Gimme Shelter

Process analyzers are critical components in many advanced process control and environmental monitoring applications. Besides being critical, they're often complex, or at least if the analyzer is not complex, the associated sample system can be. However, just as we often overlook the infrastructure necessary to support other field instruments, the same happens to the protection of the analyzer system—the shelter and its design. But designing a suitable process analyzer shelter, even for a complex system, does not have to be an intimidating task. What follows are some basic guidelines.

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:

• Correct location and orientation of sample point;
• Proper design of sample transport and sample conditioning systems;
• Availability of reliable and clean utilities;
• Environmental protection against heat/cold, humidity, solar radiation, rain, dust and corrosion;
• Ease of accessibility for maintenance of all the analyzer system components;
• Proper design of validation and calibration facilities;
• Adequate preventive maintenance.

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:

• Analyzer case—An enclosure forming part of the instrument. In other words, no additional equipment other than the case in which the analyzer is built.
• Analyzer cabinet—A small, simple housing in which analyzers are installed singly or grouped together. Due to the size of the cabinet, maintenance is carried out from outside the cabinet.
• Analyzer shelter—A structure with one or more sides open and free from obstruction to the natural passage of air, in which are installed one or more analyzers. Maintenance of the analyzer is normally carried out from within the shelter.
• Analyzer house—An enclosed structure in which are installed one or more analyzers. Either natural or forced ventilation is used. The maintenance of the analyzer is always carried out from within the house (Figure 1).

Once you know the size and type of housing the next consideration is its placement. The preferred location is always an unclassified area. This not always possible, either because of the type of process and general layout of the unit operations, or as is often the case when an unclassified area is not an option, because the lag time of the sample system and analyzer would be too long to be useful for control.

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:

• Dilute escaping vapors (those liquids which vaporize at ambient temperatures) from the rupture or failure of the most hazardous sample or service line to less than 20% LEL around any potential means of ignition;
• Dilute to below the occupational exposure limit any toxic gases/vapors introduced into the house by accidental rupture of any one sample or service line within the house;
• Dilute to below the asphyxiant level any asphyxiant gases/vapors introduced into the house by accidental rupture of any one sample or service line within the house.

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.