Water & Wastewater Treatment Magazine
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The Knowledge DfMA T he Building and Engineering Services Association 2015 report 'An Offsite Guide for the Building and Engineer- ing Services Sector' suggests design for manufacture and assembly is now used in well over 90 per cent of UK construction projects, from building to process plant. DfMA is, essentially, offsite construction and it is by no means a new concept in the process industries. Skid-mounted packages from pumps to pressure vessel assemblies such as filters and absorbers have been around for many years, but now they are becoming so common that it is estimated by the Building and Engineering Services Asso- ciation that, in some projects, up to 80 per cent of construction is carried out offsite. In the water industry, the most com- mon example is probably chemical dosing packages, which are delivered to site pre- assembled in a chemical-proof enclosure with all the required piping, valves and cabling mounted on a back board. However, the benefits of this approach mean that its use is rapidly expanding. DfMA is nothing new. Long before the acronym was coined, companies had been fabricating bespoke engineered, skid- mounted systems for specific projects across a wide range of industries. These units are manufactured offsite in the controlled, clean and safe conditions of a factory and are usually supplied complete with local control panels that can interface with all in- dustry standard distributed control systems. Factory assembly means that the com- pleted system can be dry and wet tested be- fore shipment. This eliminates installation errors, and factory acceptance tests (FAT) greatly reduce commissioning time. A standardised package means that fully detailed drawings are available even before order placement, and that makes layout design much simpler. But even with bespoke packages, 3D drawings can now be provided early in the design phase, and fly- through capability allows space constraints to be assessed and a detailed ALM (access, li—ing and maintenance) study to be under- taken before manufacture begins. The increased availability of virtual reali- ty headsets even allows operators and other stakeholders to carry out a detailed plant walk-round. Virtual reality has become a valuable tool for product visualisation, meaning significant design changes can be made well before the design is set in stone, helping to reduce costly changes later on. A skid-mounted DfMA unit can be easily relocated and rapidly commissioned, mak- ing it ideal for dealing with emergencies. For the most part, the water industry is not well equipped to deal with a Cryptosporid- ium incident and, with the current state of technology for detecting and identifying the organism, it is not easy to see a quick fix. This situation will be exacerbated with the implementation of competition and the high levels of supply switching which are likely to occur. Prefabricated, skid-mounted UV disinfection units that can be hired provide a perfect temporary solution for just such eventualities. A natural extension to skid mounting is containerisation. Constructing UV systems in standard ISO containers means that they can be completely autonomous with all nec- essary controls. The containers are provided with climate control, so can be used as a self-contained control room. They can even include a diesel generator to provide electri- cal power, so that all that is needed on site is a suitable installation area and connec- tions for water inlet and outlet. This approach is the ideal solution for emergencies such as Cryptosporidium inci- dents, because the container can be rapidly deployed to specific parts of the works, service reservoirs and distribution network, and be operational within hours. Severn Trent's 117Ml/d Mythe Water Treatment Works near Gloucester normally removes Cryptosporidium by coagulation, clarification and rapid gravity sand filtra- tion but, when levels of the protozoan are particularly high, there is an increased risk of breakthrough. In 2018, Severn Trent programme man- ager Phil Robinson tasked delivery partner CiM6 with a fast-track project to install UV disinfection to obviate any possibility of an incident. atg Evoqua was appointed as UV plant supplier and designed a system of six UVLX- 16800-20-16 reactors operating as up to five duty and one standby, located post RGF treatment and upstream of final disinfec- tion. To facilitate offsite manufacture, deliv- ery and installation, it was decided that the UV reactors would be mounted in standard 20' x 8' containers, with two reactors and their associated local control panels in each container. Daily collaborative planning sessions were held on site between the design and construction teams, and the first UV reactor container was delivered to site on 28 June only eight weeks a—er the placement of the order. The other two units followed and in- stallation and initial testing was completed by the end of July allowing the plant to be brought into use only 15 weeks a—er initial contract placement. Mobile rigs have proved to be a very ef- ficient and cost-effective way of containing an outbreak and several water companies already have emergency containers for such eventualities. But it is wrong to think of containerised units as just temporary solu- tions: they are increasingly being used for permanent installations. The lack of on-site piping and cabling together with reduced testing and com- missioning time makes them a very real alternative to skid-mounted packages installed in kiosks. Lighting the way to offsite fabrication Design for manufacture and assembly has proved its value in many industrial sectors and is now increasingly being used in the water industry. But what, exactly, is DfMA? Barry Hopton of atg Evoqua explains 24 | OCTOBER 2019 | WWT | www.wwtonline.co.uk