6 replies to this topic

[arkae]

Dear all,

If we know the boiler operating pressure, how do we calculate the design pressure?
i have gone through ASME BPVC but couldnot exactly find the margin to be considered above the maximum operating pressure.

Moreover, i asked some of my friends in other companies who have worked in boiler area. But they are having their own company standard which says that we have to consider 10% margin over max. operating pressure.

I have seen some of the datasheets of boiler and auxiliary boiler, where a margin of 10%, 7.36%, 40%, 50% have been taken. I just wanted to know on what basis these margins are taken.

Some one please suggest the exact code used.

[ankur2061]

arkae,

By boiler design pressure, do you mean the steam drum of the boiler. If yes, then according to the classic book "Steam" by Babcock & Wilcox company, the following is suggested::

As a practical matter, to avoid unnecessary losses and maintenance from frequent popping of the safety valves, the first drum safety valve should then be set relieve at least 5% higher than the drum maximum operating pressure. The ASME code stipulates that the boiler design pressure must not be less than the lowest safety valve relief valve pressure

From the above, it is clear that the minimum boiler design pressure that could be selected would be 5% of the maximum operating pressure and equal to the set pressure of the drum relief valve which is set at 5% above the maximum operating pressure.

Hope this helps.

Regards,
Ankur.

[kkala]

The 10% margin over max operating gauge pressure (or increase by 2 bar, whichever results in higher design pressure) is applied here generally, including steam generators more or less.

Besides margin of 30 ^oC over max operating temperature usually gives design temperature.

Nevertheless, an actual example from a refinery steam boiler is shown in the attached "steamcond.xls", with (normal) operating and design conditions.  We can see that the 10% margin on pressure is not precisely applied in all cases.
Too small difference between operating and design pressure will cause frequent opening of the relevant PSV.  Too high margin over max operating pressure may shift steam piping to a higher class, thus increasing capital cost.
All above refers to high pressure steam coming out of boiler battery limits, concerning its operating pressure versus design pressure. The query was about this, as understood from post No 1.

Please examine whether mentioned margins of 40% or 50% refer to cases where steam pressure has to increase under specific conditions for some reason.  Another explanation could be that design pressure of steam piping (and relevant hydrostatic test) covers conditions of higher pressure to be realized in the future.

It is clarified that design pressure of steam drum is (somehow) higher than the design pressure of steam coming out of the boiler, at least seen for boilers superheating steam.  For example see http://www.steamforu...ety Valves.pdf'> http://www.steamforum.com/pictures/Boiler%20Safety%20Valves.pdf.

Note:  Boiler design pressure can mean this of steam drum, but in this post it concerns the steam coming out of the boiler, as mentioned earlier.

Attached Files

•  steamcond.xls   20.5KB   146 downloads

[arkae]

Thankyou very much for giving reference on the design pressure of boiler drum.

[mfhasnain]

Dear Friends , 

 

I am about to carry out some calculations for changing re-heater of a sub-critical boiler , in order to evaluate its impact on efficiency can you please share any standard / guidelines / calc sheet , if you may be havin it 

[Art Montemayor]

mfhasnain:

 

Please post your specific topic in another, independent thread that you can start.

 

Your subject matter is definitely off-topic for this thread.   In order to have organization and the ability to resource all past valuable information in our Forums it is very important to keep all threads within their specific topic.

 

I will delete your posting here after a few days.

 

Thank you,

[sidibouziane]

Hi. 

 

back to mr. Arkae and thanks to all comments.

 

we are also working on a plant where a number of boiler are needed. on the design aspect, we identified the operating parameters (P,T, Q...) and we are about to contact a number of vendors for the  boilers design. i see it risky to be an asset owner and designer itself. Vendors have a huge amount of experimental data and accordingly they know how to combine standards and regulation to safely design an equipment.

 

this is my opinion.

regards