Sizing the holding tube
Holding time is tube length divided by speed, so the needed length is speed x 15 seconds. Speed is the volume flow divided by the tube's inside cross section, area = pi x diameter squared / 4.
This model uses average speed. In real tubes milk at the center moves faster than milk near the wall, so regulators require the fastest moving particle to stay at least 15 seconds and test it with the actual pump. Real holding tubes are therefore longer than this simplified estimate.
Try it: at 10,000 L/h with a 2 inch tube, the tube needs about 24 m. Switch to a 3 inch tube and it drops to about 10 m, because the milk slows down. Then lower the temperature below 72 °C and watch the valve divert.
Why 72 °C for 15 seconds
The standard US time and temperature for milk is 72 °C (161 °F) for 15 seconds, which destroys the disease causing bacteria of concern while changing flavor far less than boiling. Older vat pasteurization holds milk at 63 °C for 30 minutes instead.