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Explosion Protection For The Dairy Industry

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Fires and Explosions in the European dairy Industry were becoming an increasing problem. In the early 1990's two groups of people from the industry, insurance providers and suppliers of protection equipment came together to analyze events and case studies, to find a new approach to solve the persistent problem.

The Dairy Industry Problem

The first now statistically confirmed finding was that the problem really was a fire hazard rather than a problem belonging to the area of explosion hazards. The analysis of a statistic database on 116 incidents in the milk powder processing industry showed that five of these incidents experienced an explosion, while another three had explosion like effects that could be traced to pressure effects of an explosion. The remaining incidents were fires only, with damages ranging from medium to total loss of the drying unit.

More than 90 % of these incidents could be traced directly back to self-healing processes within the drying installation. The five (eight) explosions in most cases, if not all, were found to have been a consequence of the fire, not the other way around.

Another analysis looked at 240 incidents that occurred in spray driers in the food industry from 1953 to 1993. 20 incidents were reported to have experienced an explosion, 210 were fires only. Even though the data of this source did not allow tracing the cause, it is considered that in most cases, if not all, the fire was the initiator of the explosion. The result of the statistic analysis defined the further steps of the new approach:

 

The First Groups Findings: Ten percent of all Fires became explosions. By studying the plant details ignition sources such as internal mechanical and/or outside introduced ignition sources were ruled out. Solution: The problem could be solved if an appropriate detection method can be found to detect heating before it develops into open combustion.

 

Simultaneously a group of researchers in the UK and Ireland were studying the same phenomena. The researchers studied Irish Dairy Industry Incidents from 1980 – 1987. During that period 12 incidents were reported involving fires in spray drying plants. In 5 of these cases, explosions were reported. Also studied were UK Dairy Industry Incidents from 1972 to 1982. During that period 7 explosions were reported in spray drying plants.

The Second Groups Findings: The researchers in Ireland and the UK decided that the dairy industry required a deflagration protection system with the following capabilities:

Effective reduction of the overpressure to a safe level

Large volume protection

Non-contaminating suppressant

Non-explosive actuation

Easily maintained

Low maintenance costs

 

 

 

 

 

 

 

 

 

 

Early Solutions – Prevention and Protection

From their independent research and concerns, the result was naturally two different directions. The first group focussed on fires as the cause. Therefore they surmised that if fires could be detected and prevented very early most spray dryer explosions could be prevented. The second group understanding the problem as the event itself decided that a solution was a method to suppress the event as it developed. But a method of suppression was not yet available that meets the true needs of the industry.

Prevention

In spray driers, for the production of milk powder, fires and explosion are a serious problem. Basically all four necessary preconditions for an explosion can be present:

  • Fuel (milk powder) powder deposits and swirling dust 
  • Oxygen
  • Ignition sources – Decomposed Milk Products 
  • Confined Space

With fuel and oxygen in abundance, the only requirement to complete the combustion triangle was an ignition source. If the ignition source could be eliminated or mitigated a deflagration might be prevented in most cases.

Operating experience shows, that the primary source of ignition for fires or dust explosions in drying installations or in secondary installations such as filters and dust precipitators are smoldering spots or self-igniting milk products. Even with the best safety measures and process controls in place, it is not possible to effectively prevent the deposit of milk products in spraying drying devices or air diffusers, nor can caking on the vessel walls be avoided. The danger exists that, through long-term exposure to hot air, a thermic decomposition of the deposits will be initiated and that this will lead to smoldering spots and/or self-ignition of the products.

Depending on moisture, fat content, and on the airflow, smoldering nests in milk products will form solid and compact structures, to which newly added products will continue to adhere. Because of the bad diffusion of oxygen through the pores, the smoldering nests will expand from inside outwards rather slowly. Various conducted tests have indicated, that small smoldering spots of milk products have quite low surface temperatures and, that therefore they are not very effective sources of ignition for their dust-air mixtures. The low surface temperature makes them very hard to see with standard IR sensor technology until they break apart exposing the hot surface providing an ignition source.

As a rule, such compact glowing deposits will only become a source of ignition when, they detach themselves and fall into the lower tower areas, or are transported into secondary installations, where potentially explosive dust-air mixtures may be present. As a consequence, it is essential to detect smoldering material in an early stage, in order to be able to take appropriate measures. If a method could be developed to detect the growth of these deposits at an early stage the product flow could be stopped eliminating the hazard. The potential ignition source can then be dealt with manual and /or automatic means before product supply was returned.

Early-Warning Fire Recognition Through CO detection

Early recognition of smoldering fires at an initial stage is possible through inspection of the exhaust air from drying installations for the presence of carbon monoxide, a gas which is the product of the thermic decomposition of milk products. Because of the high airflow rate within milk powder drying installations, the produced carbon monoxide is diluted so strongly, that an extremely sensitive measuring system is required, in order to be able to detect small smoldering fires at an early stage.

With the usual exhaust air volumes up to 100,000 m3 /hour, an increase of the CO content in the exhaust air of less than 1 ppm can be an indication of a smoldering spot. On the other hand, due to environmental contamination, it is possible that the air intake to the drying installation is biased and already contains substantially higher concentrations of CO, which would lead to a false alarm from the early-warning system.

This problem could be solved by means of differential measurements between the air intake and the exhaust, where only the CO content actually produced in the drying apparatus is taken into consideration.

Infrared Gas Analysis and the Challenge

The characteristic of the heteroatomic gas CO, to absorb infrared light in specific bands between the frequencies 2.5 and 12 pm, is used in infrared spectroscopy to provide a means for determining concentration levels. With NDIR, the Non-Dispersive Infra-Red Absorber, a measuring principle is available which is suitable for the detection of traces of carbon monoxide levels. NDIR CO gas analyzers, with a measuring range of 0 to 10 ppm, have been tested in the area of emission control under harsh conditions, allowing them to be considered as reliable means for this type of measurement.

To accomplish the measurement small gas samples would be continuously extracted from the drying apparatus and pumped through a measuring cell, which has been fitted with windows that permit infrared rays to penetrate. A ray of light, which is directed through the windows and penetrates the gas, is weakened in the area of certain frequencies before it meets the detector. This absorption correlates with the CO concentration and is defined by Lambert-Beersch's law:

 

 

The required calculation of the difference between the exhaust air and the air intake of the drying process would require a procedure using one analyser that would require extensive time to review gases in a series procedure or using two analysers to accomplish a quicker comparison. Both methods would require an external processor to determine the differential result. The choice was time or cost, neither one acceptable. Researchers understood that if both samples could be compared together then the time would be cut in half with a more economical system. By using a so-called cross flow-modulation procedure the air intake of the drying apparatus is used as a reference gas. The exhaust air sample, which is to be measured, and the reference gas, is alternately introduced into the measuring cell through a micro solenoid valve. In contrast to other infrared analysis techniques, the optical path in the cross flow process is the same, for both the gas to be measured and the reference gas, considering optical cutter (diaphragm) is not required for the calculation of a difference, which reduces the signal noise and the sensitivity to contamination.

 

 Principle of Cross-Flow Modulation

As shown in the Figure below, the sample gas and the reference (zero and inlet) gas are alternately (Frequency =1 Hz) sent to the measurement cell at the specified flow rate by continuously switching the solenoid valves. In case of an incident in the spray drier, the sample gas (outlet) will contain more CO (which absorbs infra-red light) than the reference gas. The infra-red light intensity reaching the detector is therefore modulated. The magnitude of this modulation is the basis of this measurement.

 

Infra-red light generated from the infra-red source passes through the measurement cell and enters a detector containing the gas to be measured. When zero gas is sent to the measurement cell, more infra-red light reaches the detector. On the other hand, when sample gas is sent to the measurement cell less infra-red light reaches the detector. The degree of this infra-red light attenuation is related to the concentration of CO gas in the measurement cell.

The detector contains a movable membrane that detects pressure changes in the optical cell. If there is any difference in absorbed energy between the reference gas and the sample gas a pressure change will occur within the optical cell and therefore be detected. This difference is amplified and output as an electrical signal. As such, no membrane displacement occurs when the concentration of the measured gas does not change during a cycle (normal Spray Drier operation, the sample gas concentration is the same as the reference gas concentration). Therefore when the same gas is sent to both the sample and the reference lines in the APMA 370, the detector produces a zero output with essentially no drift. A detector set-up using two optical cells in series is used to sense measured components and interference components in the front cell (MAIN) and is used to sense mainly interference.

Basic Design Of An Early-Warning Fire Detection System

An early-warning fire detection system, which is based on CO detection, consists of the following main components:

  1. Gas sampling probe
  2. Sample gas preparation
  3. Analyzer
  4. Process system controller interface

To be acceptable as a Prevention Alternative a Co Early-Warning Fire Detection System must provide quick recognition of smoldering fires while avoiding false alarm activations, considering the associated process downtime cost.

First, the Process System must be analyzed to determine the proper points of sampling to get a true picture of the systems process flows. In summary, probe locations must assure all air in = all air out are measured with respect to the CO content of the air.

Because the exhaust gases are loaded with dust and have a high dew point temperature, a dust and water removal system is required. The input air to the analyzer must first go through various air preparations to remove moisture from the line and balance the flows to the analyzer. This ensures that the measured signal can be stably obtained with the minimum interference effects. The reaction time of the system is an additive combination of the time, during which the air is present in the system, the residence time in the gas sampling lines, and the device response characteristics of the analyzer and system.

To avoid a false alarm, caused by a sudden rise in CO content in the intake air (for instance as a result of heavy traffic), the transit time in the sampling lines must be balanced. The gas sampling lines to the analyzer should be kept as short as possible, in order to avoid unnecessary delay times. Note: This proved to be an insurmountable task when considering direct-fired dryers.

Since infrared gas analyzers are very sensitive to contamination, the gas samples must be carefully preconditioned, by removing humidity and product residues. H20 is the chief interferent in the NDIR measurement of CO. The output from the COMP detector is then used to correct the main signal for any concentration of this interferent which might be present. The correction is made in real-time.

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