Portable Gas Monitors- new ways to use them
All of us a ware of portable gas monitors, those instruments with the built in sensors that give out an alarm in case they detect the presence of a gas (or fail to detect a sufficient level of a gas like Oxygen). We all know how to use them. We use portable LEL meters, before starting any welding or “hot work” in a hazardous area, we use portable Oxygen gas detectors before attempting to enter a confined space area for working and so on. But can we use these meters for other purposes? Sure, we can. If you notice if you carry a perfectly calibrated Oxygen monitor to a remote unpolluted area, what reading do you expect. 20.8 % Oxygen or even slightly better (I am not considering high mountainous regions that are low in Oxygen content). What reading do you get in your plant near all your reactors and distillation towers and tanks? Maybe 20.6 % or even lesser. What does that say about your workplace? Do a little “workplace monitoring” with these things, you’ll be surprised.
Now for some fun with the LEL meters. Take one of these to all the designated “hazardous” areas (all the Division 1, Zone 1, areas) and the designated “safe” areas and monitor the LEL levels. If your area classification is still current, you should not get any surprises. If not, well, you need to really rethink.
Caveat: A hazardous area (Even a Zone 0 or Zone 1) does not mean that you will have explosive gases and vapors all the time. Of course not! Otherwise your plant is really something that can get shut down by the authorities. But if you take these readings for a sufficient number of times, you can find out if for example the boundaries of your hazardous areas are OK or should they be extended? Are your safe areas immune to those vapors that may creep in when say some manholes on nearby reactors are opened? Are your double mechanical seals on your pumps really working properly? And so on. One round in your plant with these little meters will really give you a picture of how things really are…good, bad or ugly.
Designing Safety Instrumented Systems?- Five things to watch out for
Modern chemical and hydrocarbon processing plants, oil & gas production facilities, power plants and other similar process plants all have some instrumented systems that ensure functional safety. These are known as Safety Instrumented Systems (SIS for short). This post is about SIS and how you can avoid certain pitfalls while designing them. To those of you who are familiar with design of Safety Instrumented Systems, this may sound too basic, but nevertheless its a useful checklist to have.
1. Keep the big picture in mind. An SIS is a Risk Reduction measure, not an end in itself.
Any large processing plant has a certain degree of inherent risk that is associated with operating it. There is nothing alarming about it. The principle applies to any voluntary human activity, like say driving a car. Driving a car has some risk and to counter this risk, one takes some safety measures (wear seat belts, have air bags, keep tire pressure OK,etc). Similarly one reduces the risk of running a processing plant by employing safety measures, one of which is by having an SIS. Thus an SIS is not the only risk reduction measure.
Secondly the goal of any safety measure (including an SIS) is to reduce the inherent risk of a process to an acceptable level. Keep this principle in mind before jumping straightaway into SIL calculations, quad redundant PLCs, etc. Will this system reduce risk to an acceptable level? Is this the only way to reduce the risk? Will it work? are some of the questions that you should ask.
2. Quantify the inherent risk and the acceptable risk.
Make sure that you know what is the inherent risk of your process (either by calculations, or historical records, or other data). This may be expressed in a variety of ways including FAR (Fatal Accident Rate), Undesired Events per year, reportable accidents per year, worker injuries per year and so on. Now also make sure, that you know what is the acceptable level of risk in the same units. This information can be sourced from your corporate safety department, or risk management team.
Now use the equation
Risk Reduction = Inherent Risk-Acceptable Risk
to give you a measure that will define the amount of risk reduction that your system has to be able to do.
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3. Get reliability data regarding your process equipment, instruments and systems before you start the design.
There is no sense in working with assumed or other vague figures. If at a later date the basic data was found to be erroneous, the entire exercise of calculating target SILs, verifications, etc will be pointless. Data can be sourced from manufacturers, third party database providers or your own historical data. Take the worst case figures out of the three sources, for your calculations.
4. Keep an eye on Common Cause Failures (CCFs).
It may sound simple and ridiculous, but sometimes we fail to foresee common cause failures, even in large projects that have several hundred engineers working on it. For example, is your BPCS and SIS powered from the same UPS? The same utility feeder? Could it become a CCF? Does your SIS card and BPCS card share a common backplane? What if the backplane fails-say due to ingress of moisture or rodents? Could it become a CCF? Ask these questions at the design stage itself to save yourself tears later.
For an interesting case study on how CCFs can lay low a very expensive and technologically sophisticated program like the International Space Station, here is an interesting link. A single CCF knocked off all redundant computers in the International Space Station, endangering the lives of the astronauts.
5. Keep an eye on the SIS components, especially sensors and final control elements. (Also ensure that your SIS loops do not use substandard components like cheap terminal strips, poor quality lugs, undersized signal wire and such things).
Are you aware that out of all documented failures of SIS loops, only 8% were related to the logic solvers (Safety PLCs) and fully 92% were failures related to sensors and final control elements. Contrast this with the amount of debate, discussion and time that is spent on designing the logic solver part of the SIS (heated discussions on whether we need triple redundant safety PLCs or quad redundant safety PLCs or something even more exotic).
The reality is that very few people focus attention to the non glamorous part of the SIS loop-the transmitter and the automated valves. Very likely they are the same types that are used in the “normal” loops. Is this a correct practice? Should not you be having a higher benchmark for these? Especially since their performance will ultimately decide the reliability of the SIS loop? Also be careful with your terminal strips. A poor quality termination can cause nuisance trips worth millions of dollars-have a better benchmark for these passive components in your SIS loops.
If you follow the tips above, I am sure you can have a better SIS in your plant. If you wish to learn about Safety Instrumented Systems, have a look here.
Comments are always welcome. You can also add any more tips that you may wish to share with our readers.
Cheers!
Yet another welding related fire-now in a nuclear plant!
As if we did not have enough of welding related fire accidents in conventional plants (read my last post on the issue here ), now we have a report of a similar accident in a Japanese nuclear plant.
Here is the incident reported by various agencies:
A fire broke out at a nuclear power plant in northern Japan on Thursday, injuring one worker but causing no radiation leak, the operator said.
Firefighters put out the fire about an hour after white smoke was spotted coming out of the reactor, which was already shut for a regular check-up, Tohoku Electric Power said.
“One worker sustained minor burns but was not exposed to radiation,” a company spokesman said, adding there was no leak to the outside environment either. The fire started at around 2:00 p.m. at the plant’s No. 1 reactor, which has been undergoing regular checkups since February, Tohoku Electric said.
Kyodo News Agency which first reported the incident said the worker was in a welding operation inside the building, and the filter in the air conditioning system might have caught sparks from the welding.
The plant is located in Onagawa town, some 350 kilometres (220 miles) north of Tokyo. The plant has two other reactors, which are operating normally.
The nuclear power complex, which suffered extensive damage in an earthquake last year, has been out of service and undergoing repairs.
The incident occurred just days after a Dec. 1-5 inspection by a team from the United Nations nuclear watchdog. The team of 10 experts from the Vienna-based International Atomic Energy Agency assessed safety measures designed to deal with the continuing threat of earthquakes.
I hope the investigation is completely impartial and gives us some better ideas to prevent such incidents in future. I know that the nuclear industry is a highly regulated and procedure-driven industry so this incident is shocking. Secondly, this is the second such fire in a Japanese plant (the earlier one was supposed to be because of an earthquake). However as usual, the investigation reports are pretty sketchy and certainly not as detailed as the ones from the chemical /hydrocarbon processing industry (Well, if they are I have not seen many in the public domain). I wonder what kind of combustibles are present in such installations and what kind of gas detection systems are used. Anybody from the nuclear industry who is reading this could be kind enough to throw some more light on this issue.
Comments as usual are welcome.
Explosion and fire at Buncefield Oil Storage Depot - Five companies to face prosecution
It is now almost three years since the Buncefield oil storage depot explosion took place and finally the authorities have declared, that they will be pressing criminal charges against five companies, ostensibly who have been found guilty of acts of omission.
For those of you who do not remember the case, here is a short overview. There were a number of loud explosions ( I mean really really loud-reportedly people in Netherlands and France heard it and it was recorded also a seismic event! ) and a massive fire at the Buncefield Oil Storage Depot in Hemel Hempstead, Hertfordshire, UK. Over 40 people were injured in the accident, fortunately there were no fatalities. Following the explosion, a Major Incident Investigation Board (MIIB) was established by the Health and Safety Commission, supported by the Board of the Environment Agency, UK.
There were a series of investigations and reports being published from time to time by this board and some of the significant findings were as follows -my summary of a rather long series of detailed reports:
a) No consequence analysis was done by any of the design engineers or safety experts, as to what could be the severity of the possible explosions of the flammable vapors generated from the petroleum storage tanks.
b) The level control loop, (that is supposed to control the level in the tank and prevent overfilling) on one of the storage tanks failed. It consisted of a Servo tank gauge connected to a series of valves. This failure led to overfilling and spillage of massive amounts of petroleum into the dikes surrounding the storage tank. Petroleum was being pumped in at a rate of about 550 m3/hr for more than three hours, yet the servo level gauge indication, failed to record any change at all!
However the DCS trend records could be salvaged and the above information was gleaned from them. Apparently the CCTVs were working and the footage showed petroleum overfilling and flowing into the tank dikes, but nobody was watching it at the time.
c) The overfill protection was provided by a point level switch which was supposed to be independently connected to an alarm/annunciator panel (separate circuit from the DCS loop). The panel had an override switch and it may be that the interlock was bypassed (no conclusive evidence since everything got burned in the subsequent fire, this may never be known). However it is warning to design engineers who think that by merely having a redundant level switch is good enough. Were there any common cause failures that both the continuous indication, as well as the interlock failed? Not known for sure.
d) The operators apparently did not notice anything amiss and neither was the control system very sophisticated, to tally the pumping rate into the tank to the rate of change of the level. Now here’s the cake. The pumping rate now increased to 890 m3/hr leading to the petroleum overflowing from the tanks, filling up the bunds and secondary containment areas and forming large vapor clouds. It seemed this occured because the inlet lines were common to all the tanks and the other tanks level indications were working, so the system diverted their inlets into this tank that appeared about half full (due to the faulty level indication). There must have been thousands of gallons of the stuff overflowing from all directions and nobody could notice anything! (Yes, it was about 3:00 am in the morning-but so what- were there no operator rounds of the premises or anything like that?- or it doesn’t happen on the night shifts at all?!)
e) Apparently the hazardous area classification which may have been done during the initial stages, may not have considered wind directions. The entire vapor cloud was carried across the road from the tank farm to an emergency generator building,about 100 meters away, where it is thought to have been ignited. The building apparently was not a classified (hazardous) location.
No doubt this entire catastrophic incident and the consequent investigations will have a major impact on how instruments and controls are designed and maintained in petrochemical/hydrocarbon processing plants, how operator alertness and awareness is important and so on.
More details are available at the Buncefield investigation site.
Note: All images have been sourced from the Buncefield investigation site and all copyrights belong to that site.
