This input is used for two types of ASME tubesheet geometries:
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If the tubesheet has a backing ring, then enter the G1 dimension for the backing ring. G1 is the mid point of the contact between the backing flange and the tubesheet. In this case it is a required input.
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If the tubesheet is gasketed with both the Shell and the Channel, then enter the channel gasket reaction diameter, Gc, in this input. The program computes the Shell gasket reaction diameter, Gs from the gasket/flange properties specified. In this case, this input is optional, required only if Gc is different from Gs.