It is helpful to remind ourselves the substantial benefits of insulating steam piping. Huge amounts of energy is put into producing steam and yet as we transport it from point A to point B we often allow large energy losses to radiate freely into the ambient air. Heat energy as we know moves from a higher temperature to a lower temperature and the rate of this transfer is proportional to the temperature differential and the surface area of the pipe (other factors do impact the losses such as wind speed, surface emittance, and thermal conductivity of the pipe material).
For example 100 psi steam is 338 F and if the ambient temperature is 60F, that is a 278F incentive for the heat to radiate out of the piping. Now if we use 3″ piping that is also lots of opportunity for the heat to escape. So, for every foot of 3″ piping carrying 100 psi steam and exposed to 60F ambient temperature, 778 BTUs per hour are lost. That is nearly 6.5 million BTUs or the equivalent of 21,000 lbs of steam per year lost for each foot of piping.