Confined space incidents continue to be over-represented in the health and safety statistics. Despite many years of learning what is required to prevent ill health, injury and death during confined space work, we still regularly see confined space management errors repeated.
We investigated an incident after a maintenance fitter was tasked with welding a bracket to a stanchion on the inside of a large reactor vessel that usually contained a flammable liquid. The vessel was cleaned and flushed, and the fitter used a gas detector before and during entry. He entered the vessel wearing a harness attached to a winch outside. When he started welding, what he thought was residual water around his feet ignited. The vessel filled with smoke and his legs were badly burned. The spotters outside the vessel had not been trained in the use of the hoist and rescue took an extended time.
Defining confined spaces
When establishing confined space entry procedures the word “confined” is often misconstrued, with it being thought the spaces are small and there is limited room for movement. Large bulk storage tanks on ships, silos and fuel storage tanks at refineries are confined spaces. They can also be open to the atmosphere; consider a farm slurry pit or a deep bund around a chemical storage tank. While the definitions are a starting point we should not be driven solely by these. We often classify confined spaces by the hazards and risks to which people may be exposed and these generally include fire or explosion, asphyxiation, drowning, and other acute risks. Often forgotten is that flowing solids such as grain in a silo can “drown”.
In general, confined spaces are only accessed occasionally for maintenance or inspection purposes rather than being regular work locations. However, just because a confined space is accessed frequently, it does not change it from being a confined space; the hazards and the risk remain regardless of frequency of access. A risk assessment will determine whether a work location should be treated as a confined space and what risk control measures should be in place. Local legislation should be referred to.
Risk control measures
Determining risk control measures should start with the question of whether confined space entry is necessary. With some thought, often at the design stage, it is possible to enable work to be undertaken from outside the space, for example including installing cleaning-inposition technology, using remote video surveillance or simply being able to retrieve equipment for maintenance without entering can be quite simple and low cost. For example, sump pumps can be connected to flexible or detachable pipes and attached to chains so that they can be hauled out for maintenance.
If entry is necessary, the risk assessment should not only consider hazards that are present, but also hazards that may be created by the work. If the fitter undertaking the welding in the reactor vessel had been instructed to bolt the bracket to the stanchion rather than welding, not only the ignition risk would have been reduced, but also exposure to welding fume in the enclosed space would have been avoided.
The work that is undertaken, the tools, equipment and substances taken into a confined space can create risk in what may otherwise have been a low risk environment. Two farm workers wanted to pump water from a clean underground water tank. There was no electricity supply and so a petrol engine pump was used. The pump did not have a long enough hose, so they took the pump into the tank with them. They died of carbon monoxide poisoning. A worker was asked to clean the inside of a process tank with a hydrocarbon cleaning solvent. He cleaned for 15 minutes before taking a break, leaving the rags and open solvent can in the tank. He returned to the tank and collapsed and was later found dead.
The nature of confined spaces is such that the consequences of an incident are often high. It is very common that an incident in a confined space will lead to well-intentioned, ill-prepared rescuers rushing to help, resulting in multiple fatalities.
Prevention of confined space incidents requires a clear and comprehensive risk management procedure. This will not only address the work planning and risk assessment, but also the risk control and emergency response. A comprehensive risk assessment of the proposed welding in the reactor vessel mentioned earlier would have identified the potential for ingress of flammable liquid after the flushing had been completed. It transpired that supply pipes simply had the ball valves turned to the closed position and liquid had “bled” around the valve. Isolation of all energy sources and blanking of feed pipes is an essential element of confined space entry that ensures restoration of energy and services and ingress of materials during occupancy is prevented.
“it is very common that an incident in a confined space will lead to ill-prepared rescuers rushing to help, resulting in multiple fatalities”
Correct use of a gas detector would have identified the flammable material in the reactor vessel before the welder started work. In addition to flammable substances, confined spaces may be oxygen-deficient and contain toxic gases and asphyxiants, and so gas detection is generally a critical step in a confined space entry procedure. If the worker using the solvent to clean the inside of the process tank and the farm workers using the pump had used gas detection, they would have been alerted to the increasingly hazardous atmosphere.
Gas detectors can be used for many purposes: from detecting and measuring oxygen, toxic and asphyxiant gases and vapours before entry, and sounding an alarm to warn people not to enter or to evacuate, to potentially operating a slam shut system to reduce the amount of any leaked substance.
Careful selection of gas detection equipment is essential, and choices are made after weighing options. These include the following:
Type of sensor
This is dependent on the gas that may be present and this is, in turn, dependent upon the risk assessment. The sensor types range from infrared to photo ionisation and are detailed in BS EN 50073. It is important to check that other gases that may be present will not interfere with detection of the target gas.
Diffusion devices are slower to respond than aspirated ones that draw air to the sensor. An aspirated system will detect more rapidly but can sometimes lead to erroneously high readings of flammable gas levels in the short-term.
Fixed or portable detectors
If occupancy is infrequent then portable, personal gas detectors are more suitable, whereas in more frequently accessed spaces, a fixed monitor is potentially a more reliable solution. A draw-back of a fixed monitor is that it may not detect pockets of gases that a person may move into. A combination of fixed and portable devices may be required.
Safety Integrity Level (SIL)
Depending on the way in which a fixed gas detector is used, it may be a part of a process or machine, in that it may automatically stop that process or machine. If this is true, it will be required to have a specific SIL or Performance Level (PL). This is directly associated with the reliability of the detector and associated alarms.
Point or open path
In most cases, multiple point source detectors are preferable to open path detection (these use an infrared beam across an area) that can be specific to a gas and locations.
“people who will undertake the work must fully understand the theory that underpins detector functions and their use”
Once detectors are adopted as a part of a confined space procedure, it is essential that regular checks and calibration of the devices are undertaken. Often “bump tests” of portable devices are conducted immediately before entry. This involves testing the device against a sample of the substance that it is suspected is present to confirm the device is functioning correctly. The welder in the reactor vessel did undertake a bump test. However, it is questionable whether he used a sample of the material that leaked.
Managers of the confined space entry processes and the people who will undertake the work must fully understand the theory that underpins detector functions and their use. This requires an understanding of how the hazards can cause harm and how the detectors can fail.
A welder was asked to weld in inside a pipe using an oxy-acetylene torch. The fume began to increase and so he was told to clear the air with a
blast of oxygen from the gas cylinder. An understanding of the hazards of oxygen-enriched atmospheres and the use of an oxygen detector would have alerted him to the risks. Unfortunately, he had no detector and neither he nor his supervisor understood the risk, so he re-ignited his torch and his hair and clothes immediately ignited and burned fiercely.
In another example, at a chemical process plant, a reactor was isolated, the atmosphere was checked for contaminants and oxygen levels, and the workers entered to undertake inspection and maintenance work, equipped with wearable gas detectors. The job was successful, and the workers left the reactor and the isolation was removed. It was then realised that another small, quick job was required. During the first period in the reactor, no gas was detected and so the workers assumed that all was still well and two re-entered without restoring the isolation and without gas detectors. Hydrogen had leaked from one of the pipes, it ignited and both men were killed.
The importance of using gas detection at all stages of confined space entry was illustrated by an
incident involving a maintenance worker entering a drainage pit. It was known that the pit was likely to contain hydrogen sulphide and so he was going to use self-contained breathing apparatus. In preparation for entry he removed the pit cover and descended the ladder to waist height where he was going to fit his respirator facepiece and attach his rescue harness. Had he been using a gas detector he would have been alerted to the gas emanating from the pit that caused him to lose consciousness and fall from the ladder to the bottom, where he died
A triple fatality occurred in an effluent pit at a brewery. A worker descended a ladder to collect a sample. He noticed a rotten egg smell, but it went away. The smell was hydrogen sulphide which has a low odour-threshold. However, at high concentrations the olfactory sense is rapidly overwhelmed, and people think the gas has gone. Knowledge of the properties of hydrogen sulphide and a gas detector would have alerted him to the high concentrations as soon as he commenced his descent and for as long as the gas was present. As he turned to climb the ladder he fell to the bottom, unconscious. Two colleagues rushed to help, but fell to the bottom and they too died.
The high-risk nature of confined space work and the possibility for error in the correct and full execution of procedures means that emergency plans must also be developed, and emergency response practiced. Indeed, training and education underpins the entire confined space entry process from the conduct of the risk assessment, through the isolation, blanking, lockout and tag out, gas detection and written permit completion, to the emergency procedures.
Had the welder working in our reactor vessel been trained in gas detector use and the device tested, the failure of the isolation would have been identified. Had the spotters been trained in the emergency procedure, the welder’s injuries would have been far less significant.
If all work within confined spaces was only undertaken after all alternative options had been exhausted and then adequate risk assessments, full isolation, accurate gas detection and on-going monitoring was in place, the saddening recurrent reports of illhealth injury and death might cease.