How bump testing can save time and money checking gas detectors in high risk sites without compromising safety
Whereas bump testing is a well-known and well-used procedure for testing portable gas detection instruments, it has not found favour in the world of fixed gas detectors. But as bump testing becomes more intelligent and automated, it gains the ability to save many high risk sites considerable expense in periodic calibration testing.
Many safety-critical sites, such as petrochemical works, oil rigs, refineries and so on, currently carry out a full calibration of their gas detection equipment every three months compared with every six months for less critical applications. A more sophisticated bump test could find its place in the high risk areas on the interim tests, so the full calibrations would still be carried out every six months with a bump test three months after each full calibration.
The bump test is a much quicker procedure and can be automated to speed it up even further. Critics of this say the bump test does not check the accuracy, and therefore it is not suitable for high risk areas. But that is changing with modern gas detectors, such that if these sites have a full test every six months, all that is needed for the interim test when using a smart detector with bump testing software installed is to verify that the sensors are responding and producing an appropriate output.
Fixed-point detectors
Conventionally, fixed-point detectors are fully calibrated on every occasion; a procedure that entails zeroing the sensor in clean air followed by the application of calibration gas and the calibration of both detector amplifier and the control system to which the detector is connected.
Calibration can be time consuming and costly. The first job is actually accessing the detectors. These will be all around the plant and not often easily reached; they might be under pipes, in ducts or on poles, so requiring the use of a ladder. Each detector will also be connected to a control system, so the first stage is to isolate the detector from that, to prevent them from sounding an alarm or shutting down any parts of the plant during the test.
The next stage is to check there is no gas present using a portable detector. Once that is done, the fixed detector is then set to zero and the output, usually 4mA, is checked for accuracy. This also has to be checked at the control centre to make sure that is also showing zero.
The detector is then put into calibration mode and gas is applied at about 0.5 to 1.0 litre/min. Calibration requires the sensor reaction to the gas to stabilise fully; in many case this can take 30 to 60 seconds, or more for some gases, such as ammonia. Once the reading has stabilised, the detector is adjusted to the correct reading, and the output signal is checked. This adjustment may be made using a magnetic tool, by pressing buttons or by potentiometer adjustment, depending on the gas detector type.
The control system should now be checked to see that it, too, is showing the correct reading. Where a single engineer is deployed, the gas may have to be left flowing on the sensor while the engineer returns to verify the calibration of the control panel, thus using a lot of expensive gas. If we assume the cost of the gas is around £1 per minute of flow, not an reasonable generalisation, it is easy to see how the cost of this can rapidly rack up. Having two engineers linked by radio to do this can obviously save time and gas, but this saving can be more than outweighed by the cost of the extra engineer.
Fixed-point detector calibration may also need to be conducted under the control of a hot-work permit as the detector needs to be opened to adjust internal settings. This creates considerable further work and disruption to site processes.
Although the full six-month re-zero and calibration process remains necessary for some sensor types, bump testing provides an opportunity to reduce the cost and time implications of more frequent sensor testing.
Bump testing
A bump test is a brief application of gas to a sensor for a defined period of time to invoke a prescribed reaction to confirm a sensor reacts appropriately. It is a widely understood term and accepted practice for portable gas detection instruments.
The European user standard EN60079-29-2: 2007 covers the selection, installation, use and maintenance of gas detectors for flammable gases and oxygen. It covers apparatus that can provide a means to reduce the hazard by detecting the presence of a flammable gas and issuing suitable audible or visual warnings. The standard prescribes that portable gas detection instrument manufacturers include an instruction to bump test units each day before use in the product manual. Similar requirements are very likely to follow for toxic gas detectors.
Bump testing provides a quick and direct method of verifying gas sensors are working correctly; some sensor technologies, such as catalytic beads and electrochemical sensors, do not fail-safe, in that they may become insensitive to gas but still appear to be operational.
The latest intelligent gas detectors and transmitters facilitate bump testing, so making it easier and more cost effective to ensure detectors are in full working order and significantly reducing the time personnel have to spend in hazardous locations. There are two options for bump testing – a fast (or speedy) method for simply verifying that the sensor will generate an alarm if exposed to gas, or alternatively a method for determining that the sensor response is accurate. As bump tests can be conducted without opening the detector, a hot-work permit is not needed. They also require substantially less gas to be applied, so creating further cost saving.
The “speedy bump” test verifies that a sensor is working correctly while consuming the minimum quantity of test gas. Once activated, gas at a concentration that exceeds the alarm-one threshold must be applied to the sensor using a calibration cap. The detector allows a time period – 30 seconds maximum by default – for the sensor to respond beyond the alarm-one threshold, after which the user is prompted to remove the gas and confirm when the sensor signal has reverted to normal levels. The result of the test is then displayed as a pass or fail. The analogue output and alarm relays, if fitted, are inhibited while the test is performed.
Alternatively, the “smart bump” test verifies that a sensor responds correctly to a specified concentration of test gas. In this case, gas of a prescribed concentration must be applied to the sensor using a calibration cap. The detector allows a time period, again usually 30 seconds maximum, for the sensor to respond. If the sensor signal does not change within this period, the test will be failed.
The test gas must remain on the sensor for the remainder of the test. Over the next time period, again typically 30 seconds depending on sensor type, the detector software monitors the sensor reading, and the test will be passed if the final reading falls within an acceptable range. The detector is inhibited during the test to prevent unwanted alarms, and the user is prompted to remove the gas and confirm when the sensor signal has reverted to normal levels.
The first advantage of bump testing over a full calibration is that does not need to be preceded by a zero signal adjustment. The intelligent transmitter will also inhibit itself during the bump test procedure. The user just supplies the gas until the detector shows pass or fail. The intelligent transmitter will also provide an on-screen wizard which guides the operator through the semi-automated process. Bump test functions and adjustments are made via the integral keypad without the need for special tools or a hot work permit.
Applications
For larger sites with many sensors, testing can be an ongoing operation, in that the time taken to calibrate every sensor means that once finished it is time to start again. This is because some of these sites have hundreds of detectors, with larger sites that can have more than a thousand. While a bump test, itself, takes 30 seconds, calibration can be very variable from one to many minutes if it requires the leg work back and forwards to the control system to ensure that it has the correct readings. On top of this, the initial set-up for a calibration test takes longer than a bump test. From this it is easy to see that the potential saving to some sites both in terms of cost of gas and of engineers’ time could be considerable.
Sites and facilities likely to find substantial value in intelligent detectors with bump test ability are ones where there is a serious risk of toxic exposure or a large explosion if a gas leak occurs. These are areas where typically full calibration testing is carried out every three months. These facilities are likely to include hazard zone one and two areas, where gas detection is a key safety measure against combustible gas. There is also significant risk of build-up and exposure to toxic gases; hydrogen sulphide (H2S) in particular during the upstream processes, and various toxic volatile organic compound vapours, such as benzene.
Good examples would be the continuous gas monitoring required to protect high-cost FPSOs (floating production storage and offloading vessels) and reduce the risk to personnel that operate and live on them. Another is the continuous combustible and toxic gas monitoring that is a critical facet of operation within oil refineries. Also, the combustible and toxic materials found at oil and gas processing plants are extremely hazardous to the facility and its personnel, especially in highly congested production areas that contain reactors, turbines, valves and high pressure distribution pipelines.
A typical offshore oil rig is composed of several modules, including the well bay, living quarters, process areas, power and drilling areas. The closeness of the modules on offshore rigs calls for the continuous monitoring of fugitive emissions. All these are the kinds of facilities that may well be currently calibrating detectors at 3 monthly intervals, and so could benefit from the fixed bump detection technology.
Conclusion
Although there is no legislation requiring users to conduct bump-tests on fixed-point detectors, the opportunities for time and gas savings are clear. Using the latest smart detectors, the bump test procedure is much quicker than a full calibration and only one engineer is needed to perform the tests. The time spent in hazardous locations is significantly reduced, as is amount of gas used. A full calibration can involve leaving the gas on for 60 seconds, or much longer if the engineer has to return to the control room to check the readings – for a bump test, it takes just 30 seconds.
While high risk areas will still need to do a full calibration every six months, conducting a bump test three months after each calibration will ensure that all the detectors are still performing correctly, and this can be done at great savings compared with doing a full calibration every three months.
Published in Hydrocarbon Engineering
