISA Standards S75.01:
C v = Flow ( specific gravity / pressure drop) 1/2
where flow is in US gallons per minute and pressure drop is in pounds per square inch (psi) at 60°F (16°C). A designer would also check the maximum anticipated flow rate and temperature combination to avoid choking or flashing conditions. The most extreme flow rates are likely to occur during a transient.
Bentley HAMMER CONNECT is the most versatile design tool for valve sizing because it allows you to simulate the operating conditions a valve is likely to encounter during steady-state or transient events. Bentley HAMMER CONNECT models valves differently depending on their response time. The principal difference between flow-control and surge-control valves is their response or activation time:
Flow control valves—The majority of valves in a water system are intended for on/off operation (i.e., they either allow or block flow). In addition to this, flow-control valves throttle flow using various methods that depend on the valve body, piston or pinch mechanism, and actuator. Although special trim is available to deal with sustained high-velocity or high-pressure differentials, most flow-control valves are not designed to react to or handle transient conditions for any length of time. They are typically actuated to ensure a slow opening or closure. Actuators are typically hydraulic, electric, or (less often for water systems) compressed air:
- Hydraulic actuators—Small-diameter tubes called pilots are connected upstream and downstream of the valve and the difference in pressure between these points is used to open or close it. The type of valve depends on how the upstream and downstream pilots are connected to the valve body and/or drained out of it to ambient, or atmospheric, pressure. The term piloting is often used to describe the hydraulic (and sometimes electrical) circuitry and connecting tubes.
- Electric actuators—These are motors coupled to gear works to ensure a gradual opening or closure. In water systems, electric actuators are most often used to operate large isolation valves, only some of which may be connected to backup or emergency power (for use during a power failure). Typically, a manual over-ride and hand wheel is also provided for each valve. The gear ratios are set so that a large number of turns is required on the wheel to fully open or close the valve. Even for the fittest operator, this ensures that the valve cannot be closed too quickly, to prevent water hammer.
- Compressed-air actuators—Compressed- or instrument-air actuators are far more common in industrial settings, where valves and flows are typically smaller than in water or wastewater systems (e.g., typically m3/hr. instead of m3/s, respectively). The compressed air is typically maintained at a set pressure and some reserve capacity is usually stored to allow operations to continue after a power failure. Since compressors are required to maintain pressure in a gas vessel, it is possible to use such actuators nearby, but this is rarely done.
Surge-control valves—The majority of surge-control valves are sized and actuated to respond very quickly to hydraulic transient conditions and to handle far greater flows and pressure drops than flow-control valves (albeit for shorter times). Small tanks containing compressed nitrogen or other special gases are sometimes provided to help valves open more quickly. The piloting is typically designed to respond to sudden or gradual changes in pressure or even to the rate of change of pressure. Hydraulic or compressed-air actuators are preferred because these valves are typically installed to protect against a power failure or sag, during which electrical actuators may fail to operate. Because hydraulic transients occur so quickly in most systems, the time required to bring backup power on line is often too long to be of use during transients.
Any valve can initiate a hydraulic transient if it is opened or closed too quickly with respect to the system's characteristic time, or if it is operated in an uncontrolled manner. Uncontrolled operation can occur due to a failure of hydraulic piloting to react during very high reverse-flow velocities, for example. This illustrates the importance of sizing a valve to handle the full range of flows it will encounter during its service life. Another example is that instrument-air pressure can fail to reach a valve at the correct flow rate or pressure, due to clogged filters or worn orifices, incapacitating its compressed-air actuator.
It is essential to follow the valve manufacturer's selection, sizing, and maintenance schedules to avoid specifying a valve that is unsuitable for a specific application. A critical first step in the process of sizing surge-control valves is to perform a thorough hydraulic transient analysis using Bentley HAMMER CONNECT to determine the normal and transient conditions the valve will encounter during its entire service life (e.g., for current, interim, and ultimate water-supply conditions and surge-control scenarios). Improper selection or sizing of surge-control valves can result in worse transients than if no protection were installed.