by Marc Ward
Clay Times July/Aug 1998
How Much is How Much?
When dealing with the question of capacities, the subject can seemingly make as much sense as the title of this article. Not only is it confusing, many times the subject is ignored. So, what am I talking about?
Pipes, valves, regulators, burners, all the combustion hardware of a gas fired kiln, have certain capacity ratings. Most folks know that the burners are going to put out a certain amount of BTU capacity (British Thermal Units) . They may not know what it is, but they have a sense that something, in a measured amount, is going to come out of the burner. That measured amount of gas, mixed with air and ignited, is going to produce a measured amount of heat. OK, you've built a kiln and you've considered this for a few minutes and decided to hope it's going to work. Or, you've studied the subject and know exactly how many BTU's are going to be produced by the capacity of the orifice (that tiny hole where the gas comes out inside the burner). Either way, you're dealing with the capacity of this orifice. Capacity is how much of something (in our case gas) can pass through something in a measured amount of time. When dealing with BTU's, this is usually expressed as BTU's per hour or as cubic feet of gas per hour.
Gas passes through restrictions such as orifices, valves, meters, regulators and also travels through pipes. All of these slow down or impede the flow of gas in various ways. The lower the pressure behind this gas, the more of an impact these things have. A ball valve that is the same size as the burner connection is not going to make one bit of difference to the flow of gas when it is wide open. A safety valve that has a small inlet and outlet may slow down enough gas to make a noticeable difference. A regulator that worked fine on a smaller kiln, may not work when you rebuild your kiln and add some more burners. The same is true of piping (especially with natural gas). You've doubled the size of your kiln and burner system, but have the same natural gas line running to the kiln. You may be outa' luck! With all gas, but especially natural gas, you have to be aware of the capacity of your pipe. The longer the pipe run the more gas gets "lost" because of friction. Also, a 90 degree elbow can add almost 6 feet worth of pipe distance incurred friction loss (with 2 inch pipe). To know what size pipe you'll need to use, consult a licensed plumber or a burner company.
Meters and regulators also have capacity ratings. For those that are confused about the difference between a meter and a regulator, it's simple; a meter is a regulator that knows how to count so you can get a bill. Both take a higher inlet pressure and reduce it to a lower outlet pressure. A natural gas residential meter may not have enough capacity to fire a large kiln and run the household at the same time. Consult with your gas company about your usage. The gas company is generally happy to provide you with a bigger meter. They'll be able to sell you more gas.
Now comes the hard part to understand so don't feel bad if the next sentence doesn't make much sense at first....Capacity is not a set thing, it's just a measure of how much is going through something.
Not a set thing? What does that mean? Why even bother with capacity ratings then? Think about water and this might be a little clearer. You've got a small valve on the bottom of a 10 quart bucket. Open the valve all the way and a certain amount of water is going to come shooting out. Now, take this same valve and put it on the bottom of a 1000 gallon stock tank full of water. Open the valve up now. Will the same amount of water come out? No way. You're going to have a water cannon. This valve is the same size, so how do you come up with a capacity rating? Pressure. This is the factor that has to be equal and accepted when setting capacity figures. As you can see from our water example, without a common denominator, capacity figures would be meaningless. Pressure is our common denominator, but not in the way you think.
The common pressure standard is not the pressure that's pushing the gas. Pressure is quite variable, as in our water example. The common denominator is the amount the pressure drops as it leaves our pressurized system of pipes, valves, and burners. A standard is accepted for the amount of pressure drop that capacities are then based upon. When you have 1 water column inch of pressure drop across the orifice, whatever amount is going through something, well, that's it's capacity. Clear as mud, huh?
What's this mean to the average potter? The more of a pressure drop you have (a big orifice on low pressure) the more capacity becomes available. I routinely run 750 cubic feet of gas through a valve that has a capacity rating of 402 cubic foot. You can also get more capacity above the rating of a natural gas meter if you have a greater pressure drop than 1 water column inch. There are limits to this though and pipeing's capacities also deal with friction loss. Go by the charts on plumbing pipe. You can fudge a bit on valves, meters, and regulators up to double the amount of rated capacities if you have a great enough realitive drop. After that, you start running into volume problems, but hey, I can save volume issues for another head scratching, counterintuitive article.