Building the better mouse trap
It’s an age-old quest—building the better mouse trap. The US Patent Office has issued over 4400 patents for mouse traps although, paradoxically, only about 20 of them have ever seen any commercial success.
This would lead one to think that better is a term that needs more specific definition. Smaller? Easier to operate? More mice out of service per minute of trap service? Cheaper? Limited contact with the eradicated mouse? What is better?
When it comes to the air that LM-series gas turbines ingest, better focuses on three critical areas:
- Temperature (45F to 65F usually is optimal).
- Direction (gas turbines don’t like directional disturbances of the air at their inlets).
When it comes to plant owners, they’re usually looking to keep expenditures down, but they budget for maintenance and are willing to look at reasonable improvements to their existing equipment. With that in mind, let’s agree that the third area, “direction” is off the table. Design of the inlet-duct structure is what it is. The engine has run with it to date and is none the worse off for it. It could likely be redesigned and improved, but the effort and costs would be unacceptably high.
So what about cleanliness? How can cleanliness be improved without busting the maintenance budget or incurring significant capital costs?
Gaskets. Man-doors, hatches, structural flanges all have gaskets that deteriorate over time. Once the gasket fails, untreated air can enter the duct. Filtration and/or temperature control may be adversely affected. Inspect gaskets regularly and fix them promptly when discrepancies are found.
Filters. There are times when the critical drivers of operational excellence (cost, vendor alliances, logistics, quality, product performance, product availability, etc) get out of whack. We may have sacrificed too much product performance for some other driver. If the product isn’t performing, it doesn’t matter how easy it is to get or how cheap it is. It’s not doing the job, and that is unacceptable. Make sure your product priorities are right.
Theory of operation. Without getting into redesigns or rebuilding, take stock of your current theory of operation versus your actual operating environment. Things change. Certain site elements may have not been taken into consideration that have now become apparent. Your EPC contractor may have bought the generic duct offering with no site-specific options to save money, or out of ignorance.
Example: You may benefit from adding a pre-filter element to your high-efficiency filters. This may not only improve cleanliness, but may extend the life of your high-efficiency filter elements. If you’re in a marine environment, you may benefit from the addition of demister pads at the front of the duct.
Conversely, you may benefit from the [IT] removal [RM] of a feature. In one case, the O&M team I was on dropped the overall differential pressure of the duct 0.5 in. H2O by removing a set of inertial separator panels that we found to be ineffective in our environment.
Preservation. Maintaining the integrity of structural surfaces—especially downstream of the high-efficiency filter elements—is critical. Rust and failing coatings are your (and your engine’s) enemy. Doing spot repairs is perfectly acceptable, but you have to ensure you’re doing it right.
Engines are sensitive to a lot of materials. For instance, they don’t like zinc, a common element in many primers used to protect steel surfaces from corrosion. Unlike regular paints or epoxies which resist corrosion by forming an impermeable barrier between the metal and atmospheric moisture, zinc-rich primers provide corrosion protection by electrical means: The zinc and the steel form a tiny electrical cathodic cell that protects the steel at the expense of the zinc. Of course, the zinc primer also provides a little “barrier” protection as well.
However, these same properties react poorly with the engine’s gas-path surfaces, so zinc-rich primers should be avoided. The best advice is to contact the duct manufacturer and get a primer/paint spec from them.
Instrumentation. Keeping instruments calibrated seems like a no-brainer, but in this case it is a special challenge. Pressure and differential-pressure instruments typically measure very small quantities. The structure has a habit of vibrating because of air buffeting around inside. Instruments are mounted to the structure. Vibration can wreak havoc on them, sometimes actually shaking apart the instruments. Check them regularly.
Temperature instruments may be affected by what side of the duct structure they’re located on (catching morning or late afternoon sun).
Don’t be afraid to suggest additional or more accurate instruments to refine your understanding of what’s occurring in each step of the process inside the duct. Build credibility and a database with them. Being able to examine the process at a more granular level may help justify work orders or even capital expenditures.
Controlling inlet air temperature has a direct effect on the horsepower output of your engine. Keeping the air going into the machine in that “Goldilocks” range of 45F to 65F will help the engine perform at its best. If we have a mechanical means to do this installed (chiller, evap cooler, fogging, etc) in the duct, all the better, but there are a couple of tricks we can use to our advantage when we don’t have the mechanical means at our disposal.
Coatings. Several high-tech coatings are commercially available (such as coated polymer stacks, ceramic beaded polymers, LO-MIT high-reflective coatings, etc) that reduce structural heat absorption. Much like your car sitting in the direct sunlight, duct structures often are quite a bit hotter than the ambient air temperature.
Theoretically, a metal surface in still, dry 77F air can reach 248F. Because of the variables in wind, humidity, sun angle, etc, it usually maxes out somewhere between 95F and 122F. Still, that’s a lot of heat available in the structure to bring the temperature of the air inside up. The next time you’re getting ready to paint the exterior of the duct, investigate the use of one of these coatings on the sides and top of the duct.
Lagging. An old friend from the steam world, the concept of lagging—or insulation—of the duct has proven successful. However, depending on the site environment, the cost of ongoing maintenance may be prohibitive. Coatings may be a better solution in all but the driest of climates. If your duct does have a mechanical means of temperature control installed, enclosed, or shielded insulation should be considered on the external structural surfaces between the introduction point (chiller coils, evap-cooler panels, etc) and the front of the gas turbine. It’s a low-cost way of ensuring you get all of the “goodness” out of the system.
Gaskets, preservation, instrumentation. Everything that was said above in these three areas can be equally applied to temperature control. Thus, their importance is not just doubled, but is raised by a magnitude. Keeping up in these three areas is nothing but pure goodness as far as your gas turbine is concerned.
Thus, by taking advantage of the maintenance budget you’ve been allocated, leveraging some new technologies, and applying modest amounts of capital judiciously, you can take the mediocre mouse trap you already have in your back yard and turn it into a lean, mean mouse killin’ machine. In other words, a much “better” mouse trap.