Safety Instrumented Systems

Letters To The Editor

Readers respond to overfilling, engineers in Congress and are puzzled by the Puzzler.


I read with interest a fine paper by Dr. Summers on the risk of overfilling []. Let me please share the following for your consideration. Some of what follows is a corollary to what Dr. Summers has pointed out in the "SOLUTION" section of her paper — 
Item 6:  "Determine the technology ……." (p. 33, August 2010).
Along with the technology for level measurement, (e.g., dp cells, radar, capacitance, or many others) proper installation of instruments also should be emphasized. It would not be surprising to find some plants where instrument installation is an "unguided" activity, i.e., there are no procedures for proper installation. Instrument technicians rely on their experience or learn by on-the-job observations of their co-workers. It is conceivable that poor installation practice could prove to be a "weak link" in efforts to ensure a safe system. 
In my view, well-written installation procedures (or guidelines) — not only for level sensors/transmitters but for all instruments — would serve two broad functions:  they (procedures) will minimize failures resulting from a poor installation and will help train younger instrument technicians. 
Finally, a positive note on the methodology for applying instrument systems: The comprehensive practices recommended by ISA 84.00.01-2004 (or IEC 61511) for Safety Instrumented Systems (SIS) follow a system's approach along with safety life cycle philosophy. These approaches collectively help minimize instrument failures, including those for level measurement, and can be applied to the basic process control instrumentation as well.

GC Shah
Houston, Texas


[This letter is in response to the September editorial "We Need More Engineers in Congress."] In my opinion, I think the reason few engineers want to be involved with current politics is that truth and logic rarely are used to make decisions, at least that is the view from those of us on the outside.  

In Washington, deals to support others' pet projects in order to gain support for your own pet projects seems to be the norm.  

It is rare to see any elected official (local, state or national) do "what is right for the country" if it does not gain them personal favor with their constituents.

I agree that term limits may delete a lot of memory and knowledge from Congress, but it would also force a global outlook for all members since there would be a limited time to be effective. Also, the elected officials would not spend the majority of their political career working to maintain their political career (as it is now) but instead spend time trying to do some good for the country.

Why would any logical, reasonable person (which I think describes most engineers as well as a lot of others) want to be involved in a profession that is highly criticized, blatantly out of step with the normal man, and that is usually rated with the lowest positive ratings of all groups?

Paul D. Douglas,
Shreveport, Louisiana


I know that this comment is a little late, but having just read my July issue of CP … I am somewhat concerned that actions to address the problem discussed in the Puzzler [] not lead to undesirable or even unsafe conditions.

One correspondent, Dr. W. Genck, was correct in his concern over the system suffering from a distance/resistance loss.

If the existing plant instrumentation was, as suggested by Dr. Genck, old, then it probably would not be of high impedance; which could certainly produce large errors. That does not represent, however, the substance of my concern.

Being retired I have no access to facilities whereby I could check the precision of my statements (it requires access to appropriate (NIST?) standards), so my memory might be suspect.

I believe that the required accuracy for commercial thermocouples is ±3.0°F and for premium couples it is ±1.5°F. In order to achieve any benefit from use of premium couples it would also be necessary to use premium extension (compensating) cable which is covered by the same standard. In any event, inspection of the cold junction compensation method/equipment is warranted. Last time I checked with a manufacturer (about 25 years ago) their auto-cold junction compensation accuracy was ±1.5% of full scale.

If the measurement is by RTD then similar problems exist: last time I checked commercial standard RTDs were required to be better than ±1.0°C and premium items ±0.5°C. Other problems can arise with using RTDs according to whether the measurement loops are 2, 3 or 4 wire (whereby the accuracies due to line lengths are respectively large, reduced but affected by Wheatstone Bridge arm ratios, and minimized.)

The major problem I see with either RTD or thermocouple primary devices (and compensating/extension cables) is that the above are stated (my memory limited) as "plus or minus" — thus you could get lucky by having both devices of same magnitude and polarity error, but that is highly unlikely (equivalent of 3¬4 standard deviations?). In fact, unless both high and low temperature devices (and compensating cable?) were check calibrated with high-accuracy test equipment (and skilled test technicians?) then, as I see it, the probability of the measured differential being anywhere +1.0°F is extremely low. They could be looking at a control loop thought to be controlled at +1.0°F and actually being up to 5.0°F. I do not understand the problem of nucleation of the process materials, but it seems like the existence of such errors could be the cause?

W. Brown
Katy, Texas

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