The specification of automated valves requires certain considerations that are not of concern with typical manually operated valves. The blog Valves – Automated Valves – What’s Different? provided a glimpse of automated valves with a very simple (but typical) example. There are, of course, innumerable variations used in the design of automated valves so the information here may not apply universally but is intended for awareness purposes only. Each case should be reviewed individually. This blog will endeavor to discuss in more detail what these differences mean in the actual application of automated valves.
In most cases, manual valves don’t really care much about the direction of flow. A ball valve, for example, couldn’t care less if the inlet and outlet are reversed, it will work equally as well in both directions. There are usually no serious consequences of reversed direction of flow in globe valves and gate valves although they are intended to be used in a particular orientation. If the valve includes other features like, for example, a pressure relief on the inlet or outlet or an integrated union, then orientation may be important for that feature to be functional but as far as the valve goes, it just doesn’t care.
In a majority of cases, however, automated valves are intended to and ONLY work when the flow through the valve is in a particular direction.
As the above illustration shows, many automated valves depend on the inlet pressure to assist in closing and holding the valve closed when flow is not desired. The spring force alone is often not sufficient. Similarly, if the flow through the valve is reversed, the pressure of the spring is not adequate to prevent the flow of liquid through the valve in the reverse direction. In specifying an automated valve, it must be determined that the incoming pressure will always be sufficient to assure valve closure. The valve also must not be relied upon to prevent flow in the direction opposite to that of the intended flow.
An especially good example of a potential problem with automated valves is their application in tank drain lines. Since the pressure at the drain is usually provided only by the depth of liquid above it, many automated valves can not be relied on to close and stay closed under this relatively low pressure condition. The problem becomes of even more concern if there is any potential for back pressure on the drain from, for example, other drains connected to a common manifold. If more than one drain source is flowing to the manifold at the same time, the resulting back pressure may force liquid back through a valve that is intended to be closed.
Just as there can be too little pressure for an automated valve to work properly, there can also be too much pressure at the inlet. In order for the valve to open, the actuator (solenoid or whatever) must be able to overcome the inlet pressure that is assisting in holding the valve closed. In high differential pressure applications, an automated valve with a “pilot operator” may be required. In this type of valve, a solenoid (or other actuator) first opens a small orifice which allows a sufficient flow to relieve pressure on the main orifice to allow it to be fully opened by the available actuating force.
Clearly, there is a lot more to know about automated valves than can be covered in this short blog. The important thing to take away from today’s discussion is that automated valves are different and require different considerations than manually operated valves. Additional tips on effectively using automated valves will be a subject for upcoming blogs.
– FJF –