ABSTRACT

New pipeline materials, increasing pressures in pipelines, increasing flows in existing pipelines and proposed international interconnections have raised issues in the accuracy and applicability of existing transmission factor formulae for natural gas. The results of flow tests have sometimes appeared to be contradictory and difficult to explain. The paper will describe the development of a very general formula that can be used as an alternative to the well-known Colebrook-White formula and the reasons for this alternative approach. A small amount of liquid in a pipeline can have a dramatic effect on the flow characteristics and this is explored with results from limited flow tests. Finally the paper will indicate those areas where further flow tests are required to improve our knowledge and help tune the parameters of the new formula. The work was supported by GERG Research Project 1.19. GERG (Groupe Europeen de Recherches Gazieres), founded in 1961, consists of members from eight European countries.

INTRODUCTION

"Taking the Rough with the Smooth" gives a good hint at what this paper is all about. For many years the gas industry has relied on the Colebrook-White equation to evaluate transmission factors in its network planning and design work, in fact in any gas pipeline simulation work. Some other equations have also been used, but by and large Colebrook-White and its approximations have been the most popular. Due to the high Reynolds-Number the flow type in gas pipelines is always turbulent. There are two main types of turbulent flow behaviour in gas pipelines that have to be modelled:

  • smooth pipe flow and

  • rough pipe flow

The Colebrook-White equation attempts to model both of these kinds of flow by having a gentle transition from one flow regime to the other: it takes the rough with the smooth. Flow test results sometimes indicate that this transition between smooth to rough pipe behaviour is not as gentle as Colebrook-White would lead us to believe. As we shall see, with current knowledge, there are a number of reasons why the Colebrook-White equation is flawed and can be improved upon. Much of this paper is based on research carried out as part of GERG Research Project 1.19. during 1995 to 1999. An article describing some of the findings was published in 2000 [1]. GERG (Groupe Europeen de Recherches Gazieres), founded in 1961, consists of members from eight European countries. This paper gives a background to pipe flow equations and transmission factor formulae. It then presents a new suggested formula and how it incorporates the most up to date knowledge. Finally it presents some flow test results

Background on pipe flow equations

So what is a transmission factor and what does it tell us about the flow of gas in a pipeline? A good survey of the subject is the paper presented at the 2001 PSIG meeting by Don Schroeder [2], but here is a brief survey of some basics to set the scene.

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