Processors have relied on gas chromatographs (GCs) since the 1950s to provide real-time compositional data to control systems. Today, tens of thousands of GCs serve in a diverse range of plants — making the GC the analytical workhorse for online compositional measurements. The performance of these devices critically impacts the quality of the final product, the efficiency of plant operations and, ultimately, the bottom line.
The GC’s important role means that its downtime should be kept to a minimum and planned ahead of time, rather than taking place only when a failure has occurred. While maintenance may seem like an issue for technicians, in fact, it becomes a management problem when it so directly impacts profitability. Therefore, every plant manager should understand GC maintenance and troubleshooting practices and use them to establish a meaningful maintenance program and provide effective training. Ideally, the maintenance program should identify analysis problems before they cause measurement errors — allowing maintenance to be performed on a predictive rather than an ad-hoc or breakdown basis.
Here, we’ll look at maintenance best practices for both key elements of a GC setup — the sample handling system and the analyzer.
Sample Handling System
This system not only is crucial to the correct operation of the analyzer but also accounts for most of the physical maintenance required in a GC setup.
The role of the sample handling system is to:
• take a representative sample of the gas;
• remove particulates and free liquids;
• control the sample pressure, so that it’s within the analyzer specifications, usually between 10 psi (70 kPa) and 20 psi (140 kPa);
• transport the sample to the analyzer, maintaining its gas phase and composition;
• provide switching between sample streams and the calibration gas stream; and
• vent the sample to atmospheric pressure.
If the sample handling system is performing all these functions correctly, then the GC will operate reliably as designed and only will require downtime when the diaphragms eventually wear. Maintenance often isn’t necessary for 18 months or more when the sample system cleans and dries the sample correctly. Diaphragm wear can be identified, enabling overhaul on a planned basis.
The frequency of maintenance on the sample handling system somewhat depends on the location and quality of the gas being measured. Analyzer installations that are inside a production facility usually are very susceptible to contamination because they are exposed directly to process upset; thus, they will require frequent maintenance to maintain the integrity of the measurements. Installations that are at the final delivery points of the process will need less frequent maintenance because many of the contaminants will have been removed.
The following steps provide the basis for a maintenance and troubleshooting program for a chemical plant:
1. Frequent maintenance (from daily to monthly, depending on operating conditions and previous history):
• Check sample pressure. Verify sample pressure is set as commissioned. Note that a blockage on the primary filters of the sample probe can cause a lower-than-normal sample pressure. Setting the sample pressure at the probe higher than the required pressure at the GC can overcome the pressure drop in the sample lines.
• Check sample flows. Watch the sample flow rotameter as each sample is analyzed. While different flow rates or pressures don’t affect analysis on units that equalize the sample to the atmosphere, a change in the flow rates may signal that the filters are becoming blocked. Low sample flow also can indicate a blocked or pressurized vent line.
• Check sample line heat trace. Confirm the heat trace is working. Often all this involves is verifying that the sample line at the furthest point (usually at the sample probe) is warm to the touch.
• Check bypass flow rates. The flow rate through the bypass flow loop should be double that of the sample outlet. Inspect the cleanliness of the glass in the bypass rotameter. Contamination there indicates the entire sample line from the probe to the GC is contaminated and needs to be flushed.
• Check calibration gas bottle pressure. It should exceed the minimum pressure specified on the calibration gas bottle certificate (typically 70 psi or 500 kPa).
• Check carrier gas bottle pressure. Ensure that both carrier gas bottles have sufficient pressure to last until the next routine maintenance inspection.
2. Less frequent maintenance (from monthly to every six months):
• Remove the sample probe and install a new moisture membrane filter (if fitted) and primary filter.
• Replace the particulate filter cartridge.
• Change the membrane in the moisture filter/shutoff.
3. System overhaul (done when conducting maintenance on the GC, whether planned or due to breakdown):
• Perform all the steps listed under less frequent maintenance.
• Flush the entire sample line(s) with acetone or isopropyl alcohol, and dry with helium.
• Leak-check all the connections throughout the sample handling system.
Routine maintenance on the GC analyzer (Figure 1) primarily involves performing diagnostics and making small changes to timed events to allow tracking the gradual increase in the retention time due to contamination and wear in the analysis valves. When the diagnostics indicate a large shift in retention times from the last overhaul or a dramatic increase in valve noise, then an overhaul of the analysis valves can be planned for a convenient time.