System – Sanitary Waste & Vent

• Overview

Sanitary waste and vent systems are common to all building properties.  What used to be a relatively non issue system except for random clogging and backups, is now becoming a significant headache to building owners and tenants in the form of more advanced failures.  Since they run extensively throughout the building as well as run underground, the failure of any sanitary waste or vent system represents a major repair problem.

• Common Problems

For buildings constructed decades ago, extra heavy bell and spigot cast iron pipe had been the standard used for larger diameter mains for both the waste and vent stacks.  Smaller run-out or trim pipe to the bathrooms and kitchens was then commonly installed using galvanized steel.  Following the same trend seen at other building piping systems, more recent sanitary waste systems typically use much thinner and no-hub cast iron pipe for the mains having approximately half the wall thickness of pipe installed 40 years ago.

For older building properties, age is now beginning to show its impact as these heavy cast iron waste and vent systems are naturally reaching the end of their reliable service life.  Although offering very extended service typically exceeding 80 years, many building properties of America’s older properties have now exceeded that reliable limit.

The failure mechanism for cast iron and ductile iron pipe is significantly different than for steel, copper, brass, or any other metal due to multiple factors.  That cast iron is a far lower quality piping material vulnerable to cracking and fracturing is one obvious fact.  For steel pipe, corrosion must completely penetrate the wall before a leak occurs; the volume of water lost being dependent upon the size of the hole.  For cast iron pipe, internal stresses created during its spin cast method of manufacture are held in place by the sheer thickness and strength of the pipe wall.  Once the pipe wall thickness is reduced on one side, internal stresses become unbalanced – a condition then potentially leading to much larger fractures of the pipe.

In the photographs immediately below, we illustrate the common fracture type failure mechanism of cast iron pipe.  Below left, a large section of 6 in. sanitary waste pipe has completely blown out even though its wall thickness seems heavy and more than adequate.  At center, a straight line longitudinal fracture exists from end to end of this vertical 6 in. sanitary waste stack.  At right, even the much heavier bell side of this extra heavy section of 4 in. cast iron pipe has not survived the internal forces after losing sufficient wall thickness.

Immediately left, pinholes are found throughout this section of older 4 in. cast iron pipe, a condition similarly defining the end of the piping, but generally less damaging in terms of service interruption and water related damages.

While the reaction to the need for sanitary waste pipe replacement is never well received, at least it is not totally unexpected at buildings 90 or more years old.  All four above failures of sanitary waste pipe at older buildings generally constructed in 1960 or before.  Of much greater surprise to building owners is the fact that relatively new cast iron no-hub piping installed as recently as the year 2000 is also showing evidence of widespread failure requiring replacement.

Below left we show an example of no-hub sanitary waste pipe installed in 1993 having through wall pinholes throughout most floors of a 43 story office building.  At center, pinholes are shown impacting the generally heavier P-trap connections at this year 1995 building.  At right, 4 through wall pinholes exist within a 1 linear foot section of 4 in. no-hub pipe from 2001.

Perhaps the worst indication to an impending disaster of the worst magnitude is this 10 in. main sanitary waste line to the entire building, as shown immedately left.  Even though this larger diameter pipe has a heavier wall thickness, a fracture is shown extending front to back and across 80% of its entire 10 in. diameter following only 27 years of service.  Total failure is guaranteed, and should the pipe move laterally, will dump the full contents of the building’s sanitary, or more accurately, very unsanitary waste product onto this lower floor.

Aside from a blowout type of failure, which are less frequent, the first sign of a waste pipe problem is typically due to complaints of offensive odors from building tenants.  Failure first occurs at the smallest diameter galvanized steel pipe, since this pipe it typically horizontal and offers much lower wall thickness than the larger diameter mains.  Following this initial event will be small pinholes at the cast iron mains and inevitably larger fracturing once significant wall loss has occurred.

Unlike galvanized steel pipe which has a very strong and uniformly structured composition, cast iron pipe is very brittle, as the above photographs illustrate.  This not only makes ultrasonic testing very difficult due to the varying grain structure of the material, but also makes any accurate prediction to its failure virtually impossible.

Vent systems typically suffer lower corrosion activity compared to their sanitary waste counterparts but are still vulnerable.  Often, the effort to open walls to replace the sanitary waste pipe has added to it the replacement of the vent pipe as a precautionary measure.  For vent system, significantly greater deterioration will be found at horizontal lines due to the condensation of corrosive gases and also to any more vulnerable threaded galvanized steel run-out piping.

• Major Threats

The overall threat is less only due to the fact that the piping is not pressurized.  When a larger failure in the form of a fracture and blowout occurs, the mess far exceeds the volume of water alone.  Replacing or patching pipe sections as they fail, which is the most common response, defines the greater potential for larger and more damaging failures as the building ages.  Such failures are often concealed behind walls or above ceilings and go unnoticed for a period of time.  This introduces an additional potential for mold or microbiological growth to occur and can create significant health threats.

• Testing Focus
  

Cast iron sanitary waste and vent lines present the greatest difficulty to ultrasonically inspect.  Not only does cast iron have a natural porosity and grain structure which absorbs sound, but its external surface is naturally course and rough, as opposed to steel, copper, or brass pipe having a smooth surface.  This exterior roughness interferes with introducing the sound signal into the pipe before even passing through the pipe wall.  Once reaching the opposite or inside pipe surface, this natural roughness is further corroded and pitted to produce a surface not favorable to signal reflection.

In addition, cast iron (and ductile iron) is a low cost pipe manufactured from scrap metal and will vary widely in its sound velocity.  While calibration velocity standards exist, they are generally unreliable – thereby requiring some form of field calibration to actual pipe samples.  While we have developed procedures to test pipe which many others in the field consider as impossible to test, the accuracy of any measurement is far below that of most other piping materials by approximately 20%.

Any UT investigation of a sanitary waste system should include:

• • • The risers at multiple floors and different riser stacks
    • Run-out pipe between riser and tenant space
    • Horizontal runs and offsets
    • Any galvanized pipe
    • No-hub pipe
    • SV marked pipe
    • Standard service weight pipe
    • Vent lines
    • Vent related run-out piping
    • Basement horizontal main

Ultrasonic testing should include the largest diameter mains all the way down to the smaller branch lines leaving sinks and other bathroom fixtures.  With our increasing documentation of premature failures of the no-hub style cast iron pipe available today, we strongly recommend consideration towards other materials if allowed by local code.

Corrview maintains a large photo gallery specifically related to waste and vent piping in addition to other corrosion related issues.  To visit our gallery on waste and vent systems, please click here

© Copyright 2023 – William P. Duncan, CorrView International, LLC