Cryogenic Storage of Ammonia

Transcription

1 Cryogenic Storage of Sachin D Chavan Abstract Gases such as ammonia, chlorine, LPG, propane, propylene, ethane, ethylene etc need to be stored at low temperatures. Cryogenic storage of such fluids is a well developed science with its own complexities. This article talks about critical points related to cryogenic storage, especially about ammonia. This article describes in brief typical configuration of storage system, various codes, design guidelines, commissioning and recommissioning procedures, etc. Sachin Dinkar Chavan is a chemical engineer and an alumnus of Tatyasaheb Kore Institute of Engineering & Technology. He is presently Sr Engineer-Business Development, with Uhde India Pvt Ltd. He has handled LSTK proposals for Cryogenic Storage Systems. He worked with NOCIL RCD, Melog Speciality Chemical Pvt Ltd, Ion Exchange India Ltd. 76

2 Cryogenic is a Greek word meaning the production of freezing cold, however, the term is used today as a synonym for the low temperature state. It is not well defined at what point on the temperature scale refrigeration ends and cryogenic begins. There are number of gases that require cryogenic storages e.g. ammonia, chlorine, LPG, propane, propylene, ethane, ethylene, natural gas, oxygen, nitrogen, hydrogen etc. In this article we have described about the different types of ammonia storages methods, typical configuration of storage systems, various codes and standards, design guidelines, guarantees involved, commissioning & recommissioning procedures, hazardous area classification, safety aspects, ammonia transportation challenges and opportunities for cryogenic storages. Types of ammonia storage tanks Liquid ammonia is usually stored either at ambient temperature under high pressure or at -33 o C under atmospheric pressure. In some cases, it is also stored at intermediate temperatures and pressures (semi-refrigerated). Pressurised storage (pressure: kg/cm 2 g) at ambient temperature for small capacities (up to 100 MT) is in horizontal pressure vessels. Semi-refrigerated storage is in spheres for capacities up to 2000 MT (pressure 4 6 kg/cm 2 g). Atmospheric pressure storage is at -33 o C for large capacities (2000 MT and higher). There are three different types of tanks for such container Cryogenic Storage discharge line Fig A1. Single wall with external insulation cryogenic storage, viz: A) Single Wall Tank B) Double Wall Tank C) Double Integrity Cup-In Tank Let us review each one separately. A) Single wall tank These are the tanks with one bottom and wall designed to contain the full liquid level of ammonia. is provided on the external surface to minimize heat leakage Fig A1. External containment is provided in the form of dyke wall to contain the liquid coming out due to rupture of inner tank. From Health, Safety and Environment perspective single wall tanks are not recommended and hence these are no longer built. Existing Single wall tanks are being replaced with Double Integrity Cup-In Tanks. See Fig A2. B) Double wall tank The outer tank is intended to contain refrigerated product leakage from inner tank but is not intended to contain any vapor resulting from product leakage from the inner tank which means, it will be released to atmosphere through flare. See Fig B1 and Fig B2. The outer container is normally provided in the form of Bund wall of height container Fig 1. Single wall tanks for carbon weather protection discharge line Fig A2. Single wall with external wall inner tank outer tank discharge line inner tank outer tank discharge line Fig B1. Double wall Tank Fig B2. Double wall tank with common roof Fig 2. Double wall tanks 77

3 Roof Mineral wool Suspended deck PUF with aluminium cladding Liquid Cup Outer tank Annular space PCC deck Perlite concrete Fig C. storage tank. Double integrity cup-in-tank (typical sketch) Foam Glass Sand Layer corresponding to hold equivalent amount of liquid in the storage tank. C) Double Integrity Cup-In Tank This Double Integrity Cup-In provides extra vapor space of 15 20% depending on tank dimensions which is very useful for maintaining tank pressure within operating limits especially under emergency condition. The presence of ammonia vapor in the annular space between the cup and shell acts as an insulating media and helps in reducing the heat ingress from outside. See Fig C. The tank is designed as per the API 620, Appendix R. The outer tank is insulated with rigid polyurethane foam insulation that is formed in-situ. The bottom insulation is foam glass and Perlite which is a load bearing insulation. The top of the cup is insulated by mineral wool/fiber glass wool insulation which is spread on a suspended deck. thickness is determined to limit the tank boil off within 0.04 wt% per day. The tank is equipped with two pressure relief valves to safeguard against overpressure. Isolation valves are provided with suitable mechanical interlock. Also the tank is equipped with two vacuum relief valves to safeguard the tank against vacuum isolation valves are provided with suitable mechanical interlock. The tank rests on elevated for a free passage of air from below, so as to avoid freezing of. A staircase tower with platform landing up to tank top is provided to access tank s top nozzles. System Configuration The cryogenic ammonia storage system comprises of the following: Ship Unloading Arm Cross country pipe line Atmospheric Storage Tank (Double Integrity Cup-In Tank) Refrigeration System (Includes Screw compressors, Condensers, Receiver etc.) transfer pumps Flare system drain tank pre-heater (If applicable) Road/Rail loading/unloading station (If applicable) All Utility equipment (Cooling tower with Side stream filters, Instrument air compressor system, DG System etc.) Considering stand alone storage system Electrical & Instrumentation system Applicable Codes and standards Cryogenic storage tank have to confirm to these codes and standards; ASME, Sec. VIII, Div. 1 code for pressure vessels (receiver and drain tank) and heat exchangers TEMA Class R code for ammonia heat exchangers API 619 for ammonia refrigeration screw compressors API 610, 10 th Edition for ammonia pumps API-RP 520/2000, ASME Sec. VIII, Div. 1 for safety valves 78

4 ASTM/ASME Sec. II for material specification API 620 Appendix R Feb 2008 edition + Addendum Addendum for storage tank and DIN 4119 sheet 1 & 2 (1961) for roof structure API 520/521 for flare IEC/IS standards for electrical equipment ISA and DIN standards for Instruments as per DIN/ANSI Fire protection as per local regulations Design Guidelines Design guidelines to be followed include: Pressure safety/vacuum relief valve should be interlocked in such a way that if one of the valve is taken out for maintenance then the other valve should be in line. Also they must be provided with mechanical interlock. All the first isolation valves are required to be welded type on the tank side. The flange valves always have danger of gasket failure. The vacuum and pressure safety valve shall be flange type. While cooling the tanks from ambient temperature to C the tank will undergo the differential expansion therefore the stairways and the tank protection rail shall be independently supported. The nozzle orientation of the tank particularly shall be in such a way that the liquid inlet to the tank and the vapor outlet from the tank shall be away from the each other. This will reduce the liquid droplets in the vapor line. The nozzles for the level indicator and the pump suction shall be away from each other. Pump under running condition will cause drop in the liquid level, therefore if the liquid level indicator is away from the pump suction it will have better accuracy in the level measurement. The filing nozzle pipe shall be with perforation and the filling pipe shall extend up to cup bottom so that splashing is avoided during the filling operation. Guarantees involved in cryogenic storage System Tank Capacity Tank Boil-Off Liquid ammonia receipt from ship/plant Transfer rate of liquid ammonia to user plant Electric power Commissioning & recommissioning procedure Important steps to be taken during commissioning and recommissioning. 1. Hydrotest, either up to 70 or 100%, depending on the design code. 2. Purge with nitrogen until the measured oxygen in the discharge gas is less than 4%. 3. Then purge with ammonia gas until the measured oxygen in the discharge gas is less than 0.5%. 4. Cool the tank down to as low a temperature as possible, at a cooling rate lower than 1 o C/hr. 5. Measure the temperature in the bulk volume of the tank, away from the gas inlet. 6. Within one week after commissioning and when conditions are stable, take samples for water and oxygen analysis from the ammonia liquid in the tank and analyse them. Decommissioning procedure 1. Empty the tank to the absolute minimum liquid level. 2. Evaporate the remaining ammonia in a way that ensures uniform and slow heating, not exceeding 1 o C/ hour. 3. Measure the temperature in the bulk volume of the tank, away from the gas inlet. Give careful consideration to temperature measurements at the lower levels of the tank during decommissioning. 4. Purge with warm ammonia gas or nitrogen until all liquid ammonia is removed. The bottom area may need to be cleaned before it is possible to get all the ammonia gas out. Hazardous Area Classification storage tank and associated facility falls under Class 1 and Zone-2. Class 1 For flammable liquids, gases and vapors Zone 2 Area in which explosive gas atmosphere is not likely to occur under normal operations and if it does occur it will exist for short period only. Gas Group All gases and vapors are classified in to 4 major groups namely, I, IIA, IIB, IIC as per IS The classification is based on the minimum ignition energy required to raise the temperature of gas locally to attain 79

5 its ignition temperature. falls under the gas group IIA. Temperature Class falls under the temperature class T1 Transportation A) By Ship Sizes of ships generally range between 2000 MT to MT. Ships are equipped with refrigeration facilities and pumps. Similar to those of storage tanks. They are designed for carrying multiple liquids of different density and temperature. B) By Rail/Road Cryogenic liquid is transported by the rail tankers (BTAL) for short distance. It is unloaded into the storage tank via flash vessel or surge drum to control the pressure in the tank. Generally, rail/road tanker loading station has following facilities Liquid loading arm Vapor return arm Shut-off valve interlocked with the flow controller on loading line and also with emergency stop switch Flow measurement, recorder and control system for the safe loading into the tanker Drain Pot (if required) Safety Aspects Safety of the cryogenic tanks is of prime importance. These tanks are designed for double integrity as against the normal tanks of API 650 which are designed for single containment. These tanks have inner cup to contain the liquid and also have an additional outer tank to contain the liquid in case of cup failure. Utmost care is to be taken while designing the inner cup as well as outer tank as both inner and outer tanks will be subjected to various different load combinations. These tanks are subjected to low temperatures and accordingly are equipped with various critical features like special anchorage and its attachment, ability to handle differential expansion, special insulation for outer tank/ tank bottom/suspended deck etc. In view of this, various stringent requirements for materials, fabrication/welding, inspection and testing of tank must be considered which are different and stringent from normal API 650 Atmospheric Storage Tanks. Storage tank design is required to meet high safety standards and flare system is strongly recommended. The flare system consists of an adequately sized pipeline (assessed for critical condition) routed to a distant location (as per local applicable rules) and released at a height and burnt with a pilot gas (usually LPG or NG). To limit the risk of over pressure in cryogenic reservoirs, it is essential to maintain a sufficiently low temperature (-33 o C) during storage and also while filling the tank, even if small quantities are involved relative to the volumes stored. Protective equipment such as bursting and valves, correctly dimensioned, must also play their role. Safety measures and equipment, such as emergency controls must be permanently accessible, even in degraded situations. The redundancy of the equipment reduces the probability of major accidents but does not exclude it. Multiplication of the various types of barrier (alarms, servo-system, emergency operating procedures, fail safe security system) while avoiding common sources of failure particularly when the stakes are high or the installations are particularly dangerous. These are some of the most effective ways of reducing major risks, but they do not necessary eradicate them. Bearing in mind the quantities stored in cryogenic reservoirs, the potential sources of danger in these installations are numerous; therefore the measures taken on the technical, organizational and human fronts to prevent accidents should be proportionate to these. Remote shut-off valves on liquid ammonia main inlet and outlet line to/from ammonia storage tank should be installed. Refrigeration system should be based on reliable screw compressors with stand-by options. 80

6 Cryogenic Storage Adequate fire water grid and water curtain around ammonia pumps, compressor house and loading/ unloading stations should be provided. Thermal relief valves should be installed on the ammonia lines where there is any possibility for blockage or heat ingress. leak detection system should be provided for storage installation. Lighting protection and earthing protection should be considered for storage tank. Emergency power is should be provided to one refrigeration holding compressor to maintain tank pressure during power failure. Wind direction indicator should be provided. Challenges & Opportunities Stints (That s good and bad) Storing millions of gallons of ammonia presents environmental, health & safety concerns handling is very energy intensive and competitive Energy prices are hitting ammonia manufacturing and handling very hard Many of the facilities were built in the old days and need to be upgraded to newer technology Due to the very low temperature these liquefied gases cannot be managed using normal equipments and thus require specially made equipments, known as cryogenic equipments. Commonly used cryogenic equipments are tanks, valves, compressors and pumps. The global market size of all cryogenic equipment (not limited to ammonia) is estimated to be $11 billion in Cryogenic tank has the largest market and majority of cryogenic equipment market revenue come from those tanks. After tanks, valves are the second largest revenue contributor in cryogenic market followed by compressors and then pumps. CHEMICAL INDUSTRY DIGEST Highlights - November 2012 Articles Advanced Materials Mechanical Equipment R&D/ Innovation Multivariable Predictive Control Erection & Commissioning Challenges Plus Regular Features and much more.... For Advertisement & Subscription contact: BLOCKDALE PUBLISHING 15, Purshottam, 21 J. P. Rd, Andheri (W), Mumbai Tel: / Fax: chemindigest@gmail.com 81