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2006-01-11 Rev.2006-02-25, -07-07 2008-04-08
Pressure and flow of fluids are interrelated and many points of design are similar to voltage and current with electricity. In both cases it is possible to "get away" with choices that work until a design needs to be near the limits at which point parts of the system don't work or interfere with settings on other parts. It is better to be sure.
Pressure is force per area and in American usage is commonly measured in pounds per square inch (psi) Under metric measure the unit is the Pascal which is Newtons per square meter. (See table below.)
The diameter of pipes becomes significant when the pipe is being branched off
to feed various sources. You then want to start with large pipe (lets say 3" for
air from a blower) and when you feed some off (maybe an 1-1/4" pipe), you want to reduce
the size of the main pipe also (to maybe 2".) This keeps the pressure up
in both the line and the side feed. The same thing is done with water as in
sprinkler systems and in gas with multiple appliances. Plumbing supply
places and sites will have pressure drop information. Full service
software for fluid systems is complex and valuable and thus expensive, as these
sites will suggest:
Flowmaster - Fluid thinking for systems engineers ,.
Pipe Flow Software -
Pressure Drop and Friction Loss Calculations or
Elite
Software - S-Pipe . For a discussion of home plumbing see
Properly Size Your Water Lines
If you don't do this, then when you adjust the air flow at the end of the
system, it will affect the pressure drops in the other lines and throw off your
other adjustments. Perhaps the best example of water problems is the blast
of cold or hot water when taking a shower and someone flushes a toilet - with
proper sized and arranged plumbing, this does not happen.
Also, if you are trying to balance flow or produce a specific maximum flow at
the end, you have to account for all the bends and elbows. If you apply 10 psi
to the beginning of a pipe and want a flow of 100 cfm at the other end at 5 psi,
with too many elbows, too much length, or too small pipe, you may not be able to
get it - you may get 50 cfm at 5 psi or 100 cfm but only at 2 psi. Again,
plumbing & air conditioning shops and sites will have the allowances for various shapes.
Here are more examples of problems:
A long air line is intended to deliver 90 cubic feet per minute (cfm) at 90
pounds per square inch (psi). Due to small pipe diameter and joints in the
line, when the valve at the end is opened all the way, the pressure drops to 45
psi and if the valve is closed to raise the pressure, when at 90 psi, only 45
cfm flows.
I use a garden hose that actually has four ways of controlling the water flow -
the faucet valve, a shut off valve on a quick connect in the middle, a similar
one on the end, and a spray nozzle with a lever handle snapped into the end.
If any of these valves but the last is just barely opened and the last one
closed - the static pressure of the water line builds up in the hose until the
hose expands as much as it can and the water comes to rest. If I then open
the spray nozzle, I get a huge blast of water which quickly falls off to almost
nothing because of the pressure drop and flow restriction from the barely open
valve. The pressure is static and has expanded the hose, which pushes back, for
a moment. The effect is exaggerated with the valve, but occurs similarly with
small diameter hose, etc.
A large water main delivers to a 2" primary sprinkler line on a golf course.
The intention is to run a main line that is reduced in size each time a branch
line or sprinkler head is used (which is proper design.) A hotshot
young man on the maintenance crew from a nearby college figures that he can save
money and effort by getting a bigger quantity of one size of pipe and install
the same fittings the full length. This manages to get built and when
water is supplied, the last heads have only a dribble coming out and if the
first ones are adjusted to reduce flow, the other heads all change their throw with
each adjustment.
Pressure and flow are interrelated in the following way:
Here is a section of pipe with a bell joint so the pipe gets larger if the flow
is from left to right. Liquid can not expand or
contract so how is the space created to be filled? The fluid slows down,
losing pressure.
[Wait a minute, isn't it true that hydraulic jacks work because liquids pass
pressure/force through them? Yes, but that is static force. Here we are
talking about dynamic forces - flow of fluids.]
Friction on the walls of the pipes and from turbulence going around bends
prevents the pressure applied to the beginning of a system from being maintained
to the end. Lets suppose a pipe system is 100 feet long, 1/4" inside
(small), and is filled with water. Pressure at the beginning is 20 psi and
because liquids are not compressible (much) the pressure is the same throughout
when nothing is flowing. This is called static pressure.
If we quickly open a valve at the other end, the pressure will start falling and
fluid will start flowing.
| Pipe Sizes The table at right shows the ID and area of standard water pipe with a column showing the proportion of each size to common 1/2" pipe. This shows reasoning behind the common arrangement of a 3/4" pipe (ratio almost 2) feeding two 1/2" pipes. Similarly, a 1" line is commonly required for 2 three-quarter inch lines (2.84:1.75) or 3 half inch lines (2.84:1). In the situation where all the lines are flowing, the water is (just about) evenly divided between the lines and pressure is even between them. |
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Because pressure is so widely used there are several different measurement terms in use mostly for the convenience of the user. The most direct American measure is pounds per square inch (psi) and the matching metric measure of Newton's per square meter. Most of the variations result in a simple small number for everyday use.
|
psi |
Pa |
kPa |
MPa |
bar |
mbar |
Torr |
atm |
in Hg |
WC* |
|
|
1/4 |
1.724 |
7 |
Household delivery pressure |
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|
1 |
6895 |
6.895 |
0.0689 |
68.95 |
0.069 |
2.036 |
27.71 |
One pound per square inch, conven.28"H20 |
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|
2 |
13.8 |
0.138 |
Glass studio higher pressure |
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|
14.502 |
100.000 |
0.1 |
1 |
750 |
0.987 |
29.53 |
401.86 |
One atmosphere, about, exact kPa |
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|
14.696 |
101,326.2 |
101.325 |
1.0125 |
1013.26 |
760 |
1.000 |
29.92 |
407.22 |
One atmosphere, exact definition |
|
|
25 |
172.4 |
Convenient pressure |
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|
145.03 |
1,000,000 |
1,000 |
1 |
One mega Pascal |
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|
60 |
413.7 |
High pressure local distribution |
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psi is American unit of pressure measurement, pounds per square inch |
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Pascal, metric unit of pressure measurement, Newtons (force) per sq. meter |
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kPa, common metric measure, convenient size units |
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MPa, More rarely used metric measure because so large |
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bar, weather, starting with one average atmospheric pressure |
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convenient units size |
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Torr, weather |
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atm, one average pressure of air |
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weather, USA |
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WC, Water Column height in inches cheap way to measure low gas pressures | |||||||||
| kilo |
The prefix "kilo" means "1,000" so one kilopascal = 1000 Pa.
|
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Normal atmospheric pressure is defined as 1 atmosphere. |
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Torr - |
Based on the original Torricelli barometer design, one atmosphere of pressure will force the column of mercury (Hg) in a mercury barometer to a height of 760 millimeters. A pressure that causes the Hg column to rise 1 millimeter is called a Torr (you may still see the term 1 mm Hg used; this has been replaced by the Torr). 1 atm = 760 Torr = 14.7 psi. | |||||||||
|
Bar |
(bar) The bar nearly identical to the atmosphere unit. One bar = 750.062 Torr = 0.9869 atm = 100,000 Pa | |||||||||
| Millibar (mb or mbar) |
There are 1000 millibars in one bar. This unit is used by meteorologists who find it easier to refer to atmospheric pressures without using decimals. One millibar = 0.001 bar = 0.750 Torr = 100 Pa |
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Sometimes lack of pressure or flow is inconvenient - as anyone taking a shower under a dribble can report - but other times it goes way beyond that - an eye wash safety basis that can't flush the eyes or a fire hose that can't put out enough water to drown a fire is useless and dangerous.
Hydraulics
Hydraulics are the use of liquids to apply force, usually by applying a lot
force over a moderate area. The force is built up by using a small pump
working vigorously for pressure instead of volume and the pump may be a motor
driven or a small cylinder manually pushed.
2007-04-02 Rev.
| Pneumatics is the use of a gas, normally air, to apply pressure or produce movement. The advantage of pneumatic mechanisms is that they can produce reasonably fast straight line movement and moderate force with a convenient substance that does not pollute if it leaks. The disadvantage of pneumatics is that the volume required, especially for higher forces, means that hand operation is rarely possible and devices may be bulky. The fact that pneumatic devices may provide a bouncy interface may be good (cushioned) or bad (imprecise). More | Hydraulics is the use of a liquid, normally special fluid, to apply pressure or produce movement. The advantage of hydraulics is that they can apply very high forces in a small package that can be manually run. The disadvantage is that the fluids can leak and the forces may require heavy metal structures and reinforced hoses. Further, the best fluids are flammable and the least polluting fluid, water, can not be used because it will corrode many metals. Controlling the higher pressures in a system where force and movement are both needed may require complex valving. More |
| An advantage of both methods is they can produce long straight movements in a way that electrical devices can not without considerable complications such as a continuous coil for solenoids, and wires, belts and pulleys for converting rotary motion to straight with motors. | |
Pneumatics have the advantage in the glassblowing shop of using something that is already there - compressed air provided for blow cleaning and inflating glass as well as air tools and inflating tires. Hoses can often run casually to the workbench across the floor or are dropped to a handy quick fitting just over head. If air leaks, little harm is done beyond wasting some energy used to compress it while a hydraulic leak produces a fire hazard, a physical injury hazard if it under high pressure and a slipping risk on the floor - a real mess.
While industry uses hydraulics for precise control of mechanical parts weighing hundreds to thousands of pounds, fortunately the glass shop rarely needs precise control - a moderate force applied to a move a door or mold part up against a stop is enough - and the common 125 pounds produced by shop grade air compressors is more than enough for most applications - especially with counter-balancing - and regulating to lower pressures is easy.
The
unit at right, from Brad Abrams shop, shows small direct acting cylinders
mounted on a steel plate to open 4 sides of a mold hinged at the bottom.
The temporary platform at left mounts a foot valve for closing the mold and upon
release springs in the cylinders retract the rods and open it as shown. At
upper right is a simple manifold with a regulator to control pressure. Low
pressures and small cylinders allow small reinforced tubing held with
compression fittings. (click to enlarge)
Cable pulls for door. Gravity return
The advantage of hydraulics is that they can apply very high forces in a small package that can be manually run. As a simple example, a bottle jack the size of a one liter soda bottle can lift 12 tons 5 inches with repeated pumping of a lever a foot and a half long. I used one to gradually level my house, moving it from place to place to lift brackets that would take shims underneath to hold the lift. A floor jack is used to lift cars and to support and move engines with the pumping of the handle.
For the glassblower wanting to move doors and other movement choices, among the problems are the flammability of the fluid and the investment in equipment needed for a basic setup if automatic operation is desired (like closing a door by tapping a switch) where pumps, accumulators, hoses and piping are required.