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 a few important types of pressure control valves:
Major pressure control valves are:
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 overpressurized 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.
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.
5. Unloading valve
The unloading valve operates the pump at a minimum load. These valves decrease the heat and preserve energy by draining the extra fluid to the tank when the flow is not required. The port of the unloading valve is connected to the pipeline that is to be unloaded. The pilot port detects the pressure of the system, and when the set pressure is reached, it sends a signal to initiate unloading.