What is the gas removal valve

This valve, also called a vent valve or degassing valve, removes gas bubbles from the hydraulic chamber. 

In many hydraulic diaphragm pumps, the hydraulic circuit has to be vented continuously because new gas bubbles are constantly released from the hydraulic oil due pressure changes and flow. To maintain the delivery rate and metering accuracy, these gas bubbles have to be removed from the hydraulic chamber quickly and safely. 

After resolving fault states, such as a blocked suction line or a pressure relief valve activation, gas bubbles may also still be present in the hydraulic chamber. After the fault state, the metering pump should resume running as soon as possible with full power and metering accuracy. To do so, the hydraulic oil must quickly be made bubble-free again. 

Compared to releasing the air, it takes considerably longer for the air to become dissolved in the oil again. In order to accelerate the degassing process, many hydraulic diaphragm pumps have a degassing valve. This valve removes gas bubbles from the hydraulic chamber by diverting a steady removal volume into the reservoir at the beginning of each discharge stroke. This removal volume is small when compared to the stroke volume. The gas removal valve is positioned at a point in the hydraulic chamber where gas bubbles have the potential to collect. This allows it to deliver an air/oil mixture, or just oil, thus venting the pump head.

Figure : Vent valves; left: volumetric, right: continuous

In the figure, the left shows a gas removal valve with a moving removal plunger. There are also solutions with balls and a double seat. The right image shows a gas removal valve with continuous removal by a cascade throttle. Removing equal volumes with each stroke cycle has advantages in terms of metering accuracy, but is more expensive due to the increased complexity of the valves. Continuous removal is easier, more robust and more cost-efficient in terms of design, but has disadvantages regarding metering accuracy due to its pressure dependency.