14 replies to this topic

[Arsal]

What is the maximum allowable value of following for Valve tray (Vendor : Glitsch)

 

1)Dry Tray Pressure Drop 

2)Total Tray Pressure Drop 

3)Downcomer Exit Velocity

 

Thanks in advance.

[Bobby Strain]

You should find the information at the Koch-Glitsch website. And their free tray/packing rating program.

 

Bobby

[Arsal]

Maximum allowable limits are not available at Koch-glitch website.

[Bobby Strain]

You should be able to find an old Glitsch tray design manual with Google search. This will probably help to answer your query.

 

Bobby

[breizh]

Hi,

As suggested above .

 

Hope this is helping you

 

Breizh

[PingPong]

What is the maximum allowable value of following for Valve tray (Vendor : Glitsch)

 

1)Dry Tray Pressure Drop 

2)Total Tray Pressure Drop 

3)Downcomer Exit Velocity

 

Thanks in advance.

 

I don't know what criteria are in KG or Sulzer software nowadays. In the days that we did trays designs we used following:

1) Dry Tray Pressure Drop: maximum one sixth of trayspacing. For example: if tray spacing is 600 mm (24 inch) then max allowable dry tray pressure drop is 100 mm (4 inch) hot tray liquid.

2) Total Tray Pressure Drop: that depends on what you decide to allow in your design, but in any case do not exceed 200 mm (8 inch) hot tray liquid.

3) Downcomer Exit Velocity: do not exceed 0.45 m/s (1.5 ft/s)

[Arsal]

Thanks for all.
Your comments are very helpful for me.

Further i asked to u that if the dry tray pressure drop is 3.6 in liq and as per criteria of 1/6 of tray sapcing max allowable value is 3.5 in liq. So what would be the consequences if the trays replacement is not planned.
Or is it necessary to replace the tray on the basis of 1/6 criteria.However Exxon design practice told the glitch criteria of 1/5 or 20 % of tray spacing.
And as per this criteria trays are adequate to re-use without modification.

Edited by Arsal, 24 June 2016 - 10:55 AM.

[Bobby Strain]

Maybe you should tell us all what you are doing. You might get some useful help.

 

Bobby

[Arsal]

We are doing REVAMP of CDU;due to high dry tray p.d;contractor recommended to replace the tray;however my stance is different and I assumed the trays shall be re-used without modification.the values of dry tray p.d obtained are mentioned above.and I m using 1/5 criteria....I need to know the consequences if replacement of tray will not implemented.
What u recommend?
Furthermore dry tray pressure drop is 3.6in of liq obtained from kg tower software...although in free version of software there is no warning,but M/S kg recommended to replace the tray that this is high value.

[PingPong]

We are doing REVAMP of CDU;due to high dry tray p.d;contractor recommended to replace the tray

Are you employed by the refinery?

I don't know what the contract between the refinery and the contractor states, but if the contractor is required to guarantee the performance of the CDU after the revamp then you need to follow the contractor's advice. In that case the refinery cannot dictate the contractor to accept using existing trays, but at the same time expect the contractor to guarantee performance of the column.

 

but M/S kg recommended to replace the tray

What do you mean by: M/S kg ?

 

KochGlitsch states following with regard to dry tray pressure drop:

 

"Dry Tray DP: The dry tray pressure drop is an intermediate term in calculating the total tray
pressure drop that does not include the effect of the liquid head. It can be used to provide a
relative indication of vapor velocity through the valves. A good starting point for many tray
designs is a dry tray pressure drop of around 2 inches of hot liquid. As a rule of thumb, the
dry tray pressure drop should be limited to 15% of the tray spacing when possible."

 

 

The higher the dry tray pressure drop, the more droplet entrainment (spray) there will be, hence the relation with tray spacing.

 

When in a crude column section (for example: between two pumparounds) all trays have the same bubbling area design, the top trays in that section will have the highest vapor loading and consequently the highest dry tray pressure drop. Total tray pressure drop, downcomer backup and percent flood will also be highest for the top tray(s) in a section.

 

Although I don't know the details of your crude column I expect that there are only one or two trays in each section that have a high dry tray pressure drop. Replacing only those few by a new design would not be a big deal. Moreover by using a more modern (than the old ones) tray design it could debottleneck the column capacity even further, as those top tray(s) now have highest percent flood. If not relevant now, it will be in the future.

 

Also you should ask yourself: how old are those trays? how corroded and/or damaged are they? what did they look like (photos?) the last time the column was opened? when would they have to be replaced anyway due to old age?

 

Revamps are always more difficult than a new design, because you always run against limits, and you constantly need to make a judgement call: do I replace or not. But remember: if it were easy your refinery would have hired a secretary instead of a chemical engineer, so make sure that they notice the difference.

Edited by PingPong, 25 June 2016 - 04:31 AM.

[Arsal]

Thanks for your time and valuable comments;
Let me explain little more to clear things completely.
first for the preflash column only.
We have preflash column having 15 no of trays (tray spacing: 533 mm);our EPCC contractor approach the koch-glitsch for tray hydraulics.the results shown that all the parameters are in the range except the dry tray p.d.
Tray # 2 p.d :3.6 in liq and tray #15 p.d: 2.6 in liq ;
As you defined the thumb rule that 15 % of tray spacing is maximum allowable limit then some trays trays are adequate.
However Koch-glitsch recommend to replace all the trays of columns without explaining the criteria;although we asked them but they refuse the answer by saying this is proprietary information.
Furthermore; I have mentioned above the EXXON design practice reference where criteria is 20% of tray spacing.
So there is contradiction;
Exxon : 20% of tray spacing
You : 15% of tray spacing

Please comments?

Further trays are in good conditions as inspected in three months before in Turnaround.

Edited by Arsal, 25 June 2016 - 10:18 AM.

[PingPong]

To be clear: that 15 % of tray spacing is not from me, but is a quote from the KochGlitsch document

Introduction to KG-Tower Tray & Packed Tower Sizing Software Program

See for yourself:  Introduction to KG-Tower.pdf   497.09KB   84 downloads

 

What I mentioned before is 1/6 of tray spacing, which is about 17 % of tray spacing.

 

And others, like oil companies, licensors and contractors, may have slightly different criteria.

 

Note that this is not an exact science, there is no sharp line above which it is disaster and below which it is perfect.

The lower the percentage the better, the higher the more entrainment.

 

Your dry DP of 3.6 inch liquid with a tray spacing of 21 inch (533 mm) corresponds to 17 % of tray spacing, so for me that would be just acceptable.

Edited by PingPong, 25 June 2016 - 12:45 PM.

[Arsal]

Thank u very much.

[Arsal]

Ping pong

the attached file "Intro to KG Tower" is very usefull document, Furthermore, if you have document having Design criterai of dry P.D and DC exit Velocity followed by SULZER.Please share.

[PingPong]

Sulzer uses:

 

1)Dry Tray Pressure Drop: max 16 % of tray spacing

2)Total Tray Pressure Drop: max 20 mBar

3)Downcomer Exit Velocity: should not exceed 1.5 ft/sec or create a pressure drop in excess of 1 inch of liquid.