code: def ND_all_available():
      
      ND_all_available = []
      for i in range(len(pipedb['NDinch'])):
          if pipedb.iloc[i, 4] == 1:
              ND_all_available.append((pipedb['NDinch'][i]))
      return ND_all_available * u.inch
docs: Return an array of available nominal diameters.
  
      NDs available are those commonly used as based on the 'Used' column
      in the pipedb.