5 replies to this topic

[Eprocess]

I have to specify the nitrogen requirement of a gas plant (commissioning+blanketing)to prepare an MR for the N2 package.
I was wondering how should I calculate the commissioning demand, shall I simulate the purging cycles for each vessel or piping section of the plant(considering the purging pressure,mass fraction and the number of cycles)and then report the summation as the N2 demand or is there an other easier way to perform this task? like a rule of thumb or something,
Thank you in advance

[ankur2061]

Eprocess,

A safe rule of thumb used quite often for system purge with inert gas is 8 times the system volume. I have myself used this successfully without any problems. I don't think you need to do any calculations for trying to determine the exact purge requirements based on air dispacement of the system unless your company has a penny pinching attitude.

Hope this helps.

Regards,
Ankur.

[Eprocess]

Dear Ankur,

Thank you

It was very helpful indeed. One more thing how shall I interpret this amount in term on Nm3/hr?

[Zauberberg]

Not sure where does the "8 volumes" rule come from, but make sure to account for some specific gas plant activities apart from common equipment purge, such as:

- Defrosting (includes vessels and piping as well);
- Mol Sieve sweeping and preservation;
- Plant piping (which can be substantial portion of total plant equipment volume).

I would still prefer working around with a few calculation methods and see if any rule of thumb gives reasonable accuracy when compared to calculation methods. See attached a file from Paul Ostand and another handy spreadsheet made for Nitrogen purging calculations. See what figures you get by applying rules of thumb and calculation methods, and post an update here.

Best regards,

Attached Files

•  HOW TO PURGE WITH NITROGEN.pdf   154.51KB   308 downloads
•  Nitrogen Purge D.Crowl.xls   73KB   271 downloads

[Eprocess]

Dear Zauberberg,

Thank you, for your reply and the cal sheet you supplied. I would surely do what you said about updating the post with result of comparison of two approaches as soon as possible.
But what I have in mind is how to specify a certain amount of the N2 package`s capacity in Nm3/hr for commissioning process.
As long as the plant is commissioned section-wise and in sub-sections, Is it OK to consider the demand of the largest consumer and specify the package`s capacity considering the duration of cycles in this section? I don`t know if I had any success in describing what I think.

[Zauberberg]

For such calculation you need to look at the commissioning schedule and see what activities can be run in parallel, and which are the ones going one-after-another. Certainly you can't do everything at once, and that would also call for highest N2 demand = package capacity.

Having refrigerated Nitrogen production and storage (LIN) would make things very much easier and you could go with a very small capacity unit, sufficient just to maintain average plant demand in normal operation mode. Nitrogen for purging purposes would be supplied from the LIN storage.

So, look at the schedule and see how much Nitrogen you will need during each stage, and use those calculation methods attached above and/or rules of thumb given by Ankur. Normally, the maximum expected demand during any of the stages would be your package nameplate capacity. Depending on tightness of the schedule, you may go with a smaller unit and extend some of the activities for a few days. It's not a rocket science in any way.

Cheers,