Posted by Eric Boelen on May 30, 2000 at 13:03:04:
In Reply to: Re: Effect of steam space on steam quality posted by Sanat Shetty on April 13, 2000 at 07:50:14:
: : How far above the tubes is your water level?
: : What is the diameter of the boiler shell?
: : Are you having problems with wet steam?
: : Is the water level in the water gauge sight glass normal?
: First of all, thanks for the replies.
: I am doing some theoretical work here on the steam space that would be
: available and the droplet size distribution in the entrainment under
: steaming conditions in a packaged firetube boiler.
: I have a shell diameter of 83 inches, 218 inches long.
: The water level is 2-3 inches over the tubes and under steaming conditions
: after taking into account the level swell, the steam space clearance can go
: down to less than 4 inches, especially at low operating pressures.
I saw your question and, depending on the applicable code, there is
already something wrong. ASME Code requires 4 inches
of water above the tubes for any boiler running at
more than 15 psig steam pressure. The swell you talk
about is not really going to happen, except for
start up conditions. The automatic water level controller
will not feed water until the level has dropped
down to the minimum level, so after initial swelling
the water will start dropping as steam is formed and
the water level will settle down. 10 inches above
the waterline is NOT acceptable in a 650 HP unit, I would
recommend at least 18 inches. The other devastating
effect from a too low steam space is the high horizontal
velocity of the steam toward the steam nozzle.
this will create a wave effect with a higher water
level right under the steam nozzle thus creating more carryover.
The water surface at normal water level should be such
that the steam velocity leaving the waterlevel does not exceed
10 to 15 m/minute Higher velocities will rapidly (not proportionally)
increase the carry over.
Let me know if this was helpful
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