Vertical Centrifugal Pumps 1

[nasirqa1407]

I have vertical centrifugal pumps that designed to pump water. The pumps have negative suction head because they fitted above the suction tanks. That’s why we have Automatic Aspirators to prime the pump prior to run. I heard the negative suction head pump must run at the beginning against closed discharge up to reach the working pressure. When I start to run the pump I observe the pump has cavitation for a while up to the system pressurize to 9 bar. But if I open any of the discharge lines the pump has cavitation again & the discharge pressure dropped to 2 bar. The problem I can’t control the discharge flow due to that valves are butterfly valve which definitely not use to control the flow. Unfortunately, we had two failures up to now on these pump. The shaft of the impeller got broken.
Questions:
1) What the right procedure to start up & shut down the pumps?
2) Is vertical centrifugal pump designed to be used in negative suction head cases?
3) Why the pumps start cavitation when one of discharge line opens?
4) Is there any problem in the piping system such as using the butterfly valve?
5) How I can maintain the pressure to 7 or 8 bar while discharge line opens?

Please feedback ASAP.

 

[LittleInch]

Can you post the pump curve and a sketch of the system with pressures and flow rates

From the information given my guess is
1) Looks like a good idea to me. Starting against a closed head reduces motor start current and lets flow stabilise before flow starts
2) Centrifugal pumps generally don't like not having a flooded suction before start, but if designed right then operating with an inlet pressure less than atmospheric can be fine, within limits of NPSHR.
3) Difficult to say based on your info, but maybe because your pump is not matched to your discharge (too high a discharge head) and is operating on the right side of the curve where NPSH rises rapidly and starts cavitating and vibrating badly.
4) Buttery valves are good for isolation, but not throttling control
5) Fit a control valve of some sort (e.g. globe valve ) designed to be used for throttling service

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[nasirqa1407]

Thank you LittleInch for responding.

Find out the attachments please.

 

[racookpe1978]

Are you sure you are actually operating (long term) at that 71% 265 1050 point?

Very short periods (startup) at off-conditions are certainly a different case than long-term conditions.

 

[LittleInch]

nasirqra1407,

Thanks for the info. However we still need to know what is on the discharge side - pipe size, length, height, pressure etc.

I very strongly suspect, like I think racookpe does, that essentially your system curve (plot of flow versus pressure on the system you are feeding (you don't say what it is) is either

a) intersecting the pump curve way to the right (1000+ US gpm) or
b) during startup the discharge line is empty and providing no resistance.

Therefore what you need to do is figure out a way to install a suitable control valve (could be manual) to provide this pressure restriction so that the pump doesn't run off the end of the curve. Hoving pressure readings will help, but some sort of flow meter will really help you control your pump and operate it so it doesn't break...

Basically I think you're using the wrong type of pump. Centrifugals are great pumps, but essentially work at a fixed discharge pressure +/- 10%. If your discharge pressure varies more than this they don't like it. If you have a fairly fixed flow rate, you're better off with a PD pump - piston, screw, gear, something like that which are essentially fixed flow +/- 10%.

Hope this helps youunderstand or explain to others why it isn't working....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Artisi]

LittleInch, I have to disagree with the "blanket" statement that centrifugal pumps should operate +/- 10%. Sure you need to know what you are doing with the system / pump selection but a lot depends on pump type, configuration, duty / application, speed, product, etc - IMHO, for the majority of industrial / process applications +/- 10% is far too tight a variation.
If you are talking very high energy pumps / high specific speed units, sure the range should be a studied engineering decisions regarding correct application.

As with far too many posts, like this one - we will probably be spinning wheels for some time yet before there is any meaningful data available, but helps fill in our day / evenings.

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.)

 

[LittleInch]

Ok, maybe +/- 20%, but not the 9 bar to 2 bar which the OP has quoted...

The thing I see on a lot of these systems is the designer / installer not thinking through how he is going to actually start these systems and just works on the "steady state" model provided by the hydraulics / Flow Assurance (when did this term start??) engineers, who don't think about it either....

There are in a few of these similar posts a lack of appreciation about the limits of a pump - it's as if someone sees it as a process "block" and thinks it can do anything when in reality it can't....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[miyanui]

make a simple sketch of the system, calculate the NPSH available and make sure it is more than the NPSH required.This link helped me understand, take your time and read it carefully,

http://www.engineeringtoolbox.com/npsh-net-positive-suction-head-d_634.html