Water Pressure Defined

[Alan]

I was asked about "water pressure". The question basically is, Does gravity count as pressure when a temporary (mobile) sink is fed by a large igloo cooler with a spicket in the bottom?? The argument is that the concentrated flow that is created demonstrates that the water is under pressure. Any thoughts?

Thanks........

~A

 

[Mac]

The pressure can be measured for compliance with 604.3, irrespective of the source of the pressure. The temporary sink (cooler?) compliance is a different matter.

I personally know of several homes that use water pressure provided by elevated water sources, there are naturally occurring springs located higher than the houses. When electric power is interrupted, they still have water pressure!

 

[vegas paul]

If the temporary sink is being fed by the igloo with a spigot, then the water pressure (head) is constantly decreasing as the water flows. Head pressure is basically the height of the water column (height above spigot, in feet) x 0.4333, yielding psig (pounds per square inch gage). So as the level drops in the igloo (due to flow out of the cooler) then the pressure drops accordingly.

So... you could never really specify a pressure for the "fixture" since it would depend what the level in the cooler was.

 

[Builder Bob]

In our area most water storage vessels (water towers) are at an average of 70 feet to bottom of tower......neglecting friction loss for flow, the water pressure (Static) would be approx. 30 psi if water stayed at the bottom of the tower.

Add another 40 feet in height to the top of the tank (70 + 40'), and the static water pressure goes to 47 psi.

ost houses are at 40 to 60 psi for domestic water.

BTW, this pressure would be the same in a 1/2" diamter tube or a 72 inch diameter pipe..... because this is water that is not moving (at rest)

 

[aintmisnphily]

Could the answer be 423.2 Approval?

 

[JBI]

Alan - What is the exact application for this proposal? 'Temporary' is as elusive a term as 'pressure' in this query... How long will this 'temporary' condition exist, and what is its' purpose?

 

[north star]

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Alan,

Welcome to THE codes forum!

As I understand the basics of atmospheric pressures, whether the water is flowing or not,

at sea level, it is still under pressure. The associated link from Wikipedia has some good

information information regarding pressures.

http://en.wikipedia.org/wiki/Atmospheric_pressure

and

Pressure and Fluids:

In terms of physics, both gases and liquids are referred to as fluids—that is, substances

that conform to the shape of their container. Air pressure and water pressure are thus

specific subjects under the larger heading of " fluid pressure." A fluid responds to

pressure quite differently than a solid does. The density of a solid makes it

resistant to small applications of pressure, but if the pressure increases, it experiences

tension and, ultimately, deformation. In the case of a fluid, however, stress causes

it to flow rather than to deform. There are three significant characteristics of the

pressure exerted on fluids by a container. First of all, a fluid in a container

experiencing no external motion exerts a force perpendicular to the walls of the

container. Likewise, the container walls exert a force on the fluid, and in both cases,

the force is always perpendicular to the walls. In each of these three

characteristics, it is assumed that the container is finite: in other words, the fluid

has nowhere else to go. Hence, the second statement: the external pressure

exerted on the fluid is transmitted uniformly. Note that the preceding

statement was qualified by the term "external": the fluid itself exerts pressure

whose force component is equal to its weight. Therefore, the fluid on the

bottom has much greater pressure than the fluid on the top, due to the weight of

the fluid above it. Third, the pressure on any small surface of the fluid is the

same, regardless of that surface's orientation. In other words, an area of fluid

perpendicular to the container walls experiences the same pressure as one parallel

or at an angle to the walls. This may seem to contradict the first principle, that

the force is perpendicular to the walls of the container. In fact, force is a

vector quantity, meaning that it has both magnitude and direction, whereas

pressure is a scalar, meaning that it has magnitude but no specific direction.

Gravity is a constant variable in your equation, but will decrease as you go higher. The

effect of gravity is at its greatest at sea level.

The real question in your equation is what is the minimum pressure that is desired or

"required" [ by the AHJ, ...if applicable ]? But, ' Yes ', in your scenario, the water

flowing out of the igloo IS under a small amount of pressure.

HTH

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