CHAPTER 6: Hydraulic Reservoirs

Fluid power reservoirs

Fluid power systems require air or a liquid fluid to transmit energy. Pneumatic systems use the atmosphere -- the air we breathe -- as the source or reservoir for their fluid. A compressor takes in atmospheric air at 14.7 psia, compresses it to between 90 and 125 psig, and then stores it in a receiver tank. A receiver tank is similar to a hydraulic system’s accumulator. A receiver tank, Figure 6-1, stores energy for future use similar to a hydraulic accumulator. This is possible because air is a gas and thus is compressible. A receiver tank is a pressure vessel and is constructed to pressure vessel standards. At the end of the work cycle the air is simply returned to the atmosphere.

Hydraulic reservoirs

Figure 6-1. Simple pneumatic power unit.

Hydraulic systems, on the other hand, 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. This tank may be part of the machine framework or a separate stand-alone unit. In either case, 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 provide a place to put fluid. A well-designed 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. It may give a positive pressure to the pump inlet and makes a convenient mounting place for the pump and its motor, and valves.

Some standard reservoir layouts

Pump on top. Figure 6-2 shows this common reservoir/pump layout -- used by many suppliers. The flat top surface of a standard reservoir makes a perfect place to mount the pump and motor.

Figure 6-2. Pump and motor mounted on top of tank.

The main disadvantage to this configuration is that the pump must create enough vacuum to raise and accelerate the fluid into the pump inlet. For most pumps, this is not a big problem, but it is not the best situation for any of them. Axial or in-line piston pump life can be adversely affected by medium to high vacuum at its inlet when using this layout. The piping in this configuration must be sealed, should be as short as possible, and have few or no bends.

Pump alongside tank. Figure 6-3 shows another design that is satisfactory for any type pump. (Many suppliers prefer this layout.) This arrangement is sometime called a flooded suction, because the pump inlet always is filled with fluid.

Although 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.

Figure 6-3. Pump and motor mounted alongside tank.

Notice the shutoff valve in the inlet line. This valve allows maintenance work to be done on the pump without draining the tank. Some precautions: install a free-flowing valve (such as a quarter-turn ball type) and use a valve with a limit switch to indicate full open. Wire this limit switch in parallel with the pump motor starter, so the pump cannot start until the shutoff valve is open.

Pump under tank. Figure 6-4 shows the very best pump/tank layout. This design puts the pump below the reservoir to take advantage of static head pressure. As explained in Chapter 1, there is pressure at the bottom of any column of fluid (about .4 psi per foot of elevation). With the tank above, the pump not only has fluid at its inlet all the time, but this fluid also could be at 2- to 4-psi positive pressure. (Note that this arrangement can be difficult to work on without ample headroom for the mechanic.) The same shutoff valve precaution goes for this layout as mentioned for the pump-alongside design.

Tank functions

The main reason the reservoir exists is to store fluid. The accepted 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. Some circuits may require more volume, while less fluid may be adequate for other circuits. A 25-gpm pump would work well with a 50- to 75-gallon reservoir for most circuits. With this general rule, the returned fluid theoretically will have two to three minutes in the tank before it circulates again. As Figure 6-5 shows, 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.

Figure 6-4. Tank located above pump and motor.

When a circuit has single-acting cylinders or cylinders with large rods, the volume of fluid returned on the extend stroke is greatly reduced – or even non-existent. In these cases, the tank must be larger than the general rule states to keep fluid level from falling below the pump inlet line.

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.

Another case for making the tank larger than the general rule is to add cooling capacity. All of the tank’s exterior walls can radiate heat to the atmosphere, so the larger the tank the greater the heat dissipation. Use the formula in Figure 6-6 to figure tank-cooling capacity. An example problem is shown later in this chapter. Several data books include formulas and charts showing tank-cooling capacity. These also can be used in conjunction with this manual.

Heat dissipation is the main reason for having the tank bottom off the floor and why it is important not to stop free air flow around the tank. It is not good practice to enclose a power unit to reduce noise.

Tank components

The filler-breather cap should include a filter media to block contaminants as the fluid level lowers and rises during a cycle. If the cap is used for filling, it should have a filter screen in its neck to keep large particles out. It is best to pre-filter any fluid entering the tank . . . either by a filter cart transfer unit or by a filter fill unit (as shown in Chapter 2, Figure 2-2.)

Figure 6-5. Standard features of non-pressurized reservoir designed for pump to be mounted on top.

Remove the drain plug to empty the tank when the fluid needs be changed. At the same time, the clean-out covers should be removed to provide access to clean out all residue, rust, and flaking paint that may have accumulated in the tank (and doesn’t flow out with the fluid). If this is not done, the new fluid gets dirty immediately — defeating the purpose of the fluid change. In the design in Figure 6-5, the clean-out covers and internal baffle are assembled together, with some brackets to keep the baffle upright. Rubber gaskets seal the clean-out covers to prevent leaks.

If the system is badly contaminated, it is wise to flush all pipes and actuators while changing the tank fluid. This can be done satisfactorily by disconnecting the return line and placing its end in a drum, then cycling the machine. Do not over-fill the drum during this operation or it may rupture and spill fluid.

Sight glasses make it easy to visually check fluid level. Calibrated sight gauges provide even more accuracy. If the sight glass or gauge is difficult to see or is damaged, find another way to check the fluid level.

Many sight gauges include a fluid-temperature gauge. Tanks that feel hot to the touch may actually be within operating range. The temperature gauge gives a more specific indication. On older systems where the temperature gauge may have stopped working, it’s best to check fluid temperature with some other method.