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| Hydraulic flow | |||
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| Posted by: Howard ® 01/29/2011, 07:32:50 Author Profile eMail author Edit |
We have a theriotic argument about flow. The question the argument is about, is, you have a 8 gpm pump, it is connected to a hose that will hold 2 gallons, at the end of the length of hose is a 6 gpm oriface, the 6gmp oriface is then plumed to tank. What is the flow? My answer is assuming all is perfect, a pump that never has any bypassing. The hose will hold exactly 2 gallons and has no friction and the oriface is perfectly engineered, you will expect to see 8 gpm from the pump. 6 gpm in the hose. My reasoning is, every 8 gallons the pump delivers 3/4 of the liquid is passed through the oriface holding back the remaining liquid in the hose. The other side of the argument says the flow will be 8 gpm reguardless of the oriface. |
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| Posted by: Kelly Bramble ® 01/29/2011, 08:32:38 Author Profile eMail author Edit |
Well, nothing is perfect and I repeat nothing is perfect (exact) in engineering or the physical manufactured world. Assuming an "Ideal Pump" or friction-less pipe is an academic perspective for simplify calculations and causing confusing for engineers fresh out of school. "But, I calculated it as perfect? Why do we have to test it? Why, why, why. There will be flow, pressure losses due to an imperfect pump, pipe friction, gravity, imperfect pressure regulation, regulator losses, orifice, et cetera. . Ok, I'm off my soapbox - If an 8 gpm pump is connected in series to anything there will be flow and pressure losses. The orifice is just that an orifice, so if you pump 8 gallons though it then you pump 8 gallons through it - nothing gets held back, they don't work that way and store fluid or whatever. The simple answer is that at the rated pressure of the 6 gpm orifice there will be less flow coming out than the potential gpm flow because of the losses associated with the orifice. |
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| Posted by: Howard ® 01/29/2011, 08:43:58 Author Profile eMail author Edit |
I believe I did mention this is theoretical, and the components are perfect. In a way, you just repeated my argument, just different words. Yes, the pump flow is 8 gpm. The flow out of the orifice is less, if all were perfect 6 gpm. |
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| Posted by: Pinkerton ® 01/29/2011, 10:22:02 Author Profile eMail author Edit |
I only did a quick read of this, but wouldn't the flow be 8gpm, just at a higher pressure? As Kelly says, you can't store those other two gallons. Well, not unless you live in Sedona. Dave |
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| Posted by: Howard ® 01/29/2011, 11:27:29 Author Profile eMail author Edit |
You are correct, you can not "store" per say. By adding the restriction, you inhibit full unimpeded flow. 8 gpm means you will fill a one gallon container every 7.5 seconds. If you impede the flow, I.E. hinder it, the flow slows down. That one gallon per 7.5 seconds now becomes one gallon at more than 7.5 seconds. You can no longer call the flow 8 gpm (gallons per minute) it's not, it's less. 8 gallons will still pass through the restriction, if fact hundreds of gallons can pass through the restriction, it's just going to take longer. |
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| Posted by: jboggs ® 01/29/2011, 11:43:55 Author Profile eMail author Edit |
Howard,
The mistake many people make in trying to understand fluid flow is that they think a "rating" applies in all situations. A "6 gpm" orifice only provides 6 gpm under specific conditions. An 8 gpm pump only moves 8 gpm under specific conditions. Any time you connect multiple fluid elements together into a system you have to look at the whole thing as a complete system. |
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| Posted by: Howard ® 01/29/2011, 12:08:09 Author Profile eMail author Edit |
I understand that, which is why in the beginning you will notice the disclaimer. The fact is if you install an orifice
in the line, you do what? You restrict the flow. What the flow will be depends on the flow before the orifice and the resulting pressure that develops from the restriction the orifice created. It however will be less flow than what you had before. An orifice is the simplest of any flow control device. The smaller the hole the less the flow, the bigger the hole the higher the flow. |
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| Posted by: Howard ® 01/29/2011, 12:07:52 Author Profile eMail author Edit |
I understand that, which is why in the beginning you will notice the disclaimer. The fact is if you install an orifice
in the line, you do what? You restrict the flow. What the flow will be depends on the flow before the orifice and the resulting pressure that develops from the restriction the orifice created. It however will be less flow than what you had before. An orifice is the simplest of any flow control device. The smaller the hole the less the flow, the bigger the hole the higher the flow. |
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| Posted by: Pinkerton ® 01/29/2011, 12:42:02 Author Profile eMail author Edit |
>>The smaller the hole the less the flow, the bigger the hole the higher the flow. Not so. For a constant flow you must FIRST regulate pressure. Either that or Archimedes, Bernoulli et al, all got it wrong. Why is there air?
Dave |
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| Posted by: Howard ® 01/29/2011, 15:29:42 Author Profile eMail author Edit |
Which many times is what a orifice will do. Fixed displacement pump, set rpm, set constant flow. Little hole low flow beyond little hole, high pressure before little hole. Big hole higher flow beyond big hole lower pressure before big hole. That relationship never changes. Yes, it is inefficient, it heats up the liquid, it however works and in the short run cheep. |
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