6 replies to this topic

[Jomhae]

What are the probable causes of over pressure in a high pressure steam condensate flash drum???

What are the possible actions to solve the said issue?


Thank you for answering!
Good day

[Fr3dd]

Without more specific data, I don't think anyone can help you over here. I recommend you to provide more specific data and maybe a basic scheme which help to understand your problem.

Regards,

[saeed.s]

unsuitable performance of steam traps on the condensate line can send steam to flash drum

[Dacs]

While I have something in my mind, it's much better if you have the P&ID with you and do some what if analysis (like HAZOP) to determine the cause of overpressure.

One thing that I can share is blocked outlet (valve in vapor line is closed). Of couse this is not applicable if you don't have any isolation valve in your overhead line or if the upstream pressure is lower than vessel MAWP.

Thus the need to check your specific P&ID.

[katmar]

All pressurized flash drums require some form of pressure control. This would normally be on the outlet because the inlet streams are controlled by upstream process variables. If your pressure is too high it means that condensate (and maybe steam) is entering the drum faster than it can be removed by the control system. There are therefore 3 possibilities you need to investigate:
1. The controller is not functioning properly
2. The inlet streams are more than the design value
3. The outlet stream is below the design value

Once you have worked out which of the 3 is the problem you can start looking for root causes. It is also very important to take into account whether this problem has recently arisen, or whether it was there from day 1. For example, if the outflow is below the design rate but it worked for 5 years until last Friday then you can eliminate undersized piping as a cause.

[kkala]

I agree with Fr3dd that more specific data should be known, as well as whether the topic concerns a new or existing installation. The query seems to ask for a way to specify design pressures / PSV around a steam flash drum. A basic design of numerous flash drums (2004, delivered but whole project postponed) had emerged the issues mentioned in previous posts. A summary specifically concerning these issues follows.
1. Steam condensate piping design pressure had better be same as that of condensing steam, in case there are isolation valves; http://www.cheresources.com/invision/topic/8710-steam-condensate-design-pressure. This was respected for the line upstream flash drum. Nevertheless the drum was protected by a PSV set at design pressure of MP (medium pressure) steam network connected to the flash steam. Existing installation in the Refinery had already applied this scheme.
2. Sizing of PSV covered the fire case, as well as blocked outlet downstream.
The blocked outlet case included HP (high pressure) steam escape from any single condensate pot of the relevant Units, due to its LC failure (there were LCVs, no steam traps). LCV was assumed 100% opened, with its bypass valve 50% (1.5*Cv, same valve Cv for bypass).
The latter event was judged credible during Units startup, not making a double contingency. HP steam escape (from the single condensate LCV of highest Cv) was considered under the condition that (α) only dry HP steam escapes, rest Units do not provide condensate yet, (β) dry HP steam escapes, mixed with HP condensate of all rest units. Both cases happened to result in almost same volumetric flow rate upstream PRV, yet temperature of drum feed was much higher in (α).
Note: Flash drum design temperature was specified same as of MP steam (having some superheat - backflow to drum possible), but maximum operating HP steam temperature was additionally noted as short term exposure temperature for the drum.

Attached "Fldrum.doc" tries to make above points clearer, along with this text. These are hopefully helpful as examples, considering causes of overpressure and ways out for the design of 2004. Moreover applicable measures can be different in the query case, due to complexity of each individual situation. As Dacs has pointed out, some HAZOP study should be conducted (extending to downstream equipment too) to determine the proper measures for the present case.

Attached Files

•  Fldrum.doc   23.5KB   185 downloads

Edited by kkala, 18 August 2012 - 12:32 PM.

[Jomhae]

Thank you for all your answers!!! I appreciate them a lot!

Can I ask you guys regarding steam hammering?

I'm a student currently having my internship and am assigned with this study..

What might be the probable causes of steam hammering at recovery column reboiler low pressure steam condensate line? Is the steam trap currently installed inefficient? (It is actually oversize based on my computations) Or the lift/rise on the said line can cause such?

Thanks again for your time. I hope you can also help me with this one.
God bless

Edited by Jomhae, 03 September 2012 - 06:18 AM.