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Summary. When a newly constructed natural gas pipeline is put into service, it can be safely purged of air by injection of a slug of inert gas, such as N2. The method of sizing the required slug is based on a model of dispersion in turbulent flow in conjunction with flammability limits.

Introduction

When a newly constructed natural gas pipeline is put into service, the initial air in the line must be purged. If natural gas is used directly to perform the purging, the air and the natural gas will mix, and flammable mixtures will form in the pipeline. The safety hazard such mixtures present is of concern. In the following discussion, the purging problem is analyzed. First, the hazards of flammable mixtures of the type that can form during purging of a natural gas line are discussed. Then it is demonstrated that inherent dangers are low and can be eliminated by introduction of an inert gas slug between the air and natural gas. A method of sizing the required slug is developed. Measurements taken during the purging of a 10-in. [25.4-cm] pipeline have been analyzed and are shown to be in good agreement with calculated performance.

Recommendations for future purging operations are presented. A step-by-step procedure for sizing of inert-gas purging slugs is given.

Hazards of a Flame

The composition of any mixture of combustibles, inerts, and oxygen can be represented on a ternary diagram. Fig. 1 shows such a diagram divided into three regions. Compositions in the smallest region are detonable. Those in the next largest region, which includes all detonable compositions, are flammable. The balance of compositions lies in the nonflammable region.

The danger of a gas mixture in a pipeline depends on its composition. Nonflammable mixtures present no danger. Flammable mixtures, if ignited, result in propagation of a flame and attendant temperature and pressure increases. As long as operating pressures during purging are maintained at reasonable levels, the limited stresses that might be caused by ignition of a flammable (but non-detonable) mixture are tolerable. Ignition of a detonable gas mixture, on the other hand, could lead to formation of a shock wave and large pressure increases. A detonation in a pipeline would be too dangerous to be tolerated.

A simple analysis of the effects of ignition of flammable gases tonned in the mixing zone during a hypothetical purging is presented below. Fig. 2 illustrates a purging operation. Assume that the gases in the flammable portion of the mixing zone (of length L,) are in-stantaneously bumed and result in a flame temperature about eightimes the initial absolute temperature. The increase in temperature of the burned gases will cause a proportionate increase in pressure, which will be dissipated throughout the pipeline as gas flows away from the heated region. If no gas can escape from the pipe, the maximum final pressure in the pipeline will be approximately

..........................................(1)

For example, with a flammable mixing length of 1 10 the length of the pipeline, an initial pressure of 40 psia [276 kpa], and a ratio of flame temperature to initial temperature of eight (which gives rise to a pressure ratio of eight),

pf=(40)[(0.1)(8-1)+1]=68 psia [467 kPa],

although a pressure spike of 320 psia [2.21 MPa] might be developed. Both pressures are well within the pressure rating of typical natural gas pipelines. Heat losses and the open (flow) nature of the system will cause the pressures developed in practice to be even lower. Peak temperatures are even less of a problem: if all heat is assumed to be instantaneously transferred to the pipe, calculations show that its maximum temperature increase is less than 2 deg.F [ 1.1 deg.C] for the average pressure scenario outlined. Taken together, the pressure and temperature analyses indicate that the effects of a simple flame on a pipeline are not of concern. Detonations, on the other hand, can produce pressure increases of greater than a factor of 40. The pressure rise occurs across a very thin zone traveling at supersonic speeds (i.e., across a shock wave sustained by the detonation). A 1,000-psig [6.9-MPa] -rated pipeline operating at an initial pressure of 40 psia [276 kpa] might not be able to sustain such a pressure pulse. The detonable region is therefore the most dangerous area for operation. Unfortunately, it is also the least well defined. Detonations can t>e prevented in practice, however, in several ways. For a detonation to form in a gas mixture, the mixture must be detonable, an ignition of the mixture must occur, and there must be a sufficient length of flammable gas for the flame to undergo a transition from deflagration to detonation. 3 If any one of these required conditions is prevented, the detonation cannot form. Each factor is discussed in detail.

First, the mixture must he detonable. This is a more restrictive requirement than flammability, as indicated in Fig. 1. Unfortunately, the detonable regime for gas mixtures is not well defined. Experimental data on the detonable range of individual combustible species are generally unavailable, and there is no well-established means of extrapolating the available data to arbitrary gas mixtures. As a general rule, hydrocarbon mixtures are not detonable in air at atmospheric pressure and temperature, but a gas high in hydrogen, hydrogen sulfide, ethane, propane, or acetylene content might be.

Although the formation of detonable mixtures is improbable, prevention of detonability can be guaranteed by ensuring that the gas is nonflammable. This is a restrictive approach, but assessment of flammability is a tractable problem. Unfortunately, development of a mixing zone within which the gases are flammable is virtually guaranteed when natural gas is used directly to purge air from a pipeline. Flammable mixtures can be avoided, however, by introduction of a slug of inert gas between the air and natural gas. If the slug is of sufficient size, only limited mixing of the air and natural gas will occur, and flammable (and hence detonable) mixtures will be prevented.

The second requirement for formation of a detonation is the ignition of the flammable gas. No known ignition sources exist during a pipeline purge. Nevertheless, a static discharge or a spark resulting from impact of some particle with the pipe wall might serve as a source of ignition. This is considered unlikely and is made more so by the nature of the purging operation, which generally takes place in the turbulent flow regime where ignition is more difficult. Nevertheless, ignition cannot be entirely ruled out.

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