What do you need to know about control valves?
What’s a control valve? Before we talk about control valves themselves, we’ll go over where you apply them and why
What’s a control valve?
Before we talk about control valves themselves, we’ll go over where you apply them and why you may need them. This may seem too basic for some of you, but we want to have all our readers on the same page. If you want more advanced content, then you can find it here.
The type of control you want will dictate the elements in your loop. For monitoring, you only need a measuring device to discover the data and a control system to collect and store it for you.
If your application requires control of the product to achieve a particular setpoint, then you’ll need a third element beyond the measuring device and control system, a final control element. Here, the system will have a control strategy and a setpoint for this process variable. The final control element can be a control valve, pump, motor, damper, etc.
The valve, one of the most popular final control elements, can handle products from gas to solids in a variety of segments. Simply put, a control valve creates an obstruction in the pipe. It can open and close, allowing more or less product to pass, to achieve the setpoint of the control system.
Now, let’s take a closer look at the types of control valves. The right valve for you will depend on your process and the results you want. We reviewed linear and rotating valves in this previous article, but now we’ll go in depth on each.
Linear control valves
As you can guess, a linear valve has an opening and closing element that moves in a straight line. So let’s take a look at the various types of linear control valves.
You’ll find many globe valves around the globe, and they come in two types as well, the single seat and the double seat. Let’s understand the difference between these two models.
Globe single seat
In this scenario, the fluid passing through the valve only has one orifice to pass through. Depending on the plug, you can change the valve’s condition, with the valve closed or open in the natural position. Keep in mind that the product always needs to flow against the valve in its natural position.
Globe double seat
This type of valve makes the fluid pass through two orifices at the same time. Here you can also have the natural position of the valve as closed or open. However, the plug has a different design, because it should open and close two orifices at the same time.
Globe 3 way
This variation has a completely different purpose from the single and double seat control valves. When you need to mix two products or you want to split one product into two outputs, you’ll use this one. The internal workings of the valve bodies gets more complex, depending on your application.
In the first diagram, you have fluid entering points B and C, with them mixing to exit point A You can control the mix of the products entering the valve by adjusting the plug.
The second valve splits the flow from point A into points C and B. Here, you can control the quantity of the product going to C and B by adjusting the plug.
This model comes from a very old concept, but updates keep it relevant on the market. A globe cage control valve with a single seat has a similar working principle but a different plug from the globe control valve. However, you still need to control the fluid passing through the valve against the force of the plug.
On top of that, you can get a balanced single seat, with a plug similar to what you’ll find in the double seat. The force on both sides of the plug balances, so you don’t need to apply much force to close it.
Here, we have a similar working principle of the globe control valve, but you install it at an angle rather than horizontally.
The diaphragm control valve has a few downsides, but it can save money, depending on your application. With an entirely different design from globe control valves, you’ll mostly find it with corrosive products, high viscosity liquids, and liquids with solids. Although it limits you in size and temperature, its construction forms an excellent seal against leakage.
This valve primarily offers easy maintenance for field technicians and engineers. It handles highly corrosive products, so of course you’ll need to maintain it regularly to keep it working properly.
The design of this valve makes it easy to replace the internal parts. Unfortunately, it costs more than other valves, because chemical compatibility with corrosive products means expensive materials.
Usually, guillotine valves block low-pressure gas and air flow. You can probably guess what form this one takes. You can also get single and double seat, depending on your leakage specs.
Rotating control valves
Rotating valves have become more popular, most likely due to their cost and light weight. Of course, they also have pros and cons. So let’s review.
This ring-shaped valve has a disc in the middle, working as the plug. Basically, the seat of the body is its own walls, but you can have other concepts depending on the vendor.
Usually, you’ll use a wafer process connection from 2 to 24 inches and a flange for bigger sizes. Most vendors recommend keeping its operating range between 0 to 60 degrees, considering the force necessary to open and close the valve from 60 to 100 degrees. When you work at 0 to 60 degrees, you can use a standard size. Otherwise, you need to change it. You can have variations on this valve too – different disc lining, composite seat, and more.
For a long time, you saw a lot of this type in on/off controls, but it has become more common in graduated control as well. Here, the valve has a ball as the plug, which allows full or partial flow through the valve.
It works with a double seat and has great leakage specs compared to other models. This one counts as a non-balanced valve because the fluid will try to close it and you don’t have that force balanced. But it can also work in both directions of flow.
Different types of plugs can augment this valve. One allows the full flow to pass through the valve at 90 degrees of openness. And another will limit the fluid passing through the valve at 40 percent. You can also find different types of seats that can influence your valve control.
We mentioned that valves can vary according to their plug types. So you should know the flow characteristic created by the movement of the plug from the seat. In calculations, we have constant pressure, but reality has different ideas.
We have two concepts to define flow characteristics, inherent/intrinsic and effective/installed. We base inherent flow on lab tests and the characteristics of the plug and its seat. And we derive effective flow from the real process based on the process conditions.
So let’s talk about the flow characteristic curve from the inherent flow.
Here, the control valve will offer a huge flow variation using a small variation of the plug. For instance, when you have 25 percent of the range, you almost have full flow passing through the control valve. This type of valve works well for on/off control.
This type of curve has a proportional variation of the movement with the flow, so it graphs as a straight line.
In this scenario, equal addition of the valve movement will create a percentage proportional to the addition of the flow rate. Take a look at this equation:
Q = Q0 eKx
Q0 = Initial flow
K = Log R / x max
x = Valve stroke
R = Q max / Q min
The modified parabolic doesn’t have a proper description. Basically, it represents a flow characteristic between the linear and the equal percentage.
What do I need to know for scaling out?
We won’t dive into the scaling out process step by step, but we’ll fill you in on the information necessary to scale out a new control valve. You’ll also need that information to calculate the noise level and coefficient of flow (Cv). Read more about Cv here.
- Flow range – minimum, normal, and maximum
- Pressure upstream and downstream of the valve for minimum, normal, and maximum flow
- Fluid characteristic
- Type of fluid – gas, liquid, mix of gas and liquid
- Temperature range
- Viscosity (for liquids only)
- Vaporization pressure (for liquids only)
- Pipe characteristics
We can define the leakage classification of control valves as the maximum leakage allowed to pass through a completely closed valve. This table will give you this information based on the standard ANSI-B16-104:
And this video has more details on control valves: