As the name suggests this type of reservoir is not pressurized, which means, the pressure in the reservoir will at no point of time rise above that of atmospheric pressure. Very extensively used in hydraulic systems, these reservoirs are provided with a vent to ensure that the pressure within, does not rise above the atmospheric value.

Figure 7.1 shows the typical construction of such a reservoir conforming to industry standards.

These reservoirs are constructed with welded steel plates. The inside surfaces are painted with a sealer, to prevent the formation of rust which might in turn occur due to the presence of condensed moisture. The bottom plate is sloping and contains a drain plug at its lowest point, to allow complete draining of the tank when required. In order to access all the internals for maintenance, removable covers are provided. A level indicator which is an important part of the reservoir, is also incorporated. This allows one to see the actual level of the fluid in the reservoir, while the system is in operation. A vented breather cap with an air filter screen helps in venting the entrapped air easily. The breather cap allows the tank to breathe when the fluid level undergoes changes in tune with the system demand.

The baffle plate in the reservoir extends lengthwise across the center of the tank. Figure 7.2 shows a cross-sectional view of the reservoir depicting the baffle plate function.

The height of the baffle plate in the reservoir is about 70% of the maximum fluid height. The purpose of the baffle plate is to separate the pump inlet line from the return line. This is done to prevent the same fluid from circulating continuously within the tank. In this way it is ensured that all the fluid is uniformly used by the system.

Essentially the baffle plate performs the following functions:
• It permits foreign substances to settle at the bottom
• It allows entrained air to escape from the fluid
• It prevents localized turbulence in the reservoir
• It promotes heat dissipation from the reservoir walls.

The reservoir is designed and constructed to facilitate the installation of a pump and motor on its top surface. A smooth machined surface of adequate strength is provided to support and maintain the alignment of the two units.

The return line enters the reservoir from the side of the baffle plate, which is opposite to the pump suction line. It should be below the fluid surface level all the time, in order to prevent foaming of the fluid. Similarly, the strainer or the foot valve should be located well below the normal fluid level in the reservoir and at least 1 in. or 2.5 cms above the bottom of the reservoir. If the strainer is located too high, it will lead to the formation of a vortex or crater that will permit ingress of air into the pump suction line.

The sizing of the reservoir is based on the following criteria:
• It should have sufficient volume and space to allow the dirt and metal chips to settle and the air to escape freely.
• It should be capable of holding all the fluid that might be drained from the system.
• It should be able to maintain the fluid level high enough to prevent air escaping into the pump suction line.
• The surface area of the reservoir should be large enough to dissipate the heat generated by the system.
• It should have sufficient free board over the fluid surface to allow thermal expansion of the fluid.

For most hydraulic systems, a reservoir having a capacity of three times the volumetric flow rate of the pump has been found to be adequate.