When I was a Padawan Learner of hydraulics, I found the concept of pressure compensation to be difficult to understand. Part of my difficultly was also a result of my difficulty understanding pressure drop, which is intimately related to pressure compensation.
For general pressure compensation, it describes a component that varies an orifice to maintain flow regardless of pressure differential. The most common component with this capability is a pressure compensated flow control.
In a pressure compensated flow control, a hydrostat is integrated into the valve, which is a component that measures pressure drop across the metering portion of the flow control. This could be a needle valve or other variable orifice. The hydrostat measures pressure before and after the orifice, and works to maintain a set differential.
By understanding pressure drop, you know how upstream and downstream pressure are related. For example, if you have 10 gpm going into one of two identical fixed orifices plumbed in parallel, the one with lower downstream pressure will be the one that flows more. Pressure drop is the energy used (or wasted) to push fluid through a restriction, and the higher the pressure drop, the higher the flow. If upstream pressure is 3000 psi and downstream pressure is 500 psi, this example will flow more than if downstream pressure is 2900 psi at the second orifice.
In my two examples, one orifice has 2500 psi of pressure drop to create flow with, and the other orifice has just 100 psi to of pressure drop to create flow with, which would barely allow a trickle. Adding a hydrostat (pressure compensator) to both of these orifices would provide constant flow based on the orifice setting or size, not the inlet pressure of the flow control.
The compensator in the illustration shows how pressure is measured before the orifice (internal to the hydrostat in this case) and then after the orifice. The difference between the two points measured is the pressure drop, and the compensator will attempt to maintain a particular pressure drop based on the strength of the spring holding the compensator open.
As pressure drop increases, the hydraulic pressure on the left side of the hydrostat starts to push the hydrostat closed, reducing flow available to the variable orifice, which reduces both pressure drop and flow at the orifice.
If pressure downstream of the orifice increases (say from load), then pressure drop decreases, and so does flow. But then hydraulic pressure supplied to the hydrostat from after the orifice pushes the hydrostat further open, which increases flow to the orifice. This once again increases pressure drop, which increases flow.
The hydrostat will balance the continuously higher and lower pressure drop to help the orifice maintain an exact pressure drop regardless of load-induced pressure. Flow will remain a function that the pressure drop created by the spring pressure of the hydrostat, and not vary based on inconsistent upstream and downstream pressure.
This is a straight-forward example of pressure compensation, but brings to light how important it is to understand pressure drop. Understanding of pressure drop is probably the most important fundamental knowledge required to master hydraulics, so if you’re new to hydraulics, it is where you should spend most of your time.
Hello,
I am not able to understand why there is an orifice on the left side of the hyrostat or compensating piston?
Can you please make me understand this.
Thank You
Deepak Yadav
The orifice on the left side of the hydrostat controls the speed that the hydrostat can shift, This keeps it from oscillating wildly when the conditions are “just right”, and prevents it from over-compensating based on an instantaneous short lived pressure spike.
This is perfectly explained. Tank you very much :))
It is important to remember that the fluid that is not going to the load in a simple system would be dumped over the relief valve. In this circuit we are trying to maintain a constant pressure drop across the orifice to maintain a constant flow. Given that the relief valve would be set much higher than the pressure drop required by the orifice we must drop the pressure by using a hydrostat (adjust able orifice) to provide the series pressure drop. This reduces the pressure at the inlet to the orifice.
Louis
In case of a fixed displacement pump flow is constant irrespective of load..then What is the need of pressure compensator to maintain constant pressure drop that means constant flow..??
Flow from the pump is constant in case of fixed displacement pump but the flow you are regulating through control valve varies with pressure. Practically even fixed displacement pump flow varies with pressure.
We need to have a pressure compensated relief valve additionally in case of fixed displacement pump.
It will bypass the excess flow to tank.
It would help greatly if you could label the diagram as to which is the dampening orifice, which is the operational variable, which is they hydrostat itself.
I have a compensator that is bolted directly to the pump on a boom lift and I’m trying to figure out if it is maladjusted, worn spring, etc.
the manufacturers recommended test is a deadhead on one controls and i’m only seeing half the pressure i should downstream of the compensator. The pump itself is fixed output (relative to engine rpms, but that is generally fixed by governor). And it appears although i don’t have an internal schematic that the compensator design diverts flow that does not pass through the compensator orifice to return piping to the hydraulic tank. There are two adjustments on the assembly, one that manufacturer claims adjusts operating pressure and that is already bottomed. The other is the ‘compensator’ adjustment and i haven’t cracked the cap off that yet as i’m trying to better unerstand what i’m dealing with.
Tech told me to check the compensator first, more likely problem than the pump. I don’t have any reason to believe the pump should have been subject to excessive wear, running under low oil or with clogged filter. It is 12 years old and has about 2000 hours. i’ve got 25 year old pumps for simiiar (45 HP) applications that are still going gangbusters.
Any help you could offer by labeling and further explanation of taking that diagram into a real world situation would be much appreciated.
thanks,
brian