Head & PressureIndustrial

Pumps

Outline• Different Types of Head• Head vs. Pressure• Calculate Static Head• Pressure & Head Conversions• Head Loss & Friction Loss

• Head is a term for expressing feet of a fluid column• Head can also be converted to pressure

"Head"

100 feet

43.3 PSI

Reservoir of Fluid

Pressure Gauge

Head & Pressure• Head and pressure can be interchangeable

terms provided that they are expressed in their correct units.

• The conversion of all pressure terms into units of equivalent head simplifies most pump calculations.

Head vs. Pressure• The main reason for using head instead of

pressure to measure a centrifugal pump's energy is that the pressure from a pump will change if the specific gravity (weight) of the liquid changes, but the head will not change.

• Since any given centrifugal pump can move a lot of different fluids, with different specific gravities, it is simpler to discuss the pump's head and forget about the pressure.

“Head"

Different Types of Pump Head• What is Pump Head?

• Units of Measure: In the U.S. system, head is measured either in PSI or in "feet of head" (usually abbreviated to "feet").

• Pump Head is the total resistance that a pump must overcome.

• Total Static Head - Maximum height (pressure) the pump can deliver. Total head when the pump is not running

• Total Dynamic Head (Total System Head) - Total head when the pump is running. Total energy in feet required to move water from fluid level suction side to fluid level discharge side

• Static Suction Head - Head on the suction side, with pump off, if the head is higher than the pump impeller

Different Types of Pump Head

• Static Suction Lift - Head on the suction side, with pump off, if the head is lower than the pump impeller

• Static Discharge Head - Head on discharge side of pump with the pump off

• Dynamic Suction Head/Lift - Head on suction side of pump with pump on

• Dynamic Discharge Head - Head on discharge side of pump with pump on

• Friction Head - is the amount of energy loss due to friction of the fluid moving through pipes and fittings

Static Suction Head

Negative and Positive Static Suction Head

Static Delivery Head

Total Head

-ve

+ve

What Is Total Head?• Total head and flow are the main criteria

that are used to compare one pump with another or to select a centrifugal or positive displacement pump

• It is defined as the difference in head between the inlet and outlet of the pump

• Total head is related to the discharge pressure of the pump.

• Why can't we just use discharge pressure?

• One of the reasons is that manufacturers do not know how you will use the pump.

What Is Total Head?• The discharge pressure depends

on the pressure available on the suction side of the pump.

• Positive static head at the same flow rate as negative static head will get a different discharge pressures

• Therefore to eliminate this problem, it is preferable to use the difference in pressure between the inlet and outlet of the pump.

What Is Total Head?• The manufacturers have taken

this a step further, the amount of pressure that a pump can produce will depend on the density of the fluid.

• For salt water which is denser than pure water, the pressure will be higher for the same flow rate

• Once again, the manufacturer doesn't know what type of fluid is in your system

What is the relationship between Head and Total Head?

• Total head is the height that the liquid is raised to at the discharge side of the pump less the height that it is raised to at the suction side.

• Why less the height at the suction side?

• Because we want the energy contribution of the pump only and not the energy that is supplied to it.

What is the relationship between Head and Total Head?

• First let's deal with the unit of energy.

• Energy can be expressed in foot-pounds

• The amount of force required to lift an object up multiplied by the vertical distance.

• A good example is weight lifting. If you lift 100 pounds up 6 feet the energy required is 6 x 100= 600 ft-lbs

I Lift Things Up & Put Them Down

What is the relationship between head and total head?

• Head is defined as energy divided by the weight of the object displaced.

• For the weight lifter, the energy divided by the weight displaced is 6 x 100 / 100= 6 feet (1.83 m), so the amount of energy per pound of dumbbell that the weight lifter needs to provide is 6 feet.

• This is not terribly useful to know for a weight lifter but we will see how very useful it is for displacing fluids.

• The following figure shows how much energy is required to displace vertically one gallon of water.

• This next figure shows how much head is required to do the same job.

Static Head• The other very useful aspect of

using head is that the elevation difference or static head can be used as one part of the value of total head, the other part being friction head.

• How much static head is required to pump water up from the ground floor to the second floor, or 15 feet up?

• Remember that you must also take into consideration the level of the water in the suction tank.

• If the water level is 10 feet below the pump suction connection then the static head will be 10 + 15 = 25 feet.

• Therefore the total head will have to be at least 25 feet plus the friction head loss of the fluid moving through the pipes.

Static Head

21

Pressure & Head Constants• Imagine a cube of water 1ft x 1ft x 1ft.

• Weight of water is 62.4 pounds per ft3

• The surface area of any one side of the cube will contain 144 in² (12in x 12in = 144 in²).

• The cube will also contain 144 columns of water one foot tall & one inch square.

12 in

1 in1 in

Weight = 62.4lbs/144in = 0.433lbs/in or = 0.433 psi

Therefore, 1 foot of head = 1ft ÷ 0.433psi

= 2.31 ft/psi

Constants 1ft of head = 0.433 psi, and 1psi = 2.31 feet of head

Remember these constants

• PSI x 2.31 = FEET OF HEADExample: If a Pressure Gauge Read 43.5

PSI what is the head?43.3 PSI x 2.31 FT/PSI = 100 FT of head

• FEET x 0.433 PSI/FT = PSIExample: If a vessel is 100 feet high what

is the pressure?100 FT x 0.433 PSI/FT = 43.3 PSI

How to Calculate Water Head Pressure

How to Calculate Water Head Pressure

• Measure the height or depth of the water in inches or feet

• Divide the depth in feet by 2.31-feet/psi – The result is the water head pressure expressed in

psi.

–What is the PSI?

150’150/2.31 = 65 psi

How to Calculate Water Height From Pressure

• What if you didn’t have a 150’ tape measure?• If the pressure gauge reads 65-psi• Multiply 65 * 2.31 feet/psi = 150-feet of water

height above ground.

– What is the water height if thePSI reads 85psi?

85 psi85*2.31 = 196’

What If I’m Not Moving Water?

Conversion Factors Between Head and

Pressure• PSI = Pounds per Square Inch

• Sp. Gr. = Specific Gravity which for water is equal to 1– For a fluid more dense than water, Sp. Gr. is greater than 1– For a fluid less dense than water, Sp. Gr. is less than 1

• Head (feet of liquid) =Pressure in PSI x 2.31 / Sp. Gr.– Concrete Asphalt’s specific gravity is 2.24

• Pressure in PSI = Head (in feet) x Sp. Gr. / 2.31– Zinc Oxide’s specific gravity is .4

What if tower was full of concrete – What is the Pressure?

• Measure the height or depth of the vessel in inches of feet

• Pressure in PSI = Head (in feet) x Sp. Gr. / 2.31–Concrete’s specific gravity is 2.24

–What is the PSI?• 150*2.24/2.31

150’145.45 psi

What if tower was full of zinc oxide – What is the Head Height?

• If the pressure gauge reads 145-psi and the vessel is full of zinc oxide what is the head height?

• Head (feet of liquid) =Pressure in PSI x 2.31 / Sp. Gr.– Zinc Oxide’s specific gravity is .4

–What is the Head Height?• 145*2.31/.4

145 psi837’ high

Head & Friction Loss• In any real moving of fluid, energy is

dissipated due to friction; turbulence dissipates even more energy.

• Head loss is divided into two main categories:• Major losses - associated with energy loss per

length of pipe• Minor losses associated with bends, fittings,

valves, etc. • For relatively short pipe systems, with a

relatively large number of bends and fittings, minor losses can easily exceed major losses.

• Friction head is the amount of energy loss due to friction of the fluid moving through pipes and fittings.

Calculating Friction Loss• In the same way that head

was calculated to lift a certain weight, the friction head is calculated with the force required to overcome friction times the displacement (pipe length) divided by the weight of fluid displaced. • Hazen-Williams Equation

• What are some Friction Loss Factors?– Length of Pipe– Diameter of Pipe– Number of Fittings– Types of Fitting– Pipe Material– Fitting Material– Fluid being moved

– http://www.engineeringtoolbox.com/hazen-williams-water-d_797.html

Why is friction loss knowledge important?

“Throw a bigger pump at it”