Fluid control valves are used widely in industry to enable processing plants to maintain full control of the flow of media through their pipelines at all times.
However, there are many types of fluid control valves available, and selecting the right one for your application and media type is critical to ensuring that you have the level of control you require.
We take a look in this article at how fluid control valves are used, the types of functions they are used for and we take a deeper delve into five of the most common types of the fluid control valve.
What is a fluid control valve?
A flow control valve is a mechanical device used in systems and processes to regulate the flow or pressure of the process fluid. Fluid control valves can be driven by a variety of power sources such as hydraulics, solenoids, or pneumatics and are usually activated by external controllers.
They can be manually operated, programmed, or based on inputs generated by data collection devices such as flow meters or temperature gauges.
What do they do?
Whilst there is a myriad of fluid control valves of varying scale and operation, they are all based on the same basic principle.
The smaller the internal diameter of a pipe, the lower the rate of flow of liquid through that pipe at a given pressure. Fluid control valves allow the user to limit the effective diameter at a point in the pipe (or process), tailoring the flow rate to the required levels.
Fluid control valves in a nutshell
Fluid control valves are key components in hydraulic and pneumatic systems, regulating power delivery to cylinders, actuators, and motors. A key application for fluid control valves is found in industries where processing is used; this encompasses oil and gas production, oil refinery, petrochemical, industrial chemical production, pharmaceuticals, power generation, water processing, dairy, and food and drink industries.
In these environments, fluid control valves can make up a large part of the control systems in the processing plant, where they are designed to control many process variables such as pressure, flow, level, and temperature, and to keep these measurables within an operating range to ensure the quality of the end product.
Types of fluid control valves
Ball valves use a variably positioned ball with a machined, centrally aligned cavity to control the flow of media. By rotating the ball by up to 90 degrees (a quarter turn) on the axis perpendicular to the valve’s input and output orifices (or flow direction), the effective size of the aperture of the valve can be adjusted.
This, in turn, varies the flow rate through the valve. Ball valves offer reliable, linear control with tight sealing in the fully closed position whilst allowing variable flow with partial closing.
Their inherently robust and simple design makes ball valves low maintenance and easy to operate. They are ideal for leak-tight shut-off, automated control, and the handling of corrosive and aggressive fluids.
Non Return Valves
Non Return Valves (NRVs, also known as check valves) only allow fluids to flow in one direction. The valve relies on a pressure differential, with a higher pressure required on the input side of the valve to open. When the pressure is higher on the outlet side – or, more importantly, the input side pressure is not high enough – the valve will close.
Depending on the valve type the closure mechanism can vary but they generally rely on a mechanical spring for operation, although flexible diaphragms and gravity activated swings are also used. Unlike other valves, they do not require an actuator or any other external intervention to operate.
NRVs are used in systems to prevent backflow, which could be potentially damaging to upstream equipment or processes.
Since the velocity of the fluid determines the degree to which the valve opens or closes they are simple to operate and, outside of maintenance, require no manual intervention after installation and calibration.
Their simplicity, reliability, and wide choice of material options mean that check valves are used in a wide range of applications from hazardous chemical handling, heating systems, and water treatment to food and drink manufacture.
A needle valve consists of a tapered pin that is lifted from a seat to gradually open an orifice for fine control of flow. The valve body has a relatively small egress with a tapered seat. There is a needle-shaped plunger on the end of a screw that exactly fits this seat.
The flow rate is controlled by the screw-threaded spindle attached to the needle (the valve stem), which allows a great degree of precision in adjusting the flow rate.
Since flow rates are low and many turns of the stem are required to cover the full range of operation, needle valves are not used for shutoff applications, however, they excel at gradual flow control and are perfect for high precision applications such as pharmaceutical processes.
A butterfly valve is made up of a rotatable disc perpendicularly mounted in a pipe. The disc is mounted on a rod with an external actuator handle. When the valve is closed by rotating the rod by 90 degrees, the disc is turned so that it completely obstructs the pipe.
When the valve is fully open, the disc is rotated so that it presents its lowest profile to the flow.
This allows an open butterfly valve to provide an almost unrestricted flow. There are three main types of butterfly valves for different applications and pressure handling characteristics. The simplest and cheapest is a zero-offset butterfly valve, which uses a rod to rotate the disc around a single axis into a flexible material (such as rubber) to provide the seal.
The double offset butterfly valve is offset from the centerline of the disc seat and body seal, as well as the centerline of the bore.
This creates a cam action during operation to lift the disc out of the seal, which makes the valve easier to operate in high-pressure applications and reduces wear. In the triple offset butterfly valve, the disc seat is machined so the contact axis is also offset.
The seat itself is made of metal in this design, and the additional elimination of the sliding action in combination with the lifting cam of the double offset achieves a bubble-tight shut-off even when used in higher pressure systems.
Globe valves used to consist of a spherical body containing two equally sized internal cavities, hence the name, although it now applies to modern valves that use the mechanism of two internal pressure chambers separated by a baffle. This baffle has a centrally oriented opening that doubles as a seat.
The final part of the assembly is a movable disc that is designed to press into the seat obstructing the flow. This disc is connected to a screw-threaded stem which is rotated to operate the valve.
They provide good shut-off capability at the expense of a higher operation force to seat the disc when compared to a butterfly or ball valve. They are often found in water treatment, petrochemical, and heating systems.