What if a pressure relief valve is not placed in a hydraulic system?

Parent Category: ARTICLES Category: EDUCATION Published: Saturday, 26 November 2011

Relief valve missingWell!, something is going to break. Let's see why...

In the event that the hydraulic system does not have a pressure relief valve or is not installed, we can think of two things. What type of pump the system has. Fixed displacement (gears, vanes or pistons) or variable displacement (with or without pressure compensator).


In the second case, if the system is using a variable displacement pump with pressure compensator, well! there is no problem, the system has surge protection, this is the function of the compensator.

In the event that the pump displacement is fixed, we have a serious problem.

If we analyze a simple hydraulic system as shown below, we will see:


Hydraulic system without relief valve


Once the motor or engine starts rotating, fluid is been pumped inside the system and if we move the handle of the directional valve the hydraulic cylinder is going to move against the wall. When it reaches the wall the cylinder stops because the load to move is too big but, the pump is still pumping!!

Let's see the second drawing.


Hydraulic system overpressurized


You can see the line that goes from the outlet of the pump directly connected to the back of the cylinder. Everything is "pushing" the wall. The fluid is very incompressible, so in a matter of miliseconds the weakest part of the system that is "pushing" or under stress is going to break. Either the cylinder rod, or the cylinder tube, cylinder piston, cylinder rear cover, or any of the pipes, the hydraulic valve body, the pressure gauge, or any part of the pump including the drive shaft, the shaft coupling, or even the motor(prime mover).

So, to prevent this, this is how the hydraulic system must look like:


Hydraulic system with relief valve


Now our hydraulic system is protected against overloads.

Remember: The fluid contained inside pipes or any hydraulic component behaves like liquid steel, as strong as it is.

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