By Chad Edmondson
When designing cooling tower suction piping, it is absolutely critical that engineers have sufficient NPSH to the condenser water pump and avoid any piping design errors that could cause air to come out of the pumping solution. If either of these becomes a problem, there are no easy solutions and the costs can be quite devastating.
Cooling towers tend to invite a lot of air into the circulating condenser water. This is not typically a problem as long as some basic design principles are observed. Designers primarily want to avoid anything that might cause the entrained air to “pop out” of the solution because this is where problems start. Any time the water pressure on the suction side of the pump drops below atmospheric pressure, air will come out of the solution.
Another condenser water system issue that must be evaluated is NPSH. If the pressure drop and the water temperature are sufficiently high, the water inside the pipe can actually boil and flash into steam, causing potential damage to the impeller, seal or even breakage of the pump shaft.
The key to avoiding these problems is first building in a sufficient safety factor for NPSH when you select your cooling tower water pump. Keep in mind that the further to the right a pump operates on its curve, the higher its required NPSHR (Required Net Positive Suction Head) will be. We recommend the following NPSH margins when selecting condenser water pumps:
Here are some important guidelines for designing the suction piping for cooling tower pumps:
Always apply the appropriate NPSH safety margin to your pump selection.
Use eccentric fittings to prevent air pockets from forming. Have the flat part of the fitting on top.
Use tapered Y branches instead of straight branch headers on multiple pump installations. Y-branches minimize the potential for increased turbulence and the resulting expulsion of air from the solution.
Headers should be sized for 3 to 5 feet per second at full flow wherever is possible.
Limit reducers to just one pipe size change
Maintain 5 to 10 pipe diameters of straight pipe between the tower and the pump.
Pipe velocity must not exceed the suction nozzle velocity. 5 to 10 feet per second requires 5 to 10 straight pipe diameters to the suction of the pump.
Equalizing lines are required for equal sump levels.
Make sure to specify compound pressure gauges at the pump suction. Regular pressure gauges will indicate zero pressure when negative conditions exist, making it difficult to troubleshoot or avoid problems due to low NPSH.
Try to limit the max pressure drop between pump suction and cooling tower cold water basin to 5 ft max.
Always try to elevate the tower high enough to have a positive pressure at the pump suction WHEN the pump runs at full flow.
Pitch suction pipe up to cooling tower when possible and provide a means to vent excessive air.
Rotate or exercise condenser water pumps if there is a long horizontal run between pumps or a dead leg in the suction line.