A criticism of PG&E following the San Bruno failure was their inability to promptly close mainline valves to isolate the gas flow feeding the fire. One of the factors contributing to the delay was the fact that the valves were manually operated. That wasn’t the whole story because the manual valves could have been closed quite quickly if the emergency response had been better organised. Nevertheless, in response California recently passed legislation “requiring utilities to install automatic shut-off valves on gas lines”. Exactly which valves, and what type of “automatic shut-off” is not clear.
Mainline valves on Australian pipelines are almost always provided with remote-controlled actuators so that the valves can be closed in emergency to limit the release of gas from a leak or rupture or to allow more rapid depressurising of a damaged section of pipe. However they are not always (not often?) automatic line-break valves but must be closed by a control room operator, who in an emergency would need to respond to observations of the pipeline pressures and flows. Actuating MLVs seems to have been standard practice in Australia for at least 25 years, probably a lot longer. There are exceptions, mostly on older pipelines.
One exception for which I was responsible (about 20 years ago) was 300 km of DN 150 pipeline to a mine in a very remote part of the outback. None of it passed within a kilometre or so of any building (except within the last few hundred metres near the mine) and there were not more than a handful of road crossings. During the design we found it hard to imagine anything that could cause a serious failure, and even if it did fail the consequences would be only commercial. There would certainly be no safety impacts that could be mitigated by prompt closure of a mainline valve. On that basis we decided to provide only basic manual valves and no telemetry.
Actuating a valve would have added perhaps $250 000 to the cost of each site by the time we included all the SCADA, telemetry and power supply infrastructure necessary to make it work. Against that, the value of gas that might be saved by prompt closure of valves was less than $100 000, and of course a loss of containment was highly unlikely anyway. That logic made the decision to omit actuation pretty easy.
Recently I was asked to help a client review the justification for retro-fitting actuators to mainline valves on an old pipeline in a more populated area. It turned out to be a really interesting exercise with a surprising outcome – justification on safety grounds was difficult. That was particularly surprising in the light of the Californian legislation.
The difficulty is that even in the event of a full bore rupture MLVs will not be closed instantly. It will take at least a few minutes for control room operators to positively confirm that there has been a loss of containment. Operators are reluctant to shut valves without compelling reason because of the serious disruption to supply and the flow-on effects of that (eg. purging and relighting pilots on possibly hundreds of thousands of domestic appliances …). In the recent study the consensus was that it might take 15 min before the control room operators sent the signal to close the valves.
However if there has indeed been a full bore rupture, and the release has ignited, we thought that the great majority of the harm would already have been done by the time the valves start to close. People who are close to a pipeline fire and cannot flee will suffer hospitalising or fatal burns within a minute or so of exposure. 15 min also seems quite long enough to ignite any buildings that are vulnerable to fire.
The usefulness of valves is even less compelling when the failure is a leak (even a large one) rather than a rupture. The pipeline section between valves will contain linepack that will continue to flow out through the leak (and possibly burn) for a long time after the valves have closed, perhaps for several hours depending on how long it takes to mobilise crews to start depressurising the pipeline via blowdown vents. Reducing that duration by an hour or so through the addition of actuated valves is not an impressive benefit.
Mainline valves may be more effective emergency protection if they were equipped with sensitive automatic line-break controls that would close them more quickly. But such controls are not 100% reliable, and no operator wants to be exposed to the risk of spurious trips that may cause both serious community disruption and large commercial losses. Given the low frequency of major pipeline failures in Australia, adoption of sensitive automatic line-break valves would probably result in a few spurious trips each year, with associated chaos, in order to protect against failure events that might only occur every hundred or thousand years (these frequencies are just informed guesses for illustration, not serious estimates). Even then automation would only provide some protection against rupture events, not leaks large or small.
This limited effectiveness of actuated valves is a rather unsettling conclusion. Discussion would be welcome.