 Good morning. This will be the concluding lecture in this course on an introduction to explosions and explosion safety. As we discussed during the earlier 38 to 39 lectures, something on what constitutes an explosion, what are the different types of explosions and also how to model the different types of explosions, be it a gaseous vapor mixture in confined and unconfined geometry, be it a solid explosive, be it a liquid explosion, be it something like a physical explosion. We looked at the modeling of the explosion and also determined what are the consequences, what are the damage, what is the type of blast wave, what is the type of over pressure, what is the type of impulse, what you get from an explosion namely the consequences. And by now we are very clear about explosions in general and also we talked of inheriting vents and all that to bring down the destructive nature of the explosions. And having said that, we also in the last lecture looked at quantification of damages from explosion. We looked at those response curves, we looked at the probit and now we are in a position to say well an explosion is something undesirable and an explosive, it could be a gas, it could be a liquid, it could be a mixture, it could be anything, it must be handled with care. That means the storage of the explosive may be the transport of the explosive, also may be the generation of the explosive plus storage plus transport plus the actual use that is in application must be done with some safety precautions in mind. But whatever be the type of safety criterion we use accidents do happen. Like for instance I am very careful to cross the road, I am very fearful of the traffic, I take all precautions still somebody can come and hit me and I could still get hurt. Therefore accidents are something which are unplanned activity. Being unplanned I think it is necessary to see under what conditions accidents can take place and also ensure that these accidents in the use of explosives do not generally occur. Therefore, let us get into some little more details of accidents when we say an unplanned activity is an accident and there is always a risk involved in whatever we do and there is a greater risk in use of explosives. Therefore, if there is a risk can you evaluate or identify the risk like for instance identify what is the probability with which an explosion can take place in a given set of conditions and if we are very clear about it then maybe we can take some steps to counteract the occurrence of these explosions. Having said risk and having said something about accidents, you know there is always a chance that an accident will occur. When I say chance what is it I mean? Well there is a possibility and when we say chance in Arabic the word chance is known as hazard and therefore we say a substance is hazardous. From the word hazard you say hazardous and when we say hazardous substances we say explosives are hazardous substances and have to be handled with care. When we say hazard we mean potential for an accident to take place. That means we need to reduce the potential and that is where we see all substances like liquid fuels when they are being transported they say hazardous substance please be careful and in case there is a spill maybe you could form an explosive substance. Therefore we talk in terms of hazardous substances and potential for an accident to take place is always there and that is the meaning of the word hazard and when we talk in terms of risk well the risk should be so small that means the chances of an accident to take place must be so small that maybe we should be able to ensure that an accident involving these explosives do not take place and that is why we do the risk analysis. Having said this let us also take a look what is it we are looking for. We say we cannot zero the or make the chance of having an accident to be zero. Therefore all what we can say is make it as low as practically possible or as low as reasonably possible or we call it as ALARP. This is the word we use namely we would like the risk to be as low as reasonably possible and let us work towards this and in this class today we will work on the subject of risk analysis of the hazardous substances namely explosive. You know there are different types of doing a risk analysis for instance you have methods like risk analysis maybe you have to be able to identify the potential for causing an accident let us say identify the hazard. We also need to be able to carry out the precise quantification of risk in the risk analysis. We should be also able to do what are the consequences if you do have an accident and therefore the analysis of the consequence has to be done and we must also not forget if an accident can occur more frequently well it leads to more chances of an accident taking place something like a frequency analysis has to be done and there are different ways of quantifying all this and the types of risk analysis which are which are done are something like we say you know you do you say what if analysis you know is something simple what if a spill occurs will it explode under what conditions will it explode maybe I could use this but it is not very quantitative though I could also talk in terms of something like second maybe a safety audit or a safety check in which case maybe I make sure that all the safety procedures are built in but again the safety audit cannot really quantify the type of risk involved I want to find out what is the probability of having a risk maybe the third could be we can talk in terms of failure modes effects and criticality analysis this is known as F mecha failure modes effects and criticality analysis but these three cannot directly give you what is the type of the quantification of the risk involved you know they are necessary because it helps you to understand what are the reasons for the explosion what are the safety features which need to be followed also the different failure modes effects and criticality analysis can come out with something like standard deviations and the type of probability of an explosion but what I do is I do something which is little bit different in the class today I do three types of risk analysis only let us say fault tree analysis the second one which I do is event tree analysis and the third what I do is known as hazards and operability analysis these are the three and maybe in the two we will be able to identify the risk involved and also identify in the process looking at the tree diagram what are the type of hazards and what type of hazards constitute most to having an explosion while in hazard we will be basically looking at the consequence analysis and frequency analysis and coming out with suggestions on having a plant operate under conditions under which the risk is as low as reasonably possible having introduced the subject let us go to the fault tree type of analysis in this what is done is we look at something like a deductive logic namely the final scenario what you have is an explosion taking place let us say that the final event is an explosion you know we start from the final event let us say this is the final event which is an explosion we try to identify the faults or the precursor events which ultimately lead to this explosion like for instance maybe this explosion could have isn't about of some fault over here some may be mismanagement some mishandling may be an ignition source coming in contact with the fuel which is should not have done maybe this fault could have come from some other fault let us say F1 and F2 may be these faults could have come from something like F3 fault and F4 fault or some more basic events and so on this could have come from F5 and F6 maybe these things could have come like this therefore we start from the final event and keep working out what are the basic faults which could result in this particular explosion point one point two when we look at the basic events we can also say a basic event I can always say from generalities like it occurs in nature so many times I can put a probability to the basic event and work out the probability of each of the intermediate faults or intermediate events which are taking place and arrive at the final probability of an explosion therefore you have something like branching out like something like a tree branching out and it is known this diagram is known as a fault tree having said that you know the events which are ultimately the last set of events which cannot be broken down any further are known as basic events and the intermediate faults are known as intermediate events and now the basic events lead to an intermediate event this intermediate event and base may be a basic event could lead to an intermediate event this intermediate event and some other intermediate event leads to some other intermediate event and this will finally leads to an explosion let us say maybe I could have another chain coming over here maybe another fault over here another fault over here and therefore, you have a tree and therefore you work out using deductive logic going from the final event to the basic events let Let us illustrate it through an example. Namely, let us say, I have a fire and what happens in a fire? Well, I need a fuel, I need air, maybe both have to mix. I also need an ignition source and therefore, if I say I want to create, my fire is my final goal or this is my final event is a fire and let us say my base or the basic events what I have are let us say a fuel and air which are mixed, which is available to me, which is basic. I also have something like an ignition source over here. Well, these two lead to my final event of a fire. But how do these two basic events relate to this? That means there has to be an opening for it to communicate with each other. Therefore, the basic events and also the intermediate events communicating with the next intermediate event, these intermediate events communicating with the final event is through gates. Like for instance, we enter a house through a gate. Also, a gate is an opening and the basic events are communicated to the next set of events through gates or openings. There are different types of gates which are possible and that is what makes this particular method essentially very challenging and fruitful. There are three types of gates possible. One is we can talk in terms of an AND gate. We can also talk in terms of a gate which has some conditions imposed on it. Like for instance, we can talk in terms of a conditional gate. What do we mean by these three gates are opening by which the basic events get into the final event? Let us say I have fire. To have a fire I go back to this particular example. Maybe I have a basic event in which my gas mixture is available. I have the basic event in which the ignition energy is available and I am talking in terms of an event in which I have a fire taking place. To be able to form a fire, we talk in terms of a fire triangle or an explosion triangle. We need all the three together. We need the gas, we need this, then only I can result in a fire. Therefore, we say both of these things are required in the opening to be able to form a fire and this AND gate is represented by something like an ellipse with a curve like this. This is what specifies an AND gate. Therefore, we have something like an AND gate. We also have something like an OR gate. OR gate means supposing I want to form a cloud of explosives or I am able to form it. And the different ways in which this cloud could have been formed are maybe there is a tank, an LNG tank. In the LNG tank I have the vent valve which gets opened and well the LNG rushes out, mixes with this and forms a cloud. Therefore, one of the basic event I can talk is maybe the vent valve has by mistake opened. I could also think of a situation wherein somebody tampers with the some of the check valves over here and allows the gas to pass through. That means somebody tampers with your tank and allows the gas to come. And my final event I am talking is formation of a cloud. Therefore, my final event is let us say cloud formation of a combustible cloud. Now I find if the vent valve by itself opens well I could release the gas and I could have a cloud being formed mixing with air. If somebody comes and tampers with it also it releases. Therefore, how is this going to get communicated with the vent valve? How is it going to get communicated with the tampering thing which is a basic event? It could be either one of them can result in the formation of a cloud and this which is either or or is what is the or gate and this or gate is shown in the form of a fish. This is an or gate that means either one of them can do the job. The third type of gate which we call is a conditional gate is one in which may be I have an event over here and I have the final event over here. This is the final event and before this may be unless the pressure exceeds some limit well I cannot have the final event taking place and therefore there is some condition associated or rather we say well some condition is associated for the final to go through and this particular gate is represented by a condition under which the intermediate event or the basic event can go to the final that means this is the intermediate case that means the pressure must exceed some limit temperature must exceed or concentration must exceed this will be the condition and this is known as the conditional gate. We use all these three gates to be able to draw back the diagram of the fault tree diagram which I just now said but in practice the conditional gates are not that much used and we will talk in terms of the end gate and our gate but we do recognize the final the conditional gate would also be similar in application. Now before I come back to the fault tree diagram let us assume let us see what is the distinction between an end gate in which both are required to form the event in which case one of the two events can lead to the formation of the event. Therefore how will I look at it? Let us say that the probability of an event that is a gas mixture being formed is P1 the probability of an ignition energy being available is let us say P2. Now the final probability of forming a fire is equal to has to be less than this probability and less than this probability because both are to be equal and therefore P is equal to P1 into P2 over here. In case the formation of the gas mixture is related to the energy requirements which is not possible but in case let us say one of the basic events is related to this event here well the probability will be greater than P1 into P2. In case of an end gate what is going to happen? Well let us say that the probability of forming the event valve opening is P1 somebody tampering with this is P2 this can independently give rise to this this can independently give rise to this therefore probability of forming this event is equal to P1 plus P2 this is the probability and in case this is related to this may be while tampering the vent valve automatically opens or something then in this case the probability of getting the final event based on these two events is less than P1 plus P2. Normally the events are so chosen that they are independent of each other and the probability is given by P1 into P2 in case of an end gate and P1 plus P2 in case of an or gate. Well this is about it let us now go back and try to form a fault tree and try to calculate the probability of an explosion. Let us try to do it through an example and the example which I take is may be forming an explosion from let us say a spill at home. Let us assume I have a kitchen being a winter month let us say that the doors and windows are all closed in the kitchen I have a cylinder a gas cylinder and the gas cylinder we know contains essentially butane C4H10 you have a pressure regulator you have a rubber hose connecting to my stove over here and the stove essentially consists of burner heads and you have control walls over here. Therefore this is the way I light a stove it is quite possible let us put down some clauses it is quite possible that may be the the person using the kitchen forgot to close a valve to close the valve of the stove with the result may be the gas comes and into the kitchen which is locked which is closed windows are all closed doors are closed may be in the particular volume of the kitchen I have let us say I form a gas mixture what is this gas mixture may be the butane air gas mixture scenario one scenario two you know you have the cylinders which are recycled all the time it is quite possible that the regulator of this gas cylinder is so old may be the person who supplies it gives me a defective regulator and the regulator leaks and this regulator leaks and I could also form a cloud in the kitchen coming from the regulator leak these are the two sources of the leakage and well these two sources of leakage form a cloud of butane air mixture in my kitchen and as usually you know people do not observe it well the man in the house he is used to smoking he smokes a cigarette while coming into the kitchen therefore the ignition source or energy source could be his cigarette burning or the children in the house they just come and they find some smelling kitchen something is there in the kitchen they try to put on a switch and the spark from the switch or the electrical switch can result and electrically the spark from the switch can cause the gas mixture to ignite and it explodes therefore we say well in this particular example you have an explosion taking place and the explosion could come either from a gas cloud which is formed either from a valve which was left open unintentionally you also see a regulator can cause this these two are basic events formation of a cloud is an intermediate event similarly I could also have something like an ignition source which is an intermediate event and this ignition energy could come either of the basic events like for instance a cigarette burning the man smoking or let us say a child comes and puts on a switch and releases some energy therefore these four are the basic events these two formation of a cloud and this ignition energy are the intermediate events and these two intermediate events feed into the explosion how they feed they have to feed through the gates over here gates over here I have a gate over here what should these gates be let us let us now plot it in terms of the gates we say this or this can lead to this because either the valve opening or a regulator can lead to this cloud therefore I will have something like a gate like this over here I will have either the smoker who really who has the ignition energy because his cigarette is running hot it can start a fire or it can start the explosion or the electrical spark can start an explosion I have something like an or gate over here leads to the intermediate event and I find well now I am left with this intermediate wherein a cloud of the gaseous mixture is formed I also have ignition well to form an explosion I need both a fuel triangle fuel and oxidizer which is formed over here I have the ignition source over here this is also required therefore I have an and and gate over here and therefore if I have to draw the fault tree diagram for this I will have the following I have let us say the final event is the explosion in the kitchen well I go through it through the and gate and the and gate comes from two intermediate events the intermediate events are having a combustible mixture or cloud one having the energy for ignition how did the combustible get cloud got formed through the or gate I have either the basic event of maybe a valve of the stove being left open or maybe a pressure regulator being faulty faulty pressure regulator similarly I have another or gate over here and what are the two basic events coming over here one is maybe somebody smoking the man in the house smokes and the other thing is maybe a child puts on the electrical switch and causes an electric spark therefore this is the way we are able to get a tree diagram for the final event in terms of the intermediate events and in terms of basic events through an and gate through two or gates now I want to calculate the probability of having an explosion let us say probability of the lady in the house forgetting to switch off is let us say P1 probability of the cylinder being supplied with a faulty pressure regulator is P2 then in this case the probability of forming the combustible cloud is through or gate P1 plus P2 and therefore I come up to this let us go to this the probability of something somebody smoking is let us say P3 the probability of having an electrical spark is P4 therefore the probability of having an energy release coming from both these things which are through or gate is P3 plus P4 now I have these two intermediates this is has a probability P1 plus P2 energy release has a probability P3 plus P4 this is through and gate therefore the probability of forming the final event or which is the explosion is equal to P1 plus P2 into I have P3 plus P4 now all my have to do is I want to calculate a net probability if I know P1 P2 P3 P4 I can get my net value of the likelihood of a fire or an explosion therefore let us put down some numbers we will try to see see normally when we when we do this the probability for these things can be conjectured or can be taken from the large number of data which is available the probability of somebody leaving the valve open you know absent mindedness maybe tiredness could be something like 0.01 the probability this is quite low because the pressure regulator there are some standards to which the manufacturer has to associate maybe the gas people have to give better one but still in some cases they do give faulty one let us say probability is 0.001 the probability of somebody smoking is quite large let us say 0.1 the probability that the child you know finds something smelling and tries to switch on the switch and see what is wrong could be again let us say 0.1 therefore the probability of having this is P1 plus P2 that is equal to 0.1 plus 0.1 0.2 over here the probability here is 0.01 plus 0.001 that is 0.011 over here and when I multiply these two over here P1 plus P2 into this it gives me a net probability equal to 0.011 into 0.2 which is equal to 0.0022 and this is how we get the probability of finding getting an explosion from these values of probability this is what we do in the case of a fault tree analysis when we talk in terms of fault tree analysis we find well basic events cannot be broken any further if I keep on going further and further like this if my length of my faults are progressively increased then my final probability will probably come down because you know each stage requires something and if I can calculate something like a lot of processes involved or if the final explosion involves a lot of faults then probably my probability will be lower 0.1 second you know the minimum set of these faults is what we call as a cut set and you know if we can from such type of examples we can also find out which are those pockets or which are those gates or which are those basic events which have the largest value of probability and which lead to higher probability number and we can focus on them to bring down the probability or the risk of having an explosion this is what we do in the fault tree analysis. Having said that let me take you through one more example which I show on the slide over here I consider may be a pipeline you know we talked in terms of pipeline bursting we said at propane liquid propane this happened at Port Hudson we said in in Siberia we had the LNG I am sorry gaseous methane line which gave way and there was a huge spill and therefore we are interested in finding out the final event is let us say a huge fire which results out and how did the flammable mixture get formed well in those cases it was essentially a weld failure it is also possible that you could have a manual opening you have the at some places in the in the pipeline you have see the gate valve or where valves are there somebody might have opened it out therefore you have probability that a weld failure is P11 the probability that a manual opening takes place is P12 well both of them either one of them can result in a flammable mixture that means I go through the OR gate you see the shape of the OR gate over here the probability of forming a flammable mixture is P11 plus P12 similarly may be along you have a gaseous cloud somebody can walk with a cigarette in his hand and ignite the cloud and this has been the case in case of most forest fires all over the country in different places well cigarette seems to be a cause of majority of the fires I could also have a car which travels next to the leakage line and the friction spark because of the car tire hitting some stone or hitting the road or hitting someplace creates a spark the probability of having a cigarette burning is P21 P22 therefore the possibility of having an ignition stimulus is P21 plus P22 being a OR gate and I need both these things therefore I have an AND gate which communicates to my final fireball and I have the probability being P11 plus P12 into P21 plus P22 that is the summation of these probabilities the summation of these probabilities multiplied together is what gives me the final probability and this is how we do the risk analysis for involving a fireball and explosion in other words we try to go from top down through a deductive logic come to the basic events and keep on progressing till a final explosion is formed. This is what we mean by a fault tree analysis it is very successful it has been applied for space for being able to find out what are the defects which could have which could lead to a loss of mission. Having said that let us go to the next one namely the event tree analysis you know this is also something like a tree but rather than using the deductive logic which we used in the fault tree analysis we use the inductive logic what do we mean by inductive logic what we do is we start from the basic event and proceed further till we get the final explosion. How do I do this let us again take a look at it let us say my basic event is over here and I start from the basic event I go to the next next event may be the basic event could either outcome could be let us say a successful event or a unsuccessful event let us say what is it I mean may be the I have a gas mixture it either ignites or does not ignite successful non successful from this successful I could have again another success over here other non success event over here I could again have another success or non success I could again have another one and so on till I have my final explosion and therefore I march from my basic event till the final event which an explosion and this is what is done in an event tree analysis for each of them I could have a probability of this happening over here this leads to this probability here this probability leads to these two probabilities here and I calculate my final value. Now what is done in an event tree analysis is let us try to put it down I have let me take an example and nothing better than through examples we study let us take the example of an LNG in a pipeline catching fire and exploding therefore let us say there is a leak in LNG pipeline conveying a pipeline conveying LNG this is the basic event well what happens is may be the LNG leaks forms a cloud and the cloud either ignites or it does not ignite no ignition this point one the gas that ignites could either explode that means forms a strong blast wave that means it explodes something like a detonation well it just forms a fire and burns out let us say that means it forms something like a fire or a flame when it is no ignition let us say it just disperses out I will come back to this a little later a more better example of even this disperse gases can again create a havoc let us just now put down this therefore we say well the final event is an explosion just to illustrate this is the explosion in this case it burns out no harm is done therefore I want to calculate the probability of this therefore I say the probability that an LNG pipeline coming out and ignites it is quite common may be in summer month I have a cloud it definitely is going to find an ignition source let us say the probability is point zero eight the no ignition is the balance 0.2 is the probability that it does not ignite well it just disperses off when it ignites well the possibility of having an explosion if there are lot of blockages like trees buildings and all that well the probability is point five well the probability of getting a flame is point five well the probability of forming an explosion is point eight into point five that is point four is my probability of forming an explosion in other words we start from the basic event go forward and go forward till we reach the explosion and this is what we call as an event tree analysis while in the fault tree analysis we denote the basic events by circles and the intermediate events by rectangles and the final active final event by a rectangle in case of the event tree we show all by rectangles and just keep on multiplying the probability till we come to the final event. Let me take one example a more strategic example more illustrative example of having some more links in the event tree and this I show in the slide over here you know I am considering the case since there is so much interest in hydrogen you know supposing there is an hydrogen leak the hydrogen leak comes out there are two possibilities either it finds an ignition source and gets ignited the probability of this ignition is zero point five five it does not find an ignition source and therefore it gets accumulated. Let us assume in this particular example that a cylinder of hydrogen is being carried through a tunnel and this tunnel is a long tunnel and therefore in the tunnel the hydrogen leaks like what happened at Stockholm in the in the downtown area there was an hydrogen leak and therefore you know maybe it does not ignite the hydrogen accumulates within the tunnel or within the space and this accumulated hydrogen after some time finds an ignition source and if you find an ignition source it could either explode or it could form a fire but the accumulated hydrogen is more dangerous because it has had a lot of time for it to mix and prepare itself well and when it catches when it burns in a confinement well it is generally going to explode rather than form a fire therefore the probability of forming an explosion is zero point nine five while forming a fire is lower at point zero five or maybe this accumulated hydrogen does not really meet an ignition source it just disperses well the the hydrogen leak which immediately ignites well it burns as without an explosion that means as a flame the probability of this is higher because it immediately ignites therefore it just catches fire and the probability of having an explosion is much smaller at point two therefore you find in this case if the hydrogen leak immediately ignited the probability of forming an explosion is point five five into point two which is equal to the net probability of an explosion is point five five into zero point two which is zero point one one while if it forms an explosion here well no ignition probability is point four five and point four five into point nine five gives me point four two seven five therefore this accumulated hydrogen is capable of having a higher probability of an explosion compared to the hydrogen leak which immediately ignites well this is also a case of an event tree you are going from base to this but we do find well you should not really accumulate gases especially in confined geometries or semi-unconfined geometries like a like let us say a tunnel or so in which I would have an explosion getting back to something on another example involving hydrogen usage using fault tree is maybe I could have a defective I could have a tankage of hydrogen having a defective wall human error in opening the valve forming a hydrogen air cloud whose probability through the or gate is p1 plus p2 over here I have an ignition source it could be electrostatic spark as we said smoking earlier I could have probability here p2 p21 plus p22 you have an AND gate and the net probability of forming an explosion through the fault tree is this and this and this is the basic difference between this example and an example having the event tree. Therefore these are the two methods with which we evaluate the final probability of an explosion but more importantly we also find while going through the paths which are the paths which are more likely and we try to plug in those things and ensure that the final event of an explosion does not take place with this let us come to the last part namely the hazard analysis and what is hazard analysis let us just put it on the board we have hazards hazards and operability analysis actually got started with chemical engineers in the imperial chemical industries in UK wherein they wanted to apply it for maybe making the plants more safe namely the chemical plants and therefore what is done in this method is a little different let me take you through the discussions on the slide you know in this what is done is maybe people sit through on a table discuss what are the different things which can go wrong but to be able to think and discuss a suitable number of guide words are given with which they can stimulate their thinking and discuss on the different types of processes which are possible which can lead to something going haywire something which will cause an hazard. To stimulate thinking and discussions on possible hazards at each step of the process are what is done and what they do is for each of the process parameters or each of the processes they say what are the process parameters and what are the process parameters normally well we talk of flow so much mass flow rate or so much volume flow rate we talk of temperature we talk of pressure we talk of concentration we could talk of some other process parameters for each of these process parameters we use certain guide words to find out whether some of these we want to find out whether some deviations in these process parameters and can result in something like a hazard and for that let us take a look at guide words that means at each step of the process we look at at maybe the flow the temperature the pressure the concentration using some guide words and what are the guide words guide words are no not none first guide word second guide word is more higher greater third guide word less or lower fourth guide word as well as fifth guide word part of the sixth guide word reverse and so on different guide words when I say no not or none what I mean is implying that there is no pressure that means if I go back the process parameters are there is no flow there is no temperature there is no pressure there is no concentration what would be the consequence similarly if I say guide words more higher or greater what I mean is well the flow or the pressure becomes larger when I say less or lower well the flow rate or the pressure or temperature becomes lower when I when I say as well as I mean qualitatively additional change is taking place when I say part part of it means a qualitative decrease when I say reverse well the flow takes place in the reverse direction that is the opposite of what we want to do or logical logical opposite therefore using these guidelines and the process parameters what is done is we try for each of the guide words find out well if the flow has to stop or if there is no flow what are the possible causes which can lead to this may be if there is no flow what are the consequences which will be there and if there is no flow how do I correct for it and keep listing out at each step of each of the sub processes we keep doing this for each of the guide words namely no flow higher flow less flow may be part of the flow may be less of the flow may be reverse of the flow and so on for each of the process parameters you keep looking at the possible causes possible consequences and then based on all the listing which is made may be may be make a set of inferences than what can go wrong how major would be the consequence how often will it occur that means the frequency of occurrence and based on all the listing done how to prevent its occurring these are the inferences which are done based on the guide words as applied to the process parameters once this is done you know we are interested in the consequences we are interested in the likelihood of the explosion occurring or the frequency well if you find that the consequence is severe that means based on the guide word based on the different process parameters if we find that the consequence is severe we say it is category one if the consequence is medium we say it is category two if the consequence is very low or something like not medium but low side of medium we say category three and if the consequence is very low that means hardly any consequence we say category four. So also when we see the likelihood of occurring of the explosion or likelihood of occurring of the final event which is undesirable that means likelihood is something like frequency we can also catalog the frequency as being category one when the frequency is large when the frequency is medium category two when the frequency is low medium we say category three and then the frequency is low as category four. Therefore these consequences and the frequency are put together in the form of a hazard and operability risk rating procedure wherein both the consequences and frequency are put together to get the net risk rating as either A, B, C or D categories and how do we do this well we say consequence one is more severe and if the frequency is also very severe we give it the grading D which is a grading which is a bad grading the risk is very high well I should not go ahead with it. If the consequence is let us say medium low and if I say well the frequency is still higher that means the frequency of occurrence is still very high but the consequence still I give the grading low because well the frequency is going to occur even though the consequence is less I still have the problem. If my consequence is low medium low and if the frequency is still high well I cannot operate it because my plant is going to be always shut down because the frequency of occurrence is very large but if my frequency of occurrence is let let us say if my consequence is somewhat very low and the frequency is higher maybe I could still tolerate it because you know it may just result in minor shutdowns and I can do it. Therefore according to the consequence and let us take maybe consequence one if consequence is very high but the frequency is let us say very low you know you still have a high value of B because the value the B is something which is acceptable you know once in a way you have a high consequence but this is something which is not very desirable because if I have something like a Bhopal gas tragedy which occurs which happens once in a lifetime well the the consequences are very bad therefore B is something which can be acceptable risk that means it is as low as reasonably practical it is category B but we have to take some steps such that we make some checks and balances such that we we tend to operate a plant when it is under category B but we make sure we do some checks on it when we the others are all in between when I say that the frequency is very low at 4 consequence is very low at 4 well there is no no issue at all this is a safe that is there is no risk involved when I say risk C is undesirable risk maybe I have to improve my plant operation if I say risk is D well it is a risk in which it is not worth undertaking this risk is not to be pursued on you must improve the plant to either B or preferably to A B is acceptable risk and as I said earlier as low as reasonably practical and this is category B we put all these different categories in the form of a plot in hazop that is a hazard and operability analysis and what we have is I show the consequences here the frequency over here and what I find is if the consequences are very large well the thing is the if consequences are very large but the frequency is very small then maybe after some checks I still have to accept it if my frequency is very large but the damages or the consequence is very small well I have to accept it this is the region in which I have acceptability that is region B over region C I am sorry region B is what we said earlier we said acceptable is B we have region B over here and this is the region C and D which is not really acceptable therefore the hazard plot gives the acceptable range of consequences and frequency and this is what is done in hazard analysis namely you evaluate the consequences you evaluate the frequencies and put everything together in terms of A B C and D categories and and evaluate the risk in these four categories and this is what is done in hazard analysis but we must remember that the hazard analysis is carried out maybe even before the plant gets commissioned during the design phase itself we sit on each of the processes in the plant and make sure that no untoward thing is possible each and every element in the plant is considered and also maybe provision of alarms automatic shutdown automatic starting provision of relief valves etc are considered while doing the hazard analysis and this is this is the third type of analysis namely hazard analysis therefore to summarize we in this class have done something on risk analysis evaluating the risk through three methods namely the fault tree analysis second the event tree analysis and thirdly the hazard analysis and these things give a good picture on what is the final probability of an explosion taking place what are the safety features to be put in but while talking on safety features something which we have to keep in mind is we have to keep in mind whenever we talk of explosions of the explosion triangle may be fuel may be an oxidizer may be an ignition source we talked in terms of inerting we talked in terms of critical diameter we talked in terms of maximum safe operating of operating gap such that an explosion does not occur and does not get transmitted to something else but when we talk of ignition you know it is possible to inert it is possible to vent out the gases it is possible to build in the safety features but when we talk of ignition sources especially when we talk of electrostatic sparks you know it is possible you know I work in an environment I walk the human body can get charged to a voltage of 25 kb and maybe my body when it is charged and I touch something it can also create an electrostatic spark therefore from safety point of view maybe if I can wear something like a on my wrist some something a chain which is on the ground which conducts away the charge I or I have something like a wrist stat or something like a leg stat on my leg I have a chain which is in contact with the ground and it gets grounded and no spark is formed well I can avoid electrostatic sparks but we also can remember that if I have a room and if it is more humid electrostatic sparks are more more unlikely and therefore we have we must devise means by which I can isolate an ignition source from an from form from the combustible mixture and even the combustible mixture by suitable inerting or when I have an explosion taking place by suitably venting out I can I can decrease the degree of the consequences of an explosion therefore this is all about the course on introduction to explosion and explosion safety we covered the different aspects including the safety aspects the consequences the reasons for causing an explosion and how to go about preventing an explosion thank you announcement please the last set of lectures between lecture numbers 33 to 40 covered condensed explosives the TNT equivalence of explosives and their yield atmospheric dispersion quantification of damages and risk analysis further references on these topics and a few homework problems are given in the downloads of this video course