API 610 recommends Minimum size requirement for casing connections other than suction & discharge nozzles based on Discharge nozzle size excluding seal flushing & gauge conections. Why?
What I appreciate most about API Standards, is that these standards are evolved by an organisation of pump-users, with little influence of pump-manufacturers. Size of casing drain connection is important to ensure that the casing can be drained as fast as possible, saving downtime during maintenance. Size of pressure-gauge connection is important because that automatically dictates dial-size of the gauges and in turn their accuracy and readability.
