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Gas Testing for Confined Spaces

Published: 11th Oct 2013

Warren Fothergill discusses confined space safety in the oil and gas industry.

Throughout the Gulf region, particularly within the oil and gas sector, there is a legal aspect to consider when keeping workers safe. Primarily, the industry depends on whether standards and requirements are derived from (OSHA) or from British legislation.

Depending on which standards are utilised, this will determine the process of incorporation into that country’s own health and safety regime and the legal or moral requirements imposed.

In all honesty, it shouldn’t matter. The principle objectives of both sets of legislation are the safety of the individual, and protection of the asset and the environment.

OSHA legislation OSHA 29CFR 1910.146 is primarily a standard, developed to define work plans for confined space entry, while the UK legislation, The Confined Spaces Regulations 1997 applies where the assessment identifies risks of serious injury from work in confined spaces. It also introduces specific duties to ensure safety – for example avoid entry; follow a safe system of work if entry is unavoidable; and have emergency plans in place before work starts.

The common theme is the plan and system of work aspect, which is the key concept for this paper – particularly the importance of the gas test in confined space entry.

Understanding confined spaces

When we think of a confined space, places that come to mind include a chamber, tank, vat, silo, pit, trench, pipe, sewer, flue, well or other similar space. According to The Confined Space Regulations 1997, a confined space can be any place in which, by virtue of its enclosed nature, there arises a reasonably foreseeable specified risk.

The OSHA definition is any space that is large enough and configured so that an employee can bodily enter and perform assigned work, has limited or restricted means for entry or exit and is not designed for continuous employee occupancy.

Already we begin to see that the interpretation is different and is dependent on the legislation followed.

Reasons to test for gases

Atmospheric monitoring is a process of obtaining air samples to determine conditions of the process parameters, either operable or non-operable. It also provides relevant information for any confined space entry team.

The correct method of undertaking the monitoring, using the relevant plant and equipment will provide knowledge to enable personnel to correctly and safely obtain the sample and analyse the results, providing a level of safety to people, the assets and the environment at all points in operations and entry.

What to check for

In confined spaces there are a number of atmospheres that are hazardous to health. These are:

• Oxygen enriched

• Oxygen depleted

• Flammable

• Toxic, which includes H2S

We must also identify the potential that Volatile Organic Compounds (VOCs) and Hydrocarbons may be present within the confined space.

Gas properties and structure

Gas is a state of matter, in which particles do not have either a defined volume or shape. As a structure, gas has no order, its particles are arranged at random and they are so far apart that there is a force of attraction. The particles of gas are unaware of each other’s existence unless they collide.

Gas will fill its entire container, which is a major issue when entering into a confined space, although it is easily compressed. As the temperature increases, so too do the energy and speed of the particles, which in turn results in pressure increasing.

Reasons to test for gas

One obvious answer to this question is simple: the risk and the consequence. The risk is so great that it needs to be tested for, the consequences of not testing, potentially lethal.

Studies conducted by the National Institute for Occupational Safety and Health (NIOSH) between 1983 and 1989 reviewed confined space entry deaths. In analysing 88 deaths from 55 incidents, NIOSH found that only 27% had any safe system of work for entry, and only three had training. Furthermore, another OSHA study identified that of 188 fatalities in confined spaces between 1984 and 1986, 146 deaths were from oxygen deficient air and toxic gases.

For a ten year period (1980-1989) the National Traumatic Occupational Fatalities (NTOF) unit in the United States analysed confined space related deaths. This breakdown can be seen in Figure 1. NTOF also identified that many fatalities in confined spaces are rescuers.

Atmospheric monitoring is the first and most critical rule, as most fatalities in confined spaces are the result of atmospheric problems. Remember, your nose is not a gas detector. Some hazards may have characteristic odours, but others do not. It is important that any entry to a confined space, as previously determined, is structured under a system of work, namely a Permit to Work. Personnel should have knowledge of the space itself in order to determine the hazards associated with entry.

For any required confined space entry, it is important to identify the possible gas hazards that could be encountered.

OSHA states that in every case, before an employee enters a permit requiring confined space, “the internal atmosphere shall be tested with a calibrated direct reading instrument.” This way it continues to determine the principle atmospheric conditions to be monitored.

Even when you can detect the presence of a hazard, you cannot determine the extent of that hazard. Some gases, such as hydrogen sulphide, can even temporarily deaden your sense of smell after short exposure. This deceives the worker into thinking the problem has gone away, when in fact all that went away was the ability to smell.

The only reliable method for accurate detection of atmospheric problems is instrument monitoring. Basic confined space atmospheric monitoring should include oxygen concentration and flammable gases and vapours.

Wherever you work in the world there may be regulatory limits, but such limits provide only minimal protection. Best practises dictate that any variation from normal, which is 20.9% oxygen and 0% lower explosive limit (LEL), should be investigated and corrected prior to entering the space. Elimination of the hazard is achieved, thus following the hierarchy of control principle and reducing levels of risk to as low as is reasonably practicable.

Toxic monitoring requires an evaluation of potential atmospheric contaminants before you even determine how the monitoring will be performed. Simply put, this means you must establish what you need to look for in order to determine what equipment to use.

The following digital instruments are available for common toxic contaminants:

• Electrochemical sensors measure carbon monoxide, hydrogen sulphide, sulphur dioxide, ammonia, chlorine, and other gases

• Infrared sensors measure carbon dioxide and several other gases

• Photo ionisation and flame ionisation detectors measure VOCs at the parts per million (ppm) level. This may be required if solvent vapours are present. These vapours will exceed the limits for inhalation long before they will be detected with most LEL meters

• Colourimetric tubes can be used to determine whether a toxic contaminant is present, in situations where no digital instrument is available

A thorough assessment of the atmospheric conditions in the space must be completed before entering the space, and should be continued during the entire entry.

Monitoring and testing processes

It is important to determine the three phases of monitoring the confined space:

• Prior to entry - externally

• Continuously - during entry

• Prior to any re-entry - externally

Prior to entry from an external point is critical, but how and where should we monitor within the space? That is an important aspect due to stratification, which means a state of many layers. Measurements should be taken from the top, middle and bottom of any confined space.

Measuring at three points within a confined space is critical. The stratification of confined spaces can be seen in Figure 2. Here, it is shown that methane is lighter than air, carbon monoxide is the same as air and hydrogen sulphide is heavier than air. This means that we must consider our breathing zones, and also liquids and sludges that may be present in the confined space environment.

All of this should be done externally.

Flammable vapour explosion

The following case study details an explosion that occurred at a chemical plant.

Hot work was being conducted on the Polyvinyl Fluoride (PVF) slurry tank farm, located adjacent to the manufacturing building. Contractors were tasked with repairing damaged agitator supports for slurry tanks numbered one to three, when the manufacturing process area was under a planned shutdown. Due to issues with procurement, however, the materials were not available to repair the third tank until after that period.

The manufacturing process was shut down on October 21, 2010, with slurry pumped out of the first two tanks. These tanks were then locked out by the maintenance team, cleaned and entered. Damage was identified on agitator supports following removal of a tank’s insulation, and an engineer wrote a works order for the repair of the agitator supports, with a work scope generated by construction field engineers.

The valves of tank one’s fill/discharge lines were then locked out, and following the site’s lockout and hot work procedures the contractor performed the repairs on tank two on the same day, although he had to delay tank one’s repairs because of a lack of materials.

During the internal inspection of tank two, engineers discovered a corroded seal loop on the flash tank overflow line and concluded the operation unlikely to impact on safety. This was because they did not know its actual purpose. The company determined that the repairs to tank one could be completed following a system restart later in the month.

On November 6, 2010, the manufacturing process was restarted, with the valves aligned to allow the flow to tank three, the equaliser line connected to all three of the slurry tanks. The line was not isolated or disconnected from tank one prior to the works. On November 8, 2010, the compressor within the manufacturing unit failed when the unit restarted without it, which had the effect of doubling the vapour present in the slurry flowing to the flash tank, and this mode of operation continued up to the time of the incident.

A permit to work (PTW) was in place on the day of the incident, with a lockout card completed for tank one. It indicated all five valves to and from the tank and the agitator motor were locked out, although it instructed the contractors to attempt a start of the motor before commencing work. Having no valves, the overflow line was not blanked or isolated. A hot work permit was completed, with contractors not checking valves. Gas testing was completed around the top of the slurry tanks. No flammables were present, and continuous gas monitoring was also conducted by a fixed monitor.

As we can see from this case study there were a number of failures, but while the PTW requested gas testing, it was done externally to the area being worked upon, and not in the confined space which it was impacting, e.g. heat of steel, flammable substance and oxygen.

This gave rise to an explosive atmosphere. One fatality, one severely injured individual and massive ‘farm’ damage was incurred. It only needed for the tank that was to be worked on to be checked and workers would have seen that polyvinyl chloride (PVC) was present.

Gas testing can save lives. Not gas testing correctly can kill.

Published: 11th Oct 2013 in Health and Safety Middle East

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