Hydraulic oil tank plays an important role in hydraulic circuit design. It stores hydraulic oil when it is not flowing in the hydraulic system.
Before we start looking at the hydraulic tank in more detail, it's worth reminding ourselves how the hydraulic system works and the role that the hydraulic tank plays in the overall system.
Hydraulic systems are designed to transmit power in a controlled manner through the use of pressurized fluids. There is a limited amount of hydraulic oil in the system, which is continuously stored and reused while the system is running. This makes the fluid storage tank, the oil tank, an essential part of any hydraulic system.
The hydraulic tank actually performs many functions in the hydraulic system because it:
Hold a certain volume of liquid
Transfer heat from the system
Allow solid contaminants to settle, and
Promotes the release of air and moisture from the fluid.
Although "fluid storage tank" may seem like a very simple concept, the design and implementation of the storage tank is very important. The efficiency of even a well-designed hydraulic system can be affected by a poorly designed hydraulic tank.
What are the design considerations for the hydraulic tank?
#1. Fuel tank size
The general rules for hydraulic tank size depend on whether the system is open or closed. For most industrial applications, the tank volume should be 3 to 5 times the pump flow rate per minute and at least 2.5 times the flow rate. While this is true in most cases, a larger tank than this may be required in some cases - for example, to ensure the level remains above the pump inlet line.
Instead, there may be reasons a smaller tank is desirable - for example, if the overall design is very lightweight or compact.
It is worth noting that smaller sizes may have a smaller heat transfer surface area, so it may be necessary to increase the power of the heat exchanger or cooler in the design to ensure that the fluid temperature remains within an acceptable parameter range.
In addition, the smaller tank volume makes it easier to bubble and agitate the fluid when the hydraulic fluid returns to the tank. Measures or special components to reduce the speed of the return fluid can help minimize these problems. This is important because inflatable fluids can cause pump damage, cavitation, or premature failure.
#2. Contaminants and filtration
If the fluid contamination in the hydraulic system is too high, more than half of the hydraulic components will fail.
"Older hydraulic systems have lower operating pressures, simple gear pumps, and lever-operated directional valves. Particulate contaminants may cause some degradation, but this rarely leads to intermittent or catastrophic failure."
"Modern hydraulic systems are characterized by very small tolerances between the parts inside the valve and the use of proportional electromagnets with small forces.
This means that particulate contaminants can easily interfere with normal operations.
Now, even in closed systems, fluid contamination can come from a variety of internal or external sources, but one common source is the air breather in the hydraulic tank.
Hydraulic tanks require breathing apparatus to allow air to flow in and out of the tank:
When the pump drains oil, the air is sucked into the hydraulic tank through the breathing apparatus, and the fluid is sucked into the hydraulic cylinder.
The breathing apparatus releases air as the fluid returns from the cylinder to the tank.
If the passage of air out of the tank is restricted or blocked, the internal pressure may rise above a safe level, potentially leading to a serious leak.
If too little air is allowed into the tank, a partial vacuum is created, which can cause cavitation, damage the pump, and reduce the flow rate within the system.
For this reason, mesh filters are often included in respirator designs. It has to be monitored to make sure it doesn't get clogged. The differential pressure switch indicates when the filter is blocked.
However, mesh filters do not solve all problems. In addition to filtering out dust and other solid contaminants, the filter also needs to ensure that water does not enter the tank. In hydraulic systems, water can reduce the performance of hydraulic oil and cause unstable operation or lead to component failure. Due to the moisture in the air, water can enter the system through the respirator.
The solution to this problem is to use a filter dryer respirator, which uses a fine-particle filter and desiccant to capture moisture in the air before it enters the hydraulic tank. The desiccant and filter respirator also need to be monitored and maintained, as the filter element will eventually become clogged and the desiccant will be depleted over time.
This way, you can ensure that any air drawn into the hydraulic tank space is clean and dry - thus reducing the likelihood of contaminants entering the hydraulic system.
#3. Additional filtering
Traditionally, the size of hydraulic tanks has been specified to allow contaminants to sink to the bottom of the tank, thereby preventing contaminants from circulating in the hydraulic system.
However, additional entrances and exits can be used to form separate loops. This circuit consists of a circulating pump, filter and connecting hose or pipe. In this way, the fluid can be filtered and returned to the hydraulic tank.
This additional filtration helps to further reduce the risk of contaminants in the hydraulic system. In industry, this is a common circulating filtration system (including filters, coolers, and even heaters).
#4. Monitoring and maintenance
We have already mentioned that filters and respirators must be monitored and maintained to ensure their continuous and effective operation.
The cost of sensors and communications today means that adding other automatic monitoring to your hydraulic system is often cost-effective. For example, since heat is the main cause of hydraulic oil degradation, you may want to install a temperature monitor in your hydraulic tank.
While high temperature readings require a switch to turn off safely and quickly, thermostatically controlled fluid heaters may be required in cold environments. Cold, viscous fluids can also cause damage to pump suction lines and pumps.
The level indicator is another important feature to include in the tank design. If you are not measuring automatically, specifying a switch to signal when the level becomes dangerously low will help prevent catastrophic and expensive repairs.
