Leak Checking a Power Plant Condenser with a
Helium Leak Detector
Finding Leaks in Tube-Sheets and Generator Cooling Systems
Loss of efficiency is the primary reason power plants need to leak check their condenser vacuum systems. Helium leak detectors are the most effective method of pinpointing these damaging leaks, and may be further applied to finding leaks in tube-sheets and generator cooling systems.
Phoenix L300i Product Information
Rent a Phoenix L300i Helium Leak Detector
Leak Checking the Tube-Sheets
Care must be taken not to contaminate the polished condensate with coolant. Particularly seawater coolant, in coastal-located generators, threatens the integrity of the system by in leakage leading to air in-leakage. Defects, wear points, and tube-to-tube joints are all potential leak sources. And each new leak raises the need for a costly partial shutdown and tedious work in the hot environment of the water boxes. Helium leak testing is fast and efficient and can be effectively accomplished with the Phoenix 300i Helium Leak Detector. Here is the procedure:
A fan set up to exhaust the far water box is mandatory. This not only keeps the worker in the near box cool, it pulls the helium down the tubes and exhausts it. The snuffer probe is set up in the exhaust of the Steam Jet Air Ejector or water-ring pump, just as it would be for checking the condenser vacuum system. Then, with one worker monitoring the leak detector, another climbs into the water box and proceeds to spray some helium into each condenser tube.
With thousands of tubes, this cannot be done with an air gun in a reasonable amount of time. The solution is to use tube-sheet testers; hand-held plenums that spread the helium over an area of the tube-sheet of approximately 2 square feet. Many workers like to use chalk to mark around the plenum while they are holding the helium valve open. That way, they can return to check the same tubes if a leak has been located.
In the past, two workers were required for the test, one spraying helium and one person at the helium leak detector. With the wireless remote for the Phoenix L300i, the person spraying helium can wirelessly read the results on the iPad. As helium is sprayed over a leak, it is drawn in through the leak and pumped out of the condenser by the Steam Jet Air Ejector or water-ring off-gas pump. As it passes the sniffer it will be sensed and show up on the leak detector. Time response for the helium varies, depending on the volume of the condenser and the speed of the air ejector, but 45 seconds to 1½ minutes is typical.
Once there is a response to a leak, the “inside man” should wait for it to clean up—that is, for the helium to be largely pumped out. Then he should go over the same area, at the same speed, but in the reverse direction, to locate the leak. When the response is seen again, the leak will be halfway between where the plenum was when he first heard the response and where the plenum is now. The specific leaky tube can be pinpointed either by use of a smaller plenum (perhaps 6″ square) or by careful use of a spray gun on one tube at a time.
Whether by using a spray gun or a plenum, the helium is drawn down the tubes by the suction of the exhaust fan, exposing the entire tube and tube-sheet weld at the far end to the helium. When a leaky tube is found, it can be plugged then, or temporarily blocked until all the leakers are located.
Use of the Phoenix 300i Helium Leak Detector and tube-sheet testers speeds a hot, tedious job, while providing assurance that each leak has been found.
The experiences of two fossil-fueled plants can illustrate this. One plant in the Northeast embarked on an ambitious program to minimize leakage. They charted the vacuum pump exhaust flow for each of five units on a daily basis, while using a Leybold Helium Leak Detector to locate leaks. When they found a leaky relief valve on the injection water return to condenser line and isolated it, they reduced the permanent gas flow by 30 cfm and estimated fuel savings of $35,000 per month. When they located and repaired a cracked weld in an extraction steam line, permanent gas flow was reduced from 72 cfm to 17 cfm—generating estimated fuel savings of $9,000 per day.
The second plant, a 350-megawatt base loaded New England unit, has computed their fuel cost directly as a function of suction pressure. They determined that each half inch Hg of extra suction pressure cost 4.6 barrels/hour of fuel. Their design rating pressure is 1.05 inches Hg absolute; a leak which would force the pressure to 1.55 inches Hg absolute would cost an extra 110.4 barrels/day, or $2,000/day at $18/barrel. The amount of leakage necessary to raise the suction pressure this much might be as little as 2 scfm (if a steam-jet air ejector is used) or as much as 20 scfm if a mechanical pump is used. In either case, a substantial cost penalty is borne by the operator of the leaky plant.
The Phoenix 300i Helium Leak Detector has proved its worth in many plants, nationwide and overseas. It is a fast, reliable approach, which can easily be learned by maintenance personnel. Most important, it can be applied while the plant is in operation, under load, so there is no need to wait for a weekend shutdown to find a new leak. The same system can be applied during shutdown to leaks in the tubes and tube-sheets of the condenser.