Smart Sensors for Gas Detection

• By Fawaz A. AlSahan, Ghulam Farooq, Saleh M. AlGhamdi
• Process Automation

Summary

Protect workers and equipment at oil and gas plants.

Gas detection systems are meant to protect workers in oil and gas plants by warning them if there is a toxic or combustible gas leak. Smart sensors for gas detection with analytical capabilities have been evolving, and they are bringing more benefits to the oil and gas industry. 

Poor gas detection performance and ineffective management of gas detection systems in oil and gas facilities are unacceptable. These two scenarios impose risks to workers’ lives, the environment, and plant assets in addition to dramatically increasing the operating cost to ensure the functionality of these detectors. 

Capitalizing on the fourth industrial revolution, Saudi Aramco took a holistic approach to improving the gas detection system performance in its operating facilities, where thousands of gas detection sensors are running. It reevaluated the currently installed gas detectors, reviewed the existing maintenance practices to identify improvement opportunities, conducted a benchmarking study, and evaluated and field tested the latest smart sensor technologies for toxic and combustible gas detection.

This article presents the assessment findings, the implemented improvements, the identified smart technologies, and the field-testing results. The article also emphasizes the benefits of the smart sensors in terms of safety enhancement, performance improvement, and optimization of operating costs.

Gas detection system common challenges

When addressing the design requirements, and similar to other instruments, ambient conditions (including temperature, humidity, sandstorms, and rain) affect the performance of gas detectors. The sensors must have the required ingress protection (IP) and approval for proper area classification. It is particularly important to understand the sensors’ limitations on storage and operating time and proper installation at vibration-free locations, with a sunshade and proper accessories (like a dust guard and splash guard). Being aware of poisonous gas and preventing sensors’ exposure to liquids (such as water, oil, and paint) are also important considerations that prolong sensor life. Conducting the required testing and calibration per the manufacturer recommendations in addition to using the right gas cylinders can ensure a reliable gas detector operation.

Gas detectors performance evaluation

Evaluating new technologies for gas detection needs to be rigorous to cover all important aspects, including environment, performance, and end user experience in the oil and gas industry. These steps are normally followed in evaluating the performance of new technologies for gas detection:

• Type testing, which covers unpowered storage, measurement of deviations, mechanical tests, environmental tests, performance tests, electrical tests, stability, fault signal tests, software-controlled equipment, and protection against water.
• Ingress protection and hazardous area approval. This is to ensure consistent and safe performance for indoor and outdoor installations in oil and gas facilities.
• Offshore compliance, which covers compliance with temperature (–25 to 70°C), humidity (up to 100 percent), vibration (up to 4 g), electromagnetic compatibility (radio frequency, electrostatic discharge), and enclosure (up to IP68).
• Benchmarking to probe the market and other end users. 
• Field piloting that demonstrates the actual performance of gas detectors.

Evaluation and results: Electrochemical smart sensors

An electrochemical H₂S smart sensor is a disruptive smart plant technology for H₂S gas detection. It works similarly to batteries. When the target gas is present, a small electrical charge is generated chemically between two electrodes and displayed in the measuring head. The signal strength is proportional to the concentration of the gas. The sensor has a fast and linear response within the range 0–100 ppm, compared to a metal oxide semiconductor (MOS). Also, the electrochemical H₂S smart sensor is not affected by humidity, because it contains aqueous electrolytes.

The electrochemical smart sensor was selected after conducting a comprehensive study on H₂S sensing technologies suitable for the oil and gas industry. Seventeen industry standards covering gas detection were reviewed, and a benchmark study was also conducted. About 23 potential products were reviewed. Multiple onshore and offshore field tests were conducted in collaboration with many Saudi Aramco facilities. A collaborative, solid conclusion was then made that the electrochemical H₂S smart sensor was the best available option for H₂S gas detection in the company. The new technology has been widely deployed in the company with more than 8,000 installations so far.

Electrochemical H₂S smart sensor technology has many benefits compared to conventional MOS technology. The table in figure 4 summarizes the main advantages.

Evaluation and results: Autonomous smart sensors

The autonomous H₂S smart sensor is an advanced version of the electrochemical (EC) smart sensor and the latest technology for gas detection. It has two parts: one consists of the generator, sensor, and pump, and the other of the electronics. The hardware, software, and sensor of the autonomous H₂S smart sensor comply with safety integrity level (SIL) 2 requirements.

The technology has an automatic self-generation and self-testing mechanism, where it generates H₂S (5–10 ppm) and tests itself every 24 hours. An automatic self-test with gas is performed at programmable intervals—normally every 24 hours. Also, a manual self-test can be initiated, after calibration or actual exposure to H₂S.

During self-testing, 2 mA is measured, and a distributed control system (DCS) alarm can be configured to indicate “self-testing.” When a failure takes place, 0 mA is measured, and a “failure alarm” can be generated at the DCS.

The H₂S generator is an electrochemical generator. It contains solid sulphur salt and conductive electrolyte. If current is applied to such a system, electrolysis starts, and in this case, H₂S is generated. The amount of H₂S generated is proportional to the electrical charge applied.

The criterion for the sensor test is the response time to this gas puff. The concentration of the gas generated depends on the sensor module applied. The relevant parameters driving the generator are stored in its RAM. In order to maintain adherence to the SIL 2 standard, the proof test or calibration interval must be no longer than one year.

Figure 5 illustrates the main advantages of the autonomous H₂S smart sensor compared to the electrochemical smart sensor, for gas detection.

• The autonomous H₂S smart sensor was field tested at Saudi Aramco. Two sensors were installed at Saudi Aramco Riyadh Refinery. The following is the test success criteria, developed and used by the team to evaluate the new technology:
• The response time of the sensors during pump testing satisfies T20 and T50 and exceeds the requirements in the ANSI/ISA-92.00.01 standard.
• The sensor does not fall asleep if not exposed to H₂S gas. 
• The sensor does not die or fail for whatever reason within the trial period. 
• The transmitter does not show any signs of fluctuations (i.e., it gives a steady 4-mA signal for H₂S gas-free atmosphere). 
• The transmitter does not generate a false alarm. 
• The sensor does not drift outside the detector’s specification, allowing for the accuracy of the gas cylinder. 
• Self-testing is generated every 24 hours and reflected at the DCS.
• Failure mode is tested, and a failure alarm is confirmed at the DCS.

The field testing of the autonomous H₂S smart sensor technology demonstrated exceptional performance over both traditional MOS technology and electrochemical technology. The performance of autonomous H₂S smart sensors, like other electrochemical smart sensors, exceeds the industry requirements (such as ANSI/ISA-92.00.01).

Evaluation and results: Open path gas sensors

An industrial gas detection system has always been considered a base for the safety of people, the plant, and the environment in the petrochemical and oil and gas industries. Basically, it consists of gas-sensing technologies with point configuration (metal oxide semiconductor, catalytic bead, electrochemical cells, and point infrared) and open-path configuration (infrared and laser). Some Saudi Aramco facilities are using open-path infrared detectors along with the point gas detectors. Vibration and misalignment have always been main concerns for these open-path infrared gas detectors.

To overcome these issues, Saudi Aramco evaluated a new laser-based technology, enhanced laser diode spectroscopy (ELDS), which was piloted at the RRD diesel hydrotreating and ISOM areas (figure 6).

ELDS-based technology can detect either methane (CH₄) or H₂S, or both gases simultaneously. Some of the advantages of this technology are no routine maintenance or calibration, dual-gas detection capability, inherent fail-safe design, and the ability to operate in extreme weather conditions.

Single- and multi-gas detection capability models are available in industry. Single-gas detection capability models are used to detect either CH₄ or H₂S gas leaks. Multi-gas detection capability models can detect both CH₄ and H₂S gas leaks simultaneously, so this model is also called the dual-gas detection model. 

A dual-gas detection model uses the same apparatus (one transmitter and one receiver) to detect methane and H₂S gas leaks simultaneously. In this case, one laser beam is used to detect methane and a second laser beam to detect the H₂S gas leaks. A dual-gas detection model should only be selected if both methane and H₂S gases are present in the process stream. If there is no methane present, then the model will only work if at least 750 ppm meter of H₂S concentration is present. So if only H₂S is present in the process, then a single-gas detection model specifically for H₂S detection should be used. 

Enhanced laser diode spectroscopy significantly increases the sensitivity and reliability of laser-diode-based gas detection and measurement, even in extreme environments. ELDS uses harmonic fingerprinting to achieve the earliest possible detection of gas leaks while reducing the negative repercussions of false alarms.

Using a separate transmitter/receiver configuration, ELDS systems detect and measure gas concentrations at specific target gas absorption wavelengths over distances of up to 200 meters. The detector measures absorbance changes along the line-of-sight path when a combustible or toxic gas passes through the beam. ELDS uses highly reliable, solid-state laser diode sources similar to those used in demanding telecommunications applications. Signal processing methods significantly increase sensitivity, enabling reliable detection down to fractions of a percent LEL.meter for combustible gases, and low ppm.meter levels for toxic gases.

ELDS addresses problems experienced by traditional laser diode systems including laser relative intensity noise (RIN), absorption by atmospheric gases, and coherence/fringe effects. ELDS uses a combination of techniques, which significantly enhance the ability of an open path gas detector (OPGD) to detect small fractional absorbance with an extremely low false alarm rate. ELDS techniques allow companies to finally meet stringent regulatory and safety integrity requirements with a false-alarm free system for low-level combustible and toxic gas detection.

The gassing cell is used to perform the functional testing of these ELDS OPGDs, in field-service conditions. The compact gassing cell only allows gas response checking to be performed with relatively high concentration test gases. Gases that can be satisfactorily used inside the gassing cell include methane, propane, butane, hydrogen sulfide, and carbon dioxide. For functional testing, the relevant gassing cell compatible with the excessive target gas shall be used. These detectors (OPGD) are calibrated for life by the manufacturer.

Transforming Saudi Aramco standards and procedures

Transforming the governing engineering standards and procedures for gas detection is another milestone, which was achieved. Saudi Aramco engineering standards governing gas detection systems (SAES-J-505 and 34-SAMSS-514) were enhanced to mandate the new technologies. The company also developed a new company procedure, SAEP-1029, to enhance gas detectors testing and calibration. The new procedure also covers the requirements for personnel qualification, verification of gas cylinders, frequency of testing and calibration, and life-cycle management of gas detectors.

Maintenance 4.0 for gas detection system

The new smart sensors for gas detection enable industrial mobility and wireless connectivity. For example, Bluetooth wireless connectivity is a new option for conducting maintenance for gas detectors.

An internal risk assessment on gas detectors using Bluetooth wireless technology was conducted. The risk assessment study objective was to allow Bluetooth wireless technology to gauge cybersecurity measures, and also to govern the use of mobile devices in hazardous and classified areas. 

The assessment resulted in accepting Bluetooth wireless technology after implementing the right cybersecurity measures in each facility. The measures are basically implementing the capability to restrict, manage, and monitor the use of hazardous area–classified mobile devices, properly managing the mobile devices used to connect to gas detectors, developing a process to approve and track mobile device assignments to personnel, and finally implementing the local passcode feature on all Bluetooth-enabled gas detectors.

Ending remarks

Digital transformation is inevitable. Saudi Aramco is continuously exploring and deploying smart plant solutions, including wireless sensors, autonomous smart sensors, and maintenance 4.0 solutions.

Healthy gas detectors are essential elements for HSE. Using smart sensors for gas detection will not only improve safety in the company facilities but will also improve performance and optimize the life-cycle costs. Gas detectors’ good performance is supported by type testing, benchmarking, compliance to industry standards, and field piloting.

The company is currently taking full advantage of the latest smart sensors for gas detection to enhance safety and improve reliability. Other smart technologies are still attractive options to further improve performance and optimize cost, like a single detector with two sensors, a flanged gas detector for installation inside an oil-water sump and a heater firebox, remote calibration capabilities, and a compact gas detection system with a detector, horn, and beacon all in one device.

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