Cavitation in suction lift centrifugal pumps

[flamearrow]

Hi,

To avoid cavitation in centrifugal pumps it has been stated that the static+dynamic head at suction to be greater than the vapor pressure,
my question is, if the static reading is negative and the suction velocity head is too small then the total head is negative which is less than the vapor, will it cause cavitation?. Apparently most of the suction lift pumps are running without fail.

 

[MJCronin]

For centrifugal pumps do not try to practically suction lift more than 15 feet (Water duty). If the liquid is hot... forget it !!!

Contact the pump vendor for his recommendations and specific pump limitations.

IMHO foot valves on the intake barely work at all.... some people think that they are more problem than they are worth...

Attention MBAs: Pump priming systems or self priming pumps are frequently necessary ..... Yes I know that they were not considered in the project scope

http://petrowiki.org/index.php?title=Pumps&printable=yes

MJCronin
Sr. Process Engineer

 

[LittleInch]

Errrr "it has been stated that the static+dynamic head at suction to be greater than the vapor pressure," - by whom exactly?? This is not, IMHO, correct. It misses out the pressure pushing the fluid in the first place which is atmospheric pressure and the friction losses.

You need to convert everything to head which is in absolute.

This FAQ

http://www.eng-tips.com/faqs.cfm?fid=1598

lists the key components, but in your case the calculation is (in m head of your particular fluid).

Atmospheric pressure in m head + head difference ( in your case a negative no) - friction losses in m head - vapour pressure in m head must be > head required to avoid cavitation.

Note that the head required to avoid cavitation is often higher than the NPSH figure quoted by pump suppliers by 1m or more.

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

 

[flamearrow]

Hi Gents,

Thank you for your replies,

LittleInch, you are right, when we calculate the available head, but what does the pressure gauge at suction read?
it is the static pressure resulted from all losses (velocity head to be added to get the total pressure)

so if the pressure just before the suction mouth (static head and velocity head) is greater than the vapor pressure then there will not be
chance of boiling? Am I wrong?

 

[LittleInch]

If your liquid level is lower than the pump your inlet pressure will be lower than atmospheric pressure.

If that pressure is above vapour pressure then the liquid won't boil in the pipe but still might isnide the pump.

The NPSHR figure gives you a good start point to avoid cavitation but add 1 to 2m to add some margin.

It's much better to work in absolute pressure when you look at pump inlet issues.

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