Basics of industrial final control elements 6

The control valve so far under discussion has been a globe valve. Here we will see what are the other type of control valves we have in industry, though most of the times, it is the globe valve which is used to control a process parameter. In the beginning of these series have we mentioned some valve types? Yes we did..... which were.....

Ball valves....

Butterfly valves....

Gate valves....

and of course our Globe valve....

These are also used in industrial control for different process applications.

So let's see how these valves work in industrial applications. Other most common one type of valve used in the industry is the ball valve. These are also of different types, like a floating ball valve or trunnion type ball valves as far as its construction is concerned. Then there are full bore valves, reduced bore valves and V notch ball valves to name a few. I hope you are not getting bored by this topic which is so exhaustive.

Ball valves in general do not have good control properties, but specifically to improve the control action V notch balls which have a V shaped slot are used. When the ball is opening, the starting flow will start to increase slowly compared to a full bore, or reduced bore ball valves because of this design and gives a better flow control.

These have quite good control characteristics as compared to a other type of ball valves. Siemens power gas turbines uses the V notch ball valve in some instances. It is use for basically controlling the speed of the turbine by throttling the fuel, and as per my personal experience work very efficiently. Full bore ball valves are mostly used in industry for emergency shutdown (ESD) purpose, since it has fast closing time as low as 0.2 seconds in some cases.

Butterflies are beautiful. I am talking about this insect whose name is adapted in industry because of the way it opens/closes its beautiful wings. Although not related directly, these valves have a disc in the line in contrast to a ball valve which has a ball with a hole. The disc rotates around its central axis to open or close, Butterfly valves also have different construction for specific use.

The valve is also used for control purpose in the industry. It is usually coupled with a rotary type actuator though linear actuators also are used by some manufacturers.. Since these valves have different characteristics than globe valves, it has more flow capacity with less pressure drop. So it is considered in those processes where the pressure drop can have a significant impact (specially causing cavitation or flashing) in a process or flow demand is higher.

Then comes the gate valves. In contrast to the gates in our homes which normally swing in or out to open or close, there is a sliding action within, which throttles the flow. The gate valves are mainly used for isolation (mechanical) and are least common in process control, but in fluids like powders or slurries it is a good choice.

Fluid catalytic cracking units where powdered catalyst is used, the process has gate valves, and some have it at an inclined angle. These are also termed as slide valves where the gate slides in and out to throttle the fluid control as required. Because of solid/powder services these valves have Stellite coatings which hardens the surface reducing wearing of the gate or slide. Stellite is a Cobalt based alloy with Chromium, Tungsten and Carbon. Carbon quantity is higher than in Carbon steel and sometimes Nickel and Molybdenum are also included. These valves are used mostly with hydraulic actuators (for greater force requirement) and form an important part of industrial control system.

For all these type of valves, different connection types can be observed from the above figures. It can be a wafer type construction without any flange, threaded connection for small sizes, or flanged connection with flanges corresponding to appropriate line pressure ratings. In other cases welded connections are also used specially for high pressure systems. The selection of connection type depends mostly on the process type and process conditions and hence here we have a wide variety of available options..

Concluding our discussion for the week, we can say that whatever the type, make, size or acting mechanism a valve always serves its basic function to throttle the fluids at the desired rate across its inlet and outlet, close the flow completely when required as well. The next issue will be dealing the same topic but we will see what more is there to explore.

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