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Understanding Pressure and Flow

Jul 31, 2020
Video Length:  1:04:23
Presented By:  Jake Lott

Understanding pressure and flow is crucial in irrigation design – not only in calculating what's available at the source, but in determining the demands of the design itself. We'll differentiate between static and dynamic pressure, followed by a discussion of how specific equipment types affect that pressure and how flow affects pipe sizing. You'll come away with a grasp of several important considerations for dealing with pressure issues – both before and after the system has been installed.

Webinar Contents:

Note: The following catalog of content covered in this webinar is time stamped to allow you to follow along or skip to sections of the video that are relevant to your questions. You can also search for content on this page using the FIND command in your browser (CTRL + F in Windows, Command + F in Mac OS.)

 

  • Intro/TOC
  • Pressure and Flow
    • Static vs. Dynamic
    • Available vs. Required ... and What's in the Middle
  • What Impacts Pressure and Flow
    • Equipment
    • Site
  • Becoming More Familiar with the Dialog Boxes
    • Pipe Data
    • Source Data
    • Mainline and Lateral Line Sizing
    • Valve Schedule and Critical Analysis

0:00 – 4:28: Intro/TOC

4:29 – 5:49: Pressure and Flow

Why does understanding pressure and flow matter? (4:29)

 

Static vs. Dynamic (5:06)

  • Static: Used for what is available
  • Dynamic (working): Used for design demands

 

Available vs. Required ... and What's in the Middle (5:34)

5:50 – 11:49: What Impacts Pressure and Flow

Equipment (5:50)

 

Main design components (5:50):

  • POC (available)
  • Mainline
  • Backflow assembly
  • Remote control valve (RCV)
  • Lateral lines
  • Emission device (demand)

 

Site (8:02)

Capturing source/field data (8:02)

 

Where to check and what is important (8:53)

11:50 – end: Becoming More Familiar with the Dialog Boxes

 

The Irrigation Preferences (12:23)

 

Pipe classes (13:05)

 

Pipe Data (13:30)

Pipe Data defaults (13:30)

 

Starting a project and pulling in the correct Preference Set (15:00)

 

Source Data (15:45)

Configuring a water meter (17:00)

 

Service line length and elevation change (17:50)

 

Static water pressure reading in the Source Data compared with the available pressure (19:25)

 

Adding an emission device to a project and understanding its operating pressure (21:00)

 

Mainline and Lateral Line Sizing (22:05)

Sizing the lateral pipes (23:03)

 

The Size Lateral Pipes dialog box (23:20)

 

The Pressure Variation setting (24:50)

Flow has a direct correlation with velocity. The higher the velocity, the lower the pressure (and the greater the friction loss).

 

Changing the velocity (25:44)

 

Fittings factor (29:53)

 

Elevation loss (30:03)

 

Loss through the valve (30:12)

 

Viewing the pressure and flow data of a lateral pipe using our Edit Equipment tool (31:35)

 

Sizing the Mainline (32:17)

The velocity setting (32:40)

 

Valve Schedule and Critical Analysis (33:24)

Critical Analysis (33:24)

 

The Residual Pressure Available reading (37:20)

 

Using pipe caps to test a specific demand (38:06)

 

Example system design and pipe sizing (42:36)

 

Running a Station Analysis from the Edit Equipment tool (46:45)

Sizing all lateral pipes and adjusting the velocity (47:40)

 

Sizing the mainline (48:08)

 

How to deal with the "Pressure available at the POC has been exceeded. Reduce the velocity and try again? error message (49:05)

 

Troubleshooting in the Critical Analysis (50:12)

 

Example: adjusting the static pressure in the Source Data and resizing (51:45)

 

Placing a Valve Schedule (54:32)

 

Accounting for elevation changes using our Spot Elevation tool (55:46)

 

Adding and placing a booster pump (58:40)

 

Regenerating the Valve Schedule (1:00:00)

 

Question: Don't looped mainlines typically have half the friction loss? (1:01:04)

Answer: It depends on the system.

 

Question: What happens with a drip system when you get to the Critical Analysis and you don't have enough pressure? (1:02:10)

Answer: This issue almost always occurs because of loss through a drip valve kit. In this case, you can add a standard valve and call it a drip valve.

 

Takeaways:

  • Need to know:
    • How much pressure and flow are available?
    • How much pressure and flow are in demand?
    • How are the pressure and flow affected as the system draws from what is available?
  • Velocity typically increases when the flow is increased (adding sprinklers to the line) or when a smaller pipe is used with the same flow.
  • 4 factors that play into friction loss (with corresponding flow):
    • Velocity
    • Inside pipe diameter
    • Pipe roughness (pipe coefficient)
    • Length of pipe

 

We are always thinking about the design from a worst-case scenario and what the farthest head on the zone needs. The critical station is what demands the most pressure at the POC.

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