6 replies to this topic

[murthy.nelakanti]

Dears,

I would like to understand the method of fixing the vessel or column elevation during basic design stage where NPSHr for the pump (which takes suction from this vessel / column bottom) is not available.
This is a general query which applies to process units.

Regards,
Murthy

[fallah]

Dears,

I would like to understand the method of fixing the vessel or column elevation during basic design stage where NPSHr for the pump (which takes suction from this vessel / column bottom) is not available.
This is a general query which applies to process units.

Regards,
Murthy

Using relevant previous experiences (if there is) along with considering engineering practices,we can announce vendor about NPSHA and ask him the pump with NPSHR not higher than NPSHA+Margin.

[sheiko]

The NPSH available (NPSHA) formula indeed includes the total liquid height above the centrifugal pump suction eye. This total height namely includes the MIN. liquid level in the suction vessel and, of course, the suction vessel bottom height above the pump centerline.
By drawing a sketch and writing down the NPSHa formula you should be able to identify and estimate the height you're looking for, knowing that you want a positive value for NPSHA.

You can consult your piping and installation department (see plot plan and other relevant drawings) or/and make some reasonably conservative assumptions.
You will then have to check these assumptions later in the project as the plant design progresses.

Edited by sheiko, 08 January 2011 - 12:02 PM.

[Zauberberg]

In many cases it is the other way round i.e. you look for a pump that can work with given NPSHa. It is not very likely that any tower will be elevated for additional couple of meters just because we need extra head for the pump.

For vessels - particularly reflux drums, they may be (and usually are) located on platforms a few meters above grade, depending on the overall plant layout/configuration.

Probably the best thing - as suggested above - is to look for similar installations and projects, and see which concept has been applied there. Unless you are dealing with quite specific process plant, there shouldn't be many unknowns and we are usually not required to re-invent hot water.

Some tips can be found here, page 39 onwards: http://igs.nigc.ir/ips/pr/e-pr-190.pdf

[Padmakar Katre]

Dear,
During basic design or in conceptual stage while performing the pump calculations, please have the pump models' data handy. This will help you to fixation of the vessel elevation. During detail engineering the elevation data given for vessel is well verified with the vendors and changes are made accordingly. In the previous designs I did, I have never taken the credit of the maximum liquid level inside vessel, rather Low Liquid Level.

[sheiko]

In the previous designs I did, I have never taken the credit of the maximum liquid level inside vessel, rather Low Liquid Level.

Absolutely. I have corrected my previous post.

Edited by sheiko, 08 January 2011 - 11:59 AM.

[deltaChe]

In the previous designs I did, I have never taken the credit of the maximum liquid level inside vessel, rather Low Liquid Level.

Absolutely.

NPSHr may be calucated as cavitation coefficient of thoma
multply pump differenal head.