The term Fd is defined in the Code paragraph UW-20. The design strength should not be greater than Ft (tube strength), which is p*t(do - t)Sa. This value is used to determine the minimum acceptable fillet/groove weld size that connects the tube to the tubesheet.
This value is required for U-tube tubesheet exchanger. It is optional for fixed and floating tubesheet exchangers.
For partial strength tube-to-tubesheet welds on fixed/floating tubesheet exchangers, the higher of the actual tube-to-tubesheet load and the user entered design strength will be used to size welds.
For full strength tube-to-tubesheet welds on fixed/floating tubesheet exchangers, tube strength (Ft) is used to size welds.