Introduction

Unlike fire systems with a specific standard for installation and placement, there is no similar standard for gas detection. Gas Detection systems only have industry-approved best practice guidance notes for locating, placement, and the number of gas detectors. This is due to the complexity/specialty of gas detection due to the vast amount of gases being detected,  over 400 gases with varying amounts of properties and environmental influences factors, such as heat, that affect gases in different ways.

The following article is intended to bring together UK industry legislation and guidance notes to help advise you on the consideration and placement of gas sensors or detectors. 

Gas detectors are usually categorized into three distinct groups for placement:

Plant Protection

Flammable gas detectors are typically categorized into this group. Besides asphyxiation, flammable gases are generally not directly toxic and therefore, the strategic position of detectors is where the gas is expected to accumulate based on its relative density to air (heavier or lighter).

Process Monitoring

Typically gas detection systems in this section are used to monitor and either alarm or be part of processing control. This could be monitoring VOC (Volatile Organic Compounds) levels pre and post-filtering, Solvent Vapour (LEL) monitoring in drying ovens, Ventilation control in Car Parks and buildings, etc. The location of these detectors will usually be based on the individual application, after considering the process, gases or vapors, air movement, and temperatures involved. Applications falling into this category usually need a good dialogue between Daviteq and the plant designers to ensure the best placement of the detectors for them to be highly effective.

Life Safety Systems

In such systems, the concern is that an asphyxiant or toxic gas is directly hazardous to personnel. The position of the gas detectors is based on the normal operating zone for people in the application area. At this point, there may be two sets of detectors involved with a life safety system. The first set of detectors performs a process monitoring function, with the detection based on the relative density of the gas with respect to the air. This set of detectors is typically used to perform some type of function, for example: bringing on extra ventilation to clear a spill hazard.

The second set of detectors is in the ‘life safety or breathing zone’ and alarm for personnel protection. When planning the system, careful consideration needs to be given to set alarm levels for the detectors. Where a detector is fitted into the breathing zone for a gas, which is much heavier than air, then the alarm level is set at a lower level to offset the effect of the higher location.

A typical example would be a Liquid Nitrogen Dewar fill room. In this application, we know that spills will occur as Liquid Nitrogen is decanted. In this application, the Low-Level Oxygen detectors bring on extra ground-level ventilation to clear the Nitrogen spill as it evaporates into the atmosphere. If the ventilation cannot effectively clear the spill, then the detectors in the breathing zone are used to alarm and evacuate personnel in the area until such time that the atmosphere returns to normal safe levels.

Cryogenic Gas Detection

Careful consideration is required for gas detection applications involving cryogenically cooled gases such as liquid helium or nitrogen. On initial release, cryogenically cooled gases typically have a lower temperature and higher density than their surroundings and behave differently than their gaseous state. In this instance it should be considered for two sets of detection for optimal safety, one for low-level detection in the gases cooled state and one for the life safety zone. It is recommended to survey applications involving such gases.

Area Coverage for Gas Detectors

Like smoke detectors, a gas detector can provide up to 75SQM area coverage based on a 5M radius of operation. There are many factors affecting this, ventilation air flows, gas characteristics, equipment in the area, the geometry of a room, etc.

Control Panels

Control panels should be positioned outside the hazardous area, protected by their connected gas detectors. They should be accessible such that when an alarm is triggered, it should be possible to evacuate the area and view the gas levels from the controller.

Consider the application of HMI panels, mimic panels, or Smart IoT Gateway supplied by Daviteq to provide additional remote indication/alarm through email and text notifications.

Audible Visual Alarms

As a general rule, if there is a gas detection fitted to an area, then there should be an audio-visual alarm (beacon sounder)to alert personnel who may be in the same area. Audio-visual alarms are standard beacon sounders where the sounder can be silenced from the control panel once an alarm is accepted. Daviteq offers Standard LED beacon sounder modules, which can be operated from addressable I/O points to minimize cabling.

Annunciators

Annunciators are addressable devices usually installed at door entry points. In the event of a gas alarm, a clear audible visual alarm is provided by an annunciator to warn people from entering an area where a gas hazard could be present. Annunciators are advantageous in many ways when compared to standard beacon sounders. They can be installed on standard dado trunking systems; cannot be confused with other alarms; can be fitted with slam switches, and the displayed alarm message and flashing color display are unambiguous.

Gas Collector Cones and Splash Guards

Gas collector cones can be considered for use in areas where detectors are placed above gas plants such as boilers or meters in rooms with high ceilings. They are fitted to detectors located just above the gas plants to enhance the capability of detectors to sense gas leaks.

For detectors fitted at low levels, fitting splash guards may be appropriate to protect sensors from floor washing, rain splash, dust, etc.