Technical Bulletin: PD-05
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Higher Corrosion Activity By Design
| Pipe Corrosion Problems No Water Treatment Program Can Protect Against |
Various changes have occurred to mechanical piping designs over the past few decades, with virtually all HVAC, plumbing, and fire protection systems having been affected in some way. Many changes relate to the materials themselves, and to the use of thinner piping schedules, lower grade of materials, worse pipe manufacturing quality, and different joining methods.
Other changes have been in the mechanical piping layout design itself, with the interest in reduced cost a primary factor.
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A Negative Evolution
For HVAC systems, and specifically for condenser or process water systems, changes in materials alone have had a significant impact.
Issues such as foreign pipe products, ERW welded pipe, undersized wall thickness to minimum ASTM limits, greater corrosion vulnerability, less effective chemical corrosion inhibitor protection, and overall lower pipe quality across the board. All such factors have resulted in modern piping systems where corrosion loss in just a few years can exceed the total wall loss of an equivalent 70 year old piping system.
A recent ultrasonic investigation of two separate condenser water systems, one a large critical data center having lost 0.098 in. of wall thickness in under 3.5 years, and the other a 68 year old high rise office building having lost 0.025 in. of pipe wall during almost seven decades of service, sadly illustrates the cumulative negative changes occurring to this industry. A severe 29 MPY corrosion loss for a new facility, in comparison to a 0.36 MPY corrosion loss for a decades older facility, represents an 80 times increase in corrosion activity requiring some form of explanation.
In the overwhelming majority of examples, the first knowledge to any such issue is after a piping failure or through some form of visual indication such as heat transfer loss, or large rust deposits captured in strainers.
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Future Needs
Piping mechanical design changes toward greater flexibility, total redundancy, and future expansion have greatly influenced a rise in corrosion activity; yet remain either unknown or ignored by developers, design engineers, chief engineers, and facility managers alike. For data centers and 24/7 facilities especially, the interest to operate under all possible conditions have unexpectedly and unintentionally introduced even greater corrosion problems and therefore a greater vulnerability to failure.
In contrast to older centralized refrigeration plant designs where building load and refrigeration capacity was well defined, many modern facilities consist of changing tenants of widely different cooling needs – thereby requiring mechanical expansion options into future decades. The past design of a refrigeration plant with two chillers is now often two chillers to start with, but with the capacity to add another three, or more. Multiple component pieces of equipment may be provided to reduce individual equipment loads, operating expense, and maintenance concerns, although at the same time increasing the piping layout size.
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Piping Design Changes & Errors
This changes the piping configuration from past designs whereby pipe diameter gradually reduced at each chiller and pump to current designs whereby the pipe remains at the same diameter over its entire course to introduce perhaps the most significant cause of higher corrosion activity.
Flow velocity in ft./ sec., instead of being maintained throughout the lower flow header as pipe diameter reduced, instead decreases at every take-off tee since pipe diameter remains the same. Less gallons per minute of flow through the same diameter size piping means lower velocity, and at the levels often reached in modern condenser water piping systems, the virtually limitless volume of suspended particulates begin to settle out.
A plant operating two refrigeration machines most of the time has less threat of particulate settlement than one where load only requires one of 4 or 5 machines to operate. Parallel rows of 5 chillers and 5 circulating pumps, each one having an 8 in. bottom take-off from a 24 in. main header, but with only one chiller and one pump operating because of low winter cooling demand, means lower linear velocity through the larger diameter headers and the opportunity for particulate settlement. Running lead and lag equipment, such a seasonal plate and frame heat exchangers, while moving water more actively through the individual supply and return lines, still allows low flow to exist within any larger header whether at the chillers, pumps, or cooling towers.
Adding small diameter by-pass lines with the intent to allow chemical treatment to flow, and to prevent particulates from settling is an entirely worthless effort.
In the example below, mechanical design engineers installed two consecutive sets of 12 in. bottom take-off tees at their 36 in. main condenser water pump suction header. While never a good idea under any conditions, the bottom positioning of 1st and 2nd future tees at the first location from the cooling tower and prior to operating condenser water pumps allowed all captured airborne particulates and iron oxide to settle into this dead-end area.
Worst Case Design
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Lower Flows / Dead Zones
Further compounding the issue is the introduction of variable speed / variable capacity circulating pumps operating based upon building load, and having the potential to cause further particulate settlement during low running operation. For HVAC systems whereby entire lengths of the piping layout remain dormant in its early years of operation with future connections capped and ready in anticipation of future growth and demand, those areas of pipe only exist as large settling basins to facilitate rust settlement, microbiological growths, other particulate deposition and of course – higher corrosion activity.
Once again, any suspended particulates introduced into the system through the cooling tower, or created within the system itself as iron oxide, have multiple opportunities now due to layout design and operating conditions to settle and cause significantly higher future corrosion losses. Once established, such deposits are extremely difficult to nearly impossible to remove.
End Result Of Rust Sediment
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Failed Corrective Actions
Provided as the primary safeguard to all corrosion problems for any condenser water system is the chemical water treatment program. Here, the inaccurate belief and self-prescribed hype by virtually all treatment providers that they can effectively maintain low corrosion activity under all possible conditions and to all the piping within a typical HVAC system is in itself one major source of the problem.
Once rust and other particulates have settled, however, chemical treatment inhibitors lose their effectiveness substantially or completely to initiate high and potentially severe under deposit pitting. That chemical treatment and biocides continue in their effectiveness to protect bare steel located under ½ in. or more of rust deposits and microbiological growths is a failed argument repeated endlessly by chemical treatment providers, but totally refuted by decades of ultrasonic testing results by CorrView International, LLC. It is an issue documented well beyond any debate.
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Failed Water Filtration
Water filtration, which now find greater specification in condenser water systems, is still mostly ineffective at removing the suspended particulate problem. In the overwhelming number of investigations we have been involved, the units themselves are installed so incorrectly and in such poorly selected locations as to provide absolutely no benefit whatsoever. Sand filtration, or some variant of sand filtration, is the primary type of unit specified, with full flow filtration very rarely installed.
In the below left photograph, the take-off or inlet to the sand filtration unit is installed to the inside radius to a downward elbow. All particulates passing through the nearby strainer and flowing to the right will be forced by inertia and velocity to the outer radius thereby never reaching the filter inlet.
Filtration Installation Errors
Combining a low flow of water at unnecessarily low sub-micron removal efficiency, and then placing inlet take-off to the filter unit in a position where it is virtually useless, eliminates any benefit to the piping system whatsoever. Its does, however, fulfill the specification for filtration, if in name only. Where a backwashing tank or basin is provided, finding one or two cups of rust particulates captured after 4 years of operation is in itself, a first good clue, commonly ignored, that something is wrong with the entire filtering system.
Sand filtration systems competing against each other in terms of lowest sub-micron removal efficiency, actually filter barely 5% of the total water flow, which in turn allows 95% of the particulate problem to settle within the piping and remain. Most particulates, except for those having the lightest weight and mass, will never circulate around and around the piping system forever until eventually captured, as the working assumption for all side stream filtration installations demands.
Potable drinking water is typically not filtered to the 0.6 sub micron levels often claimed by some filter manufacturers; totally unnecessary in the overwhelming number of condenser water systems. And for the majority of the filters installed incorrectly, filtering 5% of clean water while that minimal 5% of the particle load is by-passed, means a particulate removal efficiency of near zero.
Unfortunately, a critical reliance is often placed upon filtration systems which filter a far too low volume of water at far too small sub-micron retention, and then are installed in the most ineffective location to ultimately provide no benefit whatsoever to a critical piping system dependent on its functioning. Water filtration manufacturers and representatives, for reasons still unknown, seem to recognize no responsibility nor show any interest to ensure that their equipment is installed to its maximum effectiveness after purchase. Unfortunately, the difference between a useless piece of filtration equipment and an effective benefit to the building is just some better pre-planning.
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Full Flow Filtration
The only effective method of filtration is full flow of 100% of the water circulating; requiring much higher initial expenditure but with substantial benefits realized years later. Full flow units typically filter water to a far lesser particle retention, but also eliminate most of the problem of particle settlement and higher under deposit corrosion. Smaller size and typically lighter particles are more likely to remain in suspension to be captured later rather than settle out.
Various automatic units exist from relatively low to moderate efficiency centrifugal units such as Lakos of 80-100 micron and above, to manual screen/scraper equipment provided by Filtomat and others.
We consider the Zero Gravity automatic backwashing filter the best full flow, low particle retention filtration unit on the market, and recommend their installation to any new condenser water system. The combination of a more coarse centrifugal separator followed by or in conjunction with a lower micron retention full flow Zero Gravity filter is perhaps the best filtration option available.
Better Filtration Options
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Inaccurate Corrosion Monitoring
Possibly the last issue contributing to the advanced failure of newly installed condenser water systems is the total and blind reliance upon corrosion coupons to define piping system health. Documented extensively throughout this Internet site is information showing the high degree of under reporting that corrosion coupons provide.
Although still recognized as the “standard in the industry upon which pipe corrosion activity is measured,” corrosion coupons greatly under report corrosion activity from 5 to up to 1,000 times. Under reporting corrosion by 10 times is likely the most common error ratio, having those believing they have a 0.4 mils per year (MPY) corrosion rate dangerously mistaken. A corrosion coupon located inside a side stream loop absolutely does not:
Measure corrosion activity at dead end and low flow areas, crossover lines, drains, vents, piping to lead and lag equipment, etc.
Measure corrosion activity under areas of deep rust and other particulate deposits.
The strict reliance and unquestioned faith in corrosion coupons, given the generally well known limitations widely addressed and documented through this Internet site, only ensures a continuation of the problems which plague modern data centers and other 24/7 facilities expected to provide long term reliability of 20 years and greater. More typically, however, the initial understanding promoted by chemical treatment providers that corrosion coupons provide a measure of corrosion and wall loss to the piping system will dramatically change once a catastrophic corrosion problem is discovered. Only then does the treatment provider’s definition change that corrosion coupons only provide “an estimate of the corrosivity of a fluid against new carbon steel.”
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Problems Will Continue And Accelerate
Unfortunately, advanced failures and escalated corrosion problems will continue as long as such above issues remain unrecognized or ignored. In the many investigations that we have been involved with over the past few years at critical facilities, severe corrosion activity and advanced wall loss was a virtual guarantee due to decisions which took place long before water ever filled the piping.
You can view and download our two page handout on this subject below.
© Copyright 2023 – William P. Duncan, CorrView International, LLC