The manhole is cut out a big opening on the Tank shell, which weakening the shell strength. It needs to be reinforced with re-pad and extra neck thickness to accommodate the loss of the metal due to large opening.

Quote (UD1987

.......... I understand that manhole neck shall be designed only for hoop stress caused by liquid head pressure. since manhole diameter is always very very less compare to tank diameter, manhole neck shall have very less thickness compared to bottom shell course thickness for the same liquid head pressure.

can someone guide me what is concept/theory for such higher thickness considered by API 650)

It is not reasonable to say that (manhole neck shall be designed only for hoop stress caused by liquid head pressure ). You may check the minimum required thickness with hoop stress but API 650 STANDARD does not suggest any calculation procedure for the same. You are expected to comply with the table 5.4a and foot notes ( a) and (b )..

IMO, the proposed manhole neck thicknesses are not only based on hoop stresses but also reinforcement requirements .

Thanks mk3223 for your kind response.
Thanks HTURKAK for your kind response.

@HTURKAK, actually required reinforcement can also be achieved from higher shell thk & RF pad. So if we have higher shell thickness or higher RF pad dimensions then is there any clause where we can consider lower manhole neck thickness. I understand there is not reinforcement calculation as we have in asme sec viii.

Quote (UD1987

...@HTURKAK, actually required reinforcement can also be achieved from higher shell thk & RF pad. So if we have higher shell thickness or higher RF pad dimensions then is there any clause where we can consider lower manhole neck thickness. I understand there is not reinforcement calculation as we have in asme sec viii.)

There is no any clause where you can consider lower neck thickness . I assume the neck thk. provided comply with Table 5-4 values ( which are the threshold values ) but not comply with Table 5-4 foot note (b). That is , (....The minimum neck thickness shall be the
thickness of the shell plate or the allowable finished thickness of the bolting flange (see Table 5-3a), whichever is thinner, but in no case shall the neck in a built-up manhole be thinner than the thicknesses given ..)

In this case, you have two options ... either change the neck with the new one having correct thickness or speak with client/end user to accept that non conformance..

PS. : Will you please post some details of your tank ( diameter, height, 1st course thk, bolting flange thk., neck thickness ,manhole centerline ht from bottom ..? .