June 1, 2010

Real-Time Monitoring of Natural Gas Fuel Cleanliness

By David Burns, PECOFacet/Scientific Process Solutions Inc., a Clarcor Company

Method for Gas Pipeline Contaminant Monitoring

Natural gas–fired power plants utilize gas chromatographs to monitor gas quality and the energy level of fuel gas. A gas chromatograph will monitor the percentage of each of the gaseous components and can even be set up to check for the presence of gaseous contaminants like H2S, CO2, and nitrogen. However, a chromatograph can not be used to accurately monitor the presence of pipeline liquid and solid contaminants. Gas chromatographs utilize filtration systems to protect their gas separation columns by removing such contaminants prior to analysis.

A technology has been developed to monitor solid and liquid contaminant levels in high-pressure gas pipelines. Contaminant monitoring in high-pressure volatile gas pipelines produces a special set of design challenges. These two design challenges have eliminated many promising technologies from the application of particle contaminant monitoring and measurement in gas systems:

• The gas must be sampled and analyzed at pipeline pressure and temperature. Changes in pipeline pressure and temperature will condense or vaporize some gas components and may change the size and number of other contaminants.
• Contaminants coat sensor surfaces over time, resulting in output signal changes.

When selecting a technology to perform contaminant measurement, the fluid mechanics of how contaminants behave in a pipeline must be considered. Understanding the combined effects of turbulent and laminar barrier flow on typical pipeline contaminants is key. For example, the data in the table for Plants 1 and 2 are very typical for gas turbine fuel. The data from these plants tell us that most fuel gas lines are fully turbulent. We also have calculated that droplets larger than typical household dust are going to be sheared and atomized into smaller aerosol droplets. If the liquid contaminant content is high enough, a liquid film will build up in the laminar flow boundary layer. The laminar to turbulent transition layer is going to place the gas liquid interface under constant shear conditions. Gas liquid interface sinusoidual wave fragmentation will occur.

Therefore, we concluded that if liquid is present, it will be in aerosol form at low liquid concentrations and a combination of aerosol and film flow at higher liquid concentrations. In either case, aerosols will be present.

A contaminant measurement and monitoring technology must be used that can detect and measure aerosols as well as solid particles, if they are present. The technology must also handle contaminant coating and perform its analysis at pipeline pressure and temperature. As particle size measurement was determined to be the most important parameter to monitor, laser-produced light-scattering technology was selected for the application. Laser particle counters have gained wide acceptance as clean room dust and aerosol monitors (Figure 2).

2. Tracking down contaminants. The PlantGard high-pressure gas pipeline contaminant monitor can detect and measure aerosols and solid particles. Data from the monitoring system are normally communicated by serial or analog cable to plant digital control systems. Source: PECOFacet/Scientific Process Solutions Inc.

The system developed includes a sampling probe that is inserted about one-third of the way past the pipeline wall. The sampling probe will flow a continuous sample of gas up through a sampling line that feeds the laser particle-counting flow cell. After passing through the flow cell, the discharge gas is pumped back into the pipeline with a pneumatic-driven compressor that is an integrated part of the monitoring system.

3. On the alert. The PlantGard web dashboard is shown recording changes in the natural gas’s cleanliness. Source: PECOFacet/Scientific Process Solutions Inc.

Data from the monitoring system are normally communicated to the plant digital control system (Figure 3). A real-time data stream is also web-based for secure viewing from any Internet connection. Particle count trend lines are monitored for each particle size channel or registry. Alarm level thresholds are set for critical registries. Increasing levels of contaminant will always increase the particle count. Experience has proven that film flow will produce a proportionally increasing aerosol content as the film content increases. The particle count data are used to measure the relative amount of contaminant flowing in the line (Figures 4 and 5).

4.    Clean bill of health. The clean gas flow in the pipeline illustrates normal clean gas fuel conditions. Source: PECOFacet/Scientific Process Solutions Inc.

5.    Caught in the act. The wet gas flow in the pipeline illustrates a liquid contaminant upset. Source: PECOFacet/Scientific Process Solutions Inc.

Particle size distribution data help indicate what’s happening in a pipeline. For example, a coalescer element seal failure will continuously produce particles in the 1- through 8-micron range, whereas a properly working filter coalescer will have an effluent stream containing only low levels of particles less than 1 micron. A coalescer element starting to lose its removal efficiency due to solids loading will produce a gradual increase over time of all particle sizes. Liquid slugs from the upstream pipeline result in an immediate jump in particle counts. Cataloging contaminant events to their resulting particle size distributions has proven to be a powerful diagnostic tool.

New Technology Improves Plant Performance

The innovative use of laser light–scattering technology is an effective method of monitoring liquid and solid contamination levels in high-pressure gaseous pipelines. Turbulent gaseous pipeline aerosol behavior produces a contaminant particle size distribution that can be used to diagnose process problems and filtration equipment performance. The technology’s ability to monitor fuel gas cleanliness levels is providing data that are now being used to successfully confirm the quality of gas supplies and troubleshoot plant performance issues.