By David Marlowe • Owner/CEO • DMAR Technical Training and DMAR Business Centers USA
Previously, I have written about the importance of knowing your hydraulic fluid’s viscosity and offered tips on keeping your oil contaminant-free. Now it’s time to discuss the heart of the hydraulic and lube oil system—the system reservoir. The typical reservoir is constructed of welded steel plate and is designed to store and cool the fluid that supplies the system.
There are three standard reservoir designs commonly used today:
- Joint Industry Conference (JIC)—a horizontal tank with extensions that hold the tank a minimum of 6 in. off the floor. This design increases the area of heat transfer from the bottom as well as the top and sides. The main disadvantage to this configuration is that the pump must create enough vacuum to raise and accelerate the fluid into the pump inlet.
- L-Shaped—a vertical tank mounted to one side of a wide base where the pump and motor are mounted. This design provides a large surface area for cooling. Additionally, having the fluid level in the tank higher than the pump inlet provides a positive suction head. The base is also raised 6 in. off the floor. This design is far superior to having the pump on top and can extend the service life of any type of pump.
- Overhead Stack—a horizontal tank mounted on a rack above the pump and motor. The vertical arrangement of the reservoir conserves floor space and offers easier access to the reservoir for maintenance. This is, in my opinion, the best pump/tank layout.
Allowing for heat dissipation is the main reason to design the tank bottom off the floor. This is also why it is important not to stop free airflow around the tank. Note: It is not a good practice to enclose a power unit.
A large tank is always desirable to promote cooling while allowing the separation of contaminants. The general rule for reservoir sizing is pump gallons per minute or liters per minute x 3 or x 5. If the reservoir is undersized, the fluid level will create a whirlpool effect at the pump’s inlet.
The whirlpool will affect the Net Positive Suction Head Required (NPSHR) by the pump and the Net Positive Suction Head Available (NPSHA) in the reservoir. When the NPSHA and the NPSHR are equal, cavitation will occur.
To prevent cavitation, you must have a basic understanding of NPSHA and NPSHR:
- NPSHA—a measure of how close the fluid at a given point is to boiling, and then cavitation.
- NPSHR—the head value at a specific point required to keep the fluid from cavitating.
Cavitation is the formation of vapor cavities (bubbles or voids) in a liquid. This occurs when a liquid is subjected to a change of pressure that causes the formation of these bubbles where the pressure is relatively low. When the bubbles are subjected to a higher pressure, they implode and generate an intense shockwave, causing high vibration and mechanical damage.
It is crucial to determine what size of tank your hydraulic or lube oil application will require, as the efficiency of a well-designed system can be greatly reduced by incorrect tank size.
Heat is generated in a system whenever there is a pressure drop and no mechanical work has been performed.
Use this formula to check the heat dissipation capacity of a tank and ensure it is sufficient enough to cool down the oil.
Head Dissipation, HD = 0.001 x (T1 – T2) x A, where
T1 = max. allowable fluid temperature (in °F)
T2 = max. ambient air temperature (in °F)
A = area of tank in contact with fluid (in ft2)
The table shows the heat radiating capacity, in horsepower, of commercial steel hydraulic oil reservoirs having a 6-in. space underneath and free air circulation on top and all sides.
The reservoir does much more than just provide a place to put fluid. A well-designed reservoir dissipates heat, allows time for contamination to drop out of the fluid, and allows air bubbles to come to the surface and dissipate, all while providing a positive suction head pressure to the pump.
If the standard reservoir design cannot be used or modified and a custom designed reservoir must be built, the following must be considered:
- With the reservoir in position, there must be ample clean-out openings to allow easy accessibility for cleaning and maintenance.
- If the top of the reservoir is to be used for mounting equipment, pay attention to the construction of the walls and top so they can support the mounted equipment.
- Ensure that the reservoir is provided with a fill opening, a drain connection located at the lowest point and a level gauge.
Size the reservoir for a 20% overcapacity to provide a reserve against unexpected demands on system capacity.
Filed Under: Fluid Power World Magazine Articles