The function of the leak replenishing valve

This valve type is often called a "snifting valve". 

At the end of the suction stroke, the leak replenishing valve allows the oil volume lost during the work cycle due to leakage to be recirculated from the reservoir. In principle, a leak replenishing valve is structured like a check valve. This valve opens depending on the pressure in the hydraulic part. It is opened due to the pressure difference between the reservoir and hydraulic chamber. Most leak replenishing valves are spring-loaded, i.e. they open at a certain pressure difference to the reservoir before air is released from the oil in the hydraulic part. Different leak replenishing valve designs are shown in the figure.

Figure : Leak replenishing valves; adjustable, screw-in, flange-mounted

Regarding the use and design of leak replenishing valves, another function can also be implemented, which relates to the fault state during a pressure relief valve activation. 

If the pressure relief valve opens, it causes oil circulation in the pump, since the oil fed back into the reservoir here is recycled by the leak replenishing valve. During circulation, the mechanical output from the drive is converted into heat that is first absorbed by the hydraulic oil. The more oil that is circulated in this way and the higher the pressure difference during the opening phase of the PRV in this circuit, the more quickly the oil temperature increases. Especially in the case of high-power pumps, this can lead to overheating and the decomposition of the oil as well as damage to the seals. 

In order to limit the energy input and temperature increase, the nominal size of the leak replenishing valve is chosen to be significantly smaller than the fluid-side suction valve. This means that the pressure loss in the leak replenishing valve during leak replenishment becomes relatively large if larger oil volumes are circulated. Oil is stored at atmospheric pressure in the reservoir. Consequently, the pressure at the leak replenishing valve falls significantly below atmospheric levels and a portion of dissolved air forms gas bubbles. Under normal conditions, mineral oil contains a high portion of trapped air. Gas bubbles in the oil reduce the amount of circulating oil due to their high compressibility. Finally, the dissipated power and the temperature increase are reduced as a result.