Preparing gas detection equipment for plant shutdown and maintenance
27 May 2016
Shutdown and maintenance are important periods in the yearly cycle at many facilities. Anything delaying the programmed activities can be a cause of significant cost as well as inconvenience. It is remarkable how often sites don’t prepare sufficiently well in advance of a maintenance or shutdown period. Here, Nick Dajda of Crowcon suggests a checklist of tasks to ensure gas detection assets receive appropriate consideration and attention during these events.
Plan well ahead
While this sounds obvious, the key to a smooth plant shutdown is to prepare well in advance. This allows plenty of time to get everything in place and makes it easier to split the planning into smaller, more manageable chunks. Planning the required activities, creating a checklist and assigning suitable deadlines to make sure you get your preparation completed in advance, can be the key to successful and efficient maintenance.
Check calibration dates
Gas detection equipment requires regular servicing and calibration. Calibration intervals are usually chosen as part of a risk assessment which is carried out when the equipment is first purchased and used to protect against specific gas hazards. For the most part, the manufacturer’s standard calibration interval is often used, typically six months. However this period is often shortened to three months at some sites as part of their risk assessment.
Equipment is frequently calibrated en masse, for example by a visiting gas detector service engineer. This means that the calibration for this equipment can also expire at a similar time. Make sure this isn’t going to happen during the planned maintenance period when gas detection equipment is in highest demand.
Check sensor dates
Plant shutdowns are a good opportunity to replace any sensors in fixed detectors which are close to their expiry date. It’s common practice at many sites to replace any sensor with three months or less life remaining rather than recalibrating it. The cost of a couple of months’ shelf life is considered to be outweighed by avoidance of the disruption caused by having to change a sensor during normal operation periods.
Since sensors usually have a limited lifetime, suppliers may not necessarily hold sensors in stock, so order ahead to ensure they are available when you need them.
Maintaining an up-to-date register of sensors and their expiry dates is very a convenient way of planning ahead so you can see at a glance how many sensors of which type are required when.
Have the risks changed?
The day-to-day work carried out on site can vary significantly during a shutdown compared to usual operation. Will anything change that means a risk assessment needs to be reviewed and updated?
This is a good opportunity to evaluate whether the existing portable gas detectors in use are sufficient for the number of personnel present during the shutdown, and whether they still protect adequately against the hazards present during a shutdown. Are there areas where it is now more appropriate to use a multigas portable personal monitor instead of the usual single gas?
Know what your contractors are doing
One well understood limitation of catalytic bead flammable gas sensors is their susceptibility to poisoning by some chemicals. The most prevalent of these are silicones which adhere to the surface of the catalytic bead and reduce the useful surface area available for the bead to come into contact with flammable gas. While most sensors will include a level of filtering to reduce the poisoning effect of silicones, it is still a good reason to consider regular functional checks on your flammable gas detector.
A great illustration of this is an incident that occurred at a company which replaced a window pane of the room where they stored gas detection equipment. Silicon-based sealant of a standard type was used in the process. As a result, all their pellistor sensors failed subsequent testing. Happily this company routinely tested all its sensors, or this incident could be both well-known and infamous. As it was, the problem was picked up, and no-one came to any harm.
Silicon based products, particularly sealants, are a well-used staple of the construction industry and it’s important to understand where and when they are being used, and how this could affect your gas detection capabilities. So, find out in advance what materials any building work may entail.
Control panels typically use some form of back-up battery to ensure that the safety system can still operate during short periods of power outage, typically a few hours at most. These batteries require maintenance and it is good practice to replace these regularly, every two years or so. Failing to maintain at an appropriate interval may mean the control panel is unable to operate for as long during a power outage as battery capacity tends to reduce over time.
It’s a good idea to routinely test the control panel battery to ensure it stays in good working order, and don’t forget to include control panel servicing as part of your pre-shutdown planning.
During shutdown, workers may well be operating for longer shifts than usual, for example twelve hours instead of eight. In areas where background levels of toxic gas are present for a lot of the time, it is important to ensure that portable detectors are capable of accurately measuring time weighted average (TWA) exposure levels to toxic gas. This is especially important when working longer shifts since the total exposure to toxic gas could be 50% higher for a twelve hour shift compared with a shorter eight hour.
Temporary Area Monitors
Temporary area monitors play an extremely valuable role in any plant shutdown. They are a hybrid product which combines the portability of a personal gas detector with the interconnectivity and durability of a fixed gas detection system. Temporary area monitors are much larger than personal gas detectors, since they are designed to be very visible and are usually deployed directly on the ground. Battery lifetimes are longer so they can be used over a weekend before needing to be recharged. Alarms are much louder, since workers could be some distance away, and audible alarms are complemented by highly visible, bright LEDs that can easily be observed from a distance.
What is the range of a temporary area monitor?
Despite their name, temporary area monitors measure the gas at the fixed point in space where the gas sensors are located. Monitors that use a built-in pump measure the gas at the point it enters the sample line.
Temporary area monitors really come into their own when they are grouped together to form a fence. When spaced at an appropriate distance, temporary area monitors can alert to an incoming gas cloud so long as the units are close enough to prevent a gas cloud squeezing between any two monitors. Setting an appropriate distance is important and for a variety of applications this will usually be somewhere between 10 and 25 metres. However, the best distance to use will vary by site and by the different gas hazards present. Ultimately this distance has to be decided and justified as part of the risk assessment taken prior to deploying the detectors.
Fixed system maintenance
One of the most popular uses of temporary area monitoring during plant maintenance is to act as a temporary detection system while different zones of the site’s fixed gas detection system are switched off for maintenance.
In this application, a reasonable sized area of the fixed detection system will be identified. Temporary area monitoring will be deployed around that part of the system and interconnected together to create a safe fence. A survey of the area using a portable detector may be used to check the area for existing gas, before the zone of fixed sensors are inhibited so maintenance can be carried out. When maintenance is complete and the fixed system is operational again, the temporary area monitors can be redeployed elsewhere.
It is occasionally necessary to perform hot work inside a hazardous area where a source of ignition is intentionally created. A temporary safe area needs to be created before any hot work can be undertaken, and temporary area monitors can play an important part in this. Monitors are typically deployed in a tightly-spaced closed fence. The area within the fence is often sampled using a portable gas detector prior to any hot work to ensure there is no fenced-in gas present.
Should a gas cloud come into contact with any of the temporary area monitors forming the fence, the first monitor to detect the gas will go into alarm. Any other monitors that are interconnected will also go into alert at that point, providing the hot worker with a powerful early warning of the hazard.
Another common use for temporary area monitors is for testing a known source of gas, perhaps a section of pipe work that’s awaiting repair. In this scenario, temporary area monitors are deployed in a tight fence around the source and provide an early warning that the hazard is not being contained to within the fenced off area.
There are two ways of interconnecting temporary area monitors. Cables have been used historically and are easy to set up. However, cables present a trip hazard and can also become damaged. The most popular approach now is to interconnect wirelessly. The wireless technology used to interconnect can range from a simple open network to more complex master/slave networks. Open networks are quick to setup but offer no guarantee that all detectors are actually connected together. Master/slave networks employ a master unit which keeps track of everything on the network and can alert when connection to a unit is lost. However, initial setup of the network is usually slightly more complicated.
Disposable personal monitors
When a large number of contractors need to work on site for an extended period, it may not be economical to buy in additional single gas detectors that are rechargeable and serviceable.
Disposable gas detectors, as the name suggests, are intended to be used for a period of time, usually up to two years, and then they are disposed of. They are not designed to be serviced.
These detectors contain a single use battery cell with enough capacity to operate for the lifetime of the instrument. Since the catalytic bead sensors used to detect flammable gases draw quite a lot of current, disposable gas detectors are limited to gases that can be detected with low current draw electrochemical sensors, most frequently H2S, O2 and CO.
Disposable single gas monitors are a way of ensuring that contractors receive suitable portable gas detection at a reasonable cost outlay. A lot of sites make no effort to retrieve the detector from the contractor at the end of the job making asset management much easier than booking out and tracking detectors that have been issued to contractors.
Disposable detectors are not the answer for every shutdown and should only be used as part of a suitable risk assessment.
Shutdown and maintenance periods are always going to be a busy time. But, with the right preparation you can make them go smoothly. Make sure you have the right equipment available. Confirm that it will be in good working order and compliant by checking expiry dates and when servicing is due. Do this all in good time, and hopefully your maintenance will go without a hitch and your site will be fully operational again with the least disruption.
About the author
Nick Dajda is a Business Development Manager at Crowcon Detection Instruments, responsible for product managing the company’s range of portable gas detection equipment. Nick has 13 years’ experience with gas detection and chemical analysis tools used in a wide range of different industries.
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