November 15, 2012

Plugged Diffusion Element Diagnostic Feature

Doug Simmers here. In situ analyzers operate very well in high-particulate flue gases resulting from processes such as coal or biofuel boilers or lime and cement kilns because the passive filter, or “diffusion element,” does not foul or plug easily. Since the probe sensing cell is inserted entirely into the flue gas stream, the process gasses can diffuse (migrate) into the cell area with minimal fouling of the filter media. This is not to say, however, that diffusion elements do not plug. After many months or years of operation, the operator at the DCS console may notice that the O2 measurement is not as active as previously or that the speed of response back to the process (purge time) after calibration gases are removed has increased considerably. When a diffusion element is new, it comes back to the process value in 3-5 seconds (Tinitial), and will be all the way back to the process reading in 30-40 seconds (Tfinal). As the diffuser plugs off over many months, these times will get longer and longer and can cause significant operational problems. New diagnostic technologies can help to overcome this risk. The Model 6888 probe with plugged diffuser diagnostics is currently in Beta testing, and is expected to release in January of 2013.

Another indication of a plugged diffuser is a large increase of the “cell constant” after a calibration. A slower speed of response not only delays O2 information for the operator or the automatic O2 trim control loop; it can also cause technicians to induce a calibration error while doing calibrations. Published specifications call for a 5 SCFH flow of calibration gases with a new diffuser. This slightly pressurizes the cell area with cal gas, ensuring that no flue gases mix in with the calibration test gasses. As the diffuser plugs off over time, the calibration flow rate will drop and the cell area becomes increasingly more pressurized. Pressurizing the sensing cell during the calibration procedure will induce an error in the O2 reading once the calibration gases are removed and pressures return to the normal operating duct pressures.

The O2 reading will be shifted lower by 1.5% of reading (not 1.5% O2, or 1.5% of full scale) for every seven inches (177.8mm) of water column pressure induced during the calibration.

The calibration induced error will be further increased if the calibration gas flow rate is adjusted to compensate for the reduced flow caused by the plugging diffuser. For example, an instrument technician may do a calibration on a probe with a badly plugged diffuser. He notices when he opens the bottle and sets his pressure regulator to 20 PSI that the flow meter is reading 2 SCFH instead of the normal 5 SCFH. He tries to readjust the flow rate on the flowmeter, but still cannot get the specified 5 SCFH, so he adjusts the pressure of the pressure regulator upward until he can get the 5 SCFH flow. This results in doing the calibration with a pressure on the cell, for example, of 2 PSI (approximately 56 inches/1422.4 mm of water column). When he removes his calibration gases, and the probe sensing cell returns to the normal duct pressure of -1 inch/25.4 mm of water column, the probe will be reading low by approximately 0.5% O2:

  • 56 inches (1422.4 mm) H2O pressure during calibration = 12% of reading shift (1422.4 mm H2O / 177.8 mm H2O  x  1.5%  =  12%)
  • If normal process O2 readings are 4% O2 then 4 x 0.12 = 0.48 % O2 shift downwards (3.52% O2)

Although instruction manuals are clear in warning against this situation, it still occurs.

The new Plugged Diffuser Diagnostic in the Xi electronics operates on the principle of measuring the “return to process” time during the calibration purge cycle (after the second calibration gas is removed). The return to process time measurement is used to calculate a diffuser response time (Diff T90) and to generate a warning to the user when the return to process time has exceeded 75% of the configured purge time. Default purge time is 300 seconds, but it’s important for the user to configure the actual purge time for their process conditions. A purge time that is too long will prevent a plugged diffuser alarm from triggering until the diffuser pluggage is very bad. If the purge time is set too short, and probe has not fully returned to the process reading after the configured purge time expires, then the process reading will be in error due to test gas not being fully purged from the sensing cell area. As the diffuser becomes more plugged, the time to fully purge the sensing cell area of calibration test gas will increase.

The plugged diffuser feature works best with an automatic calibration system, where the probe electronics knows the exact timing of the solenoid gas switching. The diagnostic will also work for manual calibrations by detecting milivolt changes at the sensing cell during the stop gas phase of the calibration, when the second calibration gas has been disconnected.

In addition to providing a warning of possible diffuser pluggage, this feature can also be used to minimize the usage of calibration test gas and the time to perform a calibration. This capability is enabled by configuring the Auto Advance Cal parameter. When Auto Advance Cal is enabled, the plugged diffuser diagnostic will advance an automatic calibration when the readings for test gasses and the process measurement have become stable. By doing this, the amount of time that test gasses flow and purge are kept to a minimum.