Developing a System Diagram: Untangling the Pacific Energy Center Ice Storage System

Up until this point, my “System Diagrams” string has been pretty focused on my concept of a system diagram.  In this post, I’ll (hopefully) start to get practical by talking about how to go about creating a system diagram from scratch.  My plan is to show you how I developed the system diagram for the Pacific Energy Center ice storage system.

I’ve been working with there with Ryan(the tool lending program coordinator for the PEC) for a bit over 10 years now, so I have a lot of information to work with including pictures of just about everything and a recently developed 3D model of the piping system. My theory is that the information I have accumulated will provide a fairly rich graphic environment to illustrate the discussion.

I should also note that this discussion will be in the context of how to do a system diagram for an existing system.  In other words, this is how I go about developing a system diagram for an existing building commissioning project.  To some extent, in an existing building commissioning process, you are trying to “reverse engineer” the evolution of the system you are studying;  you know what it is doing because you can measure it.  You’re trying to figure out what the design intent was behind what you are witnessing.

This is a contrast with the way I learned a lot of what I know about system diagrams.  While my first diagram was a “reverse engineering” effort, many of the ones that followed were my initial efforts to develop a system design.  What I am saying here is that a tool that serves me well today as a commissioning provider also was on the critical path for developing a design when I was working as a system designer.

In fact, had I gone to the principles at McClure Engineering (where I sprouted my designer wings) and asked for drafters to produce a project and not had a system diagram, an estimate of the loads, and some concept of how the system would be controlled, they would have turned me down.

That was because with out those items, I really was not ready to start committing  a design concept to paper, which at some level means merge a concept with the physical reality imposed on it by the building structure.

Thirty plus years down the road, with a lot of time in the field, I’m absolutely convinced they were right. Bottom line is that even though I will be discussing system diagram development as a forensic engineering effort, it (in my opinion) is also a critical first step in the design process.

That said, if you could some how magically hover in space outside the Pacific Energy Center and have super duper, selective, HVAC, X-ray vision, (its a technical term) that allowed you to see through the walls but only see the elements of the facility that were associated with the ice storage/chilled glycol system, you probably would see something like this.

For the purposes of orientation:

  • The dashed cube with the blue cylinder in it towards the upper left of the diagram is the chiller (the dashed cube) and it’s evaporator (the blue cylinder).
  • The dashed cube towards the upper right side of the diagram is the air handling unit that serves the facility.  The rectangles with all of the short horizontal lines represent the chilled water cooling coil.   The coil is actually a stack of three coils piped in parallel.
  • The big round cylinders in the lower left quadrant of the drawing are the ice storage tanks.
  • The red “things” to the right of the tanks are the circulating pumps.

With the exception of a few details (for instance, the “X-ray vision” illustration does not show the piping to AHU-2 beyond the point where it connects to the mains, it doesn’t show the expansion tank or make up connections and it doesn’t show the various temperature control system sensors) both diagrams illustrate the system piping network.

If you contrast the illustration at the beginning of the post with the 3D model, you probably are starting to see the value of the system diagram as a design, troubleshooting, and diagnostic tool.  Its likely that you can also see that the physical arrangement illustrated in the 3D model is valuable for someone who is trying to fit everything in the building and connect it in a manner that allows the desired functionality to be achieved.

In the next post, I will start to explore how, when confronted with the physical reality of the system as illustrated in the 3D model, you can go about untangling it to get the system diagram as illustrated in the opening graphic.

Until the, I hope everyone has a nice weekend as summer wanes into fall.  Personally, I’ll be occupied with some “real work” in the form of building a flagstone patio out in our back yard.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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