Trane SYS-APM001-EN manual Check valves

System Configurations

Check valves

Some designers recommend the installation of a check valve in the bypass line of a primary–secondary system to eliminate the possibility of deficit flow in the bypass line. The premise is that if there is a system problem (low temperature differential), the check valve will put the primary and secondary pumps in series and pump more water through the chiller, thus balancing the primary and secondary flow requirements. This is not universally accepted.

Coad14 states:

One constraint is that if the system is designed as variable flow and is experiencing operating problems related to low return water temperatures, the solution is not in the plant but rather in the load.

and

One solution that has been suggested is to install a check valve in the plant common pipe or decoupling line circuit

…However, to be realistic, all that the check valve can do is assure that no water bypasses the chillers, which in turn has the effect of increasing flow in the chiller. Thus with the installation of the check valve must be an algorithm to either slow down system pump(s) when flow increases beyond the design maximum for the on chiller or turn on additional chillers. Thus the check valve solution offers its own problems and many designers feel very uncomfortable with forcing pumps into series operation without the benefit of understanding the full impact thereof.

This manual does not recommend the use of check valves in the bypass line.

Figure 28. Decoupled arrangement

CV Pump

Chiller 3

CV Pump

Chiller 2

CV Pump

Chiller 1

	Production
Bypass Line	Distribution
	VV Pump ~

The unrestricted bypass line hydraulically decouples, or separates, the production and distribution pumps so that they cannot operate in a series coupled pumping arrangement.

Although the two pumping systems are independent, they have three things in common:

•bypass piping,

•no-flow static head (from the building water column), and

•water.

Changes in flows or pressures, due to variations in dynamic head or the number of chillers operating, cannot cross the bypass line.

The extent of decoupling depends solely on the restriction (or lack of restriction) in the bypass pipe. Total decoupling is accomplished only if the bypass piping has zero pressure loss at any flow. Since this is not possible, some insignificant pump coupling will exist. The important issue is to keep the bypass piping free of unnecessary restrictions such as check valves (see sidebar).

46 Chiller System Design and Control SYS-APM001-EN