Northerly islands face a burning issue
04 May 2011
The highly successful Shetland Islands waste-to-energy scheme is set for future expansion with its all-new distributed control system. The change was a difficult choice for the operators, but the flexible, open technology promises smooth legacy migration as the project progresses
In the cold of winter in the UK’s most northerly town, the people of Lerwick have cause to be glad of their district heating scheme (DHS). The Lerwick DHS pumps hot water around the clock to 1,100 homes and other buildings in the Shetland Islands’ capital. The heat, supplied by a local energy recovery plant (ERP), is generated by incinerating 22,000 tonnes of waste each year from Shetland, Orkney and the Highlands.
Shetland’s DHS and ERP form a flagship project for waste-to-energy in the UK, taking waste otherwise destined for landfill and combusting it to provide room heating and hot water. Both rely on a distributed control system (DCS) to manage the entire process, from regulating the ERP’s 1,100oC furnace to ensuring that pressure and temperature levels are correct as the water is pumped around Lerwick.
For some12 years now an ABB Advant DCS did the job. However, the Shetland teams were concerned that this system was fast becoming obsolete and would be unable to support their ambitious plans for expansion. Early in 2010 they began the hunt for a replacement, aware that the right choice was crucial if Shetland’s waste-to-energy operation was to continue and remain the success story that it has become in the eyes of the community.
Tight regulation of emissions needed in a waste incineration plant is just one of several key areas where the DCS simply has to deliver, as Shetland Islands Council’s ERP manager, William Spence explains. “It is critical. If anything is going wrong with the control system then it probably means that the plant’s emissions are not within the permitted range,” he says.
Following a tender process, Shetland selected Digital Applications International (DAI) to build a control and reporting system based on the Siemens Simatic PCS 7 platform, for which DAI is an approved integrator. Neville Martin, DHS manager for operator Shetland Heat Energy and Power (SHEaP), admitted that migrating to a new supplier was a nerve-racking prospect, even though the Siemens equipment would come in at a lower cost. “We were wary. In some ways staying with ABB seemed the easy option, even though it was going to cost us more money,” he admits.
But of more immediate importance, the project beginning in March 2010 had to be completed before winter - and thus the peak demand for heat - without disrupting the service while replacement was underway. It was now up to DAI and Siemens to ensure that migration to the Simatic PCS 7 platform went smoothly and to schedule.
Thanks to the flexibility of the Siemens platform, the DAI project team was able to build several features into their changeover strategy that kept risk of operational disruption to a minimum. For example, as the PCS 7 is based on open networking and communications standards, it was possible to retain the existing ABB I/O modules, so reducing project time, costs and the perceived risk. The level of flexibility even allowed an emergency fallback option, which would have retained the ABB’s controllers, though this did not prove necessary in the end. According to Mr Martin, DAI did a great job.
“They really went to town on understanding our system and its needs, and then coming up with their own solutions to make things work better than they did before.”
Tangible benefits
Three months after migrating to the Simatic PCS 7, the ERP and DHS were both able to report tangible benefits. At the ERP, in particular, Mr Spence noted improvements to furnace regulation and a consequent reduction in maintenance time, which also cuts the use of oil as a back-up fuel. And the plant’s operators have found the new system a pleasure to work with. “It’s very operator-friendly, which was one of the things we were concerned about before installation,” Mr Spence added. The DHS team reported a similar experience.
“There is no doubt that it works a lot more smoothly,” says Mr Martin, adding that a range of individual operations had seen benefits. “Probably the biggest single improvement is that it is regulating itself better to avoid unnecessary alarms.” By ‘unnecessary alarms’, Mr Martin was referring to those caused by problems with pressure differentials in the system; these are especially unwelcome when they occur at 6am when somebody has to be woken up to sort them out! The Shetland teams are also pleased to be supported from nearby Aberdeen rather than Denmark, as was the case with the old DCS. Siemens’ process automation business manager, Simon Ellam summarises:
“The Shetland project shows how flexible technology based on open standards can deliver smooth migration from legacy systems; for example, by retaining the existing I/O modules. We understand that legacy migration can seem like a leap of faith. The key to successful migration is the ability to move to a new platform with as little impact as possible on the day-to-day running of the facility.”
The success of the Shetland migration exposed the limitations of the old approach based on proprietary standards for each manufacturer, he added. “That was done to tie the customer in. But customers don’t want to be tied in. They want open technology which allows the flexibility of operation and maintenance that modern process manufacturing demands."
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