Hydraulic pressure relief valves are the most common type of valve found in a hydrauliksystem. Their primary function is to protect the system from pressure overloads by diverting excess fluid back to the reservoir. There are two main types of these valves: direct acting and pilot operated relief valves. Choosing the appropriate one can significantly enhance system performance and longevity.
Relief valves are normally closed. Whenever one is open, it converts the pressure energy of the hydraulic system directly to heat. This means that systems relying heavily on relief valves must also incorporate efficient heat management solutions to prevent thermal damage.
How Does A Pressure Relief Valve Work?
In open-loop hydraulic systems, pumps create flow and tolerate pressure, but they do not generate pressure themselves; pressure arises from resistance to flow. When a restriction occurs, pressure builds up rapidly. The relief valve acts as a safeguard, opening when the pressure exceeds a preset limit and redirecting fluid back to the tank.
There are three types of pressure that hydraulic pressure release valves help maintain:
- Cracking Pressure: This is the pressure where the first drop of oil starts to pass through the relief valve back to the tank.
- Full Flow Pressure: This is the pressure required to allow all the pump flow across the hydraulic relief valve back to the tank.
- Reseat Pressure: This is the pressure where the relief fully closes after being open. This pressure is typically less than cracking pressure.
Proper sizing is critical; if your relief is too small for the flow rate, you could see extremely high full flow pressures that can cause premature failure of system components.
What Is A Direct Acting Relief Valve?
A direct acting relief valve is a poppet held on a seat by a spring. The other side of the poppet is exposed directly to the high-pressure oil. When system pressure reaches cracking point, it pushes against this poppet, compressing the spring and allowing fluid to escape.
Direct acting valves tend to be very fast to respond to changes in system pressure. Their response times typically range between 5–10 milliseconds (ms), although some can react as quickly as 2 ms depending on design and manufacturer specifications.
However, they come with certain drawbacks: they tend to have a high-pressure delta between crack and full flow pressure, sometimes up to 500 PSI above cracking level. Additionally, direct acting reliefs also tend to be noisy, producing high-pitched squeals during operation.
What Is A Pilot Valve?
Pilot pressure relief valves, also referred to as balanced relief valves, use a pressure balance to control a larger internal spool. Within these valves lies a small direct acting component that senses system pressure. When this internal valve cracks open, it disrupts equilibrium and shifts a spool that opens an oil path back to tank.
Pilot operated relief valves tend to be much slower than direct acting reliefs because it takes time for the spool to move once the balance is lost. Their typical reaction time hovers around 100 ms—ten times slower than their direct acting counterparts.
Despite slower response times, they offer superior control over flow transitions: crack pressure to full flow pressure with a pilot operated relief valve is typically 50-100 PSI, which makes them ideal for protecting sensitive equipment.
The Main Differences Between Direct Acting Vs. Pilot Operated
The core distinction lies in response speed versus control precision:
- Direct-acting reliefs tend to be very responsive to changes in system pressure, opening very fast.
- However, they tend to have a high-pressure delta between crack and full flow, making them less suitable for delicate machinery.
- On noise levels: Direct acting reliefs also tend to be noisy Pilot operated make noise as well, but it is a much duller sound that is less distracting.
In contrast:
- Pilot operated valves are typically much slower The key advantage is that once unbalanced, they have a very small delta from crack pressure to full flow.
This makes pilot operated designs more suitable for applications where precise protection at specific pressures is required despite their slower activation speed.
Why Does Relief Speed Matter?
Relief speed plays an essential role in preventing catastrophic failures during sudden stops or blockages:
When you reach the end of a stroke the pump flow has been blocked the system starts intensifying at an incredible rate. These spikes can stress or destroy pumps and components if not managed swiftly.
The faster the relief opens, the lower the pressure spike will be. Data logs show spikes exceeding 6000 PSI even when using functional pilot-operated valves set at 3000 PSI cracking point due solely to their delayed response time.
To address this issue effectively: the solution is to add a small direct acting relief valve Using both types of pressure relief in the system gives us benefits of both. This hybrid approach combines rapid response with controlled flow transition—an optimal setup for most industrial hydraulic systems.
FAQ
What does cracking pressure mean?
It’s when fluid first begins bypassing through an opened valve due to reaching threshold force.
Can I use both types in one system?
Yes. Combining both provides fast initial protection (via direct acting) while maintaining stable operational control (via pilot-operated).
Why do some systems still fail despite having relief valves?
Improper sizing or slow-reacting components may not activate quickly enough during extreme conditions.
Are pilot operated valves always better?
Not necessarily—they’re better for stable operations but unsuitable where immediate response is critical without additional support from direct acting units.
POOCCA Has The Right Relief Valve For Your Hydraulics
Poocca Hydraulic (Shenzhen) Co., Ltd. was established in 2006 and has four companies in Hong Kong, Guangdong, Jiangsu and Zhejiang. With over two decades of expertise in manufacturing hydraulic pumps, motors, and valves—including advanced pilot operated relief valves—we offer tailored solutions for every industry need.
We have invested in advanced manufacturing equipment such as CNC gear grinders and coordinate measuring machines, ensuring precision-engineered products with exceptional reliability.
Our committed team includes over 300 professionals including 70 sales experts and 18 technical specialists, dedicated to providing exceptional service from consultation through post-sale support. Whether you’re designing new systems or upgrading existing ones, POOCCA ensures you get performance-driven components backed by world-class expertise. Kontakt POOCCA team for quick quote now.
