Centrifugal pump behavior when the outlet is closed

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What would happen if the water pump output or outlet is closed and pump continues running. Will the rotor or impeller inside just rotate and rotate slowly just raising temperature of stuck water in the impeller or would there be sudden increase in pressure immediately damaging the seal. This is assuming no pressure switch and water pump turn on manually (just for sake of discussion).
 
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  • #2
Baluncore
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When the outlet is turned off there will be three things happen.

1. A small step increase in outlet pressure, as the flow stops.

2. A slight increase in RPM of the induction motor as pump load and slip is reduced.
The specified 3450 RPM will rise part way towards synchronous 3600 RPM.

3. The beginning of a gradual increase in temperature of water trapped in the pump.
 
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  • #3
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When the outlet is turned off there will be three things happen.

1. A small step increase in outlet pressure, as the flow stops.

2. A slight increase in RPM of the induction motor as pump load and slip is reduced.
The specified 3450 RPM will rise part way towards synchronous 3600 RPM.

3. The beginning of a gradual increase in temperature of water trapped in the pump.
The seals of the pump wont suddenly go off? I wonder how the impeller moves against stuck water inside. Why wont impeller suddenly break? It can freely move in the stuck water? Any illustration you saw?
 
  • #4
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I have seen an industrial centrifugal pump, whose discharge was inadvertently closed off, boil the liquid in the casing. The seal was fine, probably because it had externally supplied seal flush.
 
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  • #5
Baluncore
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I wonder how the impeller moves against stuck water inside. Why wont impeller suddenly break? It can freely move in the stuck water?
The water is accelerated as it passes through the pump. The work done on the fluid being pumped is proportional to the volume moved, multiplied by the pressure difference.

When the flow stops, load on the motor and power consumption is reduced.

The maximum pressure available from a centrifugal pump occurs at minimum flow, and is proportional to the RPM multiplied by the radius of the impeller.
 
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  • #6
russ_watters
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The seals of the pump wont suddenly go off?
Water hammer can be an issue regardless of where a valve is in a system when it is closed. It is a function of the flow velocity and how fast the valve is closed. It can damage basically anything in the piping system. So the answer to your question is "maybe".
I wonder how the impeller moves against stuck water inside. Why wont impeller suddenly break? It can freely move in the stuck water? Any illustration you saw?
If you google "pump impeller", you'll get tons of photo and videos of what they look like. They aren't positive displacement; they throw water out via centrifugal force. If you shut the valve, the water just spins with the impeller, in place.
 
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  • #7
Baluncore
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There is a complexity here, but it should not occur when the outlet is closed. It can occur when pumping against a high pressure or head.

The pressure at the centre of the impeller is low on both sides while the pump is running. When the pump stops the head may pressurise the entire pump chamber. That will put full pressure on the shaft seal. A non-return valve on the pump outlet will prevent the problem.

Sometimes the non-return valve is put on the inlet because it makes priming easier. That can lead to problems. I have seen the chamber wall on the motor-side, with the shaft seal, break apart, but only when the pump was turned off, with a head present, and missing a functional non-return valve.
 
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  • #8
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Thanks for all the tips.

I plan to remove the pressure tank since water pressure is sufficient for normal use.
But ill retain the 1 horsepower water pump in the source with manual switch and not controlled by pressure switch since there would be no pressure tank.

I just want to know how many psi is a 1 horsepower water pump? For example when bathing and turning it on without any pressure tank. How many psi of water can it produce in the output, how do you complete. The plumber doesnt know how to compute.
 
  • #9
russ_watters
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I just want to know how many psi is a 1 horsepower water pump? For example when bathing and turning it on without any pressure tank. How many psi of water can it produce in the output, how do you complete. The plumber doesnt know how to compute.
Every pump is different: You will have to get a pump curve - it should be available online.
 
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  • #10
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Every pump is different: You will have to get a pump curve - it should be available online.
I read about it, actually trying over 30 mins. Such as

https://www.irrigationtutorials.com/selecting-a-pump-using-pump-curves/

But my pump has no pump curve data. It has this label on the unit though

20200824_070947.jpg


There is a Vmin . H 45 -22 m

Its 22 to 45 meters? Numbers seem to be reversed in Italy.

I just want to know what equivalent pressure you would get while bathing with the pump directly on and no water tank.

Hmm. If the pressure is 100 psi. Wont the shower jets hurt your skin?
 
  • #11
russ_watters
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I'm not familiar with that notation, but it looks more like liters per minute and head in meters. 22 meters is 30 psi and that is probably the dead head pressure..
 
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  • #12
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I'm not familiar with that notation, but it looks more like liters per minute and head in meters. 22 meters is 30 psi and that is probably the dead head pressure..
Ah. So if one puts a faucet directly to the pump. Its like the pressure or psi is from a 30 psi pressure tank? Or is the dead head pressure (when outlet is close) higher than open pressure. So if a faucet is directly connected to that pump. Pressure is like 20 psi. Well i wont actually do it. Just asking to grasp how it works.

What psi before water jet can hurt the flesh. 1000 psi?
 
  • #13
russ_watters
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Ah. So if one puts a faucet directly to the pump. Its like the pressure or psi is from a 30 psi pressure tank? Or is the dead head pressure (when outlet is close) higher than open pressure. So if a faucet is directly connected to that pump. Pressure is like 20 psi. Well i wont actually do it. Just asking to grasp how it works.
Dead head is maximum pressure. Maximum flow happens at zero pressure (with no piping attached. Finding the actual pressure and flow of a system requires the pump and system curves.
What psi before water jet can hurt the flesh. 1000 psi?
I'm not sure. The problem is that the static pressure of a jet in air is zero, so it doesn't necessarily relate to the pressure that generated the flow.
 
  • #14
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Dead head is maximum pressure. Maximum flow happens at zero pressure (with no piping attached. Finding the actual pressure and flow of a system requires the pump and system curves.
I actually tried to read many articles but still puzzled with your statement. With no piping attached, there is no water flow. So in what context you meant by maximum flow at zero pressure. Zero pressure of water or the impeller? But how can there be maximum flow with no piping or water? Kindly rewords, thanks :)

I'm not sure. The problem is that the static pressure of a jet in air is zero, so it doesn't necessarily relate to the pressure that generated the flow.
 
  • #15
russ_watters
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I actually tried to read many articles but still puzzled with your statement. With no piping attached, there is no water flow.
With no piping attached to the discharge, the water just pours out of the pump discharge, onto the floor.
 
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  • #16
Baluncore
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What psi before water jet can hurt the flesh. 1000 psi?
When I cut you, do you not bleed? There is only about 1 psi pressure across your skin, where blood meets air. That helps prevent infection by making sure bad things from the environment do not flow in.

When you are impacted by the ram pressure of a kinetic jet, (density*velocity squared), some of the fluid will enter the skin. If the jet velocity is too high or the jet is very fine it can cut and penetrate the skin.

You avoid contact with a compressed air jet because it can cause air embolism, resulting in a stroke. It is also why you do not “feel” for hydraulic fluid leaks with your hands, because it can result in traumatic amputation.

Pin holes in hydraulic systems can inject oil under the skin, which blocks blood flow and can lead to gangrene infection.
 
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  • #17
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I appreciate for all the assistances, learnt a lot. Thanks very much. Last question for this plumbing thing before I remove my thinking of it after this week. What happens if the inlet of the water pump is closed and the pump is manuallyt turned on? What would happen to impeller, same scenerio?
 
  • #18
Baluncore
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What happens if the inlet of the water pump is closed and the pump is manuallyt turned on?
Cavitation corrosion of the impeller is probable since the pump will pull a partial vacuum.
 
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  • #19
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Cavitation corrosion of the impeller is probable since the pump will pull a partial vacuum.
I plan to put sediments filter before the pump.

20200905_153512.jpg


So the water flow before pump would be slower. What would occur to the pump or impeller when the source is not instant source of water? Can it cause cavitation corrosion too?
 
  • #20
Baluncore
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Can it cause cavitation corrosion too?
There will be no problems for a short while when priming.
When you change or clean the filter you will get air in the system. That will move through the pipes until it can escape. After changing the filter you might vent the air from a valve before the pump. The air will be pushed out by mains water pressure.
 
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  • #21
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20200907_101241(0).jpg


I notice a ppr 3/4" gate valve has narrower inside compared to a 3/4" bronze gate valve which has wider inside. I guess the pressure inside the bronze gate is stronger? I wonder why would they make the ppr gate valve smaller inside.
 
  • #23
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Yeah. So white is a glove valve.

20200907_121404.jpg

Bronze is a gate valve.

20200907_121917.jpg



So it is categorical glove valve has higher pressure loss? How many percent loss compared to gate valve? Why would people use glove valve given same 3/4" pipe size?

"Unlike the gate valve, globe valve can be used for regulating flow or pressures as well as complete shutoff of flow. It may also be used sometime as a pressure relief valve or as a check valve. Compared with a gate valve or ball valve, the globe valve has considerably higher pressure loss in the fully open position."
 
  • #24
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There will be no problems for a short while when priming.
When you change or clean the filter you will get air in the system. That will move through the pipes until it can escape. After changing the filter you might vent the air from a valve before the pump. The air will be pushed out by mains water pressure.
By the way. If you put say 3 stage sediment filters and you dont use any water pump. How many percentage would it decrease the water pressure? From 18 psi.. maybe becoming 15 psi or maybe as low as 10 psi? The sellers dont know. Some claim what would come out is still 18psi even without adding any water pump.
 
  • #25
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... So it is categorical glove valve has higher pressure loss?

How many percent loss compared to gate valve?

Why would people use glove valve given same 3/4" pipe size?...
Yes.

Much higher percent.
Please, see:
https://www.engineeringtoolbox.com/minor-loss-coefficients-pipes-d_626.html

Gate valve can only isolate both sides of a pipe.
It must be either full open or full closed, otherwise gets damaged.
Globe valve can modulate or regulate flow, but makes it sharply turn several times and go through an orifice smaller than 3/4” inside its body.
When a gate valve is open, the flow barely notices it is there, while a globe valve is a significant restriction to the flow, even when full opened.
 
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