By Gary D. Nichols, PE, Principal Control Systems Engineer
Before discussing the maintenance/service and wear-out/retirement phases of process analyzer life cycle reliability, let’s devote special attention to analyzer system safety and ergonomics. While it appears intuitive that safety and ergonomics should be addressed before or during the project scope development, technical requirements and project management phases, many of the safety and ergonomics line items might have lacked meaningful context had the breadth of analyzer system technology not been discussed first. Now it’s time to circle back and look at these issues.
To frame our discussion about analyzer systems ergonomics and safety, let’s assume we have an analyzer shelter that includes an assortment of gas chromatograph systems and single-component analyzer systems; e.g., photometric, oxygen or moisture analyzers. Without becoming immersed in detail, let’s assume further that the process streams have combinations of physical and chemical properties that include flammability, toxicity, simple asphyxiant, corrosive, liquid, gaseous, condensable and other common properties and combinations that will assist us in illustrating key safety and ergonomic challenges and how they might be satisfactorily met. We shall also assume that typical process plant utilities of 480 VAC/3-phase power, UPS power, steam, water, plant air, instrument air, nitrogen, analytical gases in cylinder banks, storm and process sewers and atmospheric vent/flare headers are available.
With a typical petrochemical plant operating environment thus set, let’s focus on the twin concepts of safety and ergonomics.
Regulatory, corporate and manufacturing site safety policies include worker safety and occupational health. But ergonomics—understood as adapting the working conditions to suit the work, rather than unreasonably forcing the worker to adapt to the working conditions—must be included because requiring a worker to adapt often leads to unsafe actions and conditions. We shall not explicitly address process safety and hazards in this discussion, but remember that personnel safety and process safety are inextricably related.
Gas and Fire Detection
In most cases, analyzer shelters are considered enclosed spaces for purposes of personnel safety. We define an analyzer shelter as a four-sided, roofed building sealed to a slab, pad or foundation, or with an integral floor. The shelter has one or more doors, no windows except in the doors and an independent, dedicated HVAC system. As an enclosed space with process samples entering the analyzer shelter, combustible gas, oxygen deficiency, toxic gas, smoke detectors and heat monitors should be included in the shelter. If hydrogen is one of the combustible gases, the catalytic bead detector must be specified because the infrared type is insensitive to hydrogen. Toxic gas detectors are available for a wide variety of common industrial chemicals including hydrogen sulfide, chlorine, carbon monoxide, phosgene and hydrogen chloride. Oxygen deficiency monitors are for warning and protection should the oxygen concentration in the analyzer shelter fall below the minimum concentration—19.5% by volume—to support life without suffering debilitating effects such as impaired judgment, impaired physical coordination or death. Oxygen deficiency can occur in the absence of toxic gases when oxygen is displaced by simple asphyxiants, such as helium, nitrogen and carbon dioxide, all of which are commonly used in process analyzers. We should note that many instrument air systems are backed up with nitrogen, thereby creating an additional asphyxiation hazard. Combustible or toxic gases, such as methane, ethylene, carbon monoxide and hydrogen sulfide, similarly displace oxygen, but generally reach their lower explosive limit or minimum toxic concentrations before adversely affecting oxygen concentration.
The height and location of the HVAC fresh air intake, often 25 ft, is determined by the area classification. Gas and fire detection systems usually have a pressure switch and alarm inside the analyzer shelter to ensure that the HVAC provides sufficiently high pressure and enough air changes to sweep a hazardous atmosphere out through the ventilation louvers. Smoke detectors and heat detectors provide protection for overheated equipment and HVAC failure when the shelter is unoccupied. A fire extinguisher next to the door inside the shelter is also a good measure, though this should be implemented in consultation with site safety and fire protection personnel.
This gas and fire detection system should be powered from a UPS or the unit safety shutdown system. The signals from these devices should go to an active safety shutdown and warning system based on hardwired dry-contacts, PLCs or a dedicated safety shutdown system. A mimic panel with switches and lights for the gas and fire detection system should be located outside of the analyzer shelter next to the door so that the analyzer technician can periodically test all safety devices without entering the building. The outputs from the system should activate local warning beacons and annunciators inside and outside the analyzer house and shut down the HVAC.
Gas and fire detection signals should go the unit safety shutdown system for alarm and action as determined by required codes, regulations, hazard analysis and review and prudent safety practices. The shutdown logic should include alarms in the event of power failure of any one detector or device. Signals normally also go the DCS as a monitor and alarm function, but with the action function residing in the relay, PLC or proprietary system. Signals to the DCS should be limited to analog concentrations, alarms and system status to minimize the number of critical alarms which operators must address.