Control valves in plastic or PFA/PTFE lined.

Control valves - for precisely regulating flows

Control valves accurately regulate flows and are available in several different designs and can be adapted to different needs. Although the needs vary, the valve's task is to regulate a flow so that, for example, the desired pressure, temperature or flow is obtained. The most commonly used control valves are plug, diaphragm, ball and butterfly valves.

control system

To control the process, the control valve is connected to your control system, sometimes called a PLC or DLC. In the system, you probably have an analysis tool that checks the desired flow and compares it to the actual flow. If the flow deviates, the system sends a signal to the valve positioner, which in turn controls the valve actuator. The actual control of the valve is usually done using pneumatics, electricity or, in rare cases, hydraulics.

Ball valve as a control valve

In English, the valve is called a globe valve, and this valve has a cone that you control up and down like a wedge valve, for example. The valve's cone is mounted on a threaded and rotatable spindle that we control with a knob or with an actuator in an automated control system. The ball valve is often the best control valve as it is very precise in its regulation. Unlike many other valves, the ball valve can be modified with different sizes and designs of the ball and valve seat to obtain very precise Kv values (see below for a description of what this is).

Benefits:

• Very careful regulation
• Efficient and precise throttling

Disadvantages:

• Provides low kV
• Provides high pressure drop, requiring higher pump capacity and system wear.

Lyma can offer plastic plug control valves or PFA/PTFE lined valves in dimensions DN15-100..

Regulate flow with diaphragm valve

Diaphragm valves can also be used as control valves. In this case, we have a diaphragm that, like the ball valve, seals in a linear direction. Unlike the ball valve, we have no need for stem sealing, which makes it suitable for use with corrosive liquids.

Benefits:

• Copes well with corrosive liquids
• Can handle liquids with particles.
• Relatively cheap valve
• Easy maintenance

Disadvantages:

• Not as accurate in regulation as a plug or ball valve.

Ball valves can also be control valves

The ball valve is perhaps the most common valve type in industry. As a precise control valve it is not as well known, but it can certainly be used in some cases. Unlike the valves above, here we have a valve that opens and closes using a rotary motion. The ball that gives the valve its name has holes and it is the position of the ball that determines what flow is possible. When the ball valve is used as a control valve, we often see that the hole pattern on the ball is specially designed on control ball valves. Like the ball valves, it is possible to modify a control ball valve to obtain different Kv values.

Benefits:

• Relatively cheap
• Good flow rate when fully open
• Withstands high pressure and temperature
• Easy to manoeuvre, quarter turn

Disadvantages:

• Can cavitate
• Not suitable for liquids with particles such as slurry. This is because the valve can become blocked.

From Lyma Kemiteknik we have the opportunity to offer ball control valves both completely in plastic and as PFA/PTFE lined valves. The plastic valves are available in dimensions DN 15-50 in materials such as U-PVC, C-PVC, PP and PVDF. The lined valves are available with different PFA/PTFE lining and with their valve body in cast iron/seal iron in dimensions 15-200. Lined valves can withstand higher pressure and heat than standard plastic valves.

Larger control valves, butterfly valves

When the ball valve is no longer sufficient, we switch to the butterfly valve. Here we have a disc instead of a ball and it is this that regulates the flow. Due to its design, the butterfly valve is not suitable for precise regulation. Butterfly valves are not available in the smallest dimensions, but are available up to DN 1200.

Benefits:

• Low cost and easy maintenance
• Capable of high capacities
• Low pressure drop
• Available in large dimensions

Disadvantages:

• Risk of cavitation at low flow rates
• Are not very careful about regulation.

Lyma supplies butterfly valves in various plastic materials and also in PFA/PTFE lined butterfly valves. The advantage of plastic butterfly valves in large dimensions is that they are much lighter than valves with metal bodies. The wetted parts in a such valve can be in the PDCPD which is a corrosion-resistant polymer. Click here to access our section on butterfly valves. 

What is meant by Kv and Cv?

When it comes to valves, we often come across the term Kv. The Kv value gives the valve's capacity at a given opening, and the measure is given in m3/h for 16 degree water where the differential pressure across the valve is 1 bar. Larger valves give higher Kv than smaller ones, while a fully open valve also has a higher Kv value than a more closed valve.

Cv is the American equivalent of Kv. Instead of cubic metres, the American counts with US gallons/minute. This applies at 60 degrees Fahrenheit and a pressure differential of 1 psi (pound force per square inch).

To easily convert Kv to Cv, you can use the formula: Kv = 0.864 * Cv, or Cv = 1.156 * Kv.

Sometimes you will also see Kvs, which is the Kv value of the valve when the valve is fully open.

The characteristics of the control on your control valve

When it comes to control valves, we see concepts such as linear, equal percentage or logarithmic characteristics. Each valve has different throttling abilities and, especially with plug valves, we can design our own throttling characteristics. Because of the inherent flow characteristics in the system, we usually want to use an equal percentage control valve to regulate linearly in practice. Different valves have different characteristics and the needs of your flow system will vary. Below we see a picture with different valve types. Valves with a fast opening are best suited for on-off, while valves with equal percentage characteristics are usually better for precise control. For example, equal percentage characteristic is suitable for level control. Equal percentage characteristics are often used for flow, pressure and temperature control. So note that the inherent flow characteristics of your system affect the choice and effect of the valve characteristics.

Choosing the right valve

To choose the right control valve, we need to know what flow you have before the valve and especially what flow you need after the valve. If the valve's Kv value is too small, the valve will stop your flow and if the Kv value is too large, we will not be able to regulate your flow. Usually we go down in dimension when we use control valves compared to a shut-off valve.
Each control valve has its own Kv value and we have to look at the valve's table which dimensions are relevant in our case.

However, as a customer, this is not your task, but we will of course help you.