System Diagrams; an Industry Concept but Not an Industry Standard

As you probably have surmised by contrasting the two versions of the system diagram for the same system in my previous post, the concepts I am talking about are by no means and industry standard.  Rather, they are my take on a common concept used in the industry to convey how a system is intended to function and be controlled, but not necessarily convey how it is physically arranged.

In fact, I could arrange my own drawing in a different way that still complies with the fundamental concepts I use when I create a system diagram, but shows the elements of the system in a different arrangement.  For instance, here is my version of the Pacific Energy Center ice storage/chilled water system diagram as I presented it in the previous post.

(Incidentally, higher resolution versions of these graphics are available in a public folder in my Picasa Web Albums account if you want to take a closer look.)

This diagram represents the final version of the diagram as I originally developed it (the steps in that process will be discussed in a subsequent post).  At the time, I was trying to understand the system so I could assess the pump head it would require.  I was curious about that because the pump head on the nameplate for the pumps serving the system seemed unusually high.  If it was in fact higher than necessary, then optimizing the pumps might represent an energy conservation opportunity.

So, at the time I started the drawing, I was interested in what created pressure drop in the system from the perspective of the pumps.  As a result, I drew the drawing in a general arrangement that placed the things that caused a pressure drop in series vertically from top to bottom (the chiller evaporator, the air handling unit coils, and the ice tanks along with the associated piping and valves serving them).  The pumps, the devices that put energy back into the system and created the pressure necessary to move the fluid through the system, where shown in parallel with the equipment generating the pressure drop.

So, my diagram as originally developed,  had the “pressure makers” (i.e. the pumps) on the left and oriented so they moved the fluid in the system from a low pressure header, represented by a horizontal line on the bottom of the diagram, to a high pressure header, represented by a horizontal line on the top of the diagram.

The “pressure users” (i.e. chiller, coils, ice tanks, and related piping and valves) where shown in series on the right, between the high pressure header at the top and the low pressure header at the bottom.  This arrangement helped focus my thinking in the context of the question I was asking myself, which was:

what uses the pressure created in the system by the pumps and have the pumps been selected to provide more pressure than is necessary?

But, had I been thinking about the system in terms of what makes the fluid cold and what warms it back up, then I might have drawn the diagram more like this.

As you will see if you contrast my two versions of the diagram, the order of connection and information contained is identical.  But the arrangement has been changed to reflect a different way of thinking about the system (pressure makers and pressure users vs. cold makers and cold users).  Neither diagram is particularly right or wrong and both employ the fundamental concepts behind the system diagrams I develop (which will be discussed in the next few blog posts).

Probably the closest thing to any kind of industry standard that exists for this sort of thing is the Process and Instrumentation Diagram, a.k.a. the P&ID diagram, commonly found in the process control industry and occasionally in the HVAC industry.  (Note that P&ID should not be confused with a related acronym, PID, which stands for Proportional plus Integral plus Derivative control, which is a common type of control algorithm.)

This example was taken from the Wikipedia post on the topic and illustrates a simple Piping and Instrumentation Flow Diagram, for an evaporative crystallizer, contributed from the author’s own work.  There is even a ISA (Instrumentation, Systems, and Automation Society) standard ISA-5.1-1984 Instrumentation Symbols and Identification that includes an extensive symbol list for this type of drawing.

That said, if you look at the discussion associated with the Wikipedia post, you will find that someone commented to say:

It looks to me like the picture shown on this page is actually a PFD, NOT a P&ID…

(A PFD is a Process Flow Diagram)

and someone else to say:

I don’t know if any professional body endorses those symbols at all …

Bottom line is to say that my discussion will be about my concept of a system diagram.  I suspect the concepts could be applied to P&IDs, PFDs or just about any diagram intended to convey how a mechanical piping or duct system is supposed to work. In fact, I’m often asked if there are standard symbols and I usually say something like not really, because there is not really a standard for system diagrams.  But the ISA symbols certainly would be a good starting point.

Be it a system diagram, a P&ID, a PFD, a piping schematic, a flow diagram or any one of  the many names attached to this type of diagram, the intent is to convey the operating concept and related in formation in as clear and concise a manner as possible.  My goal in this string of posts is to convey some ideas that have really seemed to help me with developing my diagrams.  Towards that end, in my next post, I will identify begin to identify what I consider to be important system diagram concepts.  Then, I will walk you through the steps behind creating the Pacific Energy Center system diagram I have been using as an illustration.


David Sellers
Senior Engineer – Facility Dynamics Engineering

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