Hydraulic relief valves function as safety devices, letting the flow of oil return to the hydraulic tank. This action could prevent damage caused by excessive pressure. Relief valves can be classified into two types. In this article, we will show you comprehensive information about the differences between direct-acting relief valves and pilot-operated relief valves. So that you could choose the best suitable relief valves for completing your project perfectly.
What is a Direct-Acting Relief Valve?
A direct-acting relief valve is composed of a poppet fixed on a seat by a spring, and it is a normally closed relief valve. One side of the poppet is faced with inner parts, and you can see that the other side of the poppet is directly exposed to the high-pressure hydraulic oil. When the pressure reaches the required point, the poppet will be pushed away from the seal, letting it grow more and more distant. And then all the pumped oil flows into the tank.
What is a Pilot-Operated Relief Valve?
A pilot-operated relief valve is also known as a balanced relief valve. It features a unique two-level design, which is composed of a main valve and a smaller pilot valve. System pressure acts simultaneously on the main valve and the smaller pilot valve, which provides better control and efficiency for your hydraulic system. A very small direct-acting pressure relief valve is installed in the relief valve. This kind of valve is designed to sense your system pressure.
Comparison of Direct-Acting Relief Valves and Pilot-Operated Relief Valves
Next, we will show you 4 aspects of the differences between direct-acting relief valves and pilot-operated relief valves.
Working Principle
Direct-acting relief valves adopt a spring-loaded mechanism. When system pressure overcomes spring pre-load force, the doors of your valves will be directly opened to release excessive pressure. But once the pressure becomes normal, the springs will push your valves back to the closed position.
While for pilot-operated relief valves, pilot valves serve as the accurate controllers for the main valves. When the pressure exceeds the default value, the pilot valve opens first. This motion could generate pressure differentials upon the main poppet for you, pushing it to release unnecessary pressure.
As the pilot-operated relief valves rely on system pressure, not rely on the springs like direct-acting relief valves. Relying on system pressure makes the pilot-operated types provide more precise control and a higher flow rate than direct-acting types.
So when you need fast yet low-flow-rate working conditions, you can choose direct-acting relief valves. But when you need a steady and high flow rate working condition, you should choose pilot-operated relief valves in the first place.
Pressure Relief Speed
Direct-acting pressure relief valves have an extremely quick pressure relief speed. The theoretical pressure relief time is 2 ms, and when you really operate them, the practical pressure relief speed is tested between 5 to 10 ms.
Pilot-operated relief valves have much slower pressure relief speed than direct-acting ones. Their responding pressure relief time is about 100ms, around ten times that of direct-acting relief valves.
So, just consider the pressure relief speed, direct-acting pressure relief valves have super quick speed, and pilot-operated pressure relief valves are much inferior to them. You can first choose direct-acting relief valves for time-critical emergencies.
Application
Direct-acting pressure relief valves are cost-effective and have simple operations. So they are normally applied in handy hydraulic equipment, small industrial devices, and relatively small machine industries.
Pilot-operated relief valves perform better in severe working conditions where precise control and a high flow rate are needed. So they can be used for huge industrial hydraulic equipment, such as power generation equipment, heavy industrial gear, and more.
Cost
Direct-acting relief valves have a simpler structure for low-flow-rate and low-pressure hydraulic systems, so they are much cheaper. While pilot-operated relief valves provide you with a more expensive initial price. And due to their complex structure, they have an overall higher operation and maintenance cost for you. But they could stand for much higher intensity applications.
So when you want a cheap but low configuration pressure relief valve, then you can take a look at direct-acting relief valves. Pilot-operated relief valves are suitable for high configuration, but their price is much higher.
Conslusion
Direct-acting relief valves are cost-effective but can’t withstand a severe environment. While pilot-operated relief valves cost a much higher price, they can offer high-level functions and configurations. If you are looking for the best pressure relief valve for your project, Target Hydraulic is always here, as we can recommend the best suitable hydraulic relief valves based on your requirements and your project. Free to contact us now!
FAQ
What kinds of pressures do relief valves need to withstand?
First, your relief valves need to uphold the cracking pressure. This kind of pressure is generated when the first drop of fluid begins to flow back into the tank through the safety valve. Next, the valves should bear full flow pressure. This is the pressure required to return all the pump oil gets through the hydraulic safety valve back to the hydraulic tank. Finally, the reseat pressure. This is the force required to fully close the pressure relief valve. And it is typically less than the opening pressure.
What are the symbols of direct-acting and pilot-operated relief valves?
You can see that there are different symbols for these two relief valves. The symbol of a direct-acting relief valve is like a square with diagonal arrows. And the symbol of a pilot-operated relief valve is decorated with a small triangle shape.
