pump selection for a boiler feedwater pump, pressure balance system
pump selection for a boiler feedwater pump, pressure balance system
(OP)
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,
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,





RE: pump selection for a boiler feedwater pump, pressure balance system
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).
RE: pump selection for a boiler feedwater pump, pressure balance system
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.)
RE: pump selection for a boiler feedwater pump, pressure balance system
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
RE: pump selection for a boiler feedwater pump, pressure balance system
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.
RE: pump selection for a boiler feedwater pump, pressure balance system
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
RE: pump selection for a boiler feedwater pump, pressure balance system
Do you have any information concerning other changes to the pump?
rmw
RE: pump selection for a boiler feedwater pump, pressure balance system
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