Laclede Gas Company is a natural gas distribution company of 543 thousand meters. Symbolic of all large metropolitan areas, it has a cast iron system in the inner city area and a steel system in the surrounding suburban areas. Annually, it is necessary to prepare distribution calculations to determine system adequacy for five different pressure level systems.
Cast iron low pressure - 8.3" W.C. to 5.0" W.C.
Cast iron medium pressure - 25 psi to 4.0 psi
Steel intermediate pressure - 60 psi to 5.0 psi
Steel commercial feeder - 100 psi to 40 psi
Steel supply feeder - 300 psi to 40 psi
Typically, Laclede experienced the acceptance of natural gas as a heating fuel and the need for reinforcing a cast iron low pressure system built for domestic usages. Next, came the tremendous expansion of the steel intermediate system. Because of the Arab oil embargo and the deregulation of natural gas, we are now in an era of conservation. Customers are converting gas lights, insulating better, and doing anything, they can to conserve. From 1971 to 1981 the average annual use per residential heating customer has decreased 20 percent, while the cost per therm has increased from l2¢ to 4l¢. This year it is anticipated that the cost will rise to 50¢. To further complicate matters for the designer, high interest rates have forced gas utilities to push their distribution systems to their limits. At Laclede, our intermediate system once operated at 30 psi to 35 psi with regulator capacities 1000 MCF/HR maximum. Today, this system is being designed to operate at 60 psi and regulator capacities were over 1500 MCF/HR. No distribution calculation is perfect. There are and there will always be differences between operating and calculated results. Some kind of allowance should be made for these differences. You can overstate your design hour or you can add pressure drop allowance. In any event, some suitable allowance should be made or recognized. Presently, at Laclede we are using a drop allowance. Since our design max./hr. Is based upon a once in 40 years occurrence, the management of our company, to the chagrin of the operators, many times will gamble on the need of a reinforcement. A distribution system network analysis is made up of the following components:
Main lengths, main diameters and main efficiencies.,
Customer loading calculations.
Locating the customers on the piping network.
Main lengths and diameters is a manner of maintaining good records. However, pipeline efficiency factors used is a distribution system network analysis are not easily maintained. Constant testing may not be practical in an integrated system, but periodically the mains should be tested under operating conditions.
Customer loading calculation is the most ambiguous and weakest link in any distribution calculation. At Laclede, we are presently utilizing monthly billing factors used for estimating customer bills. A brief explanation of normal and A.D.D. factors is attached.