PLANNING AN INDUSTRIAL GAS DETECTION SYSTEM


There is a popular belief that when you install a gas detection system you make the installation safe.
This incorrect and dangerous statement cannot be made further from the truth.
An installation cannot be made safe by such a simple action; The safety of any installation begins at the design stage, good engineering and materials, and follows through construction to the development of safe operating practices undertaken by trained and competent personnel.
After the correct engineering and development of safe operative practices only then can gas detection accept a role to assist and improve the overall safety of the installation.
A fixed gas detection system has one prime role, that of a safety device.
In the normal operation of a process the gas detection system should never be activated, its initial task is that of a 24 hour ’sentinel’, monitoring chosen atmospheres looking for the occurrence of an abnormal event which could indicate the failure of a plant item or an incorrect and potentially hazardous operating practice.
Under such circumstances, the second task is for the gas detection system to provide a rapid response to be abnormal event, so that remedial action can be taken before the developing gas or vapour cloud can become of a size that a catastrophe becomes a possible result.
A gas detection system of the type envisaged for an installation should be never used to monitor of a process or procedure, which is the role of a process analyser.
In the event of a gas detector being used to monitor a process, there is a very real risk that operating personnel will grow accustomed to a regular activation of gas detection system due to ‘normal’ operations.
Under such circumstances there is a very real danger that the occurrence of an abnormal and hazardous event will pass unnoticed and not differentiated from a ‘normal’ operation.

Gas Detection System Size:

There is no ‘correct’ size for a gas detection system.

It is clear that the larger the system becomes for a fixed site volume, the greater is the probability of detecting a small toxic or flammable gas cloud and that the time taken to detect will similarly reduce as the system size increases.
This logic however, excludes both the capital cost of such a system and its ongoing cost of ownership.
A compromise therefore becomes necessary between the benefit of a system and its cost.
This compromise is by definition subjective; it can be based on many factors example:
A sum of money within a budget
A philosophy, such as perimeter monitoring at, say, 20 metre intervals
Mathematical predictive modelling to assess the time to respond to known events and providing a gas detection system to be faster in response than the required time to respond.
Personal opinions of experienced plant personnel.
The analysis of the perceived hazard on an area by area basis.

In reality, the final decision as to system size is usually combination of these factors.

Deciding What’s ‘Essential’

Whilst is an ideal world we would aim to detect gas leaks, from financial and engineering constraints fixed sensors are usually placed so as to detect gas accumulations which could create a ‘Significant Hazard’.
At this stage therefore, we need to define what is a ‘Significant Hazard’? Whilst there will be many different views, one definition could be an event where people would get hurt and plant/buildings get damaged.
Within the oil and gas industry there is some support for defining this as the energy released when a 5 metre diameter gas cloud containing a stoichiometric gas mixture ignites approximately 10% natural gas in air for a high methane based natural gas.
Remember you only need an overpressure of approximately 150 millibars within a normal building to damage the structure.
Following this regime, one would be required to place a gas sensor in a horizontal 4 metre triangular matrix in ‘Special Hazard’ areas.
In addition to the flammable hazard of the many different hydrocarbons processed within a complex, there is also invariably the toxic hazard of hydrogen sulphide and in my opinion, when the volumetric concentration of hydrogen sulphide in high pressure gas system is greater than 500 ppm H2S then the area becomes a hazard from toxic gas viewpoint.

Where to Position Gas Sensors?



Factors which should be taken into account in determining suitable locations of sensors are discussed as follows:

Are we are monitoring an indoor site?
Indoor sites with mechanical ventilation enable objective and precise sensor location based on measured air movements through the building – use ‘smoke-tubes’ to monitor air movement.

Outdoor monitoring is not so easy, where variations in wind direction and speed beneficially require the use of gas detectors based on modern optical technologies equipment.

What are that the potential sources of gas and vapour leaks ?
At any location where is a break in the pipework system there is potential for leakage and such components as valves, flanges, pumps seals, pressure regulators, sight glasses, compression fittings and plant subjected to a large temperature excursions and high levels of vibration are all vulnerable to leakage.
Clad pipework and components in many cases increases the perceived hazard, as the inventory of leakage products can be high relative to plant where containment is unlikely.

What hazard high pressure gas leaks or vapour clouds ?
Gas leaks from valves and flanges where the pressure is greater than 2-3 bar tend to form diluted gas/air clouds through turbulent diffusion with the surrounding air and once away from pressure jet effect, the gas cloud tends to be close to neutral density and moves under the action of prevailing air movement.
The very opposite of this scenario applies to vapour clouds formed from evaporating or boiling liquids, for example liquid propane and butane.
These vapour clouds obey the laws of density and flow as heavy concentrated clouds, influenced by the topography of the site and local air movement.
These very hazardous vapour clouds flow into voids, spaces and drains and in many circumstances represent a far more serious fire and explosion hazard than high pressure gas leak which, if uncontained and you’re lucky, rapidly dilute themselves and dissipate in prevailing air movements.
As air movement and direction is important in both of these considerations, the assistance of the local meteorological station to obtain a ‘wind rose’ for the area, which defines a prevailing wind direction and strength over a defined period of time, is useful.

Which Gas Sensor is best?


Catalytic Flammable Gas Detectors
For many decades the catalytic flammable gas detector or ‘pellistor’ as it is colloquially known, has been the workhorse for flammable gas detection on industrial plant.
It is also recognised there are many failure mechanisms for these sensors which are ‘fail-to danger’ in principle. Whilst modern catalytic sensors show a far greater resistance to this effect it must still be perceived as a severe weakness which is ameliorated only by labour intensive
maintenance, notably by exposure of the sensors to standard test gas mixtures at regular intervals and confirming correct functioning.

Infra red Flammable Gas Detectors
As an alternative to the use of pellistor technology in the detection of flammable gases, sensors based on light beam absorption have been developed, offering the characteristics of a potentially long life, fast response time and fail to safety or more correctly, fault indication.
Most hydrocarbons absorb infra-red radiation in the 2.3 or 3.3 micron range as this corresponds to a harmonic or fundamental frequency of vibration of the C-H bond in its stretch mode.
The sensor measures the amount of radiation that is absorbed at these wavelengths by a volume of gas and this absorption gives a measure of the amount of hydrocarbon present.
Normally a reference sensor is also employed operating at a nearby wavelength where absorption by the gas being detected does not occur.
From this technology, over the past 20 years both ‘point’ detectors and Open Path or Line of Sight (LOS) infra-red gas detectors has been developed, where a modulated infra-red source and its detectors are separated by distances of up to 150 meters.

Acoustic gas detectors
A recent development in the armoury of gas detectors is the ultrasonic gas detector whereby the sound at ultrasonic frequencies from a pressure gas leak is detected by a sensitive microphone, the signal from which is then analysed by intelligent electronics.
I have had recent and most successful experiences of this principle with a Danish developed detector, where sensitivity and selectivity is achieved by setting the alarm level just higher than the ambient background level of ultra-sonic noise, correlating the most appropriate frequency band and the duration of the leak signal prior to activating an alarm.

Electro-Chemical Cells
An electrochemical cell is a type of gas-battery developed from fuel cell research.
These devices which, are self-powered micro fuel cell, use a diffusion barrier to limit the amount hydrogen sulphide reaching a special permeable gas sensitive electrode surrounded by an acid electrolyte.
These sensors are usable in temperate climates, but their main weakness is their failure at high temperature and with an upper temperature limit of circa 40 °C; this prevents their use in hot climates where frequently sour gas is in abundance.

Semiconductor Gas Sensors
One of the few options for hot climates, where electrochemical cells are inappropriate, is the semiconductor gas sensor.
These detectors are hot running devices and so are undamaged by high operating temperatures and respond to the presence of gas in air by alteration of surface conductivity. These sensors are sturdy and inexpensive, the sensing element of these gas sensors now in service, use a tin dioxide substrate.
In Conclusion ….
When either installing a gas detection system as new or improving an existing installation, financial constraints will probably make necessary the formulation of an ‘Essential Detection’ regime, that is, a gas detection system based initially and critically on ‘Special Hazards’,

’High Hazards’ and ‘Medium High Hazard’.