Consulting-Specifying Engineer - April 2008 - (Page 34) PT Plug 8 in. PT Plug South cell High and low level alarm sensors 6 in. 8 in. PT Plug 6 in. PT Plug 8 in. 6 in. 6 in. North cell Make up from back flow preventer and meter PT Plug Chiller 2 Dr PT Plug PT Plug Chiller 2 Dr PT Plug 8 in. CT-1 (North CT-2 cell simliar) BAC 15146 Induced draft 123 tons (15,000 btu/ton-hr) 67 F 246 gpm from 90 F to 75 F 7.5 hp 460 vac 3 phase motor with VFD (each cell) 4,500 lb operating weight Future side stream filter connection point 5 pipe diameters minimum between tee or elbow and pump inlet if no suction diffuser CWP-2 (CWP-1 similar) Bell and Gosset 1531 3 AC End suction, close coupled 246 ngpm at 35 ft.w.c and 2.91 bhp 5 hp, 1,750 rpm, 3ph, 480v, with VFD CWP-1 6 in. 6 in. All temperature sensors are to have test ports located in their immediate vicinity to allow testing and calibration 6 in. Figure 4: The system diagram for the library cooling towers Source: David Sellers manufacturers’ recommended operating level and the bottom of the overflow connection was 3 in. Given that a float valve is a proportional control, there will need to be a change in level from the set point to fully open the valve. For the valves on the library project, the water level needed to change by more than an inch (32% of the difference between the desired level and an overflow condition) to completely open the makeup valve. If the tower makeup valves were not adjusted properly and coordinated with each other, the overflow connection, the high and low alarm settings, and the operation of the pumps, including the impact of a pump starting (and associated draw down of the basin to fill drained piping) and stopping (and the associated drainage of piping that is filled when a pump starts). 2. If a check valve was not holding on one of the parallel pumps serving the towers (see Figure 4), then that could cause a basin overflow problem because discharge water from the operating pump would tend to recirculate through the inactive pump and pressurize the line leaving the cold basin. In addition, when the pumps both shut down, water that would normally be trapped in the piping above the basin level would drain back into the basin through the leaking check valve, causing it to overflow. 3. If there were obstructions in the piping to the hot distribution basins, then that could set up the problem the team was observing. This would tend to be more pronounced at the flow rate associated with two chillers. The obstruction could have several forms: • Isolation valves that were not fully open even though they were commanded to be fully open. • A valve disc on an isolation or service valve that had came loose from the shaft so it appeared open based on shaft position, but wasn’t • Debris hung up on the valve discs in the manual valve or the control valve serving a basin. Even though the piping had been flushed at one point, it probably is worth checking one last time. Strange and unexpected obstructions to flow can be found in the piping on new construction projects including cans, welding gloves, chunks of wood, and deceased animals. 4. If the hot basin screens were obstructed, flow balance problems would arise, leading to basin level control problems. 5. In theory, the hot basin distribution nozzles should be clean if the hot basin screens are clean. But, if construction debris or manufacturing debris moved into the hot basin at some point, the nozzles could be obstructed even though the strainers were clean, causing problems with water distribution and basin level control. 6. Flow may not equally distributed to the hot basins. This is hard to assess, but it can be done by comparing the water levels in the basins. If the towers are identical and the nozzles are clean and free of obstructions, then the water level in the distribution basins should run at about the same depth on all cells if they are receiving the same amount of water. If one cell is deeper than the others, it’s receiving more water. Solve this problem by throttling flow to the basis with more flow. But getting it right at one flow condition does not ensure it will be right under other conditions if the towers are not piped symmetrically. 7. If the pumps are not balanced yet and have more capacity than necessary, they could be running out their curves and moving more water than required, causing level control problems and making the issues set up by the non-symmetrical piping worse then they would be under design flow rates. 8. Another issue is if there is something blocking the cold basin screens or obstructing the lines leaving the cold basins. The potential obstructions are similar to those discussed for the hot basin. 9. If the equalization line was plugged or the valve disc on the service valve was not open even though the handle/operator indicated it was, that certainly could be an issue. When the commissioning team dug into the problem at the library, they discovered that there where two root causes. One was that the 34 Consulting-Specifying Engineer • APRIL 2008
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