Water Pressure and Flow Analysis

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# Water Pressure and Flow Analysis - PowerPoint PPT Presentation

Water Pressure and Flow Analysis. Engineers Without Borders South Central Regional Workshop November 12 th , 2011. Brad Appel, Texas A&M University. Purpose. Survey the concepts and equations to calculate the flow properties of a pipeline

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### Water Pressure and Flow Analysis

Engineers Without Borders South Central Regional Workshop

November 12th, 2011

Purpose
• Survey the concepts and equations to calculate the flow properties of a pipeline
• Practice these equations by comparing to a test flow rig
• As a reminder: The EWB-USA website has a great list of resources

Log-in Project Resources  Technical Resources

Overview
• Flow rate and pressure units
• Pump curves
• Friction pressure drop (Darcy-Weisbach)
• Friction pressure drop (Hazen-Williams)
• Minor Losses
• Experiment Competition
Flow Rate Units
• In general:

where = mass flow rate, = fluid density, and

= fluid velocity

• For room-temperature water:

where = volume flow rate

Pressure Units
• Metric: 1 Pascal (Pa) = 1 Newton / m2 = 10-5 bar
• English: 1 psi = 144 psf
• Head is a way to represent the height that a column of fluid will reach under a particular static pressure.
• Head is widely used because it will not change with fluid density.
Bernoulli’s Equation
• Conservation of energy for an inviscid fluid along a streamline.
• If we neglect energy changes due to friction or heat transfer, the total head of a fluid will remain constant, and can only be traded between dynamic, static, and potential energy.
• Bernoulli’s is particularly useful for predicting the static pressure after a flow area change.
• When working with head, gravity is the simplest quantity to calculate.
• The head difference is independent of the path between the two measurement points.
• This is the most common driver for rural water distribution systems.
• To convert back to pressure:

1

2

Gravity-Driven System Example
• EWB-TAMU project in Costa Rica: 2000 people supplied with water without any pumps.
Pump Curves
• A given pump will have a characteristic curve showing the flow rate it can produce when operating over a particular head rise.
• The operation point on the curve will shift to match the pressure drop of the system.
Friction Pressure Drop
• Method 1: Darcy-Weisbach
• Friction factor f can come from a Moody Chart, where the Reynolds number is needed:
• Values for surface roughness are tabulated and widely available.
Friction Pressure Drop
• Method 2: Hazen-Williams (this is the metric version)
• C factors are typically between 70 and 150, and are widely available for different materials.
Minor Losses
• We already know how to calculate the pressure drop due to wall friction (Darcy-Weisbach and Hazen-Williams).
• For everything else (including bends, valves, orifices, expansions & contractions, etc…), we use minor losses.

Typical k values can vary from 0.2 to 20

All this…wrapped into “k”

• The available head in a pipeline is commonly graphed on a energy/hydraulic grade line chart.
• Bernoulli’s equation (velocity head), friction losses, minor losses, gravity head changes, and pumps can all be visualized.
Flow Networks
• While hand calculations for flow through a single pipeline can be easy, a pipe network quickly turns into a large system of equations.
• Parallel flow circuits require iterative solving.
• For student EWB chapters, EPANET is a great tool for simulating large networks.
• EPANET model of San Juan de PenasBlancas includes 500 nodes and pipes, 7 tanks, and stretches over a 10 km2 area.
• Solves for a 24 hour demand pattern in a matter of seconds.
Experiment Competition!
• Break into 4-person groups
• Use either Darcy-Weisbach or Hazen-Williams
• Collect flow rate data
• Predict the head on the second piezometric tube
Experiment Setup

Piezometer tubes

Pump

Vinyl tubing

Tee

Tubing length = 84 cm

Tubing inner diameter = 0.4318 cm