1 reply to this topic

[aniljani]

Hi friends,

We have distillation Column and it is operating at 5psia (vacuum). Column reboiler is of vertical thermosiphon and a control valve in steam line which is cascaded with feed flow to the column (means feed low low and steam cut off) Steam supply pressure to the reboiler is 175 psig and saturation temp is 373 degF. There is a control valve in the vapour line from reflux accumulator which goes to the vacuum pump. This control valve maintains the pressure at the top.

Set pressure of the PSV is 50 psig. PSV is in overhead vapour line.
B.pt of overhead product (Pure component) is 482 F at 50 psig.

Note: No lighters present in the column overhead

Now, my problem is, We are using steam with 373 F saturation temp, Will column overpressure in case of reflux failure, Air cooled exchanger failure, power failure, steam control valve failure (since at 50 psig, B.Pt of overhead product 482F is more than steam supply temp 373F)?
What we think is only fire case is applicable

Thanks in advance

AKP

[Lowflo]

Now, my problem is, We are using steam with 373 F saturation temp, Will column overpressure in case of reflux failure, Air cooled exchanger failure, power failure, steam control valve failure (since at 50 psig, B.Pt of overhead product 482F is more than steam supply temp 373F)?
What we think is only fire case is applicable

Aniljani....you've analyzed this correctly, and you answered your own question. The temperature pinches out before the column pressure reaches set pressure. Therefore, the scenario is not credible. Copy what you've just said into you documentation file, and proceed with determining if any other scenarios are credible.

Based on the information you provided, I'm guessing that this material has a very high flash point. It's probably a Class IIIB material, which is normally not considered to be a fire risk. However, regardless of its classification, it's a fire risk if the operating temperature is higher than the flash point. So, check this out to determine if fire needs to be considered a credible scenario. Also, check to see if there are any flammable or Class IIIA materials in the area, which could create a fire risk for this column.

If you find that there are no credible scenarios, then there's no sizing required. In such a case you're free to choose the PSV size. Refer to my comments in this posting for more explanation on that.

http://www.cheresour...lant-fire-case/