HI ..

iam doing a piping system which is fabricating in china as a module and installing in Indian site.here ambient temperature of fabrication yard is 15 deg and site ambient temperature is 24 deg.

In Caesar which ambient temperature i should consider for this paticuler system (15 or 24)

if it is 15 why?

Dear,
The system has to be installed where ambient temperature is 24°C. So you should go for 24 rather than 15°C!

Try this...
Set your ambient temperature to 15 and run a thermal analysis at 24. This will give you the changed position of your module connections at the site. Do they change much? If the difference is not significant in terms of field connection, then you probably need not worry about it.
To me, there are two issues in setting ambient temperature - the temperature of the pipe when it is cut and the temperature of the system when it is buttoned up. When these two temperatures differ, such questions arise. But is it significant in light of all the other uncertainties in field assembly? That depends...