Cryogenic Storage

Chemical Industry Digest. October 2012

CMYK

Cryogenic Storage

76

Cryogenic Storage of Ammonia Sachin D Chavan

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 handledLSTK proposals for Ammonia Cryogenic Storage Systems. He worked with NOCIL RCD, Melog SpecialityChemical Pvt Ltd, Ion Exchange India Ltd.

Abstract

Gases such as ammonia, chlorine, LPG, propane, propylene, ethane, ethylene etc need to be storedat low temperatures. Cryogenic storage of such fluids is a well developed science with its own com-plexities. This article talks about critical points related to cryogenic storage, especially about am-monia. This article describes in brief typical configuration of storage system, various codes, de-sign guidelines, commissioning and recommissioning procedures, etc.


CMYK

Cryogenic Storage

77

Cryogenic is a Greek word meaning the produc-tion of 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 stor-ages e.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 configura-tion of storage systems, various codes and standards, de-sign guidelines, guarantees involved, commissioning &recommissioning procedures, hazardous area classifica-tion, safety aspects, ammonia transportation challengesand opportunities for cryogenic storages.

Types of ammonia storage tanksLiquid ammonia is usually stored either at ambient

temperature under high pressure or at -33oC under atmo-spheric pressure. In some cases, it is also stored at inter-mediate temperatures and pressures (semi-refrigerated).

Pressurised storage (pressure: 12 18 kg/cm2 g) atambient temperature for small capacities (up to 100MT) is in horizontal pressure vessels.

Semi-refrigerated stor-age is in spheres for ca-pacities up to 2000 MT(pressure 4 6 kg/cm2

g).

Atmospheric pressurestorage is at -33oC forlarge capacities (2000MT and higher). Thereare three differenttypes of tanks for such

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 tankThese are the tanks with one steel bottom and wall

designed to contain the full liquid level of ammonia. In-sulation is 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 En-vironment perspective single wall tanks are not recom-mended and hence these are no longer built. ExistingSingle wall tanks are being replaced with Double Integ-rity Cup-In Tanks. See Fig A2.

B) Double wall tankThe outer tank is intended to contain refrigerated prod-

uct leakage from inner tank but is not intended to con-tain any 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 height

Carbonsteelinner tank

Insulation

Concretefoundation

Carbon steelouter tank

Ammoniadischarge line

Carbonsteelinner tank

Insulation

Concretefoundation

Carbon steelouter tank


Fig B1. Double wall Tank Fig B2. Double wall tank with common roof

Fig 2. Double wall tanks

Carbonsteelcontainer

Insulation

Concretefoundation

Ammoniadischargeline

Carbonsteelcontainer

Insulation

Concretefoundation


Concrete for carbonsteel weatherprotection

Fig A1. Single wall with external insulation Fig A2. Single wall with external wall

Fig 1. Single wall tanks


CMYK

Cryogenic Storage

78

corresponding to hold equivalent amount of liquid in thestorage tank.

C) Double Integrity Cup-In TankThis Double Integrity Cup-In provides extra vapor

space of 15 20% depending on tank dimensions whichis very useful for maintaining tank pressure within op-erating limits 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 bear-ing insulation. The top of the cup is insulated by min-eral wool/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 safe-guard the tank against vacuum isolation valves are pro-vided with suitable mechanical interlock.

The tank rests on elevated foundation for a free pas-sage of air from below, so as to avoid freezing of founda-tion. A staircase tower with platform landing up to tanktop is provided to access tanks top nozzles.

System ConfigurationThe cryogenic ammonia storage system comprises of

the following:

Ship Unloading Arm

Cross country pipe line

Atmospheric Ammonia Storage Tank (Double IntegrityCup-In Tank)

Ammonia Refrigeration System (Includes Screw com-pressors, Condensers, Receiver etc.)

Ammonia transfer pumps

Flare system

Ammonia drain tank

Ammonia 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 system

Electrical & Instrumentation system

Applicable Codes and standardsCryogenic storage tank have to confirm to these codes

and standards;

ASME, Sec. VIII, Div. 1 code for pressure vessels (re-ceiver and drain tank) and heat exchangers

TEMA Class R code for ammonia heat exchangers

API 619 for ammonia refrigeration screw compressors

API 610, 10th Edition for ammonia pumps

API-RP 520/2000, ASME Sec. VIII, Div. 1 for safetyvalves

Fig C. Ammonia storage tank. Double integrity cup-in-tank (typical sketch)

Suspended deck

PUF Insulation withaluminium cladding

Annular space

PCC

Concrete deck

Sand Layer

Foam Glass

Perlite concrete

Outer tank

Cup

Mineral wool

Roof

Liquid Ammonia


CMYK

Cryogenic Storage

79

ASTM/ASME Sec. II for material specification

API 620 Appendix R Feb 2008 edition + Addendum1 2009 + Addendum 2 2010 for storage tank and DIN4119 sheet 1 & 2 (1961) for roof structure

API 520/521 for flare

IEC/IS standards for electrical equipment

ISA and DIN standards for Instruments

Insulation as per DIN/ANSI

Fire protection as per local regulations

Design GuidelinesDesign guidelines to be followed include:

Pressure safety/vacuum relief valve should be inter-locked in 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 0C the tank will undergo the differential expan-sion therefore 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 suc-tion shall be away from each other. Pump under run-ning condition 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 Capacity

Tank Boil-Off

Liquid ammonia receipt from ship/plant

Transfer rate of liquid ammonia to user plant

Electric power

Commissioning & recommissioning proce-dure

Important 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 oxy-gen in the discharge gas is less than 0.5%.

4. Cool the tank down to as low a temperature as pos-sible, at a cooling rate lower than 1oC/hr.

5. Measure the temperature in the bulk volume of thetank, away from the gas inlet.

6. Within one week after commissioning and when con-ditions are 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 en-sures uniform and slow heating, not exceeding 1oC/hour.

3. Measure the temperature in the bulk volume of thetank, away from the gas inlet. Give careful consider-ation to 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 ammo-nia gas out.

Hazardous Area ClassificationAmmonia storage tank and associated facility falls un-

der Class 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 major

groups namely, I, IIA, IIB, IIC as per IS 2206. The clas-sification is based on the minimum ignition energy re-quired to raise the temperature of gas locally to attain


CMYK

Cryogenic Storage

its ignition temperature. Ammonia falls under the gasgroup IIA.

Temperature Class Ammonia falls under the temperature class T1

Ammonia 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 dif-ferent density and temperature.

B) By Rail/Road Cryogenic liquid is transported by the rail tankers

(BTAL) for short distance. It is unloaded into the stor-age tank via flash vessel or surge drum to control thepressure in the tank.

Generally, rail/road tanker loading station has follow-ing facilities

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 sub-jected to various different load combinations.

These tanks are subjected to low tempera-tures and accordingly are equipped with vari-ous critical 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 mate-rials, fabrication/welding, inspection and test-ing of tank must be considered which are dif-

ferent and 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 pipe-line (assessed for critical condition) routed to a dis-tant location (as per local applicable rules) and re-leased at a height and burnt with a pilot gas (usuallyLPG or NG).

To limit the risk of over pressure in cryogenic reser-voirs, it is essential to maintain a sufficiently low tem-perature (-33oC) 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 de-graded situations.

The redundancy of the equipment reduces the prob-ability of major accidents but does not exclude it. Mul-tiplication of 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. Bear-ing in mind the quantities stored in cryogenic reser-voirs, the potential sources of danger in these instal-lations are numerous; therefore the measures taken onthe technical, organizational and human fronts to pre-vent accidents 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.

80


CMYK

Cryogenic 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 am-monia lines where there is any possibility for block-age or 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 refrig-eration holding compressor to maintain tank pressureduring power failure.

Wind direction indicator should be provided.

Challenges & Opportunities Ammonia Stints (Thats good and bad)

Storing millions of gallons of ammonia presents envi-ronmental, health & safety concerns

Ammonia handling is very energy intensive and com-petitive

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.

81

Cryogenic Storage

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 - 400058 Tel: 26207402/26259421

Fax: 022-26249725

Email: [email protected]

Highlights - November 2012

CHEMICAL INDUSTRY DIGEST

Cryogenic Storage

Documents

Transcript of Cryogenic Storage