Understanding Primary Secondary Pumping Part 5: Best Practices for Piping

By Chris Edmondson

Piping in a primary secondary pumping systems as a whole can be quite complex, but when it comes to the “primary” part of the system, there are really only two approaches:  Dedicated or Non-dedicated.

Figure 1 shows the chillers in a primary secondary system piped with one pump per chiller.  If the chillers are to operate in stand-by mode, a butterfly valve can be installed in the shared pipe between the two chillers.  Either way, the pumps are dedicated.

Figure 1

Figure 2 shows a non-dedicated piping arrangement, with the primary pumps manifolded together.  To protect the chillers, this approach requires two-way isolation valves on the return side of the chillers and flow limiters on the supply side.

Figure 2

In either of the above piping arrangements, you can see that the primary and secondary loops are decoupled from one another by virtue of the common pipe.  This calls to mind a question we frequently get when it comes to these primary/secondary systems:  How long (or short) can the common pipe be?  In other words, how much pressure drop can we tolerate through the common pipe before it creates flow problems? 

Bell & Gossett actually conducted extensive research on this topic and determined that the maximum pressure drop should not exceed 1.5 ft.  They also determined that the minimum length should be no less than 3 pipe diameters.  The common pipe is typically the same diameter as the chiller supply piping.

Another question that engineers often ask concerns the location of the air separator on a primary/secondary chilled water system.  Air separators will do their job in any number of locations, but they will be most effective when they are located in the return piping from secondary loop, close to the common pipe.  (See Figure 3)  This is where the water is the hottest and at the least pressure.  This is the most opportune location for air removal since the hotter the water, the least air it can hold.