Hydraulic System Components: Case Drains, Valves, Filters, Lines, and Pumps

Hydraulic system components must be sized and arranged so flow, pressure, cooling, filtration, and return paths stay within safe limits. Case drain lines, valves, filters, hoses, tubes, and pumps all affect each other.

A small sizing mistake can lead to excessive case pressure, heat, aeration, cavitation, unfiltered oil, or pump failure.

Case Drain Line Sizing

Many hydraulic motors and pumps use case drain lines to return internal leakage oil to the reservoir. If a piston pump case drain line is too small, case pressure can rise above the allowable limit.

As pump efficiency decreases with wear, case leakage can increase. Extreme case pressure can lift piston shoes from the swash plate and damage the pump. Use the case drain port size as the starting point, and up-size the line when the design requires it.

Conditions That Raise Case Pressure

High case pressure can be caused by an elevated reservoir, a pressurized tank, oil that is too cold for its viscosity, or oil viscosity that is not suitable for operating temperature.

Designers should confirm case pressure limits rather than assuming a small drain line is acceptable.

Aeration and Cavitation

Aeration occurs when air enters the pump inlet and mixes with fluid. When bubbles collapse on the pressure side, they can erode internal surfaces and damage components.

Cavitation occurs when low inlet pressure creates vapor bubbles in the fluid. Both conditions increase noise and can eventually destroy the pump if the system continues to run.

Valves, Filters, Lines, and Heat

Lines, filters, and valves must be sized for the expected flow. Undersized lines create heat, undersized filters can open bypass valves or collapse elements, and poorly sized valves can restrict flow.

Pressure control valves protect the system from leaks, burst hoses, and failed tubing, but restrictions can waste energy. For example, throttling flow while the relief valve opens at high pressure turns energy into heat that damages components and reduces efficiency.