September 2017

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Retrofitting a BAS

– The Devil’s In The Details

Ira Goldschmidt

Ira Goldschmidt, P.E., LEEDŽAP
Engineering Consultant,
Goldschmidt Engineering Solutions

Contributing Editor

As published
Engineered Systems 
September Issue - BAS Column

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Older BAS’s are typically replaced when BAS controllers and/or operator interface (the “brains”) are at the end of their useful life.  This typically means that the “brains” are no longer supported by the manufacturer.  BAS’s are custom built-up using many components besides the “brains.”  These include actuators/valves, sensors/switches, various types of wiring, conduit/EMT/J-boxes, enclosures including power supplies/transformers, etc. and more.  For the sake of this subject, I will refer to these components as “field devices.”  How much of the system beyond the “brains” should also be replaced?

The simplest approach to designing a BAS retrofit is, of course, to specify that everything is replaced.  However, this may not provide the best value to the owner.  The cost of field devices represents a substantial portion of a BAS installation, yet they are neither part of the system’s “brains” nor do they all share the same expected length of useful life.  Instead, field devices need to be evaluated to determine the optimum combination of replace vs. reuse…the devil’s in the details.  The following are suggestions about how to negotiate these details without the need for excessive design efforts.

How Do You Judge the Useful Life of Field Devices?
There is no easy answer to this question.  I usually start with 20 years and then look at various other factors to decide if it should be more or less.  These factors include the physical condition, the owner’s experience with the reliability of the device, whether it is readily compatible with a modern BAS, and any experience I may have had with the device’s accuracy/dependability.

Field Device Compatibility With a New BAS?
Most field devices will work with any brand or generation of BAS’s.  The obvious examples include conduit/EMT/J-boxes and power supplies/transformers.  Point wiring can also be a safe bet if it is 18ga. UTP, but not communications wiring (especially if the retrofit involves a change in the communications protocol).  Most sensors/switches/relays are also compatible unless something one-off was used.  Enclosures seem like a no-brainer (and their useful life can be quite long if treated gently), but reuse may still not be a good idea.  If in doubt contact a couple of BAS contractors that you trust and discuss your device compatibility concerns or just specify that they are replaced.

contemporary Additional Advice For Some Specific Field Devices

  1. Actuators/Valves – Replace pneumatic actuators where possible regardless of the age (the electric-to-pneumatic transducer & often-needed pilot positioner adds complexity & reduces reliability, and they don’t easily allow for position feedback).  Electronic valves should be observed for smooth end-to-end operation.  Valves (especially those with integral actuators) can have a greatly shortened life – observing their operation via the BAS (i.e., to see if they open and close fully) helps greatly.
  2. Temperature Sensors – Thermistors or 1KΩ RTD’s are most commonly used.  Their point signaling is via a varying resistance, and there are many different types of sensors each with a different resistance curve.  Some BAS’s can be readily programmed with any sensor’s resistance curve while others not so much.  Again follow the above “if in doubt” advice.
  3. Conduit/EMT/J-Boxes - Generally I see no reason not to allow the re-use of these regardless of age unless they look in bad shape.
  4. Enclosures – Regardless of the condition of an enclosure I’ve seen bad results when they are reused.  The reason is that they will be rebuilt in the field (using as much of the existing innards as possible), which typically leads to shoddy results.  New shop-built enclosures are probably best.

Final Word
Regardless of the above, there’s no reason for the specification to require the contractor to re-use anything if they are concerned that it might be a weak link in their system.  Just say “may be reused” and leave it up to the contractor to make the final decision.


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