Hot Steam Turbine Thrust Bearing

 "It is difficult to bring people to goodness with lessons, but it is easy to do by example.:" - Lucius Annaeus Seneca

 Today's pic of the day is the Manchester Street power station in Providence, Rhode Island.  Initially, this was an old coal-burning power house with a very tall single stack.  It was re-powered in the mid 1990's with three Siemens V.83 gas turbines and attached boilers, and - unusual in the modern era - aesthetics were kept in mind.  It still looks like a lovely early 1900's power station on the outside.  The plant is configured as 3 1x1 units for a total plant output of 495 MW.  

Fun fact:  I briefly worked with a guy who did the start-up commissioning on this power station.  He was a chain-smoker, and he was required to go outside to smoke.  One night he was out in a nasty rain and wind storm having a smoke, and noticed that rainwater was being sucked in and out of a crack in the concrete foundation for one of the gas turbine stacks.  The stack was tilting the foundation as the wind buffeted it.  I'm not sure what measures they had to take to reinforce the foundation after the discovery, but his bad habit and awareness may have saved the day.

Onward.

I don't discuss ongoing issues at the power plant, because it's never good to air dirty laundry in public.  I'd assume if someone was interested enough, they could figure out who I am, where I work, and ruin my life.  The plant's owners or the operating company that I work for would likely not appreciate my sharing operating issues.  I don't comment on chronic equipment issues unless the problem has been resolved.  Once the equipment issue has been resolved, it's good engineering practice to share the solution. 

With that said, I'll share this:  For several years the plant has been experiencing very high temperatures on the Steam Turbine active thrust bearing.  I've posted images of that bearing before.  The image below was from an outage in 2014, showing heavy wear, chunking, and oil breakdown and coking.

 

While the active face of the steam turbine thrust bearing ran much hotter than the others, there were times when the temperature would really soar.  The boiler is equipped with supplemental duct firing to increase steam production when needed.  This additional steam flow generates more thrust.  The Low Pressure (LP) section on this turbine is not opposed-flow, so you get significant axial thrust with added steam flow.  Steam tries to shove the shaft towards the condenser as it pushes through the LP end.

In recent years, the thrust bearing temperature would limit how hard the duct burners could be fired.  Management set a maximum bearing metal temperature that we were allowed to operate at - well above the alarm setpoint, mind you.  The goal was to keep the bearing metal temperature below the value that the oil would break down at. 

You had to feather the duct burner fuel flow quite a bit after nightfall, as the ambient temperature would drop.  Gas turbine output increases with the added mass flow of the denser air.  This added mass flow increases boiler steam production, adding to the total thrust (and heat) on the steam turbine bearing.  It's a bit stressful having to worry about smoking a bearing due to ambient temperature swings, but I suppose that's part of what we're paid for :)  

Below:  The Steam Turbine Thrust Bearing after assembly this outage.  The copper-colored segments are the bearing pads.  The active face is on the right side, and the inactive face is on the left side.  A journal bearing (not yet installed) sits between the thrust bearings.  The thrust collars on the shaft are to the outside on each side of the assembly

On to the point:

The final night that I worked on this outage, one of the millwrights made a great find:  One of the bearing drain flex hoses was kinked.  

Below: The right-most flexible hose is kinked where it crosses over one of the others.  I'm willing to bet this kinked hose is the active thrust bearing drain.   Three oil drain thermocouples are at the left in this image.  They monitor the temperature of the oil leaving each bearing.

Below:  A view closer in.  There is little doubt in my mind that this kink was restricting oil flow coming out of the bearing.  With any luck, our days of nail-biting and nervously monitoring the thrust bearing temperature during duct firing are over with.  Yes, all that junk in the bottom will get cleaned up before the cover is replaced.  There's no help for stuff falling in during an outage.  In the summer, it's usually bugs.

Anyway, huge props to the millwright who noticed this issue.  It was a great catch, and not something that anyone would expect to find.