The purpose of this circular is to raise awareness of the factors to consider when operating storage tanks containing hazardous substances. Information was drawn from a series of site inspections ranging from refinery tank farms, large speciality chemical storage facilities as well as a number of smaller sites where bulk storage tanks were used to store raw, intermediate and final product materials.
While some of the information is general and can be applied to a wide range of mechanical plant, other parts of the report are equipment-specific. It is intended as a reference aid to assist regulators and those concerned with the operation, and in-service integrity of storage tanks, by identifying good practices. These are set out in Section 5. Other sections of the report cover issues such as general integrity procedures, damage mechanisms, manufacturing and repair defects, inspection practices, non-metallic tanks, venting and relief, coatings and linings, and hydrotesting. It concludes with a summary of the most relevant codes and standards.
1 During the 2005/2006 work year Inspectors in HID CID carried out a number of interventions examining the way in which the integrity of atmospheric storage tanks was managed. This guidance has now been updated (2011) to include current best practices and advances in technology.
2 The aim of this SPC is to provide Mechanical Engineering Inspectors with information for use when inspecting atmospheric storage tanks at COMAH sites and other hazardous installations.
3 Regulatory Specialists are not expected to attempt a detailed technical inspection using this guidance. It may be useful, however, in supporting interventions associated with the HID Chemicals Industry Strategy particularly Key Goal 1 ‘Prevention of Major Accidents’.
4 The SPC also gives a summary of the principal management arrangements to reduce the risk of loss of containment from storage tanks that were identified as representing good practice for this sector. HSE expects duty holders to adopt relevant good practice as a minimum. Therefore these measures, or others that have been demonstrated to be at least as effective, would normally be considered enforceable as part of the package of measures that reduce risks ALARP.
5 The objectives of this SPC are to:
6 The scope of the interventions was limited to in-service issues relating to tanks containing hazardous substances, their appurtenances, and piping up to the first isolations. The principal focus was on those measures to prevent loss of primary containment.
7 Most designs of tank were included in the exercise e.g. flat-bottomed vertical cylindrical, skirt and leg supported domed end, and horizontal. Tanks constructed from non-metallic materials such as GRP and HDPE were also studied in the project and appear to be gaining popularity for certain applications, notably the storage of mineral acids.
8 No tanks constructed from aluminium were inspected nor were any with double skins or double bottoms. A number of stainless steel tanks were seen during the course of the initial project and many claimed to have been designed and manufactured generally in accordance with BS 2654 ‘Specification for Manufacture of vertical steel welded non-refrigerated storage tanks with butt-welded shells for the petroleum industry’’
1 Stainless steel tanks fall outside the scope of this standard. Appendix S of API 620 or Underwriters Laboratory Inc. Standard for safety, UL 142, ‘Steel aboveground tanks for Flammable and Combustible Liquids’ may have been more appropriate.
9 Key issues examined included:
Note: because of the wide range of topics areas some were intentionally left out of scope, for example:
However the information within the SPC may be relevant to these areas
10 This SPC identifies reasons why the integrity management arrangements of atmospheric storage tanks are usually not as comprehensive and robust as for other process plant and equipment. The following list summarises good practices identified during the interventions together with existing guidance and standards:
11 Storage tank integrity needs to be well managed since tanks can contain large inventories of hazardous liquids and their failure has the potential to result in serious and dramatic events.
12 Many of the tanks around the UK are still in operation after well over 50 years of service. Use of riveted tanks installed in the first half of the 20th century is still widespread. The oldest tank seen still in service was of riveted construction, manufactured in 1893, and used to store diesel oil.
13 Failures, though not as common as those in piping systems, occur regularly. Permeable bunds are still found at many sites and it is by no means unknown for a tank to be leaking through the base without any indication or the operator’s knowledge.
14 Tanks are often perceived as simple structures that require little attention. Nevertheless damage mechanisms associated with them can be complex and varied. The measures operators have in place to maintain tanks in a safe operating condition were found to be variable and in some cases fundamentally inadequate.
15 Atmospheric tanks are similar to much process piping in that they are non-codal type equipment (i.e. not subject to PSSR 2000 regulations). Integrity management regimes are now gaining maturity, but in many cases still lag that of the well developed systems as seen for pressurised plant. In the UK there is no specific duty or regulation that sets out prescriptive rules for storage tank inspections, though clearly duties under HSW, COMAH and PUWER regulations can apply. The generally high reliability of tanks has meant that in many instances maintenance approaches have tended to be reactive. Usefully, there is extensive guidance available on periodic/proactive tank maintenance and inspection practice. References to ‘Supporting Guidance’ are given in Section 21.
16 It is often the case that thorough inspections of tanks are only carried out if the tank can be taken out of service, which, in many instances, can only be achieved by agreement between the Operator and their customers who are renting the space under 3rd party tank lease agreements. Clearly this can be a major problem where shortage of tanks or lack of flexibility is an issue.
17 At many sites, tanks are not dedicated to a single substance and may have been used to contain a range of products. This can have a significant effect on their integrity, not only from the threat of direct chemical attack and corrosivity (e.g. incompatibility between the substance stored and the tank material of construction) but from other factors such as product density, chemical stability, volatility, concentration, temperature and erosion.
18 At the best-managed sites, this difficulty is clearly recognised and strict procedures are in place to assess the properties of different products. These controls were not in place on many of the less well-managed sites.
19 Despite their importance storage tanks are frequently perceived as ‘infrastructure’ and not fundamental to process. This can lead to issues such as poor record keeping, insufficient design, construction and condition information, unsatisfactory design drawings, process diagrams and P&ID’s. Often repairs, modifications and other changes, some which have been substantial, have not been documented and captured on records.
20 Well-produced policy and procedural documentation describing the systems in place to provide integrity assurance of storage tanks were seen at the best managed sites. These specified the principal requirements of an integrity management regime and were used to underpin all of the good practices seen at these sites. The documentation often included references to relevant guidance and standards.
21 Key information in good procedures included administrative and organisational information, statements on general integrity strategy, roles and responsibilities, personnel qualifications, tank databases and operational duties.
22 High-quality procedures often incorporated annexes giving details of inspection scope and techniques associated with the main elements of tanks, e.g. foundations, floors, roofs, and shell. Where relevant, additional detail would set out particular requirements (e.g. for inspection and maintenance of floating as opposed to fixed roofs, or requirements for insulated tanks). Individual sections included different requirements for external and internal examination. Guidance was also given on the format, production and retention of records along with corrective actions and review procedures.
23 Other general information incorporated elements such as schemes of examination specifying the frequency and scope of inspections (in and out-of-service), recommended NDT techniques, cleaning and surface preparation standards, retirement limits, level and settlement surveys, repairs and hydrotests.
24 In the best cases, procedures also defined the requirements for maintenance, inspection and testing of protective devices such as pressure/vacuum valves and emergency relief.
25 Procedures often specified inspection strategies drawn from the three leading guidance documents: API Std 653 ‘Tank inspection, repair, alteration and reconstruction’, API RP 575 ‘Guidelines and Methods for Inspection of Existing Atmospheric and Low-Pressure Storage Tanks’ and EEMUA 159 ‘Maintenance and inspection of above ground vertical steel cylindrical storage tanks’.
26 Other specific guidance material, such as HSE PM 75 ‘Glass reinforced plastic vessels and tanks: advice to users’, National Sulphuric Acid Association (NSAA) ‘Recommended guidelines for the bulk storage of sulphuric acid, oleum and liquid sulphur trioxide’ and HSG 235 ‘Guidance on the storage of Hydrochloric and Nitric Acid in tanks’ were referred to where relevant.
27 Use was also made of HSG 176 ‘The Storage of Flammable Liquids in Tanks’ which makes an important recommendation that there should be a ‘written scheme of examination’ (WSE) agreed between the user and competent person that should include the scope and frequency of the thorough examination. It is worth noting that although the WSE and the scope of inspection may well be similar to that employed on pressure plant; this WSE is not, from a regulatory perspective, the same as that required under Pressure Systems Safety Regulations 2000.
28 The best operators maintained tank records that contained information such as original design codes and construction data, date of installation, operational history, piping systems or process schematics, drains, foundation data, repairs and modifications, maintenance and inspection histories. In the best cases the key document was the scheme of examination, each adapted to the individual tank. These schemes identified the foreseeable deterioration mechanisms and then specified the maintenance and inspection practices to prevent or control them.
29 Some of the less well-managed sites had little if any formal in-service inspection and maintenance procedures for their tanks. Indeed in many cases, tanks were not even individually identified in plant registers or included in any proactive inspection and maintenance regime, even where these contained hazardous substances.
30 While it is essential to examine tanks on an individual basis some useful information can be obtained from sample inspections. Guidance on the circumstances where the results from sample examinations can be helpful in modifying recommended inspection frequencies or the scope of an inspection are given in API Std 653 and EEMUA 159.
31 A wide range of damage mechanisms can cause a storage tank to deteriorate and fail. Many can operate simultaneously. This section cannot identify all of these, but does aim to address those that pose a threat to the integrity of the structure. The following section on inspection methods will describe how the failure mechanisms can be detected.
32 The vast majority of storage tanks are constructed from carbon steel and corrosion is a prime cause of deterioration of them and their accessories. It can be associated almost equally from external attack (atmospheric side) or from an internal (product side) mechanism. By way of example, tanks in crude oil service can be particularly susceptible to sulphate reducing bacteria (SRB) attack.
33 Corrosion is rarely uniform, though this is not unknown. However, random, localised, pitting corrosion attack, particularly of flat-bottomed tank floors appears to be the most common failure. This can be topside down (especially where there is an aqueous phase) or underside up. Product temperature appears to be an important element. The condition and materials of construction of tank base along with the effectiveness and durability of the floor to base seal, and the slope angle of the berm or tank pad away from the base are crucial factors in prevention of bottom up corrosion.
2 Although it is common to refer to some tanks as flat-bottomed, the floor may actually be designed cone-up or cone-down. Cone-up floors are the most common and allow settled water or bottoms product to gravitate to sumps around the periphery of the tanks. Cone-down floors normally have a sump at the centre of the tank.
For tanks in crude oil, or other liquid hydrocarbon, service with the possibility of entrained water in the product or entering through seals or natural breathing, water will naturally collect as a layer in the bottom. This is often referred to as a ‘water bottom’. Along with any sediment, it can often contain aggressive compounds and in some instances monitoring the pH of drained water may be required as a corrosion control. It is important that operators adopt good drainage procedures where water can accumulate in the bottom of tanks.
Corrosion leading to small leaks in floors can potentially go undetected for a period or time. In some cases this has lead to foundations been washed away, causing the tank to become unstable, leading to catastrophic failure of the tank.(Belgium tank incident)
34 Corrosion attack of the lower shell strakes is also common, as is attack of annular plates where these are fitted. This is often, though by no means universally, caused by poor bund drainage. Edge lamination of annular rings is also commonly seen in these circumstances. BS EN 14015 recommends tanks > 12.5m diameter should be constructed with annular ring plates. API 650 takes a different approach and uses allowable stress to determine when annular plates are necessary.
35 For the reasons outlined above, many operators arrange to have the bottoms and typically the first metre of the shell of their tanks painted or coated to provide increased corrosion protection. Linings and coatings for tanks is a subject in itself and some further detail on coating protection is given later in the report.
36 Deterioration and failure of tank drains is common. Drains are often the single most vulnerable point on a tank, especially those that run through channels under the floor. They are regularly overlooked, difficult to inspect and the culverts are often submerged in rainwater and full of debris. At one site, the channel under the tanks and the drains had simply been concreted in.
37 Water draw-off sumps are also vulnerable parts of the tank, particularly those which are centrally located on cone-down type floors. While wall thicknesses are generally greater than that of the floor, both the inner and outer walls are at potentially greater threat from corrosion. Guidance on inspection techniques for sumps is poor and it is arguable that full ultrasonic thickness scans should be carried out here. API 650 gives information on centre draw-off sumps and drains. In addition EMMUA 159 provides guidance on this topic.
38 Examples were seen where preferential attack had occurred to lower shell strake welds and in another case the floor directly under the dipping point on one tank containing diesel oil had been pierced. This may be attributed to fretting-related corrosion (a mechanism seen with floating roof support leg striking plates) where repeated contact removes any protective layer of rust scale that may have formed.
39 External corrosion is often also found where fittings are welded to the tank shell or at water traps for example around and below wind girders, stairwells and vertical/spiral stairway connections.
40 Other areas of increased corrosion attack are typically at the liquid/vapour interfaces and areas around vents and breathers where oxygen and humidity levels can be higher.
41 Care must be taken walking on tank roofs where there are doubts about its condition. It is by no means unusual that the first sign of deterioration found is when daylight is seen through holes in the roof at the time of internal entry. Internal damage can occur where the product is more corrosive in its vapour phase and due to general condensation in the roof space. It can be more prevalent around vents and breathers. Some Operators highlighted roof walkways by painting them in a different colour or used a non-slip coating and carried out additional inspections in these areas.
42 There are a number of different designs of floating roofs and inspection of their many features such as rolling ladders, deck plates, seals, drains, pontoons and guides is carried out from the top. Like that for fixed roofs care should always be taken when walking on them and, if the tank is in service, access is normally restricted if the roof is below a certain level (normally top dip or no more than 2m from the top). Access to pontoons or the inter-space of double deck roofs should also be restricted if the tank is in-service.
43 The other most common external corrosion attack is CUI (corrosion under insulation). Here the operating temperatures are critical and many tanks contain liquids that are stored in the ‘highest risk’ range of 50 to 100°C. Also where insulating material is high in chlorides. Again the damage tends to manifest itself as pitting attack predominantly around the bottom edge particularly if the insulation is not cut away causing a ‘wicking’ effect. More information is contained in HSE Guidance SPC/Tech/Gen 18
44 Effective preventative measures here include the application of good insulation and cladding standards, effective sealing at vulnerable areas for example the shell to roof joint, insulation supports, small-bore penetrations and include an insulation holiday for about the first 8 to 10” (200 to 250mm) from the tank base. Recommendations for the design and application of insulation can be found in Appendix Q of BS EN 14015. In addition, EMMUA 159 Section 15 makes comment on inspection of insulation.
45 Chloride stress corrosion cracking is the main cause of degradation with stainless steel tanks. It is temperature dependent and the main problem areas are the nozzles and attachment points around steam heating coils. Further guidance can be found in HSL research report ES/MM/09/48 and HSE research report RR298 Chloride Stress Corrosion Cracking in Stainless Steels.
46 Erosion is generally found at points of increased flow or change in flow direction particularly adjacent to fill and discharge nozzles. It is especially common in tanks containing sulphuric acid. In many cases increased thickness of materials or backing plates are used where erosion is identified as a threat. One instance was found where damage to a tank sump, which eventually leaked, was caused by poorly installed vibrating piping which eroded the side wall of the sump.
47 Erosion of a floating roof tank inner shell wall was identified at one site where the tank was sited downwind in a particularly dusty location. It has also been observed directly under a mixer on a tank in crude oil service.
48 Cold creep (or stretching over a period of time) is a well-understood deterioration mechanism but is limited to non-metallic thermoplastic tanks. These tanks can be temperature sensitive as well, with HDPE in particular losing strength as temperature increases.
49 Fatigue does not appear as significant a potential failure mechanism as it is for pressure vessels, since the membrane stresses are generally much lower. Nevertheless for tanks in high cyclic fill/empty service areas of high stress such as the floor to shell/annular to shell welds could be threatened as well as some areas around nozzles. It is common to feel tank floors flex when walking across them. Many operators utilise dye penetrant and MPI NDT techniques to inspect for fatigue induced defects.
50 Fatigue is recognised in GRP tanks and a service life is defined.
51 The threat to tank integrity through chemical attack and incompatibility between the products stored and the material of construction is obvious. The most common construction materials are carbon and stainless steels, thermoplastics, polypropylene, PVC, PVDF, HDPE, and laminates such as GRP. Aluminium tanks are used more commonly for hoppers and silos and in low temperature applications but appear less general for normal liquid storage. Tanks utilising a variety of lining and coating materials are used, although metallic clad tanks are quite rare.
52 Carbon steel is suitable for the storage of a large number of chemicals, however, some, such as dilute acids will react with carbon steel.
53 Stainless steels are often used where purity and quality of the stored chemical is paramount. However welding of stainless steel tanks and system components is critical as these can be susceptible to stress corrosion cracking.
54 Additional information on HDPE and GRP tanks is given later in the report, however poor high temperature properties, UV light degradation and environmental cracking are all well understood failure modes for non-metallic tanks.
55 It is essential that the materials used for the construction of the storage tank are compatible with the chemicals to be stored. Additional consideration may be required where chemicals are stored at elevated temperatures. The best operators had robust systems which identified the issues as well as applying formal change procedures to assess the effects of any anticipated product/ tank changes.
56 Mechanical damage to storage tanks can be caused by impact, differential or non-uniform settlement, over-pressurisation, vacuum, excess dead loads such as snow or ice, and wind inflicted damage. Buckling of shell plates and other externally inflicted damage should be identified from good inspection. Shell buckling can have severe effects where tanks are fitted with floating roofs as can differential settlement and tilt.
57 Snow and wind loads appear to be the most common threats to a tank’s structure and buckling of the shell due to wind gusts is relatively common. Buckling of shells and subsequent catastrophic failure have been caused by internal vacuum where vents have become blocked. Serious damage has also occurred on floating roof tanks where rainwater drains have blocked or failed.
58 Subsidence can occur, particularly on weak and compressible ground. Many tank farms are sited on recovered land, often alongside rivers and other environmentally sensitive areas. All of the tank codes stress the importance of operators/designers understanding the subsurface conditions and the soil properties.
59 Frost heave and frequent freeze-thaw of the ground can effect tank foundations as can exceptionally high tides in tidal areas. This can lead to uniform or differential settlement, planar tilt and edge settlement. In addition to roof binding on floating roof tanks and cracking of welds, settlement can also affect connected piping systems, many of which are fitted with bellows or flexible joints of varying design.
60 The interventions revealed that many operators did not consider it necessary to carry out periodic verticality and settlement surveys.
61 It is worth noting that anchoring flat-bottomed tanks using bolts is not a mandatory design requirement. Many factors are taken into consideration, such as tank foundation type, deadweight, product weight, pressure uplift and roof frangibility. These must be balanced with other risks e.g. tanks floating if bunds fill with rainwater or leaking product or the effects of wind loads. Where anchor bolting is fitted the design codes do specify a minimum diameter and spacing. Clearly, condition evaluation of anchor bolts is fundamental if they are fitted. This can be carried out in conjunction with the foundation inspection.
62 The poor impact resistance of non-metallic tanks is well recognised. Mishandling during transport and poor installation have resulted in their premature failure
63 Brittle failures of tanks have occurred shortly after construction, especially during hydrotesting or on the first filling in cold weather. Experience has shown that once a tank has demonstrated satisfactory service, the risk due to brittle failure in normal operation is minimal. More information on tank hydrotesting is given later in the report.
64 Section 5 of API Std 653 is devoted to an assessment procedure for tanks in brittle fracture service.
65 This section of the report gives examples seen where original manufacturing defects or questionable repairs and alterations have been carried out.
66 Examples of loss of containment have arisen from weld defects, particularly from poor standards of single pass butt welding in floors, shell to floor welds, and from fatigue cracking of welds in the floor and around nozzles.
67 Detailed guidance on the repair and modifications of tanks can be found in both EMMUA 159 and API653 respectively. Patch over plating of metallic tank floors is common, as is whole floor replacement. It is not unusual for floors to be replaced with thicker plates e.g. 6mm being replaced with 8mm plate. Floor replacement also allows the opportunity for foundation material checks and any remediation to be carried out. This is often carried out by jacking the whole tank up, or by cutting out letterbox type sections in turn. Jacking can also be used to correct uneven settlement. Good advice on tank jacking is given in EEMUA 159. In addition a HSE Safety Notice (HID-2-2011) on the jacking of tanks warns of the dangers of getting this procedure wrong.
68 Annular plates typically suffer from corrosion attack around the outer exposed plate edge, which, in extreme cases, can lead to a ‘laminated’ appearance. In addition, both top-side and bottom-side corrosion are relatively common. At one site, through-thickness cracking occurred from a region of undercut along the inner shell to annular plate fillet weld. Most of these problems are caused, or at least exacerbated, by inadequate foundation support under the annular plate and poor drainage. It is also possible that cyclic loading could play a part. Modifications to improve the drainage in this region based, on the recommendations in CIRIA 598 ‘Chemical storage tank systems – good practice: Guidance on design, manufacture, installation, operation, inspection and maintenance’ are often made at the time when new annular plates are installed.
69 Some tanks have been designed to a known standard, yet do not have an annular plate. These tanks may have increased stresses along the floor to shell weld and in the area of this weld, known as the ‘critical zone’. When undertaking repairs in this area without an annular plate, care must be taken not to induce further areas of stress concentration which may initiate potential damage mechanisms.
70 In one other case it was found that a tank floor and annular plates had been replaced and when the tank was subsequently hydrostatically tested, it was reported that a lower shell plate moved inwards towards the tank centre and became distorted, along with two other plates higher up the tank shell. This was attributed to an incorrect alignment between the lower shell course and the new annular plates prior to welding.
71 General shell wall thinning, and subsequent failures are much less frequent than floors but can occur on external floating roof tanks. Articulated roof drain failures are also common on this type of tank.
72 Repairs to lower shell strakes generally involve cutting out and re-welding areas of replacement shell plate (including one or more entire shell plates or full height segments of shell plate) or welding into place lapped patch plates. It should be noted that lapped patch shell repairs are considered to be an acceptable form of repair for butt-welded, lap-welded and riveted tank shells in API Std 653, but only under specified circumstances. If more extensive areas of shell plate require repair then the use of butt-welded shell replacement plate is preferred. The repair of a buckled tank shell should always be done using insert plates.
73 At one site, repairs had been made to corrosion holes in two tank shells using epoxy or polymer based type materials. No formal assessment of integrity threat had been carried out or any indication given of when these would be replaced by a proper repair.
74 The installation of a second drain line into a tank required a repair and modification. Unfortunately, to accommodate both drain pipes in the sump one of the vertical sections of the new steel drain pipe had to be located in close proximity to the sump wall. This eventually resulted in perforation of the sump and a considerable quantity of product was lost. 75 Roof repair procedures are described in API Std 653. If it is necessary to replace roof plates, consideration should be given to strengthening areas used as designated walkways (subject to proper design assessment).
76 Repairs to wind girders are also covered briefly in API Std 653, as are repairs to other features such as penetrations. This document makes frequent reference to the original design code (i.e. API Std 650). In addition, if the original design code was a British Standard then this needs to be used when carrying out calculations or repairs to these areas. The number of wind girders required on tanks should be checked and calculated using the original design code.
77 During the course of the project numerous examples were seen where the competence of those organisations engaged to carry out repairs of storage tanks varied markedly.
78 A number of Operator’s employed Type B, or 2nd party, inspection bodies to carry out the inspection activities while others employed Type A, or 3rd party, inspection bodies. In the case of the Type A bodies, some were independent sole traders (not UKAS accredited) while in other cases the inspectors were the engineer/surveyors from the major engineering insurance companies accredited as Full or Associate members of the Safety Assessment Federation (SAFed).
79 HSE has growing concerns over the differences in performance of the inspection bodies, particularly in respect of COMAH major accident sites. Contacts with the representative trade associations have become more regular recently, in order to address matters of common interest. There is additional guidance in a CA paper, Competent Authority Mechanical Integrity: Use of third party expertise on high hazard sites
80 It was not uncommon to find sites that did not employ dedicated tank inspectors or utilised competent contractor support. Inspector qualification and competencies are covered later in this section.
81 The competencies of those involved with tank inspection varied markedly. In the organisations with the better integrity management programmes, examinations were conducted using a range of techniques, by inspectors qualified to the standards required by API Std 653 or EEMUA 159. There appeared to be widespread knowledge of the EEMUA 159 guidance document, even if not all operators followed its recommendations rigorously.
82 HSE recommends that only qualified and competent persons are engaged for the inspection of atmospheric storage tanks. Provider information is available on the internet giving details on formal courses which train inspectors to either API Std 653 or EEMUA 159 standards.
83 Examples were seen which demonstrated that the inspections performed by single operator inspectors, or others employed by more reputable bodies were not of a high standard and many had never received any formal training.
The following issues were identified:
84. This section gives an account of the main inspection practices seen during the project. There is an abundance of guidance and other publications on this topic and only a brief description is given here.
85. “Vertical, cylindrical, above ground, welded, atmospheric storage tanks located in the UK are designed, fabricated and tested to the rules given in national and international codes. The majority of tanks comply with the rules given in BS 2654, EN 14015 or API 650. Design and construction to the chosen code rules provides a fit for purpose tank. In addition we would advise that inspection works carried out during new build or repair is done by a competent 3rd party inspection body independent of the manufacturer / repairer. The basic philosophy adopted by the code writers means that some aspects of the rules vary from code to code. Obvious examples of this are material designations and minimum shell plate thicknesses.
86. Storage tank inspection, maintenance, modification and repair codes, standards, guidelines and practices are generally linked to the national or international design and fabrication document. For example, EEMUA 159 is supported by BS EN 14015 and API 653 by API 650. Each of the tank assessment and inspection documents list the codes to which they apply in their introduction sections.
87. Prior to assessing an in-service tank to EEMUA 159 or inspecting to API 653/ 575 the tank’s principal design code should first be identified and safety margins established. The design code is usually noted on the tank’s nameplate and construction drawing. The subsequent assessment or inspection of the tank should then be completed to the inspection document that compliments the tank’s principal design code.”
88. The interventions revealed that thorough tank inspections were often very limited in scope before the 1990s. In many instances they were visual only. This situation is improving and numerous techniques were encountered with most operators carrying out external visual checks as part of the routine maintenance function. Nevertheless, examples were found where no proactive inspections of the internal condition of the tanks were being undertaken.
89. Where both proactive and reactive inspections have shown that repair / modification to tanks has been required, we would strongly advise that on completion of works the tank assessor is provided with all relevant documentation / information of the works completed to allow them to complete a final assessment that the tank is fit to be returned to service and signed off.
90. It is worth stating that thorough internal inspections of storage tanks can involve a very high investment of time and resource. Opening up a tank for safe entry and to allow meaningful inspection may require months of preparatory work. The costs can be very substantial and a tank may be out service for extended periods of time.
91 As well as the provision of good access equipment such as ladders, scaffolding, machines etc. it is emphasised that detailed visual internal inspections of tanks can only be carried out under adequate lighting conditions. BS EN Standard 970:1997 ‘Visual Examination of Fusion Welds’ recommends a lighting intensity level for inspection of no less than 350 lx while 500 lx is preferred.
92 Tank cleanliness is also essential, and, depending on the product stored, may involve many weeks of intensive cleaning effort. Other preparatory work can include supporting floating roofs and removing equipment such as heating coils and drains.
93 Frequencies of external inspections varied widely. In some cases a number of external visual inspections would be carried out before one internal thorough inspection was completed. Generally, however, inspections alternated between an intermediate external examination and a thorough internal (and external) examination.
94 Visual inspections of shells on insulated tanks can be problematic. Unless there was evidence of cladding or insulation damage, discolouration etc, little was removed. In some cases Operators required areas to be stripped though the procedures in general permitted a considerable degree of latitude.
95 The most common approach was to remove sections of cladding and insulation adjacent to spiral staircases. It is essential however, that careful examination is made at other vulnerable areas such as below wind girders, other penetrations like pipe stabbings, handrails etc. and at the roof to shell joint. One example was seen where the Operator had incorporated purpose-designed sealed vertical strips into the insulation at the cardinal points around each insulated tank. Each of these contained a number of removable plugs that permitted direct access to the shell. This allowed easy access for repeat thickness checks and also some limited opportunity to check for CUI.
96 The principal codes do not give detail on the amount of insulation removal required; however excellent advice is given in Section 7 of API RP 575 ‘Inspection of Atmospheric and Low Pressure Storage Tanks’.
97 This technique was not encountered during the interventions and appears to be restricted to inspections carried out during construction, or following repairs and modifications.
98 Use of spot pulse echo ultrasonics to carry out thickness checks of shells, floors and roofs became more widespread from 1990 onwards. It is arguably the most practical way of measuring the thickness of the shell, roof, floor and nozzles of a tank. Generally tank floor inspections consisted of a pattern of 5 inspection points per floor plate.
99 Other methods such as long-range ultrasonics have proved popular for checking corrosion deterioration on annular rings. At present this technique cannot be universally applied since there are limitations in regard to the minimum length of projection of the annular and its topside surface condition.
100 Crawler type scanning techniques can be used on shells but are not the norm.
101 The use of floor scanning techniques using magnetic flux leakage and saturated low frequency eddy currents appear to be gaining acceptance as the preferred method for detecting and recording tank floor corrosion, even to the extent that, where it is practical operators now remove hot water, steam and electrically heated pipes temporarily in order to permit the technique to be used in heated tanks. There are a number of specialist providers and confidence in the techniques is now high.
102 There can be limitations in areas of coverage e.g. adjacent to the shell. However it is a relatively simple task to cover small localised gaps by hand scanning.
103 HSE has just supported a research project on magnetic floor flux scanning, ‘Recommended practice for magnetic flux leakage inspection of atmospheric storage tanks floors’, Research Report RR 481, which has concluded that the technique has many advantages over spot ultrasonics.
104 An important consideration when carrying out MFL and SLOFEC inspection techniques on tank floors is the level of cleanliness of the floor. This should be clear of magnetisable debris and fit for a detailed visual inspection as a minimum.
105 Where grit blasting is required it is generally accepted that a standard equivalent to BS EN ISO 8501-1 Sa 2 or Sa 2.5 (or NACE and SA standards) is necessary to carry out meaningful inspection. High pressure water jetting can be a practical alternative to grit blasting though generally it is not thought to be as effective in some circumstances. Depending on thickness of application the equipment can scan directly through some types of lined floors.
106 In the past these techniques have been employed selectively following repair and alterations, rather than as a primary in-service inspection practice. They are useful however where floor plates may be subject to fatigue loading, especially where empty/full cycles are high and for crack detection of floating roof plate welds. An alternative technique for flaw crack detection is ACFM, Alternating Current Field Measurement.
107 When used they tend to be applied more extensively around nozzles and at the floor to annular joint of flat-bottomed tanks. However, the general application of MPI NDT, given its capability of revealing weld defects during tank thorough examinations is an approach which HSE would support.
108 Dye penetrant NDT is used where stress corrosion cracking of stainless steel tanks is a threat. A particularly vulnerable location can be the area around steam heating coil nozzles.
109 This technique is used, but not extensively, and appears more commonly employed as a secondary, supporting technique and a screening tool. AE used at very high sensitivity can evaluate the floor condition by listening for active corrosion and leakage without taking tanks out of service. AE has also been used on cryogenic and refrigerated storage where non-invasive inspection is the norm. In certain circumstances the equipment can be installed on line to provide continuous data. When used for justification of continued service, HSE would expect to see AE used to compliment other techniques for assessing tank integrity
110 This technique, which is very effective, is used more often following a floor repair or where there is a suspected leak, rather than as a normal inspection tool, although it can be used in conjunction with traditional NDT.
111 Under floor pressurisation using low pressure air and soap solution or helium and gas sniffers have been used to detect small corrosion holes and weld defects where tanks have leaked.
112 Less common methods such as Pulsed Eddy Current (PEC) to determine wall thickness without the removal of insulation and where direct access to tank floors and shells is difficult, have been employed with some success.
113 In addition to MFL, another NDE used on tank floors, particularly local to the annualar ring is TALRUT. This uses long range ultrasonics to give an indication of % material loss in the annual plates.
114 We would advise that these newer NDE techniques are used as complementary to the more robust and well established methods available to duty holders / site operators for assessing tank condition. In addition they should never be used in isolation to support the postponement of any planned inspection / examination.
115 The API and EEMUA codes offer procedures for calculating next inspection intervals and remaining life from trendable degradation rates. Minimum thicknesses for floors, shell plates and roofs are given in both. The interventions revealed however that there is only limited use of these procedures. This is perhaps not too surprising given that the period between inspections is so wide, and the inherent difficulty in obtaining reliable and accurate data (particularly for flat-bottomed tank floors) that can be compared with previous results to derive a trend. An additional challenge arises from the unpredictability of uniform corrosion loss.
116 Whilst the periodicity of tank inspection was drawn from the guidance given in the codes it was often the case that the operators set maximum endorsement intervals for examinations less than those given in the codes. In other instances, more frequent access into tanks for cleaning and product quality purposes (e.g. aerofuels) gave opportunities for additional thorough examinations to be carried out.
117 One operator, whose inspection strategy involved external examinations every two years with a thorough internal at the 12th year, introduced a procedure making use of enhanced visual inspection supported by additional NDT at the mid-point between their intermediate and thorough inspections.
118 Section 6 of API Std 653 suggests frequencies based on service history and from corrosion rates determined by previous inspections. Where not known, it recommends external inspections every 5 years and internals at 10 years. Inspection intervals should never exceed 20 years.
119 Table B.3-1 in Section B3 of EEMUA 159 sets out frequencies of tank inspection which include external routine visuals, detailed external inspection and internal inspection. The frequencies can vary dependant on service conditions and products stored, as well as climate conditions. By way of example the maximum interval for internal inspections in temperate climates is given as 16 years.
120 Both the codes now recognise the value in adopting RBI principles to improve the availability of equipment while ensuring its integrity is not compromised, though they also highlight some limitations with respect to non-uniform degradation mechanisms and sparse data.
121 The postponement of any examination, whether it be an intermediate or thorough, needs careful consideration. The full operational parameters, inspection history and any potential or known degradation mechanisms need to be taken into account through some form of risk assessment. The consequences of a potential loss of containment to people, plant and the environment need careful considered assessment. This process may require the involvement of more than a single individual as specialist knowledge e.g. tank assessor, may be required to ensure that an appropriate decision is arrived at.
122 The quality of tank inspection reports varied widely. In a small number of cases, these were very comprehensive and included photographs, sketches or more detailed drawings along with checklists. The best incorporated floor, shell and roof sketches for vertical tanks and elevation schematics for horizontals. Many included positions of man ways, main nozzles and small-bore branches, heating coils, drains, sumps etc. Sketches were often used to identify thickness measurement locations and the results.
123 At other sites, the quality of the inspection reports and the standard of information recorded were poor. This is unsatisfactory. Thorough internal examinations of tanks are carried out infrequently, and it is their quality which offers the principal means by which the integrity of the tanks can be determined, often many years on.
124 Many examples were found in reports where unhelpful comments such as ‘satisfactory where seen’; ‘shell not visible – insulated tank’; ‘parts inaccessible’ etc. had been made by inspectors without challenge by the operators. Inspection procedures found at some sites acknowledged difficulties for examinations, for example with insulated tanks or around masked or problem access areas etc. but offered no suggestions of alternatives or of how these could be satisfactorily resolved.
125 Use of the inspection reference survey point or grid system set out in EEMUA 159 was not common, though it was employed at a small number of sites. The principle was used more widely for tank floors. There appeared great reluctance to adopt the guidance given in the EEMUA or API codes to domed bottom or horizontal tanks even when much of the detail in these was of obvious value. The use of the checklists set out in both of the codes was variable though there is some evidence that recognition of their value is increasing.
126 Frequent examples were encountered where deterioration and defects that should have been found by inspectors had been missed. This raised serious questions about the thoroughness of the process and the competence of those carrying out the examinations.
127 In other instances defects had been correctly identified and recorded during inspections but no action taken by the operators. In one case this led to the operator modifying their procedures by introducing corrective actions punchlists which then had to be signed off by a senior engineer.
128 The interventions revealed a general requirement for improved inspection reporting particularly as the period between thorough tank examinations is wide.
129 Increasing use is being made of floor scanning techniques. The quality of the reports from these surveys is generally high, providing good detail.
130 It is a commonly held misconception that non-metallic storage tanks are suitable for indefinite service and that they require little or no maintenance and inspection.
131 Four visits were included in the project to follow-up serious leaks, failures or issues associated with GRP and thermoplastic storage tanks. In three of the incidents the tanks had exceeded their original design life (calculated on the basis of creep deformation) and had split in or near to the weld seams. Some evidence of fatigue damage was also found in the shell of one of the tanks that leaked. HSE was also informed of a serious fire in a Polypropylene tank caused when an electric immersion heater failed to trip on reaching a high temperature limit.
132 Of particular concern are aspects such as material compatibility and continued fitness-for-service for tanks approaching, or even exceeding, their original design life.
133 There are a number of national and international codes and standards available and in widespread use for the design and manufacture of non-metallic storage tanks. These are referenced at the end of this document. In addition, HSE publishes two guidance documents, PM75 and PM86 which provide advice to users of such plant.
134 The arrangements for managing the integrity of horizontal tanks on most sites visited during the course of the project were poor. Sites, which had comprehensive tank integrity management programmes in place for their vertical storage tanks, had nothing so thorough for horizontal tanks. This was not uncommon.
135 Many riveted construction horizontal tanks of significant age were still seen in use. Other examples were observed where tanks clearly designed for vertical use had been modified and installed horizontally. Little or no information was available concerning the thickness of the shell wall directly under the saddle supports. On many bona-fide horizontal tanks these areas are reinforced with doubler plates.
136 A maintenance and inspection regime at one operator’s site treated horizontal storage tanks in a similar manner to that in place for their pressure vessels even though these were not in pressurised service.
137 BS EN 12285 Parts 1 and 2 ‘Carbon steel welded horizontal cylindrical storage tanks’ is a recognised manufacturing standard for atmospheric horizontal storage tanks, however it does not deal with issues of in-service inspection and maintenance matters and it has been noted that there is no other specific guidance material for this type of tank.
138 Horizontal tanks can be vulnerable to corrosion above the saddles (which may or may not be integral to the tank). In certain applications, where thermal expansion is expected, the tank may be supported on steel saddles which are not physically attached to the shell. In other instances the saddles may be fully seam welded to the shell while in other cases the welding is not continuous. Concrete type saddles often have bitumen or other packing material between them and the tank shell. These can often be a trapping point for water. The interventions revealed many instances of serious corrosion around the saddle areas.
4 At the present time it is understood that NDT techniques are not available which can detect localised shell corrosion above and around the saddle areas from outside of the tank. In most cases assessment of deterioration in these areas will require internal access.
139 Localised corrosion was commonly found around small-bore piping stubs and at gantry, handrail attachment points plus other vulnerable areas such as bottom drains.
140 At one site a tank, originally designed as a vertical tank, was being used in the horizontal position to store waste hazardous liquids without any technical justification.
141 The quality of inspection reports for horizontal tanks was, if anything, worse than that found for verticals. In only a small number of cases were drawings and sketches employed to detail inspection locations and record results.
142 Questions involving piping, bellows and other flexible joints were not central to this study. Nevertheless because of their close association with tanks some of the issues are worthy of comment.
143 It was not uncommon to find poor piping standards in tank farm areas. There seems little doubt that piping in these areas is not given the same level of attention as process piping.
144 Examples of badly aligned, supported and corroded piping were widespread. Other features such as buried piping and ‘masked’ areas through bund penetrations were also of concern due to the difficulty of carrying out meaningful inspections at these locations. Failures are not uncommon due to wall thinning corrosion of piping caused by trapped water in the bund wall.
145 It was common to find piping lying unsupported just above and on the ground. Even where there was poor bund drainage, piping was seen partially or completely submerged under water.
146 At one site, piping angular displacement and expansion directly away from the tanks was managed by employing flexible type couplings. Following a failure it was established that the coupling pressure rating was less than the maximum delivery head of the associated pumping system.
147 Bellows and other purpose designed flexible joints are often an integral part of tank installations. They are installed where settlement or other movement between the tank and the downstream piping is greater than can be accommodated in the piping design or where expansion and contraction of the shell is expected. They are often fitted at the tank side of the first isolation.
148 Inspections of these joints may be difficult owing to their construction. Often bellows convolutions are protected by steel covers. Operators should ensure that the integrity of these vulnerable components is not missed during the tank examinations.
149 Ideally they should be subject to close visual inspection and measurement (for corrosion, alignment, squirm, seizure of guides, hinges, links etc.) at intermediate inspections, and, where possible, removal with thorough NDT plus pressure testing during tank outages. Detailed inspection should also be carried out following any system excursion or abnormal external event.
150 In some instances Operator’s had established that tanks were no longer subject to settlement problems and examples were found where technical reviews were being carried out to determine whether flexible connections could be removed by utilising piping expansion loops and spring type supports.
151 Proper venting and relief is critical to protect against possible catastrophic failure of atmospheric storage tanks. The interventions revealed however that this is not a well-understood concept, and numerous examples were found of poor maintenance and inspection standards of venting equipment.
152 It is important to understand the differences between normal pressure/vacuum relief and emergency pressure relief. Normal venting rates, which cater for liquid movement pumping-in and pumping-out, plus thermal outbreathing and inbreathing, can be calculated using appropriate formula. Emergency venting in the case of external fire may require that the tank has a frangible type roof or additional equipment such as hinged lids fitted (sometimes referred to as Whessoe Lids). Advice on emergency venting and frangible roofs is given in Annex K of BS EN 14015; API 650 and EEMUA 180:96 ‘Guide for Designers and Users on Frangible Roof Joints for Fixed Roof Storage Tanks’.
153 For normal venting, tanks can be fitted with a variety of pressure and vacuum relief devices. These may range from simple open vents (swan neck or Chinese hats) to more complex combined pressure-vacuum valves.
154 Roof vents and pressure–vacuum valves should be inspected at regular intervals to ensure they function properly. It is not unknown for vents to be blocked by bird’s nests, vermin, debris and ice, or even purposely during maintenance. Open vents should be installed with wire netting with openings of at least 6mm to prevent birds nesting. Note: It is not normal to fit flame arrestors to tanks equipped with PV-valves.
155 Pressure–vacuum valves are vulnerable to blockage or mal-operation caused by contact with the vapour of waxing, sublimating, crystallising or polymerising materials. Phenol, aniline and styrene are typical examples. In some of these cases PV-valves can be trace heated and insulated and sometimes low temperature alarms are fitted.
156 Good schemes of examination adopted frequencies of examination based on the likelihood of vent or PV valve failure, and the scope included removal of the valves for checks, cleaning and resetting of pallets. Other considerations for inspection may be required if tanks are equipped with nitrogen blanketing systems.
157 All types of valve connections with storage tanks should be considered when assessing the integrity of the tank. These include, but should not be limited to:- isolation, Remote Operated shut-Off Valves (ROSOV’s), drain, flow control. The maintenance guidance for such items can be sought from original equipment manufacturer.
158 The inspections revealed that the use of second-hand tanks is common. In many cases the design and codes used for the tank construction, or the history of use were not known. This was not limited to smaller sites and in many cases re-evaluations had been undertaken on tanks where there was no history or where the history was lost when the site ownership changed.
159 In some cases, operators have been requested to carry out design gap analyses where original design information was not available, and particularly where there was little or no baseline data and tank history.
160 At one site, most of the tanks had been purchased from suppliers of second hand tanks and pressure vessels without any supporting documentation. They had then been put into service without any detailed technical fitness-for-service assessments. This is not good practice.
161 Comprehensive guidance on tank coatings and protection is given in API RP 652, ‘Lining of Aboveground Petroleum Storage Tank Bottoms’, and EEMUA Publication No. 183: 1999, ‘Guide for the prevention of bottom leakage from vertical, cylindrical, steel storage tanks’.
162 Epoxy coatings are frequently used to protect tank floors and the lower shell strakes, but care is needed to ensure that the tank surfaces are properly prepared before the coating is applied, that the correct thickness is applied and curing procedures are followed, as breaks in the coating can lead to accelerated corrosion. Other coating products are available that offer improved resistance to chemical attack and the manufacturer’s advice should be sought for particular applications.
163 Where coatings have been used, both internally and/or externally, careful inspection of their integrity should be carried out thoroughly and by a competent person. Close inspection around pipe branches, attachments, etc. and where liquid can collect or pool should be considered. The repair / replacement of such protective coatings, either for anti-corrosion or product quality purposes should be completed in accordance with manufacturer’s guidance and by approved contractors for that product.
164 Cathodic protection systems for tanks were not in widespread use on the sites visited during the course of the project. It should be noted that cathodic protection systems should not be used where the tank is founded on bitumen/sand or similar, as a hydrocarbon layer will provide a high resistance barrier to prevent the current passing through.
165 The subject of testing and examination requirements for coating repairs does not appear to be addressed by API Std 653 or EEMUA 159 but some information is contained in API RP 575 and also API RP 652 ‘Lining of Aboveground Petroleum Storage Tank Bottoms’
166 There are a number of reasons why British and API Standards require that storage tanks are filled to the top of the shells with water for testing
5 Note that, unlike BS 2654, tanks designed and manufactured to API Std 650 are designed for a stated product specific gravity. If the tank operator wishes to store products of higher specific gravity in the tank at some later stage then a lower maximum filling height should be determined to prevent the tank being overstressed (see page 51 in ’Guide to Storage Tanks and Equipment’, by Bob Long and Bob Garner).
167 Almost all hydrotesting is therefore carried out immediately after construction of the tank. The codes do however specify that hydrotesting is also carried out after significant repairs or modifications have been made.
168 Re-hydrotesting is normally required after:
169 The principal codes also highlight important factors that should be considered before carrying out hydrotests such as water quality and temperature, filling rates, hold times, filling and emptying procedures.
170 API Std 653 sets out a number of conditions which allow a tank back into service after modifications or alterations without hydrotesting. Section 16 of EEMUA 159 also gives advice on this matter.
171 It should be noted that the requirements for hydrostatic testing following repairs are more stringent in EEMUA 159 (as amended in February 2004) than API Std 653, particularly following shell jacking operations. It is also worth noting that API Std 653 (clause 18.104.22.168) permits fitness-for-service assessments to be adopted to exempt a repair from hydrostatic testing but is silent on the procedures and acceptance criteria that should be followed. This is an area that may merit further discussion.
BS EN 14105
Specification for the design and manufacture of site vertical, cylindrical, flat bottomed, above ground storage, welded steel tanks for the storage or liquids at ambient temperature and above. (this replaces BS 2654)
API Std 620
Design and construction of atmospheric and low pressure storage tanks
API Std 650
Welded steel storage tanks for oil storage
BS EN 12285-1:2003
Workshop fabricated steel tanks. Horizontal cylindrical single skin and double skin tanks for the underground storage of flammable and non-flammable water polluting liquids (this replaces BS 2954)
BS EN 12285-2:2005
Workshop fabricated steel tanks. Horizontal cylindrical single skin and double skin tanks for the aboveground storage of flammable and non-flammable water polluting liquids SS
Oil burning equipment – specification of oil storage tanks
Steel Aboveground Tanks for Flammable and Combustible Liquids
BS 13121 Parts 1-5
GRP tanks and vessels for use above ground(This replaces BS 4994: 1987, however this is still in circulation)
API RP 12R1
Fibre reinforced plastic tanks
BS EN 12573 Parts 1-4
Welded static non-pressurized thermoplastic tanks
Standard specification for polyethylene upright storage tanks
EEMUA Publication Number 159: 2003 (amended 2004 and 2005), ‘Users’ Guide to the Inspection, Maintenance and Repair of Aboveground Vertical Cylindrical Steel Storage Tanks’
API Std 653: Tank Inspection, Repair, Alteration, and Reconstruction
API RP 575: Inspection of Atmospheric and Low Pressure Storage Tanks
API RP 579-1/ASME FFS-1 2007 : Fitness for Service(second Edition)
PM 75: Glass reinforced plastic vessels and tanks: advice to users
PM 86: Thermoplastic tank integrity management : advice to uses
A number of sources of guidance and further information were consulted during the course of the project and the following were found to be particularly helpful:
Chemical storage tank systems – good practice: ‘Guidance on design, manufacture, installation, operation, inspection and maintenance’ (C598), published by CIRIA in 2003 (ISBN 0 86017 598 7).
EEMUA Publication Number 183: 1999, ‘Guide for the prevention of bottom leakage from vertical, cylindrical, steel storage tanks
HSE Research Report RR 481, ‘Recommended practice for magnetic flux leakage inspection of atmospheric storage tanks floors’
EEMUA 180:96 ‘Guide for Designers and Users on Frangible Roof Joints for Fixed Roof Storage Tanks’
‘Guide to Storage Tanks and Equipment’, Bob Long and Bob Garner, Professional Engineering Publishing, 2004, ISBN 1860584314
6 A comprehensive description of the development of internationally recognised design and construction codes for both steel and concrete storage tanks is given in Section 2.9 of this reference book.
API RP 651: Cathodic Protection of Aboveground Storage Tanks
API RP 652: Lining of Aboveground Petroleum Storage Tank Bottoms
HSL Report HSL/2006/21: Specification and Inspection of Thermoplastic Storage Tanks
HSG 41: Petrol Filling Stations: Construction and Operation
HSG 176: Storage of flammable liquids in tanks
HSG 235: Guidance on the storage of Hydrochloric and Nitric Acid in tanks National Sulphuric Acid Association (NSAA) ‘Recommended guidelines for the bulk storage of sulphuric acid, oleum and liquid sulphur trioxide’
For further information regarding this circular please contact: HID CI Mechanical Engineering Team CI 1F 0131 247 2027