Abstract

The Weymouth, PanHandle and Modified PanHandle equations were compared to the detailed Fanning friction factor method for pressure drop calculations, and appropriate flow efficiencies were derived. In general, the flow efficiency is not a constant but varies with pipe diameter, roughness and flow rate. Efficiency plots are presented for use with those equations. Rigorous pressure gradient plots, Figure 5 to 15, were also developed for use in practical design situations.

Introduction

When designing a gathering system or evaluating the efficiency of a pipeline used for gas transmission, simplified equations such as the Weymouth, the PanHandle or the modified-PanHandle equations are used. These equations use a simplified form of the friction factor relationship and account for pipe roughness implicitly through the use of an "Efficiency Factor". Generally a pipeline efficiency factor of 0.9 is considered reasonable. Frequently in a test of the gathering system or gas plant, the actual pressure drop in a segment of pipe is measured, and the flow efficiency is "backed out" of the equations. A low flow efficiency is assumed to reflect restrictions in the pipe, or the effects of multi-phase gas-condensate-water flow.

Case History

In a recent study, the flow efficiency of a pipe segment was determined from actual flow and pressure measurements of the gathering system shown in Figure 1. The PanHandle flow efficiency, EPH, was calculated from these measurements. The majority of these flow efficiencies were in the range of 0.7 to 0.8. The odd one was 0.9 and one particular portion of pipe had a flow efficiency of 0.35. The segment that was exhibiting very low flow efficiencies is marked A-B. The specific characteristics of that segment were:

d = 4.28 inches (109 mm)

Q = 20 MMSCFD (563 103m3/ d)

Δp/L = 114 psi/mile (491 kPa/km)

EPH = 0.35

Normally a flow efficiency, EPH, of 0.9 (approximately) is expected for the PanHandle equation. As shown above the measured flow efficiency was 0.35. At first this was thought to be caused by multiphase flow of gas condensate. Rigorous use of multiphase flow correlations did not alleviate the problem. The reason for the low flow efficiency, as determined from the methods to be described later was that the flow rate of 20 MMSCFD (103m3/d) was excessive, considering the pipe size and pressure levels. The simple fact of the matter was that the PanHandle equation was simply not applicable in such a situation.

Friction Pressure Drop Methods

The above case history illustrates that a flow efficiency of 0.9 is not valid for all situations encountered in practice. A detailed study comparing the "flow efficiency" method to the more rigorous Fanning friction factor method was undertaken and is the subject of this paper.

In the engineering industry, there are four common ways of calculating pressure drop for a gas flowing in a pipeline. These are the Weymouth Equation, the PanHandle Equation, the Modified-PanHandle Equation, and the Fanning friction factor method. Of these, the latter is the most rigorous, and in this presentation, is considered to be the bench mark against which the other three methods are compared.

This content is only available via PDF.
You can access this article if you purchase or spend a download.