[b]Permissible Motor Cycle[/b]

[mitcham]

Hi
I am sizing a pressure tank for domestic cold water supply system in a multi-story building. I have come to the point that I need to decide what should be the maximum permissible booster pump motor cycle for a safe and long lasting operation. whould you please give me an advice or a rule of thumb in this matter?
I would like to thank all responses in advance.

 

[PUMPDESIGNER]

How much Static Head are we looking at?
What size system?


PUMPDESIGNER

 

[mitcham]

S.P.=230 FT-W
DESIN GPM=180

regards
mitcham

 

[electricpete]

NEMA publishes suggested maximum starting duty based on motor speed, horsepower, and motor+load inertia.

http://www.joliet-equipment.com/allowable_starts.htm

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[PUMPDESIGNER]

electricpete makes a good suggestion to help, but still the problem of the pressure tank.

We use a lot of pressure tanks, but never on your type of application. Not sure of all the reasons, but maintenance upkeep is a part of the reason if not the whole reason.

We use heat purging combined with low Ns=450-700 jockey pumps to maintain pressure at low flows, say at night. We monitor volute temperature, dump a few gallons down the drain to keep the volute cool. System is infallible and does not require any site specific information. Station ramps up and down through the flow rates smoothly, with alternating main pumps and jockey pump for the low flow areas.

If you must have a pressure tank then I hope someone on this board has a source quote for you.

If no one else helps you I have a few ideas that I hesitate to post because I have no reliable solid knowledge of using a tank in your application. My first idea would be to use the largest practical tank, then control starting frequency with a minimum run timer set so that the pump is retired by the time the volute starts to heat much. If the pump is low Ns with enclosed impeller, then that run timer can be set somewhere around five minutes.

A by-pass tube is another option to extend time before volute temperature climbs.

PUMPDESIGNER

 

[PUMPDESIGNER]

I checked a captive air tank manufacturer site (Well-Mate), and they have a little form you fill out and it tells you their recommended tank size. I think all the captive air tank manufacturers have similar forms.

The page at Well-Mate is:

http://www.wellmate.com/techsupport.html

Sta-Rite recommends 1 gallon of drawdown per 1 gpm of pump capacity. Link is:

http://www.berkeleypumps.com/specs/sys_fibrewound_tank_specs.pdf

I have read all their stuff before, have no confidence in it because site conditions vary so much.

PUMPDESIGNER

 

[GaDawg]

We build these systems and there are a number of factors affecting the sizing of the tank. If you are concerned about motor cycling and actually expect some low or "no flow" periods, then use NEMA or the motor manufacturers recommendations for number of starts per hour to determine the off time required. The off time is then used to determine the storage volume by a)the system leak rate, and b)the time to refill the tank. You should also consider fixture flow rates and the amount of off time you can maintain with minimal flow. ASHRAE has information on fixture flow rates and Hunter's curve. Now, if your system is in a hospital or high use facility, don't expect much off time at all. In fact, many of these such facilities are continuous use so number of motor starts per hour isn't a factor.
Precharge and tank location should also be considered to be sure you are able to achieve the correct acceptance volume. Most tank manufacturers have capacity curves that aid in determining acceptance volume. I hope this helps.