Process Safe Limits
If the value of a variable moves outside its safe range then, by definition, a hazardous situation has been created.
Figure 1 shows a simple process sketch; it is taken from the first standard example.
Table 1 provides some examples for safe limit values for the equipment items in Figure 1. Table 1 also provides some discussion to do with each of the values, showing where they came from, and what the impact of exceeding that value would be.
Figure 2 provides another illustration of the safe limit concept (the values shown in Figure 2 could be for any process parameter such as pressure, temperature, level or flow).
Figure 2 shows three ranges for the process parameter in question. The first is the normal operating range; it lies between 2 18 and 245 (in the appropriate units of measurement). Normal operations are carried out within this envelope. If the value is allowed to go outside the range it is likely that production or quality problems will crop up. If an operating value goes outside the operating range, but stays within safe limits, then the facility is in "trouble". There is no perceived safety or environmental problem during this phase of the operation, but the facility may be losing money. Examples of "trouble" in this context include:
The third range shown in Figure 2 defines emergency conditions. If a variable value goes outside the emergency limit range, urgent action is required. It is probable that an excursion outside the safe limits will lead to activation of emergency instrumentation and mechanical safety devices (such as pressure relief valves).
Maximum Allowable Working Pressure (MAWP)
One particularly important safe limit value to understand is that of Maximum Allowable Working Pressure (MAWP) for pressure vessels. Since the concept of MAWP is so important, and since it is not always well understood, the following guidance, based on ASME terminology using V-101 in Figure 1 as an example is provided.
Because temperature affects the strength of a vessel (higher temperatures make the metal yield more easily), the MAWP has an associated temperature. The effect of high temperature on equipment strength can be very deleterious. For example, the MAWP for a certain vessel may be 150 psig at a temperature of 600F. At 1000F, the same piece of equipment will fail at just 20 psig. On the other hand, at 100F, it may be able to handle nearly 300 psig. Hence, when temperatures are changing, the nominal pressure rating can be very misleading. (In this context, metal temperature refers to the average metal temperature through its entire depth.)Although the MAWP should never be exceeded during normal operation, it may be acceptable for the operating pressure to go above the MAWP for brief periods of time, say during an emergency situation. However, following such an excursion, the vessel should be checked by qualified vessel expert before being put back into service.
If equipment and piping are designed by rigorous analytical methods, such as finite element analysis, it is possible to operate with a lower safety margin than is required by the use of MAWP.
Unsafe Mixing Scenarios
Serious accidents can result from the mixing of incompatible chemicals. Therefore, the safe limit values should include information on the mixing of the chemicals found in the process under consideration, and information as to what concentrations are allowable. Mixing tables such as that shown in Table 2 are commonly used to address this requirement.
Table 2 lists five chemicals: A - E. It shows which chemicals can and cannot be mixed with one another safely.
The symbols in Table 2 have the following meanings:
Not much publicly available information to do with safe mixing values is available. However, some information is available from the United States Coastguard Chemical Hazards Response Information System (CHRIS).
Materials of Construction Table
Many accidents result from the use of incorrect materials of construction, particularly when corrosive chemicals are being used. A Materials of Construction table such as that shown in Table 3 shows how various materials of construction can be used for containing chemicals A - E.
The symbols in Table 3 have the following meanings:� Potential problems - further information may be needed
X Not allowed
N/A Information not available
Defining and Changing the Limits
The existence of properly defined upper and lower limits for all key variables is fundamental to successful Management of Change. In practice, however, these values are often not known, and can be difficult to ascertain. It can be even more difficult to predict what will happen if the limits are exceeded.When design values are not provided, those running the facility need to have methods for determining the safe limit values and for determining what needs to be done if those values are exceeded. Typically, they will use one of the five following techniques:
Information on Safe Limits can be provided by industry specialists - particularly equipment vendors and licensors of technology. This information can be very valuable and authoritative because the company that makes a particular machine or that owns a process technology will probably have an excellent idea as to what its safe limits are likely to be.
Operating experience is probably the most widely used method for determining safe operating limits in plants that have been running for a few years. After a plant has been in operation for a few years, there have usually been enough upsets and operating excursions to provide useful information as to what the Safe Limits might be and what happens if they are exceeded. This source of safe limit information is one of the justifications for having a good Incident Investigation program, because such a program can be used to collect information about all types of upset, even those that were just "near misses".
When a number of facilities use similar process technology it is very helpful to set up a method whereby they can share this type of information with one another. Sometimes the sharing of technology is restricted for competitive reasons. However, when it comes to safety, most companies are willing to help one another. Indeed, in some areas of technology, such as ammonia manufacture, there are regular conferences at which safety-related information is shared. Similarly, companies working with hydrogen cyanide and chlorine share knowledge so as to improve everyone's safety.
Extrapolation from current conditions is another means of determining a safe limit value. For example, Figure 4 shows the reaction rate for a particular chemical reaction as a function of temperature. Also shown is the maximum safe reaction rate. Above this point the reactor could be over-pressured.
Points A and B represent the range of current, normal operation. By drawing a line through them, it is possible to predict the reaction rate at Temperature C and to determine if the operation at that point is safe.
The problem with extrapolation is that the forecast may fail to predict the introduction of some new function that creates a non-linear change in the dependent parameter. In Figure 4 it can be seen that the chemical reaction rate starts to rise quite rapidly between temperatures B and C. Clearly some change in the reaction chemistry has taken place in this temperature range.
Mathematical ModelsSometimes, it is possible to use mathematical models to predict the acceptable operating range. However, such models are usually based on observed operating data that is obtained either from the plant or from laboratory experiments. Hence, the models have the same problem as empirical extrapolation: they can only be used with confidence for interpolation, not extrapolation.
"Nudging"One of the ironies of having a successful process safety management program is that it is difficult to determine safe operating ranges because the plant will have less experience of out-of-range operating conditions. A plant that is badly operated, however, will suffer many upsets and excursions, thus providing a knowledge base as to what happens when conditions are abnormal.
For those plants that are well run and so do not have this experience, it may be possible to "nudge" a value into a new operating range. The basic idea is to change the value very gradually in small steps. At the end of each small change, the overall operation is examined carefully to make sure that no unsafe conditions exist.
For example, if the operations management wants to increase the temperature in a reactor from say 210C to 220C but has no experience of the operation over 210C, they might increase the temperature 1C per day for a period of ten days. During this period a special watch will be kept on all variables that could indicate that the plant operations are unsafe. Also, additional readings and lab samples will probably be taken so that as much information as possible is available. Once the final temperature of 220C is reached, continued special scrutiny will be maintained until everyone is satisfied that the new condition is safe and operable.
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