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Gas Detection

Gas Detection

Published by Bay Publishing in OSA Magazine 2012 09:00 GMT Thursday 1st March 2012

gas detectionGas detection technology is widely used in industry to protect people, plant and atmosphere from the damage that would take place due to release of flammable or toxic gas/vapours. A gas detector of some kind - either fixed or portable - is usually part of safety engineering system.

A gas detector is a device which detects the presence of various flammable/toxic/normal gases within the area targeted. This instrument can detect a gas leak from storage vessels, a pipeline, pump or other applicable process equipment. In event of a leak it sends the signal to a control system and processes are automatically turned shut off to safeguard the situation.

During detection it sounds an alarm also so that people may leave that area immediately. Gas detection systems form an integral part of plant safety. The main function of the Gas Detection System (GDS) in the plant is to detect the concentration of toxic and/or combustible gases and initiate an alarm or shutdown functions at a predetermined level, at a stage sufficiently early to protect lives and assets.

The gas detection system is different for different types of gases, and typically include single and multi gas detectors.

A typical gas detection system detects combustible, flammable and toxic gases, and also registers a depletion in oxygen levels. Portable gas detectors are usually battery operated. They transmit signals through a series of audible and visible devices such as alarms and flashing lights, when dangerous levels of gas concentrations are reached and thus detected.

When detectors measure a particular gas concentration, the sensor responds to calibrated gas, which acts as the scale. When the sensor’s detection exceeds a preset alarm level, the alarm or signal gets activated.

The alarms are set on LEL concentration level, generally measured as first low alarm activation on, for example, 20% of LEL and a second high alarm activation on, for example, 40% of LEL as defined in the design philosophy of the project, or as demanded by a Hazard Identification Study.

Gas leak detection is the process of identifying potentially hazardous gas leaks by means of various sensors. These sensors usually employ an audible alarm to alert people when a dangerous gas has been detected. Common sensors used today include Infrared Point Sensors, Ultrasonic gas detectors, electrochemical gas detectors, and Semiconductor Sensors. These sensors are used for a wide range of applications, and can be found in industrial plants, refineries, wastewater treatment facilities, automotive plant - and even around the home.

A complete gas detection system

A really thorough gas detection system should consist of hazardous gas detection configurations ranging from a variety of fixed gas combustible and toxic gas detectors to a complete line of display transmitters, gas controllers, power supplies, and gas control panels.

The method employed to monitor leakage of hazardous gases is to place a number of sensors at the places where any leaks are most likely to occur, as indicated in Hazard Studies or specified requirements. These sensors are often then connected electrically to a multi-channel controller located some distance away in a safe, gas free area with display and alarm facilities, and equipment such as event recording devices. This is often referred to as a fixed point system. Being a fixed system it is permanently located in the area such as an offshore platform, oil refinery or laboratory cold storage facility - wherever a gas leakage problem might potentially be in the future.

Types of devices

Gas detectors are of two types, such as portable devices and fixed gas detectors. The first is used to monitor the atmosphere around personnel and is worn on their clothing. Portable detectors are also used during shutdown operations or maintenance operations. The second type of gas detectors are of a fixed type, which may be used for detection of one or more gas types. Fixed type detectors are generally mounted near the process area of a plant or control room. Gas detection devices should appear to the distributed control system (DCS) as any other instrument with display of analogue value and alarm settings.
gas detection
The above gas detectors are divided into the following categories:

• Electrochemical

• Infrared

• Ultrasonic

• Holographic

Gas detection other than within an industrial context are equally important for maintaining safety within buildings. Certain carbon monoxide detectors can be purchased for around $20-60. Handheld flammable gas detectors can be used to trace leaks from natural gas lines, propane tanks, butane tanks, or any other combustible gas. These sensors can be purchased for $35-100.

It is important that employees are trained in the use of this equipment, and understand the importance of their maintenance.

International standards and codes

When designing any Gas Detection System (GDS) there are numerous International standards and codes to be followed:

• RP505 Recommended Practice for Classification of Locations at Petroleum Facilities Classified as Class I, Zone 0, Zone 1 and Zone 2

• BS.5345 Code of practice in Selection and Maintenance of Electrical Apparatus for use in Potentially Explosive Atmosphere (other Than Mining Applications for explosive Processing and Manufacture) Part 1-9

• IEC 61511 Functional Safety - Safety instrumented systems for the process industry sector

• IEC 61508 Functional safety of electrical/electronic/programmable electronic safety - related systems

• MIL-HDBK217 Reliability Prediction of Electronic Equipment

• OISD-STD-116 Fire Protection Facilities for Petroleum Refineries and Oil/Gas Processing Plants

• OISD-STD-152 Safety Instrumentation for Process System in Hydrocarbon Industry

• IEC 61000 Electromagnetic Compatibility for Industrial Process Measurement and Control Equipment, Parts 1, 2, 3, 4, 5, 6

Training after installation is also necessary on how to use the GDS and interpret the data recorded. Initially responsibility may lie with an Automation Contractor, but later on it would fall to Plant Management to ensure continued training for new employees, plus updated training for established users. The GDS should be supplied, engineered, integrated and tested from a vendor facility where such a system has been supplied, engineered, integrated and tested previously.

Locating the gas detector

Detectors should be located where the risk of gas is most likely to be present. Locations requiring the most protection in an industrial plant would be around gas boilers, compressors, pressurised storage tanks, cylinders or pipelines. Areas where leaks are most likely to occur are valves, gauges, flanges, T-joints, filling or draining connections.

Simple considerations that should be taken account of in determining detector locations are:

• Study how escaping gas will behave due to natural or forced air currents. Mount detectors in ventilation ducts if it is possible

• When mounting detectors consider the possible damage that may happen due to rain or flooding. For detectors located outside in the open it is preferable to use the weather protection assembly guards

• To detect gases lighter than air such as ammonia, detectors should be mounted at a high level

• To detect heavier than air gases such as SO2, detectors should be mounted at a low level

• Locating a detector in a hot climate and in direct sun, Use a detector sunshade

• Detectors should be positioned a little way back from high pressure parts to allow gas clouds to form. Otherwise any leak of gas is likely to pass by in a high speed jet and not be detected

• Plan properly for functional testing and servicing on time to maintain the detection system life

• Detectors should be installed at the designated location with the detector pointing downwards to ensure that dust or water will not collect on the front of the sensor and stop the gas entering the detector

• When siting open path infrared detectors it is important to study that there is no permanent blocking of the infrared beam. Short-term blockage from people or even birds can be adjusted

• Ensure the structures that open path devices are mounted in such a way that they are not susceptible to vibration
Important definitions

Given what a complex undertaking it is, and the extreme consequences which might ensue without a full, well-rounded understanding of this field, there are a number of important definitions you need to be aware of when designing a GDS:

• Alarm: an audible or visible means of indicating to the operator equipment or process malfunction, or abnormal condition

• Alarm system: the collection of hardware and software of an alarm which transmits the message to be displayed to the operator, records the message, and generates an alarm metrics report

• Auto-ignition temperature: lowest temperature at which there is enough heat energy to ignite vapours spontaneously

• Flammable gas: a flammable gas is one than can burn when brought into contact with heat or flame

• Flammable liquid: a flammable liquid is defined as one having a flash point below 100° F (37.8° C) with a vapour pressure not exceeding 40 psi (276kPa).

They are volatile in nature, constantly giving off heavier than air vapours that cannot be seen with the naked eye

• Lower Explosive Limit (LEL): this is the minimum concentration of flammable gas or vapour mixture that will propagate flame when exposed to a source of ignition. Commonly abbreviated LEL or LFL (Low Flammable Limit), a mixture below this concentration level is considered ‘too lean’ to burn. An increase in atmospheric temperature or pressure will decrease the LEL of gas or vapour

• Upper Explosive Limit (UEL): the maximum concentration of gas in air that will produce a flash of fire when an ignition source is present. Any higher percentage of combustible gas or lower oxygen in the mixture of the two, and the mixture will be too ‘rich’ to sustain combustion

Maintenance

All gas detectors must be calibrated under a specified schedule as per standards. Of the two types of gas detectors, portables must be calibrated more frequently. A typical calibration schedule for a fixed system may be quarterly, bi-annually or even annually. A typical calibration schedule for a portable gas detector is a daily bump test, accompanied by a monthly calibration. Almost every portable gas detector out there has a specific calibration gas requirement which is available from the manufacturer you purchased your monitor from.

Case study

What follows is a toxic gas detection process case study for the detection of Sulphur Dioxide.

A chemical plant was near a populated residential area. By the factory authorities it was warned during its annual check that SO2 level had to be monitored. Prior to this, despite many indications after the several audits the chemical plant believed that even if there were releases, the concentration was not more than its TLV (Threshold Limit Value).

One day, SO2 was released from the unit operation, though there were no major injuries or accidents. Their safety engineer was intelligent enough to determine eight monitoring points were needed and that each monitoring point would have two alarm levels to indicate the presence of low levels of Sulphur Dioxide. The engineer also judged that the range for the SO2 detector had to be 0-20 ppm.

The remote locations of the sensors had wireless transmission of data back to the plant control room. The monitor installed included non-intrusive calibration but had a range of 0-100 ppm. Timely intervention of safety engineers avoided further releases of SO2 and hence many fatalities or even injuries were avoided.

This engineer accurately studied and interpreted the gas concentration levels through the installed Gas Detection System and through his professionalism probably saved many lives - and environment damage too.
Be proactive

It’s very important to have proactive safety measures in place, as opposed to being merely active.
Generally if SO2 concentration levels become more than the allowed ppm level, they may cause:

• Suffocation

• Death

• Regulatory fine

• Major damage in the surrounding aquatic and biotic life

It can be well said that early gas detection will reduce leading your workplace into the unthinkable risk of an explosion.

OSA Magazine | Issue 9

Sanjeev Paruthi

Mr Sanjeev Paruthi is a postgraduate Chemical Engineer from Punjab University Chandigarh (India), and is presently associated with a multinational EPC Company as its HSE and Process Safety Engineer at Gurgaon-India.

His experience of six years comprises of working with Hindustan Zinc Limited, Tata Coffee Limited and with leading consulting and Training Company in the domain of Process Safety/Risk Management. He also holds PG Diploma in Business Management from ICFAI. He is also pursuing an Advance Diploma in Industrial Fire Safety Management from Mohali Punjab, India.

Mr Paruthi has wide range of consulting and training experience for working with Chemical, Fine Chemical, Refinery, Petrochemical Storage Installations, paints and allied chemical industries. He has also designed Fire Prevention and Detection systems for Refinery in his current company. He has led various HAZOP and HAZID sessions and prepared various HSE documents under a refinery project.

Mr Paruthi has also conducted Operational Process Safety Studies.

His technical expertise is in the following domain areas:
• Quantitative Risk Assessment, HAZOP Studies, Process Hazard Analysis
• Fire Risk Assessment
• Process Safety Training and Development
• Process Safety Management Studies and Audits
• SIL Studies
• Hazardous Area Classification
• Static Hazards Evaluation
• Lockout Tagout
• Chemical Handling Safety (Gas/Vapour/Dust)
 

Sanjeev Paruthi

Website:
http://

Email:
sanjeevparuthi@gmail.com

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