Condenser Vacuum Loss 2

[josh1990]

Mechanical) 24 Feb 07 14:51
We have a Westinghouse 27 megawatt machine that was installed in 1952. About 3 years ago we started having vacuum problems. We went from 28'' to about 26 '' and going from one air set to two air sets. We are still using two sides to the air ejector and our vacuum has dropped to 25''.
we have cleaned the tubesheet too many times with no increase in vac.
We are presently checking the horizonitial joint to check for tightness.
Air leaks to the condenser are a gimmy but I suspect the air sets and the loopseal back to the condenser.
Does anyone have a gauge glass on their loopseal?

In the old days with one air set the air leakage was 2.0
cu.ft. of air, with two sides of the air set I can't get an accurate reading the manometer is off the range.
Can the loopseal get so plugged up that the steam and condensate overwork the aftercondenser?
I would appreciate any help in the matter.
Also we have had to increase our steam seal pressure from 40 psi to about 100 psi,that should be a tipoff to seaking seals.
Leo

 

[rmw]

Have you done any type of leak testing (helium)?

rmw

 

[josh1990]

yes we have done test using helium a few years ago and we did seal up some leaks,

 

[rmw]

I think it is time to do it again. And after you fix the leaks you find, if you still have high back pressure, post back and we will take it from there.

rmw

PS: be sure to test up under the belly of the turbine too.

 

[josh1990]

Thank You rmw for the replies.
We found the horizionital joint had a few loose bolts, also the ruptured disc was shot and the L.P.seal piping had some holes in it.
We have hydro blasted for the past three days to clean the tubes, so this should also help.
I would like to pick your brain about Chlorine injection, years ago and I mean about 15 years ago we use to inject chlorine all year long and the tubes stayed clean, now with all the restrictions we do not inject any chlorine which I think is wrong. Do you know of any reliable chlorine injection monitoring equipment out there?
We use to use a metal wheel with a timer and holes in the disc to inject chlorine it was not state of the art but it did the job.
Again thats for your replies.

 

[rmw]

Ah the days of chlorine injection......No use crying over spilt milk.

Have you considered bromine injection? It has its own set of problems, but you don't have any chlorine residual.

Are you going to hydrotest the condenser by filling it to the expansion joint to look for leaks? If not, I recommend it if you have enough condensate stored to fill it that high. You can find a lot of leaks that way.

Speaking of expansion joints, did you take a close look at them too?

No, sorry, I don't know if any realiable chlorine injectin monitoring equipment but others might.

rmw

 

[josh1990]

We were late in getting started this morning so I don't know the status of our vacuum.
We did not fill the condenser with condensate, office says its not needed, you know how they can be.
I am working tonight and I'll let you know if we have had any improvement.

 

[rmw]

Yeah, I love the guys in the office. Upper level degrees in idiocy making technical decisions about power plants. They can run power plants but you have to wonder how they find their way home at night.

rmw

 

[josh1990]

Well the vacuum is not much better. I've noticed that the vacuum gets better at a higher load.
At 19 mws the vac. is 27'',condensate temp is 107°,hood temp 106°.
At 7.9 mws the vacuum is 25.7'',condensate temp 113°,hood temp 122°.You would think the the condensate and hood temp should be the same, sucking air in?
At 12.7 mws the vacuum is 26.1''condensate temp 120°,hood temp 123°.
I tried checking around to see if anything is obvious but nothing.

josh

 

[byrdj]

I'm not too familiar with the equipment that pulls vacuum, but is it possible to isolate from condensor and see if it is working propper. I think I have overheard talk that worn ejector nozzle, excessive steam can make SJAE not pull.

I guess saran wrap around flanges, sawdust in the cooling intake, and water in the blow out has been done.

Does this unit have a steam packing gland exhuaster, if so, when you stop its blower, is there enough sealing steam at the packing ends for steam to puff into the room.

I'm not familiar with (-W) and those seal pressure values don't make sense. where as GE is seal pressure of 3psig normal and increasing to >5psig a temp fix for damaged packing.

 

[rmw]

What ByrdJ alludes to is an old vacuum pump service engineer's trick of isolating the jets from the condenser for a short period-just long enough to take some gage readings.

Put a vacuum gage between the isolation valve and the jets themselves if one is not already there and then shut the valve and watch the vacuum.

If the jets are good, they will pull down rapidly to their design vacuum. If not, then they are the problem.

If they do, then air inleakage is your problem and you should pursue that.

It might be your inter/after condenser water temperature too. Where does that come from. Some systems use condensate and others use water from the heat sink (cooling tower, river, lake whatever). If that water is hotter than design, it can throw the jet performance off.

Give us some numbers around your jets so we can give some tips.

My guess is that if you haven't done some serious maintenance on your jets over the years, that they are just shot. They are pretty good, but they won't last forever.

rmw

 

[josh1990]

We checked the nozzels last year with a drill and maint said they were ok but the nozzels have been in there a long time.I think I was in puberty the last time they were replaced.
We use condensate from the condenser to cool down the air sets but this temp is over 120°, the condensate temp is 120°
going in and 136-140° coming out of the air set.
Our intercondenser drain valve is directly under the air set from there it goes to the loopseal, this line is ''hot'' I have tried to restablish the seal on the loopseal but there is not much volume between the loop drain and the air set,we are talking about 6'' under there.I tried to have maint install a isolation valve closer to the condenser so that if we are having loopseal problems we could fill the loopseal quickly,I have also heard that some places have a sight glass on the loop for monitoring the level.
We use two sides of the air set which does not help.
thanks for the help.

 

[rmw]

Not knowing how far past puberty you are I can only guess that the jets are plenty old.

If they are the type that have the nozzles threaded into the steam chest, then the possibility is likely that there is leakage around the threads, a common problem with threaded nozzles. If this is the case, then the leakage of motive steam around the threads becomes load for the jets robbing them of air removal capability.

The nozzles can be OK with respect to diameter and performance, but if there is leakage, then all they are doing is pumping motive steam instead load (air).

Secondarily, with your increased back pressure, your condensate temperature is up and this penalizes your inter/after coolers. If they were marginal, or if there is additional load due to motive steam leaking around the threads, then the performance can be affected.

You have to solve these one at a time.

I will guess that once upon a time, probably about the time you were dating, someone added the second set and it helped, but now both are limping along.

Start checking out each of these issues one at a time.

Are you able to isolate one or both of the jet sets to see if they are able to pull down to rated vacuum?

rmw