Diary of an engineer: The Outage

Submitted by AWL on 14 August, 2019 - 8:16 Author: Emma Rickman
cleaning

Emma Rickman is an engineering apprentice at Sheffield combined heat and power plant

Every summer, our plant shuts down for annual maintenance; fourteen days of 24-hour work on essential systems that can’t be carried out while the plant is running.

Because the plant generates energy, this shutdown is called “the Outage”.

Hundreds of workers from contracted companies set up on site to erect scaffolding, drain chemical silos, replace pipes, grind debris off tubes, weld metal sheets over holes, calibrate instruments, replace machines and clean as much as possible before the plant comes back online. There are many conflicting pressures in an outage; the main one being the city bin waste (which our plant burns to generate electricity). Anything can go into ‘“general waste’” bins, and when a lot of it is piled together it attracts pests, harmful bacteria, and generates heat as the organic matter is squashed and begins to break down.

Waste in big quantities is a fire hazard, so legally can only be stored for 12 hours before being buried or incinerated. This means that the longer the plant is offline, the more waste Veolia and the council have to send to landfill, which (rightly) incurs heavy taxes.

Veolia in turn are not paid by the government to process waste, which is their main source of revenue. Being online makes them money, being offline costs them financially and in carbon-footprint.

On top of this, while the fire is out no steam is generated. The turbine and generators are offline, no power is sold to the grid, and the plant must pay the grid to power themselves. There is no source of heat for the District Energy network, which heats most of the public buildings in central Sheffield. Veolia must buy and burn gas to power back-up boilers; another financial and carbon cost.

Putting pressure in the other direction, burning any old crap every hour for a year puts enormous strain on the plant. By law the waste must burn at over 850 degrees, and the tubes inside the furnace must absorb this heat and withstand erosion from hot particulates continuously.

These tubes – which carry the water that becomes steam – wear very thin and often burst before the annual shutdown, extinguishing the grate fire and causing an emergency shutdown. The ash that remains after incineration must be cooled and processed, funnelled through metal ducting and fed onto conveyors, which all block up or wear out. The air pollution control system, essential for preventing toxic smoke venting to atmosphere, must be similarly cleaned and repaired.

Any valves or pumps which deliver harsh chemicals or high pressure steam must be repaired or replaced. Any tiny misalignments in the turbine mean it must be stripped and overhauled. Any malfunction of the transformers, including their temperature probes or cooling systems, must be addressed to avoid turning off power to all of Veolia’s buildings in the area. The list of essential work that is “offline only” is endless.

Some of these jobs can take place simultaneously, but most of it must take place in a specific order. For example, the grate must be cooled and ventilated while workers are grinding and welding inside it.

Ventilation is provided by the plant’s Induced Draft (ID) fan which also provides the pressure differential to push chemicals around the Air Pollution Control (APC) system. This means that while the ID fan is on, no work can be done on the APC without blowing lime and carbon everywhere. The ID fan itself must be serviced at some point.

The other pressures are human. To save burning gas for District Energy, the Outage is scheduled in midsummer when demand for heat is low, and why we find ourselves in boots, overalls, gloves, helmets and face-masks in 26 degree heat, inside a confined metal space, with power tools.

The scaffolders must move heavy metal poles and wooden boards up and down flights of stairs and into the furnace, ducts and silos. The fabricators and mechanics must weld and grind within these spaces. The industrial cleaners must empty screw conveyors, silos and pipes of waste ash and APC chemicals - these are a combination of lime and carbon, which become hard as concrete when mixed with the water vapour in the air.

Everyone is working on top of each other, working for a different company and on a different work permit. But a sack of ash can’t be emptied near mechanical work; electrical testing can’t happen if equipment is isolated; gear can’t be installed without scaffolding; instruments can’t be calibrated near vibrating grinders; and everyone must have the correct materials and equipment delivered on time in a safe condition. Frequently and unsurprisingly, the service lift breaks down.

Honestly, I’m having more fun than I’ve had all year. In addition, I met another woman last week - one of the lagging team who strip and install insulation around ducting. I lost a bet with these laggers when they told me there would be an Outage Ice Cream van and I didn’t believe them.

Well, there is an ice cream van.

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