Drain Lines, Valves, Filters, Lines and Pumps in a Hydraulic System

Sizing case drain lines. Typically, hydraulic motors and pumps have hoses that run to a case drain in order to drain excess internal oil from the motor.  Having a case drain usually requires running motors in series to avoid damage.  If the case drain line is undersized on a hydraulic pump (piston), it can cause the pressure in the case to be too high.  As the pump’s life depreciates, its volumetric efficiency will decrease, which in turn will increase leakage from the case drain line.  If there is extreme case pressure, it can cause the piston shoe to lift off of the swash plate.  This will cause damage that will force the pump to stop working.  Go by the case drain port size or up-size it. It is important to make sure the case pressure is below the max rating, which, if necessary, can be adjusted at the case drain port size, or increase the size of the port.  Case pressure can also become too high at:

• The reservoir well above the pump
• The pressurized tank
• A temperature that is too low for the viscosity of the oil being used
• A viscosity that is too low for the operating temperature

Alleviate pump failure by mitigating caveated or aerated components.  If air is allowed to enter the system, the aeration will produce erosive damage when passing through the pump.  In addition, cavitation can cause insufficient pump inlet, which can damage the pump. Either of these conditions can be very destructive.

• Aeration is caused by air entering the pump inlet and mixing with the fluid.  Low pump pressure at the inlet will cause air bubbles to expand and, as the aerated fluid reaches the pressure side of the pump, the bubbles will disintegrate and implode which causes internal erosion of the system.
• Similar to aeration, extreme vacuum in a component will cause cavitation, which allows vapor bubbles to form in the fluid, ultimately damaging the pump.

Either of these conditions will cause pump noise to go up.  If the system is allowed to continue to operate the pump will eventually fail.  To safeguard against this problem in the design phase any source of air must be contained and the potential of vacuum at the inlet must be alleviated.

It’s important to design for flow amplification when sizing filters, lines and valves

If the proper size of the lines is not selected to handle a higher flow rate, unwanted heat will occur causing damage to motors or other hydraulic system components.  When designing a hydraulic system valves must be selected and sized correctly, or flow will be restricted which can cause it to unseat.  In addition, the filter must be sized correctly, or the bypass valve may open causing some of the fluid to be unfiltered or cause a flow surge that could collapse the element.

The control of pressure in a system is paramount in the design.  Within hydraulic system components and their functions, pressure control valves are essential in preventing leaks or bursting of pipes, hoses or tubing.  This is largely dependent on the proper selection of pressure control valves, which may include:

Needle valves are common in low-pressure hydraulic systems and are often automated, connected to a hydraulic motor or air actuator that automatically opens and closes the valve. Manual and automated valves of all types provide precise control of the flow rate and are used for regulating the flow of fluid in the system.  However, in some cases a needle valve can cause excessive heat to build up and waste energy if not properly designed into a system.  For instance, if the system is operating at 10 gallons per minute, pressure will flow to the actuator if the relief valve is set at less than the 3,000 PSI.  If the pump is producing 10 gallons per minute and it becomes necessary to slow the cylinder down, the needle valve must be adjusted down to restrict the flow.  As the flow to the cylinder is restricted, the relief valve will open allowing the flow to go over it at 3,000 psi wasting energy in the form of heat.  At 3,000 PSI, with as little as 2 gallons per minute over the rated flow, it can generate as much as 9,000 BTU.  Excessive heat will severely damage hydraulic system components and waste energy.