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igem news – yppc

major constituent being propane gas 14ppm, and that the odorant must be
(98 per cent). A different odorant is of suitable level where it is “distinctive
used with limited data available for the and unpleasant at 20 per cent LEL” .
1
intensity and rate of injection. The thresholds were agreed with the
supplier through their own testing
Technical compliance methods using disposable GasTec²
The Stornoway site is an upper tier tubes, whereby a vapor sample is taken
COMAH (Control of Major Accidents and, utilising a hand suction pump, the
Hazards) establishment due to its concentration level is noted.
capacity for more than 200 tonnes of To provide additional external
LPG. With this title comes frequent assurances on our operations, I engaged
interest and attention from both the with our competent authority, DNV.
HSE and the local community, who are The organisation currently provides
affected by the Public Information Zone SGN with specialist rhinology services
(PIZ). Propane gas arguably poses more including calibrated site rhinologists.
risks to gas customers than natural DNV provided us with a separate report
gas as it is 1.5 times heavier than air, concluding that the existing practices
meaning it can accumulate in low and our changeover management plan
lying areas such as basements, crawl were both compliant and adequate with
spaces or floors, whereas methane the limited data available.
will dissipate into the surrounding
atmosphere. With this characteristic, Implementation
the critical nature of supplying a well- Stock management
odorised gas is further emphasised. The Stornoway LPG site has three
The initial step in rolling out this storage pressure vessels that share the
changeover was an internal feasibility overall capacity of 540 tonnes. One
study and review of current supply key risk considered was the mixing
terminal practices. I carried out a of odorants during the changeover
number of visits to our supplier based period, which led to the generation
in Grangemouth to review existing of a controlled and structured stock
practices and monitor the different management plan for the site. The
characteristics of the new odorant, result of mixing could lead to confusion
ethyl mercaptan. to the gas consumer and potential
It was apparent that technical unidentified gas leaks on the network.
compliance and odorant concentration Once internal approvals were
STORNOWAY HARBOUR, ISLE OF LEWIS, SCOTLAND would play a vital role in the achieved, the programme began with
implementation and, ultimately, the draining stocks down one vessel at a
replacement proved a concern for the sensitivity of the gas consumer to time. However, as this was undertaken
SIU team within SGN. detecting gas leaks in the household or during the summer months to ensure
A review was undertaken on the most elsewhere on the network. With limited continued security of supply during
suitable and feasible solution, following guidance from British Standards and the busy winter period, this took a
which it was clear that moving to pre- a severe lack of similar work carried considerable length of time with low
stenched LPG was the best option. The out in the past to use as reference, the network burn rates. I produced a
decision to transition to pre-odorised initial scoping of our requirements forecasted plan that included vessel
product eliminated the ongoing risk proved challenging. swap-over dates and the first loads of
of transporting unodorised LPG across The addition of odorant to LPG EM-stenched LPG from our supplier.
Scotland with no ability to conduct leak should comply with the requirements Some mixing was inevitable during the
detection in the event of a road tanker of BS4250: Specification for process, however this was managed
failure or unexpected defect. Commercial Propane & Butane. This and minimised to one vessel, of which
Due to the critical nature of gas states that when EM is added the a phased approach was taken to supply
odorant and the fine line between content in liquid phase should not the network to again reduce the impact
over- or under-odorising gas – both be less than 14cm³/m³, equivalent to on the gas consumer.
of which carry associated risks – the
project gathered a significant amount of
internal focus and attention. FIGURE 1 GASTEC DISPOSABLE TEST TUBES
SGN is well-established in odorising
natural gas across Scotland and
southern England at a number of high-
pressure sites. As these networks are
all on the UK gas grid, the gas odorant
and injection rates have been tried and
tested for years and there is consistency
across the other gas distribution
networks (GDNs).
However, the Stornoway network is
unique in that it is fuelled by LPG, the

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