9 replies to this topic

[J_Leo]

Dear Fellow Engineers,

 

I have an existing 180gpm pump with 1750rpm. The suction drum is about 12ft above the pump centerline and supported with a structure. There is no safety margin for the current operating conditions. The pump head will be ok with increased impeller size. What to do to provide enough NPSHA for the increased pump capacity? I need approximately 4 ft more on NPSHA.

 

What I can think of,

(1) lift the drum 4 ft more

(2) Dig a hole and put the pumps inside

(3) Replace the pump

 

Generally speaking, which one might be more practical and economical?

 

Thank you for any advice.

Regards,

Leo

Edited by J_Leo, 03 April 2017 - 10:09 PM.

[Zauberberg]

Calculate actual pressure drop in suction piping and see if increasing line size would provide you with sufficient additional head.

[Zauberberg]

I have assumed that liquid phase in the feed drum is in the equilibrium with vapor phase (i.e. liquid at its bubble point, at given temperature). If you have subcooled liquid in the drum, increasing drum pressure OR cooling the liquid that flows to the pump suction will also help in increasing NPSHA.

 

Cooling will also consume additional pressure drop, so you need to be careful.

[Bobby Strain]

You should discuss the situation with your pump vendor. And, if you search you may find a correction to npshr for hydrocarbons. But your pump vendor can guide you. Maybe even provide a custom impeller.

 

Bobby

[sgkim]

Hi Leo,

 

Is the liquid boiling? or under vacuum ?  If not, then the suction thank has already about 33 ft of additional atmospheric head.  Check the both NPSH's required and available in detail.  

 

Why not maintain the liquid level more than 4 ft in the tank ? You can probably solve the problem by simply providing a suitable low level alarm and interlock. 

 

~Stefano   

[J_Leo]

Dear Zauberberg, Bobby and Stefano,

 

Thank you very much for your advice.

 

Sorry I didn't provide enough information at the first place. The suction drum is a KO drum after Tower Feed Preheater. The pump is transferring liquid feed to a tower. The liquid is at boiling point. The distance between LLL and HLL is only about 4ft. The pressure drop through the suction line was pretty small. With the suction line one size bigger, the NPSHA increased only 0.5ft.  Therefore, the proposals from Zauberberg and sgkim cannot solve the problem. 

 

As Bobby suggested, I tried to contact Flowserve today and nobody picked up the phone. I will continue to try tomorrow. I hope the vendor will have a good solution to this problem.

 

Regards,

Leo

Edited by J_Leo, 03 April 2017 - 10:07 PM.

[breizh]

Hi ,

You may check with manufacturer the possibility to add inducer .

 

Hope this helps

 

Breizh

[J_Leo]

Breizh,

 

Thank you for your advice. I called the vendor today and was told probably there is nothing they can do with this pump to lower the NPSHR. However, he agreed to review the pump data. 

 

Anyway, I am preparing a process datasheet for a new pump.

 

Regards,

Leo

[vaibhav]

LEO,

 

the pump is having  VFD ? DO you have performance curves for the pump ? in what percentage capacity of the pump is increased ?

 

normally difference between NPSHA & NPSHR shall be 1 meter for safe  / trouble free operation of the pump.  In creasing the level of the liquid in suction drum will help to increase NPSHA. or introducing nitrogen at source point @ 0.5 bar g can help to increase NPSHA artificially.

 

give me feedback on these points.

 

thanks

VP

[sgkim]

Leo,

 

If it can solve the problem, attaching an "INDUCER" in collaboration with the manufacturer, as recommended by Breizh seems to be the most simple solution.  

 

Even a brief process system configuration is not introduced, so the location with function of the KO drum calls in my question.  The pump and the drum should have installed not "after" but "in front of" the Tower Feed Preheater, which is my understanding.  

 

What will be the destination of the vapor separated from the KO drum ?  In case, the vapor is to be sent to the column, just insert an ORIFICE to the vapor line, which will make some pressure build-up in the KO drum thus can provide additional NPSH.   

 

Please comment on the above, thanks in advance.

 

~Stefano