lesson 4_.pdf

Lesson 7:Dynamic Pressure

Objective

In this lesson we will learn and study the details involved with dynamic pressure - existing as movingenergy - and friction loss inside of pipes. We will discuss the relationships between the pressure andfriction loss.

Reading Assignment

Read the online lecture as well as Chapter 6 in your textbook.

Lecture

Introduction

Water will flow downhill - with no problems - but there is friction between the water and the insidesurface of the pipe. If the inside of the pipe is extremely rough - then there is more friction loss betweenthe water and the pipe. Friction loss in a pipe depends upon the velocity or rate of flow and the size of thepipe (diameter), the length of the pipe, and the roughness of the inside surface of the pipe. The degree ofpipe roughness is called the C - Factor - which is a coefficient in the Hazen - Williams Formula fordetermining the flow. Conversely, the flow rate (quantity) varies with friction. For most pipe materials Cranges from 90 - 140. A high C value implies less friction.

As stated in the objective and the previous Lesson #3, dynamic pressure is "moving energy", while staticpressure exists when the water flow is still or not flowing. Water flow in a pipe is dependent upon howslick the surface of the pipe is, called friction loss - C factor. Pressure in a water system must be at least20 psi for residential use.

Head or friction loss for 100 ft. lengths of pipe of various sizes for a C factor of 100 are given in thefollowing tables. One table is for valves and fittings, one is for types of pipe, and one is the Hazen -Williams tables for C factors , which can be found in most all water manuals.

An example of this concept is - The head loss that would occur through 100 ft. of a 2 in. pipe, for a flowof 100 gpm is found to be, under the "US gpm" column to be 100, then moving across the "loss in ft.column for a 2 inch pipe = head loss of 35.8. Then multiply 35.8 0.433 = 15.5 psi.

Then, if the pressure in the main where a 2 in. tap is made is 50 psi, the pressure 100 ft. away (forexample, where a home service tap is to be located) is approximately 34.5 psi. (50psi - 15.5 psi = 34.5 )when the flow through the service line is 100 gpm. The friction loss would be twice as much if the pipe

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was 200 ft. and 10 times as much for a 1000 ft. length of pipe, - IF - the average flow rate is maintainedat 100 gpm.

When a pipe has a C factor other than 100, it is possible to adjust back to a C factor of 100 by multiplyingthe flow by 100/C, and then go to the table when you have a newly computed flow value. Using the sameexample from above, if the C factor was 125 not 100, then the new flow rate would be 100 100/125 =80 gpm. Then go to the 80 gpm column on the table for a 2 in. pipe, = a loss of 23.2 ft. Thus, the smootherpipe, saves 12.6 ft. of head , per each 100 ft. of pipe used. 35.8 -23.2 = 12.6 .

Also, while discussing flow, we should mention Fire Hydrants , and the rate of flow = color.

Required to be installed on a 6 in.line at a minimum.

RED = 0 ---------500 gpm

ORANGE = 500-----1000 gpm

GREEN = 1000 ------ 1500 gpm

BLUE = 1500 ------GREATER gpm

Allowance in Equivalent Length of Pipe forFriction Loss inValves and Threaded Fittings

(C = 140)

Diameterof

fitting, inches

90 std.ell,feet

45 std.ell,feet

90 side tee,feet

Coupling or straight run of tee, feet

Gatevalve,

feet

Globevalve,

feet

Anglevalve,feet

3/8 1 0.6 1.5 0.3 0.2 8 4 1/2 2 1.2 3 0.6 0.4 15 8 3/4 2.5 1.5 4 0.8 0.5 20 12 1 3 1.8 5 0.9 0.6 25 15 1 1/4 4 2.4 6 1.2 0.8 35 18 1 1/2 5 3 7 1.5 1.0 45 22 2 7 4 10 2 1.3 55 28 2 1/2 8 5 12 2.5 1.6 65 34 3 10 6 15 3 2 80 40 3 1/2 12 7 18 3.6 2.4 100 50 4 14 8 21 4.0 2.7 125 55 5 17 10 25 5 3.3 140 70 6 20 12 30 6 4 165 80

*C = 140; to set C = 120, divide by 1.32

The following formula is used for Laminar Flow: (does not create turbulence)

Use this formula to determine the pressure drop after adding the length of pipe and equivalent length ofpipe due to the fittings:

Pipe friction losses should be determined on the basis of Hazen and Williams formula:


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Where: Q = Flow C = Roughness d = Diameter L = Length

When using a Graphing Calculator, here are the steps in solving an equation with the Hazen-Williamsformula:

Pipe friction loss (P) can be determined by doing the following calculations:

First punch in 4.52 on the calculator, then the parenthesis, ( , sign, substitute Q for whatever theflow is in your equation, hit the yx key (this will allow you to add in your superscript) then key in1.85 and hit the other parenthesis, ).

1.

Next hit the divided by sign, and key in a parenthesis, ( , replace the C factor with the roughnesscoefficient in your equation. Hit the yx key and key in 1.85 and the other parenthesis, ). Then keyin a parenthesis again, ( , and replace the dfactor with the diameter coefficient in your equation, hitthe yx key again and punch in 4.87 and the other parenthesis, ).

2.

Hit the equals sign. When you get an answer, hit the multiply key and replace the L factor with theLength coefficient in your equation.

3.

This should give you the correct friction loss using the Hazen-Williams formula.

Pressure and Flow: (relationship in fixed system)

Flow and Roughness:

Flow:


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*Below are the C Values to use in the Hazen-Williams formula:

Average C Values for Certain Types of PipeType of Pipe Average C Value

Uncoated cast iron: smooth and new 125.3 Coated cast iron: smooth and new 30 years old Trend 1: slight attack Trend 2: moderate attack Trend 3: appreciable attack Trend 4: severe attack 60 years old Trend 1: slight attack Trend 2: moderate attack Trend 3: appreciable attack Trend 4: severe attack 100 years old Trend 1: slight attack Trend 2: moderate attack Trend 3: appreciable attack Trend 4: severe attack

133.3

106 90 69

49.7 96.3 77.7 57.7 39

88.3 69.7 48.7 30

Miscellaneous Newly scraped mains Newly brushed mains

115.3 103

Coated spun iron: smooth and new Old: take as coated cast iron of same age 93

Galvanized iron: smooth and new 127.3 Wrought iron: smooth and new 136 Coated steel: smooth and new 136 Uncoated steel: smooth and new Black Iron (NFPA)

140.3 120

Coated asbestos-cement: clean 132 Uncoated asbestos-cement: clean 95.7 Spun cement-lined and spun bitumen-lined: clean 98.7

Smooth pipe (including lead, brass, copper, polythene, and smooth PVC): clean 145.3

PVC wavy: clean 140.3 Concrete: Scobey Class 1: CS = 0.27; clean Class 2: CS = 0.31; clean Class 3: CS = 0.345; clean Class 4: CS = 0.37; clean Best: CS = 0.40; clean

49.3 65.7 75 82

87.3

Tate relined pipes: clean 75

The velocity pressure should be determined on the basis of:


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Steel Pipe

Nominal Size & OD Inches Schedule Numbers(1) a - b - c Wall Thick InchesInside Diameter

InchesPipe Weight

lbs/foot3/4 1.050 40 - Std - 40S 0.113 0.824 1.13

1 1.315 40 - Std - 40S 0.133 1.049 1.67

1 1/4 1.660 40 - Std - 40S 0.140 1.380 2.27

1 1/2 1.900 40 - Std - 40S 0.145 1.610 2.71

2 2.375 40 - Std - 40S 0.154 2.067 3.65

2 1/2 2.875 40 - Std - 40S 0.203 2.469 5.8

3 3.500 40 - Std - 40S 0.2161 3.068 7.6

3 1/2 4.000 40 - Std - 40S 0.226 3.548 9.1

4 4.500 40 - Std - 40S 0.237 4.026 11

5 5.563 40 - Std - 40S 0.258 5.047 15

6 6.625 40 - Std - 40S 0.280 6.065 19

8 8.625 40 - Std - 40S 0.322 7.981 29

10 10.750 40 - Std - 40S 0.365 10.020 40

12 12.750 40 - Std - 40S 0.406 11.938 54

Pressure Loss Through BackflowProtection DevicesEquivalent Feet Length (C = 120) (Conservative) Not good forTurbulent Flow

Size Rated flow Double check a,b

Avg. NFPA C = 120

Reduced pressure zone

C = 140

3/4 30 --- 20 1 50 10' 25.2 1 1/4 75 14' 32.4 1 1/2 100 18' 35.2 2 160 22' 44.8 2 1/2 225 28' 51.2 3 320 32' 57.0


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4 500 44' 61.6

Note a. ASSE 1015 Note b. ASSE 1024 Note c. ASSE 1013

Standards come from the NFPA (C = 120)

C = 140; to get C = 120, divide by 1.3

Review

In this lesson we discussed the different relationships between the sizes of pipes, the interior surface of thepipe, and the rate of flow. We learned how to use the chart for the C Factors, or the Hazen - WilliamsFormula Table, these numbers do vary for the sizes and types of pipe and the fittings in the system. Eachof these factors is important when calculating the flow within a system, and each of these factors must betaken into account when doing these types of calculations. We also discussed the color relationship of firehydrants vs the flow within the fire hydrant.

Sources

Water Distribution Operator Training Handbook - AWWA

Operator's Companion - USA BlueBook

Operator Certification Study Guide - AWWA

Assignments

Answer the following questions and either email or fax to the instructor.

The resistance to the flow of water in a pipe is called ________________.1. Does friction loss increase or decrease with velocity?2. The C Factor of a water pipe is ________________.3. Static suction head + friction suction head + static discharge head + friction discharge head make upthe _________________ of a pump.

4.


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