Check Valves For Hydraulic Applications
Figure 1: Inline check valve
A hydraulic check valve allows fluid flow in a single direction and protects important hydraulic system components. When properly installed, a hydraulic check valve can stop potentially contaminated fluid from mixing with the fluid reservoir, keep the system primed, and relieve pressure. This article will examine check valves' integral role in hydraulic systems. For a comprehensive check valve understanding, read our article on check valves.
Table of contents
- What is a hydraulic system?
- Hydraulic check valve functions
- Hydraulic check valve symbols
- Hydraulic check valve materials
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What is a hydraulic system?
A hydraulic system uses an incompressible fluid, typically oil, to transmit force from one point to another. Because the fluid is incompressible, force is undiminished across the system. Different sized pistons at different points in the system allow for force multiplication (or division). For an example, consider a simple two-piston system: piston 1 and piston 2. Piston 2 has a surface area three times larger than piston 1. If a 100-kilogram downward force is applied to piston 1, a 300-kilogram upward force will appear in piston 2.
Figure 2 shows a basic hydraulic system. The incompressible fluid flows from the reservoir (Figure 2 labeled A) by a hand pump (Figure 2 labeled B) or a motor-driven pump (Figure 2 labeled C). Check valves (Figure 2 labeled D) at the discharge point of each pump keep the fluid from moving back into whichever pump is operating. The pressurized fluid moves to the actuating cylinder (Figure 2 labeled F), where it converts from hydraulic fluid energy to mechanical energy. The pressure relief valve (Figure 2 labeled E) remains closed unless the system's pressure overcomes the valve's cracking pressure. In this event, the relief valve opens, which allows the fluid to flow back to the reservoir and reduces pressure in the system. Finally, the filter (Figure 2 labeled G) ensures that any impurities picked up by the fluid do not enter the reservoir.
Figure 2: A basic hydraulic system with components: reservoir (A), hand pump (B), motor-driven pump (C), check valves (D), pressure relief valve (E), actuating cylinder (F), and filter (G). The three lines are suction (blue), pressure (red), and return (green).
Hydraulic check valve functions
As discussed in the previous section, a critical function of a hydraulic check valve is to prevent fluid from returning to the pump that suctions it from the reservoir. Other essential functions are:
- Block pressure spikes: Directional control valves in hydraulic systems can be closed to block flow. This deadheads the pump, meaning the pump is operating but is unable to move fluid. Pressure rapidly increases in the system, and a check valve can stop the pressurized fluid from returning to the pump and damaging it.
- Keeping prime: The inside of a pump needs to have fluid for the pump to operate without risking damage. A check valve keeps fluid from flowing out of the pump when the system is off.
- Prevent backspin: A hydraulic accumulator is a component that some hydraulic systems use. It keeps the fluid pressurized while the system is off. Check valves prevent the pressurized fluid from returning to the pump and causing backspin.
- Protecting multiple pumps: Figure 2 shows a two-pump hydraulic system, which is a typical setup. Check valves at the discharge points of each pump prevents the operating pump from moving fluid into the pump that is not operating.
Pressure control: Direct-acting and pilot-operated check valves are suitable for working as pressure relief valves in a hydraulic system.
- Direct acting: A spring keeps a direct-acting hydraulic check valve normally closed and determines the valve's cracking pressure. The valve begins to open when the system's pressure surpasses the cracking pressure. The valve being fully opened is called full relief valve pressure.
- Pilot-operated: Pilot-operated hydraulic check valves are two-stage valves rated for higher pressure systems. When rated for the same flow, they are smaller than direct-acting check valves. The first stage uses a smaller spring-loaded piston. When system pressure rises enough to overcome the small piston, a small amount of fluid is allowed to flow through the valve. This creates a pressure drop that allows the larger piston to open, allowing full flow through the valve.
It is important to note that the hydraulic check valve flow direction is important, and that incorrect installation can lead to hydraulic check valve failure, which can cause system inefficiencies and potentially damage hydraulic components.
Hydraulic check valve symbols
Figure 3 shows p&id schematic symbols for hydraulic check valves: check valve with a spring (Figure 3 labeled A), check valve without a spring (Figure 3 labeled B), & pilot-operated check valve (Figure 3 labeled C).
Figure 3: Hydraulic check valve p&id schematic symbols: check valve with a spring (A), check valve without a spring (B), and pilot operated check valve (C).
Consider the valve's body and seal materials to choose a suitable check valve for a hydraulic application. For more information about materials, read our chemical resistance guide.
Suitable materials for hydraulic check valve bodies are:
- Brass: Not commonly used, but advantageous in corrosive environments.
- Stainless steel: Considered extremely suitable in highly corrosive environments.
- Cast iron: Suitable in hydraulic applications that will need many cycles and for high-pressure applications.
- Aluminum: Corrosion resistant and suitable for low-pressure applications.
Suitable materials for check valve seals are:
- Thermoplastic elastomers: Polyurethane (PU) and thermoplastic polyester elastomers (TPC) have chemical and hydrolysis resistance, wear and pressure resistance, and elasticity and flexibility.
- Rubber: Nitrile rubber (NBR), hydrogenated nitrile rubber (HNBR), and fluorocarbon rubber (FKM, FPM) are resistant to a variety of hydraulic fluids, can operate in a wide range of temperatures, are resistant to UV rays and weathering, and are resistant to wear.
- Teflon: Teflon has chemical resistance, low coefficient of friction, and resistance to wear.
- Thermoplastics: Thermoplastics are necessary when seals need to have high mechanical strength.
What does a hydraulic check valve do?
The use of a check valve in a hydraulic system is to allow pressurized fluid in a hydraulic system to flow in only one direction. This protects the system’s pump(s) and prevents contamination.
How does a hydraulic check valve work?
When the system’s pressure overcomes the check valve’s cracking pressure, the valve opens to relieve pressure in the system and protect valuable components.