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Lessons Learned in Decommissioning and Re-commissioning Process of Ammonia Storage Tanks

Ammonia Storage Tanks are integral part of any ammonia plant and the safety and integrity of these tanks are of utmost importance to the operating companies and to the communities at the location. Ammonia Storage Tanks are critical pieces of equipment considering the highly hazardous nature of the stored contents and hence ensuring its integrity is of paramount importance. Ammonia Storage Tanks are also one of the most intricate and difficult pieces of equipment to inspect. Based on AmmoniaKnowHow.com team experience, the numerous unforeseen problems might arise as the repair work progressed once the decommissioning is completed, and tank is ready for inspection.

Current international codes do not regard decommissioning of refrigerated liquid storage systems as a normal operational requirement and is only considered necessary subsequent to an upset (such as the persistent presence of ammonia in the annular space) or failure of a component in the system, where there is a need for entry). The thermal stresses induced by the decommissioning activity and the admittance of air (oxygen) into the tank could cause severe damage to an otherwise normal tank. Stress Corrosion Cracking damage once initiated is difficult, if not impossible, to rectify. Apart from the potential dangers associated with decommissioning inspection of Ammonia Storage Tanks, the prohibitive cost of decommissioning, inspection, recommissioning and tank downtime (and subsequent plant downtime in case production is supported solely by the subject tank) are serious deterrents to this method of integrity assessment.

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Some of the lessons learnt by our team members and other industry specialists during the emptying the tank as part of the overall decommissioning are collected and shared below: Preparation for decommissioning and tank drainage • Before preparing a (desk) procedure the commissioning engineer should execute a site visit to make

notice of local circumstances and discuss the planned approach. • It is advisable to make a very practicable procedure (with step-by-step actions and explanatory drawings)

instead of a desk report with theoretical assumptions and schemes. • The engineer should verify and list all the necessary measuring devices, auxiliary tools & equipment and

ensure the local availability. • Assess various de-commissioning methods together with operation team and select the suitable one

based on site circumstances and environmental constraints. • Contingency (x %) for the required period should be timely communicated and built into the proposal

towards client/owner. In many cases it proves difficult to drain the residual ammonia after the transfer pumps were stopped (in some cases in excess of 200 – 400 tons) using drainage pumps connected to drain nozzle. When company has more than one Ammonia Storage Tank, the residual ammonia from the tank being decommissioned can be transferred to the service tank. The temporary drainage pump should initially be started on recycle mode. Two consecutive samples to be analized to determine the oil content. Once the sample results confirm the oil content is below 100 ppm, the liquid ammonia is routed to the available storage tank. Period sample is taken every 4 hours (twice per shift) and oil content results recorded. Owner should prepare way of accessing the tank manholes for inspection, maintenance and for measuring purposes.

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Ammonia Tank heating up process with nitrogen vapours • Once it becomes impossible to pump the remaining liquid, tank is ready for heating up with hot nitrogen

gas. This step is required to evaporate any liquid ammonia in the inner tank. • A dedicated drawing for nitrogen heating connection shall be prepared as part of decommissioning

procedure. • Gradual nitrogen heating process shall be started following the recommended N2 temperature of N2 at

the initial stage of heating up process. • N2 flow should be adjusted according to pressure and temperature during the heating process. • It might be required that a temporary piping connection shall be installed for ammonia storage tank

heating up process. • The nitrogen temperature can be controlled by a temporary heat exchanger using hot water circulation

system. • During Tank heating & nitrogen purging all the associated liquid & vapor lines isolation valves shall be

closed (block & bleed arrangement). The positive blinding to be done prior to air purging activity. For heating & purging of Ammonia Tank with nitrogen, valve position shall be ensured / confirmed via dedicated valve position table.

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Fig 1. Sample of Heating Profile

Inerting of system and purging with nitrogen • After completing the heating operation, purging will be started by admitting nitrogen at the atmospheric

temperature inside the tank through the same system used during heating up process. • Before starting the N2 purging process the water heater & circulation pumps shall be isolated and the N2

temperature will be reduced to ambient temperature. • Ammonia vapours removed by nitrogen purging shall be diverted to flare. • Care shall be given to all liquid ammonia lines that are isolated for the Tank purging purpose. Liquid

ammonia may be entrapped in any “dead" leg section and require purging. Purging the tank with air • After depressurizing the tank, blinds shall be installed at recommended tank flanges as per

decommissioning procedure. • The blinds shall be fabricated of stainless steel or low temperature carbon steel with a thickness selected

to withstand the design line pressure. Stainless steel or low temperature carbon steel material is preferred as it is considered suitable to withstand against cold ammonia which is likely to be present should any tank side valves pass. A double block and bleed isolation is preferred where possible.

• Opening the manway after tank depressurization shall be done using full PPE and self-breathing apparatus. A normal overall PVC suit with hood made of polycarbonate/acrylic (also known as acid proof suit) is required for the first entry together with self-breathing apparatus and anti-skid boots.

• Company vessel entry procedure and permit shall be followed, and suitable PPE shall be used for the ammonia tank environment that include existing vapours inside the tank.

• Care shall be taken that no skin exposure to ammonia vapours is allowed.

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Preparations for Inspection Extensive safety checks and preparations shall be made for entry to the tank.

• A confined space rescue plan shall be prepared and provided to the team. • A confined space entry control format to be developed and administered by a dedicated safety

operator. • A suitable warning arrangement was installed inside the tank to alert personnel in case of emergency

in the complex. • One reaction fan to be kept running at the top of the tank to provide ventilation. • All personnel entering the tank shall be provided with torch lights as a protection against power

failure. • An emergency evacuation exercise shall be carried out to check the adequacy of preparations and

capabilities of the overall system. Tank Entry The tank may be ready for entry after 15 - 20 days from the beginning of decommissioning process. Initial entry shall be made using full PPE to evaluate and monitor the condition inside the tank. Ammonia concentration on the floor is expected to be around 10 ppm. The allowable exposure limit for ammonia is 35 ppm as a short 15 minutes exposure limit (OSHA standard). Oxygen levels shall be normal at approx. 20% and no less than 19.5%. Company confined space permit to entry procedure shall be used. Nitrogen purging and safety measures to avoid the risks It is to state that the presence of oxygen in connection with (liquid) ammonia can lead to a considerably increased risk of stress corrosion cracking (SCC) of tank material. Another risk is the build-up of electrical charge due to static electricity while spraying liquid ammonia into an oxygen containing tank. The explosion risk will be there when the air contains between 15 and 28 % of ammonia gases. So, first the air will be “replaced” by nitrogen, then gaseous ammonia, and later liquid ammonia, can be brought into the tank. During this process the tank temperature will be decreasing. This has to be done safely and slowly in order to let the tank “follow” the required temperature change of less than or equal to 1 C/h. When nitrogen is inserted the oxygen content of the outcoming air will be continuously measured and one can see when the atmosphere in the tank becomes ‘inert’. It is estimated that this process should take a few days. Some records show that after 4 days the O2-level in the tank shall be <1.5 %: in some instances, was measured 1.14% O2 and this is considered to be a sufficiently safe value to start with blowing in gaseous ammonia. Other crews went with oxygen level in the tank even lower, up to 0.15%, confirmed by four repeated samples.

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Fig 2. Sample of Oxygen level

At the same time the tank has to be slightly pressurized to approx. 1.15 bara in order to check whether all connections are tight (by using bells of soap). This procedure might take another day. However, to execute this nitrogen purge, one should preliminarily consider a few safety measures: • All inspections and repairs should take place according to rules and legislation; • All manholes shall be closed; • All relief valves to be checked and installed; • Availability of (enough) nitrogen for purging; • Availability of (enough) liquid ammonia for filling and cooling the tank gradually. Bringing in ammonia vapour into the tank The procedure of feeding the ammonia vapour into the tank should take several days and each time the laboratory (of the client) has to measure the ammonia content in the tank. The actual NH3-values in the tank shall be measured each day: Some reference values are listed below: 1. After one day the value was: 14 gr/m3 (~2 %) 2. After two days it was: 38 gr/m3 (~5.5 %) 3. At the end after three days the value measured was: 87 gr/m3 (~12.5 %) 4. One should take samples from the tank until the N2- content was below 5%; this is a measure to start with

bringing in liquid ammonia for filling and further cooling down of the tank. Cooling down the tank by spraying liquid ammonia.

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It is advised that the temperature drop has to be uniform and be lower than 1C/h. The ammonia will be sprayed in from the top and will liquify after a certain period on the bottom of the tank, subsequently cooling down this bottom. At the end of this activity the temperature at the bottom will be -32C, which means that with an ambient temperature of +20C, the total difference will be at least 52C. It should therefore require a cooling down time of at least 52 hours, but for safety reasons it is recommended to arrange 2 or 3 times as much, i.e. 156 hours (~6.5 days). At a carefully monitored flow rate the liquid ammonia shall be brought into the tank; during this procedure the temperature at the bottom and alongside the wall must be monitored by the shift in charge. If the temperature decreases too fast, one should immediately slow down or stop the filling procedure to avoid any risk of stress corrosion cracking on the bottom or the side walls. It may take up to 4 or 5 days of filling with liquid ammonia (approx. 20 t.) to cool down the tank to the level of -32C (measured).

Fig 3. Sample of ammonia storage tank cooling rate

Closing of the Ammonia Storage Tank Recommissioning If the recommissioning goes well, the total duration is approximately 14-21 days subject to the site condition.

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Ref 1. Decommissioning Inspection and repair of 5000 MT ammonia storage tank, Ashish A. Nair Process and Combustion Equipment, FEDO, FACT, Cochin – 683 501, India; Ref 2. Successful inspection of two large Ammonia Storage, Tanks Yusuf Abdulla, Yoga Narasimha, Gulf Petrochemical Industries Co. Kingdom of Bahrain; Ref 3. Successful and safe de- and recommissioning of a cold ammonia storage tank, L.A.J. Tol and G.J. Tol Of Continental Engineers BV - The Netherlands, in cooperation with A. Bourras of OCP Jorf Lasfar at El Jadida, Morocco Ref 4. Ammonia Storage Tank Decommissioning Procedure developed for Deepak Fertilizer by Dan Cojocaru, AmmoniaKnowHow.com