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TECHNICAL BLOG

Welcome to John’s Blog. Answers to frequently asked questions are periodically posted here. The objective is to share information about PVC pipe with readers as well as with utilities, design engineers and pipe installers. The blog will provide the latest information on PVC pipe design, installation, and application for water and wastewater infrastructure projects.

If you are interested in having the response to your question considered for posting, e-mail John at techblog@uni-bell.org

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John Houle: Technical Director

John Houle is Uni-Bell’s Technical Director. John holds a Master’s Degree in Civil Engineering from the University of Missouri and an MBA from the University of Oregon. He has more than 25 years of experience in the plastic pipe industry in applications engineering, market development, forensic analysis, technical writing, and standards development. 

 

John Houle,
Technical Director

 
PVC Pipe Print Line Valuable Information for Pipe Installers and Owners
Posted on April 15, 2014 by John Houle
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Each length of PVC pipe is required by its product standard to contain a line of print that describes the pipe.

Print information should be checked at several locations:

  1. At the manufacturing plant to ensure that the correct pipe is being shipped
  2. At the jobsite or storage yard to confirm that the pipe that was purchased is the pipe that has been received
  3. At the trench to ensure the specified pipe is being installed
  4. Any time the pipe is exposed after installation

The attached technical brief explains the details of print requirements and provides an example print line. Click here.

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Occasional Surge Pressure for DI and PVC Pipe: Advantage PVC
Posted on April 1, 2014 by John Houle
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This document compares two pipe materials for response to “occasional surge,” a term that might not be well understood. AWWA standards define “occasional surge” as:

Occasional (emergency or transient) surge pressure: Surge pressures caused by emergency operations, usually the result of malfunction (such as power failure, sudden valve closure, or system component failure).

The design assumption is that an emergency event would occur at most only a few times in the life of the pipeline. For this reason, occasional surge is analyzed differently from “recurring surge” (also known as “cyclic surge”), which is a design condition for sewer forcemains.

The ductile iron (DI) industry promotes their material as superior to PVC for surge pressures. The attached comparison uses the same set of design conditions to compare PVC and DI for occasional surge pressure. See for yourself – both products are suitable for the static pressure, but the total pressure (including surge) in the DI pipe is 495 psi compared to 282 psi in the PVC pipe.

For ductile iron the total pressure is at 110% of the allowable pressure – redesign would be required. Meanwhile, for PVC the total pressure is only 75% of allowable.

Furthermore, the 495 psi pressure in the DI pipe might damage non-pipe components such as valves, fittings, and service lines.

Click here to read.

Occasional Surge Pressure for DI and PVC Pipe:  Advantage PVC

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Changing Direction: Axial Joint Deflection Explained
Posted on February 14, 2014 by John Houle
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There are three ways to change a gasketed pipeline’s direction:

  1. Fitting (elbow, bend, etc.)
  2. Curvature of the pipe barrel
  3. Axial deflection at pipe joints

This tech brief focuses on the third method, axial deflection. Mathematically the idea is simple. For example, if you want to change direction by 5° and the allowable change in direction at each joint is 1°, then you need to deflect 5 joints.

The document explains this concept and the geometry involved. Click here to read.

Design Aid

There are two links in the tech brief -- the first is to a design aid. The designer initially inputs two variables:

  1. Total angle to be turned
  2. Allowable angle per joint (from the manufacturer of the pipe to be used)

The first output is the number of joints necessary. After adjusting the number of joints to a whole number, the designer then inputs a third variable: the distance between joints. Outputs are the offset at the end of the deflected pipe and the radius of curvature of the pipeline.

Offset Tables

The second link is to a series of tables that show the linear offsets from the original undeflected pipeline for different allowable angles.

This information should be helpful to a designer in deciding which method to use to curve a pipeline.

Changing Direction: Axial Joint Deflection Explained

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JOINT RESTRAINT OPTIONS FOR PVC PRESSURE PIPE
Posted on January 17, 2014 by John Houle
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Internal pressure causes longitudinal thrust forces in pipe systems. These forces are developed at changes in flow direction or pipe size, as well as at dead ends. To keep pipe joints from pulling apart, either joint restraint or concrete thrust blocking is required.

In the past, concrete thrust blocks were the most commonly used method to prevent movement. Recently, thrust blocking has largely given way to joint restraint.

There are several methods for restraining PVC pipe joints. In addition to the traditional external joint restraint devices, several integral internal devices have been developed.

The attached technical brief provides background on this topic. Click here to read.

JOINT RESTRAINT OPTIONS FOR PVC PRESSURE PIPE

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Important Considerations When Comparing PVC and Ductile Iron Pressure Pipe
Posted on November 13, 2013 by John Houle
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The Decline of Iron Pipe

During 60 years of sustained growth for PVC water pipe, iron pipe’s market share has decreased dramatically. The iron industry has reacted by negative attacks and by spreading misinformation about PVC pipe. I think it is time to provide more balanced information.

PVC Pipe – Healthy and Safe

PVC is used in over 40,000 municipalities in North America because it meets health and safety regulations – some 10 million quality tests have been conducted on water carried through PVC pipe since it was introduced. Health and safety are major reasons why most new installations of drinking-water distribution pipe are PVC.

Why the Move from Iron to PVC?

The major cause: corrosion of iron pipe. To make matters worse, thinner-walled ductile iron (DI) corrodes sooner than traditional cast iron pipe products.

Transitioning from Iron to PVC Pipe: A Smooth and Seamless Process

The DI industry has argued that the transition to PVC is so difficult it would be better to stay with underperforming iron. In fact, the typical move to PVC has been very straightforward:

• Same outside diameters – for the municipal water market, PVC pipe and DI pipe are manufactured in the same cast-iron outside diameter regimen.

• Same fittings, valves, and appurtenances – the same slip-on or mechanical joint fittings, valves, and appurtenances used with DI can be used with PVC pipe in the same manner. This ensures that product styles and installation procedures will be familiar to the waterworks professional.

• Same suppliers – all items needed for PVC water systems are available from the same suppliers that provide DI items.

Comparison Sheet

Attached is a comparison sheet that addresses important issues in water pipe selection and serves as a quick reference for comparing PVC and DI pressure pipe. Click here to read.

Important Considerations When Comparing PVC and Ductile Iron Pressure Pipe

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Occasional Surge Pressure for HDPE and PVC Pipe: Advantage PVC
Posted on October 30, 2013 by John Houle
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This document compares two pipe materials for response to “occasional surge,” a term that might not be well understood. So that everyone is on the same page, AWWA standards define “occasional surge” as:

Occasional (emergency or transient) surge pressure: Surge pressures caused by emergency operations, usually the result of malfunction (such as power failure, sudden valve closure, or system component failure).

The thinking is that an emergency event would occur at most only a few times in the life of the pipeline. For this reason, occasional surge is analyzed differently from “recurring surge” (also known as “cyclic surge”).

The polyethylene (HDPE) pipe industry is promoting their material as superior to PVC for surge pressures. The attached comparison uses the same set of design conditions to compare PVC and HDPE for occasional surge pressure. See for yourself – both products are suitable for the application, but the surge generated in the HDPE pipe is 341 psi compared to 282 psi in the PVC pipe. While both pipe materials would fare well, the additional 20% higher surge in the PE pipe might damage non-pipe components such as fittings and valves.

Click here to read.

Occasional Surge Pressure for HDPE and PVC Pipe: Advantage PVC

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Not Time to Abandon Conservative AWWA Tradition for Plastic Pipe Safety Factors
Posted on October 23, 2013 by John Houle
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Back in 2007, the AWWA C900 standard for PVC pipe was revised. The most important change was the reduction of the standard’s safety factor from 2.5 to 2.0. A significant factor in the decision was more than 40 years of use of PVC pipe in ASTM pressure pipe applications with a safety factor of 2.0.

Fast-forward to 2013 – the HDPE industry is proposing to reduce its safety factor for a new material from 2.0 down to 1.6. This material does not have a long history of usage and therefore has unknown longevity – paradoxically, it would also have the lowest safety factor of any AWWA pipe used for transmission/distribution applications.

The attached technical brief discusses the contrast between the evolution of safety factors for PVC and HDPE. Click here to read.

Not Time to Abandon Conservative AWWA Tradition for Plastic Pipe Safety Factors

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Design Factor and Safety Factor Explained
Posted on October 16, 2013 by John Houle
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There has been confusion recently in the plastic pipe industry regarding the terms “design factor” (DF) and “safety factor” (SF).

Mathematically, the relationship is simple: DF is the inverse of SF. At least three AWWA standards state explicitly in the definition for DF that design factor is “the inverse of the safety factor.”

The attached technical brief discusses the relationship between the two concepts in more detail. To further illustrate the point, the brief includes a table of numerical examples from AWWA product standards for plastic transmission/distribution pipe. In all cases the design factor is 0.50 and the safety factor is 2.0.

Click here to read.

 

Design Factor and Safety Factor Explained

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Special Backfill Not Required for PVC Pressure Pipe
Posted on October 2, 2013 by John Houle
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I sometimes hear the statement that PVC pressure pipe requires “special backfill," while ductile iron does not. This is a mind-set that is not based on engineering principles.

The attached “Backfill Requirements for PVC Pressure and Gravity Pipes” sets the record straight by investigating the concepts of pipe stiffness and pipe deflection. Examples are included using low-quality uncompacted backfill. Even in these extreme conditions, the result is that PVC DR18 and DR14 pressure pipes (the typical PVC municipal pipes) experience in-ground pipe deflections that are less than one-fifth of the allowable deflections.

It is clear that uncompacted native soil provides enough support to keep pipe deflections very low – insistence on costly imported backfill is a waste of an owner’s money.

AWWA Standards – Same for DI and PVC

AWWA installation standards recognize that PVC and ductile iron do not require different backfill. In fact, the AWWA C600 standard (ductile iron) and the AWWA C605 standard (PVC) each include drawings of five trench types. Close inspection of these trench types shows that the two standards have identical requirements.

PVC Gravity Sewer Pipe – a Different Story

It is likely that the “special backfill” mentality arose because of PVC gravity pipe. Gravity pipe is used in high-load conditions:

  • Extremely shallow burial (as little as 1 foot) with highway loads
  • Very deep burial (in excess of 50 feet) with significant soil loads

These high loads cause backfill material and compaction to become critical components of the engineer’s design.

For details, please see the attached technical brief. Click here to read.

Correction: In my previous blog entry on air testing of installed sewer pipe, I wrote “Gravity pipe is typically tested with low-pressure air (usually 5 psi).” The correct pressure is 4 psi.

 

Special Backfill Not Required for PVC Pressure Pipe

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Safe Pressure Testing of Installed Pipelines
Posted on September 10, 2013 by John Houle
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It is common practice to pressure test installed pipelines to ensure that the pipe materials and installation are satisfactory. In fact, it makes sense that every installed pipeline be pressure tested to ensure a leak-free system.

Gravity Sewer Pipe

Gravity pipe is typically tested with low-pressure air (usually 4 psi). The maximum recommended air pressure is 9 psi, which seems like a safe pressure. However, if an end-plug were to let go in a large-sized pipe, even a low-pressure test could pose a safety threat.

Pressure Pipe

Pressure pipe is tested at much higher pressures, often at the pressure class of the pipe. Because air is a compressible medium, air under high pressure stores a large amount of potential energy. A failure of any component in the system would result in the air’s potential energy turning into kinetic energy, with dangerous consequences. For this reason, the most unbreakable rule in the pipe industry is: DO NOT USE AIR TO TEST PRESSURE PIPE.

Click here to read.

  

Safe Pressure Testing of Installed Pipelines

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