Pressure & Flow
Pneumatics & Hydraulics

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2006-01-11 Rev.2006-02-25, -07-07 2008-04-08


Pipe Diameters
Units of Pressure
In the glassblowing shop


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: 
Plumbing joint that changes diameter 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.




Ratio to 1/2 pipe





















1 1/4




1 1/2









Pressure Glossary
Static Pressure
Pressure in a fluid system when the fluid is at rest and the pressure is unchanging or measurement that cancels the effects of motion..  A pitot tube is used on air craft, measuring static pressure on the sides and dynamic pressure on the end facing forward - the radio being interpreted as velocity.
Dynamic Pressure
Pressure due to the movement of fluid.  The force of wind, for example.
Category containing two of three common states of materials in nature, liquid and gas, which behave in many similar ways, while solids are quite different.
A state of nature when the material will flow and change shape without changing volume much, if at all, and in which the molecules are attracted to each other (unlike sand, powders, and dust.)  Compressibility is low because molecules are relatively close.
Gas or Gaseous
A state of nature when the material will flow while changing shape and also changing volume depending on pressure.  Common gases follow a fairly rigid rule in which gas volume, pressure, and temperature are interrelated.  Compression occurs because molecules are relatively far apart. 
Applying force using liquid, taking advantage of the incompressibility of liquids to change the direction and shape of the force.  Manual advantage is created by applying a levered push on a small area which is translated to a huge push from a larger area that moves a much smaller distance - valves hold the distance.  A jack the size and shape of a Coke bottle with a one foot handle can lift 6 tons 6" with a few minutes of pumping. glosshop.htm#HYDR
Applying force using a gas, usually air, to apply a force in a flexible way, dryer than hydraulic but also bulkier.  Gas leaks more easily than liquid, so pressures are usually limited to several hundred pounds rather than several thousand.  Liquid does leak and this may favor pneumatic where the air is used, either because of mess or because hydraulic fluid is flammable.  Pneumatic moves faster but is less precise because of the compression of the gas.



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.










in Hg





Household delivery pressure









One pound per square inch, conven.28"H20




Glass studio higher pressure









One atmosphere, about, exact kPa










One atmosphere, exact definition



Convenient pressure





One mega Pascal



High pressure local distribution

psi is American unit of pressure measurement, pounds per square inch

Pascal, metric unit of pressure measurement, Newtons (force) per sq. meter

kPa, common metric measure, convenient size units

MPa, More rarely used metric measure because so large

bar, weather, starting with one average atmospheric pressure

convenient units size

Torr, weather

atm, one average pressure of air

weather, USA


WC, Water Column height in inches cheap way to measure low gas pressures

The prefix "kilo" means "1,000" so one kilopascal = 1000 Pa.
Therefore 101.325 kPa = 1 atm = 760 Torr and 100 kPa = 1 bar = 750 Torr.

Normal atmospheric pressure is defined as 1 atmosphere.
   1 atm = 14.6956 psi = 760 Torr.

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) 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

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 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.


Pneumatics & Hydraulics

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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.

BGSU hotwall doors











Mold with pneumatic closing at Brad Abrams studioThe 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. 


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