System Diagrams: Untangled in Action

You probably recall from a previous post that one of the things I like about system diagrams is that they make physically complex piping arrangements easier to understand.

That’s not to say that physical drawings like a piping isometric don’t have their place.  For instance, this piping plan makes it fairly clear how the pipe, duct and equipment will fit in the mechanical space, including critical maintenance and code requirements like access for coil removal and clearance to grounded objects in front of the electrical panels.

This piping isometric further clarifies the details of the piping around the pumps.

But a system diagram can often be the key to understanding how something will (or won’t) work as was the case for the central chilled water plant illustrated in this piping plan.

I first started to look closely at the piping plan in preparation for a field trip to a project site for some preliminary retro commissioning work.  The Owner had sent me the documents they had available and I was spending a little time becoming familiar with what was there.

Of particular interest to the Owner was my take on their chilled water plant since they had operational problems with it.  For one thing, the chillers did not run well under some load conditions and tended to trip out on safeties.  And, any time they tried to use the plate and frame heat exchanger, the chillers tripped out on low flow and had to be manually reset when it came time to use them again.

As a result, they had never allowed the wet economizer process to function because the plant was not attended round the clock and they could not risk the loss of chilled water when they needed it at their Palm Springs resort and spa location.   Thus, the money they had invested in a potentially energy saving idea had yielded no fruit and they had high hopes for our retrocommissioning effort in terms of addressing that particular issue.

As I looked at this piping plan, something seemed wrong with the way the three way valve appeared to be piped.  For one thing, even though the pipe itself appeared to be drawn to scale, the actuator on the valve appeared to be a simple symbol, which, was embedded in the wall according to the drawing.

Dimensional issues like that are frequent occurrences on new construction projects.  If I’m lucky, I catch them during design review and resolve them there, hopefully addressing both a maintenance issue and the potential change order that can come up when the fitter realizes a few more fittings (and welds, which are more expensive than the fittings in this size pipe) will be required to make things fit.

But what really was bothering me was that it looked like the valve would create some sort of hydraulic short circuit if it actuated.  So, I decided to untangle things and make a system diagram.  The result came out looking like this.

While it turned out that the valve didn’t exactly short out the mains, it seemed that it would perform no really useful function.  My guess was that it had been intended to select either the chillers or the plate and frame heat exchanger associated with the wet economizer process as a cooling source.  But the mistake on the drawing that the pipe fitters would use to pipe up the system caused me to put taking a closer look at the system on my “to do” list since there now was the potential for a “built-in” problem.  If nothing else, it made me curious about what had actually been installed.

A few pages later in the same set I can across the engineer’s version of the project’s system diagram, which looked like this.

As you probably have guessed, this was a little tangled up for my taste, so I turned it into one of my system diagrams and got this.

That tended to confirm my suspicion about the intent of the three-way valve, but two things now had my attention.

One was that I had two different system diagrams that were created for the same system from the same set of contract documents by the same engineer;  one associated with the piping plan (left) and one associated with the piping system diagram (right).

The other was that the two diagrams conveyed to different interpretations of what was to be installed and how it was to be arranged.  From the designer’s piping system diagram (on the right above), it appeared that the intent was for the three way valve to select either the chillers or the free cooling process.  The designer also appeared to have intended for the chiller pumps to be interconnected on their discharge so they provided redundancy for the chillers.

The problem with the latter intent was that no service valves were shown in the interconnecting piping, so it would not be possible to isolate one pump for service or to segregate it to a chiller.  So there were potential operational issues there.

Finally,  it appeared that the designer intended to have the pump for the plate and frame heat exchanger serve the loads as well as the heat exchanger. In other words, the distribution pumps would only run when the chillers ran.

If you looked to the piping plan for clarification on the valve arrangement, you found that there were no interconnects shown at all, and that the details of the valves serving the pumps were simply not visible in plan. And the arrangement shown on the plans would require operation of the distribution pumps regardless of the cooling source.

No isometrics were provided to clarify things.  When you converted the plan to a system diagram (the left drawing above) the three way valve did not do anything useful as discussed previously.

So now, I was really curious about what I would find.  When I came across a set of control drawings, I thought I probably had the answer;  after all, the graphics there should reflect how it was actually installed.  Otherwise, how could the control contractor have made things work, right?

When I made myself a little system diagram from the control graphic, I came up with yet another variation on the theme.

In this variation, the three way valve would could impact the entering water temperature to the plate and frame heat exchanger when it was in operation.  And, depending on which ports on the valve were normally open, normally closed, and common, it could potentially create a hydraulic short circuit that would allow water from chilled water supply header to recirculate to the return header.

A decoupling bypass path is actually a design requirement for a variable flow chilled water plant, but for it to work, it needs to have very little pressure drop and always be available for flow.  That was not the case with the bypass path created through the three-way valve.

Incidentally, if you want to learn a bit more about variable flow chilled water plants, I have a PowerPoint show on my Google Docs page that looks at Variable Flow Chilled Water Plants.  It is set up to be accessible to the public so you should be able to download it if you want to.

Returning to our discussion, you will notice that the control system based version of the system diagram also shows both pumps in parallel  with each other, serving a common header to the chillers.  Each chiller is equipped with an automatic isolation valve, probably intended to prevent flow through it when the chiller is not operating.  Thus, either pump could serve either chiller, reflecting to some extent the designer’s intent as illustrated in the his piping system diagram in terms  of interconnected pumps.  But none of the drawings matched each other.

So, at this point I only really new three things.

  1. The owner reported operating issues with the central chilled water plant that kept them from using an energy saving feature.
  2. The potential for a system problem due to confusion about what was required was quite high.
  3. I definitely would plan some field time to make a field verified system diagram once I was on site.

When I got to the site and developed a system diagram of what was there, as you may or may not have guessed, it was different from all of the others.

But what it had going for it was that it represented what was actually there, which is pretty powerful.  Specifically, with it I could see that:

  • The three way valve really didn’t do much.  But, if you triggered the wet economizer/free cooling cycle (tried to use the plate and frame heat exchanger), as the valve changed position, at some point, it would cause the flow to the chillers to drop off to a point that would likely trip them off on a flow safety;  one mystery reported by the Owner was solved.
  • The check valve in the decoupling bypass could place the low head, low horsepower evaporator chilled water pumps in series with the high head, high horsepower distribution pumps under some load conditions.  While it was hard to say exactly what would happen in those circumstances, I knew from past experience that it could lead to unusual operating problems including chiller safety trips. This was a clue to resolving the other operating mystery reported by the Owner.

The bottom line is that my system diagram effort focused my field effort and ultimately helped me solve a problem that had mystified the Owner for a number of years.

In the next post, I’ll look at a case study where the order of connection issues became particularly important.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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