process safe limits

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Many aspects of Process Safety Management (PSM) requireknowledge of safe limit values for process variables. Forexample, a HAZOP team needs to know the quantifiedmeaning for terms such as 'High Temperature', an inspectorneeds to know the corrosion allowance for vessels andpipes, an operator needs to know the maximum andminimum levels in tanks, and a Management of Changereview team needs quantified information about theparameters that are being changed.

If the value of a variable moves outside its safe range then, by definition, a hazardoussituation has been created.

Figure 1 shows a simple process sketch; it is taken from the first standard example.

Figure 1Standard Example

Table 1 provides some examples for safe limit values for the equipment items inFigure 1. Table 1 also provides some discussion to do with each of the values,

Process Safe Limits http://www.stb07.com/process-safety-management/process-safe-limits.html

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showing where they came from, and what the impact of exceeding that value wouldbe.

Table 1Examples of Safe Limits

Item Parameter Units Safe UpperLimit

Safe LowerLimit

T-100 Level % 95 10

The high limit is based on operating experience; it has been found thatupsets rarely cause the level to deviate more than 2 or 3%. Therefore,keeping the level at 95% or less should minimize the chance of tankoverflow.

Minimum flow protection for the pumps is not provided so a minimumlevel in the tank must be maintained to prevent pump cavitationleading seal leaks.

P-101 Flow kg/h N/A 500

The upper limit for flow is set by the capacity of the pumps. Evenwhen they are pumping at maximum rates, no hazardous condition iscreated. Therefore no meaningful value for a safe upper limit of flowexists.

Below the prescribed minimum flow rate, the pumps may cavitate.

V-101 Pressure bar(g) 12 (at 250C) 0

The upper pressure limit is set by code.

V-101 is not vacuum-rated, and there is uncertainty about lowerpressure limit, so 0 barg (1 bar abs) has arbitrarily been set asthe lower limit.

V-101 Temperature C 250 -10

The upper temperature limit is defined by code.

Stress cracking may occur below the lower safe limit value.

Figure 2 provides another illustration of the safe limit concept (the values shown inFigure 2 could be for any process parameter such as pressure, temperature, level orflow).

Figure 2Example of Safe Limit Range


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Figure 2 shows three ranges for the process parameter in question. The first is thenormal operating range; it lies between 2 18 and 245 (in the appropriate units ofmeasurement). Normal operations are carried out within this envelope. If the value isallowed to go outside the range it is likely that production or quality problems willcrop up. If an operating value goes outside the operating range, but stays within safelimits, then the facility is in "trouble". There is no perceived safety or environmentalproblem during this phase of the operation, but the facility may be losing money.Examples of "trouble" in this context include:

Excessive steam consumption;Product quality problems;Unusually high consumption of spare parts; andProduction flow limitation problems.

The second range lies between the safe upper limit and the safe lower limit (210 -275 in Figure 2). These parameters are sometimes referred to as "Not to Exceed"limits. If the value of the parameter goes outside this range then the process is, bydefinition, unsafe, and action must be taken. The option of doing nothing is not anoption. It is likely that, once these safe limits are breached, safety devices -particularly instrumentation systems - will be activated. Operations personnel shouldunderstand the consequence of exceeding the limits; they should also be providedwith procedures and training as to what actions to take to bring the variable back intothe safe range. If the operations team wishes to operate outside the safe range, sayto increase production rates, they can only do so after implementing the Managementof Change process.

The third range shown in Figure 2 defines emergency conditions. If a variable valuegoes outside the emergency limit range, urgent action is required. It is probable that


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an excursion outside the safe limits will lead to activation of emergencyinstrumentation and mechanical safety devices (such as pressure relief valves).

Some safe limits may have no meaningful value. For example, if a pressure vessel isdesigned for full vacuum operation then that vessel has no safe lower limit forpressure. In Table 1 no value for a safe upper limit for high flow is provided becausethe system is safe even when the pumps are running flat-out with all control valveswide open.

Maximum Allowable Working Pressure (MAWP)

One particularly important safe limit value to understand is that of MaximumAllowable Working Pressure (MAWP) for pressure vessels. Since the concept of MAWPis 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.

As the process design is being developed, the process engineers require thatV-101 be designed for a maximum pressure of 95.0 psig. This is the DesignPressure or pressure rating of the vessel (it is measured at the top of thevessel).

The process engineer's target values are transmitted to the vessel engineer. Heor she designs the vessel using standard sizes for wall thickness and flangesize, thus generating the Maximum Allowable Working Pressure (MAWP), whichis the maximum pressure at which the vessel can be operated. Generally MAWPis higher than the design pressure because wall thicknesses are in discretesizes, and the designer will always choose the standard value greater than thatcalled for. In the example, since it is unlikely that he can design for exactly95.0 psig, the designer selects the next highest level, which turns out to give amaximum allowable pressure (MAWP) of 120.0 psig. Once the vessel is inoperation, the vessel can be operated at up to 120 psig without exceeding itssafe limits. MAWP is the pressure that will be used for setting relief andinterlock values.

The Test Pressure for the vessel is 1.5 times design pressure. Anytime thevessel is opened (say for inspection), it will be tested to that pressure beforethe process is restarted.

If the pressure goes above test pressure, the vessel walls are close to theiryield point. Up to twice the MAWP value the vessel or associated piping may beslightly distorted, but any leaks are most likely to occur at gaskets. At 2 to 4times MAWP, there will probably be distortion of the vessel, and it can beassumed that gaskets will blow out.

The vessel's burst pressure will typically be in the range of 3.5 to 4 timesMAWP. Therefore, for this example, therefore, the burst pressure would bebetween 400 and 500 psig. (It is difficult to predict this value accuratelybecause so few vessels actually fail, so there is not much field data.)

Because temperature affects the strength of a vessel (higher temperatures make themetal yield more easily), the MAWP has an associated temperature. The effect of hightemperature on equipment strength can be very deleterious. For example, the MAWPfor a certain vessel may be 150 psig at a temperature of 600F. At 1000F, the samepiece of equipment will fail at just 20 psig. On the other hand, at 100F, it may be ableto handle nearly 300 psig. Hence, when temperatures are changing, the nominalpressure rating can be very misleading. (In this context, metal temperature refers tothe average metal temperature through its entire depth.)

Although the MAWP should never be exceeded during normal operation, it may beacceptable 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 vesselshould be checked by qualified vessel expert before being put back into service.


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If equipment and piping are designed by rigorous analytical methods, such as finiteelement analysis, it is possible to operate with a lower safety margin than is requiredby 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 foundin the process under consideration, and information as to what concentrations areallowable. Mixing tables such as that shown in Table 2 are commonly used to addressthis requirement.

Table 2 lists five chemicals: A - E. It shows which chemicals can and cannot be mixedwith one another safely.

Table 2Mixing Scenarios

A B C D E

A -

B � -

C √ √ -

D X X � -

E N/A √ √ √

The symbols in Table 2 have the following meanings:

√ No known problems with mixing the two chemicals in any range� Problems in certain mixing rangesX Mixing creates unsafe conditions in any range of concentrationN/A Information not available

Mixing Tables usually consider only binary mixtures. The consequences associatedwith simultaneously mixing three or more materials are not usually known.

For those mixing scenarios where only certain ranges are hazardous, a safe mixingrange such as that shown in Figure 3 can be used. The shaded area represents thepredicted unsafe mixing range of the chemicals X and Y at a given temperature. (Inpractice, it is often very difficult to obtain sufficient data to construct an envelopesuch as that shown in Figure 3.)

Figure 3UnSafe Mixing Range for a Given Temperature


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Not much publicly available information to do with safe mixing values is available.However, some information is available from the United States Coastguard ChemicalHazards Response Information System (CHRIS).

Materials of Construction Table

Many accidents result from the use of incorrect materials of construction, particularlywhen corrosive chemicals are being used. A Materials of Construction table such asthat shown in Table 3 shows how various materials of construction can be used forcontaining chemicals A - E.

Table 3Materials of Construction Matrix

Carbon Steel StainlessSteel 304

StainlessSteel 316

GasketMaterial A

GasketMaterial B

A √ √ √ √ √

B � √ √ X X

C � N/A N/A N/A N/A

D X X � √ √

E N/A √ √ √ N/A

The symbols in Table 3 have the following meanings:

√ No known problems with mixing the two chemicals in any range � 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 isfundamental to successful Management of Change. In practice, however, these valuesare often not known, and can be difficult to ascertain. It can be even more difficult topredict what will happen if the limits are exceeded.

Although the engineers who designed the plant should provide values for the safeoperating range, in many cases they fail to do so. They themselves may not knowwhat the safe limits are, the best they can do is to provide a single, point value, not arange. Furthermore, if the facility is more than a few years old, it is likely that thecurrent operations differ significantly from the original design. Hence, the originalvalues may no longer be germane.

When design values are not provided, those running the facility need to havemethods for determining the safe limit values and for determining what needs to bedone if those values are exceeded. Typically, they will use one of the five followingtechniques:

Industry InformationOperating experienceExtrapolationMathematical modeling"Nudging"

Industry Information


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Information on Safe Limits can be provided by industry specialists - particularlyequipment vendors and licensors of technology. This information can be very valuableand authoritative because the company that makes a particular machine or that ownsa process technology will probably have an excellent idea as to what its safe limits arelikely to be.

It is true that any information from sources such as these is likely to have acommercial bias. Nevertheless, when it comes to safety, everyone wishes to do thingsright, regardless of commercial interests. Therefore the information provided byvendors and licensors is likely to be as complete and accurate as they can make it.

Operating Experience

Operating experience is probably the most widely used method for determining safeoperating limits in plants that have been running for a few years. After a plant hasbeen in operation for a few years, there have usually been enough upsets andoperating excursions to provide useful information as to what the Safe Limits mightbe and what happens if they are exceeded. This source of safe limit information is oneof the justifications for having a good Incident Investigation program, because such aprogram can be used to collect information about all types of upset, even those thatwere just "near misses".

When a number of facilities use similar process technology it is very helpful to set upa 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, insome areas of technology, such as ammonia manufacture, there are regularconferences at which safety-related information is shared. Similarly, companiesworking with hydrogen cyanide and chlorine share knowledge so as to improveeveryone's safety.

Extrapolation

Extrapolation from current conditions is another means of determining a safe limitvalue. For example, Figure 4 shows the reaction rate for a particular chemical reactionas a function of temperature. Also shown is the maximum safe reaction rate. Abovethis point the reactor could be over-pressured.

Points A and B represent the range of current, normal operation. By drawing a linethrough them, it is possible to predict the reaction rate at Temperature C and todetermine if the operation at that point is safe.

Figure 4Interpolation and Extrapolation


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The problem with extrapolation is that the forecast may fail to predict the introductionof 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 rapidlybetween temperatures B and C. Clearly some change in the reaction chemistry hastaken place in this temperature range.

A straight line extrapolation of the reaction rate from point A to point B shows that, attemperature C, the reaction rate is still in the safe range. However, because thereaction rate increases exponentially, the reaction "takes off", and goes well abovethe safe limit at point C.

Mathematical Models

Sometimes, it is possible to use mathematical models to predict the acceptableoperating range. However, such models are usually based on observed operating datathat is obtained either from the plant or from laboratory experiments. Hence, themodels have the same problem as empirical extrapolation: they can only be used withconfidence for interpolation, not extrapolation.

"Nudging"

One of the ironies of having a successful process safety management program is thatit is difficult to determine safe operating ranges because the plant will have lessexperience of out-of-range operating conditions. A plant that is badly operated,however, will suffer many upsets and excursions, thus providing a knowledge base asto what happens when conditions are abnormal.

For those plants that are well run and so do not have this experience, it may bepossible to "nudge" a value into a new operating range. The basic idea is to changethe value very gradually in small steps. At the end of each small change, the overalloperation is examined carefully to make sure that no unsafe conditions exist.

For example, if the operations management wants to increase the temperature in areactor 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 thisperiod a special watch will be kept on all variables that could indicate that the plantoperations are unsafe. Also, additional readings and lab samples will probably betaken so that as much information as possible is available. Once the final temperatureof 220C is reached, continued special scrutiny will be maintained until everyone issatisfied that the new condition is safe and operable.

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