"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 [email protected].
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.
The UPS does not help. Note that similar coupling of the electromagnetic pulse from nearby lightning blew out my home computer via the Ethernet cable, even though my computer was on a UPS.
A: Lightning protection is needed whenever there is no guarantee that both ends of the wire will have about the same potential if lightning strikes. If you can establish a ground plane under everything with only one connection to the earth, you might be OK. This usually isn't possible if electric power or other wires leave the area of the ground plane.
Lightning currents can be huge, so even very low resistances can develop damaging voltages from one end to the other. The magnetic field from a lightning bolt changes in sub-microseconds, as does the electric field, so you have more than enough electromagnetic energy to get through cable screens. Conduit is better if it doesn't carry lightning current.
People who are serious about lightning protection put up tall towers with excellent earth grounds that are checked regularly. Lightning will prefer to strike the tower, which provides a cone of protection at about a 40° to 60° angle from the top, enough to cover buildings.
The trick is to isolate the tower lights from any other electrical circuit. Once the lightning is in the ground, it will dissipate over an area. That is why four-legged animals will die, but humans with their feet together may live when lightning strikes a nearby tree. Being inside a building is not enough, especially if the building has exterior lights.
This is a problem in risk management. Balance the chance of a lightning strike and its cost against the cost of lightning protection and the probability that it will be effective. Fiber-optic and wireless connections remove a lot of the risk.
The PolyPhaser division of the Protection Technology Group (www.transtector.com) has specialized in lightning protection for many years, especially for antennas. Its web site has good information about lightning and grounding.
Transmitter companies such as Rosemount offer external protection devices. Talk to PolyPhaser or MTL about what to do at the marshalling panel end.
I don't know of any device that will survive a direct hit, but a grounded metal building can protect even against direct hits.
A: Internal transient protection is relatively simple to implement in a 2-wire transmitter because there are only the two wires and the case or shield connection to worry about. In my opinion, all transmitters should have internal transient protection. I include it in all transmitters that I design. In my opinion, it should not just be an option. It should be included in the transmitters of all three cases that you list. I use transient protection zener diodes with at least a 600-W peak rating, such as the Littlefuse model SMBJ33CA.
Transient protection is not protection against direct or nearby lightning strikes. Nearby lightning strikes can cause a peak current flow of thousands of amps, and protection against that is not practical to include within the transmitter (even a very beefy circuit board copper trace can be burned-off). A direct lightning strike may cause the entire transmitter to vaporize or leave a melted mass of remains, and I don't know of any device that will protect against such an event. Some optional equipment is available, as you mention, for additional protection against damage from nearby lightning strikes.
In cases where cable runs will go outside a building and not underground, then you may want to consider the additional external protection.
David s. Nyce
A: Years ago, I normally used a metal oxide varistor in combination with a zener diode (in parallel, but with a small impedance in between). That was because the MOV could take a lot of peak power, which a small, inexpensive zener could not, but the MOV by itself was too slow to protect some MOS ICs. One problem with MOVs is that they can become damaged with repeated activation within their specified rating.
Now I use a higher power zener and no MOVs, because the higher power zeners are now available with a relatively small size (smaller than an equivalent MOV) and at low cost.
When building a lightning protection circuit (as opposed to just a transient protector), the first element in the circuit can be a high-voltage fuse, then a small impedance, followed by a spark gap of around 60 V (The spark gap is for the really high currents of a nearby lightning strike). The spark gap is followed by a small impedance and a power zener diode (around 33 V). The spark gap is available as a packaged component, with an arc-over voltage rating (usually around 60 V to 90 V). Extra-heavy traces are needed on the PCB that are able to support the current that will flow through the small impedances, into the spark gap and zener, for the amount of time until the fuse will blow. The spark gap and zener can handle repeated activations without degradation or failure.
A: You ask if transient protection is required for the transmitters if your listed three conditions are fulfilled simultaneously?
If the transmitters are "smart" (i.e., microprocessor-based), I would strongly recommend the use of internal transient protection unless the following additional conditions were all met:
- Your items #1, #2, and #3 and building/equipment/raceway grounds are industrial-grade level of compliance.
- The building is steel-framed (or grounding-conductor-framed) in a way that provides all equipment with the proper cone of protection.
- Further, the power distribution system in the building has a coordinated surge protection system in place down to the branch circuit level.
- The process is not hazardous so that safety is not an issue, because a run-away reaction will not result if transmitter components are damaged.
- Loss of process uptime is not an issue.
- Damage to downstream equipment (e.g., analog inputs) is not an issue.
- Transmitters are located in the same ground plane as the devices they are wired to.
- The plant site is in a very low lightning strike frequency area.
- PRIOR USE indicates surge protection is not needed.