cswip 11-01 - plant inspection- week 2 cours

CSWIP PLANT NSPECTOR LEVEL I WEEK 2

CSWIP PLANT INSPECTOR LEVEL 1 WEEK 2

COURSE NOTES

CONTENTS

SECTION SUBJECT01 Roles and duties of the plant inspector

02 Using codes and standards

03 Introduction to pressure equipment

04 Pressure vessel inspection

05 Inspection of pipework

06 Inspection of storage tanks

07 Inspecting painting and lining

08 An introduction to RBI

09 Inspection reporting

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Section 03Introduction to pressure equipment

What is pressure equipment?The pressure equipment may differ between countries, industry and technical applications. It is a complex and ever-changing picture.Pressure equipment categories

Some characteristics of ‘pressure equipment’1. Gauge pressure Equipment is subjected to a positive gauge pressure or negative

gauge (vacuum) pressure.2. Principal stresses Components are subject to principal stresses in three dimensions, or

two dimensional membrane stresses (for thin-walled shells).3. Stored energy Pressure equipment acts to contain stored energy in use.4. Controlled manufacture Because of the potential hazard if failure occurs, pressure

equipment is subject to controls on its specification, design and manufacture. The amount of control varies, depending on what the equipment is and how will be used.

5. Factors of safety They provide a margin against unforeseen circumstances and reduce the risk of failure to acceptable levels.

6. Inspection and testing-during manufacture

They play an important part in ensuring the fitness for purpose of pressure equipment during its manufacture and before use.

7. In-service inspection It is necessary to inspect pressure equipment throughout its working life to make sure it continues to be safe and fit for purpose.

Rules and regulationsIt is important not to confuse the pressure system safety regulations (PSSRs) and the pressure equipment directive (PED).

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Section 04Pressure vessel inspection

What are pressure vessels? Remember that there is no universal definition of what constitutes a pressure vessel. Vessels may be unfired or fired. The examples above are unfired; the one below is an example of a fired

vessel (i.e. a boiler). The inspection of boilers and vessels forms a major part of a plant inspector’s role. Any power or process plant will have a large number of vessels for different applications. Some are

complex, such as those forming the component parts of steam-raising plant or large condensers; others such as air receivers and low pressure or atmospheric vessels are of relatively simple design and construction.

Vessels provide a useful vehicle for showing how different inspection disciplines mesh together in a practical plant inspection context.

Fitness for purpose criteria Pressure vessels contain large amounts of stored energy, for this reason the main (overriding) FFP criterion

is integrity-as plant inspectors to know that the vessel is safe and is not going to fail. There are many engineering aspects, which contribute to this integrity; design, creep, fatigue, corrosion-

resistance and workmanship all have an effect.The system for ‘assuring’ the integrity of pressure vessels

1. Arranging for an independent design appraisal.2. Using traceable materials.3. Applying proven NDT techniques4. Doing a hydrostatic (pressure) test

Your role as plant inspector is to verify that all elements are in place, have been completed, and that the control mechanism has indeed worked.

Some simple design aspects1. Basic construction

There are various practical requirements such as the need for cylinder end closures, holes for inlet/outlet pipes, attachments and take account of the probability of weld flaws and similar defects.

A basic feature of all vessel design codes is the limitation placed on design tensile stress (S) of the vessel material.

2. Corrosion allowance Corrosion, which occurs over the life of a vessel, is defined by a corrosion allowance(c), whose design

value depends upon the vessel duty and the corrosiveness of its contents.

3. Welded joint efficiency Most vessels codes assume that welded joints are not strong as the parent plate. This strength reduction is characterised by the welded joint efficiency:

o Welded joint efficiency (η) = joint strength / parent material strength. o It varies from 100% for a perfect weld down to 75-85 % for welds where integrity is not so assured.

4. Pressure vessel cylinders Weld types and efficiencies usually differ for longitudinal and circumferential joints, so the joint

stresses in a vessel must be designed to satisfy both requirements.For example:

o Circumferential stress (Sc) = ( p D/ 2t ) ≤ ηℓ SWhere ηℓ = longitudinal joint efficiency

ANDLongitudinal stress (Sℓ) = ( p D/ 4t ) ≤ ηc SWhere ηc = Circumferential joint efficiency

o Note that the symbol σc is used instead of Sc and σℓ instead of Sℓ and p=pressure.

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As the circumferential stress is twice the longitudinal stress, then circumferential strength is usually the controlling parameter.

5. Openings and compensationVessels need various types of openings, which weaken the shell. Compensation, or reinforcement, is the provision of extra stress-transmitting area in the wall of a cylinder or shell when an area has been removed.

Types of compensation a. Use a thicker nozzle and/or a thicker shellb. Use a compensation pad on the shell

The principle of compensationSee fig.7

Inspection openings The size and disposition of the openings depend upon the duty and size of the vessel. In small vessel a single hand hole or a flanged-in inspection opening may be adequate whereas large

vessels require elliptical access manholes, often with reinforcement / seating rings. The minor axis of an elliptical opening in a cylindrical shell generally lies parallel to the longitudinal

axis of the shell. The studs provide the initial sealing force.

Pressure vessel codesCore areas such as vessel classes, design criteria and requirements for independent inspection and certification are based on similar (but not identical) guiding principles.

Code compliance and intent

The premise behind a pressure vessel code is that equipment should meet its requirements in all respects - i.e. Full code compliance.

‘Code intent’, this means that it may comply with the code in some areas e.g. design stresses, but not in other, such as the requirements for material traceability, NDT and defect acceptance criteria.

Note that equipment built to such code intent cannot be officially ‘code stamped’.

Vessel visual / dimensional examination The visual and dimensional examination is part of the final manufacturing inspection activities carried

out on a pressure vessel. It is normal contractual witness point and is mandatory for various ‘modules’ under the new PED.

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CSWIP PLANT NSPECTOR LEVEL I WEEK 2Vessel visual examinationThe purpose of a vessel visual examination is to look for problems that are likely to affect integrity.

1. The vessel exteriorThe basic examination points are:1. Plate courses : Check the layout of the plate courses against the original approved design

drawings.2. Plate condition : Check for dents and physical damage. Look for deep grinding marks or

obvious grooves deeper than 10 per cent of plate thickness.3. Surface finish : General mill-scale on the surface of the plate is acceptable before shot

blasting. Check for any obvious surface ‘rippling’ caused by errors during plate rolling.

4. Reduced thickness : Pay particular attention to the areas around the head-shell joint; this area is sometimes heavily ground to try and blend in a poorly aligned seam.

5. Bulging : Check the whole shell for any bulging. This mainly caused by forcing the shell or head during tack welding to compensate for a poor head-shell fit or excessive out-of-roundness of the shell.

6. Nozzle flange orientation: Check that the nozzles have not ‘pulled’ out of true during fabrication or heat treatment of the vessel. This can cause the nozzle flanges to change their alignment relative to axis of the vessel.

7. Welding : Make a visual examination of all exterior welding. Watch for rough welding around nozzles, particularly small ones of less than 50mm diameter.Look for undesirable features such as undercut, incomplete penetration or a too-convex weld profile.

2. The vessel interior

1. Head-to-shell alignment : There should be an even weld-cap all the way around the seam.2. Nozzle ‘sets’ : Check the ‘set-through’ lengths of those nozzles protruding through into

the vessel.3. Weld seams : Do the same on the inside weld seams as on the outside. Make sure that

any spatter has been removed from around the weld area. 4. Corrosion : Check all inside surface for general corrosion. In general there should be

no evidence of mill-scale on the inside surfaces-if there is, it suggests the plates have not been properly shot blasted before fabrication.

5. Internal fittings : Make sure they are in the correct place with respect to the ‘handing’ of the vessel. Check the fit of the manhole door and any inspection covers.

Vessel dimensional examinationThe dimensional check Dimensional checking can be done using a steel tape measure, with the use of along steel straightedge and

large inside or outside callipers for some dimensions. There is a technical standard DIN 8570 that gives general guidance for tolerances on fabricated equipment.

The main points as follow:

1. Datum lines : Each vessel should have two datum lines: a longitudinal datum (normally the centreline) and a transverse datum. The transverse datum is normally not the circumferential weld line- it will be located 50-100mm inwards from the seam towards the dished head and indicated on the vessel by deep centre-punch marks.

2. Manway location : Check the location of the manway w.r.t the longitudinal datum line.3. Manway flange face : Check that this flange face is parallel to its indicated plane. A

tolerance of ±1° is acceptable.4. Nozzle location : Check the location of the nozzles in relation to the datum lines.

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CSWIP PLANT NSPECTOR LEVEL I WEEK 25. Nozzle flange faces : Nozzles flange should be accurate to within about 0.5° from their

indicated plane. Check also the dimension from each nozzle flange face to the vessel centreline-a tolerance of ±3mm is acceptable.

6. Flange bolt holes : Check the size and pitch circle diameter of bolt holes in the flanges. 7. Vessel ‘bow’ measurements : The amount of acceptable bow depends on the length (or height) and

diameter of the vessel. For example:A small vessel of 3-5m long and a diameter of up to 1.5m should have a bow of less than about 4mm.

Vessel markings The correct marking of vessel also has statuary implications; it is inherent in the requirements of vessel

codes and most safety legislation and directives that the safe conditions of use are clearly indicated on the vessel.

For vessels covered by the PED, nameplate marking must include the max. Pressure and min. temperature. This was not mandatory under previous schemes of vessel certification.

Typical pressure vessel nameplate content

Vessel misalignment and distortion Misalignment and distortion can affect the integrity and therefore fitness for purpose of a vessel. Misalignment between adjoining plates results in a significant increase in stresses and thermal gradients-

this is worst at the shell/head seam where the so-called ‘discontinuity stresses’ are already high as a result of the constraining effect of the less-flexible dished head.

What causes them? The shell plates have to be rolled and then persuaded into alignment with a ‘spun’ dished head. With thick materials, say> 20mm, the material is difficult to roll (it ma have to be done hot) and can ‘spring

back’ making accuracy difficult. With thin plates, which are less than 10mm thick, the main problem is a lack of rigidity-it will distort due to

heat input during welding.

Vessel toleranced featuresThere are five basic toleranced features:

1. Surface alignment : This refers to any ‘step’ that exists between adjacent plates, either head-to-shell or shell-to-shell, after welding is complete.

Allowable surface misalignment is based on a fraction of the plate thk (e). Longitudinal welds are subject to hoop stress, which is greater than the

longitudinal stresses seen by the circumferential welds-hence longitudinal seams are considered as ‘more critical’ and less tolerant to stress-inducing misalignment.

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CSWIP PLANT NSPECTOR LEVEL I WEEK 22. Straightness : Straightness is expressed as a min. taper or ‘deviation’ between parallel planes

over unit cylindrical length of the vessel. It is easy to measure using wires, straight edges and a ruler.

3. Circumference : Circumference is used as a toleranced feature to guard against vessel shells which are ‘swaged’ as a result of poor rolling or handling, or as an extra caveat against large differences in the mating dimensions of head and shell.

Thin-walled vessels that exhibit rolling swage-marks are not uncommon-check it using ‘circumferences’ req. to see if they are compliant with the code.

The easiest way is with a flexible tape, which provides a direct reading.4. Circularity : This is also known as ‘out of roundness’ (OOR).

It is the difference between the max. and min. internal diameters on any cross sectional plane across the vessel. It is a broad measure of ovality or OOR.

5. Profile : This is a ‘catch all’ requirement to put a sensible tolerance on dents, bulges

and general profile variations that could not accurately be classified as out of roundness.

It is a variation of a surface between two theoretical surfaces separated by a sphere of a diameter equal to the tolerance size.

The normal way is to use a plywood template cut to the circular profile of the vessel.

Sharp creases are outlawed by a clause in most vessel standards that require all profile variations to be gradual.

Pressure testingSome points about a pressure test (vessels or pipes)

Static, principal stresses. No cyclic stresses. It is a ‘proving’ for vessels or pipes that have not been properly checked for defect? Is it only a check for leakage under pressure?

The point of a pressure testThe pressure test is not a full test of whether the vessel will fail as a result of being exposed to its working environment. Hence, a pressure test is not a ‘proving test’ for the vessels that have not been properly checked for weld defect.The standard hydrostatic test

Guidelines when witnessing a standard hydrostatic test on a vessel

1. Vessel configuration

The test should be done after any stress relief Vessel components such as flexible pipes, diaphragms and joints that will not stand the pressure test must

be removed. The ambient temperature must be above 0°C (preferably 15-20 °C) and above the brittle fracture transition

temperature for the vessel material (check the mechanical test data for this).2. The test procedure

Blank off all openings with solid flanges. Use the correct nuts and bolts, not G-clamps. Two pressure gauges, preferably on independent tapping points, should be used. It is essential for safety purposes to bleed all the air out. Check that the bleed nozzle is really at the highest

point and that the bleed valve is closed off progressively during pumping, until all the air has gone. Pumping should be done slowly (using a low capacity reciprocating pump) so as not to impose dynamic

pressure stresses on the vessel. Test pressure is stated in PD 5500, ASME VIII or the relevant standard. This will not overstress the

vessel (unless it is a very special design case). If in doubt use 150% design pressure. Isolate the pump and hold the pressure for a minimum of 30 minutes.

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CSWIP PLANT NSPECTOR LEVEL I WEEK 23. What to look for

Leaks, these can take time to develop. Check around seams and nozzle welds. Dry off any consideration with a compressed air-line, it is possible to miss small leaks if you do not do this. Leaks normally occur from cracks or areas of porosity.

Watch the gauges for pressure drop. Any visible drop is unacceptable. Check for distortion of flange-faces etc by taking careful measurements.

Pneumatic testing

Pneumatic testing of pressure vessels is a ‘special case’ testing procedure. Common reasons are:1. Refrigeration system vessels are constructed to ASME VIII and pneumatically tested with nitrogen.2. Special gas vessels may have an unsupported structure and so are unable to withstand the weight of

being filled with water.3. Vessels that are used in critical process applications where the process of even minute quantities of

water cannot be tolerated. Pneumatic tests are dangerous because of compressed air or gas contains a large amount of stored energy. There are a number of well-defined precautionary measures to be taken before carrying out the test.

General guidelines on witnessing a pneumatic test

1. Precautionary measures before a pneumatic test PD 5500 requires that a design review be carried out to qualify the factors of safety inherent in the vessel

design. NDT requirements are those specified for the relevant application plus 100% surface crack detection (MPI or DP) on all other welds.

ASME VIII (part UW-50) specified that all welds near openings and all attachment welds should be subjected to 100% surface crack detection (MPI or DP).

It is good practice to carry out 100% volumetric NDT and surface crack detection of all welding prior to a pneumatic test.

2. the test procedure The vessel should be in a pit, or surrounded by concrete blast walls. Ambient temperature should be well above the brittle fracture transition temperature. Air can be used but inert gas (such as nitrogen) is better. Pressure should be increased very slowly in steps of 5-10%-allow stabilisation between each step. PD 5500 specifies a maximum test pressure of 150% design pressure. ASME specifies a maximum test pressure of 125% design pressure, but consults the code carefully-these

are conditions attached. When test pressure is reached, isolate the vessel and watch for pressure drops.

ASME CERTIFICATION

Some possible problems encountered during pressure vessel inspections1. Missing documents2. Incomplete material traceability3. Unrecorded repairs4. Corrosion 5. Leaks 6. Head/shell distortion 7. Unauthorised welding

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CSWIP PLANT NSPECTOR LEVEL I WEEK 2Section 05

Piping inspection

Introduction: general principlesThere are no hard-and-fast rules; pipework is normally one of the first areas of a plant to be subject to risk-based inspection (RBI) techniques.Typical process plant pipework inspection periods

API 570: what’s in it?1. Inspection and testing practices.2. Frequency and extent of inspections.3. Thickness calculations.4. Repair, alternating and re-rating.5. Inspection of buried pipelines.

The scope of API 570Included:Almost any metallic piping system1. Raw and finished oil products 3. Hydrogen/fuel/flare gas systems2. Raw and finished chemical products 4. Sour/waste systems etcExcluded:1. Water, steam and condensate 5. Domestic sewers etc2. Piping related to mechanical equipment 6. Everything < ½ in diameter3. Pumps/compressors etc 7. Non-metallic piping4. Pressure vesselsNote that it concentrates on high-risk systems rather than small bore or ‘utility’ pipework.The 11 ‘risk areas’ identified by API 5701. Injection points 7. Corrosion under linings2. Dead legs 8. Fatigue cracking3. CUI 9. Creep cracking4. Soil / Air (S/A) interfaces 10. Brittle fracture5. Erosion and erosion/corrosion 11. Freeze damage6. Environmental crackingThe injection point circuit (IPC); as defined by API 570One of the important areas is near injection points. These are a common source of corrosion.

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CSWIP PLANT NSPECTOR LEVEL I WEEK 2API 570 pipework classes

API 570 defines three levels of pipework ‘class’, they are generally applicable to many process plant types.The basis of the API 570 risk classes

Class 1: Highest risk/consequences1. Flammable services/flash-off leading to brittle fracture 4. Hydrofluoric acid2. Explosive vapours after flash-off 5. Piping over public roads3. H2S gaseous

Class 2: ‘medium’ risk1. Most other process piping 3. H2, fuel and natural gas2. On-site hydrocarbons 4. On-site acid and caustics

Class 3: ‘low’ risk1. Fluids that will not flash-off (even if they are flammable) 3. Off-site acids and caustics2. Distillate/product storage/loading lines 4.

The API 570 class inspection intervalsType of circuit Thickness measurements External visual

Class 1 5 years 5 yearsClass 2 10 years 5 yearsClass 3 10 years 10 years

Injection points 3 years By classS/A interfaces - By class

Pipeline hydrotesting The major difference between hydro-testing of pipelines and vessels is that when testing pipelines it is very

difficult t exclude all the air. A special procedure is used in which air volume/pressure drop are calculated, and an acceptable leakage rate

defined.

Pipeline hydrotest procedure1. Pressurised to the test pressure 2. Calculate the volume of water added3. Calculate the volume of entrapped air4. Thermal stabilisation5. Hold for 24 hours: record the volume of water added to maintain the pressure6. Compare the results with codes acceptance criteria

Pipeline hydrotest acceptance levelA ‘rule of thumb’ acceptance level (for pipelines which are not specified as zero leakage is:

1 gallon per inch diameter per mile of pipeline per day for each 100ft of pressure head

Equivalent to0.1 litre per mm pipe diameter per km of

pipeline per day for each 30m of pressure head

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Inspecting storage tanks

Types of tank There are two main types of storage tank; the rubble-based (old designs) and the more modern concrete based. Types of storage tank bases

Most tank designs are atmospheric, i.e. their contents are not retained under pressure. Special designs are used for the storage of chemical products under pressurised conditions, including cryogenic tanks that store chemical products at low temperatures.

Rubble-based tanks These suffer from sinkage of the foundations leading to a ‘reverse angle’, in which water gathers. This

inevitably leads to corrosion of the lower strake of the tank.Concrete-based tanks A mixture of sand and bitumen (known as ‘bitsand’ is used between the tank and the base. It is important to check the integrity of this layer. If it allows water between the tank and the base, corrosion

will occur of the tank floor plates. This leads to serious thinning of the plates and they will eventually have to be replaced.

API 653 corrosion calculationsAs a general principle, isolated pitting can be ignored and the average amount of wall thinning is calculated over a predetermined area as shown in fig.: API 653 corrosion assessment Assessment of pitting over an ‘effective length’

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Introduction to pressure risk-based inspection (RBI)

What is RBI?RBI is a method of optimising inspection effort. It is a statistical and management tool, rather than an engineering one.The principles of RBI

1. It formalises a common-sense approach.2. It considers the CONSEQUENCES and LIKELIHOOD of failure.3. Identifies the highest-risk equipment.4. Quantifies the risk.5. Reflects the risk in the written scheme of examination.

The two main approaches to RBI1. Quantitative approach

1. Based on ‘pre-set’ risk parameters 2. Considers just about everything

2. Qualitative approach (quicker)

1. Uses plant experience and feedback 2. Decides priorities, based on risk

The main steps of an RBI analysis1. List the equipment

2. Decide the consequences of failureConsequence considers:

a) Safety of plant systems (e.g. of PSVs etc)b) Fire/explosion damage potentialc)Proximity of assets and the communityd) Effect of product losse)Pressure factor (stored energy?)f) The effect on productiong) Time of rectify the component h) How good are the local safety arrangements

3. Allocate risk categories Allocate risk categories, i.e. the LIKELIHOOD of failure.

4. Compile a ‘risk matrix’

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5. make decisions based on risk categories Typical risk category decisions

Risk exposure before RBI planning

Classification Action required

1 Unacceptable

Mitigate immediately for QRR 3 or 4 Improved operating practices or controls. On-line plant monitoring (continuous or intermittent). Engineering measures to mitigate consequences. Inspection.

2 Undesirable Mitigate as above on timescale of next shutdown of the unit.

Risk exposure after RBI planning

3 Acceptable with controls Define and implement appropriate inspection strategy or other controls.

4 Acceptable No inspection or other actions are required unless to satisfy national legislative or insurance requirements.

RBI of new equipment It involves a particular approach to deciding the scope of inspection that will give the most economical and

effective assurance of fitness for purpose.a. Why it is needed? In the manufacturing works inspection environment, one of the main pressures for its development has come

from (virtually) international acceptance of the quality management standard series ISO 9000. Whatever the opinions of its ability to produce equipment which is fit for purpose it has clearly had the

effect of tidying up a lot of loose practices in manufacturing companies. This has helped to eliminate many ‘bad practice’ risks to FFP in simple equipment and general ‘low

technology’ fabricated cast and forged components. Risk-based inspection can keep the cost down and so reduce your client’s unit cost of verifying FFP. It increases confidence level all round, and used in conjunction with good general FFP principles, can help

the whole process become more effective and therefore better value for money.b. What exactly is it? Risk based works inspection is about allocating works inspection effort to those materials, processes and

items of equipment that are likely to cause the greatest risk to FFP. This three-level approach is the key to it-many many equipment items, which have high FFP risk levels do

so not because of the design of the equipment, or what it does, but because of the engineering materials and processes that are used to make its component parts.

Continuec. The technique The root of the technique is the scope of inspection that is used. There are three ‘risk’ considerations, to

deciding this scope; that owing to the materials used, the nature of the manufacturing process, and the particular type of engineering equipment in question.

It will affect the number of visits, their frequency and the timing of your inspection and review activities within the manufacturing programme.

Once the ‘risk-based’ scope of the inspection has been set, it has to feed forward into the ITPs-the mechanism by which the programme of inspection activities is planned and recorded.

The following ITP entries are important:1. The number of ITP steps that are separately identified on the plan.2. The number and position of ‘witness’ and ‘review’ points-and the relationships between the two.3. The various amounts of involvement of the inspection parties; i.e. contractor, consultant, third party

(statutory) inspector and end-user.4. The level of documentation that has to be produced (and inspected).5. The agreed mechanism for issuing NCR/CAs and for ‘closing them out’.6. The mechanism for formal release of equipment from the manufacturing’ works, and its subsequent

acceptance, in turn, by the main contractor and end-user. They become more streamlined and in harmony with the particular items to which they prefer.

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The changes are essentially related to the technical facets of the equipment-and this fundamental point- and have little to do with whether or not you trust the manufacturing; you are seeing the bigger picture.

Figures 10 to 12 show a sample with a numerical values reflecting risk levels experienced from inspection on mechanical plant in the power and process industry.

They are based purely on inspected equipment has not performed as it should. Multiplying or dividing them to reach an overall ‘risk factor’-their true effect is qualitative only, they help make up the bigger picture.

A word of caution about RBI It is wise to be careful with any technique that involves an assessment of risk. Risk assessment

is not a perfect science-whatever the background data, it still remains highly subjective. This is your assessment of FFP risk of a particular material or piece of equipment will not be

the only one-your client may well have different ideas, based on personal experience or intuition.

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