Pump behaviour 1

[dianad]

Hi,

I have a question that everyone answers me in a different way, and i hope to clarify it with everyone.

What happens if a pump is working and pumping water, and we close a gate valve 1 meter ahead the pump?
My first answer was that that pump would start to block and in the limit burn the motor and stop, because of the impossibility to pump out the fluid.
Next, i talked with my boss and he guaranteed me that i was completly wrong. The pump continues to work undefinetly without no effort.

So what is your opinion about this???

Thanks for the opinions!

 

[JLSeagull]

What type pump?
If any protection exists, -
What internal protection?
What external protection?

 

[dianad]

So, what you are trying to say is that depends on wich type of pump do we have.
I've thinked in that too, but i can't refer to a pump specificly.

Can you explain better with some examples?

Thank you soo much for the reply!

 

[quark]

There will be two dead heads. One will be the pump and the other will be of your boss.

 

[Artisi]

If you have a positive displacement pump without any high pressure protection it will immediately overload and either break or burst something or stall and overload the motor and if it doesn't have any protection it will either blow fuses or burn-out.

If it is a centrifugal pump it will pump at its highest pressure, this results in a number of possibilities;
shaft breakage, internal recirculation, increased temperature of the liquid in the pump - even to boiling point to name a few conditions - however, it will not overload the motor - it will continue to run as it is usually at its lowest power requirement.

Therefore in either pump type it is not advisable to run agains a closed discharge.

running against a closed valve is called operating at closed valve (CV) or dead headed condition.

 

[BigInch]

Ahead of the pump, you mean upstream on the suction side?

Dry seals, dry bearings, loss of lubrication, overheating, internal recirculation, possible heating and vaporization of product, rise in pressure with temperature.. none leading to short term good results.

"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics

http://virtualpipeline.spaces.msn.com

 

[hydromech]

As a water pump designer I would agree with the above comments, as the impeller is spinning it is, by design, still working and trying move the fluid.

In a previous life I designed and installed/serviced hydraulic systems. In those systems, particularly on large forging presses we used fixed displacement pumps that could not 'suck'. These pumps were 'boosted' by a centrifugal pump that would run against a dead head while the main pumps were not running. I know that the centrifugal pumps ran for at least 2 hours per day for at least 20 years with no maintenance. The head generated by these pumps was at least 50 mtrs.

That goes against everything I have learnt since then.

I believe that the ability of a centrifugal pump to run with a dead head is dictated by its design.

I have also seen, on the same forging presses 5000 KG steel tanks blown 50mtrs into the air when fixed displacement pumps are dead headed.

 

[Artisi]

" I know that the centrifugal pumps ran for at least 2 hours per day for at least 20 years with no maintenance. The head generated by these pumps was at least 50 mtrs. "

With centrifugal pumps many things are possible, I can also relate many stories of pumps running against CV for extremely long periods of time with little effect, but to make it a general statement is not in the interest of most pump users or those seeking advice.

 

[dianad]

Hi,

Thank you all very much for the contribution!
(I loved QUARK answer and i couldn't agree most!)

Now i'm more relaxed, because what was said here goes along my thoughts. In centrifugal pumps, the water stays in thr rotor and starts do overheat. That was what my boss told me and he was quite right.
And makes sense that positive displacement pumps break.

Thanks for the confirmation!
thank you all very much!

 

[quark]

Actually, that is a bit of plagiarism, as I picked up some clues from one of BigInch's earlier posts

I will agree with Artisi 100%. That is the correct suggestion in forums.

 

[rmw]

dianad,

You have to always remember what hydromech said; in a centrifugal pump the impeller is always working and even though the motor is at lower power levels at shut off conditions as Artsi has stated, the work (heat) generated by the motor goes into the fluid via the impeller. That can be accounted for with bypass orifices if a pump is known to be expected to encounter such operating conditions or piping configurations that dissipate heat. BUT, the heat is always there and biginch's comment is the most succinct.

quark's comment applies to any boss I have ever had-even me when I was the boss, maybe????

rmw