Replacement of an Ammonia John S. Shipman and Ray Davies

Storage Tank

ICI Chemicals and Polymers Ltd., Wilton, Middlesbrough, Cleveland TS90 8JA, England

In June 1994, a proposed minor modification to an ammonia storage tank led to a review of its overall de- sign and construction. This review ultimately led to the tank being removed from service and a, replacement tank being constructed.

This article describes: the design and construction of the tank; concerns that had developed in 1975, after a period of 20 years operation, consequential actions and the subsequent discovery of cracking; the 1995 fitness- for-service review; the options for replacement and dif- ferences between published recommendations; design issues and consequences for the choice of construction material; and the installation and commissioning of the tank.

OPERATIONAL DETAILS

The No. 3 Ammonia Storage Vessel (3 ASV) at ICI Billingham, England, provided a capacity of 183 te and to- gether with the No. 5 Ammonia Storage Vessel (5 ASV), a 600 te capacity sphere, was used to provide buffer storage for the site ammonia main pressure control system (Figure 1). The storage conditions were 200 mbar at - 29°C and the design temperature was set at -33°C corresponding to complete depressurization. In normal operation, it would

be 5 ASV which supplied the system with anhydrous liquid ammonia via a pair of pumps, 3 ASV being used as backup should 5 ASV be unavailable due to inspection require- ments or unforeseen circumstances. There was a require- ment for 3 ASV to be brought into service rapidly and at short notice, when up to 300 te/h could be pumped in.

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

MINOR MODIFICATION

In June 1994, a project was initiated to install remote ac- tuated isolation valves in the inlets and outlets of all am- monia users on the Billingham Site. This was to enable all ammonia inventories to be isolated and partitioned in the event of a major incident. The project required a pipework modification to enable the new isolation valve to be in- stalled. However, due to the age of the installation, the ex- isting pipework was fitted with “Sternes” flanges. These flanges are screwed onto the pipe and use a solid alu- minum ring as the gasket; this is deformed when the joint is made.

Site policy was to replace these flanges wherever possi- ble and as the 3 ASV outlet was fitted with one of these,

FIGURE 1

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Sphere (SASV) together with the replacement stainless steel tank (6ASV).

Fall, 1997 165

the decision was taken to substitute it. This constituted a registered vessel modification under ICI internal proce- dures requiring formal design verification by a nominated, specialist engineer. In preparing the design package for verification, it became apparent that there were a number of unsatisfactory design and construction features con- cerned with the tank as a whole and doubts began to arise regarding its suitability for the required service.

r

DESCRIPTION

With some exceptions, the original tank was a typical example of the vertical, conical roof, atmospheric tanks used for the storage of a wide range of liquids in the petro- chemical industry (Figure 2). Installed in 1957, no record could be found that the tank had been designed, con- structed, and tested in accordance with any national code, whereas a modern tank for this duty would be expected to comply with BS 7777 in the United Kingdom or API 620 in the United States.

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

(2) MI 620. Design and construction of large, welded, low-pressure storage tanks.

The tank was 20 ft diameter by 30 ft high with a roof slope of 1 in 5 and had been constructed from a now obse lete structural quality mild steel in accordance with BS 14.

(3) BS14. Structural steel for the pressure parts of ma- rine boilers.

This British Standard did not specify any impact test requirements.

The tank was provided with 36 holding-down bolts to prevent the occurrance of uplift due to the prevailing pres- sure conditions. This phenomenon occurs when the force on the underside of the roof, due to vapor pressure, ex- ceeds the weight acting down through the shell causing the shell to pull upward on the floor. This upward pull causes severe local deformation of the floor and rapidly leads to tearing of the shell-to-floor weld and release of the tank contents (Figure 3).

A notable and undesirable feature of the tank was the use of fillet welds for the circumferential seams between the shell strakes (Figure 4). This has been prohibited within ICI for many years and is not an approved method in mod- em British tank codes; all seam welds must be made by butt welding.

5

Liquid Hold down Bdts

/

FIGURE 3 Upward pull causes severe local deformation of floor and leads to tearing of shell-to-floor weld and release of tank contents.

Figure 4 shows a square edge butt weld as would be found on a typical 5 mm thick strake. Thicker strakes would have single or double-sided vee preparations.

A further notable feature was the absence of thermal in- sulation. This had deteriorated to a level necessitating re- moval in 1976 and had not been replaced.

HISTORICAL CONCERNS

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

(1) The materials of construction did not meet then cur- rent 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 would arise on fill- ing the ambient tank with cold ammonia-made worse by having no thermal insulation.

These issues were dealt with to the satisfaction of the raiser essentially by invoking the principle known as grandfathering, i.e., “If its been alright for nearly 20 years it will continue to be alright.” A concession was made to the thermal gradient issue by specifying that a cold heel of am-

a Fillet

Shell Strakes \

Butt-Weld

/

Original Tank Modem Requirement

Circumferential seam welding in tank shell. FIGURE 4 FIGURE 2 Diagram of tank.

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monia should always be kept in the tank to produce a con- tinuous chilling effect. This heel had a depth of 300 mm, accounting for about 20 te of ammonia.

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

Magnetic particle inspection of the inside of the tank in 1982 and 1988 revealed an amount of cracking in welds and parent plate. These were deemed acceptable by ICI materi- als engineers, although it does not appear that 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 of design and construction deficiencies and cracking in No. 3 ASV, the Maintenance Manager responsible for the equipment called for a formal fitness-for-service review under ICI procedures.

FITNESS-FOR-SERVICE REVIEW

This was carried out by a team comprising a specialist vessels engineer, a specialist materials engineer, a stress analyst, an equipment inspector, and the responsible main- tenance manager.

Two key issues emerged from the review, these being:

(1) The heel of ammonia was not doing what had been hoped for, and the tank shell warmed up to ambient about 300 mm above the surface.

(2) Using toughness data equivalent to the fully brittle condition (because nothing else was available), the critical defect sizes determined were so small that they could not all be guaranteed to be found.

It was concluded that the team could not provide evi- dence adequate to satisfy ICI’s internal pressure vessel de- sign verification policy and that the tank would have to be replaced.

REPLACEMENT OPTIONS

Two main replacement options were considered, these being:

(I) Replace the complete installation with a pressurized storage (1.38 barg) facility in a different location on the site.

(2) Replace 3 ASV with a similar tank, but designed and constructed to modem standards.

The reasons for considering the first option were twofold:

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

(b) The existing system operates at 200 mbar and therefore generates flash gas at 200 mbar. This is used at this pressure by one consuming plant on the site and if this is not available, the gas must be boosted in pressure for use by others. The higher pressure storage would, there- fore, offer greater operational flexibility.

On detailed examination, it was concluded that the advantage gained by moving towards the center of the site was offset by the fact that a leak from 1.38 bar g stor- age would be more severe, due to the generation of a greater quantity of flash gas. Furthermore, the advantages of greater operational flexibility were considered to be of little importance.

An additional factor was the recognition that due to the continuous purge from 3 ASV and 5 ASV, neither vessel had suffered significant stress corrosion cracking problems.

It was decided, therefore, to replace this tank with one essentially of the same type, but with deficiencies in the design, material specification, and welding rectified; i.e., full compliance with modem national standards with butt welded seams, impact tested material, and appropri- ate quality control. This proposal immediately ran into op- position from the responsible engineer representing ICI’s Safety and Loss Prevention Dept. over the issue of single containment.

It is quite clear that the relevant national tank standard, BS 7777 (Reference should be made to annex A of BS 7777), permits single containment of ammonia - provided certain more severe materials requirements are met. This is also the published view of the U.K.’s Engineering Equipment and Materials Users Association (EEMUA), which is an organi- zation of substantial purchasers including ICI, British Petroleum, Shell, and Exxon. However, the published rec- ommendation of the Chemical Industries Association was the use of double containment for large ammonia storage vessels and ICI is a leading member of that body. Exam- ples of double containment are shown in Figure 5 , and it can be seen that the expense and degree of sophistication is significantly greater. The desire to improve the safety of the emergency ammonia storage now threatened to turn into an expensive overkill.

After examination of the relevant standards and discus- sion about whether 200 te capacity constituted a large am- monia storage tank, the decision was made to replace it with single containment and go along the route of en- hanced material properties (see the following section).

REQUIREMENTS FOR MATERIALS

BS 7777 groups materials into 6 types and relates the choice to single or double containment and product stor-

FIGURE 5 Examples of double containment tanks.

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TABLE 1. Material Types for Tank Shell and Bottom

Typical Double Product

Single or Full Storage Product Containment Containment Temperature

Butane Type I1 Type 1 - 10°C Ammonia Type I1 Type 1 - 35°C Propane/ Type I11 Type I1 - 50°C

Ethane/ TypeIV Type IV - 105°C

LNG Type V or VI Type IV* - 165°C

propylene

ethylene

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

age temperature (Table 1). These types are related to mate- rial generic types and specific impact test requirements (Table 2). As can be seen, the use of single containment requires enhanced material requirements to ensure ade- quate safety of storage.

The introduction of the delta T term is intended to make an allowance for the reduction in toughness in the heat af- fected zone of a weld. The steelmaker is required to obtain impact energy transition curves for the parent plate and for the HA2 of a test weld in order to determine delta T (see Figure 6).

This procedure for arriving at the 27J temperature is in- tended to ensure that 275 is achieved in the HA2 of the site-made weld at 25°C + / - 5°C lower than the design minimum temperature.

The 120J requirement is to give reasonable assurance that the Cv transition temperature is at least 30°C above the im- pact test temperature.

It was realized within ICI that the enhanced material re- quirements would be a problem, because the plate ton- nage quantity was too small to form a reasonable mill or- der. Quantities such as this normally come from specialist stock holders.

Quotations for the tank in carbon-manganese steel proved this to be the case and no tank vendor quoted en- hanced grade material. In fact, the vendor most involved in

275 / I Test Temperature T 1- delta1 4

FIGURE 6 Determination of delta T

the drafting of BS 7777 stated that the enhanced Type I1 material was unnecessary because the tank was not large and quoted Type I as did the others. ICI decided that the use of Type I did not meet it’s safety requirements.

FIGURE 7 New stainless steel tank in position.

TABLE 2. longitudinal Charpy V-Notch Impact Testing

Tested Per Plate Classification Steel Type (0, (2) 40te Batch (3)

Type 1 Normalized carbon-manganese 275 at -50°C Not required

120 J Tested Per

Type 11 Improved Toughness C-Mn 27J at - 50°C-deltaT - 20°C Type 111 Low nickel steel 275 at - 80°C-deltaT - 50°C Type IV 9% nickel steel 35J at - 196°C Not required Type v Improved 9% nickel steel lOOJ at - 196 C Not required Type VI Austenitic stainless steel No impact testing Not 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 mm subsize specimens are to he used, and demonstrate 70% of the values specified in this table. For Type V steel, the value is to he 50% of

the value soecified in chis table. (3) Impact testing IS carried out on each plate to demonstrate the required impact value. In addition, testlng at a frequency of one test per 40 te batch is to be carried out to demonstrate the l20J

(4) Reference should be made to annex A of BS 7777. requirement (see annex A). The definitions of plate and hatch are given in BS EN 10025.

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FIGURE 8 New tank being lifted into position.

A BETTER SOLUTION in order to minimize disruption to the availability of 3 ASV, especially severe because of access problems, it was de- cided to build the new tank on open ground to the side of

removal of the old, the new tank was lifted into position (Figures 7 and 8).

It was decided that apart from an increase in the initial

text of the whole replacement project-the route which offered the best all-round solution was the use of austenitic stainless steel. The comparison of tank costs was $110,000

capita1 Of the tank-a percentage in the con- the storage facility, Upon completion of the new tank and

for the carbon-manganese against $208,000 for the stain- less steel tank. However, the lower cost was for Type I ma- OTHER FEATURES terial and would have increased significantly for Type 11: the delivery time would also have been quite unacceptable.

Austenitic steel is obviously a much safer material be- cause of its immunity to brittle fracture at the storage temperature of liquid ammonia-even under impacting missile conditions. In addition, the following advantages are obtained:

(1) The tank does not require the application or mainte- nance of a paint system.

(2 ) The tank is not vulnerable to the corrosion which could be expected with carbon-manganese steel under the freeze/thaw conditions of intermittent storage.

This is particularly important with regard to underfloor corrosion which cannot be seen directly.

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

( 4 ) After the initial inspection, it is not necessary to carry out an invasive inspection again. This elimination of the tank preparation and entry work is a significant cost saver for the future.

INSTALLATION

It is conventional to erect a site-built storage tank on the base where it is to remain for its operating life. However,

The new installation is monitored by CCTV from the lo- cal control room.

The new installation is protected by remote actuated iso- lation valves, which enable the tank to be partitioned should an incident occur. These can either be actuated lo- cally or from the main site operations center.

CONCLUSIONS AND RECOMMENDATIONS

(1) The discipline of putting a minor modification through a formal design verification system helped to iden- tify that an existing ammonia storage vessel was unfit for further service.

(2) The option taken and the design and specification selected for the replacement was influenced by a risk as- sessment on the probability of a major vessel failure, to- gether with an assessment of the consequences of such a failure.

This paper (33 was pwsented at the AIChE Ammonia Plants & R e lated Facilities Symposium held in Boston, Massachusetts on Tuesday, September 10, 19%.

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