Reasd the rest of that section. Peng's discussin is following general vessel connections. Here, he is saying that the HX shell is smaller than a typical vessel and it may be useful to include that HX as "pipe" in the model. And if you do, then you might as well model the connection as a tee. But the next paragraph says the HX is a "factory-manufactured" item usually with manufacturer-supplied load limits. The piper is responsible for meeting those loads and that check supercedes any guess at stresses in the equipment.
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Dave Diehl