Literature | F.A.Q.
|
|
|||||||||||||||||||||||||||||||||||||||||||
|
Uni-Bell has served the engineering, regulatory, public health and standardization communities with integrity and considerable measures of both time and resources. Whenever questions have arisen relative to PVC pipe performance, Uni-Bell members, through their Association, have responded. Pipe designers and installers have been provided with much needed research from Uni-Bell regarding pipe deflection, ultraviolet aging, tapping, cyclic surge performance, in-service durability, and safety. The following Frequently Asked Questions are provided in an effort to share information on some common topics. If you would like to discuss any of the questions or answers in greater detail, feel free to contact any of the qualified engineers on the Uni-Bell staff. |
|||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||||
|
|
|||||||||||||||||||||||||||||||||||||||||||
| Q: What are the advantages of using gasketed-joint PVC pipe? | |||||||||||||||||||||||||||||||||||||||||||
A: Gasketed-joint PVC provides
many advantages over other types of joining systems. The following
is a brief summary:
|
|||||||||||||||||||||||||||||||||||||||||||
 |
|||||||||||||||||||||||||||||||||||||||||||
| Q: What is the life expectancy of PVC pipe? | |||||||||||||||||||||||||||||||||||||||||||
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. |
|||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||||
| Q: What effect does ultraviolet exposure have on 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. |
|||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||||
| Q: What is the difference between DR and SDR? | |||||||||||||||||||||||||||||||||||||||||||
| 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. |
|||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||||
| Q: What is flexible conduit deflection and how is it calculated? | |||||||||||||||||||||||||||||||||||||||||||
| 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: DL = 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
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! |
|||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||||
| Q: What is the maximum allowable depth of bury for PVC pipe? | |||||||||||||||||||||||||||||||||||||||||||
| 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.
Calculated Deflections of Buried AWWA C900 PVC Pipe (%)
Measured Long-Term Deflections of SDR 35 (PS 46) PVC Pipe (%)
|
|||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||||
| Q: What is the recommended Manning’s ‘n’ value for 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.
|
|||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||||
| Q: What is Uni-Bell's recommendation for bell-direction during installation? | |||||||||||||||||||||||||||||||||||||||||||
| 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.
|
|||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||||
| Q: What is the difference between pressure class and pressure rating? | |||||||||||||||||||||||||||||||||||||||||||
| 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. |
|||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||||
| Q: What are the differences between different PVC pipe specifications? | |||||||||||||||||||||||||||||||||||||||||||
| 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
|
|||||||||||||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||||||||||||
| Q: Can PVC pipe withstand a vacuum? If so, what is the maximum vacuum that PVC pipe can withstand? | |||||||||||||||||||||||||||||||||||||||||||
| 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 100oF,
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. |
|||||||||||||||||||||||||||||||||||||||||||
|
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.
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.