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Development of R290 transport refrigeration system (Part 3)

Development of R290 transport refrigeration system (Part 3)

 In Part 3 of his paper, Daniel Colbourne carries on assessing the safety measures in the groundbreaking R290 project currently being trialled locally by Transfrig in association with the GIZ, focussing on risk assessment in particular.

Assessing safety

Risk assessment
With a functional prototype unit, a quantitative risk assessment (QRA) is necessary to estimate the probability of ignition and the severity of consequences. It essentially identifies the likelihood of a leak, the chance of a SOI being active and within the subsequent flammable mixture arising from the leak. This should be conducted in consideration of all the possible operating modes, load conditions, RRV states and so on.

The likelihood of leakage is considered for the various normal operating conditions (on, off, during defrost), but also when other typical faults may have occurred, for instance during failures of the engine/alternator/battery, condenser/evaporator airflow, compressor, EEV, hot gas bypass valve. In addition, the equipment also relies upon a leak control safety system to minimise the quantity of refrigerant entering the refrigerated space so a failure mode and effects analysis (FMEA) is necessary to appraise its reliability and how it could impact on the overall risk. Using the service database, it is possible to assign fault probabilities to the various elements, which provides overall probabilities of failure for the leak control system.

For the QRA, each of the possible operating conditions and failure scenarios therein are evaluated for the various situations: RRV in use or not in use; RRV being stationary (closed or un/loading) or in transit; empty, loaded or partially loaded. Considering selected leak hole sizes, the subsequent volume and duration of the flammable mixture and the type and characteristics of possible SOIs within that mixture are estimated for each relevant CV. SOIs were accounted for in terms of faults of electrical components and where relevant their external protection, but also possibility of introduced SOIs from workers, such as lighting cigarettes, use of power tools, and so forth. Using standard techniques (for example, Colbourne and Suen, 2008) and leak frequencies from a previous study on R290 in transport refrigeration systems (Jansen and Van Gerwen, 1996), ignition frequencies were determined.

An example of the results using more pessimistic assumptions is provided in Table 2, which also includes values for maximum consequences, overpressure (OP) and thermal intensity (TI). The ignition frequency is low, that is, less than one ignition event per 1 million RRV-years. The most vulnerable locations are the condensing unit and the refrigerated space. For the condensing unit, even though the likelihood of a formation of a flammable mixture is extremely low (due to high air speeds), it contains many electrical parts and if there is a fault with any of them, or if the pre-purge ventilation fails, it is assumed that the resulting spark, arc, or high temperatures could ignite a release.

Table2MM

With the refrigerated space, the possibility of a flammable mixture is far more likely due to the confinement, although the possible presence of an active SOI is relatively slim. Surrounding the condensing unit, apart from there almost always being high airspeeds, it is extremely unlikely that any SOIs would be introduced and therefore the ignition frequency is substantially lower than elsewhere. For ignition of a mixture in the condensing unit and surrounding area or in the evaporating unit, the TI imposed on a person 1–2m away is very low and would be insufficient to cause pain.

Conversely, the TI for a person theoretically standing at the door of the refrigerated space is substantially higher and implies a small (<10%) possibility of mortality. However, as was shown in the zoning tests, such a flammable mixture would not occur if the doors were open (the exception being if a person were trapped inside the space). Similar to the TI, the OP from ignition within the condensing unit and the surrounding area and evaporator unit is very low, but fairly high for ignition within the refrigerated space (when the doors are closed). The resulting OP is dictated by the values at which the doors will be forced open.

To put these values into context, they can be considered against comparable risks, such as the frequency of fires of refrigeration appliances or frequency of truck accidents. Comparing against the fire risk of comparable equipment for which data is available, the background fire risk (that is, typically due to electrical faults and not associated with the use of flammable refrigerants) for air conditioners in the USA about 2 × 10−5 y−1 (Hall, 2012) and about 1 × 10−5 y−1 for domestic refrigerators in the UK (DCLG, 2014). These fire frequencies are about 100 times higher than the estimated ignition frequency. In terms of vehicle accidents, OGP (2010) states 22 injuries and fatalities associated with goods vehicles per 109 vehicle-km travelled; this gives an injury/fatality frequency of 2 × 10−8 km−1. Assuming that RRVs travel somewhere in the range of 10 000–50 00km per year, the frequency of injuries/fatalities is no less than 1 × 10−4 y−1 per vehicle.

Even on the pessimistic supposition that every single ignition event led to an injury or fatality, the additional risk from using R290 is 1 000 times lower than the background injury/fatality risk from road accidents. Another way to gauge the calculated risk is to compare against targets from health and safety authorities. For example, the UK Health and Safety Executive considers the risk of fatality of one in a million (workers or members of the public) per annum to be extremely low and thus is deemed ‘negligible’ (HSE, 2001). Since the incidence of fatalities from ignition of gas releases is around 1 in 100 to 1 000, the additional risk posed by the use of R290 is easily considered as ‘negligible’.

Compliance with safety standards
The applicable safety standards include EN 378 and SANS 10147 — assessment against prEN 378: 2016 and SANS 10147:2009 confirmed prototype satisfies all requirements. 

 Acknowledgements
The authors would like to acknowledge the German Ministry for Environment, Nature Conservation, Building and Nuclear Safety and the Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) GmbH and Transfrig Ltd for giving permission for publication.

Guidance
Awareness of those involved with the RRV refrigeration system — primarily service technicians and the driver — is important for risk reduction. Guidance on how to service the refrigeration system safely must be provided to technicians in the form of literature and training. Additionally, truck drivers must be briefed on the topic and made aware of the leak control and safety system, so that they can take appropriate precautions if there is an indication that a major leak has occurred within the refrigerated space.

Final remarks
A prototype refrigeration system for RRVs has been developed. Major aspects of the development were significantly reducing the refrigerant charge and adopting a special leak control and safety system. A risk assessment indicates that the level of flammability safety of the prototype should not present any concerns. Now that the prototype has been developed and tested, it is undergoing field trials, so far with favourable results.

The motivation behind the project is to reduce the emissions of greenhouse gases. An initial assessment of lifecycle emissions suggests about 16% lower diesel consumption — which translates directly into reduction in ‘indirect’ emissions — and on account of the negligible GWP of R290 an elimination of ‘direct’ global warming emissions. Overall, the R290 model is expected to generate only 34% of the global warming emissions (in terms of tCO2-eq) compared to the R404A baseline model.

Read Part 1 of this article

Read Part 2 of this article

 

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