Hydraulic System Design - Technical Notes

Hydraulic Reservoirs

Hydraulic systems need a finite amount of liquid fluid that must be stored and reused continually as the circuit works, therefore, part of any hydraulic circuit is a storage reservoir or tank. Reservoir design and implementation is very important, the efficiency of a well-designed hydraulic circuit can be greatly reduced by poor tank design. A hydraulic reservoir does much more than just provides a place to store fluid, a reservoir also dissipates heat, allows time for contamination to drop out of the fluid, and allows air bubbles to come to the surface and dissipate and may give a positive pressure to the pump inlet.
Pumps that are mounted alongside of tank, assures the pump inlet always has fluid, there will be some vacuum in the inlet line when the pump is running. A pump with its inlet below fluid level no longer has to raise the fluid, but it does have to accelerate and move it. However, this design is far better than the pump on top and can extend the service life of any type pump. A shutoff valve in the inlet line allows maintenance work to be done on the pump without draining the tank. Install a free-flowing valve (such as a quarter-turn ball type full ported).

Tank Functions
The main reason the reservoir exists is to store fluid. The accepted general rule for sizing a tank is; the tank volume should be two to four times the pump flow in gpm. This is only a general rule, and some circuits may require more volume, while less fluid may be adequate for other circuits. With this general rule, the returned fluid theoretically will have two to three minutes in the tank before it circulates again. The application really determines the reservoir size, for example, in very high ambient temperatures the reservoir should be oversized if possible even with an oil cooler. It requires time to dissipate heat, allow contamination and air to escape the fluid, and the longer you can give the fluid time to rest before it circulates again the better.
On mobile equipment where space and weight are a premium, the reservoir still must provide all the basic functions, just at a lower level. More external cooling and filtration is required in most mobile applications to make up what the oil tank could do if sized larger.
The reservoir level during operation should raise and lower no more than is necessary, about 6”-8”. The more the fluid level changes, the more air is required to enter and leave the reservoir. Along with this air exchange, comes moisture which produces condensation (water) in the fluid. There also should be about a 4”-6” air pocket above the oil level at all times and sufficient level above the pump from falling below the inlet and to prevent a vortex from developing above the pump inlet allowing air to enter the system.
Another situation where a tank may need to be larger is if the circuit has accumulators. Accumulators need fluid to fill them at start up and space into which to discharge this fluid at shut down. An undersized reservoir may not have enough fluid to keep the pump inlet covered at all times.

Tank Components
A baffle separates the return line from the pump inlet line, forcing the fluid to take the longest possible path through the reservoir before returning to the pump inlet. This arrangement also mixes the fluid well and provides more time to drop contaminates and de-aerate. In addition, the fluid spends more time in contact with the outer walls of the reservoir to dissipate heat.
A formula for estimating how much heat a reservoir can dissipate is as follows:

HP = 0.001 x (Tf – Ta) x A
HP = maximum HP tank can dissipate.
Tf = maximum fluid temperature as °F.
Ta = maximum ambient air temperature as °F.
A = tank area as sq. ft. in contact with fluid.

Heat dissipation is the main reason for having the tank bottom off the floor and why it is important not to stop free airflow around the tank. No through holes are to be in the baffle, we want the oil flow to go over the top of the baffle. A small ¼” gape at each end of the baffle allows oil levels to remain equal on each side of the baffle, (majorly of flow goes over the top).
The breather cap of the correct size should include a filter media to block contaminants as the fluid level lowers and rises during a cycle. It is important to understand that too small a breather cap will cause a vacuum in the oil tank and cause cavitations in the pumps. You should never use a filler-breather type cap, as any opening where someone can pour contaminated fluid into the tank should be avoided. Always provide a quick disconnect just before the return filter used for adding or filling the reservoir, that way all fluid going into the tank is filtered.
All reservoirs must be painted inside with a hot oil resistant paint to prevent rust, never use an un-painted oil tank; it will rust within a few months of use. Clean outs (prefer two) are required to drain and open the reservoir for cleaning, (usually once every year to year and half). A simple water wash down and wipe dry is all that is required, (do not scrape the inside of an oil tank and remove any paint).
Sight glasses with a temperature gauge make it easy to visually check fluid level. Calibrated sight gauges provide even more accuracy. Tanks that feel hot to the touch may actually be within operating range. The temperature gauge gives a more specific indication of oil temperature.
All return lines should be terminated under oil level, and never closer than 2”-4” from the bottom of the tank. A 45° cut on the bottom of the return line facing towards a reservoir wall will direct the oil flow in that direction to slow oil flow and direct flow for heat dissipation against the wall. Size returns lines for maximum oil flow and speed of about 8-12 ft/sec. velocity. The pump inlet should be located at least 2” off the bottom of the oil tank.

These notes are just basic simple suggestions and are not intended to be detailed design notes in designing reservoirs. There are many other factors and design notes required to make a good reservoir, and are too many to outline is the forum. A good reservoir design is the least expensive to build, but the most critical component of any hydraulic system.