7 replies to this topic

[aju_1807]

Dear All,

We have a case where the control valve upstream system design pressure is 130 bar g and downstream design pressure is 105 barg with a RV set at 105 barg. Now, as per our philosophy we have to perform CV failure calculation from PSHH trip point which is 109 bar g (worked out based on max operating pressure at wellhead).

My problem is how to perform CV failure calculations for such application where the upstream pressure is 109 barg and if I can consider downstream pressure as relieving pressure (relieving pressure will be 105*1.16 = 121.8 barg) then there won’t be any pressure gradient for flow. Is it correct to consider set pressure of relief valve for calculating CV failure flow for such cases as relief valve will try to lift, moment it senses the RV set pressure?


2nd Query :

For column is it advisable to locate PSV on reboiler as column PSV will not able to react that quickly and due to pressure drop across the column the reboiler will be at far higher pressure than the column and reboiler deign pressure (during tube rupture scenario). Please note that the column and reboiler are design with design pressure of 6 barg and we have LP steam as a heating medium. Advise requirement of separate PSV on reboiler.


Appreciate your feedback on this issue.


Regards,


AJ

[sheiko]

Is it correct to consider set pressure of relief valve for calculating CV failure flow for such cases as relief valve will try to lift, moment it senses the RV set pressure?

Yes.

[Zauberberg]

Design pressure applies for the vessel top, and placing a PSV on reboiler doesn't make any sense - I don't remember even seeing such concept anywhere. If you are about to place the PSV on reboiler, the relief pressure - if set as design pressure or MAWP - would be actually lower than the true set point. Column top or the overhead line is, in my opinion, the most convenient location for pressure relief device(s).

[fallah]

For column/reboiler system with LP steam as heating medium,normally,for tube side protection in the tube rupture case a PSV would be installed on tube side and as Zauberberg mentioned another PSV would be installed on column top to protect both column and reboiler shell side.Of ourse,set pressure of the column PSV is a little bit lower than design pressure of the reboiler shell side in order to consider the pressure drop between reboiler and column in the case of shell side overpressure.

[aju_1807]

For column/reboiler system with LP steam as heating medium,normally,for tube side protection in the tube rupture case a PSV would be installed on tube side and as Zauberberg mentioned another PSV would be installed on column top to protect both column and reboiler shell side.Of ourse,set pressure of the column PSV is a little bit lower than design pressure of the reboiler shell side in order to consider the pressure drop between reboiler and column in the case of shell side overpressure.

Please refer attached P&ID for reference. I got it from this forum only.

Well, my concern was as the tube rupture scenario can create a pressure spyke so probably the PSV close to the pressure generating source would be better but of course the design pressure of equipmet and set pressure of RV shall be such that it should avoid overpressurization and the nuisance lifitng of the PSV.

If we rely on column RV only for this scenario then we will have reboielr at far higerh pressure depending upon the Dp through the column.

Please share your thoughts.

Rgds,

AJ

Attached Files

•  PSV_Configuration1.pdf   92.5KB   158 downloads

[fallah]

Please refer attached P&ID for reference. I got it from this forum only.

Well, my concern was as the tube rupture scenario can create a pressure spyke so probably the PSV close to the pressure generating source would be better but of course the design pressure of equipmet and set pressure of RV shall be such that it should avoid overpressurization and the nuisance lifitng of the PSV.

If we rely on column RV only for this scenario then we will have reboielr at far higerh pressure depending upon the Dp through the column.

Please share your thoughts.

Rgds,

AJ

In the case you attached (design pressure of shell side/tube side is 5barg/12barg) as per API 521 you need to consider PSV on the shell for tube rupture case,but because of LO valves between reboiler and column,you could consider both reboiler and column as one system and protect this system with the PSV on the column provided that the pressure drop between reboiler and column would has been considered in PSV set pressure.

Anyway,in most reboiler/column system protection i have ever seen,when the system is protected by the PSV just on the column the shell side had had same design pressure as tube side may cover your concern regarding the phenomena more or less similar to pressure spike created in the case of tube rupture.

[chemsac2]

I have heard of PSVs on column bottom sump, but not on reboiler. I have certainly seen PSVs on column overhead line. This is what I feel about pros and cons of two options:

Benefits of having PSV on reboiler or column sump:

- PSV being close to source of pressure (i.e. reboiler), relief flowrate does not have to flow through column internals. Too large flow through column during relief event can lead to excessive pressure drop across column which in turn can lead to uplifting of trays.
- PSV size may get reduced as latent heat of heavy components would be more, molecular weight higher and hence more density i.e. lower volumetric flowrate. However, increased relief temperature (due to higher boiling point of bottoms product) may offset this effect.

Disadvantages of having PSV on reboiler or sump:

- PSV is anyway required for reflux drum relief, power failure scenario etc. Backflow of vapour from top to bottom through internals is not desirable.
- PSV inlet line length increases as PSV needs to be located above flare header to ensure free draining from PSV outlet. This would increase PSV inlet line size.
- PSV would relieve fluid during column flooding. During column flooding liquid static head would mean PSV set pressure would reach.
- This location can not take credit of partial/total condensation of vapour in power failure cases.

Having PSV on column overhead is the norm. Although this would mean higher design pressure for reboiler, difference is not significant. As Zauberberg mentions, vessels design pressure is for top. Column bottom design pressure is top design pressure plus column pressure drop plus sump liquid static head. PSV is located at lowest design pressure component of the system.

All this is nicely covered in section 9.5 titled "Locating Column Relief Devices" in Kister's Distillation Operation. In fact chapter 9 is devoted to Distillation Overpressure Relief.

Regards,

Sachin

[sheiko]

Hi,

3 typical locations are:

Location 1

- at column overhead, upstream of condenser
- best location for columns operating above atmospheric pressure
- prevent downward vapour flow (as compared to lowly placed relief valve) which might damage the trays
- avoids discharging liquid when column become flooded - may upset downstream relief header and impede the ability of the relief valve to reduce column pressure

Location 2

- at the vapour space in the column bottom, just below the bottom tray or packing supports
- much less common
- prevents trays from being uplifted under large discharge rate
- avoids possibility of overpressure due to plugged trays

Location 3

- at the reflux drum (overhead accumulator)
- when a partial condenser is used
- reflux drum must be large enough to avoid liquid discharge

Edited by sheiko, 24 September 2010 - 01:45 PM.