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Unknown Corrosion Influences

One of the most critical responsibilities of any property owner or manager is maintaining their various HVAC piping systems.  This requires strict preventative maintenance, and for most HVAC systems, an effective chemical water treatment program.

Properly maintained and protected, most HVAC piping systems will last far longer than the expected service life of the building itself.  For standard 12 in. condenser water pipe having an initial wall thickness of 0.375 in. and a minimum acceptable thickness limit of 0.150 in., maintaining a uniform 1 MPY corrosion loss will easily provide over 200 years of reliable service.

At 5 MPY, service life is reduced to 45 years, and at 10 MPY – 25 years or less.  Quite clearly, effective chemical protection, or the lack thereof, plays a very critical role in any building’s operation.

In fact, we often document outstanding corrosion control in our ultrasound based piping investigations.  Such as extra heavy 24 in. condenser water pipe from 1958 found still at 0.485 in. and near its new ASTM wall thickness specification, and chill water and secondary systems having confirmed corrosion rates of under 0.2 MPY.

Piping failures, however, do occur, and when they do, attention immediately points to the chemical water treatment provider as being solely responsible.

While incompetence and poor chemical protection clearly exists, most water treatment providers make their best effort to reduce corrosion losses for their clients and most are highly effective toward that goal.  Quite often, however, a high corrosion condition or piping failure has been unknowingly influenced by other factors totally beyond their control.  All too frequently, the problem was never even known to exist!

• Coupon Error

First and foremost is the potential error involved in monitoring corrosion activity using standard corrosion coupons.  Installed in a side stream loop and existing physically and electrically isolated from the main system, coupons suffer almost none of the environmental influences occurring inside the piping system itself.  In fact, they only indicate the potential corrosivity of the water itself against new steel pipe.

Coupon reports are heavily relied upon by building operators, chemical treatment providers, as well as overseeing corrosion consultants – all using the same flawed testing method, and all blindly accepting the same wildly inaccurate results.  Reported condenser water corrosion rates of 0.3 MPY, practically impossible in the real world, are eagerly accepted when in fact true corrosion activity may be 10 to 100 times higher.

In these below photographs, taken from the remnants of a 6 year old condenser water system at a luxury high rise condominium, corrosion coupons consistently reported corrosion activity at below 1 MPY and even as low as 0.4 MPY.  Removed pipe samples providing clear evidence of a corrosion problem were in fact ignored due to consistently spectacular corrosion coupon results.

Corrosion Activity Measured at 0.4 MPY Using Corrosion Coupons

Ultimately, the 25 to 48 MPY true corrosion activity destroyed this relatively new piping system requiring total replacement.  Even the 1 in. steel corrosion coupon rack from which 0.4 MPY corrosion rates were derived, failed in 1 year!

As we have often stated, “It is far more preferable to advise a client to a corrosion problem than their learning of it on their own and notifying you”.   For this reason, employing supplemental corrosion monitoring and testing methods is always recommended, with ultrasound technology offering the most accurate and useful gauge of both corrosion activity and remaining service life.

• Piping Differences

The generally lower quality of today’s steel pipe plays an immense role in its service longevity.

Pipe produced 50 years ago and earlier is significantly more corrosion resistant than steel piping products produced today.  Although meeting the same ASTM specification in terms of physical and chemical properties, current steel pipe products fail to perform equally in terms of corrosion susceptibility.

Pipe is also often manufactured having less wall thickness.  It is not unusual to ultrasonically measure schedule 40 pipe from 1950 still exceeding ASTM specifications due to the manufacturing procedures of past decades to produce oversized pipe.

Yet today, most steel pipe is produced 10% or more under its stated ASTM factory specification – still within the tolerance limitation for new steel pipe, but providing the client with significantly less wall thickness than expected.  While less of a concern for larger diameter pipe having inherently greater wall thickness, a 10% loss of wall thickness at pipe of smaller diameter produces a dramatically greater impact.

Where ASTM A53 grade B seamless black steel pipe had been the standard specification for any HVAC piping system for decades, seamed or ERW welded pipe now is the predominant pipe material installed.  An often incomplete internal welding seam not only adds an immediate physical weakness to the piping system, but also provides a starting point for debris and microorganisms to attach and quickly advance a piping failure.  Any corrosion condition, and especially MIC, will typically focus its attack at this most vulnerable area.

Common to most high corrosion problems we investigate is the discovery of foreign pipe products; the lack of an ASTM stamp often signaling its presence and possibly the interest to shield such fact.  This translates to higher corrosion losses often difficult to control to reasonable rates.

Foreign pipe is especially susceptible to corrosion, and often of poor quality.  Installing pipe from Korea, Mexico, Thailand, China, and most foreign sources, for still unexplained reasons, is an almost 100% guarantee of future trouble.  Even American steel piping has shown a dramatic decrease in quality and corrosion resistance over the past few decades – resulting in overall higher corrosion losses.

Common Foreign Piping Suppliers

For any piping additions or renovations to an existing property, the new pipe will typically suffer far greater corrosion losses in comparison to older examples.  This anomaly is due to fine rust particulates from the old pipe quickly migrating into the new pipe to initiate often excessive cell corrosion and pitting at already more corrosion susceptible and possibly thinner materials.

In a nationwide piping investigation of federal facilities at 18 different U.S. cities conducted by CorrView International, all documented near 0.3 MPY corrosion rates for their earliest 1961 closed system piping systems; increasing through various facility additions to 3.6 MPY at their more recent 2005 installed lines.  The same HVAC systems, same ASTM steel piping specification, same chemicals, same operation, and same water quality.  And yet, a tenfold increase in corrosion losses.

• Engineering and Design

More complex piping designs needed to meet varying operating conditions means more opportunities for isolated corrosion hotspots.  With operational flexibility and redundancy built into all modern plant facilities, new and unforeseen threats are often introduced by the significantly higher corrosion activity at those piping areas subject to lower flows or intermittent service.  The design of variable flow condenser water systems controlled by demand is a relatively new contributor to the same problem.

In addition, surprisingly common engineering design mistakes are repeated over and over given that the end result, often failure, typically occurs many years or decades later, and therefore are never correlated back to their initiating source.

Incorporating multiple future connections into any condenser water piping system means new opportunity for debris and particulates to collect at each dead end and initiate higher under deposit corrosion.  Cross connections to maintain water movement, though well intended, still fail to stop particulate settlement and higher localized pitting – as illustrated in the below photographs.

Bottom take-off connections from a condenser water main header establishes a settling zone for all suspended dirt and particulates.  This is especially threatening to lead and lag chillers or seasonal equipment such as heat exchangers; allowing debris to collect and deteriorate the bottom of the pipe.

Failure will occur at approximately 6-8 ft. from the take-off tee as rust and particulates lose their momentum from the turbulence of the main line and settle.  Chemical treatment protects upper areas of the pipe, but provides no protection below inches of hardened sediment.  Failure is the common end result to the surprise of everyone expecting their chemical water treatment program to protect and control an impossible corrosion condition.

Installing isolation valves at a condenser water plate and frame heat exchanger 35 ft. from its take-off point, rather than isolating the line immediately at the main, creates a 35 ft. long rust settling zone.  This again creates an area of pipe prone to severe under deposit pitting, and where no amount of chemical treatment will provide any protection.

In the design of HVAC systems decades ago, pipe diameter would be reduced as flow rate decreased.  This maintained water velocity at levels sufficient to prevent particulate settlement.  Today, however, the very common design practice of maintaining pipe diameter across an entire piping header, even as flow rate decreases, sets up the furthermost areas of pipe as a virtual settling basin subject to severe under deposit pitting.

Similarly, temperature tempering cross connections and by-pass loops for cooling towers, typically remaining closed most of the time, are notorious debris settling locations common to localized deep bottom pitting and advanced failure.

Condenser water systems constructed decades ago of schedule 80 or extra heavy material having a wall thickness of 0.500 in. are now commonly standard or schedule 40.  Finding thinner schedule 30, 20, and even schedule 10 pipe at open condenser water systems is not uncommon – making the same moderate 5 MPY corrosion rate now a threat where it would otherwise be a non-issue.

Threaded pipe is commonly installed using schedule 40 stock for virtually all piping systems.  This is entirely acceptable except for open condenser water systems having always higher corrosion activity.

For a 2 in. schedule 40 steel condenser water line having an ASTM defined initial wall thickness of 0.154 in., threading removes 0.072 in. or 47% to leave only 0.082 in. of useable pipe wall.  Under an inaccurate corrosion coupon rate estimate of 0.4 MPY, such pipe would be expected to provide almost unlimited service life, but under the more realistic corrosion loss of 4-5 MPY common today, most threaded condenser water pipe will only last 20 years at best.

Threaded drain and vent nipples are common points of failure to any HVAC system due to their inherently higher corrosion attack and substantially less wall thick-ness.  While the failure of a larger section of pipe is likely to be a pinhole, many failures at smaller pipe are complete thread separations producing far greater water damage.

For this reason, we recommend schedule 80 pipe for all threaded open condenser or process water applications.  Threading steel pipe to brass or copper further introduces the potential for galvanic attack against the steel threads.  This is another common failure event again beyond the control of most chemical treatment programs.

• Water Filtration

Corrosion occurs to an HVAC piping system in two stages.  The first is the initial wall loss from the pipe to produce an iron oxide rust deposit. This is the primary focus of all chemical water treatment programs.

The second is that rust deposit being allowed to accumulate sufficiently to reduce the effectiveness of the chemical treatment program and initiate more aggressive secondary effects.  We consider secondary corrosion an often greater threat to long service life since it is the deeper and more focused under deposit pitting which almost always leads to premature failure.

For this reason, effective filtration is no longer an option, but a necessity for both open and closed systems.  Even under very low corrosion conditions, significant rust deposits can accumulate over years of service unless removed.

For example, a 5 MPY corrosion rate at 12 in. schedule 40 pipe removes an astounding 64 lbs. of steel and creates 2.6 cu. ft. of rust product for every 100 ft. of pipe – per year.  For all open condenser systems, additional dirt and debris is constantly introduced into the system through the cooling tower.

Simply installing a water filter, however, without significant thought to its most effective suction take-off location to the filter inlet, guarantees an expensive capitol cost which will never solve the problem it was designed to address.  For the vast majority of filter installations, a tangential take-off to the flow of water guarantees that any larger particulate will flow right past the inlet to settle elsewhere in the system.  No benefit is realized, and the hidden threat continues on its path toward pipe failure.

• Reliable HVAC Operations

Today, ensuring the reliability of HVAC operations requires far more than a monthly review of chemical treatment logs and quarterly corrosion coupon results; more than the assumption that chemical treatment alone will protect your valuable HVAC piping systems.  It requires more accurate corrosion monitoring, and a careful review of the piping system itself to identify areas of likely threat and therefore additional corrective actions.

You can view and download our two page handout on this subject below.

© Copyright 2024 – William P. Duncan, CorrView International, LLC