Pressure and Atmosphere

What is the definition of pressure in physics?

Pressure is force exerted over a unit of area.
Pressure = Height x Density

Imperial: lbs/in2 (psi), ounce/in2 (osi), WC/in2 (“WC)

Metric: Pa, kPa, MPa (per m2).

How is pressure calculated?

Pressure = Force / unit area.

or

Pressure = Height x Density

What happens to pressure when a large force is applied to a small area?

It results in high pressure.
3× 1lb cubes (1×1”) stacked on top of eachother makes 3lbs/in2 (or psi) force in this scenario is 3lbs.

• If you stack the sideways, it would be 1psi but still 3lbs of force.

What is the unit of pressure when one Newton is exerted over 1 m²?

Pascal.

What is the definition of force in physics?

Force is a push or pull that affects the weight, not restricted to a specific area.

Force = Area x Pressure

• Imperial : Pound force,

• Metric: newton, kilonewton.

• Newton (metric) amount of force needed to accelerate 1kg at 1 meter/sec. To determine newtons, multiply object weight in kg by gravity (9.81).

What do you need to calculate the force in Newtons?

Multiply object weight in kg by gravity (9.81 m/s²).

What are the six principles of hydrostatics?

1. Fluid pressure exerts in all directions

2. Shape of container does not affect fluid pressure.

   • pascal’s vases: if the vases have different volumes, the height will stabilize between all vases at the same level.

   • Water seeks its own level.

3. Fluid Pressure is Proportional to Depth

   • the height or depth of liquid affects the pressure exerted.

   • If a column height is halved, the pressure is also halved.

4. Fluid Pressure is Proportional to Density

   • Pressure exerted relative to height depends on the density of that fluid.

5. Horizontal runs do not increase pressure

   • Horizontal runs do not increase pressure.

6. Volume of fluid does not affect pressure

   • if 2 tanks were joined at the same height and one tank had more volume, that tank would not affect the other tank

What does Pascal's vases principle illustrate?

If the vases have different volumes, the height will stabilize between all vases at the same level.

How does the height of a liquid column affect the pressure?

If a column height is halved, the pressure is also halved. It is proportional.

How is fluid pressure related to fluid density?

Pressure exerted relative to height depends on the density of that fluid.

Hydrostatics Pressure

• Hydrostatic is study of fluids under pressure and at rest.

• If water density is 62.4 lbs/ft3, convert it to psi:

  • 62.4lbs/ft3 / 12²= 0.433 psig

  • So 1 cubic foot of water is 0.433psig.

What does it mean when fluid is in horizontal runs?

Horizontal runs do not increase pressure.

What happens when two tanks are joined at the same height with different volumes?

The volume of fluid does not affect the pressure; the tanks would have the same pressure.

How is hydrostatic pressure defined?

Hydrostatics is the study of fluids under pressure and at rest.

What is the water density converted to psi if it is 62.4 lbs/ft³?

62.4 lbs/ft³ / 144 = 0.433 psi.

What does STP stand for in pressure measurements?

Standard temperature and pressure.

What are the conditions for STP in imperial units?

60F, 29.92”Hg, air density of 0.076 lbs/ft³.

What are the conditions for STP in metric units?

0C, 101 kPa, air density of 1.29 kg/m³.

How does atmospheric pressure affect the boiling temperature of water?

Pressure can change the boiling temperature of water. Decrease in atmospheric pressure will decrease the boiling point of water.

What was the purpose of the Torricelli experiment?

To measure atmospheric pressure using a mercury barometer.

What happens to atmospheric pressure with elevation?

• Elevation vs pressure:

  • 2000ft increase, you lose 1 psi of atm press.

  • 1000ft increase you lose 1”Hg

  • 90m increase, you lose 1kPa

What is the maximum suction lift at atmospheric pressure?

Atmospheric pressure supports 760mm of mercury or approximately 10.34 meters of water.

Realistically it’s more like 23 to 26’ because no pump creates a perfect vacuum

What is siphonage in plumbing systems?

Siphonage occurs due to a partial vacuum created that allows atmospheric pressure to push water out.

What is the purpose of venting in plumbing systems?

Venting prevents siphonage by creating atmospheric pressure on both sides of a trap. This applies to systems with no trap priming feature.

What is the equivalent of 1 osi in inches of water column (in wc)?

1 osi = 1.73 in wc.

What is the conversion factor for 1 meter water column to kPa?

1 meter wc = 9.81 kPa.

What does absolute pressure refer to?

Atmospheric pressure indicated as absolute pressure, marked with an 'a'.

What is gauge pressure?

Gauge pressure has atmospheric pressure factored in a zero point.

What is the absolute pressure equivalent of 0 psig?

0 psig is 14.7 psia or 101 kPa absolute.

What is the common measurement for atmospheric pressure at sea level?

14.7 psia, 29.92”, 760 mmHg.

Define maximum suction lift.

• Atm pressure supports 760mm of mercury. How much meter of water pressure can be supported by atm pressure?

• 13.6 SG x 760mm = 10336mm or 10.336m

• A suction pump uses atmospheric pressure to lift water from shallow wells, this pump makes a vacuum then uses the atmospheric pressure to push it. This means the max theoretical depth of the well would have to be 10.34m, otherwise, the water would not reach the pump. 10.34 is too deep as no pump makes a perfect vacuum. Realistically it’s more like 7m.

How many mmHG are in 1 psig of pressure?

I usually work with “wc and I know that 1psi = 27.68”wc.

I also know that 1” is 25.4mm.

So: 27.68”wc x 25.4mm = 703.07mmWC

Now I know the SG of mercury is 13.6, so:

703.07mmWC / 13.6 SG = 51.6 mmHG for 1 psig.

How much pressure in kPa does 1m of water produce?

9.81 kPa

True or False - Only density and height affect fluid pressure?

True

True or false - container shape affects pressure?

False

True or False - 2” pipe 10m high has more pressure than a 12” pip 10m high?

False - the pressures are the same if the level is the same.

True or False - IF a million gallon tank is connected to a 10 gallon tank and the levels are the same. The pressures are also the same

True

Types of barometers (2)

• Mercury baroneters:

  • 36” long glass tube sealed on one end, filled with mercury.

  • Tube is inverted into a container of mercury. Level of mercury rises or falls based on atm press.

  • Moveable cup to adjust the zero

  • More accurate than aneroid barometers

• Aneroid Barometers:

  • Convenient and not affected by position.

  • Evacuated chamber with flexible walls, partial vacuum in bellows and flexible walls are sensitive to pressure change.

  • Essentially a compound gauge.

What is a compound gauge used for?

• Perfect vacuum is 0psia, compound gauges can read pressures below or above atmospheric pressure.

Why is mercury preffered in barometers?

A water barometer would be 34 feet height which is impractical. A Mercury one is usually 36 inches.

Minimum height for a Torricelli tube mercury barometer?

915mmHG or 36”HG. Not sure why because that’s more than a complete vacuum. - likely to prevent overflow and to allow some room to create the vacuum.

Water tank has 2.5 m2 filled to height of 8.25m. Calculate the force in newtons on bottom of tank.

I know that 1 meter WC = 9.81 kPa

8.25m x 9.81 kPa = 80.93 kPa

Force = Pressure x Area → 80.93kPa X 2.5 m2 = 202.33 Kilo Newtons (202,330 Newtons)

Common Conversions to remember to make conversions easy:

• inches to mm (length conversion)

• psi to “wc

• meter to kPa

• psi to kPa

• Know how to convert “wc to “HG etc…

1 psi = 27.68”wc

1 inch = 25.4 mm

1 meter = 9.81 kPa

1 psi = 6.89 kPa

Ex: if you needed to know “HG from “wc, do this: 27.68”wc for 1 psig / 13.6 = 2.035”Hg for 1 psig. Now you can use that and work with it.

What is the diameter of pipe if your total force is 1759 Lbs on the bottom of a pipe, and the pressure is 35 psi?

8inch:

1759lbs / 35 psi = 50.4 in2

SQRT(50.4 / 3.1416) = 4inch radius, need diameter so it’s 8inch

A piping system is filled with water to a height of 165'. What is the total force exerted on a 6" round cap at the bottom of that pipe?

165’ makes 71.5 psig pressure

6” round cap → need area so do 3² x pi = 28.27 in2

71.5 psig * 28.27in2 = 2019.9 lbs of force

Convert 9 psia to inches of Hg vacuum.

9 psia = 18.33 “Hg

29.92 “Hg absolute - 18.33 “Hg absolute = 11.6” Hg Vacuum

Air pressure decrease:

1” Hg for every ___ above sea level

1 pai for every ____ above sea level

1 kPa for every ____ above sea level

1” Hg for every 1000’ above sea level

1 psi for every 2000’ above sea level

1 kPa for every 90m above sea level

Pressure units for imperial and metric?

Imperial:

Psi, osi, “wc - using in²

Metric:

Pa, kPa, MPa (per m²)

Units of measurement for force imperial and metric

Imperial = ft/lbs or in/lbs

Metric = newtons