Finding the best place to sample heat recovery steam generator (HRSG) tubes isn’t as hard as finding a needle in a haystack (but it’s like that).
Ask most HRSG users where they take their routine tube samples for monitoring under-deposit corrosion in the high-pressure evaporator, and they’ll tell you, “first row, because it’s the hottest row.” But is the first row necessarily the row with the highest level of deposits?
Case studies show that the highest level of deposits actually may be located in a variety of places, depending on each HRSG’s configuration and operating conditions. Therefore, we recommend that HRSG users find out which row actually has the highest level of deposits in their HRSG and routinely take samples there.
The worst deposits may be located deeper in the bundle if the flue-gas stream is so turbulent that it swirls around the first row. That’s just one of the variables affecting the deposition level. Another one is flue-gas slippage, which can channel the flue gas along the sidewall of the bundle (Figure 1). Still more variables affecting the deposition level are the firing pattern of duct burners, the finning pattern on the tubes, and stratification of water and steam in the tube, caused by water-circulation problems.
1. Flue-gas slippage affects the deposition level on HRSG tubes. Courtesy: Bob Bartholomew
With so many variables affecting where deposits may be the highest, some HRSG users might feel overwhelmed by their search—as if they’re trying to find the proverbial needle in a haystack. Fortunately, they can employ some practical tools to aid them in their quest.
One of these tools is the borescope. Most HRSG users already have one on hand for gas turbine inspections, so without much trouble they can start using it to inspect their HRSG tubes. A borescope will identify the row and elevation in the bundle that actually has the worst deposits. To verify the borescope’s findings, HRSG users might need to employ a second tool: a computational fluid dynamics (CFD) study, which uses plant data and advanced software to map the precise flue-gas flow all the way from the inlet duct to the exhaust stack (Figure 2).
2. CFD studies map the flue-gas flow. Courtesy: Airflow Sciences Corp.
After HRSG users use borescopes and CFD studies to determine precisely where to sample, they need to know precisely how to sample. Here’s our brief list of how-tos—what we call our “dirty dozen” tube-sampling tips:
- Extract full-length samples, if possible. If full-length samples aren’t possible, then extract a minimum length of two feet.
- Mark the crown of the hot gas side with an “X.”
- Mark the crown of the cold gas side with a dot inside a circle.
- Mark the top and bottom.
- Mark the direction of fluid flow.
- Document the sample’s position relative to pertinent equipment, such as “two feet above bottom row of duct burners.”
- Photograph the sample in-situ before extracting it.
- Use a non-lubricated (dry) saw to cut out the sample. Don’t cut it out with torches, hammers, or pry bars because they’ll dislodge the all-important deposits the lab needs to analyze.
- Handle the sample as gently as possible. Don’t drop it to the floor or mechanically shock it because that’ll dislodge the deposits.
- Replace the extracted tube with the proper spare tube.
- Keep the sample dry and protected from dust between the time of extraction and the time of packaging.
- Follow industry recommended actions, which might include conducting a chemical cleaning if the deposit weight density exceeds the prescribed limit for your HRSG’s operating pressure and water chemistry regime.
These tips on tube sampling can help you not only monitor under-deposit corrosion in the high-pressure evaporator, but also to monitor tube condition in the intermediate- and low-pressure sections. They can even help track down the elusive problem of phosphate hideout.
—Bob Bartholomew, Sheppard T. Powell Associates, LLC; David Daniels, M & M Engineering Associates; and Rob Swanekamp, HRSG User’s Group Inc.