Basics of industrial final control elements 4

Final control elements in the industry is the basic discussion which had been the topic of our last three articles and in this one our first in line the CONTROL VALVE is further being discussed here. When we discussed the leakage class in our last articles, we have seen that the working pressure and test pressure criteria should be met to define leakage class. Pressure also has some adverse effects in valve operation. In the figure below for the single or double seated valves, as the line pressure at the inlet increases, it will tend to put pressure on the valve plug pushing it up. In some cases when valve is in the closed position the downstream pressure develops and it will not allow the plug to move up. This pressure unbalance can sometimes effect the valve performance.

To overcome the pressure in first case, we can use a strong spring in the actuator to keep the plug from lifting due to process pressure. But at the same time this tension of the spring may slow down the valve response. How to overcome this unbalancing force is what we are going to see next in this article. A challenge for instrumentation design engineers. But it is met when we used a balanced plug along with a cage. Here CAGE is the keyword. If you have read the last article thoroughly we have mentioned the terms cage and graphite, soft seals and Teflon rings when discussing leakage classes. Here we will see how these items are used not only to reduce leakages, but to overcome the process or line pressure without going to alter actuator spring. Wait a minute.... some of us can ask here that we do use a bench set for actuators. This concept however we will take when we are talking about the valve actuators. The plugs shown above are known as unbalanced plugs and these are also in use in process industry because of their simple design and lower cost of manufacturing. However process engineers have the option to use either balanced or unbalanced plugs based on different process requirements, budget and for sake of maintenance in some cases. So what are the balanced plugs and how they differ from the unbalanced ones. A balanced plug have holes drilled through them. Oh... what? Why do we need holes in a plug. A plug with holes? How it will stop (plug) the flow in closed condition? These were the questions came to my mind in my earlier years when this concept was being explained. Because a plug should stop the flow. But having holes through them???? How we are going to stop flow with a hole in the plugs?

The hole concept to balance the plug have different arrangement to stop the flow. It is the seal ring(s) which will be closing the path of flow while the hole in the plug will balance the pressure on both sides of the plug . Brilliant idea, since the pressure inside the body on two sides of the plug is neutralized. This is the whole (hole) concept. So let us have a look on the balanced plug whole construction.

When we see the whole picture, we will realize holes are not bad for our plugs. The cage has opening holes as well on the sides, not clearly shown in the figure here, but we will skip it here. Or if you are curious just scroll down where three different cages are shown. The contoured openings will allow flow to pass from inlet to the outlet once the valve plug opens upwards. The figure shows a balanced plug (see the holes on top around the stem) the cage and the seat of the valve. form a complete set.. A Fisher control valve trim set as it is normally known is shown in the next figure. Now here comes a close up of the balanced plug in real life..

The plug or rather the balanced plug as shown above has the holes, and normally cylindrical in shape. Typical Fisher control valve replacement trim is shown in the figure. Now we know how to overcome the unbalancing force in a control valve. But how do we get the TRIM characteristics namely quick opening, linear or equal percentage in these valves as the plug is just a cylindrical piston type part?

The answer lies in the figure below which shows CAGES with different trims rather than the plug. The contoured cage holes create exactly the same flow characteristics in a balanced trim system as earlier we have explained for unbalanced plug types.

Top left unbalanced plugs with three type of trims, below them balanced plug cages with same trim types and right side the characteristics flow of these plugs The three type of trims discussed in our topic with caged balanced plugs are therefore carved or trimmed as shown in the figure above with linear, equal percentage and quick opening characteristics. The plugs on the top are unbalanced as we have discussed earlier, while the cages down are used with the balanced plugs . Notice the different designs of opening which corresponds to the characteristics of each type of valve trims. Here we come to know firstly that a picture is worth a thousand words... or so the saying goes. But surely it helps in the explanation which would have taken a long long text.

Then comes the next part which is required by a balanced plug to perform its function as a PLUG. Yes it is the seal ring or rings, which are important part of a balanced plug in control valves and are manufactured using various materials and are of different types, shapes and sizes as well.

The plug as we have seen in the figure above has grooves in which seals are inserted which are necessary for the plug to perform its function as a plug. There we can have a single seal groove or multiple seal grooves usually up to four in a row. More common are single or double seal ring sets. Most manufacturers use these type of seals on the balanced plug. Some also use spring loaded seals as well like in the one shown in the photo below.

Several valve manufacturers use this type of seals as well. IMPORTANT: This is a directional seal where the spring side should face the upstream side of the plug. This will help in a better seal since the U type construction will also act as a unidirectional check valve and will expand with fluid pressure to make a better sealing arrangement. I have seen a couple of instances where the valve newly serviced, still reported having passing problem when tested for leak, and on re-inspection found the seals were not inserted the correct way. This type of seals are also common in hydraulic cylinders and actuators. There are numerous other configurations, but our main idea is to show how the fluid is sealed by a seal ring in the balanced plugs when the control valve is fully closed. Here we will seal our discussion for this week, though still many more aspects of the control valves, actuators and relevant devices will be taken up in our upcoming articles.

Newsletter INSTRUMENT REVISITED is about providing general information about the industrial instrumentation aspects. Feel free to share it if you like it. Also suggestions, comments and feedback will help to improve the articles.

Please note that though the accuracy of the information provided here is based not only on reliable sources, but also based on personal experiences, but it is for providing a general picture and the author is not liable for any loss or damages arising from the use of this information in any instance.

This and all the articles published so far are NOT artificial intelligence (AI) based, but are a result of research and personal experience or (HI) Human Intelligence based.