HAZARD & OPERABILITY STUDIES

HAZARD & OPERABILITY STUDIES

Bayu Nurwinanto

Introduction

The technique of Hazard and Operability Studies, or in more common terms HAZOPS, has been used and developed over approximately four decades for 'identifying potential hazards and operability problems' caused by 'deviations from the design intent' in both new and existing process plants.  Before progressing further, it might be as well to clarify some aspects of these statements.

Potential Hazard and Operability Problems

You will note the capitalised 'AND' in the heading above. Because of the high profile of production plant accidents, emphasis is too often placed upon the identification of hazards to the neglect of potential operability problems.  Yet it is in the latter area that benefits of a Hazop Study are usually the greatest.  To quote an example, a study was commissioned for a new plant.  Some two years previously, and for the first time, a similar study had been carried out on different plant at the same site that was then in the process of being designed.  Before the latest review commenced, the Production Manager expressed the hope that the same benefits would accrue as before, stating that "in his twenty years of experience, never had a new plant been commissioned with so few problems, and no other plant had ever achieved its production targets and break-even position in so short a time.

Deviation from Design Intent

To deal firstly with 'design intent', all industrial plant is designed with an overall purpose in mind.  It may be to produce a certain tonnage per year of a particular chemical, to manufacture a specified number of cars, to process and dispose of a certain volume of effluent per annum, etc.  That could be said to be the main design intent of the plant, but in the vast majority of cases it would also be understood that an important subsidiary intent would be to conduct the operation in the safest and most efficient manner possible.

With this in mind equipment is designed and constructed which, when it is all assembled and working together, will achieve the desired goals.  However, in order to do so, each item of equipment, each pump and length of pipework, will need to consistently function in a particular manner.  It is this manner that could be classified as the 'design intent' for that particular item.  To illustrate, imagine that as part of the overall production requirement we needed a cooling water facility.  For this we would almost certainly have cooling water circuit pipework in which would be installed a pump as very roughly illustrated below.

A much simplified statement as to the design intent of this small section of the plant would be "to continuously circulate cooling water at an initial temperature of xx°C and at a rate of xxx litres per hour".  It is usually at this low level of design intent that a Hazop Study is directed.  The use of the word 'deviation' now becomes more easy to understand.  A deviation or departure from the design intent in the case of our cooling facility would be a cessation of circulation, or the water being at too high an initial temperature.  Note the difference between adeviation and its cause.  In the case above, failure of the pump would be a cause, not a deviation.

Industries in which the technique is employed

Hazops were initially 'invented' by ICI in the United Kingdom, but the technique only started to be more widely used within the chemical process industry after the Flixborough disaster in 1974.  This chemical plant explosion killed twenty eight people and injured scores of others, many of those being members of the public living nearby.  Through the general exchange of ideas and personnel, the system was then adopted by the petroleum industry, which has a similar potential for major disasters.  This was then followed by the food and water industries, where the hazard potential is as great, but of a different nature, the concerns being more to do with contamination rather than explosions or chemical releases.

The reasons for such widespread use of Hazops 

Safety and reliability in the design of plant initially relies upon the application of various codes of practise, or design codes and standards.  These represent the accumulation of knowledge and experience of both individual experts and the industry as a whole.  Such application is usually backed up by the experience of the engineers involved, who might well have been previously concerned with the design, commissioning or operation of similar plant.

However, it is considered that although codes of practise are extremely valuable, it is important to supplement them with an imaginative anticipation of deviations that might occur because of, for example, equipment malfunction or operator error.  In addition, most companies will admit to the fact that for a new plant, design personnel are under pressure to keep the project on schedule.  This pressure always results in errors and oversights.  The Hazop Study is an opportunity to correct these before such changes become too expensive, or 'impossible' to accomplish.

Although no statistics are available to verify the claim, it is believed that the Hazop methodology is perhaps the most widely used aid to loss prevention.  The reason for this can most probably be summarised as follows :

1. It is easy to learn.
2. It can be easily adapted to almost all the operations that are carried out within process industries.
3. No special level of academic qualification is required.  One does not need to be a university graduate to participate in a study.

THE BASIC CONCEPT

Essentially the Hazops procedure involves taking a full description of a process and systematically questioning every part of it to establish how deviations from the design intent can arise.  Once identified, an assessment is made as to whether such deviations and their consequences can have a negative effect upon the safe and efficient operation of the plant.  If considered necessary, action is then taken to remedy the situation.

This critical analysis is applied in a structured way by the Hazop team, and it relies upon them releasing their imagination in an effort to discover credible causes of deviations.  In practice, many of the causes will be fairly obvious, such as pump failure causing a loss of circulation in the cooling water facility mentioned above.  However, the great advantage of the technique is that it encourages the team to consider other less obvious ways in which a deviation may occur, however unlikely they may seem at first consideration.  In this way the study becomes much more than a mechanistic check-list type of review.  The result is that there is a good chance that potential failures and problems will be identified that had not previously been experienced in the type of plant being studied.

Keywords

An essential feature in this process of questioning and systematic analysis is the use of keywords to focus the attention of the team upon deviations and their possible causes.  These keywords are divided into two sub-sets :

• Primary Keywords: that focus attention upon a particular aspect of the design intent or an associated process condition or parameter.
• Secondary Keywords: that, when combined with a primary keyword, suggest possible deviations.

The entire technique of Hazops revolves around the effective use of these keywords, so their meaning and use must be clearly understood by the team.  Examples of often used keywords are listed below.

Primary Keywords

These reflect both the process design intent and operational aspects of the plant being studied.  Typical process oriented words might be as follows.  The list below is purely illustrative, as the words employed in a review will depend upon the plant being studied.

Flow	Temperature
Pressure	Level
Composition	Separate (settle, filter, centrifuge)
React	Mix
Reduce (grind, crush, etc.)	Absorb
Corrode	Erode

Note that some words may be included that appear at first glance to be completely unrelated to any reasonable interpretation of the design intent of a process.  For example, one may question the use of the word Corrode, on the assumption that no one would intend that corrosion should occur.  Bear in mind, however, that most plant is designed with a certain life span in mind, and implicit in the design intent is that corrosion should not occur, or if it is expected, it should not exceed a certain rate.  An increased corrosion rate in such circumstances would be a deviation from the design intent.

Remembering that the technique is called Hazard & Operability Studies, added to the above might be relevant operational words such as :

Isolate	Drain
Vent	Purge
Inspect	Maintain
Startup	Shutdown

This latter type of Primary Keyword is sometimes either overlooked or given secondary importance.  This can result in the plant operator having, for example, to devise impromptu and sometimes hazardous means of taking a non-essential item of equipment off-line for running repairs because no secure means of isolation has been provided.  Alternatively, it may be discovered that it is necessary to shut down the entire plant just to re-calibrate or replace a pressure gauge.  Or perhaps during commissioning it is found that the plant cannot be brought on-stream because no provision for safe manual override of the safety system trips has been provided.

Secondary Keywords

As mentioned above, when applied in conjunction with a Primary Keyword, these suggest potential deviations or problems. They tend to be a standard set as listed below :

Word	Meaning
No	The design intent does not occur (e.g. Flow/No), or the operational aspect is not achievable (Isolate/No)
Less	A quantitative decrease in the design intent occurs (e.g. Pressure/Less)
More	A quantitative increase in the design intent occurs (e.g. Temperature/More)
Reverse	The opposite of the design intent occurs (e.g. Flow/Reverse)
Also	The design intent is completely fulfilled, but in addition some other related activity occurs (e.g. Flow/Also indicating contamination in a product stream, or Level/Also meaning material in a tank or vessel that should not be there)
Other	The activity occurs, but not in the way intended (e.g. Flow/Other could indicate a leak or product flowing where it should not, or Composition/Other might suggest unexpected proportions in a feedstock)
Fluctuation	The design intention is achieved only part of the time (e.g. an air-lock in a pipeline might result in Flow/Fluctuation)
Early	Usually used when studying sequential operations, this would indicate that a step is started at the wrong time or done out of sequence
Late	As for Early

It should be noted that not all combinations of Primary/Secondary words are appropriate.  For example, Temperature/No (absolute zero or -273°C !) or Pressure/Reverse could be considered as meaningless.

Hazop Study Methodology

In simple terms, the Hazop study process involves applying in a systematic way all relevant keyword combinations to the plant in question in an effort to uncover potential problems.  The results are recorded in columnar format under the following headings:

DEVIATION	CAUSE	CONSEQUENCE	SAFEGUARDS	ACTION

In considering the information to be recorded in each of these columns, it may be helpful to take as an example the simple schematic below.  Note that this is purely representational, and not intended to illustrate an actual system.

DEVIATION

The keyword combination being applied (e.g. Flow/No).

CAUSE

Potential causes that would result in the deviation occurring.  For example "Strainer S1 blockage due to impurities in Dosing Tank T1" might be a cause of Flow/No.

CONSEQUENCE

The consequences that would arise, both from the effect of the deviation such as "Loss of dosing results in incomplete separation in V1" and if appropriate, from the cause itself, for example "Cavitation in Pump P1, with possible damage if prolonged'.

Always be explicit in recording the consequences.  Do not assume that the reader at some later date will be fully aware of the significance of a statement such as "No dosing chemical to Mixer".  It is much better to add the explanation as set out above.

When assessing the consequences, one should not take any credit for protective systems or instruments that are already included in the design.  For example, suppose the team had identified a cause of Flow/No (in a system that has nothing to do with the one illustrated above) as being spurious closure of an actuated valve.  It is noticed that there is valve position indication within the Central Control Room, with a software alarm on spurious closure.  They may be tempted to curtail consideration of the problem immediately, recording something to the effect of "Minimal consequences, alarm would allow operator to take remedial action".  However, had they investigated further they might have found that the result of that spurious valve closure would be overpressure of an upstream system, leading to a loss of containment and risk of fire if the cause is not rectified within three minutes.  It only then becomes apparent how inadequate is the protection afforded by this software alarm.

SAFEGUARDS

Any existing protective devices that either prevent the cause or safeguard against the adverse consequences would be recorded in this column.  For example, you may consider recording "Local pressure gauge in discharge from pump might indicate problem was arising".  Note that safeguards need not be restricted to hardware… where appropriate, credit can be taken for procedural aspects such as regular plant inspections (if you are sure that they will actually be carried out!).

ACTION

Where a credible cause results in a negative consequence, it must be decided whether some action should be taken.  It is at this stage that consequences and associated safeguards are considered.  If it is deemed that the protective measures are adequate, then no action need be taken, and words to that effect are recorded in the Action column.

Actions fall into two groups :

1. Actions that remove the cause.
2. Actions that mitigate or eliminate the consequences.

Whereas the former is to be preferred, it is not always possible, especially when dealing with equipment malfunction.  However, always investigate removing the cause first, and only where necessary mitigate the consequences.  For example, to return to the "Strainer S1 blockage due to impurities etc." entry referred to above, we might approach the problem in a number of ways.

• Ensure that impurities cannot get into T1 by fitting a strainer in the road tanker offloading line.
• Consider carefully whether a strainer is required in the suction to the pump.  Will particulate matter pass through the pump without causing any damage, and is it necessary to ensure that no such matter gets into V1.  If we can dispense with the strainer altogether, we have removed the cause of the problem.
• Fit a differential pressure gauge across the strainer, with perhaps a high dP alarm to give clear indication that a total blockage is imminent.
• Fit a duplex strainer, with a regular schedule of changeover and cleaning of the standby unit.

Three notes of caution need to be borne in mind when formulating actions.  Do not automatically opt for an engineered solution, adding additional instrumentation, alarms, trips, etc.  Due regard must be taken of the reliability of such devices, and their potential for spurious operation causing unnecessary plant down-time.  In addition, the increased operational cost in terms of maintenance, regular calibration, etc. should also be considered (the lifetime cost of a simple instrument will be at least twice its purchase price for more complex instrumentation this figure would be significantly greater).  It is not unknown for an over-engineered solution to be less reliable than the original design because of inadequate testing and maintenance.

Finally, always take into account the level of training and experience of the personnel who will be operating the plant.  Actions that call for elaborate and sophisticated protective systems are wasted, as well as being inherently dangerous, if operators do not, and never will, understand how they function.  It is not unknown for such devices to be disabled, either deliberately or in error, because no one knows how to maintain or calibrate them.

Considering all Keywords - The Hazop procedure

Having gone through the operations involved in recording a single deviation, these can now be put into the context of the actual study meeting procedure.  From the flow diagram below it can be seen that it is very much an iterative process, applying in a structured and systematic way the relevant keyword combinations in order to identify potential problems.

P & ID (Hazard and Operability Studies)

CORROSION RATES AND INSPECTION INTERVALS

PIPING INSPECTION CODE: IN-SERVICE INSPECTION, RATING, REPAIR, AND ALTERATION OF PIPING SYSTEMS

CORROSION RATES AND INSPECTION INTERVALS

Oleh : Bayu Nurwinanto

CMLS (Condition monitoring location)

General

CMLs are specific areas along the piping circuit where inspections are to be made. The nature of the CML varies according to its location in the piping system. The selection of CMLs shall consider the potential for localized corrosion and service-specific corrosion as described in API 574 and API 571. Examples of different types of CMLS include locations for thickness measurement, locations for stress cracking examinations, locations for CUI and locations for high temperature hydrogen attack examinations.

CML Monitoring

Each piping system shall be monitored at CMLs. Piping circuits with high potential consequences of failure should occur and those subject to higher corrosion rates or localized corrosion will normally have more CMLs and be monitored more frequently. CMLs should be distributed appropriately throughout each piping circuit. CMLs may be eliminated or the number reduced under certain circumstances, such as olefin plant cold side piping, anhydrous ammonia piping, clean noncorrosive hydrocarbon product, or high-alloy piping for product purity. In circumstances where CMLS will be substantially reduced or eliminated, persons knowledgeable in corrosion should be consulted.

The minimum thickness at each CML can be located by ultrasonic scanning or radiography. Electromagnetic techniques also can be used to identify thin areas that may then be measured by UT or radiography. When accomplished with UT, scanning consists of taking several thickness measurements at the CML searching for localized thinning. The thinnest reading or an average of several measurement readings taken within the area of a examination point shall be recorded and used to calculate corrosion rates, remaining life.

Where appropriate, thickness measurements should include measurements at each of the four quadrants on pipe and fittings, with special attention to the inside and outside radius of elbows and tees where corrosion/erosion could increase corrosion rates. As a minimum, the thinnest reading and its location shall be recorded. The rate of corrosion/ damage shall be determined from successive measurements and the next inspection interval appropriately established. Corrosion rates, the remaining life and next inspection intervals should be calculated to determine the limiting component of each piping circuit.

CMLS should be established for areas with continuing CUI (Corrosion under insulation, including stress corrosion cracking under insulation), corrosion at S/A interfaces, or other locations of potential localized corrosion as well as for general, uniform corrosion.

CMLS should be marked on inspection drawings and on the piping system to allow repetitive measurements at the same CMLS. This recording procedure provides data for more accurate corrosion rate determination. The rate of corrosion/damage shall be determined from successive measurements and the next inspection interval appropriately established based on the remaining life or RBI (risk-based inspection) analysis.

CML Selection

In selecting or adjusting the number and locations of CMLS, the inspector should take into account the patterns of corrosion that would be expected and have been experienced in the process unit. A decision on the type, number and location of the CMLS should consider results from previous inspections, the patterns of corrosion and damage that are expected and the potential consequence of loss of containment. CMLS should be distributed appropriately over the piping system to provide adequate monitoring coverage of major components and nozzles. Thickness measurements at CMLS are intended to establish general and localized corrosion rates in different sections of the piping circuits. A minimal number of CMLS are acceptable when the established corrosion rate is low and the corrosion is not localized.

A number of corrosion processes common to refining and petrochemical units are relatively uniform in nature, resulting in a fairly constant rate of pipe wall reduction independent of location within the piping circuit, either axially or circumferentially. Examples of such corrosion phenomena include high-temperature sulfur corrosion and sour water corrosion (provided velocities are not so high as to cause local corrosion/erosion of elbows, tees, and other similar items). In these situations, the number of CMLS required to monitor a circuit will be fewer than those required to monitor circuits subject to more localized metal loss. In theory, a circuit subject to perfectly uniform corrosion could be adequately monitored with a single CML. In reality, corrosion is never truly uniform and in fact may be quite localized, so additional CMLS may be required. Inspectors must use their knowledge (and that of others) of the process unit to optimize the CML selection for each circuit, balancing the effort of collecting the data with the benefits provided by the data.

More CMLs (Condition monitoring location) should be selected for piping systems with any of the following characteristics : 

1. higher potential for creating a safety or environmental emergency in the event of a leak.
2. higher expected or experienced corrosion rates.
3. higher potential for localized corrosion.
4. more complexity in terms of fittings, branches, deadlegs, injection points, and other similar items.
5. higher potential for CUI (corrosion under insulation, including stress corrosion cracking under insulation).

Fewer CMLS can be selected for piping systems with any of the following three characteristics : 

1. low potential for creating a safety or environmental emergency in the event of a leak.
2. relatively noncorrosive piping systems.
3. long, straight-run piping systems.

CMLS can be eliminated for piping systems with any of the following characteristics :

1. extremely low potential for creating a safety or environmental emergency in the event of a leak.
2. noncorrosive systems, as demonstrated by history or similar service; and
3. systems not subject to changes that could cause corrosion as demonstrated by history and/or periodic reviews.

Every CML should have at least one or more examination points identified. Examples include :

1. locations marked on un-insulated pipe using paint stencils, metal stencils, or stickers.
2. holes cut in the insulation and plugged with covers.
3. temporary insulation covers for fittings nozzles, etc.
4. isometrics or documents showing CMLS.
5. radio frequency identification devices (RFID). 

Careful identification of CMLS and examination points are necessary to enhance the accuracy and repeatability of the data.

Corrosion specialists should be consulted about the appropriate placement and number of CMLs for piping systemssusceptible to localized corrosion or cracking, or in circumstances where CMLs will be substantially reduced or eliminated.

Inspection Intervals 

If RBI ( risk-based inspection)  is not being used, the interval between piping inspections shall be established and maintained using the following criteria : 

1. corrosion rate and remaining life calculations.
2. piping service classification.
3. applicable jurisdictional requirements.
4. judgment of the inspector, the piping engineer, the piping engineer supervisor, or a materials specialist, based on operating conditions, previous inspection history, current inspection results, and conditions that may warrant supplemental inspections. 

The owner/user or the inspector shall establish inspection intervals for thickness measurements and external visual inspections and, where applicable, for internal and supplemental inspections.

Thickness measurements should be scheduled at intervals that do not exceed the lesser of one half the remaining life determined from corrosion rates or the maximum intervals recommended in Table. Shorter intervals may be appropriate under certain circumstances. Prior to using Table, corrosion rates shall be calculated.

Table contains recommended maximum inspection intervals for Classes 1, 2 and 3 of piping services, as well as recommended intervals for injection points and S/A interfaces. Maximum intervals for Class 4 piping are left to the determination of the owner/user depending upon reliability and business needs.

The inspection interval shall be reviewed and adjusted as necessary after each inspection or significant change in operating conditions. General corrosion, localized corrosion, pitting, environmental cracking, and other applicable forms of deterioration mentioned in Section 5 shall be considered when establishing the various inspection intervals.

CUI (corrosion under insulation, including stress corrosion cracking under insulation) Inspection.

Inspection for CUI shall be considered for externally-insulated piping in areas or temperature ranges that are susceptible to CUI. CUI inspections may be conducted as part of the external inspection. If CUI damage is found during spot checks, the inspector should inspect other susceptible areas on the equipment.

Although external insulation may appear to be in good condition, CUI damage may still be occurring. CUI inspection may require removal of some or all insulation. If external coverings are in good condition and there is no reason to suspect damage behind them, it is not necessary to remove them for inspection of the equipment. CUI damage is often quite insidious in that it can occur in areas where it seems unlikely.

Considerations for insulation removal are not limited to but include :

1. history of CUI for the specific piping system or comparable piping systems.
2. visual condition of the external covering and insulation.
3. evidence of fluid leakage (e.g. stains or vapors).
4. whether the piping systems are in intermittent service.
5. condition/age of the external coating, if known.
6. evidence of areas with wet insulation.
7. the type of insulation used and whether that insulation is known to absorb and hold water.

Piping Service Classes 

General

All process piping systems shall be categorized into different piping classes. Such a classification system allows extra inspection efforts to be focused on piping systems that may have the highest potential consequences if failure or loss of containment should occur. In general, the higher classified systems require more extensive inspection at shorter intervals in order to affirm their integrity for continued safe operation. Classifications should be based on potential safety and environmental effects should a leak occur.

Owner/users shall maintain a record of process piping fluids handled, including their classifications. API 750 and NFPA 704 provide information that may be helpful in classifying piping systems according to the potential hazards of the process fluids they contain.

Class 1

Services with the highest potential of resulting in an immediate emergency if a leak were to occur are in Class 1. Such an emergency may be safety or environmental in nature. Examples of Class 1 piping include, but are not necessarily limited to those containing the following.

• Flammable services that can autorefrigerate and lead to brittle fracture.
• Pressurized services that can rapidly vaporize during release, creating vapors that can collect and form an explosive mixture, such as C2, C3, and C4 streams. Fluids that can rapidly vaporize are those with atmospheric boiling temperatures below 50 °F (10 °C) or where the atmospheric boiling point is below the operating temperature (typically a concern with high-temperature services)
• Hydrogen sulfide (greater than 3 % weight) in a gaseous stream.
• Anhydrous hydrogen chloride.
• Hydrofluoric acid.
• Piping over or adjacent to water and piping over public throughways (refer to Department of Transportation and U.S. Coast Guard regulations for inspection of over water piping).
• Flammable services operating above their auto-ignition temperature. 

Class 2

Services not included in other classes are in Class 2. This classification includes the majority of unit process piping  and selected off-site piping. Typical examples of these services include but are not necessarily limited to those containing the following :

• on-site hydrocarbons that will slowly vaporize during release such as those operating below the flash point.
• hydrogen, fuel gas, and natural gas.
• on-site strong acids and caustics. 

Class 3

Services that are flammable but do not significantly vaporize when they leak and are not located in high-activity areas are in Class 3. Services that are potentially harmful to human tissue but are located in remote areas may be included in this class. Examples of Class 3 service include but are not necessarily limited to those containing the following :

• on-site hydrocarbons that will not significantly vaporize during release such as those operating below the flash point.
• distillate and product lines to and from storage and loading.
• tank farm piping.
• off-site acids and caustics. 

Class 4

Services that are essentially nonflammable and nontoxic are in Class 4, as are most utility services. Inspection of Class 4 piping is optional and usually based on reliability needs and business impacts as opposed to safety or environmental impact. Examples of Class 4 service include, but are not necessarily limited to those containing the following :

• steam and steam condensate.
• air.
• nitrogen.
• water, including boiler feed water, stripped sour water.
• lube oil, seal oil.
• ASME B31.3, Category D services.
• plumbing and sewers.

Assessment of Inspection Findings 

Pressure containing components found to have degradation that could affect their load carrying capability [pressure loads and other applicable loads (e.g. weight, wind, etc., per API 579-1/ASME FFS-1)] shall be evaluated for continued service. Fitness-For-Service techniques, such as those documented in API 579-1/ASME FFS-1, Second Edition, may be used for this evaluation. The Fitness-For-Service techniques used shall be applicable to the specific degradation observed. The following techniques may be used as applicable.

To evaluate metal loss in excess of the corrosion allowance, a Fitness-For-Service assessment may be performed in accordance with one of the following sections of API 579-1/ASME FFS-1. This assessment requires the use of a future corrosion allowance, which shall be established.

• Assessment of General Metal Loss—API 579-1/ASME FFS-1.
• Assessment of Local Metal Loss—API 579-1/ASME FFS-1.
• Assessment of Pitting Corrosion—API 579-1/ASME FFS-1.

To evaluate blisters and laminations, a Fitness-For-Service assessment should be performed in accordance with API 579-1/ASME FFS-1. In some cases, this evaluation will require the use of a future corrosion allowance, which shall be established.

To evaluate weld misalignment and shell distortions, a Fitness-For-Service assessment should be performed in accordance with API 579-1/ASME FFS-1.

To evaluate crack-like flaws, a Fitness-For-Service assessment should be performed in accordance with API 5791/ASME FFS-1.

To evaluate the effects of fire damage, a Fitness-For-Service assessment should be performed in accordance with API 579-1/ASME FFS-1.

Piping Stress Analysis (Analisis Piping Stres)

Piping shall be supported and guided so that : 

• its weight is carried safely.
• it has sufficient flexibility for thermal expansion or contraction, and.
• it does not vibrate excessively.

Piping flexibility is of increasing concern the larger the diameter of the piping and the greater the difference between ambient and operating temperature conditions. 

Piping stress analysis to assess system flexibility and support adequacy is not normally performed as part of a piping inspection. However, many existing piping systems were analyzed as part of their original design or as part of a rerating or modification, and the results of these analyses can be useful in developing inspection plans. When unexpected movement of a piping system is observed, such as during an external visual inspection the inspector should discuss these observations with the piping engineer and evaluate the need for conducting a piping stress analysis.

See API 574 for more information on pressure design, minimum required and structural minimum thicknesses, including formulas, example problems and default tables of suggested. 

Piping stress analysis can identify the most highly stressed components in a piping system and predict the thermal movement of the system when it is placed in operation. This information can be used to concentrate inspection efforts at the locations most prone to fatigue damage from thermal expansion (heat-up and cooldown) cycles and/or creep damage in high-temperature piping. Comparing predicted thermal movements with observed movements can help identify the occurrence of unexpected operating conditions and deterioration of guides and supports. Consultation with the piping engineer may be necessary to explain observed deviations from the analysis predictions, particularly for complicated systems involving multiple supports and guides between end points.

Piping stress analysis also can be employed to help solve observed piping vibration problems. The natural frequencies in which a piping system will vibrate can be predicted by analysis. The effects of additional guiding can be evaluated to assess its ability to control vibration by increasing the system’s natural frequencies beyond the frequency of exciting forces, such as machine rotational speed. It is important to determine that guides added to control vibration do not adversely restrict thermal expansion.