Some utilities are hesitant to use PVC pipe in deep-burial applications. This reluctance is based more on misconception than on engineering principles, since PVC pipe has been used for many years at depths in excess of 50 feet.

The attached technical brief concentrates on the science of the subject. Click here to read.

PVC Pipe Stands Up to Large Earth loads

Some engineers insist on using iron pipe over a certain depth – while the rest of the sewer main is PVC – because they fear PVC won’t hold up under large dead loads.

The fact is that buried PVC pipe does not act alone – it receives support from the surrounding soil. The combined pipe/soil structural system allows PVC pipe to be buried at depths not possible for “stronger” rigid pipes.

Long Life for Deep Bury

The two example projects in the document show that deep-bury PVC pipe continues to function well over time: one installation is from 1995 (18 years) and the other from 1986 (27 years).

 

There has been discussion in the piping industry about the ability of PVC pipe to withstand internal vacuum caused by pumping from fire hydrants. In fact, some designers mistakenly contend that only thicker-walled PVC pipe (DR18 or less) can resist fire-flow requirements.

The attached technical brief provides the engineering to dispel this misconception. Click here to read.

The fact is that buried PVC pipe receives support from the soil that surrounds it. Design examples in the document assume a low value for the soil capacity – despite this very conservative assumption, thin-walled SDR41 pipe would not experience collapse due to fire-flow conditions. An engineer who designs buried PVC pipe does not need to consider collapse from fire-flow pumping.

The presence of air in a pressure pipeline is unavoidable, since dissolved air is present in the fluid being transported. This entrained air comes out of the fluid during certain operating conditions and gathers in pockets at high spots in the pipeline.

Air Pockets Increase Operating and Maintenance Costs

Research has shown that air pockets in pressure piping systems cause increased operating costs and can result in pipeline damage. Air pockets decrease flow capacity and cause significant energy losses. They can also produce or enhance surge pressures, which can cause severe damage to pipelines.

The solution is air valves, the subject of the attached technical brief. Click here to read.

Correctly designed, installed, and maintained air valves are the most cost-effective tools to control entrained air and air pockets in pressurized piping systems.

Last week’s blog entry described the importance of correctly installing PVC pipe spigots to the insertion line. This week we will look at what happens inside the pipe when the joint is assembled correctly.

Expansion Gaps: Important for PVC Pipe Joint Design

With correct installation, there is a horizontal gap inside the joint between the end of the spigot and the shoulder of the bell. This gap is the subject of the attached technical brief. Click here to read. The gap occurs in both pressure and non-pressure pipe, but is observable only during video inspection of gravity-flow sanitary sewer pipe.

The gap is necessary to provide room for thermal expansion of the pipe and to allow for angular joint deflection that accommodates any ground movement that occurs. The gap is an integral feature of the joint design and does not affect the joint’s water-tightness or its hydraulic characteristics.
 

Many pipe installers think that a PVC pipe spigot is correctly installed if its insertion line has disappeared into the pipe bell. They believe that the purpose of the line is to ensure that the spigot has been installed far enough to allow the gasket to seal properly. This is only partly correct.

As the attached technical brief explains (click here), there is more than one purpose for the insertion line:

1. The first purpose is to ensure that the gasket will properly engage with the spigot to form a leak-free joint. If there is a space between the line and the bell, sealing may be affected.
    
2. The second reason is to prevent the spigot from being inserted too far into the bell. If the line is not visible, the spigot has been over-inserted.

Thus, the insertion line is barely visible in properly installed PVC pipe.

Over-insertion is not ideal because it defeats some of the benefits of PVC pipe joint design:

• Less room for the joint to accommodate thermal expansion
• Reduced ability to axially deflect the joint (to change direction)
• Possible creation of stress in the bell wall

Sixty years of experience have shown that PVC pipe joints function reliably long-term even when not installed perfectly. However, the recommendations of the technical brief should be followed to maximize joint performance.

 

The effect of sunlight on PVC pipe has been a misunderstood topic. Negative attacks by our competitors allege that exposure to sunlight causes PVC pipe’s properties to degrade so severely that the pipe becomes unsuitable for use. The attached technical brief rebuts this contention. Click here

As with many subjects, there is a germ of truth to the allegation: it is true that UV radiation from sunlight can affect the outermost surface of PVC pipe. However, even after prolonged exposure, there is no practical effect on the pipe’s performance characteristics.

Two-Year UV Exposure Study

A two-year study has quantified the effects of UV radiation on the properties of PVC pipe. The research found that exposure to UV radiation results in a change in the pipe’s surface color and a reduction in impact strength. Other properties such as tensile strength (pressure rating) and modulus of elasticity (pipe stiffness) are not adversely affected.

It is important to realize that the average impact strength after two years of exposure still remained above the level required at time of manufacture. There are no restrictions on cutting or tapping of UV-discolored PVC pipe. As well, UV-exposed pipe can still be installed with less care than is necessary for more vulnerable clay, concrete, or mortar-lined/epoxy-coated ductile iron alternatives.

The Bottom Line

On a practical basis, exposure to UV radiation has no performance-based effect on PVC pipe.

 

Negative attacks made by our competitors allege that PVC pipe is not suitable for cold-weather projects. The attached technical brief refutes this claim. Click here.

PVC pipe has been in service in North America since the 1950s and has proven its ability to function well when exposed to the rigors of cold-temperature installation and operation. PVC is widely used in Canada, Minnesota, and the Dakotas, so cold weather is not a limiting factor for PVC pipe projects.

In fact, installation of PVC pipe is even more advantageous as temperatures decrease and workers become hampered by inclement weather conditions. PVC is ideal in these situations because it is easy to cut, easy to handle, and easy to assemble.

This technical brief discusses three aspects of PVC pipe in cold temperatures:

1. Design
    - Effects on PVC’s material properties
    - Accommodating thermal expansion
    - How to prevent freezing
    
2. Handling and Installation
    - Effects on PVC’s material properties
    - Accommodating thermal expansion
    - How to prevent freezing
    
3. Operations – no change from typical operations

There are three key messages:

1. In cold conditions, installation practices are important for all piping – abusive handling practices should not be used with any pipe material.
    
2. Worker safety should always be paramount – prolonged exposure to cold temperatures may affect workers’ productivity and performance.
    
3. PVC pipe is a logical choice for cold-weather projects.

Stay tuned for my next blog entry when I will address misconceptions about the effects of UV exposure on PVC pipe.

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

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. We thought it was time to provide a more-balanced view.

Health and Safety

PVC is used in over 40,000 municipalities in North America because it meets all 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?

A combination of two factors is driving the move from iron to PVC:

• Increased focus on replacement of our decaying and corroding water infrastructure.
• Recognition that thinner-walled DI corrodes sooner than traditional cast iron pipe products(See: Utah State University’s 2012 Water Main Break Rates in the USA and Canada: A Comprehensive Study).

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 smooth and seamless. Here are the facts:

• 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.

We encourage you to contrast and compare and to include PVC pipe in your upcoming water project bids. Join the growing number of North American water utilities that are providing greater value for taxpayer dollars by using PVC pipe – installing better-performing pipe while simultaneously reducing capital and O&M costs.

 

I would like to direct your attention to our standards sheets, one of the Association’s key resources:

• Pressure Pipe Standards (potable water & sewer force main applications)
• Gravity Sewer Pipe Standards (solid-wall and profile-wall)
• Reclaimed Water Pipe Standards (purple pipe)

Each sheet provides an overview of available PVC pipe products. Included are the applicable standards as well as size ranges, material cell classes, and joining-system types. The pressure-pipe sheets also provide outside-diameter regimens and pressure classes/ratings, while the gravity-sewer sheet includes pipe stiffness values for both solid-wall and profile-wall pipes.

Some might not realize the extent of the size ranges available. For PVC pressure pipe, the maximum size is 48-inch. For PVC gravity pipe, the largest solid-wall pipe is also 48-inch, with profile-wall pipe up to 60-inch.

Recent introduction of PP into the sanitary sewer market should be cause for concern to wastewater utilities. Unsupported claims about performance, lack of rigorous studies and testing, questions regarding joint integrity, reduced safety factor, limited options for fittings and lateral connections, all point to the need for caution when considering PP alternatives over PVC pipe.

Thermoplastic materials have different engineering properties and standards, and it's important for engineers, contractors and sewer departments to recognize them. Otherwise they risk compromising long-term performance.

Uni-Bell's PVC vs. PP comparison sheet (click here) summarizes these differences, ranging from why bell stiffeners are required for PP, to allowable deflection limits and the corresponding safety factor – confirming PVC pipe's superior performance.  

Supported by over 40 years of standards and testing, PVC pipe offers exceptional joint integrity, low maintenance and a high safety factor, backed by stringent mandrel and low-pressure air tests. With a broad assortment of fittings for connections, which help avoid compromising system integrity through use of cut-in fittings, PVC pipe is available in a wide array of options suitable for the most difficult applications.

With more than one million miles in service,  and manufactured in sizes ranging for 4-60 inches, PVC is the most widely used material for gravity sewer pipe with an estimated market share in excess of 80%.

The quality and performance of PVC have been validated by countless universities, government agencies, engineers, installers and utilities.  As well,  it was ranked highest in a 2010 buried pipe market survey by Trenchless Technology  Magazine  and rated as the most commonly specified, easiest to maintain and longest-lasting pipe material.  See pages 8-11 of the study by clicking on the following link:

http://www.trenchlessonline.com/pdfs/2010_Pipe_Materials_Guide.pdf

When compared to PVC, PP just doesn't measure up. For proven performance, sustainability and cost effectiveness, PVC is the material of choice for water and wastewater systems.

If you have any questions about the comparison sheet, please feel free to contact us.