We are looking for an inexpensive flow switch to indicate flow/no-flow in gas-filled 2-inch pipes. The gas could be air, natural gas, or LPG gas. Operating pressures are constant, but may vary from installation-to-installation between 10 psi (min) to 100 psi (max); operating temperatures are 60 to 150 degrF; flow rates could be 0 (zero) to 1500 cubic feet per minute. Sensor (switch/contact) enclosure must be suitable for Class 1 Div 1 (natural gas / LPG vapor).

Any help will be appreciated.

 

Paddle type flow switches are very economic.You can refer to www.almegcontrols.com and search for Dwyer flow switches.

 

I would recommend a thermal dispersion type flow switch. ifm-efector make a nice range.
http://www.ifmefector.com/ifmus/web/flow.htm#

This type is unsuitable however if there will be rapid changes in the gas temperature.