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Open Condenser Water Systems

Open condenser water piping systems traditionally exhibit the greatest vulnerability to corrosion; thereby producing the most trouble for any property owner or operator.  While there are very obvious reasons for many problems, such as poor or nonexistent chemical water treatment, the general lack of understanding to the many issues impacting such critical systems often plays a role.

Below are just some of the many subtle issues responsible for higher corrosion activity and the resulting premature failure  of condenser water systems.

1. Cooling Tower Is An Air Scrubber: 

Being open to the atmosphere, the cooling tower operates as a giant air scrubber to capture airborne particulates into the condenser water.  Particulate filters used at industrial facilities such as oil and coal fired powerplants spray water across their exhaust in order to remove pollutants – a process closely similar to any cooling tower.  For most cooling towers, the proximity of the cooling tower to roadway traffic ensures a significant airborne dirt and debris component.  Although the threat decreases with building height, we have documented heavy airborne particulate capture at 50 story buildings, and even identified the problem at the 111th floor of the former World Trade Center at a supplemental tenant tower.

Unless effectively removed, these particulates circulate throughout the entire condenser water system where they settle and collect to produce flow restrictions, higher pressure drops, reductions of heat transfer efficiency, and most importantly – secondary corrosion related problems.

For many properties, it is not the amount of wall loss which represents the greatest threat to the piping system, but instead the volume of iron oxide rust product created which then initiates far more destructive secondary issues.

2. Effective Water Filtration Is Rarely Provided: 

Filtration is critically important to all HVAC piping systems yet rarely installed unless a problem has exceeded a certain threshold.  In virtually all examples, water filtration is provided on a sidestream basis under the mistaken assumption that any particulate debris avoiding capture on the first pass will then be removed subsequently.  Most such systems are variations of sand filters, which provide unnecessarily low size sub-micron particulate removal for a very low volume of water.  Given that sand filters will produce clear water by removing the smallest suspended particulates, success is assumed even though the bulk of the problem still remains within the pipe.  More information on this subject is available in Technical Bulletin WF-06.

Other issues also exist.  Almost all filtration units are installed where most convenient as opposed to where they would be most effective.  It is not uncommon to find the take-off or supply of water to the filter located at the top or sides of the pipe, often at the direction of the filter manufacturer, when quite obviously, rust and other particulates congregate to the bottom of the pipe.

Full flow filtration options exist, but are substantially more expensive and require additional maintenance.  Centrifugal separators, which are the most common form of full flow filtration, don’t actually “filter” the water at all; removing only the largest and heaviest particulates of generally above 75 micron through centrifugal action.  Given that the bulk of most condenser water rust problems are under 30 micron, their effectiveness is limited.

3.  The Microbiological Impact:

Cooling towers also capture various forms of bacteria and other microorganisms which cannot be removed by any form of filtration, and which will then circulate throughout the entire condenser water system.  Controlling the growth of microorganisms is accomplished by the regular application of alternating chemical biocides, although they will not thoroughly eliminate the problem.  Periodic system sterilization is advised using higher dosages of chlorine, bromine, or ozone, but can also introduce higher corrosion rates.  Left to propagate without chemical control, microorganisms can change their morphology from aerobic to anaerobic – thereby allowing them to produce a far more damaging form of corrosion known as MIC, or microbiologically influenced corrosion.  More information on this subject is available in Technical Bulletin CT-05.

Microbiological growths are commonly associated with higher corrosion rates even in the absence of MIC.  Legionella, a bacteria most commonly associated with cooling towers and Legionnaires’ Disease, represents the greatest threat to any building property given the legal liability should illness occur to anyone in the building, as well as anyone in a surrounding building.  The ability to trace individual Legionella infections through DNA has dramatically increased the level of investigation and enforcement in many cities such as New York City.

4.  The Difference Between Rust And Scale:

Open cooling tower systems evaporate water which in turn leaves behind calcium and magnesium carbonate or hardness components which can concentrate sufficiently to produce a white scale. Controlling this condition requires a periodic or constant blowdown of the condenser water system set by the chemical treatment provider in order to maintain “chlorides” or “cycles of concentration” to below a prescribed level.

The condition within the cooling tower is equal to boiling water in a pan and continually adding more water.  As the water boils, hardness is left behind where it concentrates to the point of plating out on the pan’s surface.  For a cooling tower, the scenario is the same, except that the cooling tower evaporates the water instead of boiling it.

It should be noted that scale is an entirely different entity and concern than rust, although both terms are often and mistakenly used interchangeably.

Scale occurs over long periods of time without adequate blowdown of the system to maintain mineral content at below scaling conditions, but does not remove pipe wall. In fact, a high scaling condition will add a protective layer of calcium and magnesium carbonate to the pipe wall which actually protects it from corrosion.  Opposite of scale, rust is the end product of a corrosion condition resulting in wall loss and physically weaker pipe.  Generally, corrosion occurs under lower pH conditions, and scaling occurs under higher pH conditions.

5.   The Impact Of Pipe Size:

The inherently higher levels of corrosion activity common to all open condenser water systems has a dramatically greater impact against small diameter pipe of lesser wall thickness typically serving individual package units or heat pump systems. Given the same rate of corrosion, the impact of a 5 mils per year corrosion rate against 18 in. standard pipe having a wall thickness of 0.375 in. is substantially less than the same rate acting against a 3/4 in. drain line having 0.053 in. of wall thickness remaining at the threads.

For this reason, initial failures of a piping system almost always occur to the smallest diameter threaded pipe and then proceed to larger and larger pipe diameters over time.

Corrosion loss, which is measured in mils per year  or MPY, has a significantly different meaning and impact depending upon pipe size as well as upon whether the issue is viewed as:

(A) The consequence of a loss of wall thickness, or

(B) The production of iron oxide rust product

The same corrosion rate has a substantially greater impact at small diameter pipe of inherently lesser wall thickness. Where the pipe is threaded – even less pipe wall remains by approximately 50%.  Also representing a greater threat to small diameter pipe, the amount of iron oxide rust product has substantially lesser impact for large diameter lines which are typically under a higher flow rate.  The same rust product will typically have a much greater impact at small diameter lines where it can settle out and/or become trapped within small fittings such as drain lines, or within small diameter heat pump coils and heat exchangers.  This is and extremely common finding in our investigations.

The volume of rust generated is also related to pipe diameter due to the increasing surface area as pipe size increases.  More information on this subject is available in Technical Bulletin PI-01.

6.  Higher Acceptable Corrosion Levels:

For open condenser water systems, corrosion rates of 1-3 mils per year (MPY) are now considered normal, with corrosion rates at or below 1 MPY preferred.  In decades prior, however, corrosion rates at or below 1 MPY were the acceptable standard – a shifting definition as corrosion rates have increased.  This is for the reason that decades ago, the combination of higher quality steel pipe and more effective chemical inhibitors commonly produced corrosion rates of well under 1 MPY.  Although becoming more rare, we still document older building properties having such low open condenser water corrosion rates.

Widespread problems related to corrosion can be expected at rates of 5 MPY and greater; with rates of above 10 MPY often resulting in catastrophic damage.  Limited service life exists at corrosion rates of above 20 MPY, and above that value, no reliability should be expected.  In many investigations, our finding of multiple examples of pipe showing corrosion rates exceeding 20 MPY raises a significant concern.  Above 25 MPY, most threaded pipe will last only five years or less, and corrosion rates reaching 50 MPY, the entire piping system may require replacement.

7.   Less Effective Corrosion Control:

The earliest application of corrosion control for condenser water systems was typically using hexavalent chromium – an effective choice but with serious health concerns.  Corrosion activity could be easily maintained at near 0.5 MPY using such chemicals at a time when a corrosion rate of 1 MPY or less was expected.  Once hexavalent chromium was banned nationwide, other alternatives, which had previously been viewed as inferior in their protection, took over that role.  As chemical costs have increased, so has been the move toward cheaper alternatives which had previously been avoided due to lower effectiveness.

Add to this issue the fact that an open cooling tower system is typically maintained at substantially lower chemical concentrations than its closed chill water system counterpart even though a cooling tower system is subject to significant negative influences.  In fact, the lower chemical protection for open systems, at often 50 times less than that of a closed system, is due to the economic impossibility of maintaining a high corrosion levels where the system is constantly blown down.  This results in the generally unknown trade-off of higher corrosion activity for lower chemical costs.

8.   Corrosion Coupon Error:

It should be noted that the mils per year (MPY) calculations produced in our UT reports are highly accurate since we are directly measuring wall loss over time in service – that is, the thousandths (0.001) of an inch of pipe wall loss having occurred during exactly one year.

MPY estimates based upon corrosion coupons, however, are highly inaccurate for many reasons, and generally fail to accurately define corrosion activity by a factor of 10 and much higher.  In prior investigations we have documented that a reliance upon corrosion coupon rates have allowed the continuation of a high corrosion condition hidden in plain sight.  Nevertheless, corrosion coupons are preferred by chemical treatment providers and still remain the standard of the industry.

Although the term mills per year (MPY) represents the industry standard for the quantification of corrosion activity, it does not provide a very meaningful explanation to the various levels of corrosion which exist.  For that reason, we have published a reference table comparing the actual amount of pipe wall loss occurring for different pipe diameters under different corrosion rates.  While a 5 MPY corrosion rate sounds bad, it takes on far greater meaning under the realization that for 12 in. pipe, more than 63 pounds of steel are lost for every 100 feet of pipe – PER YEAR.  More information on this subject is available Here.

9.   Oversized And Undersized Pipe:

Unknown to most in the HVAC related business, carbon steel pipe has a very wide allowable manufacturing tolerance by ASTM of 25%, which means that it can be produced having a wall thickness +/- 12.5% of its ASTM specified value and still be considered acceptable for sale and installation.  No concern is ever raised for pipe which is manufactured over specification, since it offers a great benefit to any building property, and in fact, producing pipe to above ASTM specifications was the manufacturing standard decades ago.

Decades ago, virtually all pipe was manufactured at or above its ASTM specification.  Even today, we routinely measure high wall thickness values still exceeding new ASTM specifications at older buildings having superior quality pipe of greater corrosion resistance.  This is especially common for piping systems which traditionally show low corrosion activity such as closed hot water heating and high pressure steam piping.  Our measurement of high pressure steam pipe From 1892 showing still high wall thickness values exceeding its new pipe wall specification is just one such example.

Conversely, almost all pipe manufactured today is undersized – a directly opposite condition to what was the standard 20+ years ago.  Piping manufacturers have clearly taken advantage of ASTM’s wide 25% wall thickness tolerance, and due to the benefits of much stricter and more accurate manufacturing practices, now routinely produce pipe at or near its ASTM defined minimum acceptable limit.  With no interest or concern to “tighten” this wide manufacturing allowance, assuming that thinner pipe will be supplied for installation becomes prudent.  More information on this subject is available Here.

10.   Overall Lower Pipe Quality:

Whether carbon steel black pipe, copper, ductile iron, or even stainless steel, almost all new pipe manufactured today is of seemingly lower quality having a far greater susceptibility to corrosion.  Beginning in the early 1980s foreign sources from China and Korea began showing up at construction sites due to its significantly lower cost.  That lower cost was quickly recognized as having a consequence given soon appearing experiences with noticeably low quality, difficulty in cutting and welding, ERW seems that would split during pressure testing, and much greater vulnerability to corrosion – all of which combined to provide significantly lower service life.

The high incidence of problems associated with foreign produced pipe quickly resulted in some consulting engineering firms excluding foreign pipe products; specifying that all pipe materials must originate within the U.S.  Over time, the lower cost of foreign produced pipe products seems to have won out over the interest for higher quality.  Lower costs vs. longer service.  Also occurring, U.S. piping manufacturers  seem to have lowered the quality of their piping products.  Whether due to environmental regulations, recycling, green interests, cost cutting, or simply the need to meet the import threat – a lower quality American product has minimized the benefit which once existed.

This explains failures in new sanitary waste systems in under 20 years where 85 years was once expected, and where condenser water system failure in under 5 years is not uncommon.  Without question, the long established lifetime allowance for corrosion in an open condenser water system of 65 mils, or 0.065 in., can be easily exceeded in just one year at a newer system.  More information on this subject is available Here.

For that reason, every possible step should be taken to reduce corrosion activity at every condenser water system, and to accurately monitor that the steps taken have been effective.

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