Commercial replacement pump sizing

Herbert

Needing help to size replacement pump on a commercial boiler system with a failing 1960-ish base mounted circulating pump. No plate on pump end/volute. Motor is 3 HP; Impeller is appox 8"; 3" suction; 2" discharge; 3" main supply & return; 30' height; approx 100' loop. Supplying 2 commercial AHU's, 4-6' convectors & 2- 4'x3' cast iron radiators.
Recommendations?

Jamie Hall

Not without more information. You need to have someone calculate the total head loss in the circuit and the flow, then find a pump which will match that at or near its best operating point.

ratio

Post a pic, there's a very real possibility someone will recognize it. Suction & discharge pressures, and volts & amps will help narrow it down.

Edit the title to say 'pics added' or something to that effect. You'll get more views.

ScottSecor

I concur, photos would certainly help. Is this the only pump for the system or is there multiple pumps. If this is the only pump please post a photo of the boiler rating tag. Assuming this is the only pump, we can determine the maximum gallons per minute you will need. Are there pressure gauges before and after this pump, if so what do they display when the pump is operating, this will help us determine the feet of head your system requires.

Zman

It would also help to have pump motor RPM and the BTU output of the boiler. From what you posted, it sounds like your 3HP motor is way oversized.

EBEBRATT-Ed

3" pipe means about 140 gpm. Does the pump run and can you get gauges on it as @ScottSecor and also close the discharge and get the shut off head

retiredguy

If you know the MFG of that pump, call them or their local distributor and ask if they have records of that pump and building. Check the base of the pump for any "numbers" that may be on a tag. They may also be able to gauge the pump and impeller to find an exact replacement. Depending on the size of the job there may be building records of the engineering drawings that may tell you everything about that installation. On a commercial job, selecting a replacement pump is much more detailed than choosing a pump for a home or a single zone.

Zman

@Herbert What you have does not really add up.
I would expect a design with 3" pipe to have a required flow <87GPM (4 ft/sec). The system you describe should have fairly low head.
This selection is a total WAG with a motor < 1HP http://apps.taco-hvac.com/siteapi/PumpWizard_v2/curve_image_dev.php?curve_image_id=getid-0_CI2506D@1760&amp;mincontrolhead=&amp;flow=90&amp;head=15&amp;motor=&amp;units=&amp;numpumps=1&amp;ns=0.63&amp;img_type=pdf&amp;impdia=4.40&amp;curve1900e=&amp;curve0034e=&amp;fluidsel=Water+@+60+F%%1.0000

One option would be to install a variable speed circ (onboard ECM or external VFD) and adjust it to meet the load.

jumper

If the pump runs you can deduce system curve with pressure gages and flowmeter.
Easier than calculating.
Too bad you don't have specifications for existing pump because then you don't need flowmeter.

Almost always circulators are drastically over sized.