Basics of industrial final control elements 2

The control valves which are seen in most of the industry can be recognized by the actuators which are sitting on top of the valves (the poor worker) like strict bosses, making sure the valve behave as per their command. Here in this part we will concentrate on the lower part of the control valve (the worker) only and discuss what ways they are working to control the fluid(s). Here again the point is in using the word "fluid" so it covers flow of slurries, powders, granulated substances as well. This is just to give a hint that the final control elements are used for so many different applications and processes.

So far we have seen control valves or final control elements are the main devices whose performance determines how well a process can be controlled. Even if we use a very sophisticated control system and latest technologies to measure the process variable, it is this part of the whole control system which will determine the overall performance of the loop or a process. This emphasizes the need to properly understand the way a control valve functions and its main parts and components.

Let's first see the main parts of a typical control valve and how it is constructed. (figure below). The body which is mostly a casted material (carbon steel in most cases) houses the critical parts of the control valve, the plug, the seat, the bonnet. In the figure the valve body is highlighted yellow to differentiate from other parts.. Bonnet shown one side as blue normally is bolted on the valve body using a body (bonnet) gasket. Bonnet is also a machined piece. It houses the sealing system for the valve stem and some details about it will be coming in our next article. Plug and seat combination forms the main part which governs the flow of fluid and regulates it. Both of these parts are machined off and use various material of construction to suit any process. Mostly stainless steel 316 is the material for general use though Monel, Inconel or other alloys and Stellite (a Cobalt - Chromium alloy coating used on the plugs) coated plugs and seats for harsh or abrasive fluids may be used.

A "PLUG" is attached to the valve stem which protrudes externally through the top bonnet of the valve, passing thru a stuffing box which is required to seal the fluids inside from escaping to atmosphere when the valve is in operation. This cylindrical shaft (STEM) connects the valve to an actuator using a valve coupling. The other end of this stem at the plug side is either threaded, or welded or both or in some cases is machined as an integral part of the plug.

So we have a seat, a plug, a valve body, a bonnet and a packing/stuffing box with appropriate packing, which are common for almost all valves weather it is a control valve or a mechanical one. The only difference appears above these parts where an actuator is placed in case of a control valve.

Have ever done some gardening or have grown a beard? What is that supposed to mean in relation to control valves? Well it has a term common here which is TRIM. We trim the hedges in our garden once they out grow. Isn't it. So we do with the beard to give it a decent shape. Trim is just a term to give a specific shape to an item. Let's trim our discussion to the basic moving part the PLUG, which determines the characteristics of a control valve so we have to see in detail how the TRIM play an important part in controlling a fluid. According to the design of the plug, the contours seen in the figure below also known as valve TRIM, can be classified as:

1- Quick opening (top curve in the graph) the first type of plug in the first figure.

2- Linear (middle curve or rather a straight line) Second parabolic shaped one sitting on the seat in the middle

3- Equal percentage (bottom curve in the graph) having a specially trimmed shape for the special flow characteristics.

Here note one thing that the seat design remains the same and only the shape (trim) of the plug has affect on the valve flow regulation. Below we can see how flow is changing from zero to maximum at different valve opening (travel).

In the graphic representation the vertical axis also mentions a term Cv. This is known as valve coefficient and for each valve, it is calculated separately. Don't confuse this with the CV we normally submit for getting a job in a company, though this Cv will also give some job to a control valve based on the process for which it qualifies. It is the capacity of a control valve to allow flow and is calculated at standard condition. Regardless of the type of trim, basically it is the amount of water in U.S. gallons flowing through a control valve at 60 degree Fahrenheit when a difference of pressure of 1 psi exist between the inlet and the outlet of the control valve. (standard conditions).

Having said that now we have learned that the valve have different flow characteristics and different Cv. values. The Cv of a control valve makes it easier to compare different valves and to select the one which suits the process requirements. While the valve characteristics helps in deciding which type of flow is better for a process condition. For example a linear valve plug design suits more for pressure or level control use, where process value is linear. Equal percentage valve suits flow requirements while quick opening valves are more suitable for temperature controls because of the lag in temperature transfer or in conditions where a larger capacity load needs quicker action.. But this is not necessary that each process will follow the same rule. There are many other factors which may effect the performance of a valve like fluid density, viscosity and temperature just to name a few. It is the shape or trim of the valve which will determine the flow characteristics.

One more thing as we explore the control valve internals. We have seen different plug types. In process where there is more line pressure, turbulent flow, to support the plug from mechanical fatigue and failure, cages or guides are used. This is also shown and mentioned in the first figure where we have seen main control valve parts. Cages are also used in balanced plugs. This will be discussed in our upcoming articles.

Valves can then be classified as direct acting or reverse acting. A valve which will open with the increase in signal causing more flow to pass will be known as a direct acting valve. A valve which closes when signal is increased reducing the flow through it is a reverse acting valve.

If we use the same actuator for both, which will be pushing valve shaft down when signal is applied, in the first case (left), the valve will close gradually reducing the flow from maximum to zero. In the other figure, the pushing action will start opening the flow through the valve until maximum position and flow is reached when stem is pushed down by the actuator. Hence first valve will be classified as a reverse acting valve while the second will be a direct action valve. Though this configuration of action change is not very common, but it is explained here to clear the concept of a direct or reverse acting control valve. Normally actuators are designed to take care of the valve actions and in some cases the valve positioners also are used when a direct acting valve needs to be changed into a reverse acting valve. But let's leave this part for the time being and will be discussed when valve actuators and control valve positioners are discussed.

Here we should not confuse this with another term used for control valves which is their failsafe condition. Fail open or fail close condition of a valve which mainly depends on the actuator type and will be explained when we talk about the actuators.

So far we have discussed the general arrangement of a control valve and what are the different valve plug TRIMS and CHARACTERISTICS. What is a direct and reverse acting valve, from the way a control valves is constructed. In the next few articles we will be discussing further about the valves, actuators and the related devices, so we will be unfolding other basic information to better understand what is a CONTROL VALVE and how it is functioning to perform the desired tasks, before moving to other final control elements.

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(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)