7 replies to this topic

[Leonardo285]

Good morning!

 

I'm a Chemical Engineer from Venezuela from the USB, I have one year of experience on an engineering consultant company as process engineer.

 

I would like to request your help about an issue that I have and I'm not totally sure about the argument that the people that works with me is giving me.

 

We have to design a pump that will handle Atmospheric Residue stored in a tank and will be transferred to the Vacuum Distillation Feed Drum.

 

The main issue is that the pump suction nozzle is higher than the tank outlet nozzle, the heights over sea level (osl) are 74.90 ft and 71.42 ft respectively. Also I've noted that the tank's LLLL (4' 11") is at 73.75 ft osl.

 

Considering this, I think that the tank won't be able to operate at it this LLLL (4' 11") and it will have a lower working capacity, because the liquid will stop flowing when the tank liquid level reaches the same height as the pump nozzle.

 

The people that works with me says that the operation (lowering the liquid level under the height of the pump inlet nozzle) is possible because "the pump is able to suck the liquid if the operation is continue".

 

Also, I have another issue with this pump suction line. This line requires and expansion loop of 6 elbows (due to the reduced space in the area) and is possible that the TOP of the expansion loop will be higher than the tank outlet nozzle. Is a similar issue than the previous.

 

Thanks a lot for taking your time for answering and sharing your knowledge.

 

 

Edited by Leonardo285, 30 June 2014 - 11:33 AM.

[Bobby Strain]

Best that the pump suction is the low point. And the suction pipe slopes toward the pump suction. You shouldn't accept poor design practice in a new installation.

 

Bobby

[fallah]

Leonardo,

 

Lack of a simple sketch...

 

Anyway, appears not to be proper configuration for pump's suction line; nevertheless below LLLL pump's operation could be considered as a matter of lifting, then as long as the NPSHA at LLLL would be adequate having no cavitation and there would be no problem for priming at start-up; seems the pump has no problem to do the job... 

[Leonardo285]

Thank you for your comments.

 

Mr. Strain, the problem is that the tank is existing and the selected location for the pump station (which is new) is higher. I'm agree that the pump should be the lower point and I'm trying to correct this issue.

 

Mr. Fallah, according to the simulation results, the vapor pressure for this fluid is negligible at the operating temperature, so I don't expect to have cavitation inside the pump. I understand your point. I will consider adding a sketch for this post and for future posts.

[xavio]

Leonardo285,

 

Please inform you pump spec, it should be self-priming type.

Not sure why your system was designed like that, but a self-priming pump will do the job.

 

As Mr. Fallah said, draw a skecth and perform NPSHA calculation.

As long as NPSHA is sufficient, the pump should be ok.

 

Good luck.

 

xavio

[oscarsender]

I used to have the same scenario several years ago. Both tank and pumps are both to be reused for new service. Problem is that the pump location is higher than that of the tank LLLL. But the senior engineers working with me says its fine. But those are Piping Engineers who doesn't know the process concepts. Eventually we decided to relocate the pump near the tank where there will be no problem in the NPSH and suction nozzle is below the LLLL of tank.

[Ajay S. Satpute]

Leonardo,

 

Please also consider the case when this pump has emptied the tank (to best of its abilities) and finally loses load. At this point suction line shall be filled with air from the tank. And when you have the tank level high again and need to use this pump, pump may not operate efficiently due to air trapped within. Getting rid of the trapped air could be time consuming activity. Therefore, it is best to keep the suction line flooded all the time.

 

Regards.

 

Ajay S. Satpute

Edited by Ajay S. Satpute, 30 June 2014 - 11:07 PM.

[breizh]

Hi ,

I agree with Ajay  and Bobby!

Operability is key , suction should be flooded.

 

Breizh

Edited by breizh, 01 July 2014 - 03:25 AM.