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Thursday, 6 July 2017

WHAT IS THE FUNCTION OF A RESERVOIR IN A HYDRAULIC SYSTEM?

We were discussing hydraulic actuators, basic concepts of hydraulic accumulator, hydraulic flow control valve and direction control valve in our previous post. Today we will start here new topic in subject of hydraulic system and that is hydraulic reservoir.

So, we will discuss here the basic concepts, roles or functions, importance and important design aspects of a hydraulic reservoir.

We will start here from very basic functions and design features of a hydraulic reservoir and after completing this article, we will come to understand the importance of a reservoir in a hydraulic system.

So let us come to the main topic i.e. Reservoir in hydraulic system

In a hydraulic system, hydraulic oil is used as the working media. In order to complete the task, hydraulic oil must be stored and reused continuously in hydraulic circuit.

Therefore, hydraulic reservoir is a very important component of a hydraulic circuit or hydraulic system as it will receive the returning hydraulic oil and also it will provide hydraulic oil to the pump inlet.

Efficiency of any hydraulic system will be greatly dependent over the design of hydraulic reservoir and hence design of reservoir is very important and critical step in order to develop a correct and efficient hydraulic system.

In order to design the correct and efficient reservoir, we must have information about the various functions of a reservoir and its various design features too. Let us first see here the various functions of a reservoir in a hydraulic system.

Functions of reservoir in a hydraulic system

Reservoir will not have to only store the hydraulic fluid or oil, but also reservoir has to perform following functions too in a hydraulic system as mentioned here.

Heat dissipation or cooling the hydraulic fluid

Reservoir helps for the heat dissipation from returning hydraulic fluid to the surrounding atmosphere and hence reservoir must have large surface area to remove the heat energy from hydraulic fluid to the surrounding atmosphere.

Hence, reservoir will cool the hydraulic fluid by dissipating the heat from returning hydraulic fluid to surrounding atmosphere.

Settle down and removal of contaminants from hydraulic fluid

Reservoir will also have to reduce the velocity of returning hydraulic fluid coming to the reservoir and also to settle down the contaminants. Therefore reservoir must have enough volume to perform this task.

Removes air from the hydraulic fluid

Reservoir will also have to help to remove the air from the hydraulic fluid. Therefore reservoir must have enough volume and also air breather to secure an efficient reservoir.

Important design aspects

In order to develop correct and efficient hydraulic reservoir, following design points must be considered before designing the hydraulic reservoir.

Sizing of reservoir

There is one thumb rule to determine the volume of the reservoir on the basis of rated flow of the pump and according to this thumb rule, volume of the hydraulic reservoir must be 3 times of the rated flow rate of fixed displacement pump or mean flow rate of variable displacement pump.
Hydraulic reservoir
Reservoir must have large surface area and enough volume in order to remove the heat and contaminates from hydraulic fluid.  

Let us think that we have one hydraulic system with fixed displacement pump of rated flow rate 200 LPM, in this case we can easily figure out the desired size and capacity of the hydraulic reservoir by considering the thumb rule and it will be approximate 600 L.

We must note it here that this is only thumb rule which is usually preferred earlier by design engineers in order to figure out the volume of the hydraulic reservoir.

But these days, there are space restrictions and also some economical restrictions and therefore hydraulic reservoir designers always think to secure the optimum size or capacity of hydraulic reservoir.

Provision for removing and cleaning of contaminates from the reservoir

In hydraulic system, we know that hydraulic fluid will be continuous in use in order to actuate the actuators and hence at some point of time, properties of hydraulic fluid will be lost or not up to the desired limit and hydraulic fluid will be degraded.

Therefore, we will have to replace the hydraulic fluid according to the OEM instructions provided by the manufacturer.

There must be one drain plug in order to remove the degraded hydraulic fluid. Hydraulic reservoir must have one clean out plate in order to remove contaminates from the reservoir.

According to OEM instructions provided by the manufacturer, we will have to replace the hydraulic fluid and at that time we must have to clean the reservoir thoroughly by opening the clean out plate.

Provision for expelling entrained air

Hydraulic reservoir must have an air breather unit. It will permit the air to leave or to enter the space above the hydraulic fluid as level of hydraulic fluid goes up and down.

Hydraulic reservoir must have baffle plate

Return hydraulic fluid must be separated with the hydraulic fluid going to the pump. If return hydraulic fluid will directly go the pump inlet then it that situation, there will be turbulence effect at the inlet of the pump and also hydraulic fluid will have high temperature above desired limit.

Therefore, baffle plate is used in hydraulic reservoir in order to separate the hydraulic fluid returning to the reservoir and hydraulic fluid going to the pump inlet.

Hydraulic fluid must be cooled and free from contaminates before going to the inlet of the pump. Return hydraulic fluid will be circulated around the wall of the reservoir for cooling before going to the inlet of the pump.

Height of the baffle plate must be 2/3 of the height of the reservoir and its lower corners must be cut diagonally to allow the circulation of hydraulic fluid.

Do you have suggestions? Please write in comment box.

We will now discuss the basics of strainers and filters in hydraulic system in our next post.

Reference:

Industrial hydraulics Maintenance manual
Rexroth Industrial hydraulic manual
Image Courtesy: Google

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