A fortnight ago, there was an especially large leak on the district heating network. A pipe manifold underneath a large planter in the city centre was billowing steam into the bus interchange.
When engineers responded to the call out, they found the man-hole cover was 100 degrees. On my way home I have a look at the screens and fences they’ve used to cordon it off, and wonder why we always spring leaks in the same place, even though new pipes were installed six months ago.
That week I’m working with S in the control room, managing the network remotely. Because of the leak, the system is losing 120 cubic metres of water every day, and bleeding heat. S shows me the temperatures and pressures we have to maintain.
At peak times in the morning and evening, when everyone is turning on the heating and having a shower, more heat is being extracted from the water in the network, and the pressure drops as more fluid is diverted into the heat exchangers of each building. To make up for the heat losses, we have back-up gas boilers; and to keep up the pressure at the far end of the network, we can switch on pumps at different points.
The five gas boilers at the back of the plant are enormous, each the size of a terraced house. The cylindrical drum is lined with pipe manifolds that pump the water around a hollow area. The gas is injected at one end into this space, and instruments supply oxygen to control the flame. On start up, the air must be completely purged and replaced to ensure nothing explodes when the gas ignites.
Water from the boilers can be directed onto either of the heat networks, set at a temperature of 106 degrees. As well those on the plant, we have boilers and pumps near the university science department; frustratingly they can only be operated by an engineer at the site.
Pumps ensure that buildings up the hill towards Crookes get enough heat.
“But you don’t want the flow rate to be too high,” says S “or the water just kind of runs past – the heat exchangers can’t extract heat from the network fast enough, so the buildings get cold.”
Another thing S is watching is the pressure at our end of the system. “Pumping too much can trip the safeties and then we’ll be venting steam at our end – it isn’t good for the pipes.”
During the day the district heating team are all out of the office trying to find and repair the leaks, talking to customers and planning isolations. The next morning I come in to some very sleep-deprived civil engineers — after a night of shut-downs and eight hours of digging, they still haven’t found all the leaks.
“We dug out the planter, and several metres down the road,” says R (baggy eyes, Leeds accent) “but when we patched the holes the steam kept coming. We’re going to have to keep digging.”
In the control room, S is dismayed when the general manager explains they won’t be isolating again for two days, but the reasons are complicated. The civil engineers need permission from the highways authority to dig up each section of the road. The work means traffic diversions, resurfacing costs, the risk of digging up electrical cables – and of course turning off the heating to forty buildings for the second night in a week.
Nevertheless, there is a very public plume of steam emerging near the Crucible Theatre, which will soon be hosting the snooker.
The length of pipe leading up to the network’s largest customer, Ponds Forge leisure centre, is very old. The network engineers strongly suspect this length of pipe has several leaks, and venting steam is working its way back to the planter through the line of least resistance. Ponds Forge staff have noticed that the pool is colder in the mornings.
“I strongly suspect Ponds Forge pump room is flooded with hot water by now,” smiles S “but maybe we should turn the leak and the cold pool into a public sauna – steam bath; cold plunge!”
Ponds Forge managers really don’t want a weekend isolation, because they have athlete time trials in the pool that week.
“But we’ve got to go ahead, they’ve got to understand that.” R explains.
S and I begin preparing the system for a shut-down. All the support boilers are off, and we need to lower the temperature of the network to below boiling point, making it safer to isolate and drain down. To do this, we look at the power plant. The ERF has its own steam system which is directed into the turbine to generate power, as with any waste incinerator.
What makes Sheffield ERF different, is that we syphon off some of that steam before it makes its way through the turbine and divert it to a large header tank. This tank feeds two very large heat exchangers, which pass heat onto the district network. To moderate the temperature, the operators control the amount of steam that passes from the header to each heat exchanger by regulating the valves.
A computer screen displays a system diagram. I spend half an hour slowly decreasing the set-point temperatures from 109 to 99 degrees, two degrees at a time. The smaller heat exchanger temperature drops faster, so I wait for the larger exchanger to catch up. Once the network is at temperature from our end, it will take several hours to circulate around the city. In the meantime that steam is diverted into the turbine, and we generate an additional 4MW to the grid.
By the end of the week, Ponds Forge is connected to a transportable gas back-up boiler, and the district pipe supplying it is permanently isolated while the road is excavated.
The plant yard is suddenly full of welders preparing a completely new length of pipe. It needed replacing badly. There were seven leaks in one run.
• Emma Rickman is an engineering apprentice at a combined heat and power plant in Sheffield.