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Thursday, November 13, 2014

Steam Turbine Outage

This may or may not be an interesting topic to the random readers that drop by here.  I honestly don't know.  I've been around steam turbines my entire adult life, and have never given the shaft sealing system much thought - until we had a seal failure that ended up with the accumulation of a bit of new knowledge, which I share here.

As you would guess, steam tends to leak out of a steam turbine at the high pressure end, where steam enters the turbine, and air tends to leak in at the exhaust, where the final stage of the turbine is at 1-3 inHg Absolute pressure (25-27 inHg vacuum).  Steam turbines are not sealed like pumps.  Pumps use flexible packing or mechanical seals, that actually come in contact with the shaft.  Steam turbines do not seal this way, due to large swings in operating temperature from cold to running.  Instead turbines are sealed using steam!

I took a quite a few photos while the steam turbine was apart for the seal repair, and thought they were interesting enough to put up.  The steam turbine is a General Electric model A-10.

Most modern steam turbines are what we call "self-sealing".  What that means is that they only need to be supplied with sealing steam during startup and shutdown.  After the turbine reaches a certain load (5-20%), leak-off from the turbine supplies the seal steam, and external sources can be shut down.  If there is any excess leak-off, and the seal steam supply pressure gets above the setpoint, a seal steam dump valve opens, and dumps the excess steam into the main condenser.

The symptoms of a failed steam sealing system are these:  The seal steam dump valve is 100% open, and seal steam header pressure is still be too high.  Massive water vapor and condensation coming from the Lube Oil System vents.  Increasing Lube Oil Tank level due to steam ingress and water contamination/condensation.

Note:  Water enters the lube oil system because the turbine shell is very close to the bearing housing.  So as steam leaks out along the shaft of the turbine, it can leak right into the adjacent bearing housing.  The lube oil system is kept under a light vacuum so that air will leak into the bearing housings, rather than oil leaking out.  This will help pull in steam if the steam seal is leaking.

First of all, before any work can begin, the steam turbine must be shut down and allowed to cool.

After shutdown, the steam turbine has to be continuously rolled at low speed (typically 4-10 RPM) for about 3 days.  Steam turbines operate at very high steam pressures and temperatures, and as a result, the turbine shell is very thick at the high pressure end.  The shell is also insulated for thermal efficiency as well as for safety.  For these reasons it takes several days before the turbine is cool enough to stop turning.

The purpose of rolling the turbine is to prevent warping the rotor.  If the rotor is allowed to stop, it will develop a bow.  If a bow develops, sometimes it may work itself out when the steam turbine is returned to service; the rotor, reheated by steam, becomes ductile again and the bow mostly disappears.  Other times, with severe bowing, the machine cannot even be started due to increased vibration caused by the off-center mass of the rotor.  The damaged rotor will require machining and the addition of balance weights to compensate for the bowing, if not complete replacement.  Needles to say, it's preferable to keep the steam turbine on the turning gear until it cools down.

Interestingly, a steam turbine rotor does not sag if it cannot be rolled during cooldown.  Instead it humps up in the middle, because of the temperature differential between the top and bottom of the turbine shell.  It is not unusual to have a 200 degree temperature differential between the top of the shell and the bottom.  As heat rises inside the shell, the bottom part of a motionless rotor will contract more quickly than the top, which then bows the rotor upwards.

Onward now to the bits and pieces of a smallish high pressure steam turbine outage...

Below, a worker uses a sledge hammer to remove a nut from a through-bolt on the high pressure turbine shell.
Just above the worker's hard hat, is a black bundle of tubes that is the power supply for an inductive heater.  The inductive heater fits down inside the through-bolt (which is hollow), and heats it up.  As the bolt heats up, it stretches out, making it much easier to remove the nut.  The same thing takes place during installation, except when the bolt cools, it shrinks, which draw the upper and lower half of the turbine shell very tightly together.  Note the insulating blanket on top of the shell.  Workers didn't want their feet to get burned while removing auxiliary piping.

Below: The top of the High Pressure turbine shell.  The far end (burnt off paint) is the high pressure steam inlet.

Below, two workers removing the high pressure inlet steam seals.

Below: Another image of the high pressure turbine.  At the bottom left corner on the rotor, are holes for balance weights.  Next is the steam sealing section, then a series of discs with tiny blades at the end.  These discs are the 10 stages of the high pressure steam turbine.  The generator is the big thing to the right.  Notice how thick the steam turbine shell is - This is why it takes three days to cool the steam turbine down from 1050 degrees F.

This is a close-up of the high pressure steam sealing section, with the packing (or labyrinth seals) removed.  High pressure (up to 1800 psi) steam enters the turbine just to the left of the disk on the right hand side, and then flows off to the right. At 1800 PSI, steam would tend to leak out along the shaft and be a hazard for everyone around, as well as causing a loss of efficiency.  To prevent this, steam turbines use labyrinth seals.

Below: Side view of a labyrinth seal.  These segments slide into the grooves in the sealing section of the shell, sitting very close to, but not rubbing on the rotor.  The purpose is to make the steam pass through a "labyrinth", or a series of very tight passages, losing a little pressure with each ridge.  At the left end of the seal in the photo above, a small fan draws off the tiny amount of leak-off steam and condenses it for re-use.  Clever!

Below, labyrinth seals (or packing) installed in the grooves on the high pressure turbine.  Interestingly these are held in place by springs that are merely short flat pieces of steel These springs press against the inside of the packing gland and the outside of the labyrinth seal.  Several of these springs were found to be broken during this outage.

The picture below shows a part of the steam turbine that is contained within the lube oil system.  At the right is a journal bearing.  This supports the aft end of the high pressure turbine (which is just off the picture to the right).  You can see a wire coming off this journal bearing, which is used to transmit the temperature of the bearing metal.  To the left of the bearing is a flat metal flange on the shaft.  This is used to determine rotor expansion.  The shaft expands quite a bit as the steam turbine goes from a cold condition to the normal operating temperature of 1050 degrees F.  The coil of blue wire connects the rotor expansion position sensor to the monitoring system. The big gear is for rolling the rotor when the steam turbine is not in operation.  The big round flange with all the holes in it is the coupling which connects the generator to the turbine.

Below, the bearings at the other end of the high pressure steam turbine.  In the center is a journal bearing, with thrust bearings to either side.  The journal bearing supports the shaft, and keeps it from moving in a radial (side to side or up and down) direction.  The purpose of the thrust bearing is to keep the shaft from moving axially (left to right) .  The shiny flanges with no holes in them are a part of the rotor called thrust collars.  The shiny things just inside them are the thrust bearings.  This section of the steam turbine is continuously supplied with oil to cool and lubricate the components.

One of the pads from the old thrust bearing... not in very good shape at all.

These looked in pretty bad shape as well.  Obviously there had been some heating and breakdown of the oil into carbon deposits.

Replacement labyrinth seals installed on the high pressure turbine lower half.  Note the serpentine path that leak-off steam must travel.

High pressure steam turbine back together and being fitted with brand new insulating blankets.

One of the things you run into while starting a cold steam turbine is rotor growth.  The rotor does not contain as much material as the turbine shell, and it is completely surrounded by steam (although the steam temperature is kept as low as possible during start up).  At the same time, the turbine shell is also cold, but only has steam warming the inside of a *very* thick piece of steel.  As a result, the rotor heats up much more quickly than the turbine shell, and it therefore expands much more rapidly.

Because one end of the rotor is held in place by the thrust bearing, the other end of the rotor grows as steam heats it.  It's quite possible to have rubs if the expansion of the rotor gets too far ahead of the shell.  That's why you will find rotor expansion proximitors (Below). These little pucks monitor exactly how much the rotor has expanded.

This is one of the few outages I have had time to take pictures and learn a few things, because on scheduled outages, I usually have assignments that keep me too busy for that.  Was fun to learn, fun to share.


Johan Siverklev said...

Thanks for sharing! I found it very enlightening! Really appreciate the many pictures.

Mark said...

Thanks for commenting! Sometimes it seems like I'm writing to myself :)

Johan Siverklev said...

Hehe.. Yeah.. I noticed that date of the post and the lack of comments, so I just wanted to let you know that at least someone got something out of it.. :) In a way it only needs to be important to a single person in order for it to be worth writing, right? :) Curious person and inventor as I am, I learnt a lot from it, so again - appreciate it! :)

swaggy said...

Hi Mark,
Great article indeed. Very helpful for me since I am just beginner for steam turbine operation. Thanks for sharing valuable experience..:)

Mark said...

Hi Swaggy,

Thanks for the kind words. Glad this is of use to someone :) If you use the blog search bar and enter the term "steam", you might find a couple of other useful steam turbine and steam cycle posts.

Again, thanks for the comment!