Skylar:
Velocity pressure acts is in the direction of flow, not the direction
perpendicular to the flow. Velocity pressure ~ ( density * velocity^2/).
I am betting it was just a slip of the vernacular. I make them more often
than I can remember.
Total pressure is, per Bernoulli: velocity pressure + elevation pressure
+ work pressure (e.g. pump)
Bernoulli assumed no friction, so we enhance our hydraulic analysis to
include Hazen-Williams for obtaining that pressure-friction term.
Static pressure is what we measure from the water when there is no flow.
It is the total flow with velocity pressure = 0.
Residual pressure is what we measure from the water with a gauge attached
to a hydrant and when there is flow past the gauge. It is the total
pressure - velocity pressure
Residual pressure is not atmospheric pressure. Residual pressure is what
we measure from the water after some part of the pipe network downstream of
the gauge is opened to atmosphere.
Another inconsequential slip of the vernacular, we all knew what Steve
meant.
There is significant error in measuring water flow:
While the gauges are usually accurate (unless removed by using leverage
on the gauge dial) other waterflow test parameters are less so,
density of the water (from sweet to brackish to sea water)
turbulent eddies
human error in guessing average value from turbulent fluctuations
unknown orifice coefficient
temperature (maybe ice has narrowed the orifice diameter)
The activity of other users on the water network, which changes hour by
hour and year by year
elevation of water at the source (e.g. tower, pond )
phase of moon (if you have vertical turbine pump pulling seawater from
a tidal basin)
condition of the equipment at the muni pump station
valves closed or partly closed on the network
unknown leakage in the pipe network
tuberculation or friction in the pipes (C factor)
incorrect pipe diameter (easy to happen at larger pipe sizes)
When one sees a water supply report to accuracy smaller than a couple of
psi, I wonder why.
When I see people get ants-in-their-pants for a high degree of accuracy
with pressure measurements on water flow tests, I wonder what they are
proving.
When I hear people flying half-way around the world to verify a waterflow
test... that a contractor is motivated to get correct under penalty of
their time and money at fixing their mistake...
I think just as important as water flow testing, is the importance of
vetting the underground commodities. I think it is just as important if
not more so, to witness and verify that the underground plastic pipe is not
solarized, nor fused improperly, that the pipe is not damaged from being
unloaded from the delivery trailer, and that proper thrust blocking is in
place. After all, what is the old joke about 'how to know when a general
contractor is lying?' Things are improving with undergrouund, but there is
still alot of misinformation and poor construction out there.
1). HDPE pipe already has bar codes to better ensure high quality HDPE
fusion welds.
2). We could go one step further, and have the pipe manufacturer enter
their production lot number onto their company blockchain; this would
verify (without ability to easily change the original data) where that pipe
was born, who was the intermediate warehouse retailer and for how long, and
under whose custody the pipe was installed---cradle to grave. Energy
companies have been doing similarly already with barreled-liquid
inventory. Piping and thrust blocks are hidden from view, and if no one
knows how little quality goes into those, the contractor gets off
Scott-free when undetected before expiration of warranty. Smart developers
would encourage pipe manufacturers to offer blockchain vetting of their
inventory, and developers would do well to also write extended warranty
periods on their underground water distribution networks. Blowout of large
diameter pipe can be costly.
Scot Deal
Excelsior Risk and Fire Engineering
gms: +420 606 872 129
> -The velocity pressure is the pressure acting on anything that is
> perpendicular to the direction of flow, like one of our pitot gauges (it
> would be the pressure you would feel pushing you if you tried to stand in
> front of a flowing hydrant) Skyler Bilbo
>
> On Wed, Jan 2, 2019 at 11:33 AM Steve Leyton <[email protected]>
> wrote:
>
>> Pitot measures velocity pressure, residual is atmospheric pressure.
>> There’s not a fixed correlation between the two values – I’m guessing that
>> the main supplying the test hydrants is a very large diameter one?
>>
>>
>>
>> Steve Leyton
>>
>
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