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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

 
Time to Redefine “Large Diameter” PVC Pressure Pipe
Posted on September 30, 2014 by John Houle
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In 1975 AWWA published the C900 standard for PVC pressure pipe and fittings. The standard’s product sizes ranged from 4-inch through 12-inch. In 1988 a second PVC pressure pipe standard was published – this standard, AWWA C905, included sizes from 14- through 24-inch.

The two standards differed markedly in their product design: C900 used a safety factor of 2.5, while C905 employed an SF of 2.0. Because of this distinctly different design approach, there was an obvious break between small diameter pipe (4- through 12-inch) and large-diameter pipe (14- through 24-inch.)

In the 26 years since C905 was published, however, three major changes have occurred that make the transition point to large diameter less clear:

  1. The design methods in the two standards have been harmonized.
  2. A revision is now under way at AWWA to merge the two standards under the C900 title.
  3. The largest size being produced has more than doubled to 60 inches.

What was considered “large-diameter” in 1988 is now relatively small. Unfortunately, some specifiers still consider 14-inch as a large pressure pipe – with 60-inch as the comparison, I think it is time to revisit the definition of “large-diameter.”

Click here for my Tech Brief on this subject.

PVC’s market share for potable water mains and sewer forcemains in North America has been growing steadily for more than 50 years. The large market share gained by smaller-diameter PVC pipe is a result of the product’s outstanding performance characteristics – the same advantages that are provided by larger-diameter products. 

Time to Redefine “Large Diameter” PVC Pressure Pipe

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Time to Update Specifications for ASTM F679 PVC Sewer Pipe
Posted on September 23, 2014 by John Houle
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In 1980 ASTM published its F679 standard for solid-wall PVC pipe and fittings. The standard’s design philosophy was to allow two wall thickness options (called “T1” and “T2”) based on two values for the PVC material’s modulus of elasticity.

In 2006 this design method was changed. Gone are the “T1” and “T2” wall designations.

Instead the new system provides a minimum wall thickness table combined with minimum pipe stiffness values – the pipe manufacturer is now able to provide the targeted pipe stiffness with any combination of wall thickness and modulus of elasticity (as long as material cell class and minimum wall thickness requirements are met).

Click here for my Tech Brief on this subject.

The bottom line: project specifications that call out “T1” or “T2” walls are outdated – these references have been obsolete for about eight years.

Time to Update Specifications for ASTM F679 PVC Sewer Pipe

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Time to Redefine “Large Diameter” Solid-Wall PVC Gravity Sewer Pipe
Posted on September 17, 2014 by John Houle
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In 1980 ASTM published its F679 standard for solid-wall PVC pipe and fittings. The standard’s title, “Polyvinyl Chloride (PVC) Large-Diameter Plastic Gravity Sewer Pipe and Fittings,” was appropriate at the time, because the product sizes ranged from 18-inch to a maximum size of 27-inch.

In the thirty-four years since 1980, however, the maximum size more than doubled to 60 inches. What was considered “large” in 1980 is now relatively small.

Unfortunately, some specifiers still consider 18-inch as a large sewer pipe – with 60-inch as the comparison, I think it is time to revisit the definition of “large-diameter.”

Click here for my Tech Brief on this subject.

PVC sewer pipe has been the product of choice for sanitary sewers in North America for more than 25 years. The large market share gained by smaller-diameter PVC pipe is a result of the product’s outstanding performance characteristics – the same advantages that are provided by larger-diameter products.

Time to Redefine “Large Diameter” Solid-Wall PVC Gravity Sewer Pipe

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Pipe Stiffness Explained: PVC and Ductile Iron
Posted on June 11, 2014 by John Houle
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Some project specs require Ductile Iron (DI) pipe because of its supposed “strength” in resisting external loads. In the past it was true that iron pipe had plenty of pipe stiffness, but that was before the iron industry converted from thicker-walled Class pipe to thinner-walled Pressure Class pipe.

Conventional Wisdom Is Not Always True

This tech brief discusses how much a pipe can safely deflect – PVC is safer.

Pipe stiffness of two approximately equal pressure class pipes is compared – PVC is stiffer.

The facts:

  • DI pipe fails at a lower deflection than PVC pipe.
     
  • DI has a lower safety factor against failure.

Conventional wisdom is turned on its head: the stronger, safer pipe is PVC!

To find out more, click here for the Tech Brief.

Pipe Stiffness Explained: PVC and Ductile Iron

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Looking Back 20 Years at AWWARF’s 1994 Study on PVC Water Pipe Performance
Posted on May 14, 2014 by John Houle
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In 1994 AWWA’s Research Foundation published a report on PVC pipe titled “Evaluation of Polyvinyl Chloride (PVC) Pipe Performance.” We have now reached the 20th anniversary of the study – time for a look back to see if the research findings were accurate.

I’ve tried to keep you in suspense, but the answer is a resounding “Yes.” The report’s assertions have proven correct and time has reinforced its conclusions. For example:

  • The allegation that PVC “loses strength with time” has been disproved.
     
  • Tapping PVC pipe was not a problem back then and is less so today with improvements in hardware, procedures, and tapping.
     
  • UV exposure and chemical permeation of PVC pipe were shown by AWWARF to be non-issues 20 years ago and the passage of time has borne this out.

Click here for my Tech Brief on this subject.

PVC water pipe was in widespread use in North America 20 years ago, but has now become the product of choice – it is a well-engineered product that provides exceptional service for water transmission and distribution systems.

Looking Back 20 Years at AWWARF’s 1994 Study on PVC Water Pipe Performance

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PVC Pipe Materials: Cell-Class Explained
Posted on April 29, 2014 by John Houle
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Product standards for PVC water and sewer pipes typically require PVC materials to meet ASTM cell-class requirements. The cell class consists of five cells that designate different aspects of the material.

This tech brief discusses cell class and what all those numbers mean. It also takes issue with two industry misconceptions:

  • The 12364 cell class is “new” – the truth is that this class has been in ASTM standards for about 35 years – this is not new!
     
  • PVC cell class is 5 numbers plus one letter – this was correct in the past, but ASTM eliminated the letter more than 15 years ago – this is not new either!

To find out more, click here to read the Tech Brief.

PVC Pipe Materials: Cell-Class Explained

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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.

PVC Pipe Print Line – Valuable Information for Pipe Installers and Owners

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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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