I just read an article about a horrible injury to a little girl in
Minnesota. This happened in a wading pool and it is difficult to read
about. Here is a link to the story:
It is summer and pool time, so take care of the little ones.
Someone's head(s) should roll over that kind of sloppy oversight and
But, I was puzzled by this line from the story:
In most public pools the drain cover is screwed in and cannot be pulled
off. According to the Consumer Product Safety Commission, the pressure
on some pool drains can be as strong as 300 pounds per square inch.
I don't see how a pool drain could suck with a force of more than one
atmosphere (14.7 psi) plus about another 1/2 psi for every foot of pool
water depth above the drain.
I think they must have meant a "total force" of 300 pounds sucking
something down against a drain opening of about 20 square inches or so.
Jeff (Hoping he hasn't forgotten his high school physics.)
Yes, and if that's the case, how come a suction well pump can't lift
water more than about 25 feet and you have to use a submerged pump for a
300 psi is the equivalent of about 660 feet of water depth.
Not really. Each 33' equals 14.7psi, therefore, each foot is .4545454545
psi. The psi for 660' would be 294. Slightly different in salt water.
However, you did say "about", so you're pretty close.
I think you folks missed the "300 pounds per square FOOT"
That is a couple PSI, 55-60" of water, certainly well within the
suction head of a pool pump.
The fix is simple, a vent to air and/or a second drain path. That's
why one or both are required in the code.
In my pool I have 3 parallel drain paths and a vent.
On Fri, 06 Jul 2007 11:01:04 -0400, email@example.com wrote:
Actually after playing in my pool tonight and screwing with the vacuum
port I change my vote. You also have to take into account the inertia
of all that water in the pipe. The static suction is limited by the
head at the vent or other opening but the instant suction while that
water is slowing down is much higher than even the pump can create
approaching one atmospherre plus the water column. That is the water
I worked in the Gulf of Mexico for Norman Industries and Global Divers from
1974 to 1980. A couple of years sprinkled in there as crane operator for
Reading and Bates, and welding for assorted companies.
Suction head plus hydrostatic cannot exceed atmospheric pressure plus
hydrostatic. In fact suction head really can't get near atmospheric 'cause
you'd get cavitation. Remember, there are no negative numbers on an absolute
On Thu, 05 Jul 2007 20:17:01 GMT, "JoeSpareBedroom"
I believe that even if there were a total vacuum below the water
drain, the maximum pressure would be the weight of the water and air
above the drain. That's the principle of a mercury barometer (except
that there is a vaccum in the tube above the mercury instead of below
it), and if water barometers were made 600 feet tall, that would be
the principle there too.
I don't think suction really exists. All there is is unresisted
pressure, and the pressure is caused by the weight of the atmostphere
(and the water, when something is under water.)
Sort of like centrifugal force doesn't really exist (that is equal and
opossite to the centripetal force, as elementary school science books
sort of said). All there is is inertia that tends to make the
something go straight ahead, perpendciular to its axis of rotation,
not away from teh center of rotation.
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