The Process Piping Best Practices Series: Valve Selection

Once material selection of the process piping systems has been considered (discussed previously), valves, fittings, and other components need to be selected for the process. The following explores the subtle details of valve selection that can make a difference in the long-term success of the process piping systems and hence the overall process line itself.

The most important step is to understand the purpose of the valve. Once that is established, the material, internal function, and cost of each potential option should be evaluated. There are valves for specific functions, such as shutoff, throttling, redirecting, backflow prevention, retaining pressure, reducing pressure, releasing air, and preventing syphoning. On the surface, the purpose of a valve may seem simple and many valves may appear adequate; however, selecting the best option takes experience. The following scenario illustrates the importance of reviewing all options:

Two parallel pumps draw water from an elevated tank and pump the water to a number of process machines or systems at low pressure. To prevent water from backflowing through a pump when one is running and not the other, a check valve must be installed on the discharge of each.

There are many types of check valves, including swing, folding disc, ball, lift, y-pattern, and cone check valves. Each has strengths and weaknesses and can serve specific purposes. In addition, there are many options for modifications and enhancements, such as spring-assisted closure, external movement dampers, and adjustable cracking pressure. In the above parallel pumping scenario, as only backflow must be prevented, it would be easy to assume that any check valve would do; therefore, the tendency might be to pick the cheapest option. Other factors, however, must be considered. For example, given the fact that the tank is elevated, were a leak to develop downstream or if a valve were left open by mistake, it could flow by gravity while the pumps are not running, resulting in a flooded room. This risk could be avoided by installing a check valve with a heavy spring to set the cracking pressure at or above the head produced by the full tank of water. As long as the pump is running, the cracking pressure would be overcome and the water could flow. However, as soon as the pump turns off, the spring would force the valve closed, preventing both back flow and leakage or syphoning.

Thorough consideration of the function of the valve and all of the possible scenarios is always appropriate, even for shutoff and throttling. Valves with open/close functionality seem simple, yet the application should be considered. For example, the valve may need to open and close very quickly, or it may be critical that the valve open and close slowly to avoid shocking the piping system or to precisely control flow.

With higher velocity flow, the risk of water hammer becomes real. Despite the name, water hammer is not exclusive to water; it is caused by sudden changes in fluid flow, usually due to rapidly accelerating pumps or fast-acting valves. Water hammer can cause pipes to move violently, often leading to failures at joints. For example, it would be risky to put a manual, lever-actuated butterfly valve in a pipe with high velocity flow, as it would enable an operator to quickly shut the valve, causing water hammer and potentially damaging the valve or the piping. In this case, a valve with a multi-turn actuator, such as a gear reducer, would ensure that the valve actuation is slow and gradual.

When throttling valves, it is common to see ball or butterfly valves used, due to their relatively low cost. Unfortunately, these types of valves do not typically provide fine control of flow. The use of a valve, such as a globe, diaphragm, or gate valve, with multi-turn adjustment and a linear relationship between percent open and flow is more appropriate. These types of valves will typically have a wheel-shaped handle that allows several full turns, from fully open to fully closed. Although these valves will tend to be more expensive, the importance of flow control must be weighed against the cost.

Most types of valves can also be fitted with pneumatic or electric actuators to allow the valves to be controlled remotely or by an automation system for flow regulation or on/off service. Actuators can be outfitted with a variety of features, such as open/closed position feedback switches, analog positioners and position feedback outputs, visual position indicators, travel time adjustments, battery backups for power failure response, and more.  Wiring of these types of actuators can be done by hard wiring each input and output to the process automation system or by network or bus systems, such as Modbus, PROFIBUS, DeviceNet, or Ethernet. Using network solutions can be highly advantageous, especially with large numbers of valves and multiple control inputs and outputs per valve.

The function of the actuator is also important. Engineers must consider the torque required, rated duty cycle, movement rate, inputs and outputs available for control, materials, IP or NEMA rating, and cost. If a problem is encountered that can only be solved by replacing or upgrading the valve or actuator, the replacement may mean much more than the cost of the parts, including labor, down time, lost production, and potential fitment issues after an installation is complete and running. In many cases, replacing a valve can necessitate fully draining a process system if isolation is not possible or practical. For these reasons, proper advance consideration of all factors to correctly select application-specific valve and actuator types is essential.