Hydraulic Reservoir Guide

Fluid power systems must have a sufficient and continuous supply of cool, uncontaminated fluid to operate efficiently.

A hydraulic system has a reserve of fluid in addition to that contained in the other components of the system.  This reserve fluid compensates for changing fluid levels from system operation, loss of volume due to cooling and fluid compression from the pressure. This extra fluid is contained in a tank usually called a hydraulic reservoir.  It may sometimes be referred to as a sump tank, service tank, operating tank, supply tank, or base tank.

In addition to providing storage for the reserve fluid, the reservoir acts as a radiator to dissipate heat from the fluid.  The reservoir serves as a settling tank where heavy particles of contamination may settle out of the fluid and remain harmlessly on the bottom until removed by cleaning or flushing of the reservoir.  It also allows entrained air to separate from the fluid.

Most reservoirs have a capped opening for filling, an air vent, an oil level indicator or dip stick, a return line connection, a pump inlet or suction line connection, a drain line connection, and a drain plug.  A properly designed reservoir has internal baffles to prevent excessive sloshing of the fluid and to put a partition between the fluid return line and the pump suction line.  The partition forces the returning fluid to travel farther around the tank before being drawn back through the pump inlet line.  This does 3 things; it helps cool the fluid more effectively, aids in settling contaminants to the bottom and separates air from the fluid.  Larger reservoirs are desirable as all 3 of the above benefits are further enhanced.

As a rule of thumb the ideal reservoir will hold about 2 ½ times the pump output per minute. The benefits of a large reservoir are sometimes sacrificed due to space limitations in mobile systems.  As a minimum they must be large enough to accommodate thermal expansion of the fluid and changes in fluid level due to system operation.

Reservoirs are of two general types – non-pressurized and pressurized. Propower manufactures non-pressurized hydraulic reservoirs and pressurized reservoirs operating up to 5 psi.  Most systems are normally designed for equipment operating at normal atmospheric pressure.  This includes hydraulic systems for truck or stationary installations.  A typical reservoir for use in industrial installations is made of hot rolled steel plates, has welded seams and is not commonly used for mobile operation.  The bottom of the reservoir is often convex, and a drain plug is incorporated at the lowest point.

Most non-pressurized reservoirs are constructed in a cylindrical shape.  The outer housing is manufactured from a strong corrosion-resistant metal.  To keep the fluid clean filter elements are normally installed within the reservoir or externally to clean the returning fluid.  Reservoirs that are filled by pouring fluid directly into them have a strainer in the filler well to strain out impurities when fluid is added.  The quantity of fluid in the reservoir is indicated by a direct reading sight gauge, a clear tube, or a float/dial gauge.