Hi Guys,
You are quite correct here Steve it is a terminology thing…… and Yes Mcgyver we are indeed talking about the same thing in our own, NOT SO CORRECT WAY.
Notwithstanding this, I feel it is important, for the newbies who may be reading this, to get the actual cause correct.
As Steve has pointed out…  “Where the argued problem rears its fatal head is when there is an instantaneous event where the super heated liquid is suddenly exposed to normal atmospheric conditions such as through a catastrophic vessel failure”
Not so much an argument, but rather a poor explanation by both Myself, and Mcgyver of what actually occurs.
so lets take another quick look at the thing: -
Since PRESSURE is a MEASURE of energy density expressed as: -
P = force/area which, when related to fluids (and yes, steam is actually classified as a fluid, not a gas),  can also be expressed as P = energy/volume (Bernoulli equation).
It follows then that you cannot have a sudden change in PRESSURE, since this is determined by the other two parameters.
By definition, it cannot, therefore, be a SUDDEN DROP IN PRESSURE that instigates the problem…. a change I one of the other two parameters MUST occur first.
In the case of a boiler rupture the sequence will be as follows: -
Boiler shell fractures (cause irrelevant) which initially connects two separate VOLUMES together (in other words we have a change in the VOLUME parameter.
If the added volume is at a different PRESSURE (in this case a lower one (atmospheric))
We now have a situation where there is a PRESSURE DIFFERENTIAL, and also, by definition, an ENERGY DIFFERENTIAL, between the two connected (via the fracture) volumes….
What follows is the rapid shift of the ENERGY content, in order to eliminate/equalise the ENERGY DIFFERENTIAL…… the high energy content within the boiler volume will move towards the lower energy content of the connected, increased, volume.
As a result of this exchange/movement of ENERGY the resulting FINAL PRESSURE as a whole will drop and is in fact the LAST thing to stabilise.
(if the added volume pressure was higher (meaning there was higher energy within it), then the ENERGY exchange would occur in the opposite direction, and the resulting final pressure within the boiler would rise)
That is not to say that this happens slowly, in fact it happens extremely quickly, however, the sequence of events does not change…… just don’t be anywhere near the thing when it happens.
So you see guys, we all are wrong, yet right at the same time…. It is a complex subject, and one where terminology can often become miss-used; I for one hold my hand up to that one.
Ok chaps, lets put this to bed now, Else we will get told off for arguing by the MOD’s
; 8) ??? ???
'Spoilsports.....Can't have any fun these days' ;D ;D ;D ;D ;D
Best regards.
SandyC.