Elevating The Tank Versus Leaving Dead Volume In Tank

[BENSMIL]

Hi,

We are designing an atmospheric crude storage tank of 3000 m3 volume, 17 m dia with pumping facility. We have to provide NPSH for the crude export pumps ( centrifugal , 100 m3/hr, 50 bar g discharge pressure ) which pump crude out from the tank.

We have two options to provide the NPSH

a.Elevate the tank bottom by about 4 m height from grade by extra foundation. Leave minimum 1 m liquid in the tank.

b. Keep the tank at grade and leave dead stock of 5 m liquid in the tank. This will increase the tank volume by that much amount.

Which of these options will be economical?

How is the trade off done in such cases between tank elevating and providing extra tank dead volume?

Please advise based on your experience.



Regards,

Benoy

 

[LittleInch]

Have you considered all the options?

What about lowering the pump, using a submersible pump, a bigger line, a can pump etc. Raising the tank 4 m is not normal not is leaving 5m of storage so I would first look at much easier pump options to do your duty.

These are usually much cheaper.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BENSMIL]

Hi LittleInch,

Thank you.

We have considered the options suggested and ruled them out for various reasons as listed below.

Lowering the pump - Creates a pocket where hydrocarbons/H2S can accumulate making it potentially unsafe
Using a submersible/Can pump - These pumps are ruled out due to maintainence issues
A bigger line - We have already made it big enough. The pressure drop in the line is negligible

Now the issue is how to optimise between raising the tank and leaving dead volume storage in the tank.
How do the foundation costs compare to tank cost + dead stock cost? How does one optimise in such a case?

Regards,
Benoy

 

[LittleInch]

I still think the costs associated with any other solution will be far less than your proposed one, but if you really want to do that way then what you need to do is calcualte civil costs of the extra 4 m of foundation vursus the extra steel costs for needing to make your tank bigger than it needs to be.

Why don't you post your NPSH calcualtions and see if we can help you that way. Have you considered a low pressure booster pump in front of the main pump?, using a PD pump, finding a pump with a lower NPSHR etc instead? You seem to be trying to solve the wrong thing to me....

Having a min level for a crude tank that high (5m) is very unusual and in reality will be ignored as no one will understand why a few months after you've built it and dissapeared.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[1gibson]

C. Throttle the pump to reduce NPSHR when draining tank
D. Get a VFD so you can run the existing pump at reduced speed (with corresponding reduced NPSHR.) I've seen a lot of VFD's used for much sillier reasons. Check the economics.

Operating logic comes into play with either of these, so may be too much risk of damaging the pump if it is not followed. If it was a rare enough event and short enough duration, I'd *almost* say let the pump cavitate a bit and just make damn sure it doesn't run dry. But a 17m dia tank, those 4 meters is 908 m^3 = 9 hours @ 100 M^3/hr with insufficient NPSHA, so that is definitely not an option.

In fact, it will take longer to drain if you throttle and/or reduce speed, so that time frame might make all of these suggestions "not an option" which is why you've decided to either raise the tank or leave it partially full.

This is a perfect application for a vertical can pump, but sounds like it is a little late to consider that option. Maybe next time, but double check the economics.

 

[MartinLe]

"Lowering the pump - Creates a pocket where hydrocarbons/H2S can accumulate making it potentially unsafe"

Is forced ventilation of the pump pit an option? I'd say the cost of a blower that runs 24/7 is probably negligible compared to raising the tank.

 

[LittleInch]

BENSMIL, You appear to finding problems (issues over the pit and maintenence), which either don't exist or are being signficantly over estimated to then arrive at a solution that no one else does.

Where is this HC vapour / H2S coming from? If you have an open pit then you don't normally have an issue that isn't present on the surface. What maintenence issue for a can pump? Why are you hurting so much for NPSH - shorten the line maybe?

The issue is not how to decide between two bad solutions to the problem, the issue is that you need to properly grade and cost each potential solution and not dismiss them without proper consideration over cost and difficulty.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MJCronin]

Leaving 5 m of dead stock in the tank seems excessive.

How about the old trick of installing an elbow (pointed downward) inside of the tank, on the pump suction pipe, so that the amount of dead stock is minimized ?

See item #24 on this page:

http://www.pumpfundamentals.com/centrifugal-pump-tips.htm

With crude oil, it is my understanding that standard practice would be to leave some dead stock in the tank... Is that correct ?

MJCronin
Sr. Process Engineer
Venture Engineering & Construction

http://www.VentureEngr.com

 

[IFRs]

Can pump would be my choice, maintenance is not that big a deal.