Hello @Anonymous ,
The problem lies in the Revit family in combination with your schedule.
The Duct Size of the Duct is passed through via the connected connectors of the family.
The Size parameter in the schedule of the Duct fitting always shows the nominal Duct size of the connector. And always from large to small.
The connector Radius parameter of the connector in the family is linked to a shared parameter that you can display in your schedule.
The family has 2 connectors. If you place a transition from 710 mm to 500 mm, you will see that the parameter "R1 Connector", linked to Revit connector 1, has been given the value 355 and parameter " R2 Connector", linked to connector 2, the value 250.
Nominal Diameter 1 = 2 * R1 Connector
Nominal Diameter 2 = 2 * R2 Connector
Parameter "D1 Description" and "D2 Description" are linked via a formula to parameter "Nominal Diameter 1" and parameter "Nominal Diameter 2"
If you reverse the diameters in a transition, so a transition from 500 to 710, the transition for us is exactly the same. It should basically have the same item number.
However, that is not the case for parameter ‘R1 Connector’ and ‘R2 Connector’ in this family but that is the case for parameter "D1 Description" and "D2 Description".

Schedule:
If a parameter (Field) has multiple values in a schedule due to sorting, Revit leaves this field empty.
To solve your problem you need to sort on parameter "D1 Description" and "D2 Description".
If you have more Duct fitting family that do not have the "D1 Description" parameter, you need another way to sort your schedule.

Ingeger:
As for your other question to convert a text parameter to an Integer parameter, I can be brief. Unfortunately, that is not possible.
You can convert a length parameter to an integer.
You do this by adding a "Calculated Value" parameter to your schedule and dividing the parameter "Nominal Diameter 1" by 1mm.

If my post answers your question, please click the "Accept as Solution" button. This helps everyone find answers more quickly!
Kind regards,
Robert Klempau
Senior Consultant AEC
Cadac Group AEC BV