Cast-iron pipe failure: Are plumbing claims draining you? (Part 2)
Part 2 of 2: Learn more about the most common cause of cast-iron pipe failure and how regulations and waste management play a role.
Editor’s Note: Part one of this series covered the basics of sanitary drains and their connections as well as typical points of failure.
Often, cast-iron pipe (CIP) claims document tuberculation, the gradual accumulation of iron-hydroxide inside the pipe’s interior surface that disrupts flow, as the source of pipe failure.
Tubercles do not originate from the CIP; they form from ferrous oxide in the water. Rather than the pipe material being “removed,” as most corrosion occurs, the tubercles are deposits on the pipe walls. The tubercle begins with bacteria that form a bacterial slime layer that feeds on the manganese and iron in the water. The slime layer accumulates over time, and the reddish-brown products solidify and reduce the interior pipe diameter and flow capacity.
A three-year study of tubercle formation revealed that the accumulation rate was fastest during the first year as compared to the following two years. The study revealed that there was no relationship between tubercle height and corrosion/pitting depth on the pipe wall.
Tubercle thickness inside a pipe does not reflect the extent of pitting on the pipe wall surface. The tuberculation and accumulation can be removed by proper cleaning without affecting the structural integrity of the pipe.
The influence of government regulation
Government water usage regulations have impacted sewer lines and drainpipes in residential plumbing systems. The Energy Policy Act of 1992 (EPA92) restricted the amount of water used by plumbing fixtures such as showers and toilets.
Before EPA92, most toilets used from 3.5-5 gallons of water per flush (gpf). The new regulations mandated 1.6 gpf. The new regulations also reduced water usage in showerheads from 2.75 gallons of water per minute (gpm) to 2.5 gallons. Water usage in faucets dropped from 2.75 gpm to 2.5 gpm. The Energy Independence and Security Act of 2007 (EISA07) limited dishwashers to 6.5 gallons per cycle (gpc) in standard models and 4.5 gpc in compact models, a drop from 14 gpc before the new regulations. The EISA07 also lessened the gallons per load (gpl) from 41 gpl to 26.6 gpl in washing machines. The regulations achieved the goal of conserving water, it also had unintended consequences of less effective flushes and evacuation of solids that lead to clogging.
Individual waste management responsibility
Homeowners contribute to pipe corrosion under two circumstances: Loading characteristics and the substances discharged down the drain line.
Loading characteristics
“Load” in residential plumbing refers to the number of drainage fixture units (DFUs), such as toilets, bathtubs, sinks and washers, that contribute and guide water flow and produce a load effect on the plumbing system.
Pipe diameter selection is guided by the intended volume of water and solid matter it will transport. In a home with a kitchen and two bathrooms, the drainpipe is usually 3-inches in diameter. Outside the home, a 4-inch diameter pipe is used where several drain lines merge before discharge to the street.
The smaller the load, the poorer performance. A one-bedroom, one-bath home would not benefit from a 4-inch diameter receiving drainpipe because the volume of water and waste may not be enough to create sufficient turbulent flow to sweep the wastes to the sewer. Load is especially relevant when garages are converted to living quarters or additions constructed for tenants or an elderly parent. More DFUs may exceed the original load drainpipe capacity and amplify obstructions and create overloads. Load modifications are also important when the home is fully occupied. The association between discharge volume flow and DFUs is not uniform but varies with the number and operation of fixture units and can contribute to overflows.
Waste Management Practices
Residential sewer systems generate significant amounts of toilet paper, durable wet wipes, and other flushable consumer products (FCPs). When combined with factors such as pipe slope and diminished flush volume, major impacts can be expected on residential sewer lines.
Residential sewer lines are often clogged by the low biodegradability of toilet paper’s pulp fibers. A recent study found that 72% of solids in every liter of wastewater per person per day is comprised of toilet paper sludge, which equals more than 4 pounds of toilet paper per person per month. There is a strong relationship between the physical properties of toilet paper (polymerization, thickness, grammage and softness) and clogging experienced by residential plumbing systems.
Toilet paper additives (binders, fillers, adhesives, and color pigments) also hinder their disintegration. Other factors affect the dissolvability of toilet paper in sewage pipes, such as the level of turbulence in the water and each brand’s unique chemical properties. Premium toilet papers offer softness and durability; however, they disintegrate more slowly than cheaper and thinner brands.
Toilet paper sheet per usage has also increased, making it increasingly difficult for toilet paper to dissolve in sewer lines as the number of sheets used per flush grows. Studies have concluded that requiring a minimal increase in pipe slopes and improving the disintegration rate of FCP in consumer products can enhance the functionality of plumbing systems. Periodic maintenance to clear the build-up within these drain lines is also beneficial.
When a plumber is asked to examine a residential drain system, the following tasks will aid in your evaluation.
- Prepare a drain system diagram and locate all the DFUs. Mark the pipe diameters, system changes and additions. Opine whether each linear section of pipe is properly sized and pitched.
- Identify the recommended water use characteristics for each DFU and determine if the pipe sizing and design were appropriate.
- Estimate the drain system’s age based on the home’s age and materials.
- Calculate whether the pipe diameter(s) are appropriate for the load characteristics.
- Opine whether solids evacuation is influenced by the design or waste management or both.
- Identify whether the flow characteristics are influenced by abrupt corners, sand, flat runs or irregular fittings.
- Identify the failure characteristics associated the operator waste management (e., grease and solids accumulation)
- Determine whether the system meets the building code.
Donald Dunn is a Florida-licensed master plumber and president of the Florida-based My Plumbing Company. In addition to providing residential and commercial plumbing services, he specializes in plumbing-related forensic and diagnostic analysis and teaches basic plumbing and plumbing-related cause and origin to insurance professionals and building scientists.
Dr. Ralph E. Moon, Ph.D., is a forensic scientist and frequent speaker at insurance conferences and seminars. His current research interests are studying rates of wood deterioration and metal corrosion among building materials and plumbing components used in residential construction. He is employed by NV5, Inc. in Tampa, Florida.
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