International Fertiliser Society

FIRST PRACTICAL EXPERIENCE WITH ROBOT INSPECTION OF AMMONIA STORAGE TANKS

by

Kjetil Bakli1, Ole Nørrekær Mortensen2 and Christian Valand3 1 Yara International ASA, Bygg 18, PO Box 1123, N-3905 Porsgrunn, Norway. 2 Force Technology, Hovedkontor, Park Allé 345, DK-2605 Brøndby, Denmark. 3 Nokas AS, Træleborgodden 6, N-3112 Tønsberg, Norway.

Proceedings 749

Paper presented to the International Fertiliser Society at a Conference in London, UK, on 3rd July 2014.

www.fertiliser-society.org

© 2014 International Fertiliser Society ISBN 978-0-85310-386-8

(ISSN 1466-1314)

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SUMMARY. Refrigerated liquid ammonia in larger volumes is normally stored in carbon steel tanks. These tanks may suffer from ammonia Stress Corrosion Cracking (SCC). Oxygen needs to be present to develop this failure mechanism.

An intrusive inspection of such tanks will include emptying the tank and filling it with air. This is a very time consuming and expensive set of operations that includes the introduction of oxygen, which is not desirable due to the risk of SCC development.

Yara has, together with Force Technology, developed a concept for non-intrusive inspection. A remotely controlled robot is operated on the outside of the liquid-containing tank, inspecting the welds using an ultrasound technique (UT). In the case of a double integrity tank, the robot is operating in an ammonia atmosphere. The concept also includes deployment of the robot.

The concept has been successfully employed on several of Yara's ammonia tanks in Europe.

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CONTENTS

Summary 2

1. Introduction 4

2. When is a non-intrusive inspection appropriate? 6

3. Important considerations when carrying out a non-intrusive inspection 6

4. Important experiences to date 7

4.1. Preparations 7

4.2. Gas diving and deployment of robot 7

4.3. Materials 8

5. Non-destructive testing (NDT) technique selection 9

5.1. Robot 9

6. Validation of this non-destructive testing (NDT) technique 10

7. Results 11

8. Reference 11

Related Proceedings of the Society 11

Keywords: Ammonia tanks, stress corrosion cracking, SCC, robot inspection, ultrasonic inspection, non-destructive testing.

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1. INTRODUCTION. Refrigerated liquid ammonia (NH3) in larger volumes is normally stored in carbon steel tanks. The designs can differ. The main design difference is if the tank is single (Figure 1) or double integrity (Figure 2). The outer wall of a double integrity tank may be steel or concrete. The main purpose of the outer wall is to collect potential leaks from the inner tank.

Figure 1: Single integrity tank.

Figure 2: Double integrity tank.

Concrete bund

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Different sets of codes define design of refrigerated ammonia tanks. The most widely used codes in Europe are API 620 including appendix 'R' and EN-14620. Refrigerated ammonia storage tanks may be subjected to ammonia Stress Corrosion Cracking (SCC). Rupture of a refrigerated ammonia storage tank would most probably cause a serious disaster. The formation of SCC depends on stress level and presence of oxygen. During normal operation no oxygen will be present inside the tank. The water content of the ammonia works as inhibitor. To avoid ingress of oxygen during the inspection of the 60,000 m3 ammonia tank at the Yara Porsgrunn plant in Norway, Yara started, together with Force Technology, Denmark, to develop a robot concept for inspection of its ammonia storage tank. Force Technology and Yara have been working closely together for many years. In Porsgrunn we ended up with a full intrusive inspection. The development of the robot was more complex and took longer than we had hoped. Prior to the full intrusive inspection, the tank was used as a 'test facility' and the experience we gained was utilised for the further development of the robot concept. The development of the concept also included a verification of the inspection method (ultrasound) by The Welding Institute, Cambridge, UK. Inspection of refrigerated ammonia storage tanks can be carried out in two fundamentally different ways. During an intrusive inspection, the liquid-containing tank is inspected from the inside. This requires the tank to be emptied and free from ammonia gas. During the inspection, the tank will be filled with ambient air so that people working inside can breathe normally. The other way is to carry out a non-intrusive inspection. In this case the liquid-containing tank is inspected from the outside. Consequently, if the right equipment is available, it is possible to carry out the inspection while the tank is in service and without emptying it. The following advantages and disadvantages apply to non-intrusive inspection: Advantages: • Oxygen ingress is not likely to occur. • Less time is taken, 7-10 days. • In-service inspection, no outage required. • Total inspection cost is lower. • Fewer emissions. Disadvantages: • Some places are not accessible to the scanner, such as the bottom plating,

pipe entry points, etc. • Modifications which requires the opening of the tank will not be possible.

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2. WHEN IS A NON-INTRUSIVE INSPECTION APPROPRIATE? Not all tanks can be inspected by non-intrusive robot inspection and not all tanks should be inspected by this method. The robot has an umbilical used for robot control, data transfer, video, nitrogen purging and contact fluid. The length of this umbilical is restricted and this will limit the size of the tank that can be inspected. We installed two additional manholes in the outer tank of the Yara Porsgrunn tank to facilitate future robot inspections as this is a very large tank. The original quality control record must be available, confirming the quality of at least all T-welds in the lower three courses and the bottom to shell weld (magnetic particle inspection, MPI). Risk Based Inspection (RBI) assessment defines the tank as medium risk or lower. This implies an inspection interval of more than 10 years. The methodology is presented in 'Guidance for Inspection of Atmospheric, Refrigerated Ammonia Storage Tanks', now published by Fertilizers Europe (EFMA, 2008). The minimum critical crack size that can cause an instant rupture of the liquid-containing tank is calculated by fracture mechanics. The calculated critical crack size must be much greater than the minimum detectable crack. The tank should be 'leak-before-break'. A leak will be easily detected in the case of a single wall tank due to the formation of ice and the smell of ammonia. In case of a double integrity tank, liquid ammonia in the annular space may indicate a leak. For smaller leaks, the ammonia will most probably evaporate without creating any liquid level in the annular space. It is important to ensure that the authorities are involved at an early stage.

3. IMPORTANT CONSIDERATIONS WHEN CARRYING OUT A NON-INTRUSIVE INSPECTION.

Full documentation including a complete set of drawings must be available. This will also include reports from previous inspections. Special focus must be on compliance between drawings and real world, especially details related to the deployment manhole. The crucial point is whether enough space is available for the deployment of the robot or not. Are the inner and outer tanks concentric? Is the length of the manhole's neck according to the drawings? In Porsgrunn, we opened the manhole and checked. Close attention must be paid to possible obstacles which could hook onto the umbilical. These may be pipes, lugs etc., or left-overs from erection. A map showing where repair works have been carried is important. Because the robot is equipped with magnetic wheels, a minimum wall thickness of 9 mm is required. This is also the minimum limit for the ultrasound examination.

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Some older tanks do not have detection of liquid ammonia in the annular space. There are several reasons why a liquid level can occur there. The most usual is condensation, but a leak can also be the reason. As far as possible, checks must be carried out to ensure that any liquid ammonia is not up to the level of the manhole before this is opened.

4. IMPORTANT EXPERIENCES TO DATE. 4.1. Preparations. Carrying out a non-intrusive inspection of an ammonia tank is a set of activities that requires great care. During deployment of the robot, human efforts are needed and even the smallest problem can cause delays or dangerous situations. Consequently, thorough preparation and focus on details are very important. When we prepared for ammonia gas diving at Yara Porsgrunn, we made a very detailed procedure on how to open and close the manhole and operate the habitat. The gas divers actively participated and approved the procedure. Some important statements in the procedure are: • Participation of gas divers was based on volunteering. • Avoid the ingress of oxygen. A two-chamber lock was designed with

arrangements for N2 purging and evacuation of used breathing air. • Write a description of how to evacuate a person in the case of an

emergency. • Training was conducted and the procedure updated where found to be

necessary. • Authorities and neighbours must be informed. 4.2. Gas diving and deployment of robot. Diving in an ammonia atmosphere was new to us and safety, both related to the divers and the tank, was of major concern. When we started, we had experienced gas divers available, however not for this special task. Consequently, special precautions were taken. An ambulance was present outside the bund wall, and as long as the divers felt inexperienced we had a mobile crane available that could lift persons out of the bund. Altogether, we used four gas divers for the diving operation. Two were handling the robot in the inner chamber and one was sitting, fully dressed in gas diving suit and with breathing air, in an ammonia gas atmosphere. The reason for having one person sitting in the first chamber was so that they could intervene in the case of an emergency or in case of minor problems requiring a change of diver. One such reason could be a leak in the diving suit. The fourth diver was dressed and prepared to replace this colleague if required.

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To avoid emissions of ammonia, water curtains were arranged and showers were installed for cleaning the divers after dives. Details are very important as even the smallest mistake can interrupt the operation. All necessary tools must be present inside the lock. No kind of instrument or equipment containing electronics and plastics should be relied upon unless previously fully tested in an ammonia atmosphere for a period of time. Make sure there is nothing that can catch or snag the diving suits and create leaks. Also ensure that all sharp edges have been rounded. All normal safety procedures were still active, such as Safe Job Analysis (SJA), working in confined spaces, reporting on near misses, etc. Take your time and don't hurry! 4.3. Materials. Ammonia is very aggressive to some kinds of plastics and electronics. Consequently, everything needs to be tested over a sufficient period of time to ensure it is resistant to ammonia. We found that polymethyl methacrylate (PMMA) is resistant to ammonia gas, at least for the period of time to which we exposed it. To avoid ammonia getting into electronics and non-resistant plastics, nitrogen overpressure was utilised for the robot.

Figure 3: Gas divers removing the hatch cover.

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5. NON-DESTRUCTIVE TESTING (NDT) TECHNIQUE SELECTION. The potentially damaging mechanism is ammonia stress corrosion cracking (NH3-SCC). The inspection technique must therefore be sensitive to crack detection. Further, the inspection technique must have the ability to detect and determine the size of cracks which can run both parallel to, and perpendicular to, the weld. Because the potential cracks will appear on the opposite side (on the internal surface of the inner tank shell) of the steel plate from where the inspection is carried out (from the external surface of the inner tank shell), the ultrasound technique (UT) was chosen as it has a very high sensitivity to linear and planar defects. The typical detection setup for weld inspection consists of two sets of shear wave probes. One set is perpendicular to the weld and the other one is in a small skewed angle to the weld. The probes are placed on each side of the weld. 5.1. Robot. The initial inspection was carried out at Yara Porsgrunn, utilising the standard automated inspection systems developed by Force Technology. The experience gained in Porsgrunn led to the development of a 2nd generation robot and deployment system especially designed to work in an ammonia atmosphere and at low temperature -33°C.

Figure 4: Inspection robot with brush and surveillance camera.

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6. VALIDATION OF THIS NON-DESTRUCTIVE TESTING (NDT) TECHNIQUE.

An inspection procedure containing a detailed description of the ultrasound probes, inspection setup and evaluation of the examination data has been established for the inspection system. The inspection procedure and inspection system has been validated by producing a number of test plates with welds and artificial defects (machined notches). The test plates were made in 9 mm and 32 mm thickness (outer limits of the wall thicknesses of the Yara tanks to be inspected). The artificial defects were placed in the weld as shown in the Figure 5. The artificial defects were placed in the centre of the weld and in the heat affected zone. The depth (height) ranged from 2 mm to 5 mm. The validation was carried out by examining the test plates with the inspection system under laboratory conditions. All the artificial defects were detected as expected. Figure 5 shows an example of transverse notches in a 32 mm plate.

Figure 5: Example of test plate and positioning of notches.

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7. RESULTS. A screen shot from the scanning is shown in Figure 6; an untrained eye could easily misinterpret this plot. Consequently, knowledge and experience are needed for the interpretation of the results of the scan. Knowledge about former repairs, whether it was done during erection or later, is very important. It can provide an explanation of the scanning results.

Figure 6: A screen shot showing an example of scanning results.

8. REFERENCE. EFMA, (2008). Guidance for Inspection of Atmospheric, Refrigerated

Ammonia Storage Tanks (2008) EN http://www.fertilizerseurope.com/index.php?id=120

RELATED PROCEEDINGS OF THE SOCIETY. 307, (1991), Stress Corrosion Cracking of Carbon Steel Storage Tanks for

Anhydrous Ammonia, L Lunde, R Nyborg. 308, (1991), Ammonia Storage Inspection,

S Hewerdine.

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382, (1996), Control of Stress Corrosion Cracking in Liquid Ammonia Storage Tanks, R Nyborg, L Lunde, P-E Drønen.

401, (1997), Ammonia: Safety, Health and Environmental Aspects, K D Shah.

482, (2001), De-Commissioning of Ammonia Cold-Storage Tanks, J Kristensen, R Fogg.

603, (2007), Inspection of Atmospheric Ammonia Storage Tanks; New EFMA Recommendations, H A M Duisters.

604, (2007), Safety Issues in Ammonia Handling and Distribution, K D Shah.

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