The hydraulic oil tank plays an important role in hydraulic circuit design. It stores hydraulic fluid when not pushed through the hydraulic system.
Before we start looking at hydraulic reservoirs in more detail, it's worth reminding ourselves how hydraulic systems work and the role a hydraulic reservoir plays in the entire system.
Hydraulic systems are designed to transmit power in a controlled manner using pressurized fluids. There is a limited amount of hydraulic fluid in the system that is constantly stored and reused while the system is running. This makes a fluid storage tank – or reservoir – an essential part of any hydraulic system.
The hydraulic reservoir performs a number of functions within the hydraulic system, such as:
Holds some liquid
Transfers heat from the system
Allows solid contaminants to settle and
It facilitates the release of air and moisture from the fluid.
Although a “liquid storage tank” may seem like a very simple concept, the design and application of the reservoir is very important. The efficiency of even the best designed hydraulic system can be impaired by poor hydraulic reservoir design.
What are the design considerations when specifying a hydraulic reservoir?
#one. size matters
The general rule when determining a hydraulic reservoir depends on whether the system is open or closed circuit. For most industrial applications, the tank volume should be between 3 and 5 times the pump flow/minute, with a minimum flow of 2.5 times. While this is true in most cases, there are instances where it may be desirable to have tanks larger than this, for example to ensure that the liquid level remains above the pump inlet line, or when fire resistant liquids are used.
Conversely, there may be reasons why a smaller tank is preferred - for example, whether the overall design should be light or compact.
It is worth noting that smaller reservoirs may have less surface area for heat transfer, therefore it may be necessary to add a heat exchanger or cooler to the design to ensure that the liquid temperature remains within acceptable parameters.
Also, smaller reservoirs may be more prone to foaming and agitation of the fluid as hydraulic fluid is returned to the tank. A flow diffuser will help minimize these problems by reducing the velocity of the return fluid. This is important because aerated fluid can cause pump damage, cavitation, or premature failure.
#2nd. Contaminants and filtration
If the contamination levels of the fluid in the hydraulic system are too high, more than half of the hydraulic components will fail.
“Older hydraulic systems had lower operating pressures, simple gear pumps, and lever-operated directional valves. Particle contaminants may have caused some deterioration, but this rarely caused intermittent or catastrophic failures.”
“Modern hydraulic systems have valves with close tolerances between parts and variable current solenoids that use minimal force.
This means that particulate contaminants can easily interfere with normal processes.” (LunchBox Sessions)
Now, even in a closed system, fluid contamination can come from a variety of internal or external sources, but a common source is through the vent in the hydraulic reservoir.
Hydraulic tanks require aeration to allow air to enter and exit the tank:
When the pump is retracted, air is drawn into the hydraulic chamber through the vent and the fluid is drawn into the hydraulic cylinder;
Air is released by the vent as the fluid is displaced by the pump and returned to the chamber.
If the passage of air through the reservoir is restricted or blocked, the internal pressure can rise above safe levels, causing serious leaks.
If too little air is allowed to enter the reservoir, a partial vacuum will be created which can cause cavitation, damage the pump and reduce flow rates within the system.
For this reason, a mesh filter is often included in the ventilation design. This should be monitored to make sure it is not clogged. The differential pressure switch can indicate when the filter is clogged.
However, a network filter does not prevent all problems. In addition to filtering out dirt and other solid contaminants, the filter must ensure that water does not enter the chamber. In a hydraulic system, water can degrade the hydraulic fluid and cause erratic operation or component failure. Water can enter the system through ventilation due to humidity in the air.
The solution is to use a desiccant filter vent, which uses fine particulate filters and a desiccant bed to trap moisture in the air before it enters the hydraulic tank. A dryer filter vent will also need to be monitored and maintained as the filter elements will eventually clog and the dryer will run out over time.
In this way, you can ensure that the air drawn into the collector space in the hydraulic tank is clean and dry, reducing the chance of contaminants entering your hydraulic system.
#3. Additional filtering
Hydraulic reservoirs have traditionally been specified to a size that will allow contaminants to sink to the bottom of the reservoir and prevent them from circulating in the hydraulic system.
However, an auxiliary inlet and outlet port can be added to the reservoir to allow the addition of a "kidney ring". Such a loop consists of a circulation pump, filter housing, and connecting hose or piping. In this way, the liquid can be filtered and returned to the hydraulic reservoir.
This additional filtration helps further reduce the risks of contaminants in the hydraulic system.
#4. Monitoring and maintenance
We mentioned that filters and vents need to be monitored and maintained to ensure their effective and ongoing operation.
The cost of sensors and communications today means that adding other automatic monitoring to your hydraulic systems is often cost-effective. For example, you may want to add temperature monitors to your hydraulic tank, as heat is the primary cause of hydraulic fluid deterioration.
Although high temperature readings require switches for safe and fast shutdown, cold environments may require a thermostatically controlled fluid heater. Cold, dense liquid can also damage pump suction lines and pumps.
Finally, liquid level indicators are another important feature that should be incorporated into the reservoir design. Unless you're measuring automatically, specifying a switch that will signal when the fluid level is dangerously low will help prevent catastrophic and expensive repairs.