Trouble Shooting Thoughts - English - Lecture Notes, Study notes of English Language

Download Trouble Shooting Thoughts - English - Lecture Notes and more Study notes English Language in PDF only on Docsity! Trouble Shooting Thoughts Trying to summarize trouble shooting is difficult. There is so much trouble available. There do seem to be some general guidelines that you should go by though. They revolve mostly around common sense, understanding your process, and getting all the data. One thing that I have found is that wrong hypotheses will explain only some of the data. When you come up with the truly correct hypothesis, all the data will suddenly become clear and all the odd events will suddenly fit together. Frequently, things that you have discarded as being a different problem will turn out to be explained by the TRUTH—when you find it. New problems are often the same old problems with some new appearance. 1. The first priority should be finding out what is really going on. This is harder than you think it would be. The old story of the blind men looking at the elephant comes to mind. Everyone looks at the problem from his or her own point of view and from their own experience. Frequently people will ascribe the problem to some past problem they have experienced. The fact that this one has several different features will not deter them at all. Sometimes they will be right but often not. [Note: These episodes of tunnel vision will also apply to me—and you— from time to time.] You should be very patient with everyone who is willing to take time to tell you about the problem. You should ask questions but should probably not explain what you think is the answer until you have most or all the “facts.” This reduces aggravation and maximizes the amount of information you can get. It often helps to talk with several people if possible, but remember that some of the information you receive is not correct. 2. Compare the information you have received with what you know about the unit. Is this really a new problem? Occasionally the newness of the problem will actually be a combination of problems. Sometimes this can be really hard to separate. Review the things that can cause problems like the one that you are seeing. What activities were going on when the problem started? Is there any way that the activities can be part of the problem that you see? What do all the features of the problem have in common? Is there any way that the common feature could be causing the problem? 3. Form a hypothesis about what is going on. This should not be done too quickly. Take some time and see if there are other things going on that you have not heard of or noticed before. Most of the time you learn the control loops that interact and how they interact in your plant. See if some of that is going on. What things are looking normal and what is really not looking normal. Look for the “root cause” or the thing that could be responsible. Remember that most of the problems really stem from simple causes. Look for the simple hypothesis first. 4. See if your hypothesis can explain all that is going on. If it can’t, it is probably wrong or at best incomplete. Don’t be too attached to your hypothesis. Most of the time it is not completely right so be prepared to gracefully abandon it. Be active in trying to disprove it yourself. It is easier on your credibility to discover that you are wrong than to have someone point it out to you. docsity.com 5. If the steps above have not produced a hypothesis that is consistent with all the data, start over somewhere. Maybe you need to go all the way back to step one and look at all the data. Keep in mind that it is supposed to make sense. When you find the things that don’t make sense, these are the things that you should look at to be sure that the data is right. Remember that your instruments can lie to you, but most of the time they are telling you information that is at least close to the truth. Frequently there are ways to check on an instrument by comparing the information it is giving you with the information that you are getting from other instruments. Be sure to always try to identify cause and effect. I have seen some very good people set out on very expensive solutions to simple problems because they did not get the causes and the effects separated. This is probably the most critical item in coming quickly to the correct solution to any plant problem. Until you get causes and effects separated accurately, you cannot come to a correct solution except by accident. Now, what are some of the things that can go wrong? Temperature elements: 1. Slip out of the well. They don’t have to come all the way out—just move enough that they do not touch the end or the side and their response time (and sometimes their accuracy) will be compromised. 2. Break. Open connections usually read very high. 3. Short out. When this happens you can be “reading” the temperature where the short is rather than where the junction of the thermocouple is. 4. Reference junction problems. If all the thermocouples are off by the same amount suspect the reference junction. This is the instrument that corrects for the current generated when you connect the thermocouple’s alloy wires to the copper alloy line that carries the information to the control room. Flow elements—dP cells: 1. Zero problem. If there is a flow reading when you know that there is no flow, suspect a zero problem. However, it could be a fill problem with the low-pressure leg (positive flow) or with the high-pressure leg (negative flow) having lost some of the fill liquid. Or if it is a gas filled connection, some liquid may have gotten into the lines. 2. Orifice plate plugging. The flow will be lower than it appears on the instrument. If there is a sudden problem, look at the valve signal. If the valve signal changed abruptly to a more closed valve, it might be something in the orifice plate. 3. Orifice plate corroding out. The flow will be higher than it appears on the instrument. This will almost always be a slowly developing problem and thus hard to identify. 4. Bad flow. If you have proof that the flow is not right (material balance, reactor product distribution, etc.) it might be a bad calibration of the instrument or if it is a new loop, the whole calculation might not be correct. 5. Problem with the seal legs. These are the connections between the meter run and the dP cell. The plan is that you set them up in a way that you can maintain. You keep them full or empty based more on what you can be sure will exist than on what is nice to have. Some are the permanently sealed connections with the lines filled with an inert fluid of some sort. Even these sometimes leak. docsity.com