The Use of Particulate Emission Monitors to Support Arrestment Plant Operation and Upkeep

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Courtesy of PCME Ltd

The operation of filtration type arrestment plant can be optimised by the effective use of Particulate Emission Monitors (eg Filter Leak Monitors and Filter Performance Monitors).

Particulate Emission Monitors are installed throughout the broad spectrum of industrial processes to satisfy regulations (IPPC, Part B Guidance Notes, WID and LCPD) for emission limit enforcement. In addition, where processes are controlled with arrestment plant, these instruments also serve to provide feedback on the operation and performance of the arrestment plant itself. Specifically after filtration plant (bag-filter type), appropriate instruments may be used to reduce running costs and minimise emissions.

Bagfilters and cartridge filters, as their names imply, use bags or cartridges of various media types to filter out particulate from the exhaust stream. Filter bags are typically provided in various dimensions and are fitted over a metal cylindrical cage. Hanging vertically in rows within the-arrestment plant, filtration occurs when air passes through the outside of the bags into the middle of the cylinder and then up into the clean air plenum. Particulate is collected on the outside of the bag where it may form a !'cake!', which provides even more efficient filtration. Periodically the 'cake' must be removed from the bag so that the pressure drop across the bags does not obstruct the air flow through the filter media.

In most current filter plant, cake removal or cleaning of the filter media is carried out using a reverse jet pulse of air applied to the top of the cylinder which inflates the media. On inflation, the particulate 'cake' is cracked, the filter material's pores are opened and the particulate falls to the bottom of the filter plant hopper where it is collected. After the air pulse is removed, the filter media recovers to its original shape and resumes filtration.—During the cleaning process there is a small discharge of particulate, which shall be referred to as a 'pulse1, when the particulate in the filter (in addition to the 'cake'') has been dislodged. The cleaning mechanism in such filtration plant typically occurs sequentially within a defined time period (e.g. 30 seconds) between the cleaning of each row.

An appropriate particulate monitor fitted in the duct downstream of the filter plant can monitor the pulses of dust created by the cleaning process and discriminate this from the filter plant's particulate emission during routine operation. Consequently, the dust profile monitored by such an instrument includes periodic dust pulses resulting from the bag cleaning sequence. The magnitude of these dust peaks reflects the condition of the filter media being cleaned, with a- failing or worn media generating a higher dust pulse emission peak than those operating efficiently. The cleaning pulse peaks present in the emission profile can be used to indicate the relative condition of the different rows of filter media and, therefore, isolate which row is failing ahead of catastrophic failure. Instruments designed to locate filter leaks can display this pulse profile on a graphical display and connected to the digital controller for the filtration plant's cleaning cycle so that the row being cleaned can be identified and marked alongside the appropriate dust peak.

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