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Bump testing doesn't have to be a headache

17 September 2013

There are many reasons why a portable gas detector may not react to gas, some of which may not be evident to the user. The safest way to make sure a gas monitor is working at its optimum is to regularly test the unit. This is known as a gas or ‘bump’ test, explains Crowcon.

The following scenario highlights the importance of bump testing:
Peter works at an oil refinery and arrives on site in the morning to carry out his daily tasks. He wears all the safety equipment required for safe site entry including a single gas hydrogen sulphide (H2S) gas monitor. His job for that day is to carry out some heavy cleaning around the site using a jet washer.
At the end of his shift he returns to the changing room to clean up. He notices that some of his equipment is dirty and cleans it using cleaning agents containing alcohols and silicones.
The next day he arrives on site as usual and gathers his equipment before getting briefed on his work schedule. Later that day he is working in a low-lying area when he smells something similar to rotten eggs, but the smell quickly passes and, as his gas monitor did not react, he continues working. Not long afterwards he suddenly collapses after being overcome by high levels of H2S.
Luckily, Peter’s colleague John sees him fall and rushes to his aid. As John approaches Peter, his H2S gas monitor goes into alarm. Following the correct site procedure, he calls for safety personnel equipped with breathing apparatus to rescue Peter, who is luckily unscathed but has to take two weeks off work to recover.
What are the lessons to be learnt in this scenario?
The refinery had provided all the equipment required to keep him safe. The situation was created by Peter himself – a situation that could have been avoided if he had followed correct procedures.
Cleaning his equipment, including the gas monitor, with cleaning products that contain alcohols and silicones was Peter’s first mistake. Sensors can become poisoned or inhibited with these types of cleaning agents. Alcohol cleaning agents can also damage electro-chemical sensors, and silicone-based products should never be used with catalytic bead sensors intended for measuring hydrocarbons such as methane, pentane and propane. The silicone attaches itself to the active bead and heat causes a film to form around the active bead, disabling its ability to react to potential gases. Consequently, most hydrocarbon gases would not be detected and the unit would not react. The circuit, however, will still be complete – so the unit will continue to give a normal output to the user who would think it was working normally. This risk can be avoided by carrying out regular bump tests.
To check the lower explosive limit (LEL) sensor for silicone poisoning, first apply methane (CH4 50% LEL) to the unit. If the unit reacts, this indicates that the sensor is not poisoned and the unit is safe for use. If, however, the unit does not react, apply hydrogen (H2 50% LEL) to the sensor. If the unit responds to the hydrogen gas but not methane, then the unit has been poisoned by silicone.
Note that while most LEL sensors are fitted with a silicone resistant filter, this will only protect the sensor to a certain extent. Also, be careful when choosing the type of LEL sensor, as sensors with silicone filters will not detect, or have a slower response to, larger hydrocarbons such as ethanol, methanol and nonane. 
What should Peter have done differently?
He should have referred to the gas monitor’s manual, which would have instructed him to clean the unit by using warm water only or, if necessary, with a cleaning agent that does not contain alcohols or silicones. Gas equipment manufacturers can provide additional guidance.
After cleaning the unit, Peter should also have carried out a bump test to ensure that the sensors were responding as they should and that the filters had not been blocked or inhibited.
Upon arriving on site he should also have carried out another bump test on his gas monitor. This would have highlighted the following: 

  • The unit was not functioning correctly, indicating potentially blocked filters, poisoned or inhibited sensors or even a faulty sensor. The bump test would also have enabled him to check that the audible and visual alarms were working and that the sensors’ response times met their required T90* response time
  • Peter would have then replaced his non-compliant unit with a functioning one, which would have safely detected and warned him about the H2S gas release which caused him injury.

* T90 = the time it takes for the sensor to read 90% of the test gas concentration.