Cryogenic storage of Ammonia.pmd - Uhde India Limited

Cryogenic is a Greek word meaning “the productionof freezing cold,” however, the term is usedtoday as a synonym for the low temperature state.It is not well defined at what point on the temperaturescale refrigeration ends and cryogenic begins.There are number of gases that require cryogenic storagese.g. ammonia, chlorine, LPG, propane, propylene,ethane, ethylene, natural gas, oxygen, nitrogen, hydrogenetc.In this article we have described about the differenttypes of ammonia storages methods, typical configurationof storage systems, various codes and standards, designguidelines, guarantees involved, commissioning &recommissioning procedures, hazardous area classification,safety aspects, ammonia transportation challengesand opportunities for cryogenic storages.Types of ammonia storage tanksLiquid ammonia is usually stored either at ambienttemperature under high pressure or at -33 o C under atmosphericpressure. In some cases, it is also stored at intermediatetemperatures and pressures (semi-refrigerated).❖❖❖Pressurised storage (pressure: 12 – 18 kg/cm 2 g) atambient temperature for small capacities (up to 100MT) is in horizontal pressure vessels.Semi-refrigerated storageis in spheres for capacitiesup to 2000 MT(pressure 4 – 6 kg/cm 2g).Atmospheric pressurestorage is at -33 o C forlarge capacities (2000MT and higher). Thereare three differenttypes of tanks for suchCarbonsteelcontainerInsulationCryogenic StorageAmmoniadischargelineConcretefoundationFig A1. Single wall with external insulationcryogenic storage, viz:A) Single Wall TankB) Double Wall TankC) Double Integrity Cup-In TankLet us review each one separately.A) Single wall tankThese are the tanks with one steel bottom and walldesigned to contain the full liquid level of ammonia. Insulationis provided on the external surface to minimizeheat leakage Fig A1. External containment is provided inthe form of dyke wall to contain the liquid coming outdue to rupture of inner tank. From Health, Safety and Environmentperspective single wall tanks are not recommendedand hence these are no longer built. ExistingSingle wall tanks are being replaced with Double IntegrityCup-In Tanks. See Fig A2.B) Double wall tankThe outer tank is intended to contain refrigerated productleakage from inner tank but is not intended to containany vapor resulting from product leakage from theinner tank which means, it will be released to atmospherethrough flare. See Fig B1 and Fig B2. The outer containeris normally provided in the form of “Bund wall” of heightCarbonsteelcontainerInsulationFig 1. Single wall tanksConcrete for carbonsteel weatherprotectionAmmoniadischarge lineConcretefoundationFig A2. Single wall with external wallCarbonsteelinner tankInsulationCarbon steelouter tankAmmoniadischarge lineCarbonsteelinner tankInsulationCarbon steelouter tankAmmoniadischarge lineConcretefoundationConcretefoundationFig B1. Double wall TankFig B2. Double wall tank with common roofFig 2. Double wall tanksChemical Industry Digest. October 201277CMYK

Cryogenic StorageRoofMineral woolSuspended deckPUF Insulation withaluminium claddingLiquid AmmoniaCupOuter tankAnnular spacePCCConcrete deckPerlite concreteFig C. Ammonia storage tank. Double integrity cup-in-tank (typical sketch)Foam GlassSand Layercorresponding to hold equivalent amount of liquid in thestorage tank.C) Double Integrity Cup-In TankThis Double Integrity Cup-In provides extra vaporspace of 15 – 20% depending on tank dimensions whichis very useful for maintaining tank pressure within operatinglimits especially under emergency condition. Thepresence of ammonia vapor in the annular space betweenthe cup and shell acts as an insulating media and helpsin 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 foaminsulation that is formed in-situ. The bottom insulationis foam glass and Perlite Concrete which is a load bearinginsulation. The top of the cup is insulated by mineralwool/fiber glass wool insulation which is spread ona suspended deck. Insulation thickness is determined tolimit the tank boil off within 0.04 wt% per day.The tank is equipped with two pressure relief valvesto safeguard against overpressure. Isolation valves areprovided with suitable mechanical interlock. Also thetank is equipped with two vacuum relief valves to safeguardthe tank against vacuum isolation valves are providedwith suitable mechanical interlock.The tank rests on elevated foundation for a free passageof air from below, so as to avoid freezing of foundation.A staircase tower with platform landing up to tanktop is provided to access tank’s top nozzles.System ConfigurationThe cryogenic ammonia storage system comprises ofthe following:❖❖❖❖❖❖❖❖❖❖❖Ship Unloading ArmCross country pipe lineAtmospheric Ammonia Storage Tank (Double IntegrityCup-In Tank)Ammonia Refrigeration System (Includes Screw compressors,Condensers, Receiver etc.)Ammonia transfer pumpsFlare systemAmmonia drain tankAmmonia pre-heater (If applicable)Road/Rail loading/unloading station (If applicable)All Utility equipment (Cooling tower with Side streamfilters, Instrument air compressor system, DG Systemetc.) – Considering stand alone storage systemElectrical & Instrumentation systemApplicable Codes and standardsCryogenic storage tank have to confirm to these codesand standards;❖❖❖❖❖ASME, Sec. VIII, Div. 1 code for pressure vessels (receiverand drain tank) and heat exchangersTEMA Class R code for ammonia heat exchangersAPI 619 for ammonia refrigeration screw compressorsAPI 610, 10 th Edition for ammonia pumpsAPI-RP 520/2000, ASME Sec. VIII, Div. 1 for safetyvalves78Chemical Industry Digest. October 2012CMYK

Cryogenic Storage❖❖❖❖❖❖❖ASTM/ASME Sec. II for material specificationAPI 620 Appendix R Feb 2008 edition + Addendum1 2009 + Addendum 2 2010 for storage tank and DIN4119 sheet 1 & 2 (1961) for roof structureAPI 520/521 for flareIEC/IS standards for electrical equipmentISA and DIN standards for InstrumentsInsulation as per DIN/ANSIFire protection as per local regulationsDesign GuidelinesDesign guidelines to be followed include:❖❖❖❖❖❖❖Pressure safety/vacuum relief valve should be interlockedin such a way that if one of the valve is takenout for maintenance then the other valve should bein line. Also they must be provided with mechanicalinterlock.All the first isolation valves are required to be weldedtype on the tank side. The flange valves always havedanger of gasket failure.The vacuum and pressure safety valve shall be flangetype.While cooling the tanks from ambient temperature to-33 0 C the tank will undergo the differential expansiontherefore the stairways and the tank protectionrail shall be independently supported.The nozzle orientation of the tank particularly shallbe in such a way that the liquid inlet to the tank andthe vapor outlet from the tank shall be away from theeach other. This will reduce the liquid droplets in thevapor line.The nozzles for the level indicator and the pump suctionshall be away from each other. Pump under runningcondition will cause drop in the liquid level,therefore if the liquid level indicator is away from thepump suction it will have better accuracy in the levelmeasurement.The filing nozzle pipe shall be with perforation andthe filling pipe shall extend up to cup bottom so thatsplashing is avoided during the filling operation.Guarantees involved in cryogenic storageSystem❖❖❖Tank CapacityTank Boil-OffLiquid ammonia receipt from ship/plant❖❖Transfer rate of liquid ammonia to user plantElectric powerCommissioning & recommissioning procedureImportant steps to be taken during commissioning andrecommissioning.1. Hydrotest, either up to 70 or 100%, depending on thedesign code.2. Purge with nitrogen until the measured oxygen in thedischarge gas is less than 4%.3. Then purge with ammonia gas until the measured oxygenin 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 thetank, away from the gas inlet.6. Within one week after commissioning and when conditionsare stable, take samples for water and oxygenanalysis from the ammonia liquid in the tank andanalyse them.Decommissioning procedure1. Empty the tank to the absolute minimum liquid level.2. Evaporate the remaining ammonia in a way that ensuresuniform and slow heating, not exceeding 1 o C/hour.3. Measure the temperature in the bulk volume of thetank, away from the gas inlet. Give careful considerationto temperature measurements at the lower levelsof the tank during decommissioning.4. Purge with warm ammonia gas or nitrogen until allliquid ammonia is removed. The bottom area may needto be cleaned before it is possible to get all the ammoniagas out.Hazardous Area ClassificationAmmonia storage tank and associated facility falls underClass 1 and Zone-2.❖ Class 1 – For flammable liquids, gases and vapors❖ Zone 2 – Area in which explosive gas atmosphere isnot likely to occur under normal operations and if itdoes occur it will exist for short period only.Gas Group❖ All gases and vapors are classified in to 4 majorgroups namely, I, IIA, IIB, IIC as per IS 2206. The classificationis based on the minimum ignition energy requiredto raise the temperature of gas locally to attainChemical Industry Digest. October 201279CMYK

Cryogenic Storageits ignition temperature. Ammonia falls under the gasgroup – IIA.Temperature Class❖ Ammonia falls under the temperature class – T1Ammonia TransportationA) By Ship❖ Sizes of ships generally range between 2000 MT to46500 MT.❖ Ships are equipped with refrigeration facilities andpumps. Similar to those of storage tanks.❖ They are designed for carrying multiple liquids of differentdensity and temperature.B) By Rail/Road❖ Cryogenic liquid is transported by the rail tankers(BTAL) for short distance. It is unloaded into the storagetank via flash vessel or surge drum to control thepressure in the tank.❖ Generally, rail/road tanker loading station has followingfacilities❍ Liquid loading arm❍ Vapor return arm❖ Shut-off valve interlocked with the flow controller onloading line and also with emergency stop switch❖ Flow measurement, recorder and control system for thesafe loading into the tanker❖ Drain Pot (if required)Safety AspectsSafety of the cryogenic tanks is of prime importance.These tanks are designed for double integrity as againstthe normal tanks of API 650 which are designed for singlecontainment. These tanks have inner cup to contain theliquid and also have an additional outer tank to containthe liquid in case of cup failure. Utmost care is to be taken❖❖❖❖❖❖❖while designing the inner cup as well as outertank as both inner and outer tanks will be subjectedto various different load combinations.❖ These tanks are subjected to low temperaturesand accordingly are equipped with variouscritical features like special anchorage andits attachment, ability to handle differentialexpansion, special insulation for outer tank/tank bottom/suspended deck etc. In view ofthis, various stringent requirements for materials,fabrication/welding, inspection and testingof tank must be considered which are differentand stringent from normal API 650 AtmosphericStorage Tanks.Storage tank design is required to meet high safetystandards and flare system is strongly recommended.The flare system consists of an adequately sized pipeline(assessed for critical condition) routed to a distantlocation (as per local applicable rules) and releasedat a height and burnt with a pilot gas (usuallyLPG 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 fillingthe tank, even if small quantities are involved relativeto the volumes stored. Protective equipment such asbursting and valves, correctly dimensioned, must alsoplay their role.Safety measures and equipment, such as emergencycontrols must be permanently accessible, even in degradedsituations.The redundancy of the equipment reduces the probabilityof major accidents but does not exclude it. Multiplicationof the various types of barrier (alarms,servo-system, emergency operating procedures, failsafe security system) while avoiding common sourcesof failure particularly when the stakes are high or theinstallations are particularly dangerous. These aresome of the most effective ways of reducing majorrisks, but they do not necessary eradicate them. Bearingin mind the quantities stored in cryogenic reservoirs,the potential sources of danger in these installationsare numerous; therefore the measures taken onthe technical, organizational and human fronts to preventaccidents should be proportionate to these.Remote shut-off valves on liquid ammonia main inletand outlet line to/from ammonia storage tank shouldbe installed.Refrigeration system should be based on reliable screwcompressors with stand-by options.80Chemical Industry Digest. October 2012CMYK

Cryogenic StorageCryogenic Storage❖❖❖❖❖❖Adequate fire water grid and water curtain aroundammonia pumps, compressor house and loading/unloading stations should be provided.Thermal relief valves should be installed on the ammonialines where there is any possibility for blockageor heat ingress.Ammonia leak detection system should be providedfor storage installation.Lighting protection and earthing protection should beconsidered for storage tank.Emergency power is should be provided to one refrigerationholding compressor to maintain tank pressureduring power failure.Wind direction indicator should be provided.Challenges & Opportunities❖ Ammonia Stints (That’s good and bad)❖ Storing millions of gallons of ammonia presents environmental,health & safety concerns❖ Ammonia handling is very energy intensive and competitive❖ Energy prices are hitting ammonia manufacturing andhandling very hard❖ Many of the facilities were built in the old days andneed to be upgraded to newer technology❖ Due to the very low temperature these liquefied gasescannot be managed using normal equipments andthus require specially made equipments, known ascryogenic equipments. Commonly used cryogenicequipments are tanks, valves, compressors andpumps.The global market size of all cryogenic equipment (notlimited to ammonia) is estimated to be $11 billion in 2011.Cryogenic tank has the largest market and majority ofcryogenic equipment market revenue come from thosetanks. After tanks, valves are the second largest revenuecontributor in cryogenic market followed by compressorsand then pumps.CHEMICAL INDUSTRY DIGESTHighlights - November 2012Articles Advanced Materials Mechanical Equipment R&D/ Innovation Multivariable Predictive Control Erection & Commissioning ChallengesPlus Regular Features and much more . . . .For Advertisement & Subscription contact:BLOCKDALE PUBLISHING15, Purshottam, 21 J. P. Rd, Andheri (W),Mumbai - 400058 Tel: 26207402/26259421Fax: 022-26249725Email: chemindigest@gmail.comChemical Industry Digest. October 201281CMYK