Why operating pressure is different for propane and butane

[Jehannum]

TL;DR

In UK LPG installations, propane is regulated to 37 mbar and butane to 28 mbar. Is my guess at the reason for this correct or is it just a coincidence?

It's desirable that propane and butane be interchangeable as fuels in gas-burning appliances. However, butane has a higher calorific value. If used at the same pressure as propane it would give a higher heat output.

My guess was that the reason propane is set to a higher pressure is to offset its lower calorific value (CV).

Numerically, if you multiply 28 mbar by the ratio of CVs of butane to propane (122 / 93) you get (near enough) 37 mbar.

This made sense at first but I later realized that these are gauge pressures. I might expect this simple ratio relationship to work on absolute pressures but not gauge pressures. So now I'm thinking it's just a coincidence.

I train people on this stuff, so I need to get it right. I would appreciate any input.

 

[russ_watters]

My guess is it is a function of boiling point, not heating value. If the boiling point at the storage temperature is too high, it won't boil [fast enough].

In the US, appliances are not interchangeable between natural gas and propane; they need different valve orifices and air mixtures. But I don't know about butane.

 

[hutchphd]

Summary:: In UK LPG installations, propane is regulated to 37 mbar and butane to 28 mbar. Is my guess at the reason for this correct or is it just a coincidence?

This made sense at first but I later realized that these are gauge pressures. I might expect this simple ratio relationship to work on absolute pressures but not gauge pressures. So now I'm thinking it's just a coincidence.

Isn't the flow rate determined by the pressure differential? Seems to me that it should be gauge pressure.