When to Use hydraulic check valves？

Check valves are the simplest form of hydraulic devices in that they permit free oil flow in one direction and block oil flow in the opposite direction. Check valves may also be used as a directional or pressure control in a hydraulic system.

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In Figure 1, oil is flowing in from the left side port, through the check valve and out the right side port. If the pressure equalizes or is higher in the right side port, the check valve will close and block flow in the opposite direction.

The spring rating varies based on how the valve is used in the system. One of the most common locations for a check valve is immediately downstream of the hydraulic pump (Figure 2). Notice that no spring is shown with the check valve symbol.

When used in this application, the spring pressure rating is usually 1-5 pounds per square inch (psi) and therefore not shown with the symbol. In this case, the valve is used as a directional control in that it allows oil flow from the pump to the system but blocks flow in the reverse direction. This is commonly called a pump isolation check valve. This valve serves four purposes within the system, which are detailed below:

Block Pressure Spikes

The check valve will block pressure spikes back to the pump. Depending on the pressure, oil flows from the pump to the system at a speed of 15-30 feet per second. When a directional is de-energized to block flow or a cylinder fully strokes, the oil is rapidly deadheaded. The pressure in the line can quickly increase by two to three times. The check valve should then close and block the pressure spikes to the pump.

I recall a plywood plant changing four pumps due to cracking of the pumps’ housings. This occurred over a week’s time on the debarker hydraulics. When the plant ran out of pumps, the staff finally took out the check valve and found that the piston and spring were no longer in the valve.

This $150 check valve cost the company $15,000 in replacement pumps and another $50,000 in machine downtime. That was one expensive check valve. The truth is that if one mechanic had looked at the schematic and known why the check valve was in the system, the replacement of the pumps and subsequent expenses would have been avoided.

Prevent Oil Lines from Draining

When a system is shut down, it is important to maintain oil in the lines. In many cases, the pump is mounted below the level of the system valves, cylinders and motors. The check valve downstream of the pump will prevent the lines from draining once the electric motor is turned off. If the oil in the lines drains through the pump and into the reservoir, a vacuum will occur.

Air will be pulled into the lines through the O-rings and seals of the valves and actuators. This can create issues when restarting the system, as the air will need to be bled out.

Block Oil Flow from the Accumulator

Some systems have a hydraulic accumulator installed downstream of the pump and check valve. When the system is turned off, there is pressurized fluid inside the accumulator. The check valve will block flow from the accumulator, preventing the reverse rotation of the pump.

You can observe the pump shaft or electric motor fan to verify that the check valve is good. Please note that all systems using an accumulator should have a method of bleeding the hydraulic pressure down to zero psi when the system is turned off.

Prevent Oil Flow from the Online Pump to the Offline Pump

On many systems, one pump is used as a backup or spare (Figure 3). Each pump will have a check valve at the pump outlet port. The check valve will block flow from the online pump to the offline pump, preventing reverse rotation.

I remember being called into a papermill that kept losing one of the two pumps on its chemi-washer drives. The shaft seal of one pump continually blew out. When the mill ran out of spares, personnel had to ship their last pump by air freight to the factory in New York.

The timeline was so critical due to downtime costs that the pump was still warm when they received it back from the factory. Just prior to installing the pump, we removed the check valve in the case drain line and found it stuck in the closed position. This prevented the oil in the pump case from draining, which resulted in blowing out the seal.

Frequently, a check valve is used for pressure control. A common application is to employ it as a relief valve to protect a heat exchanger (as shown in Figure 4). In this case, the spring rating is usually 65-100 psi.

If the oil is cold, the inlet pressure to the cooler may reach the check valve’s rating. The check valve will then open and direct the pump volume around the cooler. A check valve will also provide protection for an air-type heat exchanger if the tubes become contaminated.

A few years ago while teaching a class at a sawmill, I observed the students doing their hands-on exercises on the edger. Although a check valve was shown on the schematic to protect the air cooler, the lines to the check valve were plugged off. I asked one of the mechanics about it. He said the check valve was taken off years ago and that they had changed the cooler the week before because of ruptured tubes.

When troubleshooting hydraulic systems, most everyone looks for something large to be the problem, such as a pump, valve or cylinder, but every component has a function. Be sure you understand the purpose of the check valves in your systems.

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A hydraulic check valve, also known as non-return valve (NRV), is a mechanical device designed to allow fluid (typically a liquid) to flow in one direction and prevent reverse flow; it plays a crucial role in hydraulic systems to maintain the direction of fluid movement or to control pressure or flow rate.

Generally check valves are automatically operated; human intervention or any external control system is not required.

How Hydraulic Check Valves Work

Check valves have a body (often cylindrical) with inlet and outlet ports for fluid to enter and exit; inside there’s a movable element (often a poppet, more rarely a ball) whose function is to block the flow in the reverse direction. When fluid flows in the desired direction, its pressure forces the poppet away from the seat, allowing the fluid to pass around the valve. Vice versa, if there’s an attempt at reverse flow, pressure from the fluid pushes the poppet against its seat. This seals off the passage and prevents fluid from flowing backward. In most cases, a spring helps to close the poppet quickly and effectively when the flow direction changes, preventing backflow.

Check valves must necessarily be installed in the correct orientation; in fact, they are designed to allow flow in one specific direction, so improper installation will result in valves not functioning as expected. When fluid flows in the desired direction, the valve opens, and when there’s an attempt at reverse flow, the valve closes to prevent backflow. This design is crucial for maintaining the efficiency and safety of hydraulic systems by preventing unintended fluid movement in the wrong direction.

Types of Hydraulic Check Valves

Check valves usually have a straight profile and one single body or two screwed semi-bodies. Inlet and outlet threads can be female or male type.

A special category is represented by parts where an orifice machined on the poppet allows controlled leakage of fluid in the direction in which the flow is normally prevented by the closure of the valve. This design trick makes check valves equipped with an orifice a flow control tool. There are also three-way valves (or “shuttle” valves), which have the characteristic of conveying to the return line of the circuit the greater of the pressures of the two delivery lines.

Differences Between NRV and Check Valves

Understanding the nuances between NRV (non-return valves) and conventional check valves is crucial in hydraulic systems. While often used interchangeably, these terms essentially refer to the same component. The distinctions lie in regional preferences and industry jargon. In essence, whether you call it an NRV or a check valve, the functionality remains consistent, ensuring unidirectional fluid flow and preventing backflow. Clearing up any confusion between the terms is vital for precise communication within the hydraulic engineering realm.

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Materials for Hydraulic Check Valves

In hydraulic systems, the choice of materials for hydraulic check valves is paramount, with carbon and stainless steel emerging as the most commonly utilized raw materials. These robust materials not only provide durability but also ensure the reliability and longevity of the check valves. The selection of carbon or stainless steel depends on the specific requirements of the hydraulic circuit. These materials play a pivotal role in maintaining the structural integrity of the check valves, contributing to their effectiveness in controlling fluid flow and preventing backflow. A careful consideration of these material options is essential in ensuring the optimal performance and longevity of hydraulic check valves in diverse applications.

Hydraulic Check Valve Symbol

In the realm of hydraulic systems, the symbolic language of hydraulic check valves is conveyed through a stamped valve symbol on the external body. This symbol serves as a visual identifier, facilitating easy recognition of the check valve among other hydraulic components, and greatly helps to avoid installation mistakes as it shows the closing direction of the valve. This simplistic yet informative feature aids in the swift and accurate identification of check valves in complex hydraulic circuits. It is a valuable element for engineers and maintenance personnel, streamlining troubleshooting and ensuring efficient operation. The presence of a clear and standardized valve symbol enhances the overall functionality and usability of hydraulic systems, promoting ease of maintenance and optimal performance.

Common Hydraulic Check Valve Problems

As with all complex components in a hydraulic circuit, it’s possible to experience issues with check valves too, the most common being the stuck of poppet in open or closed position, unexpected leaks, chattering or the valve not opening at the set pressure. Choosing products suiting the circuit specifications, installing them properly and performing some periodical maintenance can help prevent many of these issues from occurring in the first place.

Why Choose Faster Hydraulic Check Valve

With a long experience of design and production, Faster is the ideal supplier for top-quality check valves. Every day we deliver all types of valves (ball-type, poppet-type, shuttle-type, etc.) to our customers to allow control of directional flow, or pressure, or flow rate for a wide range of applications and for different markets.