Transient flow in low pressure distribution systems presents special problems to the distribution engineer if computer control is anticipated. Modeling low pressure transient flow requires more work than setting up pipeline problems. Transient problems in low pressure systems were unrecognized until recently. Solving these problems can be done with programs developed by Dr. Michael Stoner and Orin Flanigan, et al.


Public Service Company of Colorado is engaged primarily in distributing natural gas and generating and distributing electricity. PSCo. has thousands of miles of distribution mains and hundreds of miles of both intermediate and high pressure lines. PSCo.'s pipeline subsidiary, Western Slope Gas Company, is engaged in the transmission of gas exclusively. Like many people involved in distribution systems almost to the exclusion of anything else, I really didn't give much thought to transient problems. I first became aware of transient flow some years back at our load dispatching center in a discussion of the operation of the 10-inch high pressure line from Mesa Compressor Station, just east of Denver, to Fort Collins, approximately 60 miles to the northwest. The dispatcher said that in order to carry the morning peak in Fort Collins and the other towns between Mesa and Fort Collins, the compressor station had to be on line and running all out as soon after midnight as possible. This was the first concrete example I had seen of whats widely known as "pack and draft" a classic case for analysis by transient flow programs under pipeline conditions. At that time (late ‘60’s) I couldn't remember anyone referring to what we now know to be transient flow.


A few years later, our load control center was completely rebuilt and a new IBM machine was installed to handle low pressure distribution control and calculate city gate flows. The information was telemetered to PSCo. 's load control center, located just south of downtown Denver. The load dispatchers had been manually controlling the pressure in what is called the inches low system. They would monitor the tail end clocks and send raises or lowers to the regulator stations according to the system demands. This required a minimum of three men around the clock seven days a week. Almost simultaneously with the installation of the new IBM computer, the Load Control Supervisor retired. The man who took his place was a firm believer in computer control of distribution systems. He proposed that the inches low stations be put on computer control and, in addition, several fringe stations be put under the control of the computer. Management approved both proposals and, in due course of events, approximately 25 stations were being controlled by the new computer.


The inches system operates between 30 and 120 inches water column pressure. All customer services are regulated via service regulators to a standard metering pressure of 6" water column, or 1/4 pound. The service regulators used in the inches system require at least 1# inlet pressure for satisfactory operation.

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