You can estimate the cost of a steam leak by
comparing the vapour plume with what comes out of a domestic
kettle that's been left to boil (the photo on the right is a leak
about that size).
The typical kettle is rated at 2 kW, so that's the
rate at which energy is being delivered to evaporate
the water. Imagine that running say 8,000 hours a
year. If the steam is being generated
in a boiler at 90% thermal efficiency using gas at
(say) 2.5p per kWh, the annual gas cost for a steam leak
resembling a domestic kettle is
2 x 8,000 x 2.5 / 0.9 = 44,444p = £444
Not all leaks are visible, unfortunately. The picture on the left shows
the bypass valve on a steam trap, which has been left open (to allow air
to eascape during startup) and then forgotten. This was costing the owners
£9,000 a year (about a 20-kettle leak) with live steam escaping into the
condensate return system.
If you want to know more, the top place to learn about steam on the web is the
Spirax Sarco web site.
Other free advice on energy saving is available from the Energy Management Register