Throttling down Pumps

tombig

If a flow rate through a hydronic loop is determined to be too great, can an oversized pump be manually throttled with a balancing valve to restrict the flow rather than change the pump? It seems to me that the pump will not overheat/fail if flow is restricted with an inlet or outlet balancing valve as long` as there's some flow. How 'bout it?

scrook

don't throttle the inlet...

it may cavitate; then you'll get to install a new (maybe smaller this time?) pump! If you throttle the outlet (within reason) it'll move over to a lower flow point on its performance curve.

Jerry

Pump Speed

What about slowing the pump to reduce the flow?

Aidan (UK)

Yes

Yes.

Usual European commercial practice is to install commissioning sets/circuit setters on all the returns. These are used to proportionally balance the system, so that the percentage flow-rates to all the branches, expressed as a percentage of their design flow-rate, are equal. The total flow rate is then reduced to the design value using a regulating valve on the pump outlet.

This is wasteful of energy. The pumps put energy into the system and the regulating valve is taking it out again. Anything more than a slight adjustment would suggest different pumps are required. A variable speed pump could reduce the total flow, but you’d have to calculate life-time energy costs for the pump to justify it. A slight excess flow wouldn’t necessarily be a problem for many systems.

Consulting Engineers tend to over-size pumps. If the pumps are under-sized, they can be sued. If the pumps are oversized, they can always be throttled back with the regulating valve and the Engineers won’t have to pay the additional electricity costs. It’s also an insurance, if they’ve estimated the pump duty with a rule-of-thumb, rather than a properly calculated estimate of the system pressure losses.

Some of Dan’s books mentioned trimming the pump’s impeller, but I’d never encountered this.

hot_rod

Ideally

you could find a pump sized real close to the load. The new multi speed pumps help in this respect.

Always throttle downstream, and use a valve designed for throttling, for long term reliability and accurate adjustment. Danfoss T&A and a few others make the "right valve"

hot rod

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Jeff Andle

within reason, why does it matter?

If the water flows are in proportion, faster circulation just means more uniform water temperature in the pipes, higher return temperature and less boiler water mixed in to maintain temperature, right?

I can see where this might cause short-cycling of the boiler. Are there other problems?

hot_rod

Exactly, Jeff

within reason is the key word Not very often you find a pump sized exactly right for the job when running at the optimun spot on it's curve. Like you indicate, within reason you can balance this out. Generally all commercial work has circuit setters speced at the equipment. With the right meter you can dial in the flows exactly.

With small residential systems the heat loss calc and design will usually spell out the required flow and pressure drop for all the various zones, and generally a system total. With this info you can select the pump that best matches the system design.

hot rod

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Art Pittaway

It Depends..!

I love that term. Each of the responses has reflected the personal experience of the person. Your question could be about a small circulator on a bathroom radiant system or three 50HP pumps in a commercial building. On the small side it can be corrected or left alone as long as it's quiet and works. I have seen an oversize pump (less than 1 HP) errode the tube right out of an A.O.Smith Burkay heater with velocity in 6 months. Had to balance and set flow rate. On the big side I worked with an engineer that couldn't read a pump curve and had his own way to choose motor HP. I said 7 1/2 and he said 15HP....15's were installed. No, they will never overload. In the case of large pumps with bronze impellers, all are trimmed to fit. Look at a curve (i.e. B&G 1510) and it shows pump performance with the impeller trimmed to several different cut diameters, and HP curves to match that performance. The "balancing valve" on big stuff is called a "triple duty valve" by B&G, as it is a check valve, shut off and flow control with meter ports and gage to mark where the correct flow is. I have heard several opinions about max flow restriction, I don't like flow dropped back to less than 20% of max flow. And once again what does the system look like. 2 gpm from a Taco 007 or 200 gpm from a 1000gpm pump ??? Yup..may have oversized it, just a tad !! Art

hot_rod

Understanding cavatation

to get back to the original question regarding throttling the intake or discharge side of the circulators. Have you ever seen Dan's demo with the soda bottle, at his seminars? A classic example of what happens when you lower the pressure of the soda belows its vapor pressure. With the cap on and tightened you see liquid in the bottle. Lossen the cap (lower the pressure) and the bubbles (CO2) appear.

Same thing can happen in a hydronic system. Cavatation will begin as the water starts to boil at the impeller. Keeping in mind water will boil below 212 degrees F. In fact water will boil at 170 IF you lower the pressure to -8.7 (6 psia). This is very possible at the eye of the impeller under certain circumstancese . Reducing the flow to the inlet side of the circ may cause this, running too low of a fill pressure may cause this, pumping towards an expansion tank may cause this, plugged strainers on the suction side, any valve or device that induces turbelence at the suction side of the pump, zone valved systems with high head pumps, another one to watch for.

Ever been on a job that runs quitely at low temperature, but starts to get noisey at higher temps (solar roll with high head pumps on staple ups ) High temperature and low pressures brings you closer to cavation. If possible bumping the static fill pressure will often cure these. High head solar rolls systems generally need around 20 lb fill to keep 'em quiet. Or use heat transfer plates, and lower the operating temperature! Be careful with those small diameter tube, high head pump jobs. The line becomes much finer regarding cavatation.

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Aidan (UK)

Are there other problems?

Are there other problems?

I once had a problem with a primary/secondary chilled water system. The secondary flow was greater than the primary flow. The secondary flow temperature was higher than it should have been because mixing of the primary flow and secondary return was taking place. This was on a system with a 5" regulating valve.

Few domestic heating systems would have flow measuring devices, in which case you wouldn't want to, or be able to, throttle down the pumps' flow rate.

If Tombig has a need to throttle down a pump, there's no reason why he shouldn't.