A brief note on HAZOP HAZOP stands for HAZard and OPerability studies. It is a systematic approach to uncover any possible dangers in any engineering idea by raising many questions about the idea even before it is actually implemented. Let us be specific to process plant engineering that we are interested in. We have seen in the chapter on SCOPE of PIPING ENGINEERING various stages in the life of a process. At each of these stages, we have some data and information available to us. We use that to take certain decisions and freeze them. The project then moves to the next stage. HAZOP encourages us to raise several questions about our decisions to uncover any hidden dangers. For example, we have seen that the first diagram that gets developed is immediately after the process chemistry is conceived and tested in the laboratory. We develop a BFD or a Block Flow Diagram. This diagram tells us what are the chemicals involved, what are the main operations involved and what are the chemical steps involved in making a desired product from the raw material. We said that BFD is normally the final outcome of a Chemistry person who develops chemical route for any manufacturing process. Before proceeding further, we would like to pause and question ourselves whether we should now take this scheme further and actually develop a full scale plant for manufacturing. Is it safe to do on a large scale what the scientist could do and demonstrate on the small scale? Is it ethical for us to handle chemicals in tonnes just because we could handle them in a small test tube or a crucible in laboratory conditions? That is where we decided to go for KYC, or Know Your Chemicals. We explored certain hazard properties of the chemicals involved, especially smelling threshold, fatal threshold, reaction with water etc. This exercise itself is a part of HAZOP study. As we move forward and develop the idea further and arrive at a PFD (Process Flow Diagram), we saw that the chemical engineering inputs are in and other engineers are likely to take over further development of the project. We also argued that PFD is at best what a chemical engineer feels will happen in reality based on his idealistic calculations. In reality, we do not have control on several things which would affect our process. For example, day and night temperatures will affect heat loss from our reactors and therefore our reactor temperature will depend on atmospheric temperature. Similarly, rains and wind would directly or indirectly affect our processing. Feed quality will change. Power would fail, etc. What is the guarantee that our plant can overcome these uncertainties and still remain safe? Therefore, we should do HAZOP at a PFD stage. Similarly at P&ID stage and so on.
Safety is a permanent concern and the standard HAZOP procedure should be applied at several stages of a project. Recommended stages are given by ICI (Imperial Chemical Industries who first developed this wonderful procedure of HAZOP) and are as shown in the representative figure below. HAZOP study at the PFD stage (some people do it at P&ID stage) is normally done by what is called as Deviation Analysis. We question everything that a chemical engineer promised in a PFD. He has given us flow rates, composition, temperatures, pressures, flow directions of each and every stream in a process. He has quantified everything in the Material & Energy Balance table in the PFD. We question all that information systematically and debate as to what could happen if there is a deviation in reality from what is given in PFD. How to apply the deviations systematically? What all can be different? What can be the nature of the difference between the intention of a chemical engineer and the actual happening in a plant? All this is captured in the HAZOP very beautifully. What we do is the following. We sit with a PFD, or a flow sheet (or a PFD and P&ID). We focus our attention on one equipment in the drawing at a time. We note from the diagram all the input streams of that equipment. Note that we concentrate only on the inputs, and not on the output streams. This is ok because the outputs of this equipment will be input streams for some other equipment and these streams will be considered when we do HAZOP for those equipment. We now focus on one input stream at a time. So remember, one equipment at a time and one input stream of that equipment at a time.
Now for this stream, we know from the PFD the flow rate, temperature and pressure. We also know the composition. We also know from the diagram the direction of flow for this stream, from which equipment to which equipment. This information is given in PFD by a chemical engineer. We do not trust him. We think, things will be, or could be, different than what he says. To apply this deviation, we focus on one parameter of the stream at a time. So, we have limited our attention to one equipment, one of its input streams, and one of the specifications of that stream (say flow, temperature or pressure). Idea is to limit our attention to a subset of the whole plant and see if we have a danger hiding somewhere. Always focus on a small problem at a time if you want to solve a mountain problem. So far, we have selected one equipment, one of its input streams, and one parameter of that input stream. Now we say; this parameter is not going to be as given by the chemical engineer. It is going to deviate from what the engineer thinks. To capture all possible deviations, we now apply one HAZOP GUIDEWORD to the parameter. Each HAZOP guideword captures qualitatively one type of deviation. Recommended HAZOP guidewords are as follows. NO (or NONE) MORE OF (or MORE) LESS OF (or LESS) REVERSE AS WELL AS For example, if you are focusing on flow rate as a parameter, NO would force you to consider the possibility of flow stopping or no flow condition occurring. MORE will similarly ask you to consider possibility of flow being more than what is given in the PFD. Similarly, LESS will raise the possibility of flow being less than what is expected. REVERSE will prompt you to consider the possibility of flow direction being exactly opposite to what is shown in the PFD. That leaves only one guide word, namely AS WELL AS. AS WELL AS suggests that the composition could allow for a component that the chemical engineer says is not there in the stream. But suppose it is there, then what? That is what this guide word asks you to debate. Can there be an unexpected component in a stream? So basically, a HAZOP guideword applied to one of the specifications of one of the input streams of one of the equipment in a PFD is equivalent to imagining a specific deviation from chemical engineer s intention as recorded in a PFD. You are now supposed to ask yourself the following.
1. Is such a deviation possible in real life? If you think it is impossible, then what is the problem? Apply the next guideword to the stream specification. If you think it is possible, then ask yourself the next question. 2. What will be the consequence of this deviation? Will the plant explode or what? Even if it does not explode, what will be the consequence is a very pertinent question to ask. Will the process be affected? Will there be something wrong that may start to happen? Will the chemicals be released to the atmosphere and cause damage to environment? Will something get overheated? Will there be an emergency situation? Will things get out of control? Think through all that can go wrong. Note it down. However, if you think that the consequences are not going to be serious, then forget about it. Go to the next guideword. 3. If any of the consequences is bad from safety point of view, health point of view, environment point of view, do not ignore it and ask yourself the following question. We talked about Safety (S), Health (H) and Environment (E) in one breath. We talked about SHE. HAZOP is all about SHE. 4. What measures you should take to keep the situation under control and stop the consequences from reaching dangerous levels? You may recommend certain alarms, trips, controls, extra equipment, monitoring instrumentation, shut down, evacuation, showering the equipment with water, etc. depending on the perception of the hazard involved. If this way we exhaust all guidewords for each specification of a stream, all input streams for each equipment, all equipments in a flow sheet, we would have seen through all hidden dangers in any plant that we are engineering. HAZOP is a systematic thought experiment. You are running a plant in your thought process, even before its blueprint is ready. HAZOP is known to avert potential accidents. HAZOP is known to help quick commissioning of the plant. That is obvious because you have already run the plant in your mind once. HAZOP is known to minimize any changes at the last minute before commissioning. HAZOP is as good as a mock test of the plant. It is net practice in Cricket. It is a practice game before a big match. HAZOP is the only guarantee of process safety. HAZOP is never done alone. It is not an on-line course on Piping Engineering. It is always a team exercise. You should have in the team someone who knows process, someone who knows site limitations, someone who knows financial implications of any suggestion, and someone who knows engineering aspects of the project. Being a team exercise, HAZOP study team also needs a captain. He (or she) is called Hazop Study Leader. He is the one who will systematically raise all questions. He is the one who will ensure that the team is not taking short cuts to finish the job quickly. He is the one who sees to it that no one in the team dominates the proceedings and no one is subdued or snubbed. He is the one who ensures that the exercise is carried out in the proper spirit and meticulously. If HAZOP study is carried out only as a mandatory requirement and as a ritual, its purpose will be defeated.
HAZOP studies are very laborious and time consuming. But, what could happen if something goes wrong with your process plant is so scary, that we better fall in line and do HAZOP. Make HAZOP a habit. Next time around, whenever you do something, do HAZOP. For you to practice HAZOP a little, see a small flow sheet below. It is for making polymer from a monomer. Say PVC (Poly Vinyl Chloride) from Vinyl Chloride Monomer. It involves pumping monomer from a storage tank, heating it to a higher temperature in a heat exchanger and passing it through a reactor where it polymerizes. Assume that the composition, flow rate and pressure/temperatures of all streams are given to you in a table at the bottom of the drawing. Do HAZOP yourself. To start, focus on one equipment (say HX), one input stream (say monomer stream), one of its specifications (say flow rate) and apply one guideword to it (say NO). All that we are saying is that the flow rate of the monomer to the HX stops. Is it possible? If possible, what are the consequences? If the consequences are unacceptable, what do you suggest as a remedy so that such a thing does not happen? This can keep you busy for a while. Do it and enjoy how thoughts come to your mind automatically. Complete the HAZOP study at least for the HX. If interested, share your observations with us. If we think you have put in efforts, we will share our study document with you privately.