Storage of Liquid Chlorine GEST 73/17 7th Edition January 2014
EURO CHLOR PUBLICATION
This document can be obtained from: EURO CHLOR - Avenue E. Van Nieuwenhuyse 4, Box 2 - B-1160 BRUSSELS Telephone: 32-(0)2-676 72 65 - Telefax: 32-(0)2-676 72 41
GEST 73/17 th 7 Edition
Euro Chlor Euro Chlor is the European federation which represents the producers of chlorine and its primary derivatives. Euro Chlor is working to:
improve awareness and understanding of the contribution that chlorine chemistry has made to the thousands of products, which have improved our health, nutrition, standard of living and quality of life;
maintain open and timely dialogue with regulators, politicians, scientists, the media and other interested stakeholders in the debate on chlorine;
ensure our industry contributes actively to any public, regulatory or scientific debate and provides balanced and objective science-based information to help answer questions about chlorine and its derivatives;
promote the best safety, health and environmental practices in the manufacture, handling and use of chlor-alkali products in order to assist our members in achieving continuous improvements (Responsible Care).
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This document has been produced by the members of Euro Chlor and should not be reproduced in whole or in part without the prior written consent of Euro Chlor. It is intended to give only guidelines and recommendations. The information is provided in good faith and was based on the best information available at the time of publication. The information is to be relied upon at the user’s own risk. Euro Chlor and its members make no guarantee and assume no liability whatsoever for the use and the interpretation of or the reliance on any of the information provided. This document was originally prepared in English by our technical experts. For our members’ convenience, it may have been translated into other EU languages by translators / Euro Chlor members. Although every effort was made to ensure that the translations were accurate, Euro Chlor shall not be liable for any losses of accuracy or information due to the translation process. Prior to 1990, Euro Chlor’s technical activities took place under the name BITC (Bureau International Technique du Chlore). References to BITC documents may be assumed to be to Euro Chlor documents.
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RESPONSIBLE CARE IN ACTION
Chlorine is essential in the chemical industry and consequently there is a need for chlorine to be produced, stored, transported and used. The chlorine industry has co-operated over many years to ensure the well-being of its employees, local communities and the wider environment. This document is one in a series which the European producers, acting through Euro Chlor, have drawn up to promote continuous improvement in the general standards of health, safety and the environment associated with chlorine manufacture in the spirit of Responsible Care. The voluntary recommendations, techniques and standards presented in these documents are based on the experiences and best practices adopted by member companies of Euro Chlor at their date of issue. They can be taken into account in full or partly, whenever companies decide it individually, in the operation of existing processes and in the design of new installations. They are in no way intended as a substitute for the relevant national or international regulations which should be fully complied with. It has been assumed in the preparation of these publications that the users will ensure that the contents are relevant to the application selected and are correctly applied by appropriately qualified and experienced people for whose guidance they have been prepared. The contents are based on the most authoritative information available at the time of writing and on good engineering, medical or technical practice but it is essential to take account of appropriate subsequent developments or legislation. As a result, the text may be modified in the future to incorporate evolution of these and other factors. This edition of the document has been drawn up by the Equipment working group to whom all suggestions concerning possible revision should be addressed through the offices of Euro Chlor.
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Summary of the Main Modifications in this version Section
Nature
All
Merge with GEST 72/10 on pressure storage of liquid chlorine and adapt the terminology (refrigerated and non-refrigerated)
All
Clarification on double-jacketed storage tanks
3.2.5.
Updated thermal insulation paragraph
3.4.
Precision that stress relief is done after welding
4.2.
Precision added for cases where chlorine withdraw is from the bottom of the tank
4.5.1.
Removal of chlorinated fluorocarbons as example, as there is no practical experience mentioned
5.8.
Added paragraph on inspection (after washing)
5.10.
Addition of paragraph on fighting a big leak
Table of Contents 1.
GENERAL POINTS 1.1. The Choice between Storage with or without Refrigeration 1.2. Unit Capacities 1.3. Number of Storage Tanks
2.
BASIC DESIGN AND LOCATION OF THE STORAGE SYSTEM 2.1. Design and Permits 2.2. Principle 2.3. Location 2.3.1. Outside or Inside Location 2.3.2. Protection from External Damage 2.3.3. Distance from Rails and Roads 2.3.4. Distance from Another Operating Process 2.3.5. Distance from the Boundary of the Factory 2.3.6. Distance between two Adjacent Storage Tanks 2.4. Bunding 2.5. Emergency Capacity
3.
CONSTRUCTION OF STORAGE TANKS 3.1. Basis of Design 3.1.1. Design Pressure 3.1.2. Design Temperature 3.1.3. Corrosion Allowance 3.1.4. Thermal Insulation 3.2. Materials of Construction 3.2.1. Steel 3.2.2. Branches, Flanges, Nuts and Bolts
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7 7 8 8 8 8 8 8 9 9 9 9
10 10 10 11 11 12 12 12 13 Page 4 of 26
GEST 73/17 th 7 Edition 3.2.3. Pipework 3.2.4. Quality of Jointing Materials 3.2.5. Thermal Insulation 3.3. Foundations and Supports 3.3.1. Foundations 3.3.2. Supports 3.4. Stress Relief 3.5. Inspection and Testing 3.5.1. Inspection of Construction Materials 3.5.2. Inspection during Fabrication
4.
13 13 13 14 14 14 14 14 14 15
ACCESSORIES
15
4.1. 4.2. 4.3. 4.4. 4.5.
Branches Valves and Isolation Pipework Measuring Equipment Safety Equipment 4.5.1. Over and Under-Pressure 4.5.2. Protection of the External Shell 4.6. Filling Ratio 4.7. Thermal Expansion Bellows for Double-Jacketed Tanks 4.8. Filling, Emptying and Venting Equipment 4.8.1. Filling 4.8.2. Emptying 4.8.3. Venting
5.
15 16 16 17 17 17 18 18 18 19 19 19 19
OPERATION 5.1. 5.2. 5.3. 5.4. 5.5. 5.6.
19
Cleaning and Drying before Chlorine can be Admitted Leak Testing Commissioning Total Emptying Reactive Materials Filling and Emptying in Normal Operation 5.6.1. Quality of Chlorine Introduced 5.6.2. Temperature and Pressure of the Liquid Chlorine Introduced 5.6.3. Emptying the Tank 5.7. Venting and Isolated Systems (Cold Chlorine Storage) 5.8. Periodic Inspection and Testing 5.8.1. External inspection 5.8.2. Internal inspection 5.9. Methods of Protection and Alarm 5.10. Response to a significant loss of primary containment - Methods and Equipment
6.
REFERENCES
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25
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DEFINITIONS This recommendation concerns storage systems provided by fixed liquid chlorine storage tanks constructed to operate with or without refrigeration.
1.
GENERAL POINTS 1.1. The Choice Refrigeration
between
Storage
with
or
without
Ambient temperature chlorine storage means storage at high pressure. The advantages of ambient temperature storage are:
Simplicity of operation
Easy visual external inspection (no thermal insulation)
Lower investment cost
Refrigerated chlorine storage means storage at lower pressure, in some cases at atmospheric pressure. The advantages of refrigerated storage are:
Lower initial emission in case of loss of containment if at atmospheric pressure (lower initial flash of chlorine gas due to the fact that liquid chlorine is at lower temperatures compared to pressurized storage)
The complexities of refrigerated storage and its associated systems mean it is unsuitable for small chlorine users.
1.2. Unit Capacities In order to minimise hazard, the inventory should be limited to the minimum strictly necessary. The unit capacity is chosen taking into account:
The process requirements (operation continuity, maintenance/inspections works),
The loading/off-loading requirements; the tank must normally be larger than the contents of one mobile tanker.
Based on practical long term field experience, the probability of a total tank failure can be assumed as negligible. The most important safety feature depends primarily on the design, operation and inspection of the storage system.
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1.3. Number of Storage Tanks In order to provide the desired storage capacity and the continuity of the supply, a compromise may be required between the individual unit capacities and the number of storage tanks. It should be noted that increasing the number of tanks leads to a proportional increase in the number of accessories, with the related various risks of maloperation and mal-functioning. It is therefore desirable to limit the number of tanks, without forgetting the necessity of an available safety capacity (see point 2.5.).
2.
BASIC DESIGN AND LOCATION OF THE STORAGE SYSTEM 2.1. Design and Permits
A careful risk assessment study, periodically updated, is necessary to ensure that the required level of safety is attained. The methods used should be agreed with the relevant national/local authorities. In some countries scenarios and models are required. Some consideration is given below to the choice of the worst case scenario (see paragraph 2.2. Principles). Simple models are usually sufficient to evaluate the physical effects. However, relevant expertise is always needed to define the scenarios, to use and to interpret the results of such models correctly. To assess the effect on people (workers and neighbouring population) recognised toxicity “probit function”1 will be used. The study must show that the risk is acceptable and that adequate measures have been taken to protect people and the environment. In some cases, storage tanks may be designed with double jacket; Usually, double jacket means that the outer wall is designed to resist a lower pressure than the inner wall as it is primarily used to monitor possible small leaks of the chlorine inner storage tank (for example PN 10). The space inside the double jacket should be monitored, e.g. by a permanent flow of dry air or nitrogen with a chlorine detector at the outlet of the flushing gas with alarm; alternatively the space can be kept under nitrogen pressure with alarm.
1 The probit function is a statistical function describing the range of susceptibility in a population to a harmful consequence; it uses a criterion in the form of an equation which expresses the percentage of a defined population which will suffer a defined level of harm (normally death) when exposed to a specified dangerous load (time and intensity/concentration).. January 2014
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In some cases, the outer wall is also used as bunding and its design conditions will then be identical to those of the inner wall.
2.2. Principle Liquid chlorine bulk storage could be the potential source of the worst case of loss of primary containment. However, a chlorine storage system can be designed and operated safely so that the risk of complete tank failure and release of much of its contents can be assumed negligible. The principles which must be followed to achieve this are discussed in the following chapters. It is also recommended to use GEST 87/130 - Hazard Analysis for Chlorine Plant' for the design of the storage system.
2.3. Location 2.3.1.
Outside or Inside Location
Storage system can be located in the open air or in a closed building. The decision on this matter must be based on a careful risk assessment taking into account the advantages and disadvantages of each alternative listed in the Position paper XII - Memorandum on Confinement of Liquid Chlorine Plants. 2.3.2.
Protection from External Damage
Chlorine storage must be located in an area with protective barriers so that it is fully protected from external damage from vehicle impact. The location and design of a storage tank has to be chosen to minimise the possible effects resulting from traffic (see 2.3.3), flooding, subsidence, earthquake, fire or explosion in a neighbouring plant. 2.3.3.
Distance from Rails and Roads
Installations should be located at least 25 m from public roads and railway lines to minimise the risk of damage to the storage in the event of an accident. This distance has to be defined taking into account the local conditions, rules and regulations. 2.3.4.
Distance from Another Operating Process
If a neighbouring unit does not present risk of fire or explosion, the minimum recommended separation is 10 m taking into account the local situation.
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If a fire or explosion risk exists, greater distances or means of protection will be required. These must be established for each individual case, based on a risk assessment and taking any national and local rules and regulations into account. 2.3.5.
Distance from the Boundary of the Factory
The minimum recommended separation is 10 m taking into account the local situation and the size of the storage. In all cases, suitable fences, together with adequate security supervision, should be provided to prevent unauthorised access (see also GEST 05/316 – Guideline for site security of chlorine production facilities). 2.3.6.
Distance between two Adjacent Storage Tanks
It is recommended that sufficient distance should be provided between adjacent storage tanks, to give good access to the tanks for operation, maintenance and inspection, and to permit the passage of personnel equipped with self-contained breathing apparatus in case of incident.
2.4. Bunding All above ground storage tanks should be placed in a liquid tight bund. The volume of the bund should be calculated to receive at least the full contents of the worst realistic case scenario. A retention capacity of one storage tank is generally considered adequate. As a single bund may contain more than one storage tank, its capacity shall be based on the largest tank contained. The bund should be designed to limit the surface area in order to reduce the rate of evaporation of liquid chlorine in the event of a leakage, but without restricting access (see sections 5.9 and 5.10. The bund shall never be directly connected to a drain. Collected water (rain …) shall be removed by a pump or an ejector which shall be manually operated only after checks on the bund contents. In case of double-jacketed tank, the outer wall can be designed to provide such a bunding facility.
2.5. Emergency Capacity It is essential that a damaged storage tank can be emptied rapidly into a spare capacity, preferably a spare tank. If the emergency capacity is in the form of an empty tank, there should be a means to maintain it at low pressure, e.g. a system of degassing to an
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absorption system or a chlorine consuming unit, or a large liquefaction set, the reliable operation of which should be assured. The use of a double-jacketed storage tank with the external wall designed as bunding facility means that any chlorine leakage will be contained and will not affect the external environment. In this case it is therefore not necessary to provide spare capacity to which the contents of a leaking tank can be immediately transferred.
3.
CONSTRUCTION OF STORAGE TANKS 3.1. Basis of Design 3.1.1.
Design Pressure
The design pressure should be chosen on the basis of detailed consideration of all circumstances which will arise during operation of the storage system. The principal factor to be taken into account is the vapour pressure of chlorine at the maximum temperature to which the storage system can be subjected. Allowance must also be made for the maximum pressure resulting from the presence of any padding gas. Once the design pressure has been chosen, all reasonable measures must be taken to ensure that it is not exceeded in the course of subsequent operations. It is also necessary to consider a minimum design pressure. In case of double-jacketed tank, these minimum and maximum design pressures have to be determined both for the internal and the external shells. Storage tank (inner shell in case of double-jacketed tank) The minimum design pressure should be calculated taking into account:
the minimum temperature of the chlorine introduced
the means of emptying and venting chlorine (connection to absorption unit or suction of a compressor)
the maximum venting rate, even under accidental circumstances
the pressure from any purge gas between the two walls of doublejacketed tanks
the possible accidental presence of some chlorine between the two walls of double-jacketed tanks.
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The maximum pressure should be calculated as a function of:
the vapour pressure of chlorine at the maximum temperature to which the storage tank can be subjected, including a margin for the presence of inert gasses
the internal conditions within the storage tank when it is isolated (for example roll over of the liquid chlorine)
the hydrostatic pressure due to the height of liquid chlorine
any transient conditions during filling and emptying. External Shell (if double-jacketed tank)
The weight of the external thermal insulation, if present, shall be taken into account. If the external wall is designed as bunding facility, the minimum and maximum pressures should be identical to the inner wall to account for the extreme conditions in case of failure. 3.1.2.
Design Temperature
The minimum design temperature is minus 40°C (considering that an excess of inert gasses can lower the vaporisation temperature of the chlorine). The maximum design temperature can be calculated from possible solar heating, taking into account of any insulation. In case of a double-jacketed tank, the choice of minimum design temperature for the external shell depends on its extended purpose. As it is intended to provide secure containment in the event of a leak from the internal tank, it should be designed for minus 40°C. If it is only intended to monitor possible small leak of the internal wall, it may be designed for ambient temperatures. The overall construction must take account of differential thermal expansion between the internal tank and the external shell, particularly in relation to supports, branches and other equipment; the maximum allowed differential temperature should be defined during the design. 3.1.3.
Corrosion Allowance
A corrosion allowance of 1 mm is considered a minimum for the tank (also for the external shell if it is designed to act as a bund in the event of a leak). This value will be added to the calculated thickness before choosing the schedule just above the resulting total thickness. Thermally insulated or not, the external surface of the equipment will be painted with a corrosion protective coating.
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3.1.4.
Thermal Insulation
For chlorine storage tanks, insulation may be required for different reasons:
avoid water condensation or ice formation on equipment
protection of personnel for cold temperature
protection from possible external heating
If installed, the thermal insulation should be based on the minimum chlorine temperature and the maximum external temperature appropriate for the geographical location. The thickness is determined by, amongst others, the capabilities of the facilities to handle the evaporated chlorine gas due to the heat input (liquefaction unit, absorption unit or refrigeration unit) see section 5.7. Care will be taken to avoid any accumulation of moisture between the insulation layer and the equipment surface in order to prevent corrosion under insulation (CUI). Proper painting or coating of the equipment or piping is strongly recommended to prevent external surface corrosion, as well as good insulation surveillance, inspection and repair programs. Operating discipline must be applied to ensure that insulation stripped or removed for maintenance or inspection purposes is replaced and sealed in a timely manner. The quality of installation is important for the corrosion protection of the pipe/equipment and the tightness of the jacketing can be improved by tapes or mastic sealing; particular attention shall be paid at branching points.
3.2. Materials of Construction 3.2.1.
Steel
The plate chosen for the construction of storage tanks should be made of fine grain steel with good welding properties and which has satisfactory impact strength at minus 40°C after welding (see also GEST 79/82 - Choice of Materials of Construction for Use in Contact with Chlorine). These tests are particularly important concerning the impact strength of the metal before and after welding. If double-jacketed tank, the quality of the steel can be different for the internal tank and the external shell, to take into account of the minimum design temperature chosen for each.
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3.2.2.
Branches, Flanges, Nuts and Bolts
The metal used for flanges and branches should have the same quality as the material used in the construction of the storage tank itself. Bolting equipment should conform to GEST 88/134 - Stud Bolts, Hexagon Head Bolts and Nuts for Liquid Chlorine. The overall arrangement of flanges and jointing material should prevent the gasket being expelled by excess pressure. 3.2.3.
Pipework
Pipework used for handling chlorine should be designed to have an adequate wall thickness and should use a quality of steel suitable for the temperature and pressure of the operation. As far as possible, l00% radiography of the welds should be carried out. In circumstances where radiography is not possible welds should be inspected by a dye penetrant test, ultrasonic or magnetic inspection. For further information, refer to GEST 73/25 - Transport of Dry Chlorine by Pipeline. 3.2.4.
Quality of Jointing Materials
The jointing material used must be an asbestos free material. See GEST 94/216 - Experience of Non-Asbestos Gaskets on Liquid and Dry Chlorine Gas Service. 3.2.5.
Thermal Insulation
The material used for thermal insulation needs to be:
inert in presence of chlorine
not flammable or combustible, or at least self-extinguishing
For cold liquid chlorine, there is no predominant insulation material but the following are satisfactorily used:
polyurethane foams
foam glass
mineral wool
Other materials are sometimes used (phenolic resins …). Vapour barriers must be utilised to prevent the ingress of moisture on any insulated pipe/equipment that operates at temperatures below ambient temperature). For the cladding used to protect externally the insulation layer and to prevent as far as possible ingress of water, several materials can be used, according to
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the local environment (aluminium, stainless steel, coated carbon steel, plastic, resin, fibre reinforced resin …). For double-jacketed tanks, no insulation will be installed in the inner space.
3.3. Foundations and Supports 3.3.1.
Foundations
The design of the foundation will take into account the wind and seismic loads. 3.3.2.
Supports
Supports should be designed in accordance with a recognised standard and in such a manner that they provide overall mechanical stability for the storage tank and do not lead to any abnormal stresses on its walls. They must permit thermal expansion and contraction due to variations in temperature which can occur. They must limit local thermal losses by conduction. The design of the supports will take into account the wind and seismic loads. Special consideration may be necessary where load cells or balances are used in determining the contents of the tank.
3.4. Stress Relief Stress relieving is recommended and should be carried out, after welding, in accordance with the quality of steel used and with the method of welding. It is specifically recommended for the support legs and branches and especially for areas of greater wall thickness.
3.5. Inspection and Testing 3.5.1.
Inspection of Construction Materials
The steel plate supplied for the tank should be tested mechanically and chemically in order to confirm the material is in compliance with the specifications (in addition to checking the certificates). These tests are particularly important to establish the impact strength of the metal at the proposed design temperature. The metal used for flanges, blanks, nuts, bolts, welding rods etc. should also be subject to acceptance tests to meet a specification compatible with the above requirements.
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3.5.2.
Inspection during Fabrication
The inspection procedures during the construction of the liquid chlorine storage system should conform to the codes being applied, particularly regarding (not exhaustive and not in chronological order):
qualification of the welders and the welding procedures
100% radiography of welds
tensile strength, bending, hardness and impact strength tests on test pieces of the welds
thickness measurement and detection of cracks or laminations by ultrasonic means
pressure test
Additionally, inspection to determine gas tightness by halogen or helium testing could be realised. Conformance to the relevant codes should be confirmed by an independent third party. The objective is to guarantee a fault free construction. The quality of the construction is considered to be an essential requirement for the safety of future operation.
4.
ACCESSORIES 4.1. Branches
The wall thickness should be according to GEST 73/25 – Transport of Dry Chlorine by Pipeline. The number and sizes of branches should be limited to the minimum necessary for the installation of equipment required in the gas or liquid phase. For mechanical robustness, the recommended minimum diameter of the branches should be 40 mm. Except for the liquid chlorine extraction connection, if realised on the bottom of the tank, all branches should be installed on manhole covers. Top connection is preferred. In the case of a bottom connection it should be limited to one connection and the construction must be strong enough to withstand accidental mechanical loads. A dimension of 150 mm is considered as a maximum for the liquid phase. Larger branches such as those required for manholes, introduction of pumps etc. should be located in the gaseous phase of the storage tank. January 2014
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There should not be bi-directional connections between the gas phase of the tank and the gaseous chlorine network that could contain some hydrogen (see also GEST 08/360).
4.2. Valves and Isolation It is necessary to provide valves on each branch of the storage tank which enable it to be isolated. It is recommended that remotely operable valves be fitted at least on liquid chlorine inlet and outlet connections. All manually operated valves should be installed in a way allowing easy access. All valves installed on the storage tank should be of a design specifically developed for liquid chlorine duty. The materials of construction of these valves should correspond to the intended operating temperature and pressures. GEST 06/318 – Valves Requirements and Design for Use on Liquid Chlorine and GEST 94/204 – Pneumatically Operated Valves for Use on fixed Storage Tanks, Rail and Road Tankers and ISO-Containers for Liquid Chlorine provide specifications for valve types suitable for use on storage tanks. Other valves may be suitable but should be assessed carefully to ensure they deliver equivalent safety. When the storage tank is fitted with a bottom connection, it is normally recommended that a secondary internal globe valve is provided, capable of isolating the branch in the event of failure of the external isolation valve or of the associated joint. This internal globe valve should also be remotely operable, independently of the external valve. As an alternative to the secondary internal valve for both bottom liquid connections, the storage tank may be located in a closed and sealed building (secondary containment) connected to an absorption unit with sufficient absorption capacity to process possible chlorine leak (see 2.3.2 regarding indoor tanks). In any case, the number of connections/valves should be strictly limited. On connections to storage tanks used to withdraw liquid chlorine from the top, dip tubes will be installed.
4.3. Pipework All pipework connections to the storage tank should be designed to suit the temperature and pressure of the chlorine and should be of an adequate wall thickness (See GEST 73/25 – Transport of Dry Chlorine by Pipeline). 100% radiography of welds on this pipework is recommended. In circumstances where radiography is not possible welds should be tested by dye penetrant test ultrasonic or magnetic inspection. The specific connections between the storage system and loading or off-loading installations should conform to the recommended guideline GEST 78/73 January 2014
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Design Principles and Operational Procedures for Loading/Off-Loading Liquid Chlorine Road and Rail Tankers and ISO-Containers.
4.4. Measuring Equipment The following instrumentation is recommended to check the status of the tanks from the control room:
an indication of level (weighbridge, load cells, radioactive measuring system or other adapted for chlorine duty)
high level alarm
pressure indicator with alarm
The high level alarm should be provided by a separate device to that used for normal measurement of level and should ensure that the filling ratio indicated in section 4.6 below is not exceeded. Any hydraulic fluids or oils used in the instrumentation should be compatible with chlorine (chloro-fluorinated oil). Additionally, double-jacketed tanks should be provided with:
maximum, minimum and differential pressure alarm for the annular space
flow rate of annular purge air
humidity of the purge gas
chlorine content of the purge gas leaving the space between the inner and outer tanks
temperature measurement on the walls of the internal tank.
Temperature measurement is important to avoid thermal shock or stresses. At least four measurement points should be provided on the wall of the internal tank. The indication given by these measurements is particularly important during the commissioning of the tank to avoid the maximum temperature differential used in the differential expansion calculation for the tank, being exceeded.
4.5. Safety Equipment 4.5.1.
Over and Under-Pressure
The internal tank (and the external shell) should be fitted with means to protect them against over-pressure (relief valves, hydraulic guards, etc.). Bursting discs may be used to prevent small leaks at safety valves. In those cases the space between the valve and the disc has to be monitored closely and leaks (pressure rise) is to be alarmed. The application of bursting discs without safety valves is not recommended because chlorine flow would not stop when the pressure returns to normal value. January 2014
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If the tank is not designed for the maximum under-pressure that can occur, additional protective measures may be required (e.g. hydraulic guards, blanketing …). These systems should be capable of isolation for maintenance or after operation, by means which provide an adequate level of safety such as, for example, the use of valves which are locked open or the use of a double inter-connected valve system. Relief valves should conform to the recommended guideline GEST 87/133 – Overpressure Relief of Chlorine Installations. Where a hydraulic guard can be used, the liquid chosen should be suitable for operating in an atmosphere of chlorine, for example sulphuric acid of 92-95% strength. The operating pressure of the relief systems should be consistent with the design pressure and operating pressure chosen. The design of the relief systems should take all possible scenarios into account. The vent from a relief stream should be retained in a suitable installation - a compression and liquefaction system or an absorption system. To cope with potential pressure surges, and to avoid liquid entrainment in a system designed for gas, a large enough buffer tank must be installed downstream of the relief system. 4.5.2.
Protection of the External Shell
The annular space between the two shells should be permanently purged by dry air or nitrogen. This compensates for the effect of breathing between the two shells, and allows also confirming the absence of humidity or chlorine.
4.6. Filling Ratio The total load should not exceed the filling ratio multiplied by the volume of the chlorine tank. The filling ratio used shall comply with the applicable national or international legislation. The filling ratio usually considered within Europe is 1.25 t/m³.
4.7. Thermal Expansion Bellows for Double-Jacketed Tanks As the internal and external shells can be at very different temperature, it is essential to allow for the differential thermal expansion which may occur between the walls of the internal tank and the external shell. This should be calculated for the extreme conditions of service, including those arising during commissioning or total emptying of the storage system (with possible liquid chlorine cooled down by partial vaporisation). To do this, all branches should be January 2014
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designed to take the full variation in temperature into account. This may be achieved by use of expansion bellows. The expansion bellows should generally be constructed of high nickel alloy, without any corrosion allowance. Expansion bellows are an important feature of the construction which should be given careful study. Radiographic examination should be carried out on all welds. In addition, particular care should be taken during installation.
4.8. Filling, Emptying and Venting Equipment 4.8.1.
Filling
The transfer of chlorine into the tank is generally done through a dip pipe. 4.8.2.
Emptying
Empting by means of nitrogen or dry air pressure is often applied. Alternatively, vertical submerged pumps can be installed inside the tank or in an individual duct connected to the tank or canned pumps can be located below the storage tank. 4.8.3.
Venting
The chlorine vent gas should be retained in a suitable installation - a compression and liquefaction system or an absorption system. In either case, the chosen system must be designed to be permanently available.
5.
OPERATION 5.1. Cleaning and Drying before Chlorine can be Admitted
Before chlorine is admitted, the tank and all its accessories should be rigorously degreased, cleaned and dried (see also GEST 80/84 – Commissioning and Decommissioning of Installations for Dry Chlorine Gas and Liquid). The drying should be carried out to achieve and confirm a dew point of minus 40°C on the purge gas at the exit of the system. For all internal equipment which requires to be greased, and where there is a risk of it coming in contact with chlorine, only grease which is compatible with chlorine may be used (chloro-fluorinated greases).
5.2. Leak Testing Before commissioning, all valves and accessories should be tested to guarantee their gas tightness under the conditions of operation. January 2014
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The following methods may be used: 1. a helium test in a calm and non-ventilated atmosphere 2. testing with a chlorine/dry gas mixture with the various joints etc. checked by the use of ammonia 3. a pneumatic test using soap and water to detect leaks.
5.3. Commissioning The procedure given in GEST 80/84 - Code of Good Practice for the Commissioning of Installations for Dry Chlorine Gas and Liquid should be strictly observed. A number of precautions need to be taken before commissioning:
confirmation of the quality of the chlorine introduced o moisture content below 20 mg/kg (see GEST 10/362) o nitrogen trichloride (see GEST 76/55 - Maximum Levels of Nitrogen Trichloride in Liquid Chlorine)
check on the temperature differential from top to bottom of the tank (for double-jacketed tanks, see paragraph 3.1.2).
One method of commissioning consists of filling the tank in batches, and waiting between each introduction of liquid until thermal equilibrium has been established. It is desirable to take the system up to the maximum operating pressure for each of these introductions of liquid. It is desirable to limit the rate at which chlorine is introduced to limit the vent rate and the temperature differential between the different points of the tank (for double-jacketed tanks).
5.4. Total Emptying A number of precautions need to be taken before total emptying:
removal of the maximum amount of chlorine in the liquid phase
final dilution of the residual chlorine contained in the tank by chlorine known to have a low nitrogen trichloride level
check on the quality of chlorine remaining in the tank in order to ensure that it contains an acceptable level of nitrogen trichloride for total vaporisation
purging with dry air or an inert gas (dew point minus 40°C at atmospheric pressure), using a connection to the tank which is only made immediately before its time of use; alternatively, a permanent connection can be installed with backflow protection and double block and bleed.
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5.5. Reactive Materials All precautions must be taken to avoid the entry of humidity, reactive materials, or hydrogen into the storage tank. In addition, if inert gas is being used for the transfer or purging of chlorine, this should not be source of contamination by reactive materials (see 5.6.3).
5.6. Filling and Emptying in Normal Operation 5.6.1.
Quality of Chlorine Introduced
The chlorine introduced into a storage system should be periodically tested in particular to meet the following points:
moisture content (lower than 20 mg/kg)
nitrogen trichloride (for values see GEST 76/55 - Maximum Levels of Nitrogen Trichloride in Liquid Chlorine)
All precautions must be taken to avoid accumulating hydrogen in the gas phase of the tank. 5.6.2.
Temperature and Pressure of the Liquid Chlorine Introduced
For refrigerated liquid storage, the temperature of the liquid chlorine introduced should be sufficiently close to that of the chlorine already contained in the tank to ensure the evaporation rate in the tank does not exceed the design conditions of the system. This point is particularly important when chlorine is being imported directly from mobile containers. During the operation of filling and emptying, the connections between the cold liquid storage tank, and all other external equipment, should be carefully checked and supervised to avoid any scenario which could unintentionally connect high pressure gas (chlorine or inert gas) to the tank gas phase. To avoid thermal shock on the steel shell, it is preferable to cool down the tank prior to the first liquid introduction (using for example a cool gas). 5.6.3.
Emptying the Tank
The liquid chlorine can be extracted from the tank with the help of a pump or with padding with uncontaminated dry inert gas having a dew point below minus 40°C at atmospheric pressure. The volume of liquid removed can be compensated with dry gaseous chlorine exempt of hydrogen or with dry air or inert gas (a dew point lower than minus 40°C at atmospheric pressure).
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To prevent possible backflow, the pressure of the inert gas should be at least 1 bar greater than the operating pressure of the tank and this differential pressure must be permanently maintained. The total pressure should not exceed the design pressure.
5.7. Venting and Isolated Systems (Cold Chlorine Storage) The vent rates depend on the level of liquid chlorine (variation in surface area), the pressure of the vent system and possibly external atmospheric conditions (temperature, rain, etc). The autonomy of the storage system (the time for which it can remain totally isolated) depends on:
the ratio of surface to volume of the tank
the filling ratio
the thermal insulation chosen
the difference between the pressure at the time of the tank isolation and the maximum operating pressure.
Two days should be considered a minimum for an isolated system to be sustained.
5.8. Periodic Inspection and Testing Periodic inspection of the whole storage system is necessary. This includes the inner tank, the shell, pipework, valves, pressure relief devices, instruments, safety loops, etc. The internal inspections are done after having completely emptied, vented and washed the tanks to avoid the potential risk coming from residual chlorine. 5.8.1.
External inspection
The first inspection is recommended two years after the first commissioning. The periodicity of inspection will be determined by local regulations but should not exceed 6 years. Inspection should include the following aspects:
visual examination, particularly of the welds
(ultrasonic) thickness testing of walls, flanges and branches
verification of all accessories
inspection of lagging and painted surfaces (including under the lagging).
As a general rule, all equipment should be replaced systematically before there is any risk of it becoming defective. January 2014
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5.8.2.
Internal inspection
Here also the inspection frequencies will depend on the requirements of the authorities. Based on the inspections history with consecutive good results and other control measures in place to avoid corrosion, it is advised to discuss with the authorities an extension of the inspection interval as long as appropriate because the procedure itself introduces the risk of corrosion. Inspection should include the following aspects:
visual examination, particularly of the welds, the bottom line and around gas-liquid interface level
(ultrasonic) thickness testing of walls
verification of all internal accessories
Where a storage tank has been washed out or where a hydraulic test is imposed by local regulations, specific procedures need to be laid down in order to reduce to a minimum the effects of corrosion (see GEST 80/84 - Code of Good Practice for the Commissioning of Installations for Dry Chlorine Gas and Liquid). Too frequent hydraulic retesting is not recommended because of the risk of corrosion which is associated with it.
5.9. Methods of Protection and Alarm The following precautions should be taken:
an emergency plan, giving the detailed instructions to be followed in the event of emergency, should be permanently available and understood by plant personnel; this emergency plan should contain detailed instructions for transferring the content of a leaking storage tank into another tank
chlorine leak detectors should be provided to warn the operators of the existence of a leak; see also GEST 94/213 - Guidelines for the Selection and use of Fixed Chlorine Detection Systems in Chlorine Plants
the operator should be able to use a fixed water spray curtain or the site fire brigade should be able to rapidly use a mobile water spray curtain to control the spread of any chlorine gas cloud; this operation should avoid, as far as possible, any direct contact between the water and the liquid chlorine
for small leaks, a funnel connected with flexible hoses to an absorption system can be applied to the leak
if the storage system is installed within a bund, measures should be taken to reduce the rate of evaporation of any chlorine contained within the bund e.g. walls of the bund with a low thermal conductivity, slope in the bund which allows the liquid chlorine to accumulate in a small area, or use of a protein foam or a plastic sheet to form an insulating layer on the liquid chlorine pool
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if the chlorine cloud comes from a leakage in liquid phase, an alternative method with an emergency funnel can be used; the funnel is connected at the leakage and reduces the access of air to droplets of chlorine (less energy available and fewer droplets evaporated). The droplets coalesce to liquid chlorine in the funnel and can be collected in a (temporary) emergency tank at the outlet of the funnel, reducing the chlorine cloud
self-contained breathing sets and protective clothing suitable for dealing with a chlorine leak should be always available in various locations in the vicinity of the storage system and always accessible in case of emergency (cartridge respirators should not be used for intervention and particularly when there is any risk of a high concentration of chlorine)
a means of indicating the wind direction should be installed to tell the operator of the possible direction of dispersion of gas that will occur in the event of an accident; it is important to remember that heavy chlorine gas clouds behave like liquid and will tend to flow to the deepest area
all operators and maintenance personnel should be trained to deal with leaks of chlorine, and periodic exercises should be organised to ensure that this standard of training is maintained.
5.10. Response to a significant loss of primary containment Methods and Equipment Depending on local circumstances, several solutions are possible to respond to a significant liquid chlorine leak. Some practical examples are given below:
spread foam (medium expansion protein foam, e.g. FP 70) on the containment bund
spray water curtains around the area (do not spray into the bundle)
start air aspiration above the containment bund, and send it to the absorption unit
connect the containment bund to an underground tank to minimise the evaporation area and allow localised connection to the absorption unit (avoid sending liquid chlorine to the absorption unit)
transfer the liquid chlorine from the leaking tank into an empty spare tank; this one is vented to the absorption unit to neutralise progressively all the chlorine. This transfer can be achieved by the pressure difference between the leaking tank and the relatively depressurised spare tank or by means of a pump. Those transfer capabilities are preferably remotely controlled.
Remarks:
the foam is effective to drastically reduce the chlorine evaporation, giving time to organise the collection of the liquid chlorine, but a
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temporary increase of chlorine vaporisation occurs when the foam is sprayed
6.
the water curtains have only a local impact; by creating a significant air mixing, they reduce the chlorine concentration behind the curtain, but have no long distance impact; precautions must be taken to avoid increased corrosion by spraying water on the leak, and to increase the vaporisation by adding water into the containment bund.
REFERENCES GEST 73/25 - Transport of Dry Chlorine by Pipeline GEST 76/55 - Maximum Levels of Nitrogen Trichloride in Liquid Chlorine GEST 78/73 - Design Principles and Operational Procedures for Loading/Off-Loading Liquid Chlorine Road and Rail Tankers and ISOContainers GEST 80/84 - Code of Good Practice for the Commissioning of Installations for Dry Chlorine Gas and Liquid GEST 87/133 – Overpressure Relief of Chlorine Installations GEST 88/134 - Stud Bolts, Hexagon Head Bolts and Nuts for Liquid Chlorine GEST 94/204 – Pneumatically Operated Valves for Use on fixed Storage Tanks, Rail and Road Tankers and ISO-Containers for Liquid Chlorine GEST 94/213 - Guidelines for the Selection and use of Fixed Chlorine Detection Systems in Chlorine Plants GEST 94/216 - Experience of Non-Asbestos Gaskets on Liquid and Dry Chlorine Gas Service GEST 05/316 – Guideline for site security of chlorine production facilities GEST 06/318 – Valves Requirements and Design for Use on Liquid Chlorine Position Paper XII - Memorandum on Confinement of Liquid Chlorine Plants TSEM 90/161 - Quantitative Risk Assessment
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Industrial consumers of chlorine, engineering and equipment supply companies worldwide and chlorine producers outside Europe may establish a permanent relationship with Euro Chlor by becoming Associate Members or Technical Correspondents.
Details of membership categories and fees are available from: Euro Chlor Avenue E Van Nieuwenhuyse 4 Box 2 B-1160 Brussels Belgium
January 2014
Tel:
+32 2 676 7211
Fax:
+32 2 676 7241
e-mail:
[email protected]
Internet:
http://www.eurochlor.org
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