Over-/ under-pressure ÜU GD 

The over-/ under-pressure valve ÜU GD serves to regulate the over-/ under-pressure in the fermenter of a biogas plant. The high and low pressure protection ÜU GD 150/200 has a stainless steel container with a barrier liquid and partition plate between the gas space and airspace.

In the inspection glass the operator can see at a glance 

  • If the barrier fluid is contaminated

  • The current gas pressure

  • The current valve setting for maximum pressure

  • If gas is being expelled

The over-, under-pressure valve can be provided with an additional pressure plate, which is weight-loaded and responsive when contamination or overloading renders the actual over-, under-pressure protection insufficient and the gas pressure exceeds the permissible value. The pressure plate is used as an emergency pressure relief device and is set to a higher value than the ÜU GD valve.

Operating principle:

In the case of increasing pressure on the gas side of the fermenter, the liquid level in the gas space of the over-, under-pressure valve is pressed down until the biogas can pass under the partition plate, and escapes to the air space through and the blow off pipe. Conversely in the case of under-pressure the liquid level on the air side can be pressed down to let air pass to the gas side of the fermenter. Thus, a possible unapproved under-pressure is reduced in the fermenter. The chamber parameters are designed so that at a chosen adjustment of the set pressure for the response pressure is always twice as large. The maximum level of the sealing liquid is set by an overflow pipe. Condensation or rain water entering can run off through the overflow pipe automatically. The height of the liquid level determines the set pressure at which the gas (over-pressure) or air is allowed to pass (in the case of under-pressure. In the case of increasing pressure through accummulated condensation or rain water excess sealing liquid is first pushed out through the overflow pipe.

Important note:

The outlet pipe must always remain open to allow excess condensate to be able to drain away. Because of the risk of freezing and empty sucking by the siphon, only an open channel and not a tube should be used for removal of the condensate!


We offer two different sizes for the over and-under pressure seals:

  • ÜU seal 150 for amounts of gas up to 125 m3/h

  • ÜU seal 200 for amounts of gas up to 200 m3/h