System – Condenser Water

• Overview

Condenser water systems are undoubtedly one of the most critical piping systems to every building property and generally suffer the highest level of wall loss due to the many adverse conditions to which they are exposed.

This threat arises due to:

• • • Open to the atmosphere where oxygen is replenished to saturated levels
    • High volumes of dirt and particulates scrubbed from the air
    • Larger diameter pipe capable of producing more rust volume under low to moderate corrosion rates
    • Multiple sources of airborne microbiological growths which accelerate corrosion
    • The use of biocides of reduced effectiveness and half life
    • Roof level pipe which is exposed to external corrosion and CUI
    • Corrosion levels typically 10 times above a closed piping system
    • Lower chemical inhibitor levels providing less corrosion control
    • Less effective filtration options
    • Substantially higher corrosion levels if drained for freeze protection

Combating this higher level of threat, corrosion control chemicals are actually maintained at much lower concentrations than they would for a far less vulnerable closed piping system.  In contrast to a closed chill water system treated with molybdate at a level of 250 ppm or greater, an open condenser water system is typically maintained at just 8-10 ppm.  But why, considering the many above adverse factors?

While various arguments by water treatment representatives, some fairly technical, exist to explain or justify why providing less corrosion protection is better for an open system under much greater corrosion threat, reality in the form of hundreds of condenser water investigations performed by CorrView has proven otherwise.   In fact, the underlying reason is strictly economic – lower chemical levels defined due to the constant blowdown required for any evaporative system, and the higher volumes of chemical inhibitor that would be required to offset such large chemical losses.

As an approximation, an evaporative cooling tower requires 0.06 GPM of make-up water per ton of cooling capacity.  For a 2,000 ton cooling tower, that means a 120 gallon per minute make-up rate to offset the water lost in evaporation and wind drift.  As the water evaporates into vapor thereby removing latent heat and cooling the remaining water, any dissolved solids or scaling components remain behind and increase in concentration.  The effect is exactly the same as boiling water over and over in the same pan, with a white residue or scale eventually forming.  For cooling systems, however, increasing concentrations of scaling components adversely impacts heat transfer, and therefore maintaining that concentration below a certain level by blowing down the water is mandatory.  For a typical New York City property, given its soft water supply, the general rule is that 10 percent of the make-up water must be blown down to the drain, or in the above example – 12 gallons per minute (GPM).

And with that 12 GPM or 17,280 gallons per day – all the expensive chemical purchased to treat it!

Of course, no such loss occurs to a closed piping system, where more appropriate higher chemical concentrations can be economically maintained.  In reality, the condition of any open condenser water system is a fine balance between a reasonable chemical treatment contract cost that the client is willing to pay for – verses a much higher yet acceptable amount of pipe wall loss.  Most clients, however, have either not recognized this face or have been led to believe otherwise.  Other factors such as water filtration, pipe quality, piping layout, and system design, etc. clearly play a role.

Closely monitoring corrosion activity and minimizing its impact within such systems is therefore a much more difficult task than its chill water counterpart; an aspect of facility management and engineering which is growing in difficulty due to the far lower quality and higher corrosion susceptibility of today’s steel pipe products.

• Common Problems

Open condenser water systems remove heat energy from a building typically through a cooling tower located at the exterior of the building.  This exterior piping now becomes at risk for additional wall loss at its outer surface, as well as due to higher internal corrosion from the higher oxygen content in the water itself.

In contrast to their intended purpose, the total reliance by the HVAC industry on corrosion coupons as a measure of pipe loss has led to many of the most serious corrosion problems we have documented.  Corrosion coupons offer every building owner, manager and engineer a false sense of security that their critical condenser water system is operating within acceptable corrosion limits, in turn inferring the longevity of the piping.  This monitoring method does not account for real operating conditions and generally presents much lower corrosion estimates than truly exist throughout the system.  Unfortunately, the true higher corrosion activity to the system remains unnoticed until extensive and irreversible damage has already occurred.  For more information on the limitations of corrosion coupons please reference our Technical Bulletins section.

Of equal importance to the upkeep of the chemical treatment program is the removal of internal deposits which are created.  Substantial volumes of rust deposits are generated even under low corrosion condition.  Therefore much greater internal rust deposits are generated at an open condenser water loop which exhibits higher corrosion compared to a closed system.  At a corrosion rate of 5 MPY, 12 in. schedule 40 pipe loses an astonishing 65 pounds of steel per 100 ft. of pipe per year; that steel then converted to a far greater volume of less dense iron oxide rust product.

Effective water filtration is mandatory in order to remove such deposits.  However, most filtration systems are inadequate to control the problem as well as improperly installed – thereby allowing the overwhelming volume of rust deposits to remain within the system.  These deposits typically settle in areas of lowest flow, at by-passes, or in dead ends where they create far more damaging secondary corrosion conditions.  Such areas are always a top priority location for ultrasonic inspection due to the higher corrosion activity caused by such deposits.  Once these deposits are present, they greatly reduce the effectiveness of the treatment chemicals.

• Major Threats

There are many different designs for condenser water systems – the most common simply being an isolated loop exclusively between a cooling tower and chillers.  These system designs are typically large diameter pipe exclusively, which eliminates the risk of extensive small diameter and typically threaded pipe throughout the building.  Others are floor by floor heat pumps or package units.  This is another common design which is much more extensively run throughout a building property, having much greater risk of failure at the small diameter piping of inherently lower wall thickness.  It is at these areas which we typically see the most devastating failures occur.  For that reason, this system design requires a higher level of inspection in order to fully evaluate its condition.

The greatest threats to any open condenser water system are:

• • • • Threaded fittings

Unfortunately, the HVAC industry has still not learned of the inherent threat of installing schedule 40 pipe where threaded, and as a result only realizes the problem some time after the system is up and running, and always by surprise.  Fifty years ago, any threaded pipe installed into an open condenser water system would be extra heavy schedule 80; today its standard schedule 40.  At a 5 MPY or greater corrosion rate, not unexpected today, any threaded pipe will provide only 15 years or less of reliable service.  Add a brass valve and galvanic activity to the equation, and even less service life exists.  With the lower quality of most pipe available today, we recommend the installation of 316 stainless steel or brass pipe, with schedule 80 pipe the minimum requirement for any threaded steel lines.

• • • • By-pass and crossover configurations

Installed into most condenser water systems but rarely utilized, such temperature and flow control piping arrangements only serve to accumulate rust and captured airborne particulates.  Such features are harbors for microbiological growth as well.  Typically valved off at only one side, most by-pass piping arrangements will show substantially greater deterioration on that side open to water flow.   Once installed, there are no corrective measures available other than pipe replacement.

• • • • Corrosion Under Insulation

After 20 years of service, a visual inspection alone is usually sufficient to document that the insulation no longer is serving its purpose.  Outer aluminum jacketing typically deteriorates and becomes porous, vinyl jackets crack from UV and separate at the seams.  The silicone sealant, if even installed, will have deteriorated and cracked – no longer stopping any water or moisture from entering.  Any vertical pipe allows water to enter where it then migrates downward to pool at any valve or flow regulator flange.  Pipe supports, due to the difficulty of insulating them, collect water, as well as can produce small galvanic differences between the ground potential of the pipe and the building steel.  Add to the environmental impact is the far greater damage caused by normal tower maintenance and foot traffic.  Severe external corrosion of the pipe is common, and typically occurs at areas which are impossible to isolate without shutting down the entire system.

• • • • Futures

The installation of multiple future taps throughout a condenser water systems represents a built-in opportunity for failure.  Most are typically schedule 40 and threaded to a brass isolation valve – thereby accelerating corrosion loss due to galvanic activity.  Deposits settling in such dead end areas then produce severe wall loss along the bottom.  Adding small diameter by-pass connections are virtually worthless, as they will never move about heavier rust and particulate deposits.

• • • • Full diameter header design

Decades ago, pipe size reduced as the supply line crossed each refrigeration machine, pump, cooling tower cell, etc.  Beginning with a 12 in. main supply, pipe would be reduced to 10 in. to 8 in. to 6 in. at each of its refrigeration machines.  Today, the header stays at 12 in. along the full run.  Instead of maintaining flow velocity across the entire pipe run, this now full size header, often at 24 in. or larger, becomes a very effective settling tank during those times  when only operating 1 or 2 machines, pumps, tower cells, etc.

Recent changes in system design have indirectly favored the collection of internal deposits.  Today, a very common design is to oversized a new piping system for potential future expansion, which  is done by over sizing the mains, headers, installing future connections, etc.  This reduces water velocity which in turn favors the settlement of internal deposits.  Future connections then become ideal areas for deposits to settle.  All of these conditions than introduce the unexpected potential for much higher corrosion activity.  Anticipating such areas for ultrasonic testing becomes of great importance.

Although sometimes recognizing the threat and installing small by-pass lines between supply and return sides, the effort is worthless.  Chemical treatment professionals claim the need for constant water flow through the pipe in order for the treatment chemicals to work effectively – thereby the source of the by-pass recommendation.  In reality, it is the settlement of rust and other deposits which produce the much greater deterioration at large diameter headers.  That rust product having no chance whatsoever of reaching the midpoint of its end cap to be circulated to the opposite side.

• • • • Variable flow pumps

This is a more recent design change drive by “Green” interests.  While quite reasonable in terms of matching water delivery to the need, the critical issue of what will happen to the rust and other particulates circulating around the system when their flow rate is reduced even further has not been considered.  Nature and the laws of physics, however, typically win, as will be seen.  From purely a corrosion perspective, most condenser water systems need greater flow, not less.

• • • • Corrosion coupon monitoring

Well documented by hundreds of ultrasonic investigations, corrosion coupons only indicate the “potential corrosivity of the water,” as the formal definition states, and have nothing to do with wall loss at the pipe itself, as the client is always led to believe.  Installed into an external loop commonly made of PVC and isolated from any electrical contact via a phenolic or plastic post, the most fundamental understanding of corrosion as being an “electro-chemical” event its totally eliminated as an influence.

Most severe corrosion problems we have uncovered at condenser water systems are to the great surprise of everyone responsible for the building, having received years of favorable corrosion coupon reports.  Given that corrosion coupons are the report card upon which the effectiveness of the chemical treatment program is graded, and favored by chemical treatment contractors as well as water treatment advisors, this misinterpretation of the testing process to outright misleading of the facts will never change.

• • • • Draining for freeze protection

Common in the Northern latitudes, many smaller condenser water systems serving comfort cooling only do not operate all year round, and are therefore drained to some extent into the building in order to prevent freezing.  This introduces yet another very specific corrosion threat to the pipe which has been drained.  Steel pipe left in a wet condition has been documented to corrode at as much as 5 or more times the rate of pipe left filled with chemically treated water.  Partially draining the pipe will benefit the water filled section, but also elevate corrosion and pitting at the water line itself where expansion and contraction repeatedly washes the pipe wall.  Whenever evaluating a partially drained condenser water system it is critically important to recognize this potential threat and place additional emphasis at the roof level pipe and upper floors.

• • • • Galvanized steel piping

The most common response to a thread piping failure at a condenser water system is to replace it with galvanized steel.  This likely provided a benefit decades ago when galvanized steel pipe was of higher quality, but today, this response instead introduces a new threat potential.  Given the deteriorating quality of galvanized steel pipe we have documented advanced failures in open condenser water systems in under 5 years.  Part of this accelerated failure is due to the fact that most threaded steel pipe joins a brass isolation valve, which in turn adds a galvanic based attack upon the zinc protective finish.  In short, galvanized steel pipe should never be installed at any open condenser water system.  Our Photo Gallery on the subject well illustrates this threat.

• • • • Heat pump systems

The flexibility of multiple smaller heat pumps for comfort is offset by the greater corrosion threat to a now much larger volume of smaller diameter and typically threaded carbon steel schedule 40 pipe.  The failure to isolate such vulnerable small diameter pipe from the higher corrosion conditions of an open system will spell disaster generally within 10 years, as many of our investigations have unfortunately documented.  All heat pump systems should be isolated from the open cooling tower through a plate and frame heat exchanger and protected by substantially higher levels of corrosion inhibitor.  Effective water filtration is also mandatory to long service life.

• Testing Focus
  

Ultrasonic testing of condenser water systems has its own inherent difficulties.  Without an understanding of a condenser water system’s greatest vulnerabilities, an inspection can result in as inaccurate assessment as the corrosion coupons themselves provide.  Therefore, in-depth knowledge of the condenser water system in question is imperative in order to accurately assess its condition.  The failure to address more difficult and possibly inaccessible areas of the system likely to experience higher corrosion activity due to specific reasons is a major source of error in its assessment.

Significant areas of interest for any UT investigation of condenser water pipe include:

• • • Main risers
    • Distribution lines
    • By-pass and crossover lines
    • Threaded pipe
    • Full diameter headers
    • Galvanized steel pipe
    • Under roof insulation
    • Drain lines
    • Equalization piping
    • All futures

With any UT testing planned, all of the above conditions should be taken into account.  Any specific conditions which are present at the building property should be disclosed and discussed in advance in order to enable us to provide the most complete assessment.  This includes any previous failures or system issues which may raise a concern.  For risers which run behind walls, access should be made prior to the test date in order to not impede inspection progress.  Sections of exterior pipe associated with the cooling tower having a metal jacket must be removed of their insulation in advance.

Corrview International, LLC maintains a large photo gallery specifically related to Condenser Water piping in addition to other corrosion related issues such as Internal Deposits, Weathering, Thread Leaks, Cooling Tower Deterioration, and Piping Failures.

To visit our gallery relating to Condenser Water systems and their issues, please click here.

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