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In most installations, process analyzers are dedicated to one process stream, and if the signal is used for closed-loop control, especially if a chromatograph or analyzer with a longer analysis time is used, this is certainly true. With the increasing use of spectroscopic analysis, there's a tendency to having multiple streams connected to one analyzer, though the price is added complexity with the sample system.
When designing an analyzer system, the following points are important considerations:
Note that in this list only the first two bullets relate to the measurement itself. The balance of the list addresses what I refer to as "ancillary systems," since they're not directly in contact with the process. In general, these items are also normally associated with the analyzer housing.
Because on-line analyzers tend to be more complex and large, they usually require environmental protection in housings to ensure reliable operation. Once it's determined that some added environmental protection is required for the analyzer, the first decision that needs to be made is the form of that housing. The IEC 61831 standard defines the following four types of housings:
Air should enter and leave the house by entry and exit ports from outside the classified area. The air intake should be through a stack provided with a weather protection cowl (rain hood), and sourced from a non-hazardous area where corrosive or toxic gases don't occur. The house should be ventilated with air to keep flammable or toxic gases, which are either lighter or heavier than air, out of the house and to dissipate any leakage inside the house. This not only provides sufficient building ventilation, but it also dissipates any gases that may enter the shelter due to leaks from equipment inside the shelter. To insure that hazardous gases don't enter the shelter, the minimum differential pressure under operating conditions should be 25 pascals (0.25 mbar), effectively creating a purged enclosure. It is normal practice to operate at an overpressure of 2 mbar to 5 mbar with a typical air flow of 10 changes/hour.
The air exchange system, which is normally part of the shelter HVAC, will have to be able to:
It should be noted that many substances will reach the short-term exposure limit or asphyxiant levels long before the lower flammable limit value. As a general rule, the overall system design should eliminate or minimize the emission of hazardous or noxious gases and vapors and the possibility of liquid spillage inside the housing.
Despite the 10 air changes/hour requirement and the need to adequately purge any accidental releases, the design of the intake duct and the diameter of the stack should be sized to limit air velocity to a maximum of 15 m/s since higher velocities not only create a feeling of being in a wind inside the shelter, but also can cause quite a bit of noise. Any ducting to the analyzer house which passes through hazardous areas should be leak-tight. Ducting through Zone 1 hazardous areas should be avoided where possible.