In the gas industry engineers often have to purge gas pipes. For example, if repair work must be done on a gas line the fuel gas in the system must be removed before work can be done safely. The Institute of Gas Engineers and Managers (IGEM) publication IGE/UP/1 stresses the need to achieve a minimum purge velocity when purging, to ensure that purge flow is turbulent. Laminar flow could leave undisturbed layers of fuel gas (of a different density to the purge gas) in pipes. To help engineers achieve this IGEM provide a table that gives minimum purge velocity for a given pipe diameter. Some sample figures are below: 50 mm pipe, 0.6 m / s ... 200 mm pipe, 0.7 m / s ... 400 mm pipe, 1.0 m / s My question is this: if Reynolds number for gas in a circular pipe is proportional to pipe diameter, why is a faster purge velocity needed for larger pipes? Wouldn't it be the opposite, i.e. a larger pipe diameter would mean you could have a slower purge velocity and still get the Reynolds number you need (say > 4000) to ensure turbulent flow?