5 replies to this topic

[Mahesh@A&M]

Appreciate your inputs to resolve a low flow centrifugal pump failures due to shaft deflections.

This pump is designed for 55 gpm but operating flow is 17 gpm. This pump also has recirculation loop from discharge to pump suction ( need to calc flow rate). Today I called vendor and found out that minimum continuous stable flow is 30 GPM and he suggested me to decrease impeller size which I am not sure of. Please suggest me so I can analyse and try on stand by pump. I have attached all the details and pump curve here


Thank You

Chem roopa

Attached Files

•  PU_11FT.xls   17.5KB   79 downloads

[Mahesh@A&M]

pump curve size is too large too upload here. how can I upload by other means?

[pawan]

Today I called vendor and found out that minimum continuous stable flow is 30 GPM and he suggested me to decrease impeller size which I am not sure of

Could not understand it. Anyway when U r running the pump at lower than min cont flow MCF than probably due to reduced volume & higher internal centrifugal forces U must be facing some problem of internal vibrations which may lead to shaft deflection.

Just check it physically from outside if U have any vibration on the pump by touching its casing.

[Mahesh@A&M]

Pawan,

I joined this company a year ago and this pump is in service for several years with repetitive failures. I called Labour-Tabor applications engineer to find minimum flow recommended.( Though API says operating range of the pump). There are vibrations in the pump during shutdown and start up ( flow rate decreases to 5 gpm for 12 hrs. Recirculation loop exists) which is effecting our mechanical seal and shaft.

I want to know what kind of changes if any can I make to the pump or should I go for new pump rated for LOW FLOW.

[Art Montemayor]

Chemroopa:

I think your pump is a Labour-Taber model and their website is:

http://www.labourtaber.com/

They are a part of Peerless Pumps, another well-known specialty pump manufacturer. They both are known for designing and building good, stout pumps. I’ve used both.

You have inherited an established and tolerated problem when you joined your company. If you are a young engineer, I advise you to calculate carefully and make your best decisions based on facts and economics as they relate to your organization. It is obviously YOUR problem now, and it’s up to you to resolve it.

Please refer to a revised copy of your Excel workbook. Is what I’ve sketched exactly what you have installed? If not, could you please sketch out what you have in order for all of us to have an accurate idea of your installation? I’m calling my copy Rev1; please title your version Rev2 – that way we keep our data and revs correct.

There nomally is no problem in machining down a pump impeller. I’ve done this before in the past – on a Southbend lathe. Bronze impellers are easy to machine down. Cast Iron is a little bit more tougher.

However, when you machine down, you have to be backed up by the manufacturer’s known limits on how much diameter you can remove. Additionally, you are going to suffer in total head developed. Before you embark on such a venture I would strongly recommend you:

• Do all the related affinity curve calculations and have them checked – for accuracy and logic;
• Develop all of your system’s hydraulic profile. This means you must confirm the total developed head requirements and also a system curve that you can overlay on your resultant pump curve. These calcs should also be checked carefully.
• Make sure you have Labour’s complete, written confirmation that you can reduce the impeller and by how much; I would prefer to have Labour do the actual machining or have it contracted out by them. That way, they are responsible for the predicted future performance of the pump.
• You will have to change out the pump seals as well and start from scratch – as if it were a new pump.
• Make sure you have complete technical back up from Labour; if you can’t get them to back you up, then consider a new, accurately designed and calculated pump installation. This may be cheaper than reworking old, mis-calculated pump applications. Sometimes this is the optimum answer rather than to continue to screw around with an admitted past mistake. But make sure that you develop your system’s performance curves and expected pressure drops. Do your job in calculating all the hydraulics and especially the system curve – for all expected flow rates and pressure drops. This will now be YOUR pump, so you want to make damn sure you are on top of any potential hydraulic problem now, and in the future.

I hope this experience helps you out.
 Chemroopa_Pump_Rev1.xls   157KB   103 downloads

[Mahesh@A&M]

Art,

Thanks once again for the detailed explanation on troubleshooting. Yes when I see a problem, its my problem and I have to fix it.

I will do the hydraulics calcs step by step as you mentioned and post it tomorrow.


Thank You

Roopa