SUMMARY

Upsets in gas systems are typically Modeled assuming the gas maintains the pipeline environment temperature. Rapid line pack and draft can lead to large temperature changes that cause modeling inaccuracies. This paper studies examples of severe upsets at three pressure levels for pipe in three environments. In these, isothermal modelling errors were computed by comparison with results from a full thermal model. The short term (5 minute) errors depend strongly upon the pressure level but are relatively insensitive to pipe environment. Errors in cases at the 2000 psis level were in the 150 psi range; at the 1000 psig level the errors were in the 35 psi range, and at the 250 psig level, errors were of the order of 4 psi. It was concluded that the heat flow barrier between the pipewall and gas at low velocity is the chief source of these isothermal modeling errors.

1. INTRODUCTION

Computer modeling of the transient flow of gas has been assuming increasing importance in the design and operation of pipelines. A growing variety of uses has imposed a number of specific requirements for accuracy. The purpose of this paper is to examine the effect of the frequently applied simplifying assumption of isothermal flow in modeling transient gas flow. Historically, as the basis for a transient gas flow model the generally been made that the temperature remains close to that of In practice this normally leads to assuming that flow is isothermal ground temperature. Assuming isothermal flow (or, perhaps somewhat better, flow with an envirommentally determined schedule of temperature profile changes) may in some cases be seriously wrong and lead to significant discrepancies in the modeled results. Since the consideration of thermal effects requires a significant increase in computer resources, it is important to distinguish those situations in which thermal effects are important from those in which they are not. We present below a sequence of comparisons over a ramie of conditions to help decide when thermal modeling should be used. These comparisons are made by using a comprehensive model, the INTERCOMP/DREM program THERMAL TRANSFLOW, which includes properly the influence of all the thermal effects thought to be important in the situations studied. A specific upset of steady flow was repeatedly modeled thermally under several conditions for comparison with isothermal results. The upset situation was chosen so as to maximize the effect of possible thermal changes. The paper is in four parts. The first part is this introduction which outlines the suppose of the work. The second part describes the thermal problem, motivates the choice of the cases to be studied, and discusses the importance of the differences between the isothermal and thermal results in terms of the end uses. The third part presents the multiple case studies which examine the effects of cutoff of the inlet flow З 10-mile test section. Comparisons by difference between the variables for the isothermal and thermal cases are presented as appropriate.

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