System – Chilled Water

• Overview

Since they are typically closed to the atmosphere, chilled water piping systems should experience low corrosion activity well under 0.5 MPY.  This is similar to any closed piping system where higher chemical levels can be economically maintained, where the introduction of foreign materials is insignificant, and where the amount of rust product created as result of normal corrosion activity is light to moderate.  Like any circulating system, the accumulation of internal rust deposits is a primary threat; those rust deposits beginning the downward spiral of deterioration.

Various steps can be easily implemented – such as to perform chemical cleaning and sterilization at approximately every five years, and installing sidestream filtration to remove any deposits.  These steps, however, are rarely carried out on a precautionary basis, and are often only considered once a corrosion problem becomes large enough to make itself known.

• Common problems

Two exemptions to normally low internal corrosion activity are where the chill water system is directly connected or injected into a condenser water system for free cooling, commonly known as Strainercycle, or where there is a thermal storage system, with the storage tanks themselves typically open to the atmosphere.  Both conditions can dramatically increase corrosion activity.

Except at the tallest structures, most chill water systems are constructed using standard or schedule 40 pipe for both the risers as well as smaller threaded run-out lines.  This establishes a potential weakness as the building ages given that the same corrosion activity acting against its inherently thinner run-out pipe wall has a more pronounced effect.  Under even the lowest corrosion conditions, substantial iron oxide rust deposits will be produced over time, as one of our technical handouts on corrosion activity illustrates.  This rust product then migrates and settles into dead ends, futures, and low flow areas where it restricts the effectiveness of the chemical water treatment and establishes substantially more aggressive under deposit corrosion.

For many older building properties, the first indication to a buildup of internal rust product is in the loss of heat transfer efficiency, which then results in lower or higher supply temperatures provided in order to counteract that loss.  A common scenario is for a building to change out all its chill water related fan coils because they are filled with mud and have lost their ability to coil, with not a question ever raised as to how all that “mud” got there.  In reality, all that mud and rust is former pipe wall.

• Major threats

The potentially greatest threat to any chill water system is commonly at its outer surface due to the migration of moisture through the generally inadequate insulation covering.  The amount of deterioration is entirely dependent upon local humidity conditions, the thickness and quality of installation for the insulation, as well as the type of material itself.  Highest deterioration is typically at the supply side pipe due to 10° F. lower temperatures.  Termed CUI for corrosion under insulation, this condition is generally not recognized until substantial damage has occurred.  Unlike an internal corrosion problem that should be noticed by rusty strainers and chiller tubes, etc., external corrosion is generally hidden from view within the insulation and behind walls.

After 40 years of service and possibly before, weakness typically shows at the smallest threaded fittings.  This is especially common to bottom drain and top vents which are either un-insulated or poorly insulated – thereby constantly sweating.  The installation of drains and vents in remote areas not even known to exist increases the threat since what is out of sight is always out of mind.

Primary threats are at:

• • • Small threaded fittings
    • Bottom drains
    • Supply side pipe
    • Outdoor pipe runs
    • Un-insulated pipe
    • Thin fiberglass covering
    • Risers in open shafts
    • Old cork type insulation
    • Soft foam insulation
    • Colder than normal temperatures

Sure signs of a chill water problem are discolored insulation, crystalization deposits, and of course water droplets at the insulation itself.  Mold, which is a major health issue and one of the top litigation issues facing all buildings today, is common where wet insulation exists.  As we have documented in our Photo Gallery on the subject, the problem is especially obvious yet commonly ignored.

A supplemental outer insulation cover will be effective at stopping moisture migration if properly installed with silicone or some other sealant at each and every seam.  The application of a vinyl covering is often installed more for aesthetic purposes than it is to provide a moisture barrier, however, and in many examples we have documented, has helped to contain the condensed moisture at the pipe surface.  Soft foam insulation, as our Photo Gallery on the subject well illustrates, offers very poor insulation protection.

• Testing Focus

A periodic visual examination of the insulation for evidence of water staining is always advised.  Opening random examples of insulation for a closer look is preferred.  Ultrasonic testing becomes more important as a building ages, and should focus highly at its smallest threaded fittings most vulnerable to a more catastrophic failure.  Specific areas of interest are:

• • • All above examples cited
    • Pipe installed to high humidity areas
    • Risers for 4 pipe HVAC systems
    • Bottom drain pipe
    • Top vent pipe

Any evidence of rust in the system strainers or reduced heat transfer efficiency is a sure indicator that an ultrasonic inspection is due.  For any due diligence assessment, engineer operating logs showing a 37° F. chill water supply temperature should be interpreted as a serious internal corrosion issue which has been ignored, as well as a potential external corrosion issue.

While almost all steel pipe is painted for corrosion protection, chill water pipe is not due to the industry wide misassumption that the insulation alone will protect the pipe.  Painting any new chill water pipe with a high solids zinc based waterproof paint before insulating is therefore strongly advised.

Corrview International, LLC maintains a large photo gallery specifically related to chilled water piping in addition to other corrosion related issues such as Internal Deposits, Mold, Soft Foam Insulation, Thread Leaks, CUI, and Piping Failures.

To visit our gallery related to Chilled Water systems, please click here

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