Accuracy of fluid resistance calcs
Accuracy of fluid resistance calcs
(OP)
Hi all,
In your vast collective experiences, what accuracy can one expect between theoretical piping dynamic head loss and actual values. What is a reasonable margin of error for these sort of things. I am designing a small hydronic system and have painstakingly calculated the pressure drop at the desired flow rate around the system. Now I am selecting a pump and am trying to get as close to peak efficiency as possible. Is this a fools errand. should I select the pump with a reasonable middle sized impeller and fine tune with trimming or replacement afterwards?
cheers
In your vast collective experiences, what accuracy can one expect between theoretical piping dynamic head loss and actual values. What is a reasonable margin of error for these sort of things. I am designing a small hydronic system and have painstakingly calculated the pressure drop at the desired flow rate around the system. Now I am selecting a pump and am trying to get as close to peak efficiency as possible. Is this a fools errand. should I select the pump with a reasonable middle sized impeller and fine tune with trimming or replacement afterwards?
cheers





RE: Accuracy of fluid resistance calcs
For smaller systems I've gotten +/- 1 psia in the calibration run and +/- 3 psia in the subsequent runs.
The equations do a very good job within the constraint of the quality of the input data.
On the other hand, your goal of reaching a peak effeciency in the pump is just silly. Conditions change continiously. A section of pipe will accumulate some scale, another section will partially plug off, another section will have a bad weld create a von Karmen Vortex Street that screws up pressure drop. The best I've ever done is to design a system with the design conditions in about the middle of the pump operating range and add enough hp to operate a fair bit on either side of the design point.
David
RE: Accuracy of fluid resistance calcs
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Accuracy of fluid resistance calcs
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Accuracy of fluid resistance calcs
what is BEP/OFF? I have calculated the system loss at 83kpa or 12psi. My flow is .6 l/s. Any ideas for pump selections. I want to go for the grundfos magna series, they have a power regulation unit attached, it matches power input to system resistance. Any other ideas?
cheers
RE: Accuracy of fluid resistance calcs
BEP/off means simply that you're simply either running the pump at its Best Efficiency Point flowrate, or its turned off. You won't try to control the flow to adjust to flowrates other than that BEP rate, so no control valves would be needed, just on/off types.
There are thousands of pumps that fit this application and maybe you can find one at your local Home Center. I'd look for a small centrifugal pump (plastic components will be fine) with as close as possible to a B. E. Point of 36 l/minute (10 gpm) at a head of 8.5 to 10 meters (25 to 33 feet) and at least an operating pressure of maybe around 20-25 psig, which shouldn't be a problem. The farther away from the 10 gpm & 30 ft head B.E.P., the more likely you are to need some valve for adjustments to flow and a little bit more money for electricity costs.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Accuracy of fluid resistance calcs
When I hear "trying to get as close to peak effeciency as possible" alarm bells go off. I've seen too many "effecient" systems that couln't handle the change in load caused by a low pressure weather cell.
David
RE: Accuracy of fluid resistance calcs
RE: Accuracy of fluid resistance calcs
What pump does 36 +/- 0.005 L (no question mark).
BEP is a target. If you can't operate at 78% efficiency, OK try to hit 77% or 76 or 75. You can operate at increasingly worse efficiencies away from BEP until you run out of dollars to pay for the increase in fuel or electricity price. If pipe roughness gets too much, then you spend some money on maintenance, including looping or replacing the pipe, if that's the economical thing to do. If the impeller gets worn too much, replace it. If its still pumping at reasonable efficiency, let it go.
Simple engineering economics can tell you when its worth the cost to replace, or maintain and how far away from BEP you can afford to operate.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Accuracy of fluid resistance calcs
RE: Accuracy of fluid resistance calcs
For a 10.5 hp application I'll generally install a 15 hp motor. "Effecient" applications would install a 10.5 hp motor. "Peak Effecient" applications would install a 10 hp motor and claim it was the pump's fault that it can't meet design specs.
David
RE: Accuracy of fluid resistance calcs
RE: Accuracy of fluid resistance calcs
But ratio your same factor up for 5000 HP required at BEP and I bet you won't get your wish list filled.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Accuracy of fluid resistance calcs
RE: Accuracy of fluid resistance calcs
In other words, the more economic risk, the more Engineering is justified. I sure agree with that. I see a lot of 5-25 hp applications that are not Engineered at all. Above 250 hp the price tag goes up and the interest of Engineering management goes up with it.
David
RE: Accuracy of fluid resistance calcs
RE: Accuracy of fluid resistance calcs
RE: Accuracy of fluid resistance calcs
RE: Accuracy of fluid resistance calcs
"see a lot of 5-25 hp applications that are not Engineered at all." Most likely why you've seen a lot of them fail to meet expectations.
But, if you don't start with a BEP target, where do you start? The pumps at Home Center I've seen have motors sized for the full curve, which is why I didn't mention HP required.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Accuracy of fluid resistance calcs