How to Install a reservoir, perforated disk and leak replenishing valve to the diaphragm compressor head
In order to catch leakage occurring at the plunger seal, a collecting container is integrated into the holder between pump head and drive. The oil leakage is collected in this reservoir.
In order to prevent overstretching of the diaphragm in the direction of the hydraulic chamber, a perforated disk is installed that supports the diaphragm. After a corresponding leakage loss, the diaphragm lays on this perforated disk towards the end of the suction stroke. Diaphragm overstretching is thus prevented.
As the diaphragm would come lay on the perforated disk before the plunger reaches the RDC due to leakage loss, this would lead to a drop in pressure in the hydraulic chamber. This drop in pressure would cause air to be released from the hydraulic oil. The resulting air bubbles are much more compressible than the hydraulic oil itself. Therefore, air bubbles in the hydraulic chamber would cause a significant drop in flow rate due to the extension of the compression phase. A large portion of the plunger stroke would be spent compressing the air bubbles and would no longer be available for transporting fluid. This would result in poor pump efficiency.
The adverse drop in pressure and the resulting formation of air bubbles is prevented by installing a leak replenishing valve. This valve opens at the end of the suction stroke as a function of the pressure in the hydraulic part and allows lost oil volume to be recirculated from the reservoir. In principle, a leak replenishing valve is structured liked a check valve. Most leak replenishing valves are spring-loaded, i.e. they open at a certain pressure level before air is released in the hydraulic part.
The diaphragm pump head shown schematically in figure 1 can already function on a long-term basis, provided that no unfavorable operating conditions or fault states occur.
Figure 1: Diaphragm pump with perforated disk and reservoir
However, the design of the leak replenishing valve can cause problems. If the pressure in the working space drops significantly and too early during the suction phase, this low pressure is transferred over the diaphragm to the hydraulic oil, possibly resulting in the leak replenishing valve opening prematurely. The pressure-controlled leak replenishing valve would then open before the diaphragm has reached the perforated disk at the rear position. Then, the plunger would suck hydraulic oil into the hydraulic chamber instead of pumped fluid into the working space, and the oil volume there would increase. If this happened for multiple stroke cycles in a row, the oil volume would become so great that the diaphragm would shift forward and be overstretched in the forward direction. Because of this, the design is not secure at low suction pressures or with a closed suction line. Suction pressure that is too low, for example, may occur if filters become clogged, if valves are incorrectly closed, if pipe cross-sections are too small or if suction containers are positioned incorrectly. As a whole, this design step has the following advantages and disadvantages:
– The reservoir (the oil collecting container) catches the leakage
– The perforated disk prevents the diaphragm from overstretching in the backward direction
– The leak replenishing valve compensates the leakage and prevents outgassing in the oil
– In case of a closed suction line or low suction pressure, the leak replenishing valve opens too soon. The diaphragm is then pressed forward and overstretched in that direction. This means that the pump is not safe for operation with a closed suction line or low suction pressure.
For the next step, it must be considered how design measures can be used to eliminate the existing disadvantages and make the diaphragm pump safe.
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