FREQUENTLY ASKED QUESTIONS

1. What are the advantages of using gasketed-joint PVC pipe?
2. What is the life expectancy of PVC pipe?
3. What effect does ultraviolet exposure have on PVC pipe?
4. What is the difference between DR and SDR?
5. What is flexible conduit deflection and how is it calculated?
6. What is the maximum allowable depth of bury for PVC pipe?
7. What is the recommended Manning’s ‘n’ value for PVC pipe?
8. What is Uni-Bell’s recommendation for bell-direction during installation?
9. What is the difference between pressure class and pressure rating?
10. What are the differences between different pipe specifications?
11. Can PVC pipe withstand a vacuum?  If so, what is the maximum vacuum that PVC pipe can withstand?
12. What are the most commonly used Standards for PVC water and sewer pipe?

• Watertight Joints:  Gasketed-joint PVC water and sewer pipe joints are virtually leak-free, enabling them to easily pass post-installation air tests or leakage tests.

• Installation Considerations: Ease in installation is another advantage gasketed-joint PVC offers.  Deep insertion, push-together gasketed joints are simple and easy to assemble, and can be filled, tested and placed in service immediately after assembly.
• Thermal Expansion/Contraction:  Gasketed-joint PVC provides an excellent allowance for thermal expansion and contraction of PVC pipelines. 

A: PVC has an impressive record of long-term durability. When water utility managers and engineering firms were surveyed in a study sponsored by the American Water Works Association Research Foundation (AWWARF), they cited corrosion resistance, longevity and durability as their top reasons for choosing PVC. When these same water supply professionals were asked to rank PVC against the other common types of pressure pipes for life expectancy, PVC ranked first. [Source: Moser, A.P., and Kellogg, Kenneth G., "Evaluation of Polyvinyl Chloride (PVC) Pipe Performance," AWWA Research Foundation, Denver, Colorado, 1994.]

Uni-Bell has an extensive collection of technical papers and experience articles that cover the subject of water and sewer pipe longevity in detail. These items are available, free of charge, by contacting Uni-Bell. After reading these papers, we believe you will come to the same conclusion that we have and consider one hundred years an extremely conservative estimate for the service life of a properly designed and installed PVC pipe.

A: In order to accurately quantify the effects of UV radiation on PVC pipe, Uni-Bell members conducted a two-year study in the late 1970s at various outdoor locations in the United States and Canada.  In this study, PVC pipe samples were placed on horizontal exposure racks and placed so that they received continual exposure to the sun.  At various points throughout the study, tests to evaluate mechanical properties were performed on the portion of the pipes that received the maximum UV exposure.

The results of the study (published as UNI-TR-5, "The Effects of Ultraviolet Aging on PVC Pipe") indicate a gradual decline in the pipe's impact strength. The lowest impact strength recorded after two years of exposure was 158 ft-lbf, or 75% of the original ASTM value. Even this reduced value exceeds those of most alternative sewer pipe products. These results indicate that no unusual handling problems should be expected from PVC pipe even after long-term exposure to sunlight.

The study results also show that Modulus of Elasticity and Tensile Strength were virtually unaffected. The fact that these properties are unaffected signifies that structural integrity and pressure capacity remain unchanged. UV degradation does not continue after installation when exposure to UV radiation is terminated.

The presence of an opaque surface between the sun and the pipe prevents UV degradation, since UV radiation will not penetrate thin shields such as paint coatings or wrappings. Burial provides complete protection.

When exposure in excess of two years of direct sunlight is unavoidable, PVC pipe should be covered with an opaque material while permitting adequate air circulation around the pipe. This prevents excessive heat accumulation.

A: The terms “dimension ratio” and “standard dimension ratio” are widely used in the PVC pipe industry.  Both terms refer to the same ratio, which is a dimensionless term that is obtained by dividing the average outside diameter of the pipe by the minimum pipe wall thickness.

Dimension ratios and standard dimension ratios were developed out of convenience rather than out of necessity.  They have been established to simplify standardization in the specification of plastic pipe on an international basis. Since these define a constant ratio between outer diameter and wall thickness, they provide a simple means of specifying product dimensions to maintain constant mechanical properties regardless of pipe size.  In other words, for a given DR or SDR, pressure capacity and pipe stiffness remain constant regardless of pipe size.

Even though the terms DR and SDR are synonymous, one minor difference between them is that SDR refers only to a particular series of numbers, i.e., 51, 41, 32.5, 26, 21, etc.  This series of “preferred numbers” is based on a geometric progression, and was developed by a French engineer named Charles Renard.  These numbers are often called “Renard’s Numbers.”

The term DR became widely used, in 1975, with the publication of AWWA C900, which governs production of small diameter PVC pressure pipe.  AWWA allowed the desired pressure capacity to dictate wall thickness.  Since the OD/t values generated did not happen to fall on any of Renard’s Numbers, AWWA removed the “standard” designation from the SDR term.

It is interesting to note that the most widely used product for small diameter sanitary sewer in the U.S., ASTM D 3034, SDR 35, provides an apparent contradiction in terms.   While 35 is not a Renard Number, it is still referred to as a standard dimension ratio.  In fact, all OD/t ratios in D3034 are listed as SDRs whether they are included in Renard’s “preferred numbers” or not.  This was probably for convenience’ sake.  D3034 was written in 1972, prior to the popularization of the DR term.  Accordingly, ASTM may have allowed all OD/t ratios to be called SDRs.

The bottom line is simple: the two terms are interchangeable.  SDR=DR=OD/t.

A: A flexible pipe derives its soil load carrying capacity from its flexibility.  Under soil load, the pipe tends to deflect (reduction of pipe diameter in the vertical direction), thereby developing passive soil support at the sides of the pipe.  At the same time, the ring deflection re­lieves the pipe of the major portion of the vertical soil load, which is then carried by the surrounding soil through the mechanism of an arching action over the pipe.  Allowable limits of deflection have been set by both ASTM (7.5%) and AWWA (5%).

The Modified Iowa Equation is used for predicting deflection in buried flexible pipe:

Where:          D_L = Deflection Lag Factor=1.0 (Typical)

                     K   = Bedding Constant=0.1 (Typical)

                     P    = Prism Load=Weight of soil over pipe

                     W’ = Live Load

                     E    = Modulus of Elasticity=400,000 psi minimum for PVC

                     DR = Dimension Ratio (OD/t)

                     E’   = Modulus of Soil Reaction  

This final parameter required to determine predicted pipe deflection is the Modulus of Soil Reaction.  Amster Howard, of the United States Bureau of Reclamation, compiled a table of average E’ values for various soil types and densities.  This information is provided here.

For a more detailed explanation of flexible conduit deflection, see our publication Uni-TR-1, "Deflection: The Pipe/Soil Mechanism", on our literature page.

Click here to download External Load Design Software for Free!

A: Allowable depth of bury can be calculated based on the allowable deflection as described above. Uni-Bell member products have been installed successfully at depths of fifty feet or more. The following tables are provided as quick reference. 

Click here for Calculated Deflections of Buried AWWA C900 PVC Pipe (%)

Click here for Measured Long-Term Deflections of SDR 35 (PS 46) PVC Pipe (%)

A: The appropriate conservative "n" value for minimum slope design of PVC sewer pipe is 0.009, as correctly justified by actual test data.  The justification of this value is briefly described below.

For many years it has been popular and convenient to use a single "n" value for all pipe materials.  However, there is no data that technically supports the single "n" value approach, and the most cost-effective sewer designs are precluded by such an over-simplified design criterion.  The recognition of "n" value variations among the commonly used sewer pipe materials is long overdue.

To properly justify the use of product specific "n" values, an extensive literature research program was undertaken.  This included review of a comprehensive listing of technical studies involving sanitary sewer pipe hydraulics that have been published within the past 30 years.  A number of important conclusions have been derived from this literature review.

No published technical study has ever reported an "n" value as high as 0.013 for a PVC sewer pipeline either in-service or in the laboratory.  No published technical study has ever reported PVC sewer pipe as having the same hydraulic characteristics as clay, concrete or asbestos cement under any conditions.  The major engineering textbooks dealing with sewer design have yet to address plastic pipe hydraulics.  The ASCE and WPCF Manual for Design and Construction of Sanitary and Storm Sewers lists a range of "n" values for "plastic" pipe, i.e., 0.011-0.015, but the authors have not supplied any evidence supporting their recommendations.  There is absolutely no scientific basis or technical justification for requiring the use of a 0.013 "n" value when designing PVC sewer pipelines.

Analysis of the compiled data revealed an arithmetic mean "n" value of 0.0088 for PVC pipe with a standard deviation equaling 0.0006.  An "n" value of 0.013 is seven standard deviations above the weighted arithmetic mean.  It is unjustified and totally unreasonable to require that an "n" value of 0.013 be used for minimum slope calculations with PVC pipe. 

A: Uni-Bell typically recommends that the bell end points in the direction of work progress because, when joining pipe, it is easier to insert the spigot into the bell than it is to push the bell over the spigot.  This also reduces the risk of soil or rubble being scooped under the gasket.

However, because of the exceptional joint tightness afforded by PVC pipe, the direction of the pipe bell, relative to flow direction, should not adversely affect the performance of the pipe joint or system hydraulics.

A: The three most common PVC pressure pipe products manufactured by Uni-Bell member companies are ASTM D2241, AWWA C900 and AWWA C905.  The following information will briefly explain the differences between the “Pressure Class” and “Pressure Rating” design philosophies employed in these three standards.

AWWA C900 has a “Pressure Class” design approach based on a 2.5 safety factor.  Pressure Class is formed using a 2.5 safety factor versus the 2.0 typically used in design.  In addition to the elevated safety factor, a surge allowance equal to surge pressure created by instantaneously stopping a column of water traveling 2 fps in the system has been allowed for in each pressure class.  The motivation for this design approach is to supply a piping product that is intended for use inside the “looped” perimeter of an urban water system where piping system geometry is complex.

AWWA C905 has a “Pressure Rating” design approach based on the 2.0 safety factor. AWWA C905 is intended for use as water transmission piping where long straight runs are the norm and system geometry is more simplistic.  Surge pressures are easily predictable and should be accounted for in design.

ASTM D2241 has a “Pressure Rating” design approach based on the 2.0 safety factor.  Once again, surge calculations are the designer’s responsibility.

In closing, ASTM D1785 schedule pipe (40, 80 & 120) does not conform to the same design approach as the above-mentioned products.  The products listed above offer a pressure capacity independent of pipe size, whereas the schedule product pressure ratings vary between different pipe diameters.

A: In order to understand all the differences between various pipe specifications, a more detailed review of the specifications should be conducted.  Uni-Bell staff stands ready to discuss any specific questions you may have, but provides the following “quick reference” for your use:

PVC Pressure Pipe Standards

PVC Sewer Pipe Standards

A: Yes, PVC pipe can withstand vacuum pressures.  According to research conducted by Dr. R.K. Watkins at Utah State University, vacuum pressures cannot collapse an underground PVC pipe that is properly encased in a soil envelope and exposed to normal service temperatures.  In fact, quick calculations show that even under conditions of elevated operating temperatures of 100^oF, the pressure required to collapse most PVC pipe is greater than atmospheric.  In other words, the pipe can withstand a complete vacuum.

Vacuum pressures are generally not considered a favorable occurrence in water distribution systems of any pipe material.  However, if the effects on the entire system are taken into consideration, PVC offers adequate strength and safety to withstand vacuum pressures.

A:

PVC Non-Pressure Pipe Standards for Gravity Applications:

• ASTM D3034: Standard Specification for Type PSM Poly(Vinyl Chloride) (PVC) Sewer Pipe and Fittings
• ASTM F679: Standard Specification for Poly(Vinyl Chloride) (PVC) Large-Diameter Plastic Gravity Sewer Pipe and Fittings
• ASTM F789: Standard Specifications for Type PS-46 and Type PS-115 Poly(Vinyl Chloride) (PVC) Plastic Gravity Flow Sewer Pipe and Fittings
• ASTM F794: Standard Specification for Poly(Vinyl Chloride) (PVC) Profile Gravity Sewer Pipe and Fittings Based on Controlled Inside Diameter
• ASTM F949: Standard Specification for Poly(Vinyl Chloride) (PVC) Corrugated Sewer Pipe With a Smooth Interior and Fittings
• ASTM F1803: Standard Specification for Poly(Vinyl Chloride) (PVC) Closed Profile Gravity Pipe and Fittings Based on Controlled Inside Diameter

PVC Pressure Pipe Standards for Potable Water, Force Main, and Transmission Main Applications:

• ASTM D2241: Standard Specification for Poly(Vinyl Chloride) (PVC) Pressure-Rated Pipe (SDR Series)
• AWWA C900: Standard for Polyvinyl Chloride (PVC) Pressure Pipe and Fabricated Fittings, 4 in. Through 12 in. (100 mm Through 300 mm), for Water Distribution
• AWWA C905: Standard for Polyvinyl Chloride (PVC) Pressure Pipe and Fabricated Fittings, 14 in. Through 48 in. (350 mm Through 1,200 mm), for Water Transmission and Distribution
• AWWA C909: Standard for Molecularly Oriented Polyvinyl Chloride (PVCO) Pressure Pipe, 4 in. Through 24 in. (100 mm Through 600 mm), for Water Distribution
• ASTM Standard Specification for F1483-05 Standard Specification for Oriented Poly(Vinyl Chloride), PVCO, Pressure Pipe, 4 in. Through 16 in. (100mm Through 400mm)