CT-02:   HVAC Piping Vulnerabilities

Over 41 years of experience in chemical water treatment and ultrasonic pipe testing has provided CorrView International an excellent understanding of many common corrosion related problems found at commercial office properties and plant facilities.  Indeed, we see the same corrosion problems again and again – which indicates to us some clear and unmistakable trends for different piping systems and operating conditions, as well as a failure by many to recognize such threats.

We offer below some general guidelines of where corrosion problems might exist and why.  Many corrosion scenarios are linked to others obviously in similar cause or effect.  Although the corrosion issue is complex, and may present itself in many different forms, it often begins due to some simple initiating factor – a lack of chemical treatment, or a faulty start-up of the system, for example.  The presence of a corrosion problem in any particular area should always raise concern and prompt further investigation.

Many different factors influence the corrosion of a piping system, with the type of system often one of the largest factors.  Below we present only the most significant influences, in no particular order:

• Supply vs. Return Pipe

Higher corrosion rates are often found at the return lines for open process or cooling tower systems.  This difference does not commonly exist in closed systems.   Sorting a large set of test data based upon flow direction for a condenser water loop will often show this difference quite clearly – with 1 MPY or higher return side corrosion rate not uncommon.

One suggested cause is that the higher temperatures returning to the cooling tower naturally accelerates the corrosion activity, and/or that it provides a more suitable thermal environment for microbiological growth.  Another is that the iron oxide created at the supply side piping then has opportunity to migrate downstream to the return side – where it is more likely to deposit and create higher corrosion conditions.

• Horizontal vs. Vertical Pipe

This is an obvious problem well recognized by most experienced plant engineers and property owners – and which becomes most pronounced as either the length of the horizontal or vertical pipe run increases.  Particulates, and especially those heavier components, settle out in the horizontal lines based upon the length of pipe, flow velocity, and pipe diameter.  We consider interior deposits to be one of the most serious problems affecting any piping system due to the secondary damage usually created.  Contrary to the claims of nearly every chemical treatment provider, corrosion control cannot be provided to the underlying bare steel once high iron oxide or other deposits are allowed to collect.  This problem exists for open and closed systems both, but more so obviously at open cooling systems due to the greater source of airborne particulates, microbiological content, and due to the naturally higher corrosion rates of an open system to produce greater deposit volume.

For tall commercial building properties, the vertical risers often show substantially less corrosion and pitting than the horizontal runs.  Longer vertical runs increase the difficulty for particulates to migrate upward and therefore horizontal settlement increases at the bottom of the return line.  Larger volumes of pipe also produce a greater volume of iron oxide deposits which inevitably settles elsewhere.

For large process and manufacturing plants spanning acres of floor space, particulates entering any length of pipe may not remain suspended, but instead settle throughout the distribution lines. This problem is amplified as the piping branches off into smallest lines, and as the distance from the circulating pumps increases – thereby lowering flow velocity.  Greatest pipe wall deterioration for any large manufacturing facility is always furthermost from the circulating pumps at its smallest diameter piping.

• Roof vs. Basement Pipe

Three separate corrosion scenarios can exist to produce a noticeable difference between pipe located at the top and bottom of the same system.  The first involves the settlement of particulates into the lowest points of the system to produce an under deposit condition with substantially higher corrosion rates.  The second factor involves the layout of the piping system, and whether significant enough differences in flow rate exist to influence the effectiveness of the chemical treatment program and/or allow the deposition of particulates.

Depending upon whether the system is partially drained over the cold weather months, a third possibility may be the substantially higher corrosion which occurs at any pipe which is drained and left open to the atmosphere.  This is a common problem at roof level piping in all Northern climates – with corrosion activity at the roof level as much as 10 times higher than at the basement level.

• Drained vs. Filled Pipe

Drained pipe is usually linked to the most serious corrosion losses.  In many cases, a high corrosion condition will be traced back to years earlier – when some event required the temporary or extended draining of some or all of the pipe.  In comparison, drained piping produces significantly greater metal loss than pipe filled with fresh and untreated water.  The high corrosion activity found at “dry” fire pipe is a good example of this problem.

The degree to which draining causes damage is directly related to the infiltration of fresh air and moisture into the pipe.  Therefore, it is common to find a high wall loss at the cooling tower side, and a sudden increase in wall thickness after an isolating valve, or as the pipe travels further toward a closed end having less air movement.  Any effort to maintain a piping system completely filled with chemically treated water is highly recommended.  This includes insulating and heat tracing outdoor lines in colder climates, and installing critical isolating valves.  Another option is to use extremely effective VCI chemical inhibitors where possible.

More information on this subject is available in Technical Bulletin CT-03.

• Threaded vs. Welded Pipe

For a wide variety of reasons, threaded pipe almost always shows the first sign of a corrosion problem.  This is primarily because as much as 65% of the available pipe wall is cut away during the threading process itself on day one.  This small diameter pipe also offers inherently less wall thickness – thereby an already thin material is reduced even further.  Threaded pipe usually exists at lower flow areas, and at the furthermost extremes of the system at the process equipment or A/C units.  Here, flow rates are the lowest, or may be periodically shut down altogether – two additional factors commonly associated with higher corrosion rates.  Almost no one installs extra heavy schedule 80 as required for all open systems.

Threaded pipe almost always involves brass valves or a transition to copper pipe.  This often creates a galvanic corrosion condition at the threads since, in the majority of examples, dielectric insulators are absent.

Gaps at the pipe fittings also offer opportunity for particulates and microbiological growths to collect and produce a localized corrosion problem – once again focused at the very weakest point of the system. While adequate pipe may exist along 99% of its length, a failure at the threads usually means the end of service for the entire pipe length.

More information on this subject is available in Technical Bulletin PD-11

• Pipe Top vs. Bottom

This is similar to the differences between roof and basement pipe but is often far more subtle.  Any piping investigation, if thorough enough, is likely to identify some higher degree of corrosion at the horizontal and/or lower floor piping.  This is obviously due to the settlement of particulates in this area, and the secondary corrosion effects such particulates produce.

In many of those cases, a substantial increase in corrosion activity and wall loss can be measured at the bottom and lower sides of the horizontal runs.  Primarily dependent upon flow velocity, rust particulates produced throughout the system will eventually settle if not quickly removed.  Tubercles of iron oxide then develop to produce extremely high under deposit pitting to rates of 25 MPY or greater.

• Flow vs. No Flow Areas

No flow areas of piping can demonstrate dramatically different corrosion scenarios.  Where isolated and truly stagnant, such as exists for a fire standpipe system, a small amount of corrosion takes place and then further corrosion activity virtually ceases.  It is not unusual, therefore, to measure remaining wall thickness values near new pipe specifications in such cases.  Where new water is introduced into a dead end, by-pass, or isolated pipe section, results can be the opposite, and extremely high corrosion activity often becomes established.  By definition, such areas of pipe are never tested for corrosion – as no flow exists to route water to and to the corrosion coupon rack.

Future taps, which are commonly installed into most condenser water systems for service which will likely never occur, and extremely common locations for dirt and rust particulates to settle and produce much higher under deposit pitting.  Often, small by-pass lines are installed across such ports for the purpose of preventing debris settlement, although the effort is worthless.  By routing a small flow of water from the supply side to the return side, more debris is actually captured.

Any by-pass loops, especially those having the downstream side closed by a valve, are highest priority areas for severe pitting corrosion to develop. This occurs when particulates enter and settle out in the pipe in high quantities to produce severe under deposit pitting, which is quite common.  The now routine design of modern condenser water systems to provide higher control over water temperature through by-pass lines, and to employ full size pump, cooling tower, and chiller headers greatly favors severe corrosion activity – as we have documented in many investigations.

Pipe serving lead and lag equipment, or where the water flow is shut down when the equipment is de-energized, is especially vulnerable to much higher corrosion activity.

More information on this subject is available in Technical Bulletin PD-05

• New vs. Older Pipe

Higher corrosion rates are often found at the new pipe following a pipe repair, replacement, or extension to an existing system. This is especially common at condenser or open process water systems.  We have documented partial pipe replacement of large 12 in. condenser water pipe due to an 8 MPY corrosion rate, only to be replaced by a 40 MPY corrosion loss at the new piping.

One easy explanation is the far superior quality and higher corrosion resistance of pipe produced decades ago.  Its not unusual to measure a 1-2 MPY corrosion rate at 40 year old pipe, and a 6-8 MPY corrosion rate at newly installed pipe within the same recirculating system.  For any modified piping system, iron oxide and other particulates quickly migrate into the new pipe to produce an accelerated under deposit condition.  Where new pipe is installed downstream of older constricted pipe, the greater volume of the new pipe slightly drops the velocity and therefore allows more settling of any particulates.

Galvanic activity between new clean pipe and old deposit laden pipe is also recognized as occurring due to microvolt differences – although the mechanism is not completely understood.

More information on this subject is available Here

• Carbon Steel vs. Brass And Copper
  

Galvanic activity is blamed for many more conditions than actually exist.  For much older buildings, no one knew what galvanic insulators were at the time the systems were installed, and yet those buildings never experienced most of the problems common today.

One obvious factor is lower quality new steel pipe which is in turn far more susceptible to corrosion, as well as less effective corrosion control.  For a condenser water system with a genuine low 0.4 MPY corrosion rate, galvanic insulators are barely necessary.  But once exceeding 5 MPY, they become mandatory.  The generally higher corrosion rate common to all piping systems today make the use of galvanic insulators more important, but still mostly appropriate at open condenser water systems.

And of course they need to be installed between the steel pipe and brass valve or copper pipe to provide any benefit, not incorrectly as we commonly find.

• Further Examples

Support to the above problem areas is provided on this Internet site.  CorrView International, LLC offers a series of Photo Galleries taken from past ultrasonic piping investigations, which address the above as well as additional corrosion conditions.  A review of the different types of corrosion is often helpful in initially determining the likely corrosion cause.  In many cases, however, a combination of conditions will exist within the same piping system.

• Looking In The Right Areas

In many cases, identifying a corrosion problem is simply a question of looking in the right direction.  Quite often, no problem is known about, nor even suspected, until some failure occurs.  Under the worst case scenario, a problem may exist for years and exhibit no indication to the property owner, operators, or chemical treatment contractor.

Ultrasonic testing excels as the most valuable corrosion monitoring tool for the purpose of finding hidden faults since it provides a quick and low cost method of determining wall thickness – if performed correctly.  Coupled with a thorough data analysis, ultrasound can provide an almost complete understanding of piping system integrity.  Areas of concern raised can then be confirmed or further investigated through metallurgical testing, remote visual inspection, or other methods.

As long as sufficient testing is performed by skilled and experienced personnel, and as long as key problem areas as outlined above are addressed, ultrasound will produce a thorough and reliable system evaluation.

© Copyright 2025 – William P. Duncan, CorrView International, LLC