Sample probe insertion depth in gas pipelines with liquid contaminants.

Author:George, Darin L.

Liquid contaminants in natural gas pipelines have recently become an increasing concern. Typical liquid contaminants can include water and hydrocarbons condensed from the gas stream, glycol carryover from inefficient equipment or process upsets, methanol injected to avoid hydrate formation, and compressor oils.

These contaminants are usually separated and filtered from the gas stream before delivery to a pipeline. However, separators and filters are never 100% efficient, so there will always be traces of contaminants that can enter the pipeline, and ambient temperatures below the dew point of the gas stream can cause condensation in the pipeline itself. Only representative samples of the gas phase are required for custody transfer purposes, so the ability to reject liquids at the sample probe is of interest to natural gas pipelines.

Industry practice requires that natural gas sample probes be inserted vertically from the top of a straight, horizontal pipeline run, with the tip of the probe in the center one-third of the pipe cross-section. Both of these requirements were created to keep the probe from capturing liquids or contaminants that may be migrating along the pipe walls.

However, the probe must also be short enough to prevent flow-induced resonant vibration from breaking the probe inside the pipe. Besides leading to contaminated samples, broken probes and debris in the pipeline can damage downstream equipment. (1,2,3) The latest API and GPA sampling standards include a formula (4) to calculate the maximum allowable probe length for avoiding flow-induced vibration, but in some cases, the probe lengths specified by this formula will not reach the center one-third of the pipe cross section.

To help resolve these conflicting probe length requirements, Pipeline Research Council International (PRCI) funded a series of projects at Southwest Research Institute (SwRI). (5,6) This article describes the findings of one part of that research, which experimentally assessed the effects of probe insertion length on the accuracy of gas samples drawn from gas liquid hydrocarbon pipe flows.

The ultimate goal was to recommend minimum probe insertion depths for collecting representative natural gas samples where contaminants may be present. The tests described here evaluated the performance of a straight-cut sample probe in annular flows and annular-mist flows of methane and hydrocarbon liquids. The findings can be useful to meter station designers and others whose work involves collecting samples from gas pipelines with liquid contaminants.

Multiphase Facility

SwRI's Multiphase Flow Facility (MFF) is a closed flow loop capable of supplying various combined streams of gas and liquid to a test section for meters and other equipment. The MFF can operate with natural gas, crude or refined oil, hydrocarbon condensates, water and combinations of these fluids at pressures from 100-3,600 psig and at liquid volume flow fractions (LVFs), ranging from 0-100%. For the sample probe tests, a heptane distillate was chosen to represent a generic liquid contaminant in a gas pipeline because its presence could easily be identified through sample analysis. A supply of methane of 99.7% purity represented the natural gas stream.

Figure 1 shows the test section layout. After the gas and liquid phases were measured separately, the phases were recombined in a 6-inch-diameter line and passed through a reducer to a 3-inch-diameter flow development section. The flow development section had a length of 244 pipe diameters, long enough to produce a fully developed annular flow at an isokinetic reference probe and the sample probe under test. Downstream of the probe, an axial flow visualization spool allowed video recordings of the flow.

Sample Probes

Evaluating the performance of the sample test probe at different insertion depths required reference...

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