Hotwell Pump Thrust Bearing Failures

[knight185]

I work a power plant that has three identical units with the same style condenser, hotwells, and hotwell pumps (Worthington 6-UJJ-2). When the unit is off-line and operators are preparing for start-up, both hotwell pumps are primed and ready to go. Only one pump is needed when the generation requirement is low. The second pump is in standby mode with the suction valve and discharge valves open, the first stage vent open, and the vents for impeller stages 2 through 5 closed. There is a check valve between the discharge of the pump and the discharge valve. The OEM manuals says to leave the first stage vent closed but plant operators have been leaving it open for years because of problem experienced in the past. During the past 10 years there have been numerous thrust bearing failures on the standby pump when the pump is started.

The hotwell is very small. The suction pipes run horizontally from the hotwell to the hotwell pumps, so there is no positive suction head from any elevation difference. Both pumps discharge to a common header. Also, the condenser that the hotwell connects to is under a vacuum.

My question is while one pump is running and the other is in standby, what guarantee is there that the standby pump will not lose its prime? I am thinking that maybe the loss of a prime is causing the pump shaft the thrust when the pump is started. Another issue is whether or not the discharge pressure from the standby pump is enough to overcome the discharge check valve and the pressure downstream of the discharge valve. Today the outboard thrust bearing shoes were wiped and to me this would indicate that too much pressure built up on the discharge side of the pump.

Any ideas? This has been a problem in the power plant for many years with no one really taking the time to figure it out. They just keep replacing the bearings.

 

[knight185]

We have very limited monitoring data for this pump and bearing. Operators can see the discharge pressure for the common header and the oil temperature of the oil being discharge from the thrust bearing. The investigation has to determine whether or not is a pump operation and priming issue or if there is an issue with the bearings and the lubrication. Oil analysis from previous bearing failures shows water in the oil. The oil passes through oil coolers. Coolers have always tested out fine but water still gets into some of the oil.

The loaded side of the thrust bearing is the inboard side. The outboard is what failed today.

 

[rmw]

Is your standby pump always the same pump or do they alternate? In other words, if pump A is functioning during low load periods, is Pump B always the one in standby? Is the wear factor for each pump about the same? Can you determine the shut off head for each pump individually?

For the stand-by pump to function, it has to overcome the head developed by the running pump and open the check valve whereupon it has to back the first pump back up its curve somewhat so that they can share the load at that point of the system resistance curve. If the stand by is worn and cannot develop the head required to do that, that could be an issue.

I tried googling your pump and didn't find anything that wouldn't take a lot of digging.

Having the first stage bleed open also throws a red flag in my mind. Multistage pumps are designed to balance the thrust using the thrust of opposed stages. If the bleed is passing too much water, it could be affecting the pump's ability to balance the thrust by robbing that stage's contribution to the thrust.

There are a lot of possibilities.

Can you post a cutaway of the pump?

rmw

 

[knight185]

Many years ago they started leaving the first stage vent open. The piping for the vent is connected to the condenser. The OEM instructions say to leave the first stage closed and some other stages open.

I am not sure if the same pump serves as the standby all the time or not. If I had to guess I would say there are no rules that govern this.

I posted a drawing of the pump that had the failed bearing. The discharge is close to the coupling to the motor. The other pump's discharge is next to the thrust bearing.

There are many problems with this system. There is no way to control hotwell level and there is no recirculation.

 

[rmw]

Wow, that was quick.

OK, first stage is double suction, so that takes thrust imbalance due to the bleed off the table for me. The other stages are balanced against each other as well.

Now, your comment about no way to control the hotwell level and no circulation are troubling. The reason that the OEM wants a bleed further down the line left open is that is a form of recirculation during pump start up. Recirculating from the first stage doesn't accomplish that.

You are pumping a saturated (or close to saturated) fluid out of the condenser and if you have no level control, you could be losing NPSH. There isn't a lot of head to go around in hotwell pumps in general. They need all the static head they are due.

Are the suctions common?

It sounds like these pumps are as old as Methuselah. What is the condition of the wear rings? A badly wiped wear ring in a stage or in 2 stages both thrusting the same way can case problems.

rmw

 

[knight185]

I am not sure when the last time this pump had the internals inspected. The boiler/turbine/generator was built in the 1940s or 1950s. The oldest pump that I recently overhauled was a 1930s vintage Buffalo Forge house service water pump that had not been opened for inspection for almost 20 years. I wanted to replace it but I found out that the centerlines for the suction and discharge were not at the same elevation. So if I replaced it I would have to change the elevation of the foundation and the suction and discharge piping. I was told to go ahead with a repair.

When you look at the set-up for the hotwell and the pumps it is evident that it is a bad design. I guess there was not enough engineering studies done when they built those three turbine-generators. Unit #4 finally got it with. It's a larger turbine with a larger hotwell with vertical pumps with suctions that have enough position suction head.

In addition to low hotwell levels allowing air to get into the suction lines, how does the vacuum on the condenser affect the pump operation?

 

[knight185]

forgot to mention...operators told me that when the hotwell levels are low and one pump is in operation, you can hear the pump cavitate. They know it's a problem but that pump must run in order to keep the turbine online.

 

[rmw]

First off, air is not a problem. There shouldn't be any in the condenser, or if there is any, (there is some in all of them) it isn't enough to have any impact on the pump. It is steam vapor that is your worry. Steam vapor from either running your level too low and actually pulling live steam into the pump, or steam (water) vapor due to insufficient NPSH. The pump doesn't' care if it is pumping out of a vacuum - plenty of them do this kind of service and do it well- it cares if it has enough NPSH to be able to pump. When it doesn't water vapor bubbles are created in the low pressure suction of the eye of the pump impeller (or upstream) and when they go through the pump and arrive at a higher pressure zone, they collapse and this is the noise that is associated with cavitation. Cavitation causes unbelievable damage so you may have some internal damage contributing to this problem.

If the operating pump is already operating with insufficient head, I suspect that the sudden addition of the standby pump further lowers the level and this pump cavitates like heck or even loses prime. If it loses prime suddenly it could thrust and wipe a bearing.

Bottom line, you need to get control on the hotwell level and protect your pumps. Further, their practice of leaving the bleed open is making the situation worse in that NPSH is a function of many things, piping losses being one of them. The additional flow over and above what the boiler (deaerator) needs is adding line losses to the side of the NPSH equation where it hurts. You still didn't answer whether or not the suction piping is common piping or separate for the pumps.

Also, have you checked external things such as the coupling? Something in your system has to let the shaft expand away from the thrust bearing. Either you have a coupling that will slip, or your motor had no thrust bearing. Is everything outside the pump up to snuff?

Do you have any pump curves for this old banger? And if so, what is the NPSH Required at design operating condition?

rmw

 

[knight185]

Each pump has its own suction pipe. The pipes come off the hotwell on opposite sides at the same elevation and travel to the suction sides of the pump at the same elevation. So not only is there a small hotwell that provides a small amount of water, there is no elevation difference between the hotwell and the pump to provide any suction head.

The bleed that you are talking about....are you referring to the first stage vent?

 

[ccfowler]

How much ambient humidity is involved, and how cold are the oil coolers? I've seen enough cases of high ambient humidity levels resulting in ambient humidity being condensed in over-cooled lube oil.

 

[knight185]

Not sure what the humidity is. The plants can get pretty hot and humid during the summer. We had an issue with air heater upper support bearing oil getting contaminated with water even though the oil coolers always tested good. I asked about this condensation issue that could arise depending on the ambient temperatures and humidity. No one had really thought about that. One of these bearings (roller bearing) failed last year after being in operation for 50 years. It basically chewed itself up.