What is the best valve for pressure control?

Hey,

I have a question to anyone who can answer me.
I need to choose a valvetype which fits the most in my application and why.
I have a compressor who delivers a constant flow. On the other side I have clients who use a variable flow but NEED a constant pressure. I need to place a valve in a bypass so I make sure the pressure remains constant. What type of valve would be the best AND WHY. Now I use a butterfly valve but the problem there is due to the caracteristic that if I open it for 50%, 70% of the flow is blown off.

Can anyone help me on this topic???

Thanks a lot You did not mention what kind of flow rate, or available presure drop you need for the valve. These are some of the main criteria for selecting the valve type. In most pressure control applications a globe valve with equal percentage trim is the best choice, unless a low pressure drop is required, or if flow is too great. There are also some pretty nice v-ball valves which would be an improvment over the butterfly you now have. These are ball valves with a v shaped notch which gives them near equal percentage performance. Check with your local Fisher rep. He will give you some good advice on valve selection.
Good Luck George Griggs
Sr Process Controls Engineer I was going to suggest a globe valve with a pressure regulator, then read the word compressor and realized you might have an air (or other gas) system. Also, it somewhat depends on the size of the line and how frequently the valve has to change position. Could you provide some more details? Patricia Lougheed Destroyer!

Pressure control valves of 'ConservAir' make are best for this application. The pressure fluctuation is very narrow and is 2 psi.(A general PRV will also work but pressure drop of those are very high.)

ConservAir make valves are a costlier. You can talk to Spirax Marshall even.

You have to install these valves in main line only and not in bypass line. (forget the bypass line untill there is a problem in the mainline valve).

Regards,

Is the m3/hr the flow rate from the compressor or the amount of air needed by the user or both? I'm not clear how physically this system looks. The other thing to consider in sizing and selecting a valve is the valve turndown, what is the maximum flow through the valve and what is the minimum flow through the valve and associated pressures (inlet and outlet) and temperatures.

If your customer needs a slipstream of air, you put a control valve in the line with a controller on the downstream pressure. As the customer takes more or less air, the control valve will bleed off sufficient air to keep the pressure constant. If you are regulating pressure with a control valve you still need the by-pass line. If you use just a control valve, it will restrict flow as the demand decreases and the pressure upstream of your valve can raise and can cause problems with your pump.
If you have a by-pass line installed, you can use a back pressure regulator in the by pass to see that the pressure in your line does not exceed what you need downstream. As the demand raises and takes more of the flow (lower pressure) the BPRV will close, as the demand lessens ( higer pressure) the BPRV will open to relive through the by-pass.
You may even be able to accomplish what you need with the BPRV all by itself. Butterfly valve can be configured for good control if some sensible installation considerations are made.
1. Match the Valve size to the particular conditions you will operate at most of the time.
2. If there are operating conditions's outside the valves control zone (20-70º open), add another valve piped in parallel to the original.
3. Select an actuator control that gives you the 'stiff' performance you require.

4. See our Website (undergoing development at present), for details about our 'closed loop control' actuator. It is electronic, with built in PID control, and high speed performance. The Emech ceramic disc valve offer cavitation resistance (no elastomer seats to wear out), and ANSI Cl. IV seal integrity (bubble tight) even in dirty fluids.

Contact me if you would like to discuss your applications in more detail.

Pressure control valves have practical applications in every pneumatic and hydraulic system. To prevent damage to the hydraulic system, power wastage, and overheating of the hydraulic fluid, circuit designers use a variety of efficiently designed pressure control valves to regulate maximum system pressure and pump flow during non-action periods. Most of such valves are easy to install without consuming much space because they are smaller in size. Some pressure control valves are capable of relieving the pressure from the system, while others are good at maintaining the pressure within a certain limit. This article highlights Different Types of Pressure Control Valves:

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There are many Different Types of Pressure Control Valves, we will discuss only major one in below.

1. Pressure Relief Valve

A pressure relief valve (PRV), also called a relief valve, is a pressure control valve used to regulate or restrict the pressure in a system. The pressure may build-up, which can lead to a process, instrument, or equipment failure. Pressure relief valves make it possible to avoid this disaster. They act as a shield that limits maximum pressure in a system by detracting the excess fluid when the pressure gets too high. As fluid is diverted, the pressure inside the vessel will stops rising and begin to drop. The valve will close once the pressure drops to the valve&#;s reseating pressure. The pressure relief valve is built or designed to open at a predetermined set pressure to defend pressure vessels and other instruments from being subjected to pressures that are higher than their design limits. The difference between the set pressure and reseating pressure is called blowdown. The blowdown of some pressure relief valves varies from 2&#;20%, while some valves have adjustable blowdowns.

The basic goal of a pressure relief valve is the protection of life and property by discharging fluid from an over pressurized vessel. They should be prepared for operating at all times, especially during the time of power failure and crisis when system controls are nonfunctional. Two categories of relief valves are closing valves that include spring-loaded and pilot-operated valves; the second is non-recurring valves that include rupture disks and buckling pins. A spring-loaded pressure relief valve consists of inlet nozzle, valve seat, valve body, seat holder, bonnet, cap, spring, seal and set pressure adjusting screw.

2. Pressure Reducing Valve

Pressure Reducing Valve is an automatic control valve organized to reduce high unregulated inlet pressure to constant, reduced outlet pressure. Sometimes, it is also indicated as pressure reducing regulator. It is a self-operated system used to lower excess pressure in a system. These valves are fully automatic and do not require power from an external source. Commonly, they have applications in the gas, steam, oil, and gas industry. The pressure reducing regulators are highly reliable and easily maintainable.  Based on the mechanism of controlling the valve opening, they can be categorized as direct-acting and pilot-operated pressure-reducing valves. Direct-acting valves are compact, cheap, and easy to install. Plus, they are best for small loads. In comparison, the pilot-operated valve is ideal for big loads where precise pressure control is necessary. They are expensive and larger in size.

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Pressure reducing valves can be utilized as bypass valves to save the system from power failure. Furthermore, under defined conditions, these valves can be used for water hammer protection. Such valves have the capability to act instantly by sensing and adjusting according to downstream pressure.

3. Sequence Valve

The sequence valve diverts the flow of fluids in a predetermined sequence. They are pressure-actuated valves, which resemble pressure relief valves in construction and operation principles. A sequence valve enables the pressurized fluid to stream to a secondary circuit only after an operation has been achieved and satisfied in the primary circuit. When closed, it lets fluid flow freely to the primary circuit to accomplish its first operation until the valve&#;s pressure setting is reached. Occasionally, it is needed to slow down the switching sequence for functional purposes. In this case, the operation of the sequence valve is not pressure-dependent; rather, it is operated by the adjustable stroke of a control piston.

The fundamental function of a sequence valve is to govern the sequential operation of multiple actuators. This function is important in many different industrial systems, involving aerospace, automobiles, fluid processing, and more. Sequence valves often possess check valves, which allow reverse flow from the secondary circuit to the primary circuit. Nonetheless, sequencing action is delivered only when the flow is from the primary to the secondary circuit.

4. Counterbalance Valves

The counterbalance valve is also known as the load holding valve because it prevents the load from falling by maintaining the backpressure. It permits the fluid to flow in one direction only while restricting the flow in another direction. They are used with cylinders to carry the suspending load and handle with overload safely. The counterbalance valves have two ports; the primary port is joined to the rod end of the cylinder, while the secondary port is connected to the directional control valve. When pressurized fluid streams to the cylinder&#;s cap end, it extends and raises the pressure in the rod end that shifts the main spool in the valve. This establishes a path for the fluid to pass through the secondary port to control the valve and reservoir.

Counterbalance valves provide safety by automatically managing the descent of load. It can also be used in hydraulic motors. The design is quite simple, which allows for multiple variations to satisfy personal preferences. However, if a counterbalance valve gets stuck in the open position, it may result in significant instability, which leads to equipment failure.

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