Piping Symmetry

Q.  What is "Piping Symmetry" and what is its importance?

Piping symmetry involves the concept that all fluid users in a branching parallel piping system will get equal flow rates simply due to the piping design.  This means that every branch will split flow equally in all directions.  In order to satisfy multiple fluid users, this also will require multiple levels of symmetrical branching.  In some cases, this type of design may be simple to accomplish and prove beneficial to the stability of the process, but in many cases the design itself may be cost prohibitive as well as space-prohibitive.

Symmetrical Piping:

Let’s look at a simple example.  The system to the right is symmetrical.  We want to supply 400 gpm of water equally divided between four identical heat exchangers.  The supply and return headers are 4 inch lines.  The headers make the first level of splitting into the 3 inch laterals.  Each of those laterals supplies 2 heat exchangers, and goes through a second level of splitting into the 2 inch laterals.  The 2 inch lines then feed the inlets of the heat exchangers.

Notice that the flow naturally splits evenly since the resistance through each path is essentially equal.  This type of design requires more pipe and fitting materials, as well as additional space for installation than would be required for a similar non-symmetrical system.

Non-Symmetrical Piping:

Now, we’ll look at the same system, only this time we are splitting off of the 4 inch header directly into one long 2 inch lateral line.  We are still positioning our split in the center of the lateral line, but now the two central heat exchangers are closer to the split than the heat exchangers at the ends.  As a result, the end heat exchangers are getting about 10.5 gpm less flow rate than the middle ones.

Since the heat exchangers are designed to handle 100 gpm, then this means the end heat exchangers are providing less heat transfer because the flow rates are lower.  Because the heat load requirements are not being met, something would need to be done to adjust the flow rates such as installing expensive flow control valves and associated instrumentation and controls.  The advantage of this system is that there are less pipe and fitting requirements than for the symmetrical system.  This system also requires less space for installation.

One way to solve the issue of unbalanced flow rates, and still be able to use the less restrictive non-symmetrical piping is to reduce the amount of resistance in the lateral pipe.  In this last example, we will keep the non-symmetrical design.  But instead of splitting off of the 4 inch header line into a 2 inch lateral line, we are splitting into a 4 inch lateral line.  This will dramatically reduce the piping losses in the lateral lines, and work to balance the flow rates to all of the heat exchangers.  Note that the flow rates to the end heat exchangers are now only 0.4 gpm less than the flow rates to the middle heat exchangers.

This example describes one of the key factors in reducing the importance of piping symmetry.  If the pressure losses in the pipelines are low relative to the pressure losses through the heat exchangers, then the flow rates tend to balance out naturally.  If the pipelines are going to be smaller with higher losses, like in the second example, then it might be beneficial to go to a symmetrical layout.

There are some other factors which tend to increase the importance of symmetry.  If there is a close temperature differential between the primary and secondary fluids in the heat exchanger, then balanced flow rates through the parallel exchangers is more important.  In the same sense, if the system is a low pressure system, then the differential pressures have a much greater impact.  Once again, balanced flow rates, and a possible symmetrical design become more important.

In the end, the importance of piping symmetry is going to depend on the piping system in question.  If your system is typically steady-state with very little variability, then piping symmetry may eliminate the cost of flow measurement and control.  The examples we looked at here were for a simple system with only 4 heat exchangers.  But if you had 8 heat exchangers, and had to maintain a symmetrical design, you would have to add a third level of splitting.  Even more levels would be required for more exchangers.  Now, you would be most likely looking at a cost-prohibitive symmetrical design.  So analyze the design requirements of the system before jumping head-first into a symmetrical piping design.  You might be able to get equal or even better performance from a carefully planned non-symmetrical design.