Determine minumm tank level to prevent pump cavitation 1

[Kannn]

Hello there

I want to implement pump interlock on when the tank level reach a certain limit the pump trips.
Note : tank is not pressurized
Suction pipe length :5 meter
Suction pipe diameter 3inch
2 90 degree elbows
Suction pipe material vc
1 Suction manual valve : butterfly
Media : water

How do I know the minumm level that the pump can safely operates without cavitation

Pump manufacture :grundfos
Pump type : Crn20-4afgj-a-e-hqqe
Attached pump curve with npshr

 

[mk3223]

To prevent the pump cavitation issue, you can calculate the system available NPSHa, to compare it to the pump NPSHr, and to make sure that NPSHa is larger than NPSHr.

 

[Kannn]

How to know the figure of npshr
I have the graph, which point on the graph is npshr to build my calculations on it

 

[mk3223]

You need a hydraulic diagram from tank liquid level to the pump inlet, and then calculate NPSHa per link below and compare it with the NPSHr from the pump vendor.

https://www.pumpsandsystems.com/centrifugal-pumps/npsh-calculation-step-step-guide

On attached pump diagram, the NPSH is on the right hand side of the lower diagram. The NPSHr is about 6.5m to cover the maximum flow up to 30 m3/h.

 

[MJCronin]

Kahn

Here is a little secret .... Many centrifugal pumps, constructed with the right impellor can tolerate a tiny bit of cavitation.

Some maintenance departments allow the pump to cavitate a tiny bit when they fully empty a tank .... They then use AOD pumps to get the heel

Of course this must be a supervised activity ... we are talking about cavitation for a minute or less.

Stainless Steel impellers can tolerate this a bit better than CS or CA

... and I am not talking about multi stage pumps !!! NO BFW Pumps ... Just simple liquid transfer

Please specify more details about your pump design and impellor material.

I agree with the comments above regarding NPSHa and NPSHr ..... But you can cheat a little

You should be able to read pump curves to determine NPSHr, Kahn

MJCronin
Sr. Process Engineer

 

[LittleInch]

Your key missing data is level.

What is the elevation difference between pump centre line and liquid level?

Is pump higher, at the same level or lower than the tank nozzle?

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

 

[pierreick]

Hi,
You need to start with the basic , the definition of NPSH Available , then add some margin to make sure the NPSH A is always higher than the NPSH required ( Pump manufacturer) .
Read the handbook attached for a better understanding ,should be easy .
Rule of thumb : NPSHA -NPSHR > 0.5m always

A sketch will be helpful to better answer your query.
Good luck
Pierre

 

[Artisi]

You are missing the key point, cavitation is not the issue, the issue is air entrainment into the inlet at low tank level.
One of the most common mis-
understandings relating to pump inlet conditions, air entrainment is NOT cavitation - for a tank open to atmosphere the minimum NPSHa is always 1 atmosphere less a few inlet friction losses in you case.
Set your low level at maybe 0.5 metre above inlet level and see if this is sufficient, if pump performance drops off increase the height a little and retry.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[pierreick]

Hi,
To add to Artisi's comments , consider the paper attached .
Good luck
Pierre

 

[Artisi]

Pierreick, the paper was a good reference, but a bit of an overkill for this installation

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Kannn]

Dear all
Thanks all for you comments
I used the formula of
NPSHav greater than NPSHreq
And I was safe to run the pump with low level around 0.5 meter above pump Suction
So I put the interlock on 0.6 meter to prevent cavitation in case of low tank level

Thank u all

 

[bimr]

You also should be concerned with a possible vortex in the suction pipe. It is possible to calculate the depth of the vortex.

Link 1

Link 2

 

[pierreick]

Hi Artisi ,
Agree with you, others can take advantage ,lol .
Pierre