System Diagrams: “Untangle it; it should look like a ladder on its side”

As you may recall from the story I used to start this string of posts on system diagrams,  Chuck McClure turned me loose to make my first drawing with out much direction.  Upon discovering that I needed some additional guidance to produce the type of drawing that he had in mind, he sat down with me and the first words out his mouth were along the lines of the title of this post;  Untangled it;  it should look like a ladder on its side.

He was referring to the schematic I had found on the project drawings that was labeled “System Diagram” but did not reflect the concept he had in mind.   As I indicated in that first post, I don’t have access to the documents that Chuck and I actually worked with.  But the drawing set for the Pacific Energy Center (where I teach a lot of classes) provides a similar example and also allows me to contrast it with my version of a system diagram, which evolved from the concepts Chuck and others taught me.

To illustrate what Chuck meant when he said “untangle it”, I have placed the system diagram from the PEC construction documents (left) next to my version of the system diagram (right).  I turned of the text in mine off so you could focus on the differences in how the piping circuit is conveyed.

If you want a closer look at the diagrams, they both are on my Google Web Albums site and you can download them from there, along with the system diagrams I have used to illustrate this string of posts.

If you think about it a bit, you can see Chuck’s “ladder on its side” concept make an appearance several places in my diagram, as illustrated below.

You’re probably thinking something like those are really short ladders and for a simple system, that will tend to be the case.  But, in the screen capture below you can see a much bigger “ladder” start to emerge.

The screen capture is from the model space tab in AutoCAD Lt and it shows a fairly complex heating hot water system serving a 20+ story high rise.  The ladder with-in the ladder idea shows up in this drawing too, as you can see from the screen shot below, where I zoom in on the boiler plant, distribution pumps, and some of the radiant panel loads (the left portion of the screen shot above.)

Something that became apparent to me when I started working to apply Chuck’s “untangled” concept was that when you got things untangled, there were few if any places where the lines crossed.

Here is a closer look at the piping around the pumps and ice tanks in my system diagram for the Pacific Energy Center ice storage system (left) next to the same area on the contract document system diagram (right) to illustrate what I mean.  (The resolution on the scan I have of the contract documents is not that good, thus my labels).

To my eye, in my version of the diagram, it is much easier to see that the pumps are in parallel with each other, as are the tanks, and that the set of pumps is in series with the set of tanks.   I think its also easier to see that the three-way valve must control temperature by mixing water from the ice tanks with water bypassing the ice tanks.

Bear in mind, that some of this stuff is bordering on style and personal taste so you may not see it the same way.  But if it is easier to see technically important relationships, like series flow or parallel flow, then the diagram will make it easier to understand and communicate how the system works.

That’s not to say that you will always be able to untangle a diagram to the point where no lines cross.  The more complicated the system is, the harder that will become. For example here is a closer look at the supply headers serving the boiler plant in my high rise diagram.  Note how may line crossing there are on the supply header in particular.

The technical reason that this matters is that the temperature of the water leaving a tee as it mixes after a boiler connection is a function of the water temperature entering the tee from the other boilers and the temperature of the water entering the tee from the specific boiler that is connecting to the header at that point.  From a diagnostic standpoint, that temperature can tell you a lot about what the system and boilers are doing.

So, from a diagnostic standpoint, having the order of connection of various circuits and subcircuits correct is critical.  That means that re-arranging the order of connection on your system diagram to make it look more like a ladder on its side with fewer lines crossing is no longer a good idea if it means the order of connection you show will no longer reflect the actual order of connections in the field.  Albert Einstein’s suggestion would seem to have a place in developing system diagrams.

Things should be made as simple as possible, but not any simpler.

When I mentioned my observation about lines not crossing in an untangled diagram to my mentors, their response was that if I had a diagram that I had verified and the lines really did cross, and I couldn’t  untangle them any further, then I should pay close attention to that point in the system in the field for a number of reasons.

  1. If my diagram had been developed from a piping plan and the intent of the design had been to have a different order of connection, then I may have just found a “built-in” problem.
  2. If the piping was confusing on paper in a system diagram arrangement, then it was probably more confusing when you started working in three dimensions with real piping in a real building with construction deadlines and budget pressures.
  3. The potential for confusion sets up the potential for a piping issue that is hard to detect and as a result, causes mysterious problems.
  4. If someone “simplified” the connection in the field with out understanding how things were supposed to work in the first place, they could actually introduce a problem that would not otherwise be there had the original system configuration been preserved.
  5. If you are “religions” in your effort to develop a field verified, untangled representation of the system that still preserves the relative order of connection of the various loads and other elements, you will likely be rewarded with a diagram that allows you to solve the mystery.

In the next few posts, I will further illustrate these concepts via a couple of case studies where a field verified, untangled (to the extent possible) system diagrams helped unravel a few mysteries.


David Sellers
Senior Engineer – Facility Dynamics Engineering

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