Written by John Evans, Technical Writer & Moderator for Webcast Experts℠, a division of Krishnan & Associates
For those of you brought up in climates where it snows, you undoubtedly threw snowballs at your kid brother or sister or classmates (and teachers?). When a full snowball fight erupted, your team would scoop up loose snow and form it into snowball ammunition. You would never think about making up a supply of snowballs a day or two in advance as you might run the risk of some melting and loss of mass and density making them less effective. Think of the advantage you would have had if you brought a snow-making machine capable of forming and ejecting snowballs at the battle site!
There is a parallel argument that can be made when the ammunition is CO2 snowballs (actually CO2 ice balls) for blasting the gas-side deposits and corrosion found on the surfaces of a heat recovery steam generator (HRSG). These deposits and corrosion the HRSG tubes and other surfaces are inevitable problems and are a common cause of reduced steam production, sinking steam temperatures, and degraded combustion turbine efficiency. The effects of a fouled HRSG are also tied to reduced electricity production and lost revenue. Removing HRSG gas-side deposits should be a part of every plant’s annual maintenance program. The most effective procedure for removal of these deposits is CO2 ice blast cleaning, proven by over 20 years of industry experience and recognized by HRSG manufactures as a cleaning best practice.
CO2 blast cleaning uses small cylindrical dry ice pellets to remove fouling, rust and scale from tube and fin surfaces. The process involves conversion of liquid carbon dioxide to cylindrical solid dry ice pellets. As the ice pellets strike the fouled heat transfer surfaces, they immediately sublime and “explode” the contaminants from the surfaces. Unlike grit blasting, there is no chance of erosion of tube metal surfaces with CO2 blasting.
Now the cleaning technician in this snowball (ice ball) fight with the stubborn HRSG deposits relies on having a good supply of high density CO2 pellets. Pre-made pellets from an offsite dry ice vendor are usually 24-48 hours old before they are used and the pellets will have already experienced a loss in density. Lower density pellets will begin to sublimate prematurely in the injection hose and the remaining softer pellets will have reduced cleaning efficiency due to the inability to penetrate the interior of the bundle. The solution is to manufacture the CO2 pellets on-site which guarantees the pellet quality and density for maximum cleaning effectiveness.
A completely self-contained mobile support trailer enables on-site production of high density CO2 pellets. The trailer shown above houses a 350 psi high-pressure air compressor, air dryer/after-cooler (for clean instrument grade air with low moisture content), a liquid CO2 storage tank, a pellet conversion unit, and all necessary support systems for direct connection to site power. The trailer also carries all the necessary tools, personal protection equipment, and other safety gear to the site.
So when challenged by HRSG deposits, don’t bring snowballs from off-site to throw. Bring the equipment to give you the winning advantage!