by Marc Ward
Clay Times Aug. 1997
If there is any one single item, besides BTU's, that remains a mystery to potters, it is gas pressure. Most folks use pressure gauges on their burners, so they have some idea of the pressure they are using. I say "some" idea because gauges sometimes don't give you an accurate reading. This is especially true in low pressure situations.
What's low pressure? Low pressure is any pressure below 1 pound per square inch (PSI). When dealing with pressures below 1 PSI, you will see the term "inches" or "inches of water column". This refers to the amount of pressure it takes to raise a column of water 1 inch. There are 27.7 inches of water column (wc) pressure in 1 PSI of pressure. So 7"wc is about 1/4 PSI. This is the normal pressure that household natural gas is delivered. The normal delivery on propane (LP), for household use is 11"wc or a little more than 1/3 PSI.
Both of these pressures are the result of the regulators on the gas line. In the case of natural gas, the meter acts as a measure of volume (so you can get a bill) and as a regulator. In many situations, you'll have 25-35 PSI of natural gas coming into the meter. The meter reduces this pressure to 5-8" wc. In some industrial settings, you may be able to receive greater natural gas pressures. In a non-industrial setting, I wouldn't count on higher pressures. Propane tanks also use regulators. A full propane tank in the summer sun, may have 250 PSI worth of pressure. This is regulated down to the 11" wc, as mentioned before, or can be delivered at higher pressures with the appropriate regulator. Because the tank pressure will change with temperature and the volume of fuel it contains, you should always have a regulator on the tank. This holds true whether the tank is a small "Bar-B-Que" tank or a 1000 gallon tank the size of a small sub. Since you are not connecting to a public utility, as with natural gas, there is more leeway as to what you can do with your gas supply. Because of this, propane offers more options as far as pressure delivery. But, propane is more expensive. Currently, BTU for BTU, propane is more than twice as costly as natural gas.
Propane contains 2500 BTU's per cubic foot vs. 1000 BTU's per cubic foot for natural gas. Even though natural gas is far less expensive, it can have some disadvantages. I get to "break the news" to folks about this on a regular basis. They buy a used kiln that had been fired on propane and they now want to convert it to natural gas. That's fine, but they may not be able to use the same burner system. If that kiln had four burners that had been fired on propane at 10 PSI, they're in trouble. They want to switch from a fuel that had 250% more BTU value and was delivered at 4000% more pressure. You can see they are going to come up short with natural gas using the same four burners. No amount of drilling out orifices is going to help this situation. This isn't to say that natural gas is a bad choice. It's cost and ease of delivery, actually make it the fuel of choice. You'll just need to be aware of it's limitations if you are used to propane.
The regulated pressure on the line is referred to the "manifold pressure". This is not the pressure that your gauge may read, but the pressure that the meter or regulator is set for. Know your manifold pressure. This pressure, along with the orifice size, determines the BTU output of your burners. If you're unsure of the pressure, contact you natural gas utility or your propane supplier.