1996: replacement of an ammonia storage tank

Replacement of an Ammonia StorageTank

A review of an ammonia storage tank ultimately led to the tank being removed from service and areplacement tank being constructed. The design and construction of the tank, material choice for a

new tank, installation, and commissioning are discussed.

John S. Shipman and Ray DaviesICI Chemicals and Polymers Ltd., Wilton, Middlesbrough, Cleveland TS90 8JA, England

Introduction

In June 1994, a proposed minor modification to anammonia storage tank led to a review of its overalldesign and construction. This review ultimately led

to the tank being removed from service and a replace-ment tank being constructed.

This article describes: The design and constructionof the tank; concerns that had developed in 1975, aftera period of 20 years operation, consequential actionsand the subsequent discovery of cracking; the 1995 fit-ness-for-service review; the options for replacementand differences between published recommendations;design issues and consequences for the choice of con-struction material; and the installation and commis-sioning of the tank.

Operational Details

The No. 3 Ammonia Storage Vessel (3 ASV) at ICIBillingham, England, provided a capacity of 183 teand together with the No. 5 Ammonia Storage Vessel

(5 ASV), a 600 te capacity sphere, was used to providebuffer storage for the site ammonia main pressure con-trol system (Photo 1). The storage conditions were 200mbarat-29°C and the design temperature was set at -33°C cor-responding to complete depressurization. In normaloperation, it would be 5 ASV which supplied the sys-tem with anhydrous liquid ammonia via a pair ofpumps, 3 ASV being used as backup should 5 ASV beunavailable due to inspection requirements or unfore-seen circumstances. There was a requirement for 3ASV to be brought into service rapidly and at shortnotice, when up to 300 te/h could be pumped in.

In addition to the duty described above, both vesselscould be used as buffer storage for the site should thesite export facility fail.

Minor Modification

In June 1994, a project was initiated to installremote actuated isolation valves in the inlets and out-lets of all ammonia users on the Billingham Site, This

AMMONIA TECHNICAL MANUAL 138 1997

was to enable all ammonia inventories to be isolatedand partitioned in the event of a major incident. Theproject required a pipework modification to enable thenew isolation valve to be installed. However, due tothe age of the installation, the existing pipework wasfitted with "Sternes" flanges. These flanges arescrewed onto the pipe and use a solid aluminum ringas the gasket; this is deformed when the joint is made.

Site policy was to replace these flanges whereverpossible and as the 3 ASV outlet was fitted with one ofthese, the decision was taken to substitute it. This con-stituted a registered vessel modification under ICIinternal procedures requiring formal design verifica-tion by a nominated, specialist engineer. In preparingthe design package for verification, it became apparentthat there were a number of unsatisfactory design andconstruction features concerned with the tank as awhole and doubts began to arise regarding its suitabili-ty for the required service.

Description

With some exceptions, the original tank was a typi-cal example of the vertical, conical roof, atmospherictanks used for the storage of a wide range of liquids inthe petrochemical industry (Figure 1). Installed in1957, no record could be found that the tank had beendesigned, constructed, and tested in accordance withany national code, whereas a modern tank for this dutywould be expected to comply with B S 7777 in theUnited Kingdom or API 620 in the United States.

(1) BS7777. Flat-bottomed, vertical, cylindrical stor-age tanks for low temperature service

(2) API 620. Design and construction of large, weld-ed, low-pressure storage tanks.

The tank was 20 ft (6.1 m) dia. by 30 ft (9.1 m) highwith a roof slope of 1 in 5 and had been constructedfrom a now obsolete structural quality mild steel inaccordance with BS 14.

(3) BS14. Structural steel for the pressure parts ofmarine boilers.

This British Standard did not specify any impact testrequirements.

The tank was provided with 36 holding-down boltsto prevent the occurrance of uplift due to the prevail-ing pressure conditions. This phenomenon occurs

when the force on the underside of the roof, due tovapor pressure, exceeds the weight acting downthrough the shell causing the shell to pull upward onthe floor. This upward pull causes severe local defor-mation of the floor and rapidly leads to tearing of theshell-to-floor weld and release of the tank contents(Figure 2).

A notable, but undesirable, feature of the tank wasthe use of fillet welds for the circumferential seamsbetween the shell strakes (Figure 3). This has beenprohibited within ICI for many years and is not anapproved method in modern British tank codes; allseam welds must be made by butt welding.

Figure 3 shows a square edge butt weld as would befound on a typical 5-mm-thick strake. Thicker strakeswould have single- or double-sided vee preparations.

Another notable feature was the absence of thermalinsulation. This had deteriorated to a level necessitat-ing removal in 1976 and had not been replaced,

Historical Concerns

In 1976, the Production Area Engineer with respon-sibilities for the tank raised concerns as to its suitabili-ty for the storage of refrigerated liquid ammonia.There were three specific issues:

(1) The materials of construction did not meet thencurrent requirements for service at -33°C.

(2) The lap welded construction, as detailed above.(3) The generation of large thermal stresses in the

shell, due to the temperature gradients which wouldarise on filling the ambient tank with cold ammonia -made worse by having no thermal insulation.

These issues were dealt with to the satisfaction ofthe raiser essentially by invoking the principle knownas grandfathering, i.e., "If its been alright for nearly 20years it will continue to be alright." A concession wasmade to the thermal gradient issue by specifying that acold heel of ammonia should always be kept in thetank to produce a continuous chilling effect. This heelhad a depth of 300 mm, accounting for about 20 te ofammonia.

The tank was placed on an inspection interval of 6years and continued in service.

Magnetic particle inspection of the inside of the tankin 1982 and 1988 revealed an amount of cracking in


Photo 1. Sphere (5ASV) together with the replacement stainless steel tank (6ASV).

Liquid Hold downBolts

Concrete Base

\

Vapour Pressure

Deadweight

Base \

Figure 1. Diagram of tank. Figure 2. Upward pull causes severe local defor-mation of floor and leads to tearing of shell-to-

floor weld and release of tank contents.


welds and parent plate. These were deemed acceptable-by ICI materials engineers, although it does not appearthat any view was reported as to their cause.

In 1994, triggered by the need for the minor modifi-cation to be design verified, and the evidence ofdesign and construction deficiencies and cracking inNo. 3 ASV, the Maintenance Manager responsible forthe equipment called for a formal fitness-for-servicereview under ICI procedures.

Fitness-for-Service Review

This was carried out by a team comprising a special-ist vessels engineer, a specialist materials engineer, astress analyst, an equipment inspector, and the respon-sible maintenance manager.

Two key issues emerged from the review:(1) The heel of ammonia was not doing what had

been hoped for, and the tank shell warmed up to ambi-ent about 300 mm above the surface.

(2) Using toughness data equivalent to the fully brit-tle condition (because nothing else was available), thecritical defect sizes determined were so small that theycould not all be guaranteed to be found.

It was concluded that the team could not provideevidence adequate to satisfy ICI's internal pressurevessel design verification policy and that the tankwould have to be replaced.

Replacement Options

Two main replacement options were considered,these being:

(1) Replace the complete installation with a pressur-ized storage (1.38 barg) facility in a different locationon the site.

(2) Replace 3 ASV with a similar tank, but designedand constructed to modern standards.

The reasons for considering the first option weretwofold:

(a) The existing installation is located at the edge ofthe site close to a public road. Moving the installationto a more central position in the site may have reducedthe risk of an ammonia incident affecting the generalpublic.

(b) The existing system operates at 200 mbar and

therefore generates flash gas at 200 mbar. This is usedat this pressure by one consuming plant on the site andif this is not available, the gas must be boosted in pres-sure for use by others. The higher pressure storagewould, therefore, offer greater operational flexibility.

On detailed examination, it was concluded that theadvantage gained by moving towards the center of thesite was offset by the fact that a leak from 1.38 bar gstorage would be more severe, due to the generation ofa greater quantity of flash gas. Furthermore, theadvantages of greater operational flexibility were con-sidered to be of little importance.

An additional factor was the recognition that due tothe continuous purge from 3 ASV and 5 ASV, neithervessel had suffered significant stress corrosion crack-ing problems.

It was decided, therefore, to replace this tank withone essentially of the same type, but with deficienciesin the design, material specification, and welding rec-tified; i.e., full compliance with modern national stan-dards with butt welded seams, impact tested material,and appropriate quality control. This proposal immedi-ately ran into opposition from the responsible engineerrepresenting Id's Safety and Loss Prevention Dept.over the issue of single containment.

It is quite clear that the relevant national tank stan-dard, BS 7777 (Reference should be made to annex Aof BS 7777), permits single containment of ammonia -provided certain more severe materials requirementsare met. This is also the published view of the U.K.'sEngineering Equipment and Materials UsersAssociation (EEMUA), which is an organization ofsubstantial purchasers including ICI, BritishPetroleum, Shell, and Exxon. However, the publishedrecommendation of the Chemical IndustriesAssociation was the use of double containment forlarge ammonia storage vessels and ICI is a leadingmember of that body. Examples of double containmentare shown in Figure 4, and it can be seen that theexpense and degree of sophistication is significantlygreater. The desire to improve the safety of the emer-gency ammonia storage now threatened to turn into anexpensive overkill.

After examination of the relevant standards and dis-cussion about whether 200 te capacity constituted alarge ammonia storage tank, the decision was made to


Lap & Fillet

,.Butt-Weld

Shell Strakes

Original Tank Modern Requirement

Figure 3. Circumferential seam welding in tankshell.

replace it with single containment and go along theroute of enhanced material properties.

Requirements for Materials

BS 7777 groups materials into 6 types and relatesthe choice to single or double containment and productstorage temperature (Table 1). These types are relatedto material generic types and specific impact testrequirements (Table 2). As can be seen, the use of sin-gle containment requires enhanced material require-ments to ensure adequate safety of storage.

The introduction of the delta T term is intended tomake an allowance for the reduction in toughness inthe heat affected zone of a weld. The steelmaker isrequired to obtain impact energy transition curves forthe parent plate and for the HAZ of a test weld in

D 1 1 1 1 UExternal Insulation

Basa Insulation

r

-

jJJJJ-*-r • ~ -^J-UJpRoot

Inner Tank

i i i i i i i i r - iJ — 1 — 1 1 1 1 — 1 — 1 1 1

J

-

1—•• -

1

r

_ Cover It Required

External WeatherBarrier

Bottom TankHeater

Suspended Deck

Cover« Required

Concrete Outer Base

Cover it Required

Loose FÏH Insulation

Outer She« (not »Weto contain BquidJ

Figure 4. Examples of double containment tanks.


ImpactEnergyJ

27J

Plate HAZ

delta TTest Temperature T

Figure 5. Determination of delta T.

Picture 2. New stainless steel tank in position.

Picture 3. New tank being lifted into position.


Table 1. Material Types for Tank Shell and Bottom

Product

ButaneAmmoniaPropane/propyleneEthane/ethyleneLNG

SingleContainment

Type IIType IIType IIIType IVType V or VI

Double or FullContainment

Type IType IType IIType IVType IV(1)

Typical ProductStorage Temperature

-10°C-35°C-50°C-105°C-165°C

*For thicknesses greater than 30 mm and less than or equal to 40 mm, Type V or VI is necessary.

Table 2. Longitudinal Cfaarpy V-Notch Impact Testing

Classification Steel type Tested per plate(1), (2)

120J tested per40te batch (3)

Type IType IIType III

Normalized carbon-manganeseImproved Toughness C-MnLow nickel steel9% nickel steelImproved 9% nickel steelAustenitic stainless steel

273 at -50°C27J at -50°C-deltaT27Jat-80°C-deltaT35J at -196°C100J at -196 CNo impact testing

Not required-20°C-50°CNot requiredNot requiredNot required

Notes:(1) Energy value (Column 3) is the minimum of three specimens with only one single value less than the value specified and with

no single value less than 75% of the value specified.(2) For material thickness less than 11 mm, 10 mm x 5 nun subsize specimens are to be used, and demonstrate 70% of the values

specified in this table. For Type V steel, the value is to be 50% of the value specified in this table.(3) Impact testing is carried out on each plate to demonstrate the required impact value. In addition, testing at a frequency of

one test per 40 te batch is to be carried out to demonstrate the 120J requirement (see annex A). The definitions of plate and batchare given in BS EN 10025.

(4) Reference should be made to annex A of BS 7777.


order to determine delta T (see Figure 5).This procedure for arriving at the 27J temperature is

intended to ensure that 27J is achieved in the HAZ ofthe site-made weld at 25°C ±5°C lower than the designminimum temperature.

The 120J requirement is to give reasonable assur-ance that the Cv transition temperature is at least 30°Cabove the impact test temperature.

It was realized within ICI that the enhanced materialrequirements would be a problem, because the platetonnage quantity was too small to form a reasonablemill order. Quantities such as this normally come fromspecialist stock holders.

Quotations for the tank in carbon-manganese steelproved this to be the case and no tank vendor quotedenhanced grade material. In fact, the vendor mostinvolved in the drafting of BS 7777 stated that theenhanced Type II material was unnecessary becausethe tank was not large and quoted Type I as did theothers. ICI decided that the use of Type I did not meetit's safety requirements.

A Better Solution

It was decided that apart from an increase in the ini-tial capital cost of the tank - a smaller percentage inthe context of the whole replacement project - theroute which offered the best all-round solution was theuse of austenitic stainless steel. The comparison oftank costs was $110,000 for the carbon-manganeseagainst $208,000 for the stainless steel tank. However,the lower cost was for Type I material and would haveincreased significantly for Type II: the delivery timewould also have been quite unacceptable.

Austenitic steel is obviously a much safer materialbecause of its immunity to brittle fracture at the stor-age temperature of liquid ammonia - even underimpacting missile conditions. In addition, the follow-ing advantages are obtained:

(1) The tank does not require the application ormaintenance of a paint system.

(2) The tank is not vulnerable to the corrosion which

could be expected with carbon-manganese steel underthe freeze/thaw conditions of intermittent storage.

This is particularly important with regard to under-floor corrosion which cannot be seen directly.

(3) The tank is not prone to anhydrous ammoniastress corrosion cracking.

(4) After the initial inspection, it is not necessary tocarry out an invasive inspection again. This elimina-tion of the tank preparation and entry work is a signifi-cant cost saver for the future.

Installation

It is conventional to erect a site-built storage tank onthe base where it is to remain for its operating life.However, in order to minimize disruption to the avail-ability of 3 ASV, especially severe because of accessproblems, it was decided to build the new tank onopen ground to the side of the storage facility. Uponcompletion of the new tank and removal of the old, thenew tank was lifted into position (Photos 2 and 3).

Other Features

The new installation is monitored by CCTV fromthe local control room.

It is protected by remote actuated isolation valves,which enable the tank to be partitioned should an inci-dent occur. These can either be actuated locally orfrom the main site operations center.

Conclusions and Recommendations

(1) The discipline of putting a minor modificationthrough a formal design verification system helped toidentify that an existing ammonia storage vessel wasunfit for further service.

(2) The option taken and the design and specifica-tion selected for the replacement was influenced by arisk assessment on the probability of a major vesselfailure, together with an assessment of the conse-quences of such a failure.


DISCUSSIONR. Frey, M. W. Kellogg: Relative to single and doublecontainment storage tanks and ammonia, LPG/LNGand so forth, I think the issue is if you have a singlecontainment tank, you also should have a dike/bundaround it. In your picture, I didn't see any dikes orbunds. How did you rationalize the fact that you havea single containment tank with very high integritywithout a bund? The original tank also didn't seem tohave a dike around it.Davis: Essentially, the justification was the heavilyreduced or eliminated possibility of failure due tocauses that might be expected with this kind of tank. It

was also justified on the basis of a substantialimprovement on what had existed there already for 40years. What we did is put forward the rationale andenable the operating plant to actually make the deci-sion about whether or not to install that tank.Frey: That seems to me to be a violation of the codes.Shipman: There is actually a low-level full-contain-ment bund around this tank, which perhaps didn'tshow up in the photographs.Davis: However, it surrounds a group of tanks ratherthan a single tank.


1996: replacement of an ammonia storage tank

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