System – Storm Drain

• Overview

Rain leader or storm drain piping systems will typically provide very extended service life, and for some of the oldest building properties we have investigated dating back to the early 1900s, are still capable of providing even further service.  Virtually every type of piping material has been used in their construction – steel, galvanized steel, copper, ductile iron, cast iron, wrought iron, and even PCV or ABS where permitted.  Service weights vary greatly, standard or extra heavy for cast iron to Type L or DWV for copper.

For older building properties, the same pipe installed for the vent and waste was typically installed for storm drain, and therefore most older buildings produce excellent results when tested – the benefit of much heavier, higher quality, and more corrosion resistant materials.  Thinner materials are installed today, and the probability of finding extra heavy cast iron bell and spigot storm pipe for any building property constructed recently is slim.

• Common Problems

Assessing the vulnerability of a storm drain system generally starts with the identification of the piping material itself.  Extra heavy cast iron can easily last 100 years and more.  Ductile iron generally less due to its lower initial wall thickness.  Galvanized steel varies the most simply due to the random vulnerability defined by its galvanized finish.  As we have well documented for galvanized steel serving other services such as sanitary waste, fire protection, and condenser water, service life is primarily dependent upon its quality, which unfortunately continues to decline.

Thin wall DWV copper pipe is more highly vulnerable to acidic conditions and lower pH rain.  For most investigations, substantial differences in pipe condition can be found at all pipe closest to the roof level due to greater moisture contact.  While there are many factors that influence the corrosion of a storm drain system, environment often plays the most significant role.

• Major Threats

Few significant threats exist to rain leader systems, but they still can produce an impact.  While arguably rain water begins as distilled water, it still carries some particulates, gases, and other contaminants down with it to potentially corrode the pipe.  Over decades, and depending upon the piping material installed, random leaks can occur.  Specific problem areas are:

• • • • Thinwall pipe

While 6 in. standard weight cast iron pipe from 60 years ago still offered a significant wall thickness of 0.380 in., today’s ductile iron which has generally superseded it only has a beginning wall thickness of 0.190 in.  While older buildings would sometimes install copper pipe for rain leader service, that option no longer exists due to cost.

• • • • Product quality

The installation of generally thinner materials has a more negative consequence due to the lower quality of pipe manufactured today.  This holds true for most piping materials as we have well documented on this site.  For storm drain service, the common use of standard no-hub cast iron pipe introduces an immediate vulnerability and suggests against the 100 year service life previously expected.

• • • • Physical location

Vertical pipe always remains in better conditions due to the obvious fact that water fully drains.  Having no internal pressure, there are no physical constraints or wall thickness requirements to satisfy.  Any building offsets or setbacks will experience higher corrosion activity at its horizontal lines – especially if lacking sufficient grade.

• • • • Channeling

Under light flows, water will often spiral down the pipe rather than move uniformly across the entire surface – an effect termed “channeling.”  Over time, corrosion favors this one pathway and produces a groove to direct water more forcefully along that route.  For cast iron and ductile iron pipe having strong internal stresses, weakening one localized area can result in long fractures along the pipe.

• • • • Roof level proximity

A common finding for many ultrasonic investigations of roof level piping is the substantially greater deterioration immediately below the roof collection basin and to 1 or 2 floors below.  This is due to higher oxygen content and humidity at the open atmosphere.  Also, water may only travel down the upper floor piping during very light rainfalls and may impact its entire length.

• • • • Environment

The wide difference in weather conditions around the United States often dictates the length of service provided by a storm drain system.  More arid areas such as Phoenix will certainly receive less water, and therefore experience less corrosion.

• Testing Focus

There is little that can be done to safeguard or extend the life of a storm drain system.  No maintenance is possible other than coating or lining them as a proactive or repair action.  Areas of interest in any UT investigation of a storm drain system include:

• • • Galvanized steel systems
    • No-hub piping
    • Galvanized steel pipe having carbon steel fittings
    • Long horizontal runs and building offsets
    • Pipe closest to the roof level
    • DWV thin wall copper pipe

Service life is generally dependent upon the quality, natural corrosion resistance, and thickness of the pipe verses localized weather and environmental conditions.

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