So we are saying that we are neglecting the stresses because we do not have the full information about the element , rather than saying that such elements can be hardly overstressed and need not to be analyzed!! is this really acceptable technically!!
If some body asked what about the stresses in the valves and the flanges, simply i will say it may be overstressed or under-stressed and i do not know .I think what i should say that the stresses on the rigid elements are usually negligible due to X , Y , Z reasons!!