Plant utilities — steam, air, nitrogen and cooling water

Nothing on the utilities list makes product. Everything on it stops the plant when it fails — which is why utilities failures cause more unplanned shutdowns than process problems do.

Steam: why there are several headers

Steam carries heat, and it delivers that heat by condensing. The temperature at which it condenses is fixed by its pressure, and that single fact explains the whole distribution system.

A reboiler needing 180 °C cannot be served by steam that condenses at 110 °C, and using 40 bar steam to heat a tank to 60 °C wastes most of the energy it took to make it. So a plant runs high, medium and low pressure headers, letting steam down in stages and taking useful work out of the drop wherever a turbine driver can be justified.

Superheat is steam heated above its condensing temperature. It matters because it prevents condensation forming in the pipe during distribution, and because a turbine fed with wet steam erodes its blades.

Condensate, and the traps that return it

Condensate is hot, clean, treated water. Throwing it away means paying to treat and heat fresh water in its place, so it is collected and returned to the boiler.

Between each steam user and that return line sits a steam trap — a valve whose only job is to pass water and hold back steam.

A large insulated steam header running along a pipe rack with aluminium cladding, a branch dropping to a steam trap station at low level with trap, isolation valves, strainer and a sight glass on the condensate line, a small wisp of steam venting from a drain point. — The header is lagged; the trap station below it is not. That small assembly is where the condensate leaves — and where a stuck-open trap would quietly vent live steam for years.

Water hammer is the other condensate problem. A slug of water accelerated along a steam line by the flow arrives at a bend or a closed valve with enormous force, and it has split pipework and killed people. It is prevented by draining lines properly before admitting steam, warming them slowly, and sloping them so condensate runs to a trap rather than collecting in a low point.

Instrument air and plant air

Compressed air does two quite different jobs, and mixing them is a classic error.

Instrument air drives control valve actuators, positioners and pneumatic instruments. It is specified for dryness — a dewpoint well below the coldest ambient the line will see — and for freedom from oil and particulate.

Plant air runs tools, hoses, blowing and cleaning. It needs none of that.

Loss of instrument air is a plant-wide event. Every spring-return valve goes to its fail position simultaneously — which is why the fail position of each one is a process safety decision, and why instrument air receivers are sized to give operators time to shut down in an orderly way.

Nitrogen: the utility that kills

Nitrogen purges oxygen out of vessels and lines so that hydrocarbon and air never meet. It blankets tanks, it dries out equipment, it pressure-tests systems.

Inert means it gives no warning of any kind

Nitrogen is not toxic. It does not smell, sting the eyes or irritate the throat. A space full of it is indistinguishable from a space full of air until you breathe it, and then unconsciousness follows in a breath or two — before the body has any sensation of suffocating, because that sensation comes from carbon dioxide building up rather than from oxygen running out.

People have stepped into nitrogen-purged vessels and collapsed immediately. Colleagues who saw them fall have gone in to help and collapsed on top of them. Multiple-fatality incidents on plants are very often nitrogen.

Which is why entry requires an oxygen test rather than a look, why a purged vessel is treated as lethal until proved otherwise, and why the rescue plan is agreed before entry rather than improvised. See permit to work.

Cooling water

Most plants reject heat to a circulating water system cooled in a cooling tower, and the cooling comes from evaporating a fraction of the flow. That is what allows a tower to cool water below ambient air temperature, and it has three consequences.

Water is consumed continuously — evaporated, plus drift carried off as droplets, plus blowdown.

Dissolved solids concentrate. The water leaves as vapour; everything dissolved in it stays behind. Left alone, the concentration climbs until salts deposit as scale on the hottest surfaces, which are the exchanger tubes you least want fouled. Blowdown removes part of the circulating water and fresh makeup dilutes what remains.

It is a biological system. Warm, aerated, nutrient-rich water grows things — algae, slime, and the bacteria responsible for Legionnaires’ disease. Treatment is continuous, and the health risk is real enough that drift eliminators and dosing regimes are regulated in many countries.

An induced draught cooling tower at a process plant, with fans on the deck above and water visible falling through the fill inside the structure. — The plume is the cooling. What leaves as vapour takes the heat with it — and leaves every dissolved solid behind in the water that stays.

A fouled exchanger is very often a cooling water problem rather than a process one, which is worth remembering before anyone opens the bundle.

The other utilities

Fuel gas fires heaters and boilers, and is usually taken from the plant’s own production, which means its composition varies with what the plant is doing.

Flare and blowdown is a utility in the sense that everything relies on it — see relief and flare systems.

Potable and service water, drainage and effluent — covered in water and effluent.

What to take away

Steam pressure sets condensing temperature, which is why a plant runs several headers rather than one.
Let down through a turbine rather than a valve where the flow justifies it, and the shaft work is effectively free.
Steam traps fail both ways. Open wastes steam invisibly; shut floods the user and looks like fouling.
Instrument air is separate from plant air on both quality and availability, and losing it is a plant-wide event.
Nitrogen gives no warning whatever. It is the commonest cause of multiple-fatality confined space incidents.
Cooling towers cool by evaporating, which consumes water, concentrates solids and grows biology.
A fouled exchanger is often a cooling water problem, not a process one.