Pump Cavitation
Pump Cavitation
(OP)
Please see attached drawing for detailed information.
The naphtha pump is pumping naphtha from the decanter. The decanter colects liquid from the steam stripper overhead condensers. The pump is experiencing serious cavitation problem. The tank bottom is 10 feet higher than the pump eye. The suction piping is 4" standard and the length is about 70 feet including all fittings.
The pump data sheet shows that the normal required capacity is 87 gpm and the rated capacity is 100 gpm. The pump data sheet also requires the minimum continuous flow is 80 gpm.
There is a minimum recirculation line with a manual open/close valve (by DCS). As the outflow is controlled by the tank level, the level control valve can be closed below the minimum flow while the manual valve is closed - historical data shows that sometimes the flow below the minimum continuous flow requirement.
My questions are:
(1) is the cavitation caused be the flow below the minimum flow? or
(2) The cavitation is caused by not enough NPSH available?
For the NPSH calculation:
Since Naphtha is a mixture with a broad boiling range, I think that the vapor pressure at the pump eye can be cancelled with the pressure of the decanter tank. Therefore, NPSH available is the elevation difference substracting the friction loss (about 0.6 ft liquid) or NPSH available is 9.4 ft liquid column.
For the NPSH required the datasheet shows the required NPSH is 6 ft (water). While the specific density for naphtha is 0.74, the required NPSH for pumping naphtha will be 6/0.74 =8.1 feet naphtha.
The margin for NPSH required is small. Is this the reason causing the pump cavitation?
Thanks in advance.
The naphtha pump is pumping naphtha from the decanter. The decanter colects liquid from the steam stripper overhead condensers. The pump is experiencing serious cavitation problem. The tank bottom is 10 feet higher than the pump eye. The suction piping is 4" standard and the length is about 70 feet including all fittings.
The pump data sheet shows that the normal required capacity is 87 gpm and the rated capacity is 100 gpm. The pump data sheet also requires the minimum continuous flow is 80 gpm.
There is a minimum recirculation line with a manual open/close valve (by DCS). As the outflow is controlled by the tank level, the level control valve can be closed below the minimum flow while the manual valve is closed - historical data shows that sometimes the flow below the minimum continuous flow requirement.
My questions are:
(1) is the cavitation caused be the flow below the minimum flow? or
(2) The cavitation is caused by not enough NPSH available?
For the NPSH calculation:
Since Naphtha is a mixture with a broad boiling range, I think that the vapor pressure at the pump eye can be cancelled with the pressure of the decanter tank. Therefore, NPSH available is the elevation difference substracting the friction loss (about 0.6 ft liquid) or NPSH available is 9.4 ft liquid column.
For the NPSH required the datasheet shows the required NPSH is 6 ft (water). While the specific density for naphtha is 0.74, the required NPSH for pumping naphtha will be 6/0.74 =8.1 feet naphtha.
The margin for NPSH required is small. Is this the reason causing the pump cavitation?
Thanks in advance.





RE: Pump Cavitation
http://www.pump-zone.com/topics/pumps/centrifugal-...
You need to check the temp rise through the pump at your low flow conditions to see if you are increasing temp (and vapor pressure) too much, that is most likely your problem.
http://www.engineeringtoolbox.com/pumps-temperatur...
RE: Pump Cavitation
Johnny Pellin
RE: Pump Cavitation
The NPSHA for your pump is 6 ft of liquid, you don't need to correct for the SG of the naphtha. You can correct NPSHA for HC per 1Gibson's link but it's an empirical correction method based on recognition that cavitation with HC is less severe than with cold water.
You should have a pretty good margin, what is the pump head? Why do you think you have cavitation? Do you have anything like a suction strainer that could be partially plugged and taking more pressure drop than you think? Is there a spot on the suction that you can take a pressure reading and see how much head you have based on the pressure in the drum (use the same pressure gauge)?
RE: Pump Cavitation
Independent events are seldomly independent.
RE: Pump Cavitation
For the pump configuration, there are two pumps for this service with one in operation and one standby. For the suction piping, there is no strainer and only one block valve.
The most serious damage is the casing at the suction side. The damage is caused by wear and cavitation (there is entrained solid in the naphtha but should expect to be minor) within a few weeks. The impeller is a closed impeller. There is some damage on the impeller surface at the suction side and the impeller. Some seal has beed eaten away. The frequency for the pump failure is about 3 every three months. Even the control valve at the recirculation line experiences cavitation (?) problem.
The proposed solution is to try to keep the flow in the recirculation line or to install a VFD for the pump to reduce the pump speed.
Thanks all of you again!
RE: Pump Cavitation
RE: Pump Cavitation
Johnny Pellin
RE: Pump Cavitation
“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King
http://waterhammer.hopout.com.au/
RE: Pump Cavitation
Looks to me like excessive flow across the wear ring area, causing erosion on the impeller shroud. Cavitation would be on the inside of the impeller eye, where pressure is reduced as velocity increases. Which, from the one picture, actually looks fine.
Only one photo was attached.
RE: Pump Cavitation
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.)
RE: Pump Cavitation
It seems to me that the extensive erosion on the casing is a little bit weired because in that area the liquid should not have the highest velocity because the impeller is a closed plate. I also don't understand why the pump at the rated capacity the effeciency is only 35% (datasheet and pump curve were attached on my second post). Hope someone can shed some light on this.
If the erosion is the main issue, opening the recirculation valve to maintain the pump flow may not be the solution to mitigate the pump damage problem. Potential solution may be to install VFD to reduce the pump speed.
Thanks.
RE: Pump Cavitation
Call someone in to inspect that knows what they are looking at, and then explain your proposed repair plan to the supplier you are using for replacement parts. Hopefully it is the pump OEM.
The only potential soluiton here is to restore and verify the wear ring clearances to API minumum values + no more than .0010" (being generous.)
RE: Pump Cavitation
Even if I unwilling to admit you are right, the erosion mode predicts what you said. As a process engineer, I cannot believe that the apparant error cannot be found by our pump specialist and causing the pump failure again and again.
It is the very likely the root cause. I give you a star again.
Thanks,
RE: Pump Cavitation
Thanks.
RE: Pump Cavitation
Johnny Pellin
RE: Pump Cavitation
Where is the pump running on its performance curve?
Additional clear photo's of the damaged areas might also help.
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.)
RE: Pump Cavitation
Sorry
RE: Pump Cavitation
RE: Pump Cavitation
RE: Pump Cavitation
Johnny Pellin
RE: Pump Cavitation
Johnny Pellin
RE: Pump Cavitation
The effect of recir. other than from a hydraulic perspective, especially if there are solids in the flow is accelerated wear due to solids being pumped continuously around internally within the volute casing. In simple terms, if a solid enters into the inlet ideally you want it to pass thru' the pump only once and not over and over due to recirculation.
Pump-out vanes on impellers, anti-swirl devices all work but the greatest effect comes from operating the pump at or near to BEP.
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.)
RE: Pump Cavitation
Thanks again!
RE: Pump Cavitation
Now for the $64 question, you also get a set of stainless steel steak knives for the correct answer: Has the pump ever been flow tested in the installation, and is the minimum flow line operational and where does it return to?
I know nothing about pumping naphtha - but are there any solids in it?
I would still opt for recirculation within the casing.
Suggest you cut your losses and buy a new pump with hydraulics suited to the application - should be a lot cheaper in the long run.
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.)
RE: Pump Cavitation
Yes. There is some entrained solid in the naphtha. Even the recirculation control valve has some erosion issue.
Thanks.