GB2344058A

GB2344058A - Calibrating particle emission monitors

Info

Publication number: GB2344058A
Application number: GB9825585A
Authority: GB
Inventors: Alan David Leonard; David Whiteford Scott
Original assignee: AEA Technology PLC
Current assignee: Ricardo AEA Ltd
Priority date: 1998-11-24
Filing date: 1998-11-24
Publication date: 2000-05-31
Also published as: GB9825585D0

Abstract

Particulate emissions eg from an incinerator 12 are filtered by a baghouse filter 16 (or electrostatic filter, not shown) and monitored by an instrument 22 to ensure they do not exceed specified levels before being released to atmosphere. To calibrate the instrument, a variable fraction of the particle-laden gas stream from the incinerator 12 is arranged to bypass the filter and merge with the filtered gas in outlet duct 18 to provide various particle concentration levels for the instrument to measure. During shutdown of the incinerator 12, two bag filters 32 are removed and replaced with adjustable orifice units (40, Figs 2 and 3), which provide a variable size flowpath bypassing the filter. By adjustment of the orifice units with the incinerator operational, it is possible to achieve readings at the instrument over the whole range of expected particle concentration levels. Each reading is then compared with the particle concentration as measured by a standard reference method, to calibrate the instrument.

Description

Callbratina Particle Monitors The present invention relates to a process and to an apparatus for enabling particle emission detecting instruments to be calibrated, in particular to enable such instruments to be calibrated in situ.

Industrial plant and processes which emit particulate emissions into the environment, for example incinerators, are usually regulated. The regulation typically requires the emission of particles to be monitored continually to ensure that the emission levels are below an authorised maximum level. There is also a requirement for the particle monitoring instruments to be calibrated regularly in order to ensure that the recorded data is valid. If this calibration is performed at just one particle concentration level (such as the normal operational level), this may lead to serious inaccuracies if levels fluctuate; however calibration over a range of different concentration levels is difficult to achieve while the monitor is in situ, as it is difficult to adjust the level of particle emissions.

According to the present invention there is provided a procedure for enabling a particle emission detecting instrument to be calibrated, the instrument being installed in an outlet duct carrying a gas stream downstream of a particle-removing means, the gas stream entering the particle-removing means being laden with particles and most of these particles being removed by the particle-removing means, the procedure comprising arranging for a fraction of the particle-laden gas stream to pass through a duct bypassing the particle-removing means and to flow through the outlet duct along with that part of the gas stream which has passed through the particle-removing means, the bypass duct containing an adjustable restricter so that the said fraction can be adjusted, and adjusting this restricter so as to achieve desired particle concentration levels in the gas flowing through the outlet duct.

Preferably the restricter is adjusted so as to achieve at least three different particle concentration levels, and at each level the reading given by the instrument is compared to the particle concentration as measured by a standard reference method (such as that specified by BS 6069 or BS 3045). In a preferred procedure four different particle concentration levels are achieved in turn: a level near the normal operating level, a level somewhat below the authorised limit, a level somewhat above the authorised limit, and a level corresponding to that which would occur if part of the particle-removing equipment were to break down.

The procedure is applicable where bag filters are used to remove dust in the gas stream from an incinerator. In this case the bypass duct may be provided by removing one (or more) of the bag filters, and replacing each one with an adjustable restricter.

This restricter may comprise a gate valve, with a valve plate which may be rotatable or movable linearly.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a diagrammatic side view of an incinerator plant which includes a bag filter unit (shown in section) ; Figure 2 shows a side view of an adjustable restricter of the invention ; and Figure 3 shows a view in the direction of arrow A of figure 2.

Referring now to figure 1, an incinerator plant 10 includes an incinerator unit 12 with an air inlet 13 and an exhaust gas outlet duct 14. The duct 14 communicates with four bag filter units 16 (only one of which is shown) operating in parallel in which particulate material is removed from the gas stream, and the cleaned gas streams are then passed via a common outlet duct 18 to a stack 20. The level of particulate material in the cleaned gas stream is continuously monitored by an instrument 22.

The bag filter unit 16 is of a conventional type and consists of a rectangular chamber 24 divided by a horizontal plate 25 into a lower portion 26 and an upper portion 28. The lower part of the lower portion 26 tapers to form a hopper 30 in which dust collects during operation, and at its base is a removable door 31 by which the dust can be extracted. The plate 25 supports 100 cylindrical fabric bags 32 in a rectangular array (only four are shown). Each bag 32 passes through a respective circular hole in the plate 25 and includes two spaced-apart snap rings 33 near its mouth which seal against the top and bottom surfaces of the plate 25, so holding the bag 32 in position. A hatch cover 34 enables operators to access the upper surface of the plate 25. A baffle plate 36 is supported adjacent to the inlet duct 14 to distribute the gas stream throughout the lower portion 26. The particle-laden gas stream from the incinerator unit 12 is passed through the inlet duct 14 and a damper valve 38 into the lower portion 26; some sedimentation of the larger dust particles occurs, while the remainder of the dust is collected as a cake on the e outside of the bags 32, and filtered gas passes into the upper portion 28, and so emerges through the outlet duct 18.

The instrument 22 monitors the particle concentration in the cleaned gas stream continuously, the typical level being about 1 mg/m, and this level must not exceed an authorised limit which might typically be 10 mg/m. At intervals it is necessary to ensure that the instrument 22 is correctly calibrated. This is done using the following procedure: two bag filters 32 from a unit 16 are removed (for example during maintenance shutdown of the plant 10), and are replaced with adjustable orifice units 40 as shown in figures 2 and 3 to which reference is now made. Each orifice unit 40 comprises a generally cylindrical aluminium cup 42 which is of the same diameter as a bag 32 and can be sealed to the plate 25 with a snap ring 33. The base 44 of the cup 42 defines a semicircular aperture 45 (partly shown in broken lines in-figure 2). A generally semicircular valve plate 46 is pivotally connected to the centre of the base 44, and its orientation can be adjusted, using a handle 48, so it obstructs some or all of the aperture 45.

Initially the valve plates 46 on the orifice units 40 are both set so the aperture 45 is closed. The plant 10 is then started up, and the flow of gas through the bag filter unit 16 is allowed to stabilise. When the gas flow is stable and the particle level concentration is typical of normal operation, the instrument 22 is calibrated. This is achieved by comparing the measurement as shown by the instrument 22 with the actual particle concentration measured by a standard extractive reference method, such as that described in BS 6069.

The gas flow into the bag filter unit 16 is then closed off using the damper valve 38, and one orifice unit 40 is adjusted to provide a small bypass flow of particle-laden gas (the operator gaining access via the hatch cover 34). The damper valve 38 is then opened again, and once the flow of waste gas has stabilised measurements are again taken, both of the reading of the instrument 22 and of the actual particle concentration measured by the standard extractive reference method, if the particle concentration has a desired value ; if it does not have a desired value then a further adjustment is made to the orifice unit 40.

This procedure is repeated a number of times, the preferred particle concentrations being respectively: -between two and five times greater than the' normal particle concentration; -about 25 percent less than the authorised limit; -about 25 percent greater than the authorised limit; and -at a particle concentration corresponding to the level which would occur if a filter bag 32 were to burst (typically 100 mg/m).

It may be possible to achieve all these levels of particle concentration by adjustment of just a single orifice unit 40, or its may be necessary to use both orifice units 40 to achieve the higher levels. The measurements obtained enable the instrument 22 to be calibrated over the entire range of expected particle concentrations. When all these measurements have been taken, the gas flow into the bag filter unit 16 would then be closed off again using the damper valve 38, and the orifice units 40 replaced either with filter bags 32 or with blanking caps (not shown).

It will be appreciated that the process of the invention is applicable to a wide range of particlegenerating industrial processes, and that it might be applied where other particle-removing plant (such as electrostatic precipitators) is in use. It will also be appreciated that the adjustable orifice may be of a different type to that described above, for example it might include one or more linearly movable gate-valve plates.

Claims

Claims 1. A procedure for enabling a particle emission detecting instrument to be calibrated, the instrument being installed in an outlet duct carrying a gas stream downstream of a particle-removing means, the gas stream entering the particle-removing means being laden with particles and most of these particles being removed by the particle-removing means, the procedure comprising arranging for a fraction of the particle-laden gas stream to pass through a duct bypassing the particle-removing means and to flow through the outlet duct along with that part of the gas stream which has passed through the particle-removing means, the bypass duct containing an adjustable restricter so that the said fraction can be adjusted, and adjusting this restricter so as to achieve desired particle concentration levels in the gas flowing through the out'et duct.
2. A procedure as claimed in claim 1 wherein the restricter is adjusted so as to achieve at least three different particle concentration levels, and at each level the reading given by the instrument is compared to the particle concentration as measured by a standard reference method.
3. A procedure as claimed in claim 2 wherein four different particle concentration levels are achieved in turn: a level near the normal operating level, a level somewhat below the authorised limit, a level somewhat above the authorised limit, and a level corresponding to that which would occur if part of the particle-removing equipment were to break down.
4. A procedure as claimed in any one of the preceding claims wherein the particle-removing means comprises a plurality of bag filters.
5. A procedure as claimed in any one of the preceding claims wherein the restricter comprises a gate valve.
6. A procedure as claimed in claim 5 wherein the valve has a rotatable valve plate.
7. A procedure for calibrating a particle concentration instrument substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.