27 June 2009
The following article is based on an extract from Wet News (November 2008 edition) the original article is about work done by water and highways/earthworks Contractor May Gurney to refurbish and improve the current Severn Trent Coalport, anaerobic digester, which digests sewage sludge.
I was very interested in the information about the biogas gas holder, which they also call a gas-bag due to its flexible construction. We are told that, the installation of a new, larger gas bag will add to efficiency of the site.
The new spherical gas holders are the most immediate visible sign of the state of the art solutions that have been implemented at the Severn Trent sites. These are golfing ball white spheres which are created from 2 polyester and PVC-based skins, the outer being inflated by air pressure and holds up the structure. The inner skin is postponed inside he outer skin and contains the biogas.
The wonderful thing about these new biogas holders is that they are made from materials which are immune to the corrosive nature of the unscrubbed biogas, which with its hydrogen sulphide content produces feeble sulphuric acid on contact with the water in the saturated gas. This would simply severely damage an unprotected steel container.
The writer informs us that there are apparent savings from a gas holder bag compared against a traditional rigid gas holder. A traditional steel gas holder also needs a significant civil structure which is full of water and contains what amounts to a steel bell, while a gas bag simply sits on a concrete base. The bag also needs less upkeep and isn't subject to freezing in winter. This produces a far smaller carbon footprint and a reduction in capital and operational spend.
The new gas holders are also in truth crucial pieces of process plant, instead of the mere storage vessels their name advocates. They maintain a consistent system pressure needed for the proper operation of engines, boilers and waste gas flare stacks. They also have level instruments that measure the height and volume of the inner gas bag to provide signals for process control, so they have to be simple, trustworthy and tough.
In-house Design Process and Safety
We are also told that further efficiencies and economies have been driven into the Severn Trent project thanks to the proven fact that May Gurney handled the complete design process in-house, from taking the outline design produced by Severn Trent's framework designer, thru to completion. While many main contractors might outsource the design part of such projects to an external consultant, May Gurney has its own expert team, so both reducing cost - making economies of scale and avoiding passing on fee-on-fee margins to the customer - and also reducing risk thanks to better control of safety in design risk assessments. Derek Shepherd is May Gurney's design chief, who is in charge of design coordination.
He explains the benefits of the full service approach : "By taking more control for Design Management upon themselves, and not passing it to a third party, they are hey believe particularly assured of coverage in all sides of the projects.
There's less risk, Derek points out, and by not having to confirm someone else's design we also save time without doubling up effort." Better still, the in-house design team have made a contribution to better environmental performance.
Derek Shepherd believes that, by having an independent and unbiased designer, they managed to identify all products, materials and providers based on performance and an overall design approach to the system. This was instead of it being based on any existing commercial relations.
Source: Wet News
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