Steam Turbine Valve Maintenance

"Withdraw into yourself, as far as you can.

Associate with those who will make a better man of you.

Welcome those whom you yourself can improve.

The process is mutual; for men learn while they teach." - Lucius Annaeus Seneca

We are currently in the middle of a steam turbine outage.  The High Pressure Control and Stop valve and both Reheat Intercept and Stop valves are being overhauled.  It's a difficult process, as many of the fasteners are breaking off, leaving the millwrights to deal with drilling out and tapping threads in the holes.  It's adding time to the outage that was not planned for.  A couple of the steam seals are also damaged, and this is very likely due to the thrust bearing clearances opening up.

 Below:  Looking down at the Left side Intercept/Stop valve, with the hydraulic actuator on top of it.  The insulating blankets have been removed to allow the metal to cool more quickly.  These valves are a bit funky.  There is another hydraulic actuator underneath.  The combined valve is actually two valves in one body.  The upper valve does throttling, and the lower valve is positive sealing and acts as a stop valve.

Below:  The single High Pressure Stop/Control valve with actuator still installed.  Same arrangement on this one.  It also has a lower actuator for the stop valve.  Note the size is slightly smaller, but the bolting is spaced more closely and the bolts are larger in diameter.

 

 

Below:  One of the millwrights is removing the spring housing for the hydraulic actuator.  These valves are opened by hydraulics against spring pressure, so they are fail-shut.  Failing open could allow the turbine to overspeed.

Below:  The left and right reheat control valves and stems.

 

 

Close up of the reheat control valves with a boot for perspective.  A bearing cap is at the top of the image.

 

 

Below:  A couple of images of valve closure springs.  The top one is pretty large.

Below:  Top-down image of the reheat valve body on the left, and the high pressure steam valve body on the right.  Reheat steam pressure is lower than high pressure steam, so it requires a larger diameter pipe and valve body so as not to restrict the flow.

Below:  One of the millwrights working on the bolting for the valve bonnet.  This guy is over six feet tall (188cm for my metric-unit readers).

Below:  The High Pressure steam valve body.  The upper part of the valve is called the "bonnet".  Four of the bolts that hold the bonnet in place are broken and will have to be drilled out.  The more difficult issue is that while trying to replace the valve seat down inside, every single one of the bolt heads snapped off.  Each will need to be precisely drilled down the center with a magnetic-base drill - so as not to damage threads in the valve body.

 

 

Below:  What will be going in the hole above at some point - a replacement stellite valve seat.  The small hole is for a lifting eye bolt - one of three.  None of these parts are light.

 

Below is the rupture disc (disk?) for the main condenser.  This is a thin round sheet of aluminum that in the event of a steam overpressure condition in the condenser of about 5 psi, the sheet will flex outward. It will then contact an installed circular knife, and split open, allowing steam to escape safely upwards - to prevent the main condenser splitting open at a seam.  Main condensers are internally braced against vacuum, but cannot withstand much internal pressure.

 Below: A close-up of the circular knife, once the assembly was flipped over and the rupture disk removed.

Below:   Looking down from where the above rupture disk is normally installed - The steam turbine exhaust.  There are a few deposits, nothing major or abnormal.  The pipe at the right is venting for the aft steam turbine support bearing.  All of this stuff gets inspected each year for signs of failure.

 

 

Below: Steam turbine seals.  The steam side is to the right here, so steam will tend to flow along the shaft from right to left.  The gland exhaust fan takes a suction from the channel between the two seals, pulling a small amount of air in from the left, and a small amount of steam in from the right.  The steam is condensed in the gland exhaust condenser and that heat is recovered to improve efficiency.  Works great unless the seals are damaged.  We discussed the turbine sealing system in a bit more detail here.

Below:  Yes the seals are looking a bit rough.

Below:  Rough.  It's difficult to see, but the knife edges are actually rolled over to the right, as the shaft has clearly rubbed on the seals.  Not supposed to happen.

Very rough indeed.  Chunks of seal are brittle and coming off.  Possible heating issue due to steam leak-by.  Which came first?  The chicken or the egg?  Did the damaged seal allow too much heat to escape, or did the heat escape damage the seal?

 

Possible source of the trouble.  Thrust collars damaged.  Remachined.

 

Below:  Interesting spring weather!