Control valves basics
We’ve all used a valve or two in our lives, right? I hope so! Sink taps, shower taps, and so on are all valves that we use in our daily routines. Yes, I know some are hands-free, but most still aren’t. Anyway, these aren’t control valves. You have to manually set the position of these to get your desired flow.
In industrial plants, valves perform the same function as shower and sink taps, to control the amount of flow in processes. However, valves here need more precision, so we let a control system operate them rather than doing it ourselves.
So without further ado, let’s have a deeper look into control valves!
A sink tap valve has two basic parts: the body, which allows or blocks the flow, and the actuator, which you move to control the body. Usually on a tap valve you have a rotary actuator, and you turn it to let more or less flow through the pipes. We’ll talk more about actuator types in a bit.
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. We’ll get to that in a second too.
So the body of the valve dictates the movement of the flow through the pipeline. Some common valve bodies are ball, butterfly, gate, needle, and diaphragm. Each type has its own mechanical system to control flow. These variations can have a big influence on a process, but going over each type would take a whole other article. If you want to know more, feel free to drop us a message!
Now, let’s have a closer look at the second element of a control valve.
The actuator opens or closes your valve according to the change you want in the flow. You can have linear or rotary actuators, depending on your valve’s body type. For example, a butterfly valve requires a rotary actuator, and a globe valve can work with a linear actuator.
You 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?
Some actuators, like pistol actuators, can come in linear or rotary versions. As you can see, 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.
This video has visual representations of valve actuators:
The positioner receives signals from the controller and adjusts the actuator to the desired position. Today you’ll see a lot of digital positioners, but you can still find pneumatic positioners in the field. To explain how the system works, we’ll run through the pneumatic mechanism first, then go into how a digital one differs.
The pneumatic positioner
In a pneumatic system, the air supply is the main element. The instrument input for pneumatic systems ranges from 3 to 15 pounds per square inch (psi), just like analog electronic control systems have the good old 4-20 milliamps (mA). The output – air, in this case – moves the valve actuator.
In the instrument input, the air pressure acts first in the input bellows, making it either expand or compress. This bellows connects to the beam, which measures the feedback from the valve stem through the input cam. The flapper also connects to the beam, so it also moves when the input bellows inflate or deflates. You can get a better idea of what I’m saying – writing rather – in this picture.
As the beam moves, it shifts the flapper toward or away from the nozzle. This causes the relay to either increase or reduce the pressure on the valve actuator. When the valve stem moves the rotary shaft arm, it makes the cam rotate, which sends feedback to the beam. And the cycle goes on and on.
This process continuously adjusts the valve position based on the input signal. Depending on the relative position of the follower assembly, you’ll have either a direct or reverse action. Usually, if the follower is on the left side of the beam, then you get reverse action. On the right, you get direct action.
This video shows how a pneumatic positioner works:
The digital positioner
Besides the all-mechanical valve positioner, you’ll also find pneumatic valve positioners with transducers and smart or digital valve positioners.
In a pneumatic positioner with a transducer, the valve itself works the same way. However, it receives a different signal from an analog system. Therefore you need the transducer to convert the analog signal to 3 to 15 psi to operate the positioner.
In a smart or digital valve positioner, a microprocessor communicates with the control system via digital field protocols. It checks the position of the valve using the feedback arm, either with a change of resistance from a potentiometer or with the Hall effect. A relay controlled by the electronics reduces or increases the output to the valve. A big advantage here is that smart positioners offer diagnostics for you to check.
The video below explains more about digital valve positioners:
The flow coefficient (Cv)
Before we wrap up this topic, we need to talk about Cv, the flow coefficient of the valve. It applies the factor of the pressure drop (ΔP) or head drop (Δh) over a valve with the flow rate Q. The formula can differ depending on the product the valve regulates, such as gas, liquid, or steam.
Also, the Cv varies depending on the control valve, since each valve has its own way to let flow pass through it. The difference of Cv relates to the position as well as the type of the valve. When you have to scale out a valve, you’ll need to know the Cv to pick the right valve for your application.
Some vendors have standardized the flow coefficient (Kv). They created this standard using such reference conditions as water at a specific flow rate, temperature, and pressure drop units. Keep in mind that Kv uses metric units and Cv imperial. To convert them, you’ll need these formulas:
Kv = 0.865 · Cv
Cv = 1.156 · Kv