Re: [PipingDesign] Re: Fire Case

From: <Rajesh.Balapure@ril.com>
Date: Tue Sep 09 2003 - 06:07:00 EDT

I came across an example where in the line is from knockout drum to flare header ,design temp.(Fire case) is as high as 300-350 deg c & operating temp. as low as 65 to 70 deg c.Now the problems are occurring at the time time support design as Nozzle load check found o.k.The supports are lifting at some locations in fire case causing the higher loads at the other support locations,for which the foundation required will be very huge & the support locations can not be shifted and At some locations pipe span is higher than the pipe span considered for the design of civil support at an initial stage.Civil wants the loads to be within as designed limits which is based on the basis of sustained loading condition. we have three options :-
1)Either provide Springs at that location and reduce the loads transferred to adjacent locations.Addition of springs will cause a cost implication but reduces the cost as compared to civil support design for higher loads.   or
  2)Neglect the fire case for all design,which will avoid spring support & reduces the cost implications too.
or
3) Treat the fire case as occasional case & check the stresses only & check Nozzle loads & support Loads as occasional Loads.which will be an Intermediate between normal operating case & a Fire case.

Since there are no guidelines available for not considering fire case ,and no such case of few tapping coming to header do have a higher temp. than the header design temp.
As talked to process the case is going to happen rarely or occasionally but the time period for the happening of temp.can not be predicted or told .So In view of the above Info.& more over the responsibility of the Analysis Temp. lies with the Stress Engineer & Not the Process Engineer. How can one justify this analysis ?What Engg. judgement could have been taken??
1st ,2nd,3rd or any other Engg- judgement??? Engg Judgements are purely based on the past experience & one who do not hold any such experience ,what parameters /Info.will help him to take such decisions other than experimenting it ??

Could you please throw some light on the above.

Regards,
Rajesh V Balapure

                       "Tushar
                       Rajyaguru"               To:       "piping design"
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                       tindia.com>              cc:       (bcc: Rajesh V
Balapure/JAMNAGAR/RIL)
                                                Subject:  Re: [PipingDesign] Re:
Fire Case
                       09/08/2003 04:56   Importance: Normal  Sender's OU:
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The discussion is getting really interesting. I would further clear some points.

  1. Analysis Temperature : Let me clarify this exercise may be carried out only for the cases where there is significant difference in operating and design conditions. There is no thumb rules, no written guidelines. Purely engineering judgement and common sense.

With higher temperature you will end up with large gaps or else, increase the support loads (if higher frequency is to be maintained) and design heavy supports. This one is cost implication. This support may not be active in very low operating cases and sometimes very purpose of analysis is killed. Intension is not to simply analyse the line in operating cases , but to arrive at some justifiable and logical temp. which may actually be there in worst situations.

I can give you some practical examples. In one of project flare design temperature is 350 deg. C. while operating temp. was around 70. We just inquired from where this magic figure of 350 came ? The process philosophy is design temp should be considered as temp. of max. temp. tapping coming to flare. Moreover fire case etc. was also considered. Now my logic is out of say 100 tapings, 90 tapings are below 100 deg c and only due to 2-3 tapings of high temp. is it wise to design entire flare header as well as all connected tapings for this temperature ? In such cases each tapping shall be reviewed and with help of process people you can arrive at some lower temp. For header it is difficult to determine the exact gradient, so we can vary temp. according to design temp. of tapping from one tapping to next tapping and make some temp. zones.

Now let us come to fire case. The purpose of fire case is see that piping material should withstand some minimum temperature in case of fire and not to see that when fire is on, plant should run normally and all support as well as nozzles are safe. In case of fire, plant will be immediately shut down and there is no point in analysing such hypothetical conditions.

Another interesting case is analysis of reciprocating compressor lines. the design temp. of suction line was around 180 deg while operating just 40 deg. The minimum frequency to be maintained is 23 Hz. Just imagine, you need to hold each and every corner to reach this frequency in operating case. And, think what would be result in design case ? If you want to satisfy both conditions, just provide loops everywhere and kill the piping. The logic of such a high dgn temp. was spill back from discharge line to the suction line whose dgn temp. was 180. Now if suction itself comes at 180, discharge will reach beyond 250 and there is high temp. trip of 160 deg. in discharge line. so there is no possibility of suction temp. can reach beyond 70 as per our calculations. So analysis temp .can be set as 70 for suction line.

Conclusion, apply your mind, engineering is not all about rules, regulations and bid requirements. We should use our own engineering judgement and common sense. If you are true technically bid itself can be changed.



"NO EFFORT IN LIFE GOES UNPAID, HENCE GO AHEAD"

Tushar K. Rajyaguru
Plant & Piping Engineering,
Larsen & Toubro - Chiyoda Ltd.,

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