If you are operating a vane or gear pump, in most cases the controls are fairly simple. The pump is either loaded (doing work) or unloaded (all flow going back to the reservoir). These are primarily fixed displacement devices so the amount of the flow is only based on the input RPM, otherwise, the flow remains constant. This is referred to as an open loop system. After doing the work, the flow is open back to the tank. In a closed loop system, after doing the work, the flow is returned to the pump. Closed circuit pumps offer additional, but different, control options.
Open Loop – Variable Displacement Piston Pumps
Variable Displacement Piston Pumps offer an array of controls based on pressure, flow, HP, or a combination of all of these. I’ll run through the basic types and reasons that you would use each.
One concept which needs to be explained first is the variable displacement. The amount of flow that each pump can provide is dependent on a rotating group of pistons. By varying the stroke of the pistons, we adjust the displacement of the pump. In a variable displacement pump, we vary the angle of the rotating group, which is done by tilting the swash plate.
Pressure Compensation – Reduced Flow After Reaching Set Pressure
Pressure compensated control is the most basic control for a variable stroke piston pump. The swash plate of the pump is operated with a heavy spring and a piston. When the prime mover, electric motor, or another device, turns the pump shaft, the pump will produce maximum flow. The system pressure flows against one side of an internal piston, which is being held by the heavy spring. When the force of the system pressure is high enough to move the piston and overcome the spring pressure, the swash plate angle is changed and the pump flow is reduced. The pump will maintain the set pressure, producing very little or no flow, until the load varies, at which point the swash plate angle changes and allows the pump to produce flow again.
This is a very simple control method and, in certain applications, this is all you need. You can adjust the spring tension, but that’s it. Remember, the flow of the pump is not adjusted until you have built pressure at full displacement. You must have enough HP to take the pump to full pressure at full flow. If there is not enough HP, the prime mover will slow down or stall before the pressure begins to compensate and lower the flow.
Application example: you are using a hydraulic motor to operate a conveyor. The load is constant and the motor requires about 1500 PSI to handle the load. You set the piston pump compensator at 1600 PSI and let it run.
Your system will also need a safety relief in case of emergency. System pressure is adjusted using the pump compensator and the system relief should be set a few hundred PSI higher than the pump compensator. If they are set too close, they can fight each other, causing the pump to go on and off stroke and/or the relief to open and close, causing inefficiency, heat, and vibration.
Load Sense – Let the Load Determine the Flow
Another option is to utilize a load sense compensator. With a load sense compensator, this compensator will include a lighter spring setting to control the swash plate. Upstream pressure is ported into a load sense port on the pump, as the pressure requirement increases, the pressure acts against the load sense piston. Once the pressure requirement is higher than the offset, the pump swash plate angle changes and the pump begins to increase flow, by increasing the swash plate angle, until we have enough pressure to balance the piston. Once balanced, the flow remains steady until the load changes.
The offset pressure is normally 200-300 PSI. With a load sense compensator, the pump produces what the load requires plus the spring offset, normally 200-300 PSI.
This system will also utilize a standard compensator so if the system pressure increases enough, the pressure compensator will take control and reduce the swash plate angle to reduce the pressure.
Let’s look at my initial application but this time, it has a varying load. They conveyor requires 1500 PSI to move 50% of the time, but the balance of the time the system requires between 2250-2500 PSI to move the load.
With a standard pressure compensator, you would have to set the pump at 2600 PSI to accomplish the work. When the work only requires 1500 PSI, the pump will be trying to produce 2600 PSI. Fifty percent of the time, your system will be operating at 1100 PSI of inefficiency, which means heat. With a load sense compensator, when the load requires 1500 PSI, the pump will actually produce about 17-1800 PSI. Yes, this is 300 PSI inefficient, but that is much better than 1100 PSI inefficient.
Flow Control – Adjust the Load Sense Using a Proportional Throttle
With a varying load, the load sense is a much better system. For additional control, you can utilize an electronic proportional flow control or throttle. You can use an electrical signal to vary the hydraulic signal which is received by the pump’s load sense line. This would give you full electronic control of the amount of flow the pump produces.
There are additional control options which allow you to remotely control the pressure compensator. With this remote compensator control, you can set 2 or more different system pressures. With the ability of a variable piston pump to build 5,000 or more PSI; the additional setting can be used when operating components with a much lower pressure requirement.
HP Control – Maximize HP Utilization
The next control is a torque limiting or HP limiting control. By adding an additional spring and piston, you can set a pump to always maximize its allowable input torque, therefore, maximizing output flow and pressure at a defined setting.
This gets a bit more complicated, but here is an example to demonstrate how the control works.
In this application, you are operating large bore, long strong cylinder. The cylinder has a 10” bore and 150” stroke. During most of the stroke, the cylinder is not doing very much work and can operate at 800-1200 PSI. During the last 20” of stroke, we want to hit our system pressure of 4500 PSI, but we can move much slower.
Our pump has an output of 15 CIR, a maximum flow of about 113 gallons at 1750 RPM. Our prime mover is an electric motor, 75HP with a 1.15 service factor. I want to keep my cylinder moving as fast as possible, but I also want to ensure that I never exceed a power demand 82 HP.
At 82 HP, the pump can produce 1254 PSI at full output, 113 GPM. As the load requires more pressure, the pump will begin to reduce flow and increase pressure. At 90 GPM flow, the system will produce about 1560 PSI; at 60 GPM we can get almost 2350 PSI. At 4500 PSI, the pump flow will be reduced to about 31 GPM. The advantage of this pump is that the internal controls of the pump are adjusting to maximize flow and pressure at all times without exceeding the available HP.
If I wanted to use a pump which could produce 113 gallons of flow at 4500 PSI, I would need 296 HP. If I choose a 75 HP motor with a pressure compensated variable piston pump, the motor would stall before the pressure compensator could kick in and reduce the pump flow. Depending on the load, a load sense pump could also stall the 75 HP motor if the load pressure is high enough to use up the HP before the pressure compensator kicks in. With a torque limiting (HP) control, we utilize the full limits of the prime mover and maximize power usage.
When beginning to work on a new application, call a certified hydraulic or fluid power specialist to help you pick the correct pump for the job!
