Don’t forget the cooling tower

During major outages, plant personnel typically focus on the gas and steam turbines, whose shafts produce marketable product—the kilowatt-hours, the juice, the ‘trons. But, as the Second Law of Thermodynamics reveals, only a portion of the heat released by combustion of fuel can be converted to shaft work. The rest must be rejected to an energy sink at a lower temperature. In your competitive zeal for short, low-cost outages, do not neglect the system handling heat rejection.

That system is the cooling tower for most combined-cycle/cogen plants in North America. Reason: Once through cooling systems—which pump river, lake, or ocean water directly through the condenser— were no longer environmentally acceptable when these plants were permitted, and air-cooled condensers— which push ambient air across externally finned tubes to condense the steam (in a direct ACC) or cool the condenser water (in an indirect ACC)—come with disadvantages in capital cost and thermodynamic efficiency. Cooling towers can be constructed of concrete, metal, or wood. The latter continues to be one of the most widely used materials in this country, because of its relative availability and low cost.

All cooling towers should be inspected on a regular basis. Because wood is susceptible to warping, cracking, and decay—conditions that are exacerbated by daily cycling—towers containing wooden elements should be inspected at least every two years. Inspection results should be documented, and items needing attention identified and repaired. Estimated cost to fully inspect one cooling tower, including a final report with detailed recommendations, is $5000.

Below is a checklist of concerns that the cooling-tower inspection should address. It was compiled by Timothy L Brunette, PE, a project engineer (civil) in Reliant Energy’s technical services department, and included in his presentation before a hundred eager participants in the Generic Roundtable at the Fall Turbine Forum recently conducted by the Combustion Turbine Operations Task Force in Tempe, Ariz.

In addition to an inspection, a performance test should be performed on a regular basis—perhaps every four years or so. Estimated cost of a performance test for one tower is $4000. Older cooling towers also may be in need of a dimensional control survey, which can reveal structural problems in the fill area not able to be seen during a visual inspection. Cost of a dimensional control survey is about $1500.

Inspection checklist

  • Environmental, safety, and health. This category should be the starting point for all O&M tasks in a powerplant. Identify all potential environmental, safety, and health hazards associated with your cooling towers and then define how each hazard will be eliminated or controlled.
  • Previous inspection results. Verify that items identified in the previous inspection as needing repair havebeen corrected. Also pay attention to items identified as needing to be checked in the future, to ensure that they have not deteriorated further.
  • Tower casing. Inspect for leaks, cracks, holes, or general deterioration, including air leaks between adjoining panels. Make sure that hardware is tight and in good condition. Ensure that access doors are in good working order, and that they can be shut tightly when the tower is in operation. Tight door seals are important to achieving proper air-flow distribution, which is one of the key factors in cooling- tower performance.
  • Structure. Regardless of the timber species, all wooden elements require the proper selection, penetration, and retention of a chemical preservative, if they are to meet structural integrity and longevity targets. A major plant outage provides your best opportunity to monitor the performance of that preservative. Inspect for signs of wood deterioration, including throughcracks (Fig 1), fractures, or decay in wood members. Inspect wood visually and by tapping with a hammer or probing with an ice pick. Check the tightness of bolted joints and the condition of joint connectors (Fig 2). Look for bowed, bent, or buckled columns (Fig 3). Look for split or missing splice blocks.
  • Ladders (referring to the rungand- rail assemblies providing vertical access on the outside or inside of the tower). Check general condition of the base material, and make sure that all connections between the ladder and the tower are tight and in good condition. Make sure ladder cages are installed where required for personnel safety, and that they are in good condition.
  • Fan deck. Check the general condition of the fan-deck material, noting any steel corrosion or wood decay. Loose fan-deck overlays are trip hazards. Oil leaks or spills are slip hazards. Look for sagging or dips in the fan deck, which may indicate buckled columns in the structure.
  • Stairway (referring to the treadand- riser assembly providing angular access to and from the top of the structure). Check for wood decay, loose treads, unsecured handrails, or deteriorated stringers. Make sure bolted connections are tight and that all of the hardware is in good condition.
  • Interior walkway. Check for broken or deteriorated treads and rails. Check for tightness of connections between the walkway and the tower structure. Check for any damage or deterioration that may pose a potential safety hazard.
  • old-water basin. Look for and remove any excessive buildup of sludge and accumulated debris. Check basin material for any signs of leaks or breakdown of sealing material. Inspect anchor castings and bolts. Make sure debris screens are in place and in good condition.
  • Hot-water distribution basin. Check for wood decay, leaks between adjoining panels, and tightness of bolted joints. Ensure nozzles are in-place, and check them for internal wear and clogging. Remove any debris from the basin. Inspect distribution boxes for leaks or other signs of damage.
  • Header pipe. Inspect iron piping for corrosion or loss of coating material. Look for deterioration of PVC and fiberglass piping. Inspect all piping supports, and verify their integrity for continued service (Fig 4). Check bell and spigot joints for separation or blown-out seals.
  • Flow-control valves. Other than fill-bypass systems designed for freeze protection, regulation of the cold-water return temperature is accomplished through modulation of air flow, not water flow. Flow-control valves (Fig 5) are fixed in the positions that balance water flow to each basin. The major outage provides an opportunity to inspect valve internals for corrosion or signs of wear. Unlock each valve and operate it manually through its full range of travel. When the tower is returned to operation and you can see the level in the hot-water basin, reset the valves to balance water flow to all basin sections. Then relock valve positions.
  • Fill. Of the two types of fill— splash and film—film is more common in today’s combined-cycle/cogen plants because of its ability to expose greater water surface within a given packed volume, hence deliver better heat transfer (Fig 6). However, its narrow passages are inherently more prone to clogging. With either type of fill, inspect for sagging, broken, or decaying splash bars, or excessive buildup of scale. Look for fallen or misplaced splash bars. Besure that all supporting grids are in-place, evenly spaced, and firmly attached to support member. Closely inspect film fill for signs of clogging.
  • Drift eliminators. Make sure that all air passages are clear of debris and as clean as possible. Check the condition of seals, to ensure that water is not bypassing the eliminators. Make sure water drain passages are clear.
  • Louvers. Make sure that all louvers are in-place, and inspect their base material for deterioration. Check the condition of louver support members and the connections between the louver supports and the tower.
  • Fans. As mentioned above, coldwater return temperature is controlled by modulation of air flow. Thus the condition of the fans and their drive systems is critical to cooling-tower performance. Check for broken or missing fan blades. Look for corrosion or erosion of the blades. Check the condition and tightness of the hub and the bushings between the hub and shaft. For adjustable-pitch fans, check the pitch angle and readjust to the manufacturer’s recommended angle if necessary. If pitch is readjusted, check motor amps at full speed to make sure that the nameplate amperage rating is not exceeded. Examine the hub connections between the hub cover and the fan hub. Check blade track and adjust to manufacturer’s recommendations if necessary. Check drain holes in the blade tips to make sure they are open. Make sure no walk boards or scaffolding is left under the fan, following completion of maintenance or inspection. Check for vibration of the fan throughout its full range of operating frequencies.
  • Fan belt-drive system. Check the condition of the pulleys. Make sure that the bushings holding the pulleys on the shafts are tight and in good condition. Check for proper belt tension and alignment. Check the fanshaft bearing lubricant and re-lubricate— being careful to use only the proper type and amount of grease. Make sure the shafts and seals are in good condition. Check for loose or damaged bearings. Make sure that all connections between the bearing housing and the support are tight and in good condition. Check the support for corrosion or other damage, and check the tightness of connections between the support and the tower structure.
  • Gear box. Check the oil level and add or replace oil as necessary. Check for oil leaks around the seals, and replace any seal where leakage is excessive. Check the backlash by rotating the pinion shaft back and forth, noting the amount of free rotation before the gear teeth fully engage. Check end play by pulling up and down on a fan-blade tip and noting the amount of movement in the gearbox output shaft. Make sure that the outside of the case is free of excessive deposits, and that all hardware connecting the gear box to the support is tight and in good condition. Check the oil drain pipe, fill lines, and hoses for leaks.
  • Fan direct-drive system. Make sure the fan bushing is tight on the motor shaft, and that the bushing and hardware are in good condition. Check the joints between the motor mount and support.
  • Drive shaft and couplings. Check alignment using dial indicator or optical device. Look for evidence of corrosion or other damage on the tube, particularly near any welds. Check all connections between the tube and the flex elements, as well as between the coupling halves and the motor and gearbox shafts. Examine metallic flex elements very carefully for signs of corrosion or fatigue. Examine elastomeric flex elements for cracks, brittleness, or other signs of wear. Make sure shaft guards are in place to retain the shaft in case of coupling failure. Check the input pinion shaft for signs of bearing wear and seal leakage. Make sure the coupling guard at the motor is in-place. Check for vibration of drive shaft and motor.
  • Fan stack. Check the overall condition of the stack material. Check the condition and tightness of all assembly and hold-down hardware. Look for leaks between adjoining stack segments. Measure the fan-blade tip clearance all around the cylinder, using the longest fan blade. Adjust the tip clearance by adjusting the cylinder or support, according to the manufacturer’s instructions. Check lightning rods and ground cables for breaks or looseness.
  • Mechanical equipment support. Check all steel components for corrosion or loss of metal. Check tightness of all connections. Check vibration switch to make sure it is operational. ccj oh