calculation for hot tap

Hi all,

has anyone some suggestions for a calculation of following problem
with caesar II or some hints to any threads in this forum?

-hot tap (welding) of two nozzles with valves (10") in one 360°C hot 16"-pipe
-connecting this nozzles with one cold 10"-bypass
-the whole system is on springs

Thanks in advance

The biggest thing to keep in mind is that your load cases become more complicated.

You are going to need to know where the existing piping is anchored (which you would always want anyway when making a connection to existing piping). In your modeling, you are going to need a "normal" case where the existing piping doesn't "heat up" as it is already in position and won't grow more (maybe). The new pipe will heat up. From this standpoint, your tie-in is effectively an anchor point. You need to make sure that the new pipe has enough flexibility on its own as there will be no balancing from the existing header.

Then, it gets worse! You need to build another case, this time, the new pipe needs to be set "ambient", and the existing pipe will need to be coded to contract back to it's installed position. Hot Tap arrangement are where I find myself going for direct coding of the alpha value instead of the temperature (you then will also directly code the allowable stress value).

Finally, you need to take the difference between those two cases in order to see what the full displacement stress range is.

Actually, your case looks to be even more complicated as you're not making just one tie in, but putting in a whole new bypass connecting to the main line in two places. Hopefully the two connections are fairly close together, as that will help you.

I guess my main point is that you need to sketch out and understand what the various temperature cases can been in each line and make sure you account for them in the model.

I guess you have an existing Caesar II calc or similar for the 16" piping since springs have been designed. If not, model it. You will need to consider the extra weight from your valves and bypass, and how that affects the springs.

You'll also need to think about temperature and expansion stresses due to your bypass. When it's cold and the header hot there should be no thermal stress in the bypass or connections. With the whole system cold, the bypass will be stressed by header contraction. When all hot, the bypass will be additionally stressed. You could use equivalent temperatures to represent these relative changes. Be careful since thermal expansion is not exactly linear with temperature change.

Some years back there was an article in our Newsletter on this subject. Read this, starting on page 13.