HYDRAULIC RESERVOIR DESIGN

figure-1(a typical reservoir cut away view)

Introduction:- 

The main function of hydraulic reservoir in hydraulic system is to store and supply the hydraulic fluid to be used in the system.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.

Functions of Hydraulic reservoir:- 

1. Storage of hydraulic fluid.

2. Acts as a heat exchanger.

3. Acts as a De-aeration system.

4. Can be used to support the hydraulic components to save  floor space.

5. Acts As a fluid conditioner.

Components of Hydraulic reservoir:- 

A typical industrial reservoir is made up of steel sheet metal fabricated to a definite shape according to requirement.In case the reservoir is used in any mobile equipment like mobile cranes or material handling equipments the the reservoir sizing and weight is very critical.

A reservoir has different components and accessories(see above picture).

1. Reservoir(sheet metal fabricated structure to suit particular requirement)

2. Strainer

3. Drain plug

4. Baffle plates 

5. Suction pipes (towards the pump)

6. Return line pipe.(from all the return line manifolds)

7. Return line filter

8. Breather

9. Riser.

10.Man hole or clean out plates(for cleaning the reservoir) 

11.Filler cap.

12.Oil level gauge.

13.Temperature gauge.

14.Hydraulic oil cooler (it is not an integral part of reservoir)

15.Oil heater (in case of cold countries)

All the above components are very essential for a good hydraulic tank design as every component has its specific function.

Strainer:-

It is fitted to the suction line inlet.It provides the filtered fluid to the pump.It normally contains some filtering screens (for particular size particulates) which filters the contaminants before it goes to the pump.It is replaced time to time to ensure blockage and reduced flow.

Drain plug:-
It is normally fitted at the bottom of the tank to drain away the fluid while overhauling or

cleaning.The drain plug size should not be so large or so small.one or more drain plugs can be provided depending on requirement.Normally drain plugs are fitted to the welded flanges.Most important factor is to choose proper fittings for the drain plug to avoid leakage issues.

Baffle plates:-

baffle plates are normally the steel plates which are incorporated in the reservoir to divide the fluid in to different chambers so that the fluid has to travel through other ways to get to the suction chamber.Baffle plates divide the reservoir into to chambers return flow chamber and suction chamber.From the suction chamber pump suction lines start. The return line flows are  put into the return chamber so that the return flow has to flow certain distance before it reaches the suction line and in the way get cooled and contaminants get to the bottom of reservoir.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” gap 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).

Suction pipe:-

Suction pipes generally have larger diameter as compared to return lines.Suction pipes have smaller length so as to facilitate lesser loss and cavitation.

Return pipe:-

Return pipe comes directly from the return line manifold or the return line filter if provided.The return line end should not be too high from the bottom of the reservoir as too high outlet will cause turbulence to the output flow.Normally the return line ends are taper cut to facilitate the flow direction towards the wall side.This will help the fluid to travel larger distance so that it gets more time to cool,purify and de-aeration.

Breather:-

 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.

 Filler cap:-

 The filler opening is often a part of breather assembly.The opening has removable screen that keeps  contaminants out of the tank while fluid is being added to the reservoir.A cap that would provide tight seal should be chained to the reservoir.
 Oil level gauge:-
 To check the fluid level of the reservoir without opening it and preventing it from contaminants oil level gauge is used.
 

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.

Heat dissipation by the wall surface of reservoir:-

 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.

Reservoir sizing :-
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.