Used in the conventional way, an essex flange is installed near the top of a HW cylinder, with its pickup pipe pointing slightly upwards. [See
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This enables water to be drawn from near the top of the tank, but without sucking the vent pipe dry - which might happen with a normal top connection and powerful pump.
HOWEVER, in the application under discussion, the flange would be installed very near to the bottom of the tank - probably with the pickup pipe on its side, so as to be at the same level as the hole. The normal reasons for using an Essex flange don't apply - the *only* reason being that it can be installed without having access to the inside of the tank.