When I have done hydraulic calculations for pumps and piping systems, I
have done it in "psi" (pounds of force per square inch), "feet of
water", dynes per square cm, and pascals. Mostly that was in nuclear
power plants (submarine) but once or twice that was done later in
civilian applications.
Head is a pressure. One can look at suction head (minimum required and
actual), pump head, head losses due to flow, head changes due to
elevation, etc. Often, in water systems, folks divide all the terms in a
loop law equation by the acceleration due to gravity (g) and density
(rho) and then they are working in "feet". If density varies within a
loop, one cannot do this without a bit of hand waving to "normalize"
each term.
Hydraulic loop equations are analogous to loop equations used in
electrical circuits.
The use of "feet (of water)" in hydraulics is the same as the use of
"inches (of water)" or "millimeters of mercury" in hospitals. The former
is used to indicate the pressure of oxygen systems at the point of
delivery to the patient and the latter is used to measure blood
pressures (and perhaps spinal fluid pressure).
Jim
Pierre Abbat wrote:
On Thursday 25 September 2008 17:11:24 Martin Vlietstra wrote:
Hi Pierre,
As I said, I am not a hydraulic engineer. However, you stated that there
are many different types of "head". Can you add these types of head up
(either as scalars or as vectors)? If so, they must all be in the same
units - pascals. If not, then there has been some messy overloading of
names.
You can add them as scalars, and they are all in the same unit. When using
pounds, head is in feet, and the author of the hydro book states that head is
measured in meters in the metric system. The author, however, is not a native
metric speaker (e.g. he gives the specific weight, not density, of water at
various temperatures and the one at 4 °C is wrong). So I'd like to know
whether hydraulic engineers who work in metric use head, or what do they use
instead.
Once the prof worked an example on the board involving oil, getting the head,
then converted it to "feet of water", stating that that is the standard. That
made no sense. Head is used because, where a still container has an open top,
it is visible as the elevation of the top. A head of oil is not a head of
water, and if you're going to compute one from the other, you need to compute
the pressure at the oil-water interface.
Pierre
--
James R. Frysinger
632 Stony Point Mountain Road
Doyle, TN 38559-3030
(H) 931.657.3107
(C) 931.212.0267
