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CEMs Drift Adjustment

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Calibration Failure on CEMs Unit

  1. Determine which part of the calibration failed. It is easiest to use the print out to see which one and how much.
  2. Check the calibration bottle pressures outside of the shack. One could have run out.
  3. Check alarms to see if there was any flow or pressure alarms on Dahs computer in control room.
  4. If there is a Low Flow Alarm then follow these steps:
    Note: See attached diagram.
    1. Check the calibration bottle pressure of the gas with the flow alarm. It should be above 200 lbs.
    2. Take the appropriate control (SCR or Stack) out of service.
      1. Go to the control screen by selecting its button.
      2. Select the In Service button along the bottom of the screen.
    3. Select the down arrow.
    4. Select the on/off button for the gas with the low flow alarm from the buttons along the bottom of the screen.
      1. Check that the flow meter labeled calibration flow has gas flow above 6 clpm. Do not adjust with the dial on the flow meter. If necessary, adjust the pressure regulator on the bottle outside.
      2. If there is no flow, it may be necessary to check the gas solenoid. You should hear it click when the gas button is selected.
    5. When the alarm is satisfied, continue with the next step - keeping the control out of service.
  5. If there are no alarms or the alarms have been satisfied, follow these steps:
    1. Select out of service on the appropriate control.
      1. Go to the control screen.
      2. Select the In Service button along the bottom of the screen.
    2. Select the down arrow.
      1. Starting from the left, each of the gasses will be cycled one at a time to adjust the calibration. It is very important that after any gas is cycled, the unit not be placed into service until at least 90 seconds after the gas is stopped so the analyzer can begin to sample the correct gasses from the HRSG before going into service. It is also important to note that adjusting the zero will also adjust the span. Therefore, after adjusting any zero, the span must also be adjusted. This is why it is good to cycle through all gasses from left to right.
      2. Select desired gas.
        1. Allow at least 90 seconds to pass for gasses to saturate sample line.
        2. Verify the gas value with the analyzer reading. Other than the known zero, there will be an asterisk flashing beside the appropriate gas values that should be displayed on the analyzers after each gas is selected.
        3. If the displayed value is out of the allowed range, make necessary adjustments. See table for allowable drift:

          Allowable drift on Stack control

          Nox Low

          Nox High

          CO Low

          CO High

          O2

          Zero

          ± 0.5

          Zero

          ± 12.5

          Zero

          ± 2.0

          Zero

          ± 40.0

          Zero

          ± 1%

          Span

          ± 0.5

          Span

          ± 12.5

          Span

          ± 2.0

          Span

          ± 40.0

          Span

          ± 1%


          Allowable drift on SCR control

          NOx High

          COHigh

          O2

          Zero

          ± 10.0

          Zero

          ± 20.0

          Zero

          ± 1%

          Span

          ± 10.0

          Span

          ± 20.0

          Span

          ± 1%

          1. Rosemount
          2. Siemens
        4. Again, if the zero is adjusted, the span will also require adjustment. Avoid going back and forth between gasses.
      3. After each gas has been adjusted:
        1. Stop the gas flow.
        2. Wait 90 seconds for HRSG gas to saturate sample lines.
        3. Select the Out of Service button.
  6. Select the Cal Off button at the bottom of the screen on the desired control and a new calibration will be started. Verify that the calibration is accurate and works normally. It will automatically cycle through all the calibration gasses from left to right in the control screen and the asterisk will flash next to the values of the corresponding gas.

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