08-16-2016 01:08 PM - edited 08-16-2016 01:09 PM
This is an issue that has gone back and forth for many years. The best answer is "it depends". First off, what engine are you talking about?
The concerns about having a secondary means of gas shutoff actually comes from older engine technology, in modern engines with electronic controls and actuators, when a control problem occurs then pretty much everything shuts off, and the fail mode of the actuators and valves without power is to close. But if you have a unit with a hydramechanical governor, or even a 2301A and a hydro-electric actuator, certain failure modes would cause the actuator to fully open the throttle, if the upstream gas shutoff valve failed, then either the engine could overspeed, or you could cause raw gas to be pumped into the cylinders and out into the exhaust (if you had a failure of the ignition). To make a bad situation worse, if you had a 3 way catalyst in the exhaust at that point you could cause a pretty spectacular exhaust explosion.
So you best bet may be to actually do a true hazard risk assesment based on your engine type, installed equipment, aftertreatment (if equipped), and your installation particulars. as you perform the risk assesment also keep in mind you may be called upon to actually test and demonstrate depending on the requirements of your local regulatory people, so a test plan would also be a good exercise as well.
Without knowing more about your actual installed unit and system at this point its hard to provide a better answer.
Hope that helps, MikeL.
08-15-2016 10:34 PM
In Australia, there is a boiler code that requires natural gas consuming appliances to install additional protection on the fuel gas train to positively prove isolation of the fuel gas line. In a boiler situation this makes sense but in a combustion engine it seems misplaced.
There is provision in the code to risk assess the standard in lieu of compliance. As part of this process, I would like to show whatever gas can bypass the engine while the engine is off, will be below the lower explosive limit.
My understanding is that the only raw gas that could escape into the exhaust is due to a burnt or bent exhaust valve, and where such damage has also impacted the intake valve.
My questions are:
1. Is this credible for a regularly serviced engine?
2. Does the engine management system detect a reduction in performance and cause alarm?
3. Are there other controls in the engine while the engine is off, that might block fuel gas entering the cylinder and further reduce credibiity?
Thanks in advance. I appreciate your time and insight.