Commissioning of Two 50,000 MT Ammonia Storage Tanks

This paper will discuss in briefQAFCO's ammonia storage systems, selection of the unique tank design (steel tank surrounded by a fill/height concrete wall) and in detail the commissioning process of the two recently installed anhydrous ammonia storage tanks from planning to commissioning and

handover o/the tanks to production.

Iftikhar Hussain Turi Qatar Fertilizer Company, Mesaieed Qatar

Introduction

Storage tanks are an integral part of the industrial landscape. In Urea fertilizer plants surplus ammonia is stored in large cryogenic storage tanks at -33°C (-28 OF) as it is the most cost effecti ve method of ammonia storage.

Selection of ammonia storage tank design can be: a l:hallenge due: to the high cost and the potential of hazards (release of large quantity of ammonia) associated with storing a high volume of toxic material. While selecting the new tank des ign, QAFCO adopted a Risk Based Approach (REA) and set up a task force team to come up with the safest design. The most common design alternatives were evaluated, single containment (single wall) with dike (Fig-I) double containment (double wall) with full dike concrete wall (Fig-2) and single containment with full dike concrete wall (Fig-3). It was concluded that new storage tanks should have a single wa ll surrounded by an independent self-supporting concrete outer protection wall (Fig-3) capable to withstand external impact of blast. The unique design was the best fit for QAFCO's requirement to choose the safest design while keeping in mind the space constraint in QAFCO because selecting first option will take much more space compared to the third option.

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Figure J. Single or double waif tank with concrete wall and sand support

Figure 2. Double containment with filII dyke concrete wall

Figure 3. Single containment wilhfull dyke concrete wall

AMMONIA TECHNICAL MANUAL

QAFCO ammonia storage facility

Qatar Ferti lizer Company (QAFCO) located in the industrial town of Mesaieed is operating six ammonia and six urea plants. Its average annual production is about 3.8 million tons of anunonia and 5.6 million tons of urea making QAFCO world's largest single site producer of ammonia and urea. In QAFCO surplus ammonia is stored in four refrigerated storage tanks to be used internally to make Urea and the rest is sh ipped to externa l consumers for further processing. With the increase in production capacity QAFCO onsite anhydrous ammonia storage capacity has also increased gradually by adding more tanks. The fi rst anhydrous ammonia storage tank (T 120 I) was constructed in 1972 to store excess anhydrous ammonia produced by QAFCO- I ammonia plant. The storage capacity of thi s tank was about 20,000 metric tons of ammonia with an internal design pressure of 0.8 psig. The tank was a re fri gerated single wall with suspended deck des ign. The second anhydrous ammonia storage tank (T 1202) was constructed in 1977 as a part of QAFCO-2 expansion. The storage capacity of this tank was about 28,000 metric tons of anhydrous ammonia with an internal design pressure of 0.8 psig. This tank was also refrigerated single wall with suspended deck design. These two tanks were decommissioned and demolished later after installing bigger tanks due business requirement and safety issues (single wa ll vs double wa ll tank design). The third ammonia storage tank (T600 I) was constructed in 1997 as a part of QAFCO-3 expansion project to store surplus ammonia produced by ammonia -3 plants. The storage capacity of this tank is 20,000 metric tons of liquid ammonia at atmospheric pressure. The fourth ammonia storage tank was constructed as a part of QAFCO-4 expansion project in 2004. This tank when built was the second largest ammonia storage tank in the World, with storage capacity of 45000 metric tons. Both the storage tanks are of double wall suspended deck design elevated on piles for air circulation.

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Before QAFCO-5 expansion, the company had two refrigerated ammonia tanks with a combined capacity of 65000 tons to store surplus ammonia. Recently, the storage capacity in QAFCO has increased to 165000 tons by adding two new storage tanks T6003 (Tank 3) & T6004 (Tank 4) having a storage capacity of 50000 metri c tons each. This gives QAFCO the flexibility to keep all the urea plants in service depending on the avai labili ty of carbon dioxide produced by the ammonia plants even when any of the ammonia plant goes down. The new tanks are of single wall design surrounded by a full height independent concrete outer protection wall. The inner shells of the tanks are fu ll integrity, self-supporting, and open top with suspended decks with dome roofs. To avoid freezing of the tanks foundation, the tanks are elevated on piles for air circulation. These two tanks are the biggest in the world.

Tank design selection

In order to select the location and safest tank des ign a team was nominated to study the pros and cons of different tank design and recommend the most suitable scenario by taking into consideration the environmental impact, operation safety, external risk factors (blast over pressure waves), stress corrosion cracking . tank over pressuri zation and tank inspection without decommissioning. For the Quantitative Risk Analysis (QRA) study only three tank des igns i.e., single wall with independent concrete wall and double integrity tank with low and high bund wall were taken into consideration. After considering the pros and cons of the three designs, a single wa ll tank with independent concrete wall was recommended, because it was concluded that it could withstand the impact of debris from explos ions in nearby facilities. As the outer wall wi ll not be linked to the inner pressure shell , hence the outer wall will not sustain damage in case of inner shell damage due to sudden pressure increase caused by ingress of warm ammonia. Also, there are

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inspection possibilities with ultra-sonic equipment on inner tank due to access to the annular space. Also, it will occupy less plot space compared to double integrity tank with low bund, since no bund is required.

Location selection, emission calculation, physical effect and individual risk

To select the location of the tank the prevailing wind direction were taken into consideration by using emission calculations and individual risk curves. (Appendix A. Fig- l1 &12) For each different tank option a gas dispersion calculation were done for the case of instant collapse of inner tank. The total area for evaporation from a bund is the total surface area in contact with liquid, except inner tank. Two types of bund walls i-e low bund with the surface area of 107 10 m2 and high bund wall surface area of 255 m2 were simulated. Emission calculations were performed by Y ARA international wi th the help of TNO the Netherlands with the following conditions:

Weather stability E, temperature 20 degrees C and 80% relative humidity and alternative temperature 50°C (122°F) (and 30% relative humidity. From the probability of wind direction in the different directions a risk contour map can be constructed for each tank type at the distance affected LCd50 in the case of total sudden collapse of inner tank for existing case with two double integrity tanks with low bund wall and the new cases high bund wall.

For emission scenarios blast explosions curves that are based on 1O-4/yr probabili ty explosion cases were used and for tank damage scenario a normal standard case of rupture equivalent to 50 mm diameter and 5 minutes release of inventory was considered. For each configuration instantaneous released amount of 60,000 tons of liquefied ammonia was considered. For dispersion study and consequence calculations

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and modelling the following configurations were used. Single wa ll with low bund wall (100 x 100m) (bund height ~ 7.5 m) Single wall with concrete bund wall (A = 2500 m') Double integrity tank, without bund (concrete wall) Single wa ll tank, with concrete wall (A ~ 2500 m2) and sand/gravel support. For physical effect calculations the following were considered. Temperature: -33.43 °C (-28. 17°F) @ atm Pasqual stabi li ty class (atmospheric turbulence): E; Wind speed: 3 mls at 10m height Ambient conditions (summer / winter) 20 & 50 °C (68 &122°F) Capacity / tank inventory: 60,000 metric tons Dimensions: H: 36 m, D : 54 m. Distance to a concentration of 4500 mg/m' (5929 ppm) Distance to a concentration of 300 mg/m3

(395ppm) Distance to a concentration of 30 mg/m3 (39 ppm). (Appendix B.Fig-13&14)

Based on the team feedback it was decided to build two 50,000 mc:LTit: Lons ~Loragc: tanks dose to the sea. The tanks should be built as per API 620 APP. R. Each tank should have an independent pre-stressed reinforced concrete outer wall to minimize the consequence in case of inner tank leak or rupture. As in this case the secondary containment cannot be lost if the inner tank collapse due to pressure increase, the emission of ammonia gas will be limited compared to other scenarios.

Construction of new tanks

After awarding the contract to CB&I the construction work for both the tanks started on March 2008 and completed in October 20 I O. It took about 41 months to complete the project. Construction was done in parallel for the both the tanks. The civil work including erection of concrete piles and external concrete walls and roof fixing took about 20 months to complete. The roof was li fted pneumatically, welded to the

AMMONIA TECHNICAL MANUAL

shell and tanks were preserved with nitrogen. Tanks commissioning activities lasted for about 30 days including air nitrogen exchange, nitrogen-ammonia exchange, and tanks cool down. The total commissioning period including refrigeration system (BOG) and utilities was three and half months. About 100,000 m3 of nitrogen was used in the process of nitrogen purging. All the phases of the project from construction, pre-commissioning and commissioning were carried out without any lost time accident.

Figure 5. Construction of tank I side walls.

Figure 6. Tank I and 2 after completion 0/ constrllction

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Tanks specifications and general description

Capacity (each) Gross Capacity Dimension (ID/H) Design Pressure (-0.08/ 2.17 psi)

50,000 mt 80,500 m' 50 m /40.5 m -0.6 /15 Kpa (g)

Operating pressure 2.5 / 4.0 Kpa (g) (0.36/ 0.58 psi) Operating temp; -33 ' C (-28' F) Design code API 620 App. R Both the tanks are single wall (carbon steel) with outer pre-stressed concrete wall . The inner metallic tank containing the liquid ammonia has suspended insulating cover deck and made of steel suitable for the design temperature. The outer wall is designed to contain the entire liquid ammonia in case of leakage from the inner tank. The space between inner tank and outer container is. around 15m. The foundations of the tanks are on piles approximately 75 feet deep and have about 250 piles per tank.

Figure 7. Tank/oundation piles.

Commissioning approach

Planning

The construction activItIes for both the tanks were planned in two phases. In the first phase tank T6003 (Tank I) and T6004 (Tank2) were constructed, hydro tested, cleaned and preserved under N2 atmosphere by reducing the oxygen content below 4% in the tanks. In the second phase associated piping, transfer pumps and

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BOG equipment (set of three, shared by both the tanks) were installed and commissioned. Afte r commissioning and testing all the required equipment including the ammonia flare, it was decided to purge tank (T6004) with vapor ammonia to replace nitrogen and finally to cool down the tank by charging liquid ammonia. The same procedure was fo llowed for tank (T6003).

Major construction activities timeline

Site handover May 08, 2008 TK I pi ling completed April 26, 2009 TK 2 piling completed April 29, 2009 TK I foundation completed May 27, 2009 TK 2 foundation completed June 24, 2009 TK I concrete wall completed Dec 19,2009 TK 2 concrete wall completed Feb 11 ,2010 TK I shell erection completed Feb 11,20 10 TK 2 shell erection completed Mar 16,20 10 TK I roof air raise Marl I, 20 10 TK 2 roof air raise Apri l 03, 2010 TK I hydro-test completed June 14,2010 TK 2 hydro-test completed June 24, 2010 TK I mechanical completion Oct2 1,20 10 TK 2 mechanical completion Oct2 1,20 10

Commissioning team

A commissioning team compnsmg of vendor representative and QAFCO was fonned for close co-ordination and execution of all the activities as per vendor guidelines and QAFCO standard operating procedures and guide lines.

Documents preparation and review

All the documents including commissioning procedures (nitrogen purging, replacing nitrogen with ammonia and step by step cool down of the tanks, BOG system testing) recei ved from vendor were reviewed and at the same time operating procedures were created and discussed with vendor. As the tank commissioning is considered a critical phase

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with respect to the chances to develop stress corrosion cracking, all the steps of the commissioning phase including purging with nitrogen, tank cooling and charging ammonia were fo llowed diligently without any deviation from the pre defined procedures.

Hydro test and final inspection

After successful completion of all the civil and mechanical work, both the tanks were cleaned, inspected and prepared for hydro testing. The tanks were filled up to 70% with desalinated water and after hydro test the tanks were drained and made ready for fina l inspection. The hydro test was conducted only for the inner tanks.

Functional test a nd commissioning

Before starting next step all the accessories including safety valves, isolation valves, BOG system, flare system and control system were tested and made ready to be taken in service.

Air - nitrogen exchange

The first step after completion of the mechanical and civi l work to prepare the storage tanks for receiving ammonia was to replace air with inert media (nitrogen) and isolate the tank from outside by installing man ways covers, and installing all the relief valves, level indication system and other instrumentations. Nitrogen purging for both the tanks were conducted by connecting tanks with the nitrogen tankers through vaporizers. Nitrogen was injected at the bottom of the tank through the outlet nozzle and vented at the high elevations from the top of the tank by providing exhaust stacks on the purge ports to divert the inert to a safe location. A sample port was provided at the top of the tanks to collect samples. Air-nitrogen exchange for tanks T6003 started on October 09, 2010 using liquid nitrogen tanker and completed on October 15, 20 I 0 by bringing down the oxygen content to below 4% in the tank. Air-nitrogen exchange for tank T6004 started on April 22, 20 I I using the same

AMMONIA TECHNICAL MANUAL

setup and completed on April 28, 2011 by bringing down the oxygen content to below 4% in the tank.

----- ---'---------

/ , --Figure 8. Nitrogen purging: up flow.

Nitrogen - ammonia exchange

To reduce the chances of stress corrosion cracking oxygen content in the tanks was further reduced to

Tank 6004 cooling down

Tank T6004 cool down with ammonia started on July 01 , 2011. Cooling down of the tank was completed on July 08, 2011 with some intcmlptions in between due to ammonia shipment as the same line is used for export. Tank cool down was stopped after getting level in the tank. The total cool down duration was about 8 days including the interruptions. (Appendix C F-15)

Tank 6003 cooling down

The same procedure was followed for tank T6003 cool down. It took about five days to complete the cool down process. Cool down started on July 10, 20 t I and completed on July 14, 2011 by injecting liquid ammonia from the top through three inch line equipped with the splash plate. Tank cool down was stopped after getting level in the tank. (Appendix C. F-16)

Safety consideration One of the significant safety hazards, during tank commissioning is the purging operation which involves the replacement of oxygen inside the tank by nitrogen venting. To mitigate the ri sk of creating an oxygen deficient environment, exhaust stacks on both the tanks were provided to divert the inert to a safe height away from personnel working in the area. Also oxygen meters were provided during sample collection. As a best practice American Gas Association procedure "Purging Principal and Practices" was adopted. This procedure provides instructions specific to purging tanks with inert gas.

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Operation

After successful commissioning of the new storage tanks, all the four tanks are integrated through interconnecting piping having a provision of leve l transfer among the tanks and a facility of sending product ammonia to any urea plant and for export when required. All the four tanks, BOG system and export system are being operated from the same DCS panel.

Conclusion

During tank cool down oxygen content must be brought down to acceptable level « 2.0%) to eliminate the chances of stress corrosion cracking and cool down should be closely monitored and controlled to maintain the required (I °Clhr) temperature drop. Care should be taken during inert purging to prevent personnel access to locations where the atmosphere may be oxygen deficient as purging invo lves a replacement of oxygen inside the tank. This can be done by providing exhaust stacks on all the purge nozzles to diverted gases to a safe location. During sampling oxygen meter should be used for on time warning.

AMMONIA TECHNICAL MANUAL

Appendix-A: Individua l risk and emission calculation.

Ca l c u l ation I ndiv idu a l Risk '0<

S i ngle VVa l! , Ex i sti ng 2

Oouble d ouble S i ng l e VVa l! Sing l e VVa ll

I ntegrity High Bund.

i n t e gri ty VVi nd Lo"", Bund H i gh Bund Incl ude "",a ll

N o Q u n d ta n ks "",i t;h O i rec::t: i o n h ea t; capacity

LOVV Bund

SOO~ 200~ 87000 en 800~ 800~ 2.00E- OS 2.00E- OS 2.00E- 09 2.00E - 09 4-.00E - 09 1

3.33E- OS 3.32E- OS 3.32E-09 3.32E-09 6.6SE- 09 2

3.02E- OS 3.02E-OS 3.02E-09 3.02E-09 6.04E- 09 3

6.0SE- OS G.OSE - OS G.OSE- 09 6.0SE- 09 1 . 2 1 E - OB 4

1.23E-07 1.23E-07 1.23E OS .1.23E 08 2.4 SE-OS 5

S.48E- OB S.4BE- OB S.4BE- 09 S.4BE- 09 1.10E - OB 6

S.30E- OS S.30E-OS S.30E-09 S.30E-09 1 .06E- OS 7

3.43E- OS 3.43E- OS 3.43E- 09 3.4 3E-09 6.S7E- 09 8

3. 1 3E-OB 3. 1 3E - OB 3. 1 3E- 09 3. 1 3E-09 6.2SE- 09 9

2.G9E - OB 2.G9E- OB 2.G9E - 09 2.G9E - 09 S.3BE - 09 10

2.3SE- OS 2.37E- OS 2.37E- 09 2.37E-09 4-.7SE - 09 11

9.37E- 09 9.37E- 09 9.37E- 09 9.37E- OG 9.37E-10 1 2

Figurell. Individual risk calculations. The Weather number 1 - 12 refers to direction starting with I or 12 as north. 3 as east. 6 as south. 9 as west.

A..o .. , or A .. _ i. Di" .. ed .. ~-.3 «:>00100000 _

, Do.hle ",.eviI) . ...... 0 •• (I ~ 10.1.)".) 87000 .. ( 20 ' C) 87000 _ (20 ' C) >100000 .. >11MKKlO .. >100000_ bo .... O .. 12!1OOO .. (50' 0 12!1OOO .. >100000 .. >100000 .. 82700..., • .., _.,

• Do.hle "'tetril)· .... k cloo .. ['·'pOntio i. SEA ,,~. ,,~. > 100000 .. >11MKKlO .. >100000 .. :> 100000 .. .0SEAW .... (I ~ iO.7f,T) J1300 1

Appendix-B: Individual Risk Curves

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Figure J 3. Individual Risk Curves for existing ammonia tanks plus Case 1 (Double integrity tank with low concrete bUild) with location option 3

Figure 14. Individual Risk Curves/or existing ammonia tanks plus Case 2 (Single Wall Tank with reinforced high concrete Wall) with location option 4

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Appendix-C: Tanks cool down DeS printout

1