Last updated on March 5th, 2025 at 11:13 am
Ever wondered how hydraulic systems stay steady under different loads? Knowing the difference between pre compensation and post compensation makes it much clearer!
Both types of compensation system help to maintain a stable flow as pressure varies.
But before we get into the details, Let’s first understand what ‘compensation’ really means!
Fig. 1 shows the symbols for a flow control valve and a pressure-compensated flow control valve. Notice the highlighted arrow in the pressure-compensated symbol, which indicates the added pressure compensation feature.
Hydraulic systems work based on two fundamental factors: pressure and flow.
Here’s an important fact: A system operating at high pressure will have less flow, while a system with low pressure will have high flow.
Fig.1: Symbol for Flow Control Valve vs Pressure Compensated Flow Control Valve
Fig.2: Conveyor in a Food Plant
Now, imagine this scenario: In the conveyor system shown in Fig. 2, let’s replace the food items with heavy motors. Based on the above principle, if there are only a few motors on the conveyor, it will move quickly. But if the conveyor is loaded with many motors, it will slow down.
This variation in conveyor speed can lead to increased work hours, which ultimately affects productivity and costs. Ideally, we want the conveyor to move at a consistent speed, no matter how many motors it is carrying.
To achieve this, we can use a flow control valve to regulate the flow to the conveyor. However, manually adjusting the flow control setting every time the load (number of motors) changes is neither practical nor efficient.
This is where a pressure-compensated flow control valve comes into play. This type of valve automatically maintains a constant flow, regardless of pressure variations caused by changes in the load (in this case, the number of heavy motors).
There are two types of these flow control valves:
- Pre-compensated
- Post-compensated
Each has its own role in maintaining a stable hydraulic system.
Why do we need a Compensation System?
Let’s break it down with the help of three hydraulic circuits.
Pressure Drop across orifice with 250bar Relief Setting
Pressure Drop across orifice with 300bar Relief Setting
Pressure Drop across orifice with 150bar Relief Setting
In above examples, the pump and motor size, orifice size, and load on motor kept the same, but the relief valve setting has been varied.
As shown in condition 1 and 2, as we increase the relief valve setting from 250 to 300Bar, the pressure drop across the orifice goes up (ignore the negative sign) too. This happens because the pressure before the orifice (set by the relief valve) increases, while the pressure after the orifice (load on motor) stayed the same.
- With a higher relief valve setting, the flow is forced through the orifice since it can’t escape through the relief valve. And as a result, the flow to the motor increases, which you can see on flowmeter 2.
In the third circuit, when we lower the relief valve setting, the pressure drop across the orifice decreases. This happens because the pressure before the orifice (set by the relief valve) decreases, while the pressure after the orifice (load on motor) stayed the same.
- With a lower relief valve setting, more flow goes through the relief valve, so ultimately flow to the motor decreases, as shown on flowmeter 2.
This leads us to an important insight: oil flow is directly proportional to the pressure drop across the orifice.
(Note: Do not compare Pressure Drop with Pressure)
So, if we can maintain a constant pressure drop across the orifice, we’ll have steady oil flow, regardless of how much the load changes. This is the key to achieving stability in hydraulic systems, and it’s what compensation is all about.
As we’ve seen in the previous scenarios, fluctuating flow is something we clearly want to avoid in hydraulic systems. That’s where pre-compensation and post-compensation come into play—they step in to ensure stable flow and smooth operation, even when the load changes.
In fact, many systems use a combination of both pre- and post-compensation to ensure optimal performance under varying conditions.
(Recommendation: After reading this post, make sure to Checkout L90LS Catalogue from Parker, which presents both pre compensation and post compensation into single valve block)
Together, they help balance stability and responsiveness in hydraulic circuits, ensuring consistent operation no matter the load
Pre Compensation Hydraulic System
Now, imagine motor is involved into system – the varying flow would cause the motor’s speed to change, which is clearly something we want to avoid.
What we need is for the motor to run at a consistent speed, regardless of the load placed on it.
To achieve that steady motor speed, we need to ensure that the flow remains constant without any fluctuations.
And as discussed: To keep the flow steady, we must maintain a consistent pressure drop across the orifice.
To do this, we can introduce a normally open 2/2 valve with a spring setting that matches the pressure drop needed at the orifice. This valve will sense the pressure upstream and downstream of the orifice.
Now as we add load to the motor, motor’s speed doesn’t change. This stability is the result of the constant pressure drop across the orifice.
And just this simple setup is known as pre-compensation.
Pre Compensation Hydraulic Circuit
It’s important to note that we don’t necessarily have to place the 2/2 valve (so called compensator) directly below the orifice. It can be above orifice, and system will work the same.
The key is that the valve should be normally open and it only activates once a specific pressure drop—determined by the spring setting—is reached across orifice.
As pressure threshold arrives compensator close such as to adjust the flow. This means the system can essentially function as a “flow limiter,” ensuring that the flow is restricted or regulated once the pressure conditions met.
System with Multiple Function
Now that we understand how pre-compensation works, let’s explore what happens when multiple actuators, like two motors, are operating at the same time.
When the load on both motors increases simultaneously, the first motor (with the lighter load) receives more flow, while the second motor (with the heavier load) receives less flow. After some time, as we continue increasing the load, the second motor eventually stalls, and all the flow is diverted to the first motor.
This demonstrates that pre-compensation isn’t ideal when dealing with multiple operations under different loads. The goal is to have both motors running at the same speed—meaning they receive the same flow—regardless of the load.
This is where post-compensation comes into play. With post-compensation, even if both motors are under significantly different loads, they’ll still run at the same speed. As the load increases further, both motors will slow down, but the key point is that they slow down together.
Now, let’s dive into how post-compensation works.
Pre-Compensation Hydraulic System with Two Motor
Post Compensation Hydraulic System
Now, let’s tweak the circuit a bit. This time, we’ll place the compensator above the orifice, but in a normally closed position.
Our goal is to maintain a constant pressure drop—let’s say 10 bar. If the inlet pressure is 250 bar across the orifice, and we want a 10 bar drop, we’ll need 240 bar at the outlet.
To achieve this, we’ll set the compensator spring to 240 bar on one side, and connect the other side to the outlet of the orifice. This creates a backpressure of 240 bar on the orifice, ensuring that the pressure drop across the orifice stabilizes at 10 bar (250 bar at the inlet minus 240 bar at the outlet).
Alternatively, instead of using a 240 bar spring setting, we can simply supply a pilot line at 240 bar to the compensator, and it will work in the same way.
This post-compensated system acts as a “pressure inducer,” ensuring the pressure downstream of orifice doesn’t fall below a specific level, which is defined by the supplied pilot pressure—or, in our simplified example, by a very strong spring.
Post Compensation Hydraulic Circuit
System with Multiple Function
Now that we’ve covered how post-compensation works, let’s see what happens when multiple actuators—like motors—are running at the same time.
In this scenario, we will analyse working with two motors, each with a different load. Initially, only the first motor is running, so all the flow is directed to it.
Next, we turn on the second motor, which has a lighter load compared to the first one. As you can see, both motors are running at the same speed, though the first motor’s speed has slightly decreased.
- Let’s say Motor 1 is demanding flow based on pressure (load on motor) of 200bar and Motor 2 is demanding flow based on pressure (load on motor) of 80bar. Now to keep same pressure drop across orifice, both motor will get flow based on 200bar
Now, as we demand higher RPM from both motors, the pump has to work harder to supply more flow and eventually it becomes saturated. At this point, the pump is delivering its maximum flow, and although both motors are still running at the same speed.
- Let’s say Motor 1 is demanding more flow (demanding higher RPM) based on same pressure (load on motor) of 200bar and Motor 2 is also demanding more flow based on same pressure (load on motor) of 80bar.
- Now both motor demanding more flow, which makes pumps to saturate. And to keep same pressure drop across orifice, our pump has to decrease it’s outlet pressure , which make decrease in pressure drop across orifice.
(Note: Demand Flow is inversely proportional to System Pressure)
(Reminder: Demand Flow is directly proportional to Pressure Drop across Orifice)
Post Compensation Hydraulic System with Two Motor
When the pump is not saturated, pressure drop equals the pump’s compensator setting, which in this case let’s take 24 bar. This is the situation in the first and second scenarios, where the pump is able to meet the demand.
However, when the pump becomes saturated, pressure drop decreases, but as long as it remains equal across all orifices in our flow-dividing system, the flow-sharing condition is still met. In the third scenarios, the pump is saturated, so the pressure drop across each orifice drops to 12-15 bar.
What would happen if we changed the compensator of the second motor to a normally open configuration?
The second motor would now behave as a pre-compensated system. And because it has the lightest load, all the flow would be directed to it.
So, it’s not that pre-compensation is ineffective; it’s just more useful when you need to prioritize flow to the actuator with the least resistance. On the other hand, post-compensation is ideal when there’s a need for flow sharing, regardless of the load on each actuator.
In hydraulics, knowing how these compensation strategies work can mean the difference between keeping things running smoothly or dealing with expensive downtime. That’s why it’s essential for anyone in the field!
Conclusion
Understanding the differences between pre-compensation and post-compensation helps you choose the right approach for your hydraulic system. Pre-compensation is great when you need to prioritize flow to lighter loads, while post-compensation ensures equal flow sharing, regardless of load differences.
In many real-world applications, to make systems work even better, both pre and post compensation are combined. This blend allows for both flow priority and stability, giving you the best of both worlds—efficient and reliable system performance.
