12 replies to this topic

[benoyjohn]

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

 

[Zauberberg]

Hi Benoy,

 

I know how these things are done in Crude Tank farms, and volumes of those tanks are usually in the range of couple of tens of thousands cubic meters - thus making the dead volume even bigger. That would be a 3rd option for you.

 

The concept is to employ booster pumps (with up to 10 bara discharge pressure) and transfer the feed stock to the main transfer pumps (high head pumps) downstream. I believe this would be less costly than elevating the tank, but I might be wrong.

[chemmu]

Hi,

 

1. You have to thoroughly evaluate the BOM required for elevating the tank to meet the NPSH requirement. (But have to thing much as the tank has to be elevated to 4 m) .

2. Based on the fluid cost, dead volume can be decided. As if fluid is costly, we can not keep....

3. Please compare the costing between 1 and 2,and then decide.

4.To avoid the NPSH problem, the suction line size can be increased and line length can be decreased and too much fittings can be avoided

5. Is it possible to keep the pump in the down from the base grade?.

6. we can combine the point no 4 and 5 to meet the NPSH requirement.

 

some ideas on this. this may be useful..

 

Reagrds,

Chem.M

Edited by chemmu, 12 November 2013 - 07:04 AM.

[curious_cat]

Is keeping the pump in a below grade sump an option? The cost of a 4m deep pump pit might be cheaper than elevating the entire tank. 

 

Just a thought. 

[Sharma Varun]

Option which Zauberberg has described, is what I have also witnessed.

And as explained by others suction pipe length shall be kept low & you may keep the booster pumps below grade

[ankur2061]

Benoy,

 

My 2 cents on this problem:

 

Elevation of the tank is a one-time fixed cost. Providing booster pumps is a fixed cost plus operation cost (power / maintenance) over the design life of the facility. Additional pumps (Booster pumps) also mean more instrumentation, both as process as well as safeguarding. This directly implies more monitoring and adds human cost.

 

Prima facie to me it appears that the option of providing booster pumps is costlier. Since this is a new design as per my understanding from your post elevating the tank is a realistic option.

 

There is one other problem with large dead stocks of crude in tanks. If the crude is untreated and contains high levels of BS&W, there will be very rapid built-up of a sediment layer in the tank leading to operational problems. If it is treated crude then you can ignore this part of my reply.

 

Hope this helps.

 

Regards,

Ankur

[Zauberberg]

That's a good point. If Crude contains BS&W, it is not very clever to locate the outlet nozzle very close to the the tank bottom.

[fallah]

Benoy,

 

IMO, elevating such huge tank cannot be a reasonable solution. To wrap up the NPSH issue, in addition to the solutions were submitted, an inducer can be applied at the pump suction or a vertical can type/low NPSHR pump might be used... 

[ankur2061]

Benoy,

 

IMO, elevating such huge tank cannot be a reasonable solution. To wrap up the NPSH issue, in addition to the solutions were submitted, an inducer can be applied at the pump suction or a vertical can type/low NPSHR pump might be used... 

Fallah,

 

A flow inducer cannot be a solution for very low differences in NPSHa & NPSHr. In the oil & gas industry, the norm is to use pumps conforming to API 610 standards, at least in the part of the world where Benoy's tank is located.

 

Another question is how will you configure a Vertical Can pump for an above ground storage tank for crude oil with the crude oil draw-off nozzle located on the bottom of the tank shell?

 

There is also the question of cost of the vertical can pump. Is it cheaper than a conventional horizontal pump conforming to API 610? As per the Gould's brochure vertical can pumps are state-of-the-art design. In such a case, they should be very expensive compared to a standard horizontal pump designed as per API 610.

 

Oil & Gas applications are simple and time-tested & I can't see much scope for high-tech innovation and experimentation.

 

Regards,

Ankur.

[ankur2061]

Is keeping the pump in a below grade sump an option? The cost of a 4m deep pump pit might be cheaper than elevating the entire tank. 

 

Just a thought. 

The option of keeping pumps in a pit or sump below grade is full of safety hazards as well as cost implications.

 

1. All electrical equipment inside the pit or sump will need to be classified as Zone 1 compatible as per IP15. In other words it is a lot of money spent trying to install explosion proof electrical equipment in a pit or sump

 

2. Even if the crude oil has low levels of H2S (heavier than air), the chances of its accumulation in a pit or sump much higher than in an above ground open installation. Add the cost of installing H2S detectors in the pit or sump where the pumps are located and the extra precautions the operator has to take while accessing the pit or sump.

 

3. Any leakage or spillage is a major operational problem if the pumps are located in a below-grade  pit or sump in terms of scavenging the leaked and accumulated crude oil in a confined pit or sump.

 

This idea of providing crude oil pumps in a below grade pit or sump would not pass muster in any Design Review or HAZOP Review both from the operational and safety view point even if somebody could show a cost saving.

[benoyjohn]

Dear all,

 

Thanks for the insights on the issue.

 

Few clarifications:

 

The fluid is treated crude with 0.5% B&W.

Vertical can type pump is not a preffered option for us due to maintainence issues and lack of support from the rotating function.

 

The only conclusion I could make from the discussion is that we need to optimise between options a and b based on cost.

 

Regards,

Benoy

[fallah]

 

Elevation of the tank is a one-time fixed cost. Providing booster pumps is a fixed cost plus operation cost (power / maintenance) over the design life of the facility.

 

Ankur,

 

Elevating the tank, in addition to higher fixed cost, can lead to increasing the operation cost due to more head/power requirement to load the tank ...

[fallah]

  

A flow inducer cannot be a solution for very low differences in NPSHa & NPSHr. In the oil & gas industry, the norm is to use pumps conforming to API 610 standards, at least in the part of the world where Benoy's tank is located.

 

Another question is how will you configure a Vertical Can pump for an above ground storage tank for crude oil with the crude oil draw-off nozzle located on the bottom of the tank shell?

 

 Ankur,

 

Yes, of course. I did merely suggest using inducer or can type pump as general solutions. Obviously, when it's decided to be considered as an option should be carefully investigated in technical/operational/maintenance standpoints...

 

The main point is: In such low NPSHA situations one should investigate, as far as would be able, to select/apply a solution to avoid elevating such huge storage tank. If there wouldn't be any possible solution, then elevate...