PROTECT AIRBLOWN ROOFS IN WIND AND EXTREME SNOW
How do you protect airblown roofs from snow and wind? In biogas plants and wastewater treatment, airblown systems such as double-layer membrane roofs and gas holders are commonly used. These are technically designed to withstand local weather conditions. Rarely does the weather turn out to be so exeptional that additional measures are required. But when it does occur, it is good to know what to do and how to act. This blog provides some tips for these extreme conditions.
DOUBLE MEMBRANE COVER ON BIOGAS TANK
Before we can explain how forces work on a double membrane biogas roof, it is useful to know how this roof is designed. Between the two membranes, air is blown by a fan. This air flows through the space between the two membranes and is discharged into the environment by means of an outlet valve on the opposite side. This overflowing air refreshes the air between the membranes. By adding a pre-calculated weight to the release valve, a pressure is mechanically created between both membranes. This pressure, usually 2.0 mbar, causes the upper membrane to remain convex.
The lower membrane can move freely to react to the increased or decreased production of biogas. This is a dynamic system that has been used for decades. This system has also proven to be able to withstand extreme weather conditions. In the standard version, it is suitable for temperatures between -30 °C and +70 °C, UV radiation, storms, snowfall, varying biogas pressures and sulphurous biogas. The exact design parameters are included in the static calculation accompanying the cover.
FLEXXOLUTIONS’ DESIGN CAN WITHSTAND EXTREME WIND SPEEDS AND PRESSURES
Double-layer membrane covers are made from oval/curved sections of foil to obtain their convex/round shape, which is calculated using 3D modelling software. In these 3D calculations, all extreme situations with regard to deformation due to storms, biogas pressure etc. are calculated. For a standard roof in, for example, the Netherlands and northern Germany, wind speeds of 22.5 to 25 m/s and a maximum wind pressure of approximately 0.74 kN/m2 are considered. In the extreme, this can result in a deformation as shown in the accompanying picture. If you see this in practice, it is therefore an ‘acceptable’ situation!
EXTREME SNOWFALL
With a pressure between the membranes of 2.0 mbar, the upper membrane can carry a snow load of almost 20 kg/m2. In the case of freshly fallen loose snow, 20 kg/m2 corresponds to approximately 10 cm of snow. 2] If this is wet snow, the weight per cm increases very quickly; with water, 20 kg/m2 is already reached with a 2 cm layer of water! Sticky snow is not only heavier, but it also lingers more easily and can cause the supporting air to no longer be able to hold up the upper membrane and to sink slowly downwards.
In the Netherlands, the standard maximum snowfall is 70 kg/m2 , so both membranes can lie on the underlying support structure, which will start to sag under this weight. This sagging is shown in the accompanying illustration. The ‘sticking’ of the snow will mainly occur if the silo on which such a roof is placed is cold. (Often the case with after storage or ‘post-digestion’ facilities).
EXTREME DEFORMATION IN THE EVENT OF HEAVY WIND IN A GAS HOLDER
Due to the large surface area of gas spheres, which ‘catch’ the wind, the deformation during a storm can be even more extreme. This can be clearly seen in the accompanying picture of a gas holder with a diameter of 30.5 metres. This extreme deformation with a sideways displacement of more than 3 metres has also been calculated.
ADDITIONAL MEASURES IN CASE OF EXTREME SNOWFALL
If the expected snowfall is greater than calculated in the accompanying static calculation, a number of additional measures can be taken to prevent, if possible, circumstances such as those described in the adjacent newspaper report [1]:
- 1. Keep the lower membrane filled with biogas as much as possible. (up to maximum pressure in static calculation) This allows the lower membrane to “help carry”.
- 2. If an inlet dust filter is fitted, check it for contamination and clean if necessary.
- Check pressure release valve for proper functioning. Check for fouling.
- Check the fan for function and possible soiling or snowing in.
- Keep emergency power available if necessary. Check whether a spare fan is available.
- Check the pressure relief valve for proper functioning. (Also check the anti-freeze liquid!)
- In case of very extreme expectations, the pressure between the membranes can be temporarily increased in consultation with the supplier.
The above applies to double-layer membrane covers as well as to double membrane gas holders.
ADDITIONAL MEASURES IN THE EVENT OF EXTREMELY HIGH WINDS
If wind speeds or pressures are expected to be higher than in the static calculation, for example hurricane force winds, all of the above measures may also be taken. The most important thing is to ensure that there is always air pressure available. So proper operation of the fan is the most crucial precautionary rule here.
WHAT IF THE ROOF HAS ‘COLLAPSED’?
If, due to extreme snowfall and despite any extra measures taken, the cover or gas holder has ‘collapsed’, great care must be taken to remove the snow from the cover. In the case of ‘loose’ snow, leaf blowers sometimes work well. In the case of sticky ‘wet’ snow, this should be removed manually using soft, non-sharp objects. During snow removal, adequate precautions should be taken to ensure safe working on and around the cover. Simply heating from the inside with hot air usually does not work in practice, as it requires an extreme amount of heat and the melting snow forms “water pockets” that can cause damage depending on the design of the substructure.
Especially to prevent damage in extreme cases, Flexxolutions always designs its understructures for a minimum snow load of 70-85 kg/m2 and equips them with a net on the understructure as standard to distribute extreme forces as much as possible.
Door: Martin Nieuwmeijer, MSc Chemical Engineering.
No rights can be derived from this document.
References:
[1] Agrarheute: ‘Schneemassen: Biogasanlagen drohen einzustürzen’
[2] nl.wikipedia.com ‘Sneeuwbelasting’
