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. 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.
Reservoir size is just part of the equation. Location of the reservoir inlet in relation to the outlet going to the circulating pump also is important. The reservoir inlet line should be below fluid level to reduce turbulence and aeration. Baffles aid in retention time by forcing the fluid to take a longer path from inlet to outlet. The reservoir outlet would be optimally located on the other side of a baffle and on the opposite end of the same tank wall that has the inlet. This forces the fluid to take the longest path around the baffle through a gap at the opposite side of the tank. If the reservoir does not contain a baffle then inlet and outlet locations should be as far away from each other as possible.
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 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. If the pump is located above the reservoir fluid level the pump needs to create an adequate vacuum to feed the pump inlet. The circulating pump inlet should be as short a distance as possible to the reservoir outlet.
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. The most common source of dirt, debris and water in a circulating system is through breathers and seals. Proper breathers, especially in wet or dirty environments, are important for protection from entry of outside airborne contamination. As the fluid level changes in a reservoir air is pulled in as fluid levels decrease. A two-stage breather system that utilizes a filter to block airborne particles and a desiccant to remove airborne moisture is most effective. If there is a location where fluid leaks out of a system through seals then outside environmental contaminants can also enter into a system through the same path. This usually occurs when the system cools during shutdown and pressures in the system change from high to low.
Though taking a sample from the reservoir to monitor the fluid in a hydraulic or turbine oil circulating system may not be the most ideal 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 reservoir. Avoid straight lengths of piping. Locations with turbulent flow, such as bends, provide the most representative sample.
David Doyle, CLS, OMA I, OMA II