Pump Cavitation 3

[rutherford703]

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.

 

[JJPellin]

The case damage is classic 'steel worms' caused by turbulence. We have solved this using swirl bars and HVOF tungsten carbide. Sulzer is very familiar with this and has a well proven solution. The impeller damage looks like simple erosion, cutting vanes from below.

Johnny Pellin

 

[JJPellin]

The primary seal plan (12) does not appear to be working well. Solids are accumulating in the seal chamber and eroding the seal gland. I do not recall plan 12. I would consider plan 13 or add a cyclone to remove solids. I prefer plan 13 in erosive service.

Johnny Pellin

 

[Artisi]

It would still be interesting to establish where the pump is operating on its performance curve. It may well be well back to the left causing a lot of recirculation within the casing, this is shown by the eroded surface on outer shroud of impeller and the "hole" thru' the impeller blade..
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.)

 

[rutherford703]

Thanks everyone for the valuable opinions. As Artisi required, I attached the pump performance curve. The rated point is far away from the BEP. The pump casing eroded as in the photo only lasted for three weeks running.

Thanks again!

 

[Artisi]

An absolute shocker selection for the duty, the curve is so flat at this point the slightest increase in head could push the duty point even closer towards CV. You say that it has a minimum flow line - does it work, is it utilised properly etc etc?

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.)

 

[rutherford703]

Good point Artisi. You deserve a star.

Yes. There is some entrained solid in the naphtha. Even the recirculation control valve has some erosion issue.

Thanks.