Pressure Compensators: What are they and How do they Work?

Hi hydraulic fans, Tomas here with a new topic this week: Pressure Compensators.

Tomas: Hi Fraser, regarding pressure-compensators, which pumps/products have these, and what do they do exactly?

Fraser: Pressure compensation simply means "takes pressure into consideration". Which can mean in valves that it adjusts to pressure changes to maintain the same performance, or far more commonly in pumps where it means to reduce pump flow based on system pressure. For a pump to reduce flow it must be a variable pump. Mostly piston pumps with a swashplate. However there are some variable vane pumps, but these are a legacy technology. Variable motors can also be pressure compensated.

Tomas: Would most variable pumps come with a pressure compensator by default then?

Fraser: Most open loop for sure. It is a very simple energy saving feature. But for closed loop it is less common, especially if there is an electronic control. Maybe 50/50. On motors it is one type control out of many. It is not an energy saving feature in a motor.

Tomas: What type of applications would benefit most from a pressure compensator, and which applications would it not make much of a difference?

Fraser: Think of the compensator as a thermostat in your house. The most efficient system would be a furnace that runs 100% of the time because the temperature outside, thus the heat required inside, never changes. But that is impractical and so you need a furnace that can still heat the house on the coldest day and when it is summer the furnace will turn off entirely. Pressure compensation is the same but for pump flow. If one minute you want high flow and then another minute you don't need any because the pressure is still high, then you want a pump that can sense the system pressure and turn off if no more flow is required into the system.

Tomas: What happens to a variable pump when it reaches its pressure compensated setting?

Fraser: It is modulating meaning it moves from on and off position and everywhere in between. And it is definitively turning the pump off. Compensation is not turning the pump on, it only turns the pump off. What turns the pump on is a mechanical device called a bias spring. So the pump is "biased" towards being on, meaning it wants to be on.

Tomas: So naturally, a pressure compensator could not work on a gear pump, correct?

Fraser: I'm not aware of any gear pump that is variable. Most gear pumps are cheap alternatives. There are some exceptions like some crescent gear pumps for very demanding fluids and applications like an aluminum die casting machine.

Tomas: What is the most common problem, or misconception that people have about pressure compensators?

Fraser: That it sets the pressure of the system. It can limit it, but it can not raise pressure. A misunderstanding of the fundamentals of hydraulics.

Tomas: Right, so it's not a safety feature like a relief valve, just an efficiency feature?

Fraser: I would say that it being a safety feature is probably the 2nd biggest misconception! Yes it is strictly an efficiency feature. For instance it cannot, in open loop pumps, remove oil from the system. This means if you have a cylinder and all of a sudden pressure builds on it, the compensator will not remove oil from the system to reduce the pressure. That feature is reserved for the relief valve.

Tomas: Are there "third party" or universal pressure comps? Or do Linde/Parker/Etc. require specific ones for their respective designs?

Fraser: The controls (compensator being one of them) are usually modular within their respective series, but I have never seen a controller work across multiple series. Even the Parker P2 and P3, which appear very similar, have unique controllers.

Tomas: What is the most common "symptom" that differentiates pressure comp issues from other common problems?

Fraser: Common problems are the pressure comp set too high so that it hits the system relief before the pump compensates (destrokes). This will generate a lot of heat. The other problem is that because it modulates, it will shift frequently and can be very small movements. This means it will wear out and need to be replaced. Last problem I often see with them is that they get damaged in transit or service. The compensator sticks out from the unit and has an adjustment screw that gets bent.

Tomas: What would be your "simple" tip for customers so they can have the easiest, least-fuss experience with their pressure compensators/settings? 

Fraser: It is a straightforward device. Not much fuss. There are some simple ones like on a P1/PD and some very elaborate and high performing ones like on a Gold Cup.