September 1, 2010

Taming Condenser Tube Leaks, Part I

By David G. Daniels, Mechanical & Materials Engineering

Hide and Go Seek

If the decision is made that the plant must take an immediate forced outage, then there is also a concern about whether the leak can be quickly found and plugged. The ability to detect a leak is the product of its size, the conductivity or sodium in the cooling water, and the method used to detect the leaks.

When half of the condenser can be safely isolated while the unit is running (or at least while there is still vacuum on the condenser) any number of methods have been tried to find condenser tube leaks. Some operators have used food wrap, candles, rubber stoppers, and shaving cream. However, helium detectors are more sensitive and reliable and can quickly locate even very small condenser tube leaks. The latest helium leak detectors have become much more portable and easier to operate.

Though the design of each plant is somewhat different, an approximation of the size of a condenser tube leak can be made by looking at the cooling water chemistry, the feedwater chemistry, and some basic fluid dynamics.

As an example, consider the typical cooling water analysis summarized here, where the water comes from a local surface source and the pH of the cooling water is adjusted with sulfuric acid:

• Specific conductivity (µS/cm): 4,000
• Calcium (ppm as CaCO₃): 770
• Magnesium (ppm as CaCO₃): 350
• Sodium (ppm): 340
• Chloride (ppm): 370
• Sulfate (ppm): 1,200

Let’s also assume that the cooling water pressure in the water box is 25 psig and that the unit produces about 1 million pounds of steam an hour, meaning that the flow through the hotwell is also about 1 million pounds per hour, or 2,000 gpm.

Suppose that there were a sudden increase of sodium at the condensate discharge of 5 ppb caused by a condenser tube leak. The increase of sodium in the condensate would be unmistakable, as it is far above any normal fluctuation from other potential sources of contamination. Such a leak would also produce feedwater chloride of approximately 5 ppb and sulfate of 16 ppb. Although it is difficult to predict precisely at these low levels, you may also see an increase of about 0.2 µS/cm in cation conductivity from this leak. In cases where the cation conductivity is stable, such an increase would be seen as unusual and warrant further investigation. In cases where makeup water or organic treatment chemical additions vary, it may be more difficult to determine if the increase in cation conductivity is from a condenser tube leak or some other cause.

Although an increase in sodium at the condensate pump discharge may signal an analyzer problem, and an increase in cation conductivity could have a number of possible sources, if there were an increase in both, then the probability of a condenser tube leak is very high. This is why both cation conductivity and sodium analyzers are must-have analyzers on both a steam sample (reheat or main steam) and at the condensate pump discharge.

Given the levels of sodium and cation conductivity in this example, the change in chemistry suggests a leak of approximately 100 ml/minute—a leak in a single tube smaller than the period at the end of this sentence. It would be very difficult to find such a leak with anything but a helium leak detector.

However small the leak, its effect is substantial. Chlorides at 5 ppb in the feedwater would concentrate considerably in the boiler water and would likely increase to well above 1 ppm chloride in most utility boilers that normally keep the continuous blowdown line essentially closed during operation. A level of even 1 ppm chloride would be intolerable for a unit operating on any boiler water treatment except, perhaps, a high-level phosphate treatment. Even with the blowdown open and continuous high levels of phosphate feed, the potential for underdeposit corrosion, such as hydrogen damage forming in the boiler, is substantial.

The Electric Power Resesarch Institute (EPRI) Chemistry Guidelines (on the EPRI website) provide charts showing the acceptable levels of chloride and sulfate for various operating pressures. When a condenser tube leak is suspected, analytical methods must be in place to analyze chloride and sulfate levels in the boiler frequently. The boiler must come off line if the established limits cannot be maintained not only for pH, but also for chloride and sulfate.

Make Your Decision

With proper analytical monitoring techniques, such as a low-level sodium analyzer at the condensate pump discharge and a continuous chloride analyzer on the boiler water, you may be able to detect very small condenser tube leaks in time to let you run to the weekend and avoid a forced outage. If so, operating the boiler to minimize attemperation flow will minimize contamination of the steam and hence minimize the chance for later boiler and steam turbine problems. However, if the chloride and sulfate levels in the steam or boiler quickly rise (when the condenser leak is large), the unit must immediately come off line to prevent substantial and costly damage to the unit. In either case, good analytical instruments that are properly placed in the boiler and cooling water systems are vital to making informed decisions.

One final thought: Remember to drain and flush the hotwell and drain the boiler when you come down for a condenser tube leak in order to remove any accumulated contamination. Be especially vigilant during the subsequent start-up to ensure that the right tube was plugged and that the chemistry is quickly returning to normal. In any case of severe contamination, a chemical cleaning of the boiler should be scheduled for the next outage.

In Part II, we explore the different mechanisms that cause condenser tube leaks.