By D. Balachandran
Fire protection technique can be broadly defined as four categories. 1. Fire prevention 2. Early detection and Alarm 3. Fire suppression and extinguishing 4. Evacuation.
In fire Extinguishing Technique, water is very widely used as fire extinguishing agent globally for many applications such as fire hydrant, Fire hose reel system, Sprinkler, Medium velocity water spray system and High velocity water spray system.
However due to complex nature of fire in the present scenario, other extinguishing agents are required to extinguish fire more effectively and also to overcome the limitation of water.
Dry chemical powder, foam and gas based extinguishing system are used widely as per requirement and application. Carbon-dioxide and Halon were used very widely during 1980's.
However due to environmental impact of Halon, Halon alternative gases are used in Fire Protection.
In NFPA-2001 application on clean agents (Halon Alternative) are very elaborately discussed.
Presently Carbon-dioxide, inert gas and Halocarbon gases are used in gas based extinguishing systems.
The gaseous systems are used to protect totally enclosed area. This is called total flooding system and some of the gaseous systems are used as local application where total enclosure is not possible. Either total flooding system or local application system, design concentration of extinguishing gas is one of the important criteria for suppression and extinguishing the fire.
While designing the gaseous system, the important factor to be considered is design concentration. The design concentration is arrived by various testing methods approved by NFPA, UL, FM, LPCB, Vds and other standards.
Depending on the size of area, type of protection and design concentration, the quantity of gas is arrived. For total flooding system, volume of the area protected is considered whereas for local application, square meter of the area is considered. It is very obvious that efficiency of the extinguishing agent depends on the particular area.
If the gas quantity is reduced due to leakage or any other reasons, then the system loses the efficiency of extinguishing the fire in the protected area. Because of this specific reason, NFPA and other standards insists specifically that quantity of gas has to be measured at least once in 6 months.
For a practical reason, Gas quantities area are always measured in terms of weight and inert, halocarbons are measured in terms of weight as well as pressure.
For CO2 system it is always necessary to monitor the gas quantity by measuring the weight. Measuring the pressure through pressure gauge to ascertain the quantity of CO2 gas may not be a proper method since CO2 gas exists in the container both in liquid as well as in vapor form.
Vapour pressure of CO2 at 21.1°C is 5826kPa absolute.
As per NFPA, when the weight of the CO2 gas is reduced below 10% then cylinder shall be refilled.
Similarly for Halocarbon if the weight of gas is reduced below 5% gas cylinder shall be refilled or replaced.
In the case of Halocarbon unlike CO2 and inert gas, agents are pressurized by Nitrogen. Monitoring the pressure through pressure gauge is one of the most important requirements for Halocarbon gases in addition to weighing agent.
While checking the quantity of gas once in 6 months or 3 months, the system should be switched off and alternative temporary Fire protection measure shall be arranged in the protected area.
Each cylinder shall be removed from the cylinder bank, by disconnecting the discharge hoses and weighed. After weighing these cylinders, again cylinders are to be fitted back and reconnected.
The whole exercise, amounts to almost recommissioning the entire bank. Since proper operation of systems, depends on actuation and discharge hoses, it is very important to ensure refitting is done properly through competent persons.
It is obvious that removing the cylinder and refitting cylinder back to the system requires skilled and trained manpower in addition to shut down the system during this period. This is time consuming and uneconomical engineering practice.
To overcome this problem, presently Online monitoring system is used in the Gas based extinguishing system. This online system is basically an alert system. This is not a weighing system. When set level is crossed online monitor will provide audio and visual alert indications.
On receipt of low weight alert indication It is the maintenance team’s responsibility to check the exact
weight of gas content by using reliable weighing machine and if needed the cylinder is to be refilled.
Presently new ranges of weighing devices are available for continuous monitor of gas weight.
SYSTEM DESCRIPTION:
This system consists of 1. Online monitor, 2. Online monitor control panel.
WORKING PRINCIPLE:
Each cylinder requires one gas weight online monitor. The cylinder should be fixed in hanging condition through online monitor. Depending on type of weight monitor, Low weight is indicated. When the weight of the gas is reduced below 10%, the low weight is indicated by means of counter weight position or LED indication or digital display of weight and LED indication.
Onilne monitor control panel provides required supply for weight monitoring unit. This panel also supervises the wire connection for open and short condition. On receipt of low weight indication from the cylinder bank, the panel gives audio and visual indication. Two set of potential free NO and NC contacts are available for remote indication or any other relevant signaling.
Types of Devices: Online monitor Unit
The following types of Onilne monitoring units are available,
1. IH006R – Online Monitor – Mechanical Indicator Type (working on counter weight principle – position of counter weight will indicate the 'Low Weight')
2. IH007R NI – Online Monitor with Normal Indication – Analogue LED Type (Working on spring displacement principle. LED will be 'ON' to indicate 'Low weight'.
These types of devices are cost effective and required low maintenance.
Other methods of gas quantity checking are being used. Liquid level indicator type or load cell principle with electronic unit are also being used.
Maintenance cost of other methods may not be cost effective compared to system cost.
As per manufacturer's recommendation, one monitor is required for one cylinder and 24v DC supply, Audio & Visual alarm will be provided through continuous online monitor control panel.
The author of this article is,
The Chief Executive of M/s. Agni Controls, Chennai, Hon. President of Institution of Fire Engineers(UK)
– Southern India Branch,
Hon. Founder President of M/s. Fire & Security Association of India (FSAI)