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Can I Mount my Opacity Monitor on a Positive-Pressure Duct?

Opacity monitors are used to measure emissions of smoke and dust from combustion sources. They are mounted directly onto the stack or duct, so a constant flow of purge air is needed to protect their delicate optics from the hot, dusty, and corrosive stack gases.

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Most stacks have a negative pressure, with the stack’s draft causing it to operate slightly below the surrounding ambient environment. However, some installations require the instrument to be mounted earlier in the process, such as before the induced draft (i.d.) fan. Such locations often have positive pressure relative to the local ambient air.

The purge blowers supplied by AMETEK Land are suitable for almost all stack pressures and temperatures. A single blower can provide sufficient purge air for both transceiver and retro mounted on a duct with up to 6” water gauge (15 mb) positive pressure. For higher pressures, separate blowers should be installed on the transceiver and retro. Note that the flange seals may leak if the process pressure is greater than 10” water gauge (25 mb), though such applications are extremely rare.

One of the biggest practical problems for an opacity monitor mounted on a positive-pressure process is what happens when the instrument is swung open for a calibration audit or for servicing. In these circumstances, the hot, toxic flue gases will escape from the process, endangering both the instrument and the service engineer.

4500 MkIII (Compliance Opacity)

To protect both the instrument and the service personnel, we recommend that an isolating device is provided on all positive-pressure installations. A manually operated ball valve with minimum 60 mm (2.5 inch) clear aperture can be welded in place while the standpipes are being installed, but a more elegant solution is to include fail-safe shutters. These are sandwiched between the standpipe and the transmissometer flange, and they open as soon as power is applied to the instrument. They close automatically if  power is lost or if they receive instruction from the 4500 MkIII via the AFU-APS-I/O unit. The instrument instructs the shutters to close if it detects a loss of purge air and also during the daily calibration check. Closing and reopening the shutters each day reduces the likelihood that they will become stuck in the open position so that they will not close when required.

The protection offered by the fail-safe shutters is especially important on positive-pressure applications. If the instrument’s purge air supply fails on a negative-pressure duct, the pressure difference will draw a small flow of ambient air into the duct. On a positive-pressure duct, the opposite occurs, and the flue pressure will immediately force a flow of hot, corrosive stack gases into the instrument’s purge housings and through the purge blower. As a consequence, severe damage can occur after even a brief purge failure.

An added benefit of shutters is that they can be closed before manual service, thereby isolating the instrument from the stack. This allows the service or audit to be conducted safely and without risking damage to the instrument.

In conclusion – an opacity monitor can be installed safely and effectively on a positive pressure stack, and a single blower provides sufficient purge air in most circumstances. Fail-safe shutters provide additional protection to both the instrument and to service personnel during routine maintenance, and in case the purge air fails.

Click here to learn more about the 4500 MkIII


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