ABSTRACT

Predicting flow temperature and pressure changes has become increasingly important for use in both the design and operation stages of multiphase pipelines. This study has presented simple analytical equations for predicting these parameters at any point along the two-phase, gas/gas-condensate transmission lines, considering basic laws of momentum and energy conservation. The Masjed Soleiman to Mahshahr gas/condensate pipeline, located in the south west of Iran, used as an example to show the impact of the present explicit method on proper prediction of flow temperature and pressure profiles in the pipelines carrying gas and condensate.

INTRODUCTION

To transport natural gas, pipelines must often span great distances across undulating terrain in varying ambient temperature. Liquid condensation in pipelines, however, commonly occurs because of the multicomponent nature of the transmitted natural gas and its associated phase behavior, as well as the inevitable temperature and pressure variations that occur along the pipeline. The presence of condensates creates several operational problems such as hydrate, wax deposition, corrosion, etc. Such problems can be avoided if the flow pressure and temperature profiles are determined. In fact, in order to better size of the pipelines and downstream processing facilities, these parameters must be predicted along the pipeline1. Many attempts have been made to predict flow conditions in two-phase pipelines. However, few efforts have been reported for the use of explicit methods for this purpose2. The focus of this paper is on gas/condensate systems and consequently describing a simple, easy-to-use analytical method to predict flow parameters of gas/condensate transmission lines.

Modeling Annular Flow

In this flow regime, calculation of pressure gradient is performed similar to stratified flow. In other word, Equations 12 and 13 are equally valid for annular flow regime with the exception that the gas does not wet the pipewall (τG = 0) and the film thickness is used instead of liquid level 15,16.

Discussion of Results

The present method was tested to perform a prediction on the Masjed Soliman to Mahshahr gas/condensate transmission pipeline (168 km long and 48.26cm in diameter) that traverses a hilly terrain with 14 elevation changes. The gas-condensate composition is given in Table 1. Also Table 2 reports the respectively the results of the temperature and pressure profiles of Masjed Soleiman to Mahshahr two-phase pipeline calculated by two different methods. As shown in these figures, results of the present explicit method show good agreement with the obtained results of Shariati et al.17 implicit method. Also, Figure 1 shows that the flow temperature drops significantly, as much as 25°C, below the temperature of the surroundings, due to Joule-Thomson cooling.

Conclusions

The main objective of the present study has been the development of a relatively simple explicit method for predicting flow temperature and pressure profiles in twophase, gas/gas-condensate transmission pipelines.

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