[modula id=”4206″]

Dual Temperature Piping Systems

From a corrosion perspective, every piping system exhibits certain characteristics in terms of wall loss, the potential for failure, and impact to building operations.  Those characteristics are further influenced by engineering design, materials used, insulation, chemical protection, temperature, and maintenance, among other variables.

• Pipe Service Life Varies

A brass domestic cold water system can last 100 years, as can an extra heavy cast iron storm drain line.  Chill water, hot water, and other closed systems should provide 65 years of service.  Wet fire sprinkler systems can easily last 75 or more years.  Condenser water pipe can provide 60 years of service at the larger pipe; 20 years at the smaller threaded sizes, if provided effective chemical treatment.  Proper maintenance is always an important variable in obtaining long pipe life, along with initial pipe quality.

Many piping systems were designed based upon certain assumptions into the future which have proven reasonable and correct.  The use of extra heavy or schedule 80 pipe in steam condensate applications, and where corrosion and pitting activity has always been recognized as higher, will often provide extended service life to well beyond 50 years.  In contrast to such logic, the use of thin wall, low quality, ERW seamed schedule 10 pipe in dry fire sprinkler systems, and where severe pitting at and below the air/water interface is common, may fail in 10 years or less.

• Engineering Assumptions

While most piping designs have been well proven with time, some assumptions and their resulting engineering designs have not worked out favorably for building owners and operators and are only now being discovered.  The extensive use of galvanized steel in domestic hot water systems throughout the Chicago area has resulted in advanced and widespread system failure due to heat destroying the internal zinc coating.  This material selection error, unanticipated at the time and extensively adopted into commercial and residential building specifications, has resulted in widespread damage requiring the replacement of much of Chicago’s building piping infrastructure.  Unknown to anyone 40 years ago, today it is a growing problem requiring high remediation costs.

• Other Examples Exist

Directly cross connecting open condenser water into the chill water system for “free cooling,” and filtered to an inadequate degree, allowed large volumes of rust, airborne particulates, and microbiological contamination under lower chemical treatment conditions to dramatically shorten the service life of the inherently weaker chill water system.  The design saved millions of dollars in energy costs initially and was praised by the building owners and operations community.  Now 40 years later, massive unexpected and unnecessary pipe replacement in the millions is required.

• Another Widespread Hidden Threat

A major problem now appearing at many residential apartment and condominium properties relates to the primary dual temperature heating and cooling piping system critical to building occupancy.  This design, which is very common to such properties built over the past 50 years, provides heating and cooling to the building from a central chiller and boiler where it is distributed through multiple riser sets typically located at the exterior support columns.

More common two pipe systems alternate service by proving cold water in the summer months and hot water in the winter.  Four pipe systems run exclusively cold or hot water in their own riser set to provide greater temperature control, especially during spring and fall weather fluctuations, but are far less common.  Such 2 or 4 pipe systems are fundamentally simple and easy to construct – allowing tenants to individually control their climate temperatures, room by room, through multiple window unit fan coil units.  A separate 3rd or 5th line exists for both systems to provide condensate drainage during the cooling season.

Common maintenance problems generally relate to the window fan units with normal use and wear to the motors and controls.  Filters require regular cleaning or replacement.  Condensate pans may occasionally clog, or the internal coils clog to reduce heat transfer if not properly chemically protected.  But it is the typically carbon steel pipe hidden within the building walls which represents the greatest threat to such systems due to an engineering design which failed to take into account a very obvious threat.

During the cooling cycle, chilled water is typically provided to the building at near 45° F.  That cold temperature is intended to condense moisture from the air at the fan coils to reduce tenant space humidity as well as cool the air.  The condensate is then collected by a pan and removed through the drainage system.  To prevent similar moisture condensation at the cold pipe within the walls, insulation is applied and herein begins the problem.

Most individual dual temperature risers, 2 or 4 pipe, are constructed of carbon steel pipe of relatively small diameter – typically 1-1/4 in. to 3 in.  Designs are typically reducing and reverse flow – meaning that supply and return lines feed from opposite top and bottom levels, and that the pipe reduces in diameter with every few floors as less water flow is required.  Virtually all older dual temperature piping systems are constructed from standard carbon steel black pipe.

• CUI The Problem

Such designs at the time employed standard paper backed fiberglass insulation of less than satisfactory thickness.  Although claimed to offer a moisture barrier by their manufacturers and assumed as such by design engineers, standard paper backed fiberglass insulation, especially in the 1 in. thickness typically applied, is entirely inadequate to prevent moisture from reaching the bare steel pipe.  Copper riser pipe is relatively exempt from this problem although vulnerable to other issues such as mold.

Additional factors influencing pipe deterioration are that the pipe is never coated with any form of anti-corrosion coating, and that proper and effective sealing between insulation sections and especially at the run-out tee connections is often absent.  It is not uncommon to find no insulation at the take-off tee at all, or loose insulation wrapped and tied – in effect providing zero protection.  This pipe then sweats equally to the fan coil itself to produce varying degrees of pipe wall deterioration.  Given that virtually all dual temperature piping is located at the exterior building columns, it is exposed to greater moisture infiltration through the building envelop.

Similarly insulated dual temperature pipe will suffer far greater outer surface corrosion located within the outer building envelope, as opposed to being located within an interior air conditioned space.  The colder supply side pipe will also corrode more severely than the warmer return lines – thereby producing dramatic differences in condition for some building properties.  Small threaded pipe nipples commonly installed between the riser tee or threadolet and copper run-out lines is always the most vulnerable and the first item to fail.  One very common but incorrect conclusion from this first failure at a small threaded nipple is that it is an isolated failure, when it is a severe deterioration to the entire riser system which is to blame.  As shown below, this problem is well hidden and unlikely to be discovered until the first major piping failure occurs.  Also illustrated is the mistaken belief that flexible foam insulation provides superior pipe protection.

Common Result

Fiberglass	Flexible Rubber

• Threat Varies Greatly

Unlike the internal corrosion of dual temperature systems which is typically uniform throughout, external corrosion varies dramatically depending upon insulation type and thickness, sealing effectiveness, joints, pipe temperature, and area humidity.  Supply side pipe, with its 10° F. lower temperature, facilitates far greater corrosion loss in comparison to the warmer return side.  Water temperature is a major factor.  For properties operating at much higher chill water temperatures of 55° F., outer surface corrosion may not be a problem at all, and may only produce a some minor cosmetic rusting to the outer surface.

It is not uncommon to find dramatically different external corrosion conditions within the same general area.  The net result can be minor surface rust having no substantial impact to the pipe, or severe deterioration close to the point of failure.  In comparison, external corrosion activity is almost always more severe than internal corrosion, and leads to far more extensive failures.

Such high external deterioration is simply due to the past failure to specify and install the proper insulation of adequate thickness.  Intentions at the time may have been to provide a piping system capable of providing 40-50 years of service, which has been achieved in most examples, but ultimately a system wide pipe failure problem will surface where the facility has remained in service beyond that initial expectation.  As buildings remain in service for longer periods, such corrosion problems inevitably appear for those now maintaining them to resolve.

• Obvious Causes

Such tremendous deterioration is due to the very common use of thin wall fiberglass insulation, poorly installed and sealed, and totally unprotected in any way against the higher moisture content inside the area it has been installed.  Pipe is never protected by any form of anti-corrosion coating as required.  For many properties, a total lack of proper installation is obvious at pipe hidden behind walls, and where no one ever expected it to again see the light of day.

The use of soft flexible foam rubber insulation offers no benefit, and in fact has been documented to harden, crack, shrink, and fail at an accelerated rate to produce even more detrimental results.  Chemical by-products of the foam deterioration have been suggested to accelerate pipe deterioration.  This condition is shown above, with further examples of high pipe wall deterioration at foam insulated pipe provided in our Corrosion Photo Gallery.

In many examples where the pipe insulation has been saturated with water, mold can develop, and under certain conditions migrate to other areas of the building to produce health related concerns.  Mold has become a major secondary implication caused by poor insulation protection and wet pipe conditions; in itself forcing major building renovations where present.

• Few Options Exist

Repair options do not exist other than to replace the pipe.  External corrosion due to moisture infiltration is rarely an isolated condition – thereby requiring the replacement of the entire riser system and its associated piping over all floors.  For a typical 25 story property having 12 or more sets of risers, this means tremendous tenant inconvenience and the opening of each riser wall to access the pipe – all at extremely high cost.  Ultrasonic testing provides the best overall assessment tool in determining pipe condition but requires access to the pipe behind the walls and therefore substantial pre-planning and contractor involvement.  For approximately 25% of our ultrasonic investigations into the condition of dual temperature piping systems, however, the pipe has been too severely deteriorated to even allow safe and accurate wall thickness measurement.

An internal epoxy coating, were it to actually be possible to apply at the piping configurations common to dual temperature piping systems, offers no benefit where the pipe wall itself has been destroyed from the outside.  And no form of corrosion control exists to stop such external wall loss once established.

The worse examples of dual temperature pipe corrosion require pipe replacement of each and every riser, with the only reasonable alternative being to abandon the dual temperature system in its entirety and convert to electric heat pump units.  Where the pipe is heavily deteriorated but still capable of providing some future service, it is often less costly to cut out and replace the pipe rather than rehabilitate it through rust removal and the application of a protective coating.

The widespread failures now arising at dual temperature piping systems could have been easily avoided.  For any new such installations, we recommend the exclusive use of hard cell 3 in. thick foamglass insulation, sealed at each and every seam using a mastic type adhesive, and further covered in plastic or wrapped in self sealing tape in order to prevent moisture infiltration.  We also recommend coating any bare steel pipe first with an effective rust control product as its 1st line of defense.

• Further Information

CorrView International, LLC offers a series of Photo Galleries taken from past ultrasonic piping investigations, which address the above and additional corrosion conditions.  A review of the different types of corrosion is often helpful in initially defining the likely corrosion cause of a problem.

Our Technical Bulletins section also address the issues of CUI, insulation failure, external pipe rust, mold, galvanized pipe deterioration, dual temperature piping, and the threat due to cross-connected “free cooling” systems.

It should be noted that for many clients, a combination of problem conditions will exist within the same piping system.  View our extended Photo Galleries of different corrosion types and failure conditions.

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