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Severe Chill Water Pipe Corrosion

Operating refrigeration equipment is a major budget expenditure for any building property.  In winter climates especially, running large and expensive refrigeration chillers when the outdoor temperature is likely colder than the chill water produced seems a major contradiction in logic.

Back in the 1970’s, the solution was to directly connect the cold condenser water into the chill water piping system – thereby eliminating the use of the centralized refrigeration chillers under certain temperature and humidity conditions.  At the time, the only concern for this modification was the introduction of captured airborne particulates into the chill water system and the potential for fouling.  This issue was presumably resolved by adding a full flow water filter at the cross connection line between the condenser water and chill water piping – in reality, a superbly inadequate solution now showing up 30-40 years later in the form of major chill water pipe corrosion.

Virtually all such “free cooling” units were retrofit modifications to the existing refrigeration plant; with such cross system operation never an original design concept.  While a popular idea for many large building properties in the Northeast New York City area, and possibly elsewhere, such early “free cooling” installations had serious flaws which were unfortunately overlooked at the time.

Overlooked Aspects Of The “Free Cooling” Design

The following are the most significant aspects of the older directly connected “free cooling” systems which were either not considered at the time, overlooked due to the impossibility of being addressed, or ignored.

• Filtration & Particulate Removal

The typical 100 to 120 micron retention rating for the “free cooling” filter or strainer elements was entirely inadequate to remove those particulates common to an open condenser water system.  Larger airborne debris were captured, but the majority of the threat typically below 50 microns in size passed straight through the filter elements.

Once entering the chill water system, most deposits would settle due to lower flow velocity and more extensive horizontal lines, and remain trapped.  Higher than normal corrosion and pitting more indicative to an open system was the common net result.

• Pipe Size

Unlike larger diameter condenser water pipe running directly between the cooling tower and refrigeration units of typically 8-24 in. and above, chill water piping reduces as it distributes to its individual fan units.  Smaller pipe size means inherently lesser wall thickness and therefore less steel to corrode before reaching minimum acceptable thickness standards or failure.

• Biological Contamination

Microbiological activity is recognized today as a major source of high wall loss.  Yet, this very major contributor to higher corrosion activity was entirely overlooked or ignored decades ago.  At the time, reliance for biological control on more toxic sodium chromate chemical treatment inhibitors negated a threat now known to be a major source of higher wall loss.  Biocides which were added to open condenser water systems, were not normally added to the chill water side.

Although carried over from the condenser water side during “free cooling” mode, biocide dosages were rarely increased to account for the much greater volume of two major piping systems combined.  In addition, once both systems were separated, no further anti-microbiological chemical treatment of the closed chill water piping was maintained to control the residual contamination.

Sterilization of the chill water system using chlorine or bromine, the only method to not just suppress but actually kill microbiological activity, was never performed unless a problem developed.

• Pipe Schedule & Wall Thickness

Although most chill water and condenser water piping systems are today constructed from standard or schedule 40 pipe, it was common to install schedule 80 or extra heavy pipe for condenser water systems built in the 1970’s and earlier.  This was due to an understanding that substantially higher corrosion activity affected open cooling tower systems in comparison to closed chill and secondary systems, with corrosion activity at open systems commonly 3-5 times greater than its closed system counterpart.

Pipe wall thickness for larger condenser water systems installed in 1960 would have likely been extra heavy material having a wall thickness of 0.500 in.  Even after 50 years of service, ultrasonic investigations will often show the steel pipe still near schedule 40 specifications and capable providing 75 or more years of additional service.

Chill water systems, on the other hand, were almost always schedule 40, threaded at its final run-out connections, and of inherently lesser wall thickness.  In contrast to larger condenser water pipe at 0.375 in. or above, smaller 2 in. schedule 40 pipe would have an original wall thickness of only 0.154 in.  With 0.072 in. cut away for threading, only 0.082 in. would actually exist under the same higher corrosion conditions of an open condenser water system.

Closed chill water and secondary systems, during their “free cooling” mode and thereafter, were exposed to substantially higher corrosion activity for which they were never designed nor intended.

• Chemical Concentration

Today, similar to decades ago, substantially less corrosion control chemical are provided to an open condenser water system in comparison to a closed system.  Where chromate concentrations of 100 PPM would have been maintained in a cooling tower in the 1970’s, 1,200 PPM might be maintained at the chill water side.  For molybdates used today, 8-10 PPM is a common dosage for cooling towers, and 100-200 PPM at the closed chill water and secondary lines.

This is not due to a lesser need for corrosion control chemical at an open system exposed to substantially greater stresses and environmental threat, but due to the financial impossibility of maintaining and paying the high cost of proper chemical levels in a system with constant water loss and blowdown.  Although multiple excuses and pre-formatted explanations are provided for such lower chemical levels, the issue is simply the cost.  And under lower corrosion treatment levels, higher wall loss will occur.

Therefore, the low chemical concentrations maintained for any open condenser water system are not due to design, but to a compromise between cost and a higher but still acceptable level of corrosion loss.  Under “free cooling” conditions, such higher loss also extends into its closed piping system(s) as well.

• Water Flow Velocity

Water velocity on the chill water side is typically lower than an open condenser loop, which allows most suspended particulates to settle out under “free cooling” conditions.  This is especially true at longer horizontal lines and in areas furthermost from the circulating pumps.  Unlike large diameter condenser water lines maintaining the same size between the tower and refrigeration machines, chill water pipe can be reduced to 2 in. diameter and smaller.

When operated in “free cooling” mode, the same one or two pumps that provided condenser water circulation between cooling tower and chillers then typically added the entire chill water loop to its load and the chill water pump is shut down.

Eliminating the energy expense of running condenser water and chill water pumps was another cost reducing feature of the direct “free cooling” system, but with again a hidden threat not realized at the time.  In fact, this reduced flow velocity at the chill water system further enabled the settlement of all particulates.

• Rust Migration

Higher corrosion activity at an open condenser water system means greater rust product created.  At a corrosion rate of only 1 mil per year for 12 in. schedule 40 pipe, an amazing 12.8 pounds of steel is lost per year per 100 linear feet, with 0.31 cubic feet of rust product produced from its original steel.  Some component of the corrosion product is blown down, lost in overspray, settled in the tower pans and removed, or captured in strainers and filters.  The remainder attaches to the pipe wall to create under deposit pitting or potentially migrates elsewhere under changing operating conditions.

When directly connected into the chill water piping, some component of this rust product inevitably migrates into the often pristine chill water pipe to initiate higher corrosion losses.

Once passing through the coarse 100 to 120 micron filtering unit, such rust particulates rarely stayed suspended in solution on its travel back to the cooling tower, and instead settled out to produce substantially greater under deposit or cell corrosion within the chill water system.

For any installation of new pipe to an existing system, whether open or closed, old rust product, unless it is removed, will inevitably migrate to contaminate the new bare steel.  In many cases, the result is a corrosion rate for the new pipe far exceeding that of the original pipe.

• Large Potential Losses

The net result following years and possibly decades of free cooling operation is substantially higher corrosion pitting activity at those areas of any building operation previously considered to be trouble free.  Our investigations of older building properties where “free cooling” systems had been operational have all identified substantially greater chill water side corrosion activity similar to the open cooling tower.  For some, corrosion rates at near 3-5 mils per year and greater have been documented.

For 12 in. schedule 40 condenser water pipe having an initial wall thickness of 0.406 in., such wall loss is still quite acceptable and will provide extended service life.  For smaller threaded schedule 40 chill water pipe, however, such higher deterioration means failure within 25 years.  Such theoretical estimates are proven through ultrasonic investigations of chill water and secondary water systems prompted by the first few failures.

Today, virtually all such “free cooling” systems have been removed from service; its “free” aspect now returning in the form of major pipe replacement.  High efficiency plate and frame heat exchangers, used instead, provide nearly the same benefit, but without mixing both waters.  Yet the threat from such operation decades ago still remains.

Effective side stream filtration and sediment removal to any direct “free cooling” closed loop is an absolute necessity, as is also a thorough ultrasonic investigation to determine the degree of negative impact and any areas of potential threat.  Where chill water is directly injected into secondary, or dual temperature loops, then those closed system will show the same negative effect.

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