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Dead Zones

Open condenser water systems inherently exist under a significantly greater corrosion threat. Even though they typically consume the bulk of any chemical water treatment budget, they still fall victim to the majority of corrosion related problems and piping failures.

Despite significantly greater efforts and expense to control corrosion through highly automated chemical feed and bleed systems, water filtration, computer monitoring, and independent corrosion consultants, we have documented a growing incidence and severity of system failures which arguably should never have occurred.

• Issues Misunderstood

Multiple factors influence such higher deterioration of condenser water systems, but are rarely addressed adequately through either their design or operation. Issues such as lower quality and undersized pipe, for example, cannot be controlled. Corrective measures incorporated into most open systems, whether they are proactive or in response to a specific problem or pipe failure, are often inadequate or even worthless; believed to work based solely upon speculation, unsubstantiated claims, anecdotal belief, and/or questionable science.

Elsewhere, it is an absence to the understanding that multiple corrosion scenarios exist simultaneously; each impacting a different area of the piping system at different levels of threat.

And of course greed, and the often sole interest toward profit rather than solving the problem, can often deter the correct course of action – thereby allowing potentially years of additional piping damage to occur before the proper solution is applied.

• Problem Sources

As essentially a giant air scrubber, cooling towers capture airborne particulate debris and microbiological matter where it is then widely distributed throughout the piping system. A secondary source of particulates are also generated internally as the steel pipe itself inevitably corrodes.

Typically it is not the loss of wall thickness that dooms a condenser water system, but the far greater volume of iron oxide rust product created from that loss of metal which leads to more aggressive deterioration in random but predictable locations.

• The True Meaning of MPY

To even those in the business employing the term on a daily basis, mils per year (MPY) is often an intangible concept. A corrosion rate of 4 MPY is far worse than 1 MPY, but what exactly does it mean in real terms? A typical 12 in. carbon steel condenser water system running the height of a 30 story office building offers an ideal illustration.

A true corrosion rate of 5 MPY, not uncommon for open condenser water systems today, can be viewed from different perspectives. In terms of wall loss alone, new 12 in. ASTM schedule 40 pipe of 0.406 in. wall thickness has an available 0.256 in of potential loss before reaching a minimum suggested operating wall thickness of 0.150 in.

At a corrosion rate of 5 MPY, or the loss of 0.005 in. of pipe wall per year, we can still estimate long service life of 51 years or more. This simple estimate, however, critically fails to consider the volume of rust product produced by such corrosion activity – in reality the primary threat to all open systems.
In terms of actual pipe lost, a 5 MPY corrosion rate translates into the removal of 63.8 lbs. of steel from the system per every 100 linear feet. And that is every year. Extrapolated over the full run of supply and return pipe for our typical high rise office building, then times its age, and the true potential threat of this moderate corrosion rate takes on dramatically new meaning.

For just the 850 linear feet of 12 in. condenser water riser pipe in the above example, we can estimate losing 542 lbs. of steel every single year due to our still rather typical 5 MPY corrosion rate. Even at half that corrosion rate, the result is still significant.

Worse, the rust product generated by the corrosion of steel produces approximately 10-12 times a greater volume of less dense iron oxide, which for this same example, means the production of 1.6 cubic feet of rust deposits per 100 linear feet. And that statistic is again every year.

So for our above building property example – 13.6 cu. ft. of new rust is produced annually. And this figure still does not include captured airborne debris and microbiological loading!
Typically overlooked or considered to be a non-issue, generated rust product combined with airborne captured particulates actually present the primary threat to any open condenser system.

• The Real Threat Ignored

While corrosion activity in MPY is the absolute focus of attention in all chemical water treatment programs, rust deposits, and specifically where they settle, not only can’t be monitored by corrosion coupons, but are generally ignored. Yet, such deposits are responsible for most of the larger and totally unexpected piping failures which occur.

Most generated corrosion product remains on the pipe wall – producing deep pitting below its characteristic tuberculation nodules. The remainder, however, circulates throughout the piping system to settle out in any low flow or dead end area where it produces a greater threat often 10-20 times exceeding the general corrosion rate.

• Top Failure Locations

Number 1 for the most severe of all pipe deterioration due to rust and particulate settlement is at any crossover or by-pass line. Designed into most condenser water systems to allow tempering of the supply water temperature to the equipment, by-pass lines are, in fact, rarely used.

With only one automatic control valve installed typically downstream, the open end of the line provides the ideal opportunity for any suspended material to migrate, settle, and accumulate to cause trouble.

At right, 23 years of inactivity allowed approximately 4 in. of rust product to migrate and settle 3-4 ft. from the 12 in. take-off line between the main 24 in. riser and tempering valve. Such heavy deposits eliminated any benefit from the chemical water treatment program; resulting in a bottom corrosion loss of 15.7 MPY and a low pipe wall thickness of 0.102 in. – a condition completely hidden from anyone until it was revealed during an ultrasonic investigation.

Another very common location for advanced and unexpected failures is at dead end pipe leading to a spare pump, chiller, heat exchanger, or cooling tower, where again the lack of water movement produces the same opportunity for particulates to settle. Standby and lead/lag equipment introduces the exact same threat.

Here, the condenser water side valve to a combined spare condenser water / chill water pump remained closed for over 20 years; allowing rust and particulates at the 10 in. open condenser water pump supply header at left to enter and settle. While ultrasonic testing identified very low system wide corrosion activity of under 0.5 MPY at the risers and almost all other areas of the system, a 13.5 MPY corrosion rate was identified midpoint of this dead end line where it reduced bottom pipe wall thickness to only 0.093 in.

Another common threat location is at any type of future connection. Designed into most condenser water systems to serve needs which don’t yet exist, futures function as built-in traps for particulates – with significantly higher under deposit corrosion resulting.

In this prime example at right, a 12 in. downward tee for a future pump off a 36 in. condenser water supply riser is the absolute worst design layout possible since it becomes the first opportunity for rust and captured airborne debris from the roof to collect. Ultrasonic testing identified extremely low wall thickness of 0.285 in. after only 3 years of operation – translating to a severely threatening corrosion rate of 40 MPY.

Extremely common at main risers where they are left valved off for decades, such futures exist under even greater threat due to their often being installed using threaded schedule 40 steel pipe.

Adding further injury to the problem is the typical use of threaded brass valves which introduce an additional wall loss due to galvanic activity – always identifiable by the blue-green deposits on the valve body. This very common combination of all four negative factors can produce complete pipe separation failures at the threads. Far worse than a pinhole at a riser, a thread separation failure will drain the cooling tower and entire condenser water system onto the floors below with always catastrophic impact.

Illustrated at this 5 year old 2 in. riser future, galvanic corrosion has added to the sediment threat at its threaded joints. Having only 0.014 in. of pipe wall thickness remaining at the valve side, and leaks present at both threads, total failure is inevitable.

Other common sites for advanced under deposit corrosion activity are at:

• • • • Large diameter piping headers where limited running equipment minimizes flow velocity
      • Long horizontal distribution lines to lead and lag equipment
      • Cooling tower equalization lines
      • Bottom drain / strainer blowdowns
      • Pipe furthermost from the pumps
      • Variable flow piping systems
      • The bottom upward elbow of the return riser
      • Any dead end piping

Quite often, multiple corrosion scenarios exist simultaneously to inflict a more than cumulative negative impact against the piping system.

• Unknown / Ignored / Un-Addressed

While still unknown to exist, such common piping vulnerabilities are also just simply ignored. Typically favorable corrosion coupon results indicating a 0.32 MPY corrosion rate inherently dismisses any interest to look further for problems.

Corrosion coupons in near pristine condition, and often still exhibiting their polished surface months into the test cycle, suggest the impossibility of a hid-den problem elsewhere within the piping system.

• Inadequate Response

For most facilities, such lurking disasters remain completely hidden from view up until the instant the failure occurs. Elsewhere, preventative actions may have been taken, but are often entirely insufficient to produce any useful benefit.

Unquestionably, the greatest misunderstanding within the building maintenance and chemical water treatment industries relates to water filtration. While finally viewed as a necessity by some for open condenser water systems, most filtration units installed fail to address even a small percentage of the problem.

• Side Stream Filtration

Most water filtration systems provided for condenser water service are side stream units of various size, efficiency, and configuration. Some draw from the piping system while others from the cooling tower pan or sump.

Sand filtration units, the most commonly employed installation for side stream use, are promoted based upon the totally unreasonable and incorrect belief that rust and other particulates not captured on their first pass will remain in circulation to be removed on subsequent laps. They don’t!

Consider for a moment if air filtration were based upon that same flawed principle, and if only 5% of supply air to tenants were filtered.

Selected primarily based upon a competition for lowest sub-micron particle retention size, most sand filtration units address a very small volume or percentage of the circulation rate at an unnecessarily high capture efficiency.

• Improper Installation

Making it even worse, most units are installed incorrectly. With take-offs to the filter often oriented tangentially to the side of the pipe, and even on top, only the smallest particulates of lightest mass will stop to make the 90 degree turn into the suction inlet of the filter for removal. Due to their size, weight, velocity, and inertia, larger particles typically flow right by the filter inlet to settle elsewhere; never returning even a second time.

At left, installing the take-off to the sand filter inlet at the inside radius of this 90 degree 10 in. elbow guarantees that all particulates will be thrust by inertia and centrifugal force along its outer radius and away from capture. An expensive filter now rendered almost useless.

Even more astonishing than the fact that this installation flaw had been specified by the filter representative was the fact that it existed undiscovered for over 6 years. With a massive corrosion problem at hand, not one plant engineer, facility manager, P.E., or critical facilities advisor questioned the placement of the filter take-off, or asked why the backwash holding tank had never needed cleaning.

This small roof level side stream cartridge filter installation illustrates another surprisingly common error. In addition to the take-off to the filter being positioned at the top of the pipe, the downward flow of water forces rust and particulates through gravity, velocity, and inertia along the bottom outer radius of its elbow and away from capture.

Proof to the ineffectiveness of most automatic filtration units is made obvious by the fact that many are backwashed directly into a floor drain. Had they been successful at removing the hundreds of pounds of heavy particulate matter necessary to maintain a clean condenser water system, a new clogging problem would be created for the waste lines.

Even the most obvious proof to a minimally functioning filter typically remains un-noticed. A 500 gallon backwash holding tank with just a few pounds of captured rust product following a year of operation defines poor capture efficiency.

Meanwhile, the far greater volume of heavier particles are captured by gravity inside the pipe itself.

Instead, the effectiveness of any side stream sand filter is completely misinterpreted by all those involved based upon visually cleaner water. In fact, only those finest particulates of lightest mass circulating in suspension and responsible for color and turbidity will be captured by the filter.

At left, the 1 micron side stream filter effectively re-moved all such particles reaching it. Unfortunately for the property, the overwhelming bulk of iron oxide rust product and airborne particulates settled out in its smaller lateral distribution runs – causing the destruction of all but the main risers in under 8 years.

• Other Failed Solutions

Efforts to prevent the settlement of particulates in such low flow areas through the installation of small diameter cross-connect lines between supply and return futures to maintain a movement of water is not only useless, but can even further accelerate particle deposition. A 2 in. blowdown at the end of a 40 ft. long 24 in. pump suction header similarly provides no benefit whatsoever.

• The Problem Will Continue

A careful review of any condenser water layout with a good understanding of its flow characteristics, corrosion vulnerabilities, and operating parameters will always spotlight areas of concern. Correcting or preferably avoiding the threat, however, requires a substantial change in thinking.

The much lower quality of today’s often undersized steel pipe, the greater utilization of low cost foreign pipe, the now standard use of ERW seamed pipe, all now protected by less effective corrosion control chemicals, helps to explain the acceleration of a problem we encounter on a regular basis.

• Solutions

Full flow filtration, the only possible solution for open condenser water systems, is extremely expensive, and again forces the compromise between water flow rate and particulate removal efficiency. The higher water flow of any 8 in. or larger condenser water system virtually excludes sand filtration as a viable option.

Centrifugal separators and screen based units offer an excellent option toward reducing the loading of larger size particulates. They do not, however, provide the low micron removal efficiency required to maintain a truly clean system.

Ultimately, closing the system through the installation of plate and frame heat exchangers at the cooling tower offers the only true solution. Significantly higher chemical levels ensure low corrosion activity. Airborne contaminants are eliminated, oxygen is reduced, and side stream filtration down to micron size levels now means something.

As a closed system, the extended layout of all critical piping is safeguarded, with remaining vulnerability now dramatically limited to a far shorter run of larger diameter and heavier walled pipe between the open cooling tower and heat exchangers.

Effectively employed as a solution for highly corroded condenser water systems where total pipe replacement had been the only anticipated solution, it seems that the right time to consider this design option is actually from the very beginning.

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

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