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Components for our double-membrane system

Fastening system made of bolted stainless steel clamping rails

The fastening system ensures gas-tight mounting of the double membrane on the reservoir crown and it also enables mounting of the substructure on the upper edge of the tank. The double membrane is fastened by means of threaded anchors cemented into the concrete or, in the case of steel tanks, with bolts. The seal consists of a foamed, self-adhesive PE strip. In the case of concrete tanks, the lower clamping rail provides a support for the double membrane and it features mounting points (flaps) for the substructure (strap layer). The double membrane is held in place by the upper clamping rail in both concrete tanks and steel tanks. All metallic components are made of corrosion-resistant stainless steel.

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Stainless steel centre support with apex crown ring

The centre support combined with the crown ring supports the substructure and is used to fasten the strap layer. The centre support is mounted to the tank floor with four bolt anchors. All the components are made of corrosion-resistant stainless steel.

The crown ring is mounted on the centre support. The straps are placed around the upper circumferential ring and routed to the edge of the tank. This makes it possible to tension the straps evenly from both ends. In addition, the even spacing prevents friction between the straps. All the components are made of corrosion-resistant stainless steel.

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Strap substructure

The substructure is used for mounting the double membrane and it prevents the double membrane from becoming immersed in the substrate when the gas pressure drops for operational reasons. The substructure consists of a polyester strap layer fastened at the centre of the tank to the crown ring and to the upper tank edge. For concrete tanks, the fastening at the upper tank edge involves flaps on the lower clamping profiles, whereas for steel tanks it involves strap mounting brackets fastened to the tank wall. All metallic components are made of corrosion-resistant stainless steel.

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Support air supply

The support air fan increases the pressure in the support air space. The volume of air required is conveyed through the air inlet line between the membranes with the help of one or two fans. Excess air can escape through a safety valve in the air inlet line or through an air outlet. The air inlet and air outlet can be completely or partially closed using gate valves. We customise the support air supply according to our customers’ specific operating parameters (gas operating pressure, support air pressure, gas withdrawal, gas production etc.) and taking the effects of weather fluctuations into account.

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Protection against over/under-pressure

The protection against over/under-pressure is a safety device that protects the roof system against damage caused by unacceptably high excess pressure or low underpressure in the gas chamber. The protection against over/under-pressure is installed on the outside of the tank wall. The pressure protection is provided by submersible drums with a fluid reservoir. The fill level in the fluid reservoirs can be checked at the inspection tubes or viewing windows.

Our protection against over/under-pressure is available in the following versions: type L DN 150 with a discharge capacity of around 450 m³/h and a maximum gas pressure of 5 mbar, or type XY DN 200 with a discharge capacity of around 950 m³/h and a maximum gas pressure of 8 mbar.

The protection against over/under-pressure can be upgraded to include activation monitoring in accordance with TRAS-120 and it can also be fitted with heating systems.

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Gas level measuring technology

dbds offers various solutions for gas level measurement, each offering different benefits:

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Measuring strap indicator with weight

We often employ the widely used measuring strap, also known as a cable sensor, for shallow gas storage tanks with continuous withdrawal of gas. This is a simple and pragmatic means of measuring the gas fill level. The measuring strap is worked onto the inner membrane during manufacturing and production. It is affixed to one side of the tank together with the components that make up the construction set, guided through hemstitch seams over the apex of the gas tank membrane and is then fastened to the above-mentioned steel weight in order to indicate the fill level. The steel weight is enclosed in a transparent tube and its height changes proportionally to the gas fill level. If necessary, we can also provide an electric evaluation unit with a 4 to 20 mA signal that can be integrated into your plant control system.

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The GasHmeter from Seitz Electric GmbH is a measuring device for recording the height position of the gas tank membrane within the double-membrane storage tank. It enables precise measurement of the height, especially for double-membrane systems with higher profiles.

The measuring principle is based on hydraulic force transmission. A pressure transducer is attached to the end of a tube filled with fluid. This pressure transducer measures the resultant pressure difference between the top and bottom ends of the tube and it generates a 4 to 20 mA signal that is proportional to the height.

The pressure transducer is normally installed at the lowest point of the object to be measured with the free end of the measuring tube at the top end. This enables precise measurement of the height of the membrane. To measure the fill level as precisely as possible, we recommend using two to three GasHmeter units for each gas tank so that a meaningful average value can be calculated even when the gas tank membrane rises unevenly.

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Laser distance sensors

As an alternative to the GasHmeter, we also use compact laser distance sensors that have been optimised for biogas; these provide contactless and precise measurement of the distance between the gas tank membrane and the outer membrane. The laser is bolted onto a flange, preferably at the pole and possibly at additional points, and it is fitted with a protective cover against rain. A lens heater prevents condensation on the lenses from affecting the measurement. The key benefit of this measuring technology is that the inner membrane can move without contact while still enabling precise measurement. In this case, we also recommend measuring from multiple points and calculating an average. 

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