Successful and safe de- and recommissioning

of a cold ammonia storage tank

Abstract

The paper that Continental Engineers BV and OCP present discusses the decommissioning procedure

to carefully and safely empty and inertize a cold ammonia storage tank and the problems that could and have occurred. The decommissioning took place in April/May/June 2004 and all together it covered a period of 6 weeks (empty-

ing 4 weeks, inertizing 2 weeks). It also discusses the recommissioning of the same tank, which took place several months later.

For the recommissioning first the oxygen had to be removed until a level of <1.5 volume % had been reached and after that a controlled filling with gaseous ammonia should take place in order to avoid too cold spots in the steel bottom or wall which could cause stress cracks. Recommissioning took a period of 2,5 weeks and went very successfully thanks to the excellent preparation and help of OCP management and employees.

The tank was cleared and finally accepted by OCP on 19 October 2004 as being successfully recommissioned.

Authors:

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

1.0 Introduction

ffice Cherifien des Phosphates Group (OCP) was founded in 1920 and its achievements are:

75 % of world phosphate reserves (98% in the middle of the country, 2% in the south part)

85 % of the world phosphate’s production is intended for fertilizer production.

OCP’s production sites, production levels and posi-tion are as follows:

1. Production sites • 4 phosphate mines in the country

(Khouribga, Benguérir, Youssoufia and Boucraâ/Laâyoune).

• 2 coastal sites of chemical transforma-tion (Safi and Jorf Lasfar)

• 4 shipment ports (Casablanca, Jorf Lasfar, Safi and Laâyoune)

2. Production levels:

• Phosphate production: 23 Mt (52% are transformed in phosphoric acid and fertilizers).

• Exportations: - Phosphates: 12.0 Mt - Phosphoric acid: 1.6 Mt P2O5 - Fertilizers: 2.2 Mt

3. Position in the world market: • Leader in phosphate’s exportation un-

der all its forms • Leader in phosphoric acid exportation

To produce their fertilizer, OCP imports ammonia

(NH3) by ship in the total amount of 400,000 t/a. A great deal of this ammonia is transported to

OCP’s site of Jorf Lasfar at El Jadida, a place 100 km from Casablanca and located near the Atlantic coast and stored in two big tanks with a volume of 23,000 m3.

OCP wanted to overhaul one of these tanks in 2004 (for the first time since the tank was put into operation in 1987) and they therefore inquired early 2004 at Con-tinental Engineers BV (C E) at Zaandam, The Nether-lands, as reputed ammonia-engineers whether they could prepare a solid procedure for the safe decommis-

sioning of the cold ammonia storage tank IR5502. After that, inspection and re-insulation should take place and the tank would have to be recommissioned, i.e. safely refilled with ammonia until the fluid phase of –33oC was reached.

In March 2004 Continental Engineers BV (C E) provided OCP with the required procedures for the de- and recommissioning of the mentioned am-moniastoragetank nr. IR5502, of which some data are given below:

TankData: Liquid ammonia: NH3anh. Temperature of liquid: -33 oC Diameter: 35 m Height of wall: 24 m Material of wall: ASTM A537 Height of dome: 6 m Material of dome: ASTM A516-70

Design code API 620 app. R

Volume: 23,000 m3

Insulationlayer: 120 mm (polyure-thane).

Annular space between tank wall and concrete out side wall: 1.20 m

The tank is placed on a concrete ring foundation which is protected against freezing by a system of heating coils. The center of the ring is filled up with a layer of sand and a layer of 200 mm foam glass.

2. Preparing the Procedure: 2.1 Preparatory observations and actions: Before the tank can be inspected, it has to be emp-

tied and all liquid and gaseous ammonia would have to be removed from the tank. The pump suction line was however located somewhat above the bottom of the tank (300 mm) which means that a certain amount of liquid ammonia remains in the tank, amounting to, by estimation, approx. 300 t. It is important to state that there were no manholes available and there was not a provision for draining below tank bottom level. See fig. 1.

The route to use aqueous ammonia or to flare was

not feasible at OCP’s location which meant that all re-

O

maining ammonia would have to be removed by evapo-ration and drawn off as vapour.

Purging of the tank with nitrogen prior to entrance of oxygen (air) was necessary to avoid the (explosion) risk of ammonia in an oxygen-rich environment and which could occur in case of a 15-28 vol-% ammonia concentration. However, to minimise eventually the re-quired heat for evaporation one could first reduce the liquid level in the tank by increasing the operating pres-sure of the tank (see below).

Possible options to remove the remaining ammonia

as considered by C E are: 1. A combination of pumping liquid ammonia out

of the tank through a flexible hose via the suctionline and evaporating the remaining liq-uid with “hot” ammonia vapour;

2. Using only “hot” ammonia vapour; 3. “Natural” evaporation; 4. Adding water to the tank and pumping aqueous

ammonia out of the tank; 5. Evaporating the remaining liquid by heating

the annular space between the concrete wall and the storage tank (in that case the insulation had to be removed).

2.2 Be well prepared before you start!:

Necessary equipment & tools

Before entering into the right procedure and

executing the necessary activities, certain provi-sions have to be made such as:

• a storage facility for liquid ammonia, • availability of ‘hot’ gaseous ammonia (45

oC), • availability of nitrogen for purging pur-

poses, • flexible hoses and suitable pumps, (spare)

heater(s) for superheating ammoniavapour, • a fan for ventilation purposes etc. etc.

Practicle calculations Increasing the pressure by increasing the set point

of the compressor, taking into account that the pressure relief valves at the tank were set at 1.16 bara. Accord-ing to our calculations indicating a pressure increase of 0.03 bar, which was fully acceptable, the pump could remove an extra 200 t. of liquid ammonia at reduced flow and the required time for that should be approx. 50 hours.

See fig. 2 with schematic diagram.

A quantity of 100 t ammonia should then remain in the tank and after that we could pump out another part of the ammonia by means of a flexible hose which could result in an extra reduction of at least 50 t. The left over then would have to be evaporated

For each of the aforementioned options, calcula-tions were made and the required “decommissioning” time was estimated resulting in:

Option no.: Estimated time/period for only empty-ing: NB: 150 kg/h approx. calculated quantity of vaporized ammonia

1. 12 days (pumping 60 t. out in 1 day and 11 days

for evaporation of 40 t.) 2. 27 days (100 t. / 150 kg/h) 3. 16-32 days (boiling off rate 0.02%-0.04%, dependant of insulation quality of tank and ambient conditions) 4. not applicable nor feasible at OCP’s (see also point 2.1) 5. 10 days, however not practible nor feasible Conclusions on methods for emptying:

• Execution by method 1 was considered not to be safe, because of our (previous) bad experience with bringing in flexible hoses

• Methods 2 and 3 were considered as feasi-ble methods on OCP’s site

• Method 4: not applicable because of lack of enough water and OCP was not in a po-sition to use aqueous ammonia

• Method 5: not feasible because of safety but also for practicable reasons

Based on the required time, the required invest-

ments and, most of all, practical considerations C E ad-vised OCP to use method 3 “natural evaporation” for emptying the tank.

2.3 Purging requirements: After all the liquid NH3 is evaporated, purging with

N2 of the tank can be started. For this purging the avail-ability of an amount of 30,000 m3 of nitrogen was re-quired. The nitrogen should have to be carefully in-serted since the layers of ammonia and nitrogen clouds can mix up with eachother when this is done "too wild"; this because of their different densities. While pushing out the ammonia the concentration of it should be measured at the outlet of the tank towards the refrigera-tion compressor. Assuming a low line velocity of 3 m/s through a DN250 line the expected purging time should amount to 60-70 hrs (< 4 days).

2.4 Air ventilation before inspection of the tank: Before inspection by maintenance people could

take place, ventilation by air has to be applied. This could be performed by means of a high perfomance fan of 50,000 Nm3/h blowing through a manhole. The nitrogen content has to be measured and when the O2-concentration is in the range of 20 to 21% ventila-tion can be stopped and maintenance and inspection personnel can then safely enter the tank.

The required time for air ventilation based on com-parable situations was estimated at 7 days. In total the required, estimated time period for empty-ing, nitrogen purging and air ventilation would cover probably 30 days. 2.5 Permission to start:

Continental Engineers BV produced based on its

experience, the facts and the figures, a detailed proce-dure for the overall decommissioning of the ammonia storage tank. This procedure was presented to OCP’s management in both English and French language. Based on this extensive procedure, OCP ordered C E to start with the proposed activities mid April 2004.

See fig. 5 for overall schematic diagram of the ammo-nia storage system for cooling, loading and emptying

3. Decommissioning and encountered problems, or in other words: Reality Check of Practice vs. Theory

The assumption was that 100 t. of ammonia should re-main in the tank after pumping out a part by means of pressure increase in the tank. However our commission-ing manager could not check nor verify the level of ammonia in the tank. OCP could also not give a reliable measurement of the amount in the tank, so the first un-certainty arose. The measured levels varied from 28 cm to 45 cm liquid and the question was:

Is the content 100 t. [~145 m3] or is it (much) more than the measurements indicated (250 t.= 38 cm height)? A lot of time was spent to in-stall a reliable measuring device by means of a floater, a thick shelf and finally steel buses. This was all done while not in operation.

Furthermore, despite the advice of C E to use option 3, but mainly because of the assumed higher content of ammonia, it was decided to blow in “hot” ammonia in order to speed up the evaporation rate. Flexible hoses were brought in into the tank, one through the under-pressure relief valve and the other through the overpres-sure relief valve. However, one of the flexible hoses brought in from the top was too short.

On the other hand emptying the liquid out of the tank by using the pumps was not successful since we did not get out the expected amount possibly due to gassing.

Parallelly it was locally investigated whether one

could use the “watermethod” described under option 4, in order to come to a 2 % solution of ammonia and wa-ter for disposal off to the sewer of the harbour, but this meant that approx. 7500 m3 of (demi)water should be needed. A 25% aqueous ammonia could however not be handled by OCP and so both ideas had to be aban-donned.

Because of the “forced” methods to at first remove a big amount of ammonia would not fulfil the require-ments to empty safely and adequately the tank, it was decided to go back to the original method, i.e. emptying by natural evaporation.

The expected rate of the liquid level reduction was approx. 1 cm/day equal to 6.5 t ammonia/day, but in practice it did not work that way. Perhaps the tank was too well insulated. To improve the evaporation rate of ammonia OCP removed the insulation at several places (which had to be removed anyway for renewal) by taking out 2x2 m spots allowing entering of heat through the wall.

The first conclusions of the reality check were: • The pumps did for some unsolved reasons not

fulfil the expected duty (possibly gassing) • We could not check, by removing parts of the

insulation shield, from the outside of the wall what the level was (one could not observe a distinct colour difference)

• There was not 100 t of ammonia in the tank but (much) more (some 250 t)

• Flexible hoses should be available and if yes they should be of enough length; it is however sometime quite difficult to insert these properly into the tank and liquid.

• One could not adequately measure the liquid level i.e. the remaining quantity of ammonia

• The alternative methods could not be applied or did not work

• After all this, the schedule (30 days) could not be maintained (i.e. more time was required, in this case ~45 days).

3.1 Lessons learnt for both parties at the phase of emptying the tank as part of the

overall decommissioning:

1. Before preparing a (desk) procedure the engi-neer should execute a site visit to make notice of local circumstances and discuss the planned approach.

2. It is advisable to make a very practicable pro-cedure (with step-by-step actions and explana-tory drawings) in stead of a desk report with theoretical assumptions and schemes.

3. The engineer should verify and list all the nec-essary measuring devices, auxiliary tools & equipment and ensure the local availability.

4. Looking back the method with inserting “hot” gases had to be preferred.

5. Contingency (x %) for the required time period should be timely communicated and built in to the proposal towards client/owner.

6. Owner should, at least for future to build tanks, make a drain beneath the tank bottom level to secure emptying the liquid from the tank by means of pumps.

7. Owner should install accessible manholes in the tank for inspection, maintenance and for measuring purposes.

3.2 Nitrogen purge & Air Purge Purging with nitrogen (N2) is based on the principle

of creating a difference of density in the layers of NH3 (molecular weight 17) and nitrogen (molecular weight 28). For environmental reasons the tank is connected to the other tank because of ‘pushing out’ the NH3.

This can be achieved by adding nitrogen very slowly (1.5 m/s, DN 250) in order to avoid turbulence via the manchet in the suction line to the bottom of the tank and take off the ammonia vapor with the compres-sors. When this is done very carefully, there is a sharp separation between the two cloud layers. When the level of nitrogen is about 1 meter above tank bottom, the amount of nitrogen can be increased (v=3 m/s), and when the level is about 2 meters above the tank bottom, the amount can be increased to maximum capacity (~800 m3/h).

One should monitor the pressure in the tank (which in the OCP case has to stay below 100 mbar).

We then could calculate how many hours it should take to reach the top of the tank (24 m). We needed an amount of approx. 30,000 m3 of nitrogen and it took eventually 2 days to feed this amount (in stead of 4). When this level was reached, the two tanks could be separated and the mixture of ammonia and the last ni-trogen could be blown through a water tank in order to avoid environmental hazard.

After two days already we could install the air fans [ ] for the purge with air via the manholes on the top and via the suction line at the bottom, in order to ensure a safe environment for maintenance and inspection per-sonnel. See fig. 3 for a schematical diagram of air ventilation

On the top of the tank we installed oxygen detectors

to measure O2-content of the gas/air mixture. When the oxygen level is approx. 20 % (to be measured with Dräger tubes or similar) and the NH3-concentration is < 20 ppm (MAC value) the environment is safe for enter-ing the tank. The air purge period took approx. 3 days.

Before entering the tank first of all enough lighting should be brought in and men should first inspect whether there was oil or other dirt on the bottom, which should be removed first.

Apart from the measurement by means of official detection the rabbit proof was also applied by the client. This is an almost fail safe test to check whether a man can safely enter such a tank The method is simple: catch a rabbit and let it in into the tank and look if it survives after a while so: See fig. 4: Schematic drawing of a successful rabbit proof.

It was agreed between parties that when the rabbit survived at least 15 min. the environment was to be considered safe for mankind. While the detectionmeters at top, bottom and manholes indicated an oxygen level between 20.3 and 20.7 %, which was o.k., the rabbit in-deed survived this exercise.

The client undersigned mid June 2004 the taking

over protocol stating that the decommissioning, how-ever with some delay, was finalised. In stead of the es-timated 4 weeks the total decommissioning period took 2 weeks extra, but at least everything went in a con-trolled and a safe way.

4. Inspection and re-insulation OCP executed after the decommissioning the in-

spection of the tank which gave no major problems for the maintenance department. The re-insulation was exe-cuted by a Morroccan company and the whole wall and dome , approx. 4000 m2 , were covered with a new layer of 120 mm polyurethane.

5. Recommissioning and its features In September 2004 OCP was ready with the re-

insulation and was ready to recommission the tank i.e. it could be filled with ammonia gas which should slowly be turned into cold liquid ammonia at –33 oC.

5.1 Nitrogen purging and safety measures to avoid the risks First of all, all the air in the tank should be evacu-

ated by feeding nitrogen into the tank, but why? It is to state that the presence of oxygen in connection with (liquid) ammonia can lead to a considerable in-creased risk of stresss 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 explo-sion risk will be there when the air contains between 15 and 28 % of ammonia gases.

So, first the air will be “replaced” by nitrogen and just after that gaseous ammonia, and later liquid ammo-nia, 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 oC/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 ‘in-ert’. C E estimated that this should take a few days and actually after 4 days the O2-level in the tank was <1.5 %: we actually measured 1.14% O2 and this is con-sidered to be a sufficiently safe value to start with blow-ing in gaseous ammonia.. At the same time the tank had to be slightly pressurized to approx. 1,15 bara in order to observe and check whether all the connections are tight (by using bells of soap) and which took an extra day. However, to execute this nitrogen purge, one should take into account a few safety measures up front:

• All inspections and repairs should have taken 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.

5.2 Bringing in ammonia vapour into the tank

The procedure of feeding the ammonia vapour into the tank (at a rate of 500 Nm3/h, temp. ~40 oC)) should take several days and each time the laboratory (of the client) has to measure the ammonia content in the tank. The ammonia gas was taken from the other storage tank [nr.

1] on the premises of OCP and the liquid ammonia was later taken from the sphere. The actual NH3-values in the tank were measured each day: After one day the value was: 14 gr/m3 (~2 %) After two days it was: 38 gr/m3 (~5.5 %) At the end after three days the value measured was: 87 gr/m3 (~12.5 %)

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 cool-ing down of the tank.

Note: No major incidents or problems occurred during this phase of recommissioning; only one relief valve from a HE-vessel opened and blew off above the annular space. Because of the well taken safety precautions, i.e. no persons were allowed by OCP in the neighbourhood, there was only waiting time to let the ammonia vapour-ize. The total ammonia vapour time amounted to approx. 6.5 days.

5.3 Cooling down the tank by spraying liq-

uid ammonia. It is to be advised that the temperature drop has to

be uniformly and be lower than 1 oC/h. The ammonia will be sprayed in from the top and will liquefy after a certain time period on the bottom of the tank, subse-quentely cooling down this bottom.

At the end of this activity the temperature at the bottom will be -32 oC, which means that with an ambi-ent temperature of +20 oC, the total difference will be at least 52 oC. It should therefore require a cooling down time of at least 52 hours, but for safety reasons we have chosen 3 x as much, i.e. 156 hours (~6.5 days).

At a flow rate of 70 to 120 kg/h the liquid ammonia has to brought into the tank; during this procedure the temperature at the bottom and alongside the wall have to be carefully monitored by the responsible people. If the temperature decreases too fast one should immedi-ately slow down or stop the filling procedure to avoid any risk of stress corrosion cracking of the bottom or the side walls. It took 4 to 5 days of filling with liquid ammonia (approx. 20 t.) to cool down the tank to the level of -32 oC (actually measured).

5.4 Closing of the recommissioning

Due to excellent teamwork of all people involved, the recommissioning went very well and in total it covered approx. a time period of 18 days. No major problems occurred during this phase except for the blow off of a relief valve of a heat exchanger vessel.

We can conclude that this recommissioning was in-deed safely and successfully executed.

6.0 Summary and conclusions

Thanks to the excellent cooperation between the com-missioning manager of C E, the management and the operating personnel of OCP on site the decommission-ing, although quite some (unforeseen) problems oc-curred, went well and after that the recommissioning even went perfectly.

It is of major importance to prepare a procedure, which is supported by experienced commissioning per-sonnel and which is to be accepted by the clients’ repre-sentatives on forehand.

Also up front the client should take the necessary measurements on site in order to secure a smooth exe-cution of the activities and OCP did so. There will be however always (unforeseen) problems on site, but only by good communication, expert consultation and enough patience, one can succeed in achieving the set goals.

Continental Engineers likes to thank OCP that they

were trustworthy to give the order for this decommis-sioning as well as the recommissioning to them. Also our de/recommissioning manager, Mr. Gerrit Tol has to be mentioned since he was the ‘solid and cool’ factor for C E on OCP’s site during periods that the inside and outside temperatures were quite high…..

Finally C E like to thank OCP, especially Mr. Abdeljalil Bourras, manager projects dpt., for giving us the oppor-tunity and his cooperation to prepare and present this paper for the interested audience here at Toronto. See fig. 6: Photo of mr. Bourras of OCP (r.) and mr.

L. Tol (l.) of C E.in front of the ammoniatank.

-o-o-o-o-o-o-o-o-o-o-o-

Attachments to paper 523 – AIChE Toronto Page 1

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Fig. 2: Schematic image of increasing the pressure in tank 2

Tank 1 pump filter tank 2 p=5 mb p=150 mb

open closed

compressor

Attachments to paper 523 – AIChE Toronto Page 2

Fig 3.: Extensive air ventilation by means of (5) air fans, exhaust via nozzles

(all valves were removed). After 15 minutes…

Fig. 4: The rabbit proof for checking safe environment in tank after purging with nitrogen resp. air.

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Attachments to paper 523 – AIChE Toronto Page 4 (Photograph)

Fig. 6: Photograph in front of the big ammonia storage tank of Mr. A. Bourras of OCP (r.), director of projects, and the author of the paper, mr. L.A.J. Tol (l.) of Continental Engineers BV, The Netherlands at the location of El Jadida, Morocco.