Hello,

I'm analyzing a steam vent line which vents to atmosphere. My question is how do I set up the load cases. Due to design pressure for steam venting, there is a 55,000 LB thrust force at the vent pipe exit, which results in an F1 in my design operating load case. But what about in the Sustained Case? Should it include the thrust force (F1) or remain W+P1?

My load cases are below, but I'm questioning the need for the force to be considered in the sustained case.

1 W HGR
2 W+T1+P1 HGR
3 W+T1+P1+H+F1 OPE
4 W+T2+P2+H OPE
5 W+P1+H+F1 SUS
6 W+P2+H SUS
7 W+T1+P1+H+U1+F1 OPE
8 W+T1+P1+H+U2+F1 OPE
9 L3-L5 EXP
10 L4-L6 EXP
11 L7-L3 OCC
12 L8-L3 OCC
13 L11+L5 OCC
14 L12+L5 OCC

Thanks,
Dmitriy

Dmitriy,

Your steam vent is not always poped up even if plant is not operating, is it?

Then its load is occasional, and occasional load is not included in Sustain case(and operating case either).

By the code, wind and seismic are not considered as acting simultaneously. That's why we make load cases for both seismic, and wind.
I guess vent pressure is same, so in your load case you need to consider F1 and U1, U2 individually.

BTW P2... is that a pressure for special case?
You only use T1, P1 for occ cases in your load cases.

3 W+T1+P1+H OPE
4 W+T2+P2+H OPE
5 W+P1+H SUS
6 W+P2+H SUS
7 W+T1+P1+H+U1 OPE(-U1 also need to be considered)
8 W+T1+P1+H+U2 OPE(-U2 as well, unless U1 and U2 is on same direction and opposite sign)
9 W+T1+P1+H+F1 OPE
10 L3-L5 EXP
11 L4-L6 EXP
12 L7-L3 OCC
12 L8-L3 OCC
13 L9-L3 OCC
14 L12+L5 OCC(seismic U1)
15 L13+L5 OCC(seismic U2)
16 L14+L5 OCC(PSV pop up)

Above is just brief adjustment from your original load cases. You can't use it, just take the concept.
and don't forget to set the combination methods for Load case 14 to 16 to Scalar.

MK,

Thank you for the reply. I agree that it is an 'occasional' load, but my concern most of all are the nozzle loads on the tanks I am connecting to, so therefore I added the F1 to my operating case.

P2 regular operating conditions in which the vent is open to atmosphere and therefore has no pressure. I ignored this case for the occasional load cases in order to not have too many load cases and results to review. A have found that the higher temp/pressure combination will almost always produce higher stresses and loads.

I appreciate the help. Thanks again.
Dmitriy

Dmitriy,

To make the load case correctly, even if you only want to know nozzle load on the tanks at the occasional case, you still need normal operating case.

and Sustain case include Weight and Pressure(and locked spring hanger weight), not with vent thrust force.

Please refer to Userguide, sec 8.

This is where I was unsure and where I decided to put the F1 into the SUS case. The reason is because if/when the design pressure is attained (P1), there will always be a thrust force present. So having a sustained case which shows me the stresses and loads due to only pressure and weight without the thrust force is unrealistic. I will look into the user guide as you suggested.

Thanks,
Dmitriy

You're right, thrust vent force is not a self-limiting, if it always occurs when system reaches design pressure it should be on the operating categoty.
It's not a temporary loading...

This might helpful,

http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=6771

Still not sure with putting thrust vent force into Sustain case. But expert's opinion would be needed, not mine.

good luck with your work,

First - what do you mean by a vent? Is this a connection open to atmosphere with a block valve? If so, you'll want to check with your process and client people on when that valve is operated. It is highly unlikely you will be dealing with design pressure in that case.

If you have a vent line open to atmosphere that has a rupture disk set to fail at a set pressure - you are going to want to find that set pressure to use as the basis for your pressure thrust.

In any case - the load will be subject to occasional loading requirements. If it is a vent to relieve pressure for vessel entry, the pressure will rapidly drop off as contents are vented. If it's a rupture disk, popping one is going to catch people's attention and things are going to get shut down fairly soon to keep from venting product indefinitely.

Also in either case, "design pressure" is not the number you should be concerned with.