Purging gas flow rate design criterion

[sfacchin]

S!
I have to estimate the flow rate of purging gas, air in this case, to keep some sighting glasses and instrumentation wells in a furnace. An older engineer told that is necessary to keep a superficial velocity (flow rate/internal area of the pipe) equals to 1 m/s, but he is not sure about this value. During my calculations I've found that the total amount is, im my opinion, too large. Just for example, in the smaller furnaces the flow rate for the purging gas is about half of the air needed for the combustion. The furnaces are specially sensitive to excess of air. It are sulfur recovery furnaces, so the air flow rate must be very well measured and controlled.
The smaller furnace, a in line reheater, has a total of five locations that need to be purged: 2 flame detectors (2 in), 2 sighting glasses (2 in) and 1 pilot/igniter well (3 in).
Does anybody knows a rule of thumb or a typical design value for purging gas velocity?

 

[CJKruger]

sfacchin, I seem to recall instrument purge rates are typically 1-5 fps. If too much air is an issue, you can use nitrogen.

 

[dcasto]

whatever velocity guarentees turbulent flow is what you need. depends on pipe diameter and roughness, but around 0.5 feet/second. Just to make sure, use at a minimum 1 foot/second as pointed out earlier. We always used the 1 foot/second as a minimum purge rate on a flare header.