Glyn,
 
If you like, I can send a videotape to you (NTSC format) of the flushing
of a wet system.  There is a lot of rust in those pipes, and that rust
would form part of the spray going on to collections during normal
operation of a sprinkler system.  The normal operating condition for a
sprinkler head is what we would term an emergency.
 
Dry systems are tested before acceptance.  The test can be performed with
air pressure or water pressure.  If a water pressure method is used,
dissolved oxygen in the water creates rust in the pipes which will flood
out on collections if the heads are tripped by fire (or a cigarette
lighter/candle when the heads are within arms reach; and I have seen them
within arms reach in museums).  Dry systems pressure tested by air
contain oils introduced during the pipe threading process which are not
removed prior to installation.
 
The foregoing presumes iron pipe/galvanized iron pipe.  Copper pipe is a
possibility.  The joints of copper pipe are sealed with solder which may
contain copper as an additive, but is, nonetheless, more other metals
than copper.  Bi-metalic corrosion does not require water to weaken
joints which may fail when called upon to perform.
 
On balance, pressurized systems, properly maintained (and this includes
annual flushing to confirm system integrity and remove accumulated rust),
using sprinkler heads which produce a fine mist, are the safest.
 
Here, I am speaking as one who has had to go into a collection area with
a hard hat and flashlight as a conservator, as an electrician, and as one
who has received training as a firefighter.
 
Jack C. Thompson
Thompson Conservation Lab.
Portland, OR
 
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