Does a change in flow lead to an instantaneous flow downstream?

[CTXG]

Hi,

I have a pump with VFD, followed by pipelines downstream to point A. For example, the pipeline to point A is around 100m. If I increase the flow by changing the pump setpoint, does downstream pipeline register an increase instantaneously? Or is there a time lag?

Thank you in advance.

 

[itsmoked]

There will certainly be a delay. 1) Always the speed of sound in the fluid at the pressure it's currently at. 2) The VFD will accelerate the motor up to the new speed over some finite period of time. 3) There is inertia involved in the mass of moving fluid. It needs time to respond.

All that applies.

A 100m long line would would probably show the beginning of a rise in pressure in about a second with a step increase in the VFD speed command.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Latexman]

For most practical purposes, it can be assumed instantaneous. Few applications require one to look at the dynamics. What is your specific concern?

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[hydtools]

Responses will be different, positive displacement pump vs. rotodynamic pump.

Ted

 

[bimr]

Depends on the definition of instantaneous, but there may be a few seconds delay. There will be a short delay as the VFD ramps up. VFD's have a preset ramping speed. Depending on the pump, there may he an effect due to the inertial of the pump and motor, where pumped discharges follow the rotational speed behavior.

 

[Compositepro]

Yes, but the delay that bimr is referring to is due to the time it takes to get all of the fluid in the pipe to accelerate up to the desired flow. There is no difference in flow rate from one end of the pipe to the other, for all practical purposes. That would require a change in density of the fluid. So with air as the fluid that is possible.

 

[TenPenny]

Doesn't it depend on the compressibility of the liquid to some extent?

 

[zdas04]

The bulk modulus of water is 319,000 psi, you would have to REALLY be increasing pressure for compressibility to be a factor.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[CTXG]

Hi all, and thank you to all.

Firstly, it is a centrifugal pump with VFD and medium is water. Secondly, I recognise that the VFD takes time to ramp up. However, I would like to ignore this factor. My query is more of due to the length of pipe/inertia/flow, will there be a lag time?

Someone mentioned that once the pump ramps up, it immediately pushes the water before the pump and hence even at 100m downstream, it will instantaneously see an increase in flow.

Another mentioned that when the pump ramps up, the increased flow will require time to travel 100m downstream to see a change in flow. Hence there is a time lag.

 

[LittleInch]

Like latexman and itsmoked say above, for a 100m long pipe ( 100m isn't really a pipeline) then for all practical purposes the fluid (100% water) given the length can be thought of as incompressible and hence any change to flow at one end will reflect in a change at the other end within 1 second or less.

However if instead of 100m you had 100km then those small changes in compressibility, the speed of sound and the mass of the fluid start to become appreciable. If you increased the speed of a VFD at one end of 100km then the pressure rise there would be much faster than at the other end. You would see a small jump based on the speed of sound ( around 2-3000m/sec) then a much slower increase in flow at the far end.

If you increased flow by say 100% for the 100km long line I would estimate that to get to steady state flow again would take about 5 minutes.

So 100m, ignore any time impact, 100km factor it in.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

In an ideal world, there is no delay in hydraulic systems (liquids cannot be compressed).

However, you mention "pump".

In your question, you would also have to exclude pump inertia. Pump inertia is the resistance of the pump to acceleration or deceleration. Pump inertia is constant for a particular pump and motor combination. The higher the inertia of a pump, the longer it takes for the pump to stop spinning following its shutoff and vice versa. Larger pumps have more inertia because they have more rotating mass.

You would also have to exclude the pump pressure. As the flow capacity of a centrifugal pump increases, the available pump head capacity is decreasing. The combination of lower available discharge head and higher pressure loss due to flow will lower the pumped flow rate.

 

[LittleInch]

Just to be a bit pedantic here, liquids can be compressed and pipe can expand under pressure, it is just that these effects are very small and for short systems can be ignored for most practical purposes, but you can't say that " liquids cannot be compressed". They can and are, but compared to gases the difference is many orders of magnitude.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

Believe that liquids are incompressible in an "ideal" world, which of course does not exist in our four space-time dimensions.

From the comments, I have surmised that the OP is interested more in the "ideal world" than in the "real world"

 

[IRstuff]

Speed of sound in water at 20C is about 1480 m/s, so a pressure wave should traverse 100 m in under 70 ms.

TTFN (ta ta for now)
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