On 4 Mar 2006 14:19:50 -0800, "Aidan" wrote:



Kin 'ell! Is this a wind up? Leave it alone.

Immersion heater contacts frequently weld themselves together, so the
heater fails on. The open vent would then discharge steam. If you block
it, the steam will blow the contents of the cylinder into the storage
tank. If that discharge route is blocked, this happens;

http://www.waterheaterblast.com

That is a small water heater (12 US gallons). Drivel loves posting that
link to promote his thermal stores, but someone proposes plugging the
vent on a domestic hot water storage system and he makes no comment; he
is a dangerous idiot.

Unvented water heaters are good, but they have at least 3 sequential
safety devices for each heating system. They are costly. A vented
cylinder cannot be converted.


An interesting story is the sinking of the Fleur de Lys, a fishing
vessel.

This one went down, fortunately not killing anybody, when its 120
litre HW cylinder, a class 4 type (weakest rating normally used for
open vented systems) was being used in a pressurised system.

A firm of plumbers who didn't know what they were doing had replaced
the cylinder and fitted the wrong type.

http://www.maib.gov.uk/cms_resources...ety_504222.pdf


The cause of the explosion was listed as follows:

"The hot water storage cylinder ruptured because the water in it
overheated and steam generated, causing overpressure. The rupture of
the cylinder probably occurred at about 3.4bar, four times working
pressure of the cylinder. The pressure relief valve must have been set
at this pressure or above, or it was seized in the closed position.

The cylinder probably overheated when the immersion heater thermostat
failed to shut off the electrical supply to the heater. There were no
safety devices fitted to the system to prevent overheating and
overpressure as a result of thermostat failure."


Returning to the original point of blocking the vent - it is quite
common to have an isolating gate valve etc. on the cold feed to the
cylinder. If this is closed when the heat source is still on and
the vent is blocked, a potential bomb will have been created.

We can see from the fishing boat example that there does not need to
be a great deal of pressure at all to achieve an unfortunate
situation.









--

..andy