ALS Tribology eSource
September 6, 2011

Effects of Reservoir Design on Hydraulic and Turbine Circulating System Performance
by David Doyle, CLS, OMA I & II
Vice President & Operations ManagerReservoir

General reservoir design is important to maintaining lubricant health in clean hydraulic and turbine oil circulating systems. The reservoir is not solely for storing fluid for a circulating system, it is also an important component in the proper operation of the system. A good reservoir design will greatly enhance equipment reliability and fluid service life

Reservoir size should allow for several minutes of residence time to allow the fluid to cool, settle out contaminants, and eliminate entrained air. This is generally dictated by the gpm of the pump. Reservoir size is part of the equation. Location of the reservoir inlet in relation to the pump outlet is important. Baffles also aid in retention time by forcing the fluid to take a longer patch from inlet to outlet. The reservoir inlet line should be below fluid level to reduce turbulence and aeration. The pump outlet line should be as short a distance to the pump as possible and optimally located on the same end of the tank as the inlet line, but on the other side of the baffle. This forces the fluid to take the long path around the baffle through a gap at the opposite end of the tank. Regardless of reservoir size, if fluid is not kept at an adequate level then the lubricant will not adequately cool, settle contaminants and dissipate air.

As indicated, the reservoir is important to system temperature control. Maximum fluid temperature should be kept below 54C (130F). If fluid temperatures are not allowed to cool sufficiently before returning to the circulating system then excessive operating temperatures can occur. Heat exchangers can aid in this process under excessive conditions. Not only is the reservoir important to heat control but in cold environments preheating of the fluid in will aid in preventing pump starvation and component wear at startup.  

Pump design and location can affect proper flow and pressure to feed a circulating system, especially if the pump is located above the fluid level and adequate vacuum is required of the pump inlet to feed the pump.

Clean systems require basic components in a reservoir design. Cleanout access, sight glass, and drain ports are all important components of a clean system design and proper reservoir maintenance. Proper breathers with a desiccant are important, especially in dirty environments. The most common source of dirt, debris and water in a circulating system is through breathers and seals. If there is a location where fluid leaks out of a system there’s no reason why environmental contaminants cannot enter into a system through the same path (usually when the system cools during shutdown).

Though taking a sample from the reservoir to monitor the fluid in a hydraulic or turbine oil circulating system may not be the best location, many times it is the only practical option. Care should be taken not to introduce contamination into the reservoir while obtaining the sample. Avoid taking the sample from the very top or very bottom of the fluid level. Half way to two thirds of the way down from the top of the fluid level is best. The best location for taking a fluid sample is directly from the return line to the reservoir just before the filter. Avoid straight lengths of piping. Locations with turbulent flow, such as bends, provide the most representative sample.


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