pump selection for a boiler feedwater pump, pressure balance system

[pbrod]

Hi,

We need to make a selection for our new boiler feedwater pump. The pump is a 750m (75bar) high pressure pump working at different flow rates.

20m³/h for 2000h/y
40m³/h for 3000h/y
60m³/h for 3000h/y

the pump is stopped aprox 20 times a year.

We got several offers with different designs:
- disk balancing system
- disk balancing system with lift off device
- drum balancing system
- opposed empeller design.

The pump which is installed now has a drum balancing device with the balancing line connected to the feedwater drum. This design failed allready several times in the past (probably due to the axial force balancing which was not good).

Could anyone give advice in selecting the best balancing device for the operating conditions? are there guidlines for this? What is the experence with feedwaterpumps working in different flow conditions?

Thanks in advance,

 

[DubMac]

As with most important questions, there is no one right and simple answer to your question. All of those designs can work just fine if they fit your application properly and maybe more importantly: fit your Maintenance/Reliability Dept's practices.

From a flow and head perspective, your conditions are not too severe, and fall easily within a broad range of pump designs. Adhering to the "kiss" maxim (keep it simple stupid), I would prefer to go with the opposed impeller, multistage splitcase, volute design.

Unless you have a fairly sophisticated Maintenance shop with expert mechanics and hydraulic engineers to back them, or are willing to pay the big bucks to sub-out repairs; I would be less willing to go with a diffuser and balancing device design. Too many parts and pieces.

Things might sway toward a barrel type diffuser pump if you had to run at elevated speeds, but it looks like this machine can be easily fit into a synchronous speed (3600rpm).

 

[Artisi]

for the varying flow conditions you need to look carefully at the pump selection and its capability of such a wide flow range, it doesn't look like much difference on paper - buit if you take the 60m3/h as the main operating point and select the pump to be running at near to BEP for this duty, then for the 20m3/h it is actually only 1/3 of the BEP flow -maybe this is the reason for failures - operating too far left on its curve, that's unless you have a bypass line to keep the pumps flow up and closer to BEP.



It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[JJPellin]

Based on the description you provided, I would not expect the failures to be associated with the balancing device. I would suspect that your lower flow condition is too low for a high energy pump. I would be more concerned about the design of a minimum flow spill-back rather than looking for re-design of the balance device. If the pump is running close to BEP, any well designed balance device should work.

With that said, I have no experience with pumps that have no thrust bearing and rely only on the balance device. But, based on what I know of that design, I don't care for it. I prefer a nice hydrodynamic thrust bearing in conjunction with a well-designed balance disk or drum. We have one pair of pumps with a spring-loaded thrust bearing designed to open up the balance disk for start-up and coast down. We have had failures on these pumps associated with this mechanism. There was a good case study at the last International Pump Users Symposium in Houston regarding a similar failure. This case was related to shaft material, heat up rate and thermal growth. You did not list the water temperature. But, I would caution you to be very careful about warm up procedures.

I agree with DubMac. I see nothing in the description of this service that could not be served well with an opposed impeller, axial split case multi-stage pump.

If you can provide more details on the failures that were experienced with the installed pump, this might shed light on the root cause. Knowing the root cause of the failures of the existing pump is important if you want to be certain that you will not have the same problems with a new pump.


Johnny Pellin

 

[pbrod]

Thanks for the information.

The current pump is a Byron Jackson (flowserve) 4 x 4 x 10 SVMX – 10 Stage.
The pump works at 183°C.

There have been several problems with the pump but there was never a clear cause found for the failure. I'm also rather new in the company.

What I can say is that the pump is now working at 45m³/h and there are no problems. The failures have been at start up and at high flow.

Tests in the installation indicate that the power consumption is higher than according to the pump curve and the efficiency tests.
41 m3/uur 750 m liq 170 kW
And according to the pump curve 140kW would be the power. Therefore the pump was destaged from 10 to 9 stages.
A possible explanation would be that the blance flow is higher. In the installation the blance line is directed to the water drum.

From the inspections after failure it seems that the bearing failed which caused a total loss of the pump.

Once a the axle was broken near the balance drum. Then there ware traces of high metal temperatures (change in metal colour, O-ring failure due to high temperature). According to flow serve the failure was a fatigue failure of the axle.

The balance drum was also HVOF coated to reduce the space and the flow in the blance line.

I can also ad that a feedwater investigation showed that there was a particle contamination of 5mg/l of iron parts.

I'm also going through the vibration measurement history for the moment.

Hope this information can help a bit.

 

[JJPellin]

The key phrase that I am focused on is the fact that the failures occurred at start-up and high flow. The function of this thrust balance device depends on discharge pressure to counter the thrust of the 10 (now 9) impellers. Without discharge pressure, it cannot function and the thrust is not properly compensated. If this pump were started-up against a slack line (not flooded), it would run off the end of the curve (at high flow) but build very little discharge pressure. This pump should only be started up well flooded against a discharge restriction and allowed to come up to full speed and pressure before the flow is gradually increased up to normal levels.

I am also still very concerned about thermal growth. Are the shaft and casing made of similar materials for thermal growth? The case study I referenced above was related to a pump constructed from 316 SS that was converted to a 4140 carbon steel shaft. Check the thermal expansion rate of the case and shaft materials. Check the proper function of the devices that allow for axial thermal growth of the casing. I am not familiar with this exact model. But, there should be guides, pins, keys, slots, etc. to allow the case to grow axially. The hold-down bolts on one end should be sleeved, slotted or left with gaps under the washers so it is free to move axially. If these devices are not functioning correctly, the distortion could cause axial and radial rubs.

It is still very important that the pump is warmed up gradually before it is started. Even if this temperature is below the limit that the manufacturer would require warm-up, I would still suggest it. Warm the pump up to within 50 °F of the feed drum temperature before starting. Verify that the top and bottom of the case are within 50 °F of the same temperature. Starting up a cold pump in a hot service can result in transients that result in axial or radial rubs.


Johnny Pellin

 

[rmw]

I don't have access to my B-J manuals (packed for recent office move and stored away right now) so I can't see the exact configuration and the curves, but I'm focused on the "10 now 9" statement. If the original pump was properly balanced with 10 stages, one less stage could make a difference if something else wasn't done to compensate for this missing stage.

Do you have any information concerning other changes to the pump?

rmw

 

[JJPellin]

rmw had the right idea. I should have cracked open my old BJ catalogs before commenting. I have attached a drawing of this pump model. This is a stacked diffuser pump with external tie-rods. This configuration opens up the door to a lot of potential problems. The sketch I attached is for the 10 stage version. They also had a 9 stage version. If you took the 10 stage and just removed an impeller, this could create problems with the thrust balance. But, there are a lot of other possible explanations for the failures. And, it sounds like they had the problems before they destaged.

This is a stacked rotor where all the impellers are stacked together with a single nut at the balance drum clamping it all up. This is generally a bad idea. We had a similar Sulzer pump that had a number of failures. We converted it to a design with the impellers individually located on split rings. This can greatly improve the run-out of the assembled rotor. With the original design, it was very important that the rotor was stacked up bare and run-out was checked. Then, when the rotor is stacked for the final time with the diffusers, it is very important that it is stacked exactly the same way. Key locations, impeller orientations, nut torque must all be the same in order to have a good chance of repeating the run-out result. Once the pump is fully assembled it is impossible to verify that the shaft run-out in the center is still within tolerances.

The Sulzer version of this pump configuration that we use is longer (13 stage). The longer pumps have so much rotor sag that they deliberately design in a wedge shaped diffuser so that the diffusers sag as much as the rotor to reduce rubs.

I think you have the right idea about replacing this pump. But, as long as you go with a more conventional pump that has individually located impellers and no external tie rods, you probably won’t have any problem with the balance device.

Johnny Pellin