Introduction to Fluid Reservoirs and Hoses
When construction machines and construction equipment are in the design stage, considerable thought is given to the type, the size, and the location of the hydraulic oil tank. Once the machine or the equipment is in operation, the hydraulic tank functions as a storage place for the hydraulic oil. The hydraulic tank functions as a device in order to remove heat from the oil. The hydraulic tank functions as a separator in order to remove air from the oil.
Hydraulic Tank
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| Illustration 1 | g01061321 |
The main function of the hydraulic oil tank is to store oil. The tank also removes heat and air from the oil.
Tanks must have sufficient strength. Tanks must have adequate capacity. Tanks must be able to keep dirt out. Hydraulic tanks are not always sealed.
The following tank components are shown in Illustration 1:
- Fill Cap - Keeps contaminants out of the opening that is used to fill and to add oil to the tank and seals a pressurized tank.
- Sight Glass - Used to check the oil level. The oil level should be checked when the oil is cold. The oil level is usually correct when the oil is in the middle of the sight glass.
- Supply and Return Lines - The supply line allows oil to flow from the tank to the system. The return line allows oil to flow from the system to the tank.
- Drain - Located at the lowest point in the tank, the drain is used to remove old oil from the tank. The drain also allows for the removal of water and for the removal of sediment from the oil.
Pressurized Tank
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| Illustration 2 | g01061324 |
The two main types of hydraulic tanks are pressurized tank and vented (unpressurized) tank.
The pressurized tank is completely sealed. Atmospheric pressure will not affect the pressure in the tank. When oil is sent through the system, the oil absorbs heat and will expand. The expanding oil compresses the air in the tank. The compressed air forces the oil out of the tank and into the system.
The vacuum relief valve serves two purposes. The vacuum relief valve prevents a vacuum that limits the maximum pressure in the tank.
The vacuum relief valve prevents a vacuum, by opening and allowing air to enter the tank when the tank pressure drops to 3.45 kPa (.5 psi).
When pressure in the tank reaches the vacuum relief valve pressure setting, the valve opens and vents compressed air to the atmosphere. The vacuum relief valve pressure setting may vary from 70 kPa (10 psi) to 207 kPa (30 psi).
The following items are other tank components that are shown in Illustration 2:
- Filler screen - keeps large contaminants from entering the tank when the fill cap is removed.
- Filler tube - allows the tank to be filled to the correct level, but not over filled.
- Baffles - prevents the return oil from flowing directly to the tank outlet, allowing time for bubbles in the return oil to rise to the top. Baffles prevent the oil from sloshing which helps reduce foaming of the oil.
- Ecology Drain - prevents accidental spills when removing water and when removing sediment from the tank.
- Return screen - prevents larger particles from entering the tank, but does not provide fine filtering,
Vented Tank
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| Illustration 3 | g01061325 |
The vented or the un-pressurized tank differs from the pressurized tank. The vented tank also has a breather. The breather will allow air to enter and to exit freely. Atmospheric pressure on the top of the oil forces the oil out of the tank and into the system. The breather has a screen that prevents dirt from entering the tank.
ISO Symbol
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| Illustration 4 | g01061327 |
The ISO symbol for the vented and the pressurized hydraulic tanks are shown in Illustration 4.
The vented hydraulic tank symbol is merely an open topped box. The pressurized tank symbol is drawn as a completely closed box or a rectangle. Tanks are shown with hydraulic lines to enhance understanding.