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:
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, Jim. That was one of the things I looked at. The problem with apply ing the formula for velocity head is that it assumes the flow is constricte d (in a pipe, for example), while the expanding volume of a compressor invo lute presents an entirely different situation.
I'll bet I've read 100 pages over the past three days. No joy yet.
ys. 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 se cond stage.
a good explanation.
Try not to make it complicated. Say you have a centrifical pump rated at
5 gal per minute at 10 psi pressure. Well the pump when used really has a input pressure of about 14.7 psi. That is just atmospheric pressure. Now add a second pump with the same specs. Put the two pumps in parallel and you get 10 gal per minute at 10 psi output pressure. Put them in series a nd you have 5 gal per minute at 20 psi. It really does not depend on them being centrifical pumps. The same applies to gear pumps , turbine pumps, diaphragm pumps, etc.What goes on in the second stage is exactly what goes on in the first stage .
Dan
e:
f identical sections all turning at the same speed. Each stage increases t he pressure. So you might have a 6 stage pump with each stage increasing t he pressure by 10 psi. Which makes for a fairly efficient pump which will supply water at 60 psi. Google it.
tage 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 befor e making its way to the discharge on the outer diameter.
es. For higher flow output, impellers can be connected parallel.
feedwater pump. For example, a 350 MW unit would require two feedpumps in p arallel. Each feedpump is a multistage centrifugal pump producing 150 l/s a t 21 MPa.
rgy driving the impeller. This can be measured at isentropic compression, r esulting in a slight temperature increase (in addition to the pressure incr ease).
days. None of them really explain it. To say that the energy is derived fro m the impeller is axiomatic. It doesn't explain what's going on inside the second stage.
nd a good explanation.
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 wate r.
umps threw me. Since water isn't compressible, I don't see how the multi-st age pumps work. For gas, no problem, but I don't get it for liquids.
e/volume relationship, though, isn't in agreement with Boyle's law. Gases a pproximate it. It's easy to imagine a multi-stage non-positive-displacement compressor that keeps building pressure in a material that obeys Boyle's l aw. It's much harder to imagine it with liquids.
t hold pressure that way.
kinetic aspects of a turbo pump (velocity) and the potential aspects (press ure). A turbine pump that's pumping a liquid must be producing potential en ergy from kinetic energy.
that an ordinary turbine pump can hold the pressure generated by a previou s stage, unless the entire thing is kinetic, which we're then measuring as potential energy (pressure).
So what do you think happens if you have a centifical pump with a pressure gauge on the output and the output blocked off so there is no flow?
Dan
Thanks, Jim. That was one of the things I looked at. The problem with applying the formula for velocity head is that it assumes the flow is constricted (in a pipe, for example), while the expanding volume of a compressor involute presents an entirely different situation.
I'll bet I've read 100 pages over the past three days. No joy yet.
So what do you think happens if you have a centifical pump with a pressure gauge on the output and the output blocked off so there is no flow?
Dan ======================== That depends on the geometry of the impeller. A centrifugal fan may stall, not couple as well to the air and draw -less- power. The discharge curve gives the relationship between flow rate and pressure rise.
-jsw
snipped-for-privacy@gmail.com wrote on 5/29/2017 2:03 PM:
ote:
of identical sections all turning at the same speed. Each stage increas es the pressure. So you might have a 6 stage pump with each stage increa sing the pressure by 10 psi. Which makes for a fairly efficient pump whi ch will supply water at 60 psi. Google it.
istage centrifugal pump. The impellers may be mounted on the same shaft o r on different shafts. At each stage, the fluid is directed to the center before making its way to the discharge on the outer diameter.
ries. For higher flow output, impellers can be connected parallel.
r 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.
nergy driving the impeller. This can be measured at isentropic compressio n, resulting in a slight temperature increase (in addition to the pressur e increase).
w 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 insid e the second stage.
find a good explanation.
don't understand is the effect of feeding one with a high pressure head o f 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.
A diaphragm pump may be a different story. The diaphragm in the second stage or even further down the chain may not be able to handle the pressure and then rupture.
Similarly, a piston pump may reach its pressure limit and either gets stuck or breaks its push-rod.
A centrifugal pump is spinning the fluid in a closed system. As long as the pressure doesn't burst the casing, then it doesn't really know or care the pressure of the fluid it is spinning.
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.
ure/volume relationship, though, isn't in agreement with Boyle's law. Gases approximate it. It's easy to imagine a multi-stage non-positive-displaceme nt compressor that keeps building pressure in a material that obeys Boyle's law. It's much harder to imagine it with liquids.
n't hold pressure that way.
e kinetic aspects of a turbo pump (velocity) and the potential aspects (pre ssure). A turbine pump that's pumping a liquid must be producing potential energy from kinetic energy.
ay that an ordinary turbine pump can hold the pressure generated by a previ ous stage, unless the entire thing is kinetic, which we're then measuring a s potential energy (pressure).
e gauge on the output and the output blocked off so there is no flow?
You have static pressure. That, too, is axiomatic. But this is a dynamic ma chine.
Right. So what is the condition inside of one involute in the second stage? Is it completely full when it's operating?
A centrifugal pump is spinning the fluid 'round and 'round inside an enclosed housing. It will never stall unless you feed solid debris like straws and sticks into it.
A centrifugal pump is generating pressure by spinning the fluid to generate centrifugal force. It is different from a turbo pump.
pumps threw me. Since water isn't compressible, I don't see how the mult i-stage pumps work. For gas, no problem, but I don't get it for liquids.
ure/volume relationship, though, isn't in agreement with Boyle's law. Gas es approximate it. It's easy to imagine a multi-stage non-positive-displa cement compressor that keeps building pressure in a material that obeys B oyle's law. It's much harder to imagine it with liquids.
ldn't hold pressure that way.
the kinetic aspects of a turbo pump (velocity) and the potential aspects (pressure). A turbine pump that's pumping a liquid must be producing pote ntial energy from kinetic energy.
way that an ordinary turbine pump can hold the pressure generated by a p revious stage, unless the entire thing is kinetic, which we're then measu ring as potential energy (pressure).
sure gauge on the output and the output blocked off so there is no flow?
You are talking nonsense. If you feed the output to storage tank, can you tell the difference whether the storage tank is pressurized by a turbo pump, a piston pump, a diaphragm pump, or a centrifugal pump?
=
A centrifugal pump is a type of turbomachine.
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.
ure/volume relationship, though, isn't in agreement with Boyle's law. Gases approximate it. It's easy to imagine a multi-stage non-positive-displaceme nt compressor that keeps building pressure in a material that obeys Boyle's law. It's much harder to imagine it with liquids.
ldn't hold pressure that way.
the kinetic aspects of a turbo pump (velocity) and the potential aspects (p ressure). A turbine pump that's pumping a liquid must be producing potentia l energy from kinetic energy.
way that an ordinary turbine pump can hold the pressure generated by a pre vious stage, unless the entire thing is kinetic, which we're then measuring as potential energy (pressure).
sure gauge on the output and the output blocked off so there is no flow?
My response to Dan's question is exactly correct. Static pressure cannot "f eed the output" to anything. To "feed," it must be dynamic.
On Monday, May 29, 2017 at 3:34:50 PM UTC-4, snipped-for-privacy@gmail.com wrote: sw
It is exactly the same as the first stage.
Dan
snipped-for-privacy@gmail.com wrote on 5/29/2017 5:13 PM:
e:
ge pumps threw me. Since water isn't compressible, I don't see how the mu lti-stage pumps work. For gas, no problem, but I don't get it for liquids .
ssure/volume relationship, though, isn't in agreement with Boyle's law. G ases approximate it. It's easy to imagine a multi-stage non-positive-disp lacement compressor that keeps building pressure in a material that obeys Boyle's law. It's much harder to imagine it with liquids.
ouldn't hold pressure that way.
t the kinetic aspects of a turbo pump (velocity) and the potential aspect s (pressure). A turbine pump that's pumping a liquid must be producing po tential energy from kinetic energy.
no way that an ordinary turbine pump can hold the pressure generated by a previous stage, unless the entire thing is kinetic, which we're then mea suring as potential energy (pressure).
essure gauge on the output and the output blocked off so there is no flow ?
t "feed the output" to anything. To "feed," it must be dynamic.
Think, Ed. If a centrifugal pump is registering 50 psi at the gauge while the output valve is closed, what do you think the gauge at the storage tank will register after it has been pressurized by the centrifugal pump? It will be 50 psi, Ed. How is it different from any other pump, Ed?
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?
Why does that matter? The pump will force out all the air it can and compress any that's trapped. If you want to build up pressure you have to restrict the outlet, according to the discharge curve. The Grundfoss manual showed that two identical stages will add their pressures. Are you being distracted by imagining an improper mismatch?
-jsw
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