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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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#81
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Centrifugal pump question
On Monday, May 29, 2017 at 7:20:01 PM UTC-4, wrote:
If there is no flow, there will be no pressure differential. Pressure will be the same throughout the volume of liquid from inlet to outlet. -- Ed Huntress If you have a column of water, the pressure at the bottom of the column in higher than the pressure at the top. So there is an example of no flow with a pressure differential. If you take something like say a large nut and tie a string to it and whirl it around your head, then you have a force on the string and a velocity of the nut. But the nut does not go flying off , unless you let go of the string. In the same way with the outlet blocked off on a centrifugal pump. the water has a velocity , but it just goes around and around. Produces a pressure, but no flow from the pump. Think of a centrifuge with test tubes in it. The material in the test tubes are subjected to force, but there is no flow. Dan |
#82
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Centrifugal pump question
On Monday, May 29, 2017 at 7:23:55 PM UTC-4, wrote:
On Monday, May 29, 2017 at 5:37:34 PM UTC-4, wrote: On Monday, May 29, 2017 at 5:13:41 PM UTC-4, wrote: On Monday, May 29, 2017 at 3:34:50 PM UTC-4, wrote: sw Right. So what is the condition inside of one involute in the second stage? Is it completely full when it's operating? -- Ed Huntress It is exactly the same as the first stage. Dan And what is that condition? Are the involutes completely filled? And, if so, how is that possible unless the velocity is the same from the input port to the periphery of the wheel? -- Ed Huntress First some centrifugal pumps do not have involutes. Most do, but anything that will accelerate the flow to the periphery will do. Second are you thinking there could be air in the pump? THa woud be bad as you would have cavitation. Not necessarily. The air could come from partial filling, which almost certainly is the case at start-up. Once the pump is running, apparently the involute spaces fill. The pump would be completely filled. How could it not be completely filled? Easy. Low input pressure; no back pressure; vanes sling the small amount of water to the periphery, with no resistance. And why would the velocity have to be the same from input port to the periphery? The velocity doesn't have to be the same. But if it's higher at the periphery, the involute space doesn't fill. If it's higher at the inlet, you have a possible energy-conservation dilemma, unless the volume in each space from center to periphery increases as to the square of distance from the center. I haven't done the math on that but it's easy to tell. This is the outward radial velocity we're talking about here. The tangential velocity increases by definition. The flow would be the same, but the passages vary in cross section, so there is no way the velocity could be the same. Again, it's a matter of which "velocity" you're talking about -- radial or tangential. Dan |
#83
Posted to rec.crafts.metalworking
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Centrifugal pump question
On Monday, May 29, 2017 at 7:39:40 PM UTC-4, wrote:
On Monday, May 29, 2017 at 7:20:01 PM UTC-4, wrote: If there is no flow, there will be no pressure differential. Pressure will be the same throughout the volume of liquid from inlet to outlet. -- Ed Huntress If you have a column of water, the pressure at the bottom of the column in higher than the pressure at the top. So there is an example of no flow with a pressure differential. Oh, right. I was thinking of externally applied pressure. Gravity or centrifugal force would result in a differential. Any pressure applied from the inlet, as in the case of a multi-stage centrifugal pump, would be the same throughout the volume within one involute. But the centrifugal force added by the spinning rotor would be greatest at the periphery. If you take something like say a large nut and tie a string to it and whirl it around your head, then you have a force on the string and a velocity of the nut. But the nut does not go flying off , unless you let go of the string. In the same way with the outlet blocked off on a centrifugal pump. the water has a velocity , but it just goes around and around. Produces a pressure, but no flow from the pump. Again, the velocity is the tangential velocity. There is no radial velocity. Think of a centrifuge with test tubes in it. The material in the test tubes are subjected to force, but there is no flow. Dan |
#84
Posted to rec.crafts.metalworking
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Centrifugal pump question
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#87
Posted to rec.crafts.metalworking
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Centrifugal pump question
On Monday, May 29, 2017 at 8:51:06 PM UTC-4, tyre biter wrote:
On 5/29/2017 5:32 PM, wrote: On Monday, May 29, 2017 at 7:27:02 PM UTC-4, tyre biter wrote: On 5/29/2017 5:12 PM, wrote: On Monday, May 29, 2017 at 6:21:04 PM UTC-4, tyre biter wrote: On 5/29/2017 3:54 PM, wrote: On Monday, May 29, 2017 at 5:31:58 PM UTC-4, tyre biter wrote: On 5/29/2017 1:32 PM, wrote: Your brain must be the size of a pea, Biter. Your pallid attempts at fighting back are hilarity, crazy eddy. Ha-ha! What is there to fight? Then why do you? If I was fighting, weasel, you'd know it. You're on your own, weasel. Don't drown in your own drool. Beg me Crazy Eddy...do it...NOW! What a trashy, low-class troll you are. LOLOLOL!!!! ....a trashy troll with a Tourette syndrome...sad... -- Ed Huntress |
#88
Posted to rec.crafts.metalworking
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Centrifugal pump question
On Mon, 29 May 2017 14:44:27 -0400, lqDXsb?? ?????? ? ??????? ??ZsAaIo
wrote: wrote on 5/29/2017 2:03 PM: On Monday, May 29, 2017 at 1:55:58 PM UTC-4, XCjEwC?? ?????? ? ??????? ??CkYyoU wrote: wrote on 5/29/2017 1:04 PM: On Monday, May 29, 2017 at 8:13:36 AM UTC-4, wrote: On Sunday, May 28, 2017 at 4:04:30 PM UTC-4, wrote: On Sunday, May 28, 2017 at 11:35:39 AM UTC-4, wrote: You are over thinking the situation. A multistage pump has a bunch of identical sections all turning at the same speed. Each stage increases the pressure. So you might have a 6 stage pump with each stage increasing the pressure by 10 psi. Which makes for a fairly efficient pump which will supply water at 60 psi. Google it. From Wik Multistage centrifugal pumps Multistage centrifugal pump[5] A centrifugal pump containing two or more impellers is called a multistage centrifugal pump. The impellers may be mounted on the same shaft or on different shafts. At each stage, the fluid is directed to the center before making its way to the discharge on the outer diameter. For higher pressures at the outlet, impellers can be connected in series. For higher flow output, impellers can be connected parallel. A common application of the multistage centrifugal pump is the boiler feedwater pump. For example, a 350 MW unit would require two feedpumps in parallel. Each feedpump is a multistage centrifugal pump producing 150 l/s at 21 MPa. All energy transferred to the fluid is derived from the mechanical energy driving the impeller. This can be measured at isentropic compression, resulting in a slight temperature increase (in addition to the pressure increase). Dan Thanks, Dan. I read that -- and maybe 100 more pages over the past few days. None of them really explain it. To say that the energy is derived from the impeller is axiomatic. It doesn't explain what's going on inside the second stage. Rather than try to go through it in detail, I'll post something if I find a good explanation. You don't have enough smarts to process too much information at once, ED. Please do yourself a favour by looking at this impeller of a simple centrifugal pump: https://d2t1xqejof9utc.cloudfront.net/screenshots/pics/bd60670ae9dc5e37fdc08142df0462d2/large.JPG To give your brain a chance to comprehend, please imagine all the valves are closed. The fluid is trapped inside the centrifuge, with no way in, and no way out. The impeller keeps spinning 'round and 'round. The impeller is trying to throw the fluid (which is slotted in the empty space in the impeller) radially outward. Because of this, the fluid is exerting pressure radially outward against the wall of the casing. Do you still have difficulty, Ed? Thanks, Wannabe, but I understand how a centrifugal pump works. What I don't understand is the effect of feeding one with a high pressure head of water. It is the same. The pump doesn't know (and doesn't care) what's being fed to it. It will spin and throw the fluid radially outward, resulting in a higher pressure than it was fed. Imagine the output valve is closed, in order not to complicate things. This is true..until the input pressure is higher than the possible output pressure..and now the pump acts as nothing more than a restrictor mechanism...and the flow becomes restricted --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus |
#89
Posted to rec.crafts.metalworking
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Centrifugal pump question
On Tuesday, May 30, 2017 at 1:00:01 AM UTC-4, Gunner Asch wrote:
This is true..until the input pressure is higher than the possible output pressure..and now the pump acts as nothing more than a restrictor mechanism...and the flow becomes restricted Sorry, but that never happens. The output pressure will always be higher than the input pressure as long as the pump is running and nothing has failed because of the high pressure. Dan |
#90
Posted to rec.crafts.metalworking
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Centrifugal pump question
On Tuesday, May 30, 2017 at 8:34:06 AM UTC-4, wrote:
On Tuesday, May 30, 2017 at 1:00:01 AM UTC-4, Gunner Asch wrote: This is true..until the input pressure is higher than the possible output pressure..and now the pump acts as nothing more than a restrictor mechanism...and the flow becomes restricted Sorry, but that never happens. The output pressure will always be higher than the input pressure as long as the pump is running and nothing has failed because of the high pressure. Dan And as long as the pump is fully filled. g I finally found the info I was looking for, and it's kind of interesting. Basically, a regular centrifugal pump has to be fully filled to work. Some have priming pumps and other priming gadgets built-in, so they're essentially self-priming, but they only produce more than a small amount of pressure when they're filled. Until they's filled, they're likely to cavitate, so it's only something you can do for seconds at start-up. From one source: "There are selfpriming centrifugal designs that can lift liquid an average of 15 feet when partially filled (13 hg vacuum)." BTW, the internal flow in a centrifugal pump is so complex that they can't model performance with computers. They have to build an actual model and test it. That satisfies my curiosity. The thing that threw me off is that illustrations of such pumps working often suggest something *less* than complete filling. I suspect that's because it's easier to illustrate the liquid in the pump that way, but it's misleading. -- Ed Huntress |
#91
Posted to rec.crafts.metalworking
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Centrifugal pump question
On Tuesday, May 30, 2017 at 10:27:07 AM UTC-4, wrote:
On Tuesday, May 30, 2017 at 8:34:06 AM UTC-4, wrote: On Tuesday, May 30, 2017 at 1:00:01 AM UTC-4, Gunner Asch wrote: This is true..until the input pressure is higher than the possible output pressure..and now the pump acts as nothing more than a restrictor mechanism...and the flow becomes restricted Sorry, but that never happens. The output pressure will always be higher than the input pressure as long as the pump is running and nothing has failed because of the high pressure. Dan And as long as the pump is fully filled. g I finally found the info I was looking for, and it's kind of interesting. Basically, a regular centrifugal pump has to be fully filled to work. Some have priming pumps and other priming gadgets built-in, so they're essentially self-priming, but they only produce more than a small amount of pressure when they're filled. Until they's filled, they're likely to cavitate, so it's only something you can do for seconds at start-up. From one source: "There are selfpriming centrifugal designs that can lift liquid an average of 15 feet when partially filled (13 hg vacuum).." BTW, the internal flow in a centrifugal pump is so complex that they can't model performance with computers. They have to build an actual model and test it. That satisfies my curiosity. The thing that threw me off is that illustrations of such pumps working often suggest something *less* than complete filling. I suspect that's because it's easier to illustrate the liquid in the pump that way, but it's misleading. -- Ed Huntress While we're at it, regarding Eric's original question about whether his 10-psi pump with 80 psi input will produce a 90 psi output, the answer is "yes," disregarding friction losses. -- Ed Huntress |
#92
Posted to rec.crafts.metalworking
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Centrifugal pump question
On 5/29/2017 10:39 PM, wrote:
On Monday, May 29, 2017 at 8:51:06 PM UTC-4, tyre biter wrote: On 5/29/2017 5:32 PM, wrote: On Monday, May 29, 2017 at 7:27:02 PM UTC-4, tyre biter wrote: On 5/29/2017 5:12 PM, wrote: On Monday, May 29, 2017 at 6:21:04 PM UTC-4, tyre biter wrote: On 5/29/2017 3:54 PM, wrote: On Monday, May 29, 2017 at 5:31:58 PM UTC-4, tyre biter wrote: On 5/29/2017 1:32 PM, wrote: Your brain must be the size of a pea, Biter. Your pallid attempts at fighting back are hilarity, crazy eddy. Ha-ha! What is there to fight? Then why do you? If I was fighting, weasel, you'd know it. You're on your own, weasel. Don't drown in your own drool. Beg me Crazy Eddy...do it...NOW! What a trashy, low-class troll you are. LOLOLOL!!!! ...a trashy troll with a Tourette syndrome...sad... Hey Crazy Eddy...guess what...you can't quit me, brokeback...LOLOL! |
#93
Posted to rec.crafts.metalworking
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Centrifugal pump question
On Tuesday, May 30, 2017 at 2:20:43 PM UTC-4, tyre biter wrote:
On 5/29/2017 10:39 PM, wrote: On Monday, May 29, 2017 at 8:51:06 PM UTC-4, tyre biter wrote: On 5/29/2017 5:32 PM, wrote: On Monday, May 29, 2017 at 7:27:02 PM UTC-4, tyre biter wrote: On 5/29/2017 5:12 PM, wrote: On Monday, May 29, 2017 at 6:21:04 PM UTC-4, tyre biter wrote: On 5/29/2017 3:54 PM, wrote: On Monday, May 29, 2017 at 5:31:58 PM UTC-4, tyre biter wrote: On 5/29/2017 1:32 PM, wrote: Your brain must be the size of a pea, Biter. Your pallid attempts at fighting back are hilarity, crazy eddy. Ha-ha! What is there to fight? Then why do you? If I was fighting, weasel, you'd know it. You're on your own, weasel. Don't drown in your own drool. Beg me Crazy Eddy...do it...NOW! What a trashy, low-class troll you are. LOLOLOL!!!! ...a trashy troll with a Tourette syndrome...sad... Hey Crazy Eddy...guess what...you can't quit me, brokeback...LOLOL! Why would I want to? It's entertaining to see what a cowardly weasel like you can say with a vocabulary of fewer than 1,000 words. My wife teaches 2nd grade. They have larger vocabularies and they're much more creative than you, but it's encouraging to watch you try. Tourette can be serious. I hope you're getting treatment for it. -- Ed Huntress |
#94
Posted to rec.crafts.metalworking
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Centrifugal pump question
On 5/30/2017 2:15 PM, wrote:
On Tuesday, May 30, 2017 at 2:20:43 PM UTC-4, tyre biter wrote: On 5/29/2017 10:39 PM, wrote: On Monday, May 29, 2017 at 8:51:06 PM UTC-4, tyre biter wrote: On 5/29/2017 5:32 PM, wrote: On Monday, May 29, 2017 at 7:27:02 PM UTC-4, tyre biter wrote: On 5/29/2017 5:12 PM, wrote: On Monday, May 29, 2017 at 6:21:04 PM UTC-4, tyre biter wrote: On 5/29/2017 3:54 PM, wrote: On Monday, May 29, 2017 at 5:31:58 PM UTC-4, tyre biter wrote: On 5/29/2017 1:32 PM, wrote: Your brain must be the size of a pea, Biter. Your pallid attempts at fighting back are hilarity, crazy eddy. Ha-ha! What is there to fight? Then why do you? If I was fighting, weasel, you'd know it. You're on your own, weasel. Don't drown in your own drool. Beg me Crazy Eddy...do it...NOW! What a trashy, low-class troll you are. LOLOLOL!!!! ...a trashy troll with a Tourette syndrome...sad... Hey Crazy Eddy...guess what...you can't quit me, brokeback...LOLOL! Why would I want to? True, you groove on being humiliated. It's entertaining to see what a cowardly weasel like you can say with a vocabulary of fewer than 1,000 words. It's amusing to see least coast lackwit try and keep up. My wife teaches 2nd grade. No doubt serving her well in dealing with you... They have larger vocabularies and they're much more creative than you, but it's encouraging to watch you try. And the worst thing is you torture a lame analogy like it was the neighbor's cat. Tourette can be serious. I hope you're getting treatment for it. Project much Crazy Eddy? Laugh, laugh, laugh, laugh! |
#95
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Centrifugal pump question
On Tuesday, May 30, 2017 at 4:49:14 PM UTC-4, tyre biter wrote:
On 5/30/2017 2:15 PM, wrote: Laugh, laugh, laugh, laugh! As I said, get treatment for that Tourette syndrome. You could wind up completely debilitated and wind up like tyre biter. -- Ed Huntress |
#96
Posted to rec.crafts.metalworking
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Centrifugal pump question
On 5/30/2017 2:56 PM, wrote:
On Tuesday, May 30, 2017 at 4:49:14 PM UTC-4, tyre biter wrote: On 5/30/2017 2:15 PM, wrote: Laugh, laugh, laugh, laugh! As I said, get treatment for that Tourette syndrome. You could wind up completely debilitated and wind up like tyre biter. Don't the neighbors have another cat for you to torture? This tautology was sponsored by Fresh Step Litter - because your argumentative turds really stink. LOL! |
#97
Posted to rec.crafts.metalworking
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Centrifugal pump question
On Tuesday, May 30, 2017 at 5:11:05 PM UTC-4, tyre biter wrote:
On 5/30/2017 2:56 PM, wrote: On Tuesday, May 30, 2017 at 4:49:14 PM UTC-4, tyre biter wrote: On 5/30/2017 2:15 PM, wrote: Laugh, laugh, laugh, laugh! As I said, get treatment for that Tourette syndrome. You could wind up completely debilitated and wind up like tyre biter. Don't the neighbors have another cat for you to torture? I only bother with weasels, like you. That, and termites. -- Ed Huntress |
#98
Posted to rec.crafts.metalworking
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Centrifugal pump question
On 5/30/2017 3:21 PM, wrote:
On Tuesday, May 30, 2017 at 5:11:05 PM UTC-4, tyre biter wrote: On 5/30/2017 2:56 PM, wrote: On Tuesday, May 30, 2017 at 4:49:14 PM UTC-4, tyre biter wrote: On 5/30/2017 2:15 PM, wrote: Laugh, laugh, laugh, laugh! As I said, get treatment for that Tourette syndrome. You could wind up completely debilitated and wind up like tyre biter. Don't the neighbors have another cat for you to torture? I only bother with weasels, like you. That, and termites. -- Ed Huntress Poor Crazy Eddy, snip and run and all for a lame analogy - again! You can't quit me, brokeback! |
#99
Posted to rec.crafts.metalworking
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Centrifugal pump question
On Tue, 30 May 2017 05:34:01 -0700 (PDT), "
wrote: On Tuesday, May 30, 2017 at 1:00:01 AM UTC-4, Gunner Asch wrote: This is true..until the input pressure is higher than the possible output pressure..and now the pump acts as nothing more than a restrictor mechanism...and the flow becomes restricted Sorry, but that never happens. The output pressure will always be higher than the input pressure as long as the pump is running and nothing has failed because of the high pressure. Dan Thats hardly true. Stick a fire hose into a 1" pump and let her rip...what you get out..wont be what you put in. Been there, done that. Old oil field hand..remember? --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus |
#100
Posted to rec.crafts.metalworking
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Centrifugal pump question
On Tue, 30 May 2017 15:46:34 -0700, Gunner Asch
wrote: On Tue, 30 May 2017 05:34:01 -0700 (PDT), " wrote: On Tuesday, May 30, 2017 at 1:00:01 AM UTC-4, Gunner Asch wrote: This is true..until the input pressure is higher than the possible output pressure..and now the pump acts as nothing more than a restrictor mechanism...and the flow becomes restricted Sorry, but that never happens. The output pressure will always be higher than the input pressure as long as the pump is running and nothing has failed because of the high pressure. Dan Thats hardly true. Stick a fire hose into a 1" pump and let her rip...what you get out..wont be what you put in. Been there, done that. Old oil field hand..remember? Actually what we remember is a poor, old, broken down, liar. -- Cheers, Schweik |
#101
Posted to rec.crafts.metalworking
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Centrifugal pump question
On Wednesday, May 31, 2017 at 11:12:16 AM UTC-4, Neighborhood number 3 wrote:
Thats hardly true. Stick a fire hose into a 1" pump and let her rip...what you get out..wont be what you put in. Been there, done that. Old oil field hand..remember? In 16 years you still haven't figured out how to make water drain into the ground. Yet now you think there must be someone who'll take advice from you about fluid dynamics, based on some of your wildly exaggerated "experience." LOL Well have to confess I was only thinking of a pump being used to pump. Incidently you can put water into a centrifugal pump outlet and it will spin the pump. Dan |
#102
Posted to rec.crafts.metalworking
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Centrifugal pump question
On Sunday, May 28, 2017 at 8:35:39 AM UTC-7, wrote:
On Saturday, May 27, 2017 at 6:57:12 PM UTC-4, Ned Simmons wrote: On Sat, 27 May 2017 12:20:00 -0700 (PDT), wrote: On Saturday, May 27, 2017 at 2:26:40 PM UTC-4, Steve W. wrote: wrote: If a centrifugal pump with a maximum pressure of, say, 10 psi is supplied with water at 80 psi will the water pressure coming out of the pump be 90 psi? I think the pressure will be 90 psi. Am I wrong? That's close. A pump on atmospheric air (1 bar) that makes a 10 bar output, will be different, because the density of inlet air determines the force on the motor to spin it up. Feeding in 10bar air, you have higher density, and the motor torque and work done will rise (because an AC motor operates at constant speed). Water, being nearly incompressible, doesn't create any such scaling issues with inlet pressure. That's exactly what I thought, but Jim's reference to multi-stage pumps threw me. Since water isn't compressible, I don't see how the multi-stage pumps work. For gas, no problem, but I don't get it for liquids. For liquids, the multistage pumps I've used have pistons, and the low-pressure pistons stall out when backpressure is high, then the same motor moves smaller high-pressure pistons to achieve higher fluid pressures with lower volume displacement (but nearly equal motor power). I suspect there's a differential gear driving all the pistons with different mechanical advantage, and a reverse-inhibiting clutch on the low pressure pistons. This is where I have trouble. Assuming these are regular centrifugal turbines, the outlet of the first stage is fed into the axis of the second stage. The pressure from the first-stage outlet is retained at the second-stage inlet, but from there it feeds into the whirling blades of the second stage, the outlet volume of which is LARGER than the inlet volume between any two blades. Why? Several stages of centrifugal pump in series for a liquid would have same volume flow, and same work per shaft would deliver the same head, at each stage. So, they'd be the same size rotors, too. Maybe different shaft seals, though. |
#103
Posted to rec.crafts.metalworking
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Centrifugal pump question
On Wednesday, May 31, 2017 at 7:35:13 PM UTC-4, whit3rd wrote:
On Sunday, May 28, 2017 at 8:35:39 AM UTC-7, wrote: On Saturday, May 27, 2017 at 6:57:12 PM UTC-4, Ned Simmons wrote: On Sat, 27 May 2017 12:20:00 -0700 (PDT), wrote: On Saturday, May 27, 2017 at 2:26:40 PM UTC-4, Steve W. wrote: wrote: If a centrifugal pump with a maximum pressure of, say, 10 psi is supplied with water at 80 psi will the water pressure coming out of the pump be 90 psi? I think the pressure will be 90 psi. Am I wrong? That's close. A pump on atmospheric air (1 bar) that makes a 10 bar output, will be different, because the density of inlet air determines the force on the motor to spin it up. Feeding in 10bar air, you have higher density, and the motor torque and work done will rise (because an AC motor operates at constant speed). Water, being nearly incompressible, doesn't create any such scaling issues with inlet pressure. That's exactly what I thought, but Jim's reference to multi-stage pumps threw me. Since water isn't compressible, I don't see how the multi-stage pumps work. For gas, no problem, but I don't get it for liquids. For liquids, the multistage pumps I've used have pistons, and the low-pressure pistons stall out when backpressure is high, then the same motor moves smaller high-pressure pistons to achieve higher fluid pressures with lower volume displacement (but nearly equal motor power). I suspect there's a differential gear driving all the pistons with different mechanical advantage, and a reverse-inhibiting clutch on the low pressure pistons. This is where I have trouble. Assuming these are regular centrifugal turbines, the outlet of the first stage is fed into the axis of the second stage. The pressure from the first-stage outlet is retained at the second-stage inlet, but from there it feeds into the whirling blades of the second stage, the outlet volume of which is LARGER than the inlet volume between any two blades. Why? Several stages of centrifugal pump in series for a liquid would have same volume flow, and same work per shaft would deliver the same head, at each stage. So, they'd be the same size rotors, too. Maybe different shaft seals, though. The issue I was trying to resolve was the question of whether the pump(s) in series were, each, completely filled. Illustrations often show centrifugal pumps as being less than completely filled. But I realized that they have to be completely filled, especially for a multi-stage pump. If those spaces aren't completely filled, the outlet pressure from one stage, which would be the inlet pressure of the next stage, wouldn't communicate to the output of that following stage. It would be like pushing on the end of a rope. But I finally found some good explanations. -- Ed Huntress |
#104
Posted to rec.crafts.metalworking
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Centrifugal pump question
wrote on 5/31/2017 8:08 PM:
On Wednesday, May 31, 2017 at 7:35:13 PM UTC-4, whit3rd wrote: On Sunday, May 28, 2017 at 8:35:39 AM UTC-7, wrote: On Saturday, May 27, 2017 at 6:57:12 PM UTC-4, Ned Simmons wrote: On Sat, 27 May 2017 12:20:00 -0700 (PDT), wrote: On Saturday, May 27, 2017 at 2:26:40 PM UTC-4, Steve W. wrote: wrote: If a centrifugal pump with a maximum pressure of, say, 10 psi is supplied with water at 80 psi will the water pressure coming out of the pump be 90 psi? I think the pressure will be 90 psi. Am I wrong? That's close. A pump on atmospheric air (1 bar) that makes a 10 bar output, will be different, because the density of inlet air determines the force on the motor to spin it up. Feeding in 10bar air, you have higher density, and the motor torque and work done will rise (because an AC motor operates at constant speed). Water, being nearly incompressible, doesn't create any such scaling issues with inlet pressure. That's exactly what I thought, but Jim's reference to multi-stage pumps threw me. Since water isn't compressible, I don't see how the multi-stage pumps work. For gas, no problem, but I don't get it for liquids. For liquids, the multistage pumps I've used have pistons, and the low-pressure pistons stall out when backpressure is high, then the same motor moves smaller high-pressure pistons to achieve higher fluid pressures with lower volume displacement (but nearly equal motor power). I suspect there's a differential gear driving all the pistons with different mechanical advantage, and a reverse-inhibiting clutch on the low pressure pistons. This is where I have trouble. Assuming these are regular centrifugal turbines, the outlet of the first stage is fed into the axis of the second stage. The pressure from the first-stage outlet is retained at the second-stage inlet, but from there it feeds into the whirling blades of the second stage, the outlet volume of which is LARGER than the inlet volume between any two blades. Why? Several stages of centrifugal pump in series for a liquid would have same volume flow, and same work per shaft would deliver the same head, at each stage. So, they'd be the same size rotors, too. Maybe different shaft seals, though. The issue I was trying to resolve was the question of whether the pump(s) in series were, each, completely filled. Illustrations often show centrifugal pumps as being less than completely filled. But I realized that they have to be completely filled, especially for a multi-stage pump. If those spaces aren't completely filled, the outlet pressure from one stage, which would be the inlet pressure of the next stage, wouldn't communicate to the output of that following stage. It would be like pushing on the end of a rope. But I finally found some good explanations. All my posts in this thread should be a good read. The impeller of a centrifugal pump forms compartments inside the tight fitting casing to trap incoming fluid so that when the impeller spins, it is forcing the fluid in all the compartments to go in a circular motion. The centrifugal force from the rotating fluid generates the pressure radially outward against the outer wall of the pump. A very basic impeller: https://upload.wikimedia.org/wikipedia/commons/thumb/6/6b/Impellerrad.jpg/1280px-Impellerrad.jpg |
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