Pump dividing walls in sump
Pump dividing walls in sump
(OP)
I have an existing sump (open pool) with three (3) mixed flow, vertical pumps drawing from it. Each pump has a wall between it and the adjacent one forming a three-walled vault around each pump. At any time only 2 of the 3 pumps are running and the combinations are rotated in order to give each pump a day or two off. We are starting to see flow rate decreases in two of the three pumps. We are looking at several things and the sump dimensions is one of them. The pump vault dimensions meet the recommendations from the Hydraulic Institute except that there is no "break-thru" between the vaults. The walls are structural for the walkway above it, so removing the walls is not an option. Should I expect that modifying the existing vaults ($) to add "break-thru" will provide a significant change to the flow rates? Are there any suggestions on how to increase the flow rates? Thank you.





RE: Pump dividing walls in sump
What is the service and how long has the installation been in place? Have any of the pumps been pulled and inspected for wear? Does the vault need a good cleaning?
If the flow reduction is a new thing to an existing (proven) installation I'd pay more attention to the pump and piping unless there's enough crud built up in the vault the may be causing problems.
RE: Pump dividing walls in sump
There are vortices visable on the surface of the water and the pump currently being rebuilt shows cavitation wear on the impellor.
The pumps are generally rebuilt each year, but no specific inspection reports have been kept on previous rebuilds.
We are doing a pipe inspection next month to look for obstructions and/or scale. We will clean the vault at this time also.
I should also have mentioned that we utilize trash screens in the sump as well as strainers on the inlets. Is this redundant? Should the mixed flow pumps handle anything that passes through the trash screen?
RE: Pump dividing walls in sump
You have not explicitly stated how far the pumps' inlets are off the bottom of the sump, or where the water enters the sump. Is it flowing into the sump parallel to the three vaults, or perpendicular? Is there any difference in pump performance characteristics between the 3 pumps/vaults?
RE: Pump dividing walls in sump
Honestly, my pump trouble shooting can be documented on a sicky note but it sounds like an NPSHa vs NPSHr situation. Lowering the pumps or increasing the liquid level may help more than trying to get more flow to the pumps.
Vortices aren't good but a litte swirl is better than a full blown "whirlpool" that lets air into the suctions.
Pulling the suction strainers certainly wouldn't hurt as long as the trash sreens are effectively "filtering" out the size of junk that'll give the pumps grief.
Combing over the original design package and comparing it to what you actually have would be a good start. It would be worth a call to pump manufacturer to see what they have to say.
RE: Pump dividing walls in sump
a
-------
| <-|
b| |d
|123 |
-------
c
The flow enters from side d (at the arrow) parallel to side a in a CCW flow. The pumps are lined up along side c in the corner by wall b. There is some natural circulation that we will be addressing with flow straighteners. Pump three performs close to rated (14400 gpm). Pump two is just a little worse and pump one is down near 70%. I'm sure it's not a coincidence that it is the one in the corner.
Sorry for the confusion about the flow loss. The decreases in flow are noticed just after reinstallation. We get close to rated flow after reinstallation and then the flow will drop off steadily. Lately the level has dropped so low that we leave pumps 2 and 3 on constantly and don't even use 1.
RE: Pump dividing walls in sump
Phitsanulok
Thailand
RE: Pump dividing walls in sump
Your explanation would make it seem like the flow comes in on side D "at" arrow with references to some circulation. If I understood it correctly, then that is at the root of your problem.
Also, if you don't have enough submergence to prevent any or all vortexing, then you have a problem with your submergence. Have you checked your pump manufacturers requirements regarding the minimum submergence, distance from the floor, and distance from each respective wall in the individual pump sumps? I suspect that something might be off there too.
What about sedementation? Is that a factor? Build up can change chamber dimensions.
Camerons Hydraulic has some information if you have a situation where your pump mfg. has disappered or is unable to help you.
I was once associated with the old BJ pump co before they were absorbed and they would go postal over the parameters I mentioned above. I have seen them ready to walk away from business rather than put their pumps in a poorly designed pit and have to fight the problems the rest of the pumps life.
rmw
RE: Pump dividing walls in sump
RE: Pump dividing walls in sump
Certainly a star award answer.
I would add thaat the presence of sub-surface vorticies can also be a major problem as they are not always evident from the surface but if there, can be a major source of low performance and vibration and can be difficult to eliminate
Phitsanulok
Thailand
RE: Pump dividing walls in sump
RE: Pump dividing walls in sump
With a new pumping station, we proposed a sump with a plan area half that achieved with a standard design, significantly reducing the construction costs. From these experiences, I would heartily recommend model testing if you can find a good modelling consultancy.
RE: Pump dividing walls in sump
The above posts contain much useful advice and should provide a good start for you.
You obviously have an expensive problem on your hands. If you can see any evidence of vortices at the surface, you can be quite certain that the pump inlet conditions are far from ideal.
If I had to deal with this problem, I would want to first consider the full economic implications of the existing situation compared to that of a properly running pumping system. I would look at the differential power costs, costs of disrupted operation, costs of excessively frequent rebuilds, etc. It would be no surprise to find that the potential savings can provide funding for significant study and modification efforts to correct this problem, but this is not a certainty. Your economic analysis could show that although not ideal, the costs of living with this problem may be less costly than fixing it. Since it is likely that significant energy savings may be involved, there may be some additional economic incentives available.
If the economics are not sufficient to justify a complete fix, you may be able to implement some measures to mitigate the problem. Paying closer attention to the details of future pump rebuilds, and some modest revisions to your intake structures may pay for themselves very quickly. Fully correcting an existing, operating system can be difficult, expensive, and physically almost impossible.
I would pay close attention to potential means to minimize vortex formation not only at the pumps but upstream of them, too. Once formed, vortices can be very persistent, so it is best to avoid their formation well upstream of the pumps.
RE: Pump dividing walls in sump
Flow into the suction bell of each pump has to be uniform, laminer, and free from vortices to prevent any problems. If you see surface vorticies, you definately have a problem with submergenace and probably getting some air induction and possible caviatation. If you are shutting off one pump, and flow is entering from non symetrical directions, you will likely need straighting vanes installed. I recommend you contact someone like at Clemson Engineering Hydraulics to physically model you intake and they will definately solve your problem.
RE: Pump dividing walls in sump
@14500 gpm:
NPSHa = 42.32 ft
NPSHr = 29.76 ft
NPSH does not seem to be an issue right now.
RE: Pump dividing walls in sump
I disagree that NPSH is not an issue. At NPSHa = NPSHr, the pump is already suffering from performance deficiencies due to cavitation. The cavitation free NPSHr is normally substantially greater than the nominal HPSHr. Determining the precise cavitation free NPSHr is a messy matter, but you can find considerable discussions on this topic with a modest search effort. These discussions can give you some guidance on this subject as it applies to your application. Don't be surprised to find that the cavitation free NPSHr may be in the range of 2 to 4 times the nominal NPSHr.
You should consider that the two most common standards for determining the nominal NPSHr are based on the point at which the pump's performance is suffering by either 1% or 3% due to cavitation.
Probably the most important thing to do to minimize vortex formation is to minimize abrupt accelerations in the flow anywhere upstream from the pump inlet. Once formed, a vortex can persist within the flow to cause trouble at the pump.
Depending upon the actual dimensional constraints of your situation, you may be able to ease submergence problems by adding a device to further control the acceleration of the flow approaching the pump inlet. From information above, I assume that there may be some potential problems with solids in the flow, and the implications of these should be thoroughly considered. For a situation such as yours, I would consider attaching a fairly wide flange to the suction bell and make modifications as necessary to assure that the floor of the sump beneath the area of the pump and the attached flange is as near to perfectly level as possible. Also, I would consider installation of a small cone on the sump floor along the pump's axis to help redirect the converging inward flow toward the pump inlet. The practicality of such a device can depend heavily upon the proximity of the adjacent walls.
RE: Pump dividing walls in sump
Are your pumps rotating counter-clockwise. If so, the inlet configuration you sketched out above looks like it would encourage pre-rotation.
RE: Pump dividing walls in sump
Q. Our plant is operating pumps on cooling tower service which are experiencing cavitation damage to the cast iron impellers. The pumps are operating close to the best efficiency point and the NPSH available is slightly higher than the NPSH required by the pump. Should there be cavitation damage in the pump impellers under these conditions?
A. There is a common misconception among pump users that if the NPSH available from the system is greater than the NPSH required by the pump, that the pump will operate free of cavitation. This is not so. In order for a pump to operate cavitation free, the NPSHA must be from 2 to 20 times greater than the NPSHR of the pump. By definition, NPSHR is measured when the pump total head is reduced by 3% due to cavitation. For satisfactory operation, some NPSH margin over NPSHR must be provided by the system.
The Hydraulic Institute has recently published a new standard on this subject namely, ANSI/HI 9.6.1-1998 Centrifugal and Vertical Pumps for NPSH Margin. According to this standard, for cooling tower service, NPSHA should be 1.3 to 2.0 times NPSHR, depending on suction energy level, which is also defined in the standard.
I will look into this new standard and see where my suction energy level is. FYI, my ratio is 1.42.