System Diagrams: Physically Complex; Schematically Simple

This is another post in my series on system diagrams.  In this post, I will start looking at the concepts I consider to be key in terms of developing a system diagram.  I will be illustrating these concepts using the system diagram for the Pacific Energy Center ice storage system.

As you may recall, higher resolution versions of many of the graphics I will be using are available in a public folder in my Picasa Web Albums account if you want to take a closer look.

Before I get into my first concept, I should say that  it’s important to realize that a lot of this stuff borders on style vs. engineering purpose and the points I make when contrasting one of my system diagrams with one drawn by someone else are not so much a critique of what they have drawn as they are an explanation of why I would have drawn it differently.  Some of that will be style and some of that will be engineering purpose.

The style may or may not matter.  The engineering purpose does usually.  But even then,  the technical points can likely be achieved with a different style.  I guess what I am saying is that there is a bit of art and creativity associated with this process, which I actually enjoy.  So when you try your hand at it, you may discover a new creative outlet that also serves you well in technical endeavors.

One of the things I noticed at McClure Engineering, which is where I first learned to do this and where my style evolved, was that even though our (McClure’s) system diagrams had the same general style, you could tell who developed and drew each one once you started looking at the details of the style.  I think that differentiation is probably the line between style for engineering purposes and style for a consistent look and personal preference.

My waterside style is based on what I saw Jerry Williams and Phil Sutherlin doing, with input from Chuck McClure and Bill Coad.  My airside style is probably driven by what I saw Jim Brooks do (he was our lead field and balancing technician) with input mostly from Phil and Jerry and some from Chuck and Bill.

Having said all of that, I’ll move on to discuss my first concept.  For me, one of the biggest advantages of a system diagram is that it can take something that is physically, quite complex, and present it in a manner that makes it much easier to understand.  For me, this also means that it is easier to troubleshoot a problem using the system diagram.

To illustrate that consider the following graphics.

The lower graphic is the system diagram for the piping shown in the upper picture.  Both illustrate the piping and gauge manifold for one of the pumps in the Pacific Energy Center’s chilled glycol/ice storage system.

To my eye, and I suspect others, the system diagram presentation makes it much easier to understand how the different valves on the gauge manifold can be used to connect different gauges and instruments to different sensing points associated with the pump and suction diffuser.

As a side note, the gauge manifold on this pump is more complex than most.   A more typical arrangement is illustrated in the system diagram for the other pump in the system.  That type of installation frequently will look like this.

The intent of the arrangement is to allow one gauge to be used to measure pressures at multiple points on the pump assembly.  Since from an operational perspective, you are interested in the difference in pressure readings, using one gauge and subtracting the readings eliminates gauge error, improving the accuracy of the results.

Obviously, the arrangement on the pump in the picture at the Energy Center is much more complex than the more typical arrangement in the picture above.  The reason for the added complexity is that the Energy Center is a training facility.  The more complex arrangement allows us to connect multiple gauges and instruments to the pump and compare the results.  For instance, we can install a high accuracy, large gauge with some nice features like the minimum indication needle (the red needle on the gauge in the first picture) with low cost, low accuracy gauges like the smaller ones and compare the results.

But I digress. I’ll do a post focusing on pump tests and gauge accuracy after I get done with my system diagram series.  My point here is to illustrate how a system diagram makes physically complex piping easier to understand.   That can be important when you are trying to diagnose a problem.

In fact, the system diagram of the pump at the Energy Center reveals the reason behind an operating problem that showed up when we modified the gauge manifold from more standard simple arrangement to the more complex arrangement shown in the picture. Here is what the gauge manifold piping looked like before we made our changes.

All of the modifications occurred in the instrument piping connected to the pump suction tap, the pump discharge tap, the suction diffuser tap, and the existing differential pressure switches. Note that originally, it was not possible to measure pressure ahead of the suction diffuser.

As part of a hands-on existing building workshop class we conduct at the Energy Center, we optimized the ice storage system by trimming the impeller on one of the pumps to more closely match the actual pump head required by the system. We did not trim the impeller in the 2nd pump for a number of reasons including economics and training opportunities.  As a result, the pump with the trimmed impeller needs to operate as the lead pump to realize the savings and the pump with the untrimmed impeller serves as a back-up pump if the lead pump fails.

When the pump modifications were made, the software controlling the pumps was also modified to provide the pump sequencing I described above. Subsequent testing and observation over the years indicated that the software worked as intended; i.e. the unmodified pump only came on if the pump we had modified was not operating when it should have been.

A couple of weeks ago, when I was updating my system diagram to show the recently implemented gauge manifold piping, I noticed that both pumps were running. At first, I thought that perhaps someone had placed one of the selector switches in “Hand” or “Manual”. But Ryan (the person I do the class with at the Energy Center) checked and they both were in “Auto”. As a result, we thought that some sort of bug had come up in the software.

As a temporary measure, Ryan placed the desired lead pump in “Hand” and shut down the other pump at the starter by turning the selector switch “Off”.  But, as I finished my my system diagram update effort, I realized that the problem was probably not a software problem; rather, it was likely related to the piping modifications.

For the time being, I’ll leave it to you to figure out how and why did the piping modifications introduced an operational problem into the pump sequencing software and what steps need to be taken to restore the system to its intended operating sequence.  Assuming my theory is correct, you should be able to correct the problem without making a modification to the software.  At some point, I’ll reveal the answer on the blog.

Meanwhile, Ryan and I challenged the current group of students in the Existing Building Commissioning Workshop to figure out the answer, including the possibility of a valuable prize for the first person to come up with the solution.  And I think we have a winner, but they have to be present to collect their prize, so we’ll see what happens when the class meets next month.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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