O&M

Condenser performance monitoring (Part 2)

The second half of this two-part series outlines fundamental methods in condenser performance monitoring.
Condenser performance monitoring (Part 2)

By Brad Buecker – Buecker & Associates, LLC

In Part 1 of this series, we examined the importance of heat transfer in steam surface condensers,
where waterside microbiological fouling and scale formation, or excess air in-leakage on the stem-side, can greatly inhibit energy transfer.1 Waterside deposits may also generate under-deposit
corrosion of condenser tubes that can lead to premature failures.

Accordingly, condenser performance monitoring is a critical tool to detect problems and respond accordingly. This article outlines fundamental methods to do so, some of which the author reported on in Power Engineering over three decades ago,2 but which are still valid today.

Condenser heat transfer

Figure 1 outlines the basic schematic of a two-pass condenser. Three temperature readings are
required for a perfunctory performance analysis, inlet water temperature, outlet water temperature
and the hotwell temperature.

Figure 2. Simple diagram of a two-pass steam surface condenser.3

Under normal conditions, the hotwell temperature is equivalent to the steam saturation temperature,
as the cooling process extracts the amount of heat necessary to convert the turbine exhaust steam to
condensate but does not sub-cool the condensate. (Some sub-cooling may occur in winter without
adjustments to the inlet water flow.)

Seasonal changes in the inlet water temperature will, of course, influence the hotwell temperature
(and the pressure in the condenser, often referred to as the backpressure) and the outlet temperature. Comparison of the inlet to outlet temperature is not effective for tracking performance, but a
measurement that does remain relatively constant over time, in the absence of tube fouling or excess
air in-leakage, is the terminal temperature difference (TTD); the difference between the hotwell
temperature and the outlet temperature. Regular TTD monitoring provides a simplified method for
tracking condenser performance, however, sometimes seemingly minor TTD increases can go
unnoticed when in actuality the onset of some heat transfer degradation issue is underway.

Condenser cleanliness factors

As outlined in references 2 and 4, the author was introduced to an excellent method for tracking
condenser performance per a training module developed by the General Physics Corp. (now GP
Strategies Corp.) utilizing data supplied by the Heat Exchange Institute. I first put the calculations
into BASIC language and then, as spreadsheet software emerged, converted the program to that
format. The program utilizes the three temperature readings mentioned above plus the following
data:

  • Cooling water density
  • Cooling water flow rate
  • Circulating water correction factor
  • Condenser tube correction factor
  • Number of condenser tubes
  • Number of tube passes
  • Inside tube diameter
  • Outside tube diameter
  • Tube length
  • A “C” value from tables given in the GP course

Because the tube dimensions and correction factors are constant for any particular condenser, over
half of the items above can be initially incorporated into the calculations with no further
modifications. Also, because water density does not change very much over ambient temperature
ranges, an average value can be utilized without compromising the calculations.

The program calculates an actual and a design heat transfer coefficient, where the ratio of actual to
design is the cleanliness factor. Because tube surfaces are typically coated with an oxide layer, the
calculations are designed to give a maximum factor of 85% for clean tubes, although this is not an
absolute guideline. Best is to establish baseline values after a condenser tube cleaning with the unit
at full load, and with the condenser air ejector system operating properly. Then track performance
over time to observe changes.

The program proved to be excellent for monitoring performance, as the following case histories
indicate.

Case histories

The following histories come from work at my first utility, City Water, Light & Power (CWLP)
in Springfield, Illinois. These will be followed by an additional example from my second utility.
All examples are from two-pass, once-through condensers, but the method is equally effective on
systems with circulating water supplied by cooling towers.

Case History #1

I had been performing thrice-weekly cleanliness factor analyses on the largest condenser, rated at
1,000,000 lb/hr at maximum load. The values remained very steady in the mid-70% range for
several months, but suddenly within two days dropped to 45%. Waterside fouling does not occur
this rapidly, and such drastic changes are more indicative of excess air in-leakage. Visual
inspection revealed a large crack in the condenser shell where a heater drips line penetrates.

Once maintenance sealed this crack, the cleanliness factors returned to previous values where
they remained for another two months until suddenly dropping again. The seal had failed. The
maintenance crew then welded a collar around the drips line, which totally sealed the crack and
cured the problem.

Case History #2

I had been collecting thrice-weekly readings on two, 690,000 lb/hr condensers. Suddenly, one
condenser began performing erratically. At maximum unit loads, the cleanliness factors ranged
between 70% to 75%, but at low loads the factor dropped as low as 18%. Again, such
fluctuations could not have been the result of waterside fouling. Plant management brought in a
leak detection firm to look for air leaks. The inspectors employed helium leak detection to
completely check the condenser and low-pressure end of the turbine. They classified leaks as
large, medium, and small, and found over a dozen leaks, including two large ones, one of which
was from a crack in the expansion joint between the turbine exhaust and condenser.

Maintenance crews repaired all leaks, but this did not solve the problem. Finally, an operator
discovered that a trap on a line from the gland steam exhauster was sticking open at low loads.
The trap and line are designed to return condensed gland steam from the condensate subcooler to
the condenser, but vent gases to the atmosphere. When the trap stuck open, the strong condenser
vacuum pulled outside air in through the vent. Once maintenance personnel replaced the trap,
the condenser performance problems disappeared. This is a classic example of the many
possibilities for condenser air in-leakage.

Case History #3

This history illustrates how the program detected a problem that had never occurred before. (It
can be quite significant in systems with cooling towers, where the circulating water typically
operates at several cycles of concentration.) The 1,000,000 lb/hr condenser from Case History #1 had been in operation for 10 years but had never suffered from scaling.

During one very dry summer, the lake volume decreased dramatically, and lab chemists calculated that the dissolved solids concentration in the lake increased four times over normal values. However, no thought was given to the possibility of scale formation. Throughout the summer the cleanliness factor declined slowly but noticeably from around 80% to 45%. When the unit came off line for an autumn outage, an inspection team found that the waterside of the tubes was completely covered with a layer of calcium carbonate (CaCO3), less than one millimeter in thickness. The deposits were a direct result of the drought. Plant management brought in a firm to mechanically scrape the tubes.

We observed an interesting peculiarity during this event. The condenser that scaled was
equipped with 90-10 and 70-30 copper-nickel tubes. The two other condensers, both tubed with
Admiralty brass, did not show scale buildups, even though operating temperatures were similar.
We surmised that heat transfer in the one condenser was just great enough to push the CaCO3
saturation index over the edge.

Case History #4

The program is very useful for detecting the onset of microbiological fouling, but if quick action is
not taken and microbiological colonies become established, heat transfer degradation may be very
swift. Also, deposit removal may be difficult. One year, when I was monitoring performance of the
1,000,000 lb/hr condenser from Case History #1, the cleanliness factor dropped from around 80% in
the early spring to 40% by early summer. Malfunction of the biocide feed system for a two-week
period proved to be the problem. Unfortunately, by the time the system was repaired the slime layer
produced by the microbial colonies inhibited the effectiveness of the biocide. (Additional details
regarding such issues are available in Reference 3.)

In mid-summer, we shock chlorinated the condenser, but this only restored the cleanliness factor to
around 65%. Visual inspection revealed that although the microorganisms had been killed, much of
the slime layer tenaciously remained. Again, plant management employed an outside contractor to
mechanically scrape the tubes.

Recognizing the data

At my second utility, which I joined approximately 10 years after performing the work above, I
found that a condenser cleanliness program had been incorporated into the distributed control
system (DCS) logic of each of the two units. It provided results that consistently matched my
spreadsheet program. But it became obvious that the plant staff was too busy to keep close track of
the data. After more attention was given to the program’s value, we observed the onset of
microbiological fouling in the condensers (again due to a biocide feed system malfunction) and a
sudden occurrence of excess air in-leakage in one condenser. Unfortunately, I do not recall the
issue that caused the air in-leakage difficulty.

A key takeaway from this example and Case History #4 above is the criticality of biocide feed
system design and diligent maintenance. Once microbes settle on cooling system surfaces, growth
can be extremely rapid.

Figure 3. A microbiologically-fouled condenser.5 The slime layer collects silt to produce a mud-like substance that can sometimes close off tubes. Under-deposit and microbiologically-induced corrosion can become very problematic in fouled heat exchangers.

Conclusion

Part 1 of this two-part series illustrated the significant penalties possible due to condenser upsets.
This part outlines reliable techniques for tracking condenser performance. The program that
colleagues and I developed at CWLP per training provided by GP Strategies proved to be very
valuable on numerous occasions. Even though coal-fired power plants have declined in number,
condensers remain a critical component for heat recovery steam generators at combined cycle and
co-gen facilities.

References

  1. B. Buecker, “Condenser Performance Monitoring – Part 1”; Power Engineering, August
    2023.
  2. B. Buecker, “Computer Program Predicts Condenser Cleanliness”; Power Engineering,
    June 1992.
  3. B. Buecker (Tech. Ed.), “Water Essentials Handbook”; 2023. ChemTreat, Inc., Glen
    Allen, VA. Currently being released in digital format at www.chemtreat.com.
  4. B. Buecker, “Condenser Chemistry and Performance Monitoring: A Critical Necessity
    for Reliable Plant Operation”; from the Proceedings of the 60th International Water
    Conference, Pittsburgh, Pennsylvania, October 18-20, 1999.
  5. Post, R., Buecker, B., and S. Shulder, “Power Plant Cooling Water Fundamentals”; pre-conference seminar for the 37th Annual Electric Utility Chemistry Workshop, June 6,
    2017, Champaign, Illinois.

About the Author: Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing. Most recently he served as Senior Technical Publicist with ChemTreat, Inc. He has many years of experience in or supporting the power and industrial water treatment industries, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company’s (now Evergy) La Cygne, Kansas station. Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry. He has authored or co-authored over 250 articles for various technical trade magazines, and has written three books on power plant chemistry and air pollution control. He is a member of the ACS, AIChE, AIST, ASME, AWT, the Electric Utility Chemistry Workshop planning committee, and he is active with the International Water Conference and Power-Gen International. He may be reached at beakertoo@aol.com.