Cooling Tower and Condenser Water Design Part 8: Pump Suction and Discharge Piping

By Chad Edmondson

Correctly piping the suction and discharge of your cooling tower/condenser water pump is critical.   Some fairly catastrophic events can occur if either is improperly installed.

Starting with the suction piping, it is important to take precaution to avoid air pockets – which can destroy a pump in pretty short order.  Remember, cooling tower water is full of air, so air in the piping is a given.  You just have to make sure to avoid any sudden pressure drops between the tower and the pump suction, otherwise the air could “pop out” of the water, enter the pump, and destroy a pump shaft.  Another potential problem, depending on the temperature of the water, is the possibility of cavitation, should the pressure drop be so great that some amount of water flashes to steam upon entering the pump. 

Here are some best practices for suction piping:

  • Make sure that the eccentric fittings are located at the top of the suction piping so that the top of the piping is flat.  Eccentric fittings are used where the diameter of the pipe on the upstream side of the fitting is larger than the down stream fitting.  Eccentric fittings (also called eccentric reducers) are installed on the suction side of the pump to ensure that air does not accumulate in the pipe.   Figures 1 and 2 shows the “Recommended” and “Not Recommended” installation of eccentric fittings.
  • Try to keep suction pipe as straight and unfettered as possible.  Valves, strainers and fittings are best located on the discharge side of the pump.  Ideally there should be no more than 5 feet of pressure drop between the tower and the suction flange.  Remember, any pressure drop in the water will cause entrained air to bubble out.
  • Be aware that slightly negative pressures at the pump suction will occasionally occur.  This is most likely to occur when the pump is first turned on and is the result of the small, momentary pressure drop in the piping between the cooling tower and the pump.   One or two psi of negative pressure is quite common and not typically a problem.  Just keep in mind that if the pressure drop is great enough and the water temperature is high enough, the water could boil, and that means pump cavitation.
  • Install compound gauges on the pump suction!  How are you going to know if you have a negative pressure if your gauge only measures positive pressures?  A zero reading can only be trusted if it’s a compound gauge!  

Figure 1

Part 8_Recommended_Suction.jpg

Figure 2

Part 8_ Not Recommended.jpg

On the discharge side, the primary concern is making sure the tower doesn’t get flooded if the pump is turned off.   The best way to avoid tower flooding is to install a water leg to trap all the water on the condenser side.  The proper length water leg will only allow a minimum amount of water to drop into the tower.  (Figure 3).

Figure 3

Part 8_Water Leg.jpg

Here are some best practices for discharge piping:

  • Put any horizontal pipe on the discharge side of the pump below the overflow level.  If this is not possible be sure to install a water leg to prevent drainage into the cooling tower.
  • Install a non-slam check valve on the pump discharge to prevent any back drainage.
  • Size the pump discharge piping per ASHRAE 90.1-2010 design tables.  Remember, ASHRAE now assigns a maximum pipe size on cooling tower/condenser water systems.
  • Include a fouling factor on pipes when sizing your pump.  Open systems like cooling towers get dirty, so some degree of increased pressure drop is bound to occur with time.  A 5 to 10 ft. safety factor is generally acceptable.

For more information, please view our video series on Cooling tower and Condenser Water Piping Design.