I recently received a phone call and subsequent email stating “my self-priming pump is not priming”. Both the phone call and the email were detailed with pictures and appropriate information to start the troubleshooting process.
The pump was a model 2796 MTO 6 X 6 – 13 with a 10.625” diameter impeller turning at 1750 RPM. The design point was 1000 GPM at 85 feet of head and the liquid was pond water at ambient temperature. The 6-inch suction piping was approximately 60 feet long and extended down to the pond with a check valve located only 30 feet from the pump. We did not know the depth or submergence of the suction piping into the pond. The suction “lift” was 23 feet. The issues with this application are at the end of the article.
In pump school we review self-priming “do’s and don’ts” and have developed
a check list of items to review if you are having a problem:
And Now for the Rest of the Story –
The Issues with the Application
A wise man once told me you can not violate the rules of physics
and 95% of pump problems are on the suction side of the pump.
Issue #1 – NPSHA is ? 6.5 feet. NPSHR at BEP is 14.6 feet. NPSHA must always be more than NPSHR with as much margin as possible.
Issue #2 – Suction pipe is ? 60 feet. Too much air to evacuate.
Issue #3 – Submergence Unknown. Minimum submergence required is ? 8 feet (without a bell mouth setup).
Issue #4 – Check / Foot Valve located 30 feet from the pond. If you are going to use a foot valve it should be located at the bottom end of the suction pipe.
As a general guideline, if your pump takes more than four minutes to prime than you should shut the pump down and look for and correct the cause of the problem.
For More Information
Reference These Pump & Systems Articles
by Jim Elsey:
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