Steam Turbine Preservation and Lay-Up: Protecting an Idle Machine So It Can Run Again

A steam turbine that stops running does not stop deteriorating. It changes which mechanisms are working on it, and several of the ones that switch on at standstill are faster and less visible than anything that happens in service. Plants find this out at the restart, which is the worst possible moment: the steam is ready, the load is waiting, and the machine that was sound when it stopped now has pitted blades, a sticking stop valve and water in the oil.

Long idle periods are normal in the markets we work in. Plants sit through conflict, through fuel shortages, and through the months between sugar crushing campaigns when the cogeneration set has nothing to do. None of those shutdowns was planned as a lay-up, and most of them start as "a few weeks" that become a season.

This article covers why idle turbines deteriorate, the lay-up methods that work, and how to bring the machine back without discovering the damage at speed.

Why an Idle Turbine Deteriorates Faster Than a Running One

In service, the steam path is hot, dry where it matters, and full of steam rather than air. The IAPWS guidance on steam purity for turbine operation makes a point that surprises many operators: the liquid films on blade surfaces during operation contain almost no oxygen, and pitting is unlikely to originate while the machine runs. It originates at shutdown.

When the machine stops, the casing cools and air is drawn in. EPRI's shutdown guidance describes turbines as particularly susceptible for exactly this reason: the oxygen in that air meets condensate collected in low spots and cavities that cannot drain. Six things then happen, at different speeds.

Deposits turn hygroscopic. Salts deposited on the blades during operation absorb moisture from the air and form small, conductive, oxygenated pools at the deposit and metal interface. That is where pitting starts.

Pits become cracks. Pitting is the usual precursor to corrosion fatigue and stress corrosion cracking, particularly in the low pressure stages where the steam first condenses. The pit forms at standstill; the crack grows after the restart, under running stress. A machine can be laid up badly, run for two years, and then lose a blade without anyone connecting the two events.

Bare surfaces rust. Seal fins, diaphragm surfaces, casing joints and valve seats corrode wherever moist air reaches them.

Bearings and oil degrade. Condensation forms in bearing housings and the oil reservoir. Water settles in the oil, and journals and thrust collars that sat in a humid housing for months come back etched.

Valves and linkages seize. Stop and control valve stems, extraction non-return valves and governor linkages stick on corrosion and deposits. IAPWS notes that even small deposits on the stem of a turbine check valve can interfere with its function, and a valve that fails to close after a trip can lead to an overspeed.

The rotor takes a set. A heavy rotor resting in one position for months can sag, and a rotor stopped while hot and left stationary bows thermally. Either shows up as vibration on the first run.

Our guide to common steam turbine failures covers how these end. The point here is timing: all of them are controllable during the lay-up, and none of them once the machine is back on load.

Six things that start working on a turbine when it stops. All are controllable during lay-up and none once the machine is back on load. Source: MIMAH engineering practice, after IAPWS TGD5-13 and EPRI 1021406.

Short-Term or Long-Term Lay-Up

EPRI divides shutdowns into three bands. Short-term means overnight or through a weekend, typical of cycling plant. Intermediate runs from a weekend to about a week. Long-term means anything longer than a week, up to and including mothballing. Its short-term shutdown guidance for steam turbine-generators covers lay-ups from two days to 26 weeks.

Duration is only one input. The others are how quickly the unit must be able to return, whether equipment will be opened for maintenance during the outage, what the site climate is actually like, and how certain the restart date is. EPRI makes the point that a unit shut down for economic reasons may be needed sooner or later than planned, and that the uncertainty itself has technical consequences.

Our view is blunt. The most expensive lay-up is the one planned as a fortnight that becomes eight months with nothing done in between. If the restart date is unknown on the day the machine stops, treat it as long-term from that day.

Dehumidified Dry Air: The Default for Long Lay-Up

EPRI describes two dry options: blanketing with nitrogen, or circulating dry air. Nitrogen is not used for long-term lay-up because of the effort needed to keep the gas in the system. Dehumidified air gives the same protection with an advantage that matters in practice: the machine stays dry even when it is opened for inspection or maintenance, and it is cheaper to run over a long period.

The humidity target in the published guidance is below 40 percent relative humidity. IAPWS gives the reason: the corrosion reactions at the deposits usually do not take place when the air flowing over the blades is kept below that level. EPRI's guiding principles are tighter for the boiler, deaerator and condenser, at below 35 percent. Treat these as typical practice; where the manufacturer specifies a different figure, the manufacturer governs.

The method, in outline, follows EPRI's description. Drain the machine and condenser while still hot so residual heat helps dry them. Open drains, and add temporary drains at low points that have none. Inject dry air at the high pressure end, where the remaining metal heat warms it and speeds drying, and keep the gland seal system working through the cooldown. EPRI suggests starting the dehumidified air after the turbine has been shut down for 24 hours and no later than 72 hours after shutdown, subject to the manufacturer's times and temperatures, and warns against thermally quenching a hot machine. For long lay-ups the main stop and control valves are jacked open so dry air can reach the whole steam path.

Measure humidity where the air leaves the machine, not at the dehumidifier outlet. The dehumidifier always reads well.

One nuance matters for dry climates. EPRI notes that at arid or semi-arid sites, forced dehumidified air may not be cost effective, and that if plant air stays below 40 percent relative humidity for the entire outage, ambient air can be circulated instead. Much of Sudan qualifies for much of the year. Not all of it, though: the rainy season, river-side and irrigated sites, and cool nights against a cold casing all break the assumption. Log the humidity for a few weeks before deciding the dehumidifier is unnecessary.

Nitrogen, Inhibitors and the Boiler Side

Nitrogen blanketing suits short periods on airtight equipment. EPRI considers purity of at least 99.5 percent suitable for plant use. The hazard is asphyxiation: nitrogen supply lines must be positively isolated, and spaces tested, before anyone enters equipment that was under a blanket.

Vapour corrosion inhibitors release a vapour that condenses as a thin protective film on metal. IAPWS is clear that they should be used only in addition to dehumidified air on a turbine, never instead of it, and that the containers must be tightly managed so every one is removed before the restart. EPRI adds that coverage inside a turbine is hard to confirm, and that any product used must be low in silica, chloride and sulphate, because residues end up in the steam.

Isolation from the boiler and feed system keeps someone else's lay-up out of the turbine. Boilers and feedwater heaters are often laid up wet, and moisture or chemicals must not migrate through extraction lines, drains or leaking valves into a steam path that is meant to be dry. Condenser steam and water sides need their own drying and monitoring.

Mothballing for an indefinite period adds steps. IAPWS suggests cleaning and re-drying a turbine that has heavy hygroscopic deposits before laying it up, and considers moving rotors and upper casings to controlled storage. Any protective oil coating applied to them must be completely removed before reassembly.

The three main preservation methods for a turbine steam path and where each belongs. Humidity targets are typical practice; the manufacturer's figure governs. Source: EPRI 1021406 and IAPWS TGD5-13.

The Lube Oil System and the Rotor

For long-term dry lay-up, EPRI's practice is to run the oil system and turning gear once a week for one hour. Running the oil keeps bearings, journals and the governor mechanism covered with an oil film, and turning the rotor spreads that film and stops the rotor settling into one position.

Keep the oil itself in condition. Run the purifier or dehydrator, keep the reservoir vapour extractor working, and sample the oil for water on a schedule rather than waiting for the restart. Oil that has sat wet for months is an argument for a change, not a top-up. Bearings deserve particular care during any long stop, and our guide to steam turbine journal bearings covers what standstill damage looks like on a journal and a babbitt liner.

Two rotor practices sit alongside the weekly run. Leave the rotor in a different angular position after each turning, and record it. And after any stop from load, keep the machine on turning gear until the casing has cooled to the manufacturer's limit before stopping it. A hot rotor left stationary will bow, and a bowed rotor makes the eventual restart a balancing exercise. Smaller machines without turning gear are barred over by hand, on the same principle. On the generator side, EPRI recommends removing exciter brushes to prevent pitting of the collector rings.

Keeping a Preservation Log

A lay-up without a log is an assumption. EPRI states that every lay-up condition, wet or dry, should be monitored continuously or periodically, and IAPWS recommends periodic inspections to confirm the preservation is working.

A useful log records humidity at each outlet point with date and time, dehumidifier running hours and alarms, nitrogen pressures where used, oil water content, each turning with its duration and the rotor's resting position, water found at drains, every breach of the dry envelope for maintenance, and an inventory of everything placed inside the machine: blanks, desiccant bags, inhibitor containers. The inventory is what makes the restart safe.

The log also tells you where to look later. Three weeks above the humidity target in the rainy season points the restart inspection at the low pressure end. Periodic looks through an inspection port, using the same approach as our turbine inspection checklist, catch problems while they can still be dried out.

Returning to Service

The restart after a long lay-up is a procedure, not an event. The sequence below is typical; the manufacturer's start-up procedure for a machine after extended standstill takes precedence.

Read the log first. Humidity excursions, water in drains and oil results decide how deep the inspection goes.

Remove everything that was put in. Blanks, desiccant, inhibitor containers and temporary drains, counted out against the log inventory. Protective coatings removed completely.

Inspect. A borescope survey of the accessible stages, valve internals where the log gives reason, and bearings opened if the oil showed water. If the findings are extensive, the restart has become an overhaul, and our overhaul guide covers that scope.

Flush and sample the oil. Circulate through the filters until cleanliness and water content meet the manufacturer's limits.

Exercise valves and prove the protections. Stroke every stop and control valve through its full travel. Test the trips, including the overspeed trip by the manufacturer's method. A protection device that has sat idle for a year is unproven until it has operated.

Slow roll. Put the rotor on turning gear and check eccentricity against the manufacturer's limit before admitting steam, then follow the cold start curve. Hold steam admission until steam purity is within your chemistry limits, because corrosion products from the boiler side arrive first.

Take a new vibration baseline. Compare it with the readings from before the lay-up. A change in amplitude or phase points at rotor bow, bearing condition or alignment.

A typical sequence for returning a turbine to service after a long lay-up. The manufacturer's start-up procedure after extended standstill takes precedence. Source: MIMAH engineering practice.

Plan It Before the Machine Stops

Preservation is cheapest when it is decided before the shutdown rather than improvised afterwards. Dehumidifier hire, the power to run it, a weekly hour of an operator's time and a log are small items next to a blade failure. Lay-up belongs in the outage plan alongside everything else, and our guide to plant shutdown and turnaround planning covers how to build it in.

Where a site may lose power or access for months, the preservation plan has to say what happens when the dehumidifier stops, because at some point it will. Decide in advance who turns the rotor, who reads the humidity, and what the restart inspection will be if nobody could.

MIMAH's rotating equipment team, which has rehabilitated 14 MW steam turbines in Nigeria, plans lay-up and return to service for turbines facing long idle periods, and inspects machines that have already sat idle without one. If you have a turbine that is stopping, or one that has been stopped for longer than anyone intended, get in touch.