"Ask the Experts" is moderated by Béla Lipták (http://belaliptakpe.com/), process control consultant and editor of the Instrument Engineer's Handbook (IEH). I am recruiting contributors for the 5th edition. If you would like to participate in this project, please write to me. Also, if you have questions for our team of "experts," please send them to me at firstname.lastname@example.org.
Q: I have two questions.
First, I would like to know how you visualize the filling station of the future and what control challenges that implies?
Second, now, that the presidential commission on the BP accident reported some of its findings, do you still believe that properly implemented process control could have prevented that accident?
A: As to the service station, process control will be key because I believe that, by the end of this century, we will be driving electric vehicles (EV), and will refill them at "battery swap" filling stations. I also expect that, although initially the electricity for recharging the batteries will come from the grid, in the long run, our energy supply structure will become wireless and we will use a more localized mode that depends mostly on distributed local power generation.
At that time, the filling station will have a large, underground, double-walled, cryogenic hydrogen storage tank, which provides the fuel to large fuel cells converting the chemical energy in the liquid or high-pressure hydrogen gas into the electricity needed to charge the depleted batteries of the previously refilled EVs, making these batteries ready to be swapped into the EVs just driving into the station. This design is the most likely to emerge because this way the car can be refilled in a couple of minutes, and can travel at least 100 miles or more before the next battery swap. I also believe that good American process control can help to regain our global leadership, and create an economic boom as we develop the "computer of the 21st century"—the tools of the coming solar-hydrogen economy.
As to your second question, the answer is yes. The simple detection of the presence of oil between the drill hole and the casing pipe and the immediate activation of the safety procedures would have saved 11 lives even if both the BOP and the ROV failed. If you want to refresh your memory about my proposed safety controls see www.controlglobal.com/articles/2010/OilBlowouts1008.html
Q: Today, pressure, DP and temperature transmitters are coming with internal transient protection as an option from most manufacturers. There are also external transient protection devices for transmitters from companies such as MTL. I would like to learn if it is necessary to provide transient protection for transmitters when all three of the following conditions are fulfilled simultaneously:
- Transmitters are located inside a covered building with both a lightening arrestor device and proper grounding, so there is no chance of lightening striking the instruments.
- Cables used for the transmitter are twisted-pair, shielded-screen cables (both individual pair shielded and overall shielded), and screens are kept open at the instrument end and connected to an electronic earth pit (screen earth) at the panel end, so that voltage surges induced in the signal cable due to starting heavy electrical equipment is grounded through the screen earth, and there is also no chance of ground loop current
- The transmitter is powered from IGBT-based, industrial grade uninterruptible power supply (UPS).
Please advice me how to take care of the transient protection for four-wire transmitters, such as magnetic flowmeters, ultrasonic level transmitters, radar transmitters, vortex flow meters and mass flow meters where the power supply is 240 VAC, 50Hz from UPS. We get 4-20mA signal output from the 4-wire transmitters.
Checking the catalogs of Yokogawa, Siemens, E&H, Krohne, etc., I found that internal transient protection is not available in them. Considering that these manufacturers do not provide internal transient protection for four-wire transmitters, do you believe that it not suitable for them?
A: The three conditions are necessary to meet the needs of intrinsic safety (IS), but IS is not always used. Transient protection is required to protect against the accidental application of AC line voltage to the signal wires or shields of the instrument cable.
The purpose of all protection is to keep high energy off the instrument cable. The reason for using IS barriers (even when the field wiring and instrumentation itself meets IS requirements) is that dangerous AC voltages are present in the nominally safe area of the control room or cable marshalling room. Although it is never intended that AC ever be applied to instrumentation cabling, it does happen due to human error. When it does, the IS barrier routes that current to earth/ground.
A: I strongly agree that transient protection is needed. Most of my transmitters were operating at 24 V. I placed 50V metal oxide variable resistors (MOV) at each end of any long or exterior cable runs. On 10-V pressure transducers, I use 15-V MOVs. If there is a lightning strike anywhere near a metal tank or tower, the metal structure will act as an antenna. A voltage spike will couple into any transducer signal or power cables that are run in close proximity to the structure. The lightning does not have to hit the metal structure. Most instrumentation can only handle 10 to 30 volts of common mode signal, so the coupled signal will blow the transducers and maybe the power supply.