Biogastechnik Süd GmbH
Am Schäferhof 2
D 88316 Isny im Allgäu



phone+49 (0) 7562 970 85 – 40
fax+49 (0) 7562 970 85 – 50



+49 (0) 7562 970 85 – 555


Service Biogas plants and components



+49 (0) 7562 970 85 – 412


Service Digestate evaporator and Biogas - small plants

+49 (0) 7562 970 85 – 512
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Biosecure over-/underpressure protection

Biosecure over-/underpressure protection type ST-DS - make biogas plant operation as safe as possible

To make biogas plant operation as safe as possible, we have designed and built our Biosecure ST-DS overpressure/underpressure protection system in accordance with the most stringent safety requirements and after careful selection of the relevant harmonised standards and other technical specifications. Thus, it meets the current state of the art and guarantees a maximum level of safety.

It can either be controlled mechanically through a biogas storage membrane or using weight-operated immersion cups. The ST-DS housing is made of stainless steel; the immersion cups and the steering linkage are plastic. We use water as sealing liquid to which antifreeze is added at low outside temperatures. As an option, an insulating jacket or an electric heating rod can be installed to prevent the water from freezing when no antifreeze is added. The functioning inside the housing can be checked through the built-in sight glass.
Biosecure over-/underpressure protection type ST-DS
Biosecure over-/underpressure protection type ST-DS
The ST-DS overpressure/underpressure protection monitors and regulates the positive and negative pressure in biogas digesters. It protects the biogas storage membranes of digesters, post-digesters and digestate storage tanks against inadmissible loads.
The ST-DS has been designed as a safety component in the sense of the 2006/42/EC Machinery Directive.

However, the safety we intent to achieve when operating a biogas plant can only be accomplished in practice if all necessary measures are taken. It falls within the operator's duty of care to support these measures and to check their execution.

Structure of the biosecure over-/underpressure protection ST-DS

The ST-DS overpressure/underpressure protection (3) is mounted to the digester wall (5). The gas inlet and air outlet pipe (7) connects the overpressure protection with the inside of the digester. The air inlet (4) at the bottom side of the housing permits air through the overpressure connection into the inside of the digestate storage if necessary. The gas outlet pipe (1) allows excess gas to escape from the digester if necessary.

The overpressure protection can optionally be controlled pneumatically or pneumatically/mechanically by means of a plastic tension strap.

The mechanical control mechanism comprises a star-shaped plastic tension strap (6) connected to the ST-DS (3) above the gas storage membrane (2) of the digester (5).

Components of the biosecure over-/underpressure protection ST-DS

Counter weight (1), screw-type connecting flanges for heating rod (2), eye for plastic tension strap (3), strap guide (4), connecting flange for gas outlet pipe (5), connection for pressure sensor (6), filler neck for sealing liquid (7), test port (8) sight glass (9), stainless steel housing (10), steering linkage (11), air inlet (12) and linkage for overpressure bell (13).

Control of the biosecure over-/underpressure protection ST-DS

a. Mechanical control using the plastic tension strap

When the gas pressure in the digester is too high, the gas storage membrane extends upward. This causes the steering linkage (2) with counter-weight (1) suspended to it to be lifted by the plastic tension strap (3). Bell I (4) of the overpressure relief valve (5) is pushed upward to rise above the sealing liquid I (6). The excess biogas from the digester flows through the gas inlet opening (8) into the overpressure relief valve and further through the gas outlet neck (7) in the atmosphere. After the gas has escaped, the gas storage membrane, the plastic tension strap and the steering linkage are lowered again. The bell re-enters the sealing liquid I and prevents further gas from escaping.

b. Pneumatic control by high pressure in the digester

If the pressure in the digester or in the ST-DS exceeds a specified value, the contact pressure acting on the weight-loaded bell I (4) causes it to lift. The biogas can escape through the overpressure relief valve (5). After the pressure compensation, the bell is lowered again and prevents more gas from escaping.

Underpressure regulation:

If the pressure inside the ST-DS falls below the set value, outside air is sucked in through the air inlet opening (12). The existing contact pressure causes the weight-loaded bell (9) of the underpressure valve (10) to rise out of the sealing liquid II (11) and allow air to flow in. After the pressure compensation, the bell II (9) is lowered again and prevents more air from flowing in.

Extension for ST-DS

Insulating jacket (insulation)

The optionally available insulating jacket prevents cooling down and freezing of the sealing liquid.
The back wall of the digester and the gas inlet opening transfer heat to the sealing liquid in the overpressure protection. Depending on the substrate temperature inside the digester, the insulating jacket is able to prevent freezing of the sealing liquid down to an outside temperature of -20°C.

Weight plates

Our 150 g weight plates allow increasing the overpressure or underpressure.

Electric heating rod

To prevent freezing of the sealing liquid at very low temperatures, the ST-DS overpressure/underpressure protection can be retrofitted with an electric heating rod. The screw-type flange for the heating rod is integrated in the casing and makes for easy retrofitting.

Technical data biosecure over-/underpressure protection ST-DS

Pickup pressure of the ST-DS

(the values are set ex factory)

overpressure (283 g equivalent to 1 mbar) in overpressure valve pneumatic 3.8 mbar at 1075 g weight of the bell

underpressure (300 g equivalent to 1 mbar) pneumatic -1.0 mbar at 300 g weight of bell II in the underpressure valve

Excess filling: mechanically via plastic tension strap

Gas volume flow: 200 m³ per hour

Outlet pipe flange: DIN 150 (stainless steel)

Extension: with PEHD pipe, OD 160, max. length 3 m

Sealing liquid

If the digester internal temperature is sufficient and a heat insulation (insulation jacket) is installed on the outside: pure water
Without heat insulation on the outside and during winter operation: water with addition of antifreeze agent

Option: electric heating rod with freeze guard: water

Sight glass: soda lime glass similar to DIN 8902 with wiper made of PTFE

Connection at digester wall: DN 200 (standard), DN 150 (option)

Dimensions: Length 860 mm x Width 750 mm x Height 1370 mm

Downloads for biosecure over-/underpressure protection ST-DS

Datenblatt Über- und Unterdrucksicherung ST-DS_DE
Datenblatt Über- und Unterdrucksicherung ST-DS_EN
101 002 02 BRO ST DS DE KD.pdf
101 019 01 BRO ST DS FR KD.pdf
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