Instrument Damping Question (with Answer & Explanation)
Question – Instrument Damping:
Which of the following is the most correct situation to configure 30 seconds of damping in the level safety transmitter described below?
Assumptions: Magneto-strictive type safety level transmitter with a 4-20mA output feeding High-High & Low-Low shutdown trips for a horizontally arranged vessel.
A) When indicated levels are fluctuating severely due to liquid sloshing in the vessel because of heavy seas (such as on a marine vessel or floating oil platform)?
B) When a magneto-strictive type level transmitter is experiencing high vibrations and producing short high level spikes that could potentially cause a shutdown.
C) When the normal fill rate for the vessel is slower than 25% per minute.
D) Other - Explain your answer in comments.
*For an extra challenge, explain the problems with the less-correct answers.
Answer / Explanation:
Answer A is incorrect. Here is why:
Damping – is a natural phenomenon of most natural measurements that are being acted upon by an outside source. Anytime a change is self-limited it will exhibit this basic characteristic. In engineering & physics this is referred to as a 1st order filter. Damping tends to “smooth out” noisy signals.
Contrary to common misconceptions, instrument damping doesn’t work like averaging and it is definitely not a pure time delay.
The trend below shows the impact of various degrees of damping/filtering during a manual bump-test that altered output from 40 to 50% and observed response.
The Blue line is the real signal (PV) shown with no noise.
The Red line shows typical process noise on the signal. For example temperature fluctuations due to the fact that not all water flowing through a heat exchanger is equally heated and this causes small variations in the temperature reading.
The Green line shows a well-filtered (properly dampened) signal. Note how the signal still rides the proper overall response and is indicative of the actual process variable, but the variations (noise) has been drastically reduced.
The Yellow line shows what happens when a signal is excessively filtered (i.e. too much damping). Note how the signal is very smooth – but also note how it doesn’t provide accurate indication of the process variable.
Answer B is also incorrect. The right answer is to fix the real problem – not mask it by altering the time response of the signal.
Answer C is also incorrect. There is no engineering reasoning (or adequate data) to justify this answer and there is not enough info to justify a very bad practice of excessive damping in a transmitter.
Answer D is the best answer since all others are incorrect. The only situation I have personally seen where a 30-second damping was justifiable was in some chemical reaction measurements (in this case pH and ORP at a WWT plant where the process reaction cycle times were on the order of several hours). Transmitter damping should only be used to eliminate the NOISE from the real measurement signal. In layman’s terms, I tell people do a bump-test in manual and observe the response. You’ll see a graph similar to the first one shown in this article. The job is to dampen enough to ‘mow the grass’ off the hill, but do NOT excavate the hill (or slow the response as shown in the yellow line).
Note – This questions is a bit vague and open because I am hoping to cause discussion, thought, and to bring attention to the issue. This is a super common mistake / problem across many industries and one that will (or possibly has) killed people. Few managers have much understanding of what I&C professionals actually do. For that reason, much of our work is treated as black-box or magic. The assumptions that smooth trendlines = good control or that ‘if you can prevent the alarm or trip the plant must be safe’ are rampant across many industries and is only getting worse due to the growing skills gaps in many industries.
A couple of related notes on this topic:
1) Getting the input signal damping set PROPERLY should be one of the first tasks when looking at any controls efforts (optimization, tuning, startup, commissioning, etc.). Likewise, ensuring the final control elements are all working properly is paramount – but that’s another blog…
2) Improper damping can be a huge safety risk (every bit as dangerous as a bypass or override).
Instrument and signal damping is one of many common problems / misconception areas that we cover thoroughly in our I&C courses. It is important to understand these concepts, because of the frequency of mistakes and because of the potential magnitude of the consequences.
About the author
Mike Glass
Mike Glass is an ISA Certified Automation Professional (CAP) and a Master Certified Control System Technician (CCST III). Mike has 38 years of experience in the I&C industry performing a mix of startups, field service and troubleshooting, controls integration and programming, tuning & optimization services, and general I&C consulting, as well as providing technical training and a variety of skills-related solutions to customers across North America.
Mike can be reached directly via [email protected] or by phone at (208) 715-1590.