We’ve all used a valve or two in our lives, right? Sink taps, shower taps, and so on are all valves, but they are not control valves because you have to set the desired amount of flow manually. A control valve can do this automatically or at least remotely. It is a widely applied device and this article will tell you all you need to know about it.
What is a control valve?
In industrial plants, valves perform the same function as shower and sink taps: they control the amount of flow in processes. However industrial valves require more precision, so we let a control system operate them rather than doing it ourselves.
The parts of a control valve
A sink tap valve has two basic parts: the body, which allows or blocks the flow, and the actuator, which we move to control the body. Usually, on a tap valve, we have a rotary actuator, and then we turn it to let more or less flow through the pipes.
Control valves, on the other hand, have three main parts: the body, the actuator, and the positioner. This part receives commands from the control system and moves the actuator accordingly. Then there are smaller, less important parts, such as bonnets, valve plugs, seat rings, and gaskets.
Control valve body types
There are many types of control valve bodies on the market. Depending on your application, one body type may offer a better performance than another.
The body part of the control valve stays in contact with the product. To handle the temperature, pressure, and chemistry of a product, we need to scale out the right material and CV (coefficient of the valve).
When we talk about valve types, we mean control valve body types. We can divide them into two groups: linear and rotating. A linear control valve uses the straight-line motion of a sliding stem to reduce or increase flow. And a rotating control valve has an element that turns within the stream to reduce or increase it.
Linear control valve body
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.
Globe control valve body
We’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 control valve body
In this scenario, the fluid passing through the valve only has one orifice to pass through. Depending on the plug, we can change the valve’s condition, with the valve closed or open in the natural position. We need to keep in mind that the product always needs to flow against the valve in its natural position.
Globe double seat control valve body
This type of valve makes the fluid pass through two orifices at the same time. Here we 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 control valve body
This variation has a completely different purpose from the single and double seat control valves. When we need to mix two products or we want to split one product into two outputs, we’ll use this one. The internal structure of the valve bodies becomes more complex, depending on the application.
Globe cage control valve
This model comes from a very old concept, but updates keep it relevant. A globe cage control valve with a single seat has a similar working principle but a different plug from the globe control valve. However, we still need to control the fluid passing through the valve against the force of the plug.
On top of that, we can get a balanced single seat, with a plug similar to what we’ll find in the double seat. The force on both sides of the plug balances, so we don’t need to apply much force to close it.
Globe angular control valve body
Here, we have a similar working principle of the globe control valve, but we can install it at an angle rather than horizontally.
Diaphragm control valve body
The diaphragm control valve has a few downsides, but it can save money, depending on the application. With an entirely different design from globe control valves, we’ll mostly find it with corrosive products, high viscosity liquids, and liquids with solids. Although it limits us in size and temperature, its construction forms an excellent seal against leakage.
Bi-party control valve body
This valve primarily offers easy maintenance for field technicians and engineers. It handles highly corrosive products, so we’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.
Guillotine control valve body
Usually, guillotine valves block low-pressure gas and air flow. We can probably guess what form this one takes. We can also get single and double seat, depending on your leakage specs.
To know more about the application of solenoid valves into control valves, you can read the Visaya Article here
Rotating control valve body types
Rotating valves have become more popular, most likely due to their cost and light weight. They come in the following types.
Butterfly control valve body
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 we can have other concepts depending on the vendor.
Usually, we’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 we work at 0 to 60 degrees, we can use a standard size. Otherwise, we will have to change it. We can have variations on this valve too – different disc lining, composite seat, and more.
Ball control valve body
For a long time, we 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. We can also find different types of seats that can influence the valve control.
Flow characteristics and plug types
We mentioned that valves can vary according to their plug types. So we 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 a control valve?
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. We’ll also need that information to calculate the noise level and coefficient of flow (Cv).
- 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:
Now, let’s have a closer look at the second element of a control valve.
The control valve actuator
The actuator opens or closes your valve according to the change you want in the flow. We can have linear or rotary actuators, depending on the valve’s body type. For example, a butterfly valve requires a rotary actuator, and a globe valve can work with a linear actuator.
We can also have three different systems to make this movement happen – pneumatic, hydraulic, and electrical. Pneumatic actuators convert air pressure to a linear or rotary movement. Hydraulic do the same with liquids instead of air. And electrical actuators use motors to convert electrical energy into mechanical torque.
All these systems work forward and backwards, or in direct or reverse actions. In direct actions, the input will push the actuator down the valve stem, and the spring will push it back up. For reverse actions, the input pushes the valve stem up, and the spring pushes it back down. Does that make sense?
To know more about control valve actuators, you can read the Visaya Article on the various types of control valve actuators
Some actuators, like pistol actuators, can come in linear or rotary versions. A lot of variations exist, but they all do the same thing in the end – move the valve to either increase or decrease the flow of the process fluid. Very simple, yet very important.
To know more about control valves, you can check out the video below or can get in touch with our engineers and we will be happy to help.