1983 Survey of Refrigerated AmmoniaStorage in the United States and Canada

Results of the third industry survey of actual design practice and experience,based on analysis of a questionnaire submitted to owners of refrigerated am-monia storage tanks in North America.

C. Clay Hale, Huxtable-Hammond Engineering Co., Kansas City, Kansas 66106

INTRODUCTION

The importance of refrigerated ammonia storage to thefertilizer industry results from several factors inherent intheir market and in the design of large capacity ammoniaplants.

• About 75% of the anhydrous ammonia produced eachyear is used for fertilizers. This is a seasonal salesmarket. Direct application by the farmers is usedeach year for a few weeks in the spring with a secondapplication in the fall.

• The large ammonia manufacturing plants operatemost economically on a continuous 24 hour per daybasis, year-round.

• Anhydrous ammonia storage tanks must provide thecapacity to accumulate the ammonia during the off-season periods from the "continuous type plants" andship-out ammonia at a high rate during the peakseason.

• For practical reasons, the ammonia plants are local-ized near natural gas supplies, while about half of thestorage tanks are distributed throughout the farmingareas. Load out of th_e inventory from many of theseterminals may be in less than two weeks, withrefilling over a six month period.

• Storage terminals remote from the manufacturingplants in these farming areas receive product bybarge, rail cars, trucks, or pipeline. The primaiy loadout is by tank trucks directly to the distributor in thelocal farm area.

In 1938, a Canadian company in British Columbia pur-chased a low-pressure storage vessel (sphere) with a capac-ity of 455 tons. This tank was designed for only 2 bar, ascompared with a design of about 18 bar, for ambient-temperature pressure storage vessels. At this reducedpressure of only 2 bar, the ammonia liquid equilibriumstorage temperature is about—9°C. In 1941, four additionalrefrigerated storage tanks were built by the same com-pany. These were designed to operate at essentially atmos-pheric pressure, operating at a temperature of about—33°C. On the basis of tons of ammonia storage per ton ofsteel required, these low-pressure storage tanks were farmore economical than pressure storage. Three of theseoriginal tanks are still in service.

As a result of this change, the unit cost for storing ammo-nia has been reduced significantly. However, refrigera-tion systems are needed to recover boil-off vapors from the

low-pressure tanks. Table 2 below compares the threetypes of storage tanks from an operating pressure and tem-perature standpoint. The type of refrigeration system re-quired to hold each type of tank at reduced temperatures isalso listed for comparison. For the larger capacity tanks thesavings in steel cost alone more than offsets the addedcomplication of a refrigeration system.

The operating temperature of the refrigerated storagetank at near atmospheric pressure is a function of operatingpressure and terminal elevation. Typical ranges are listedin Table 3 for comparison. This temperature affects thechoice of steel for the storage tank and auxiliary equip-ment. Another factor in the design of the refrigerated stor-age tank is the maximum tank operating pressure. Theoriginal tanks with capacities above 4550 tons were de-signed for operation at a pressure as low as 1.5 kPa pres-sure above atmospheric. This presented operating prob-lems with the compressor control to avoid vapor loss. Thenext design range was double this or 3.0 kPa. Now, most ofthe 30,000-ton tanks are designed for operation to 6.9 kPa(1.0 psig). A few larger tanks (40,000 ton) at seaports are de-signed for pressures between 9 and 10.3 kPa. This higherpressure reduces the added compressor capacity neededduring filling from ships or for sudden drops in the baro-metric pressure which are common on the coasts, (abovevalues are gage pressures).

TABLE 1. TYPICAL AMMONIA STORAGE CAPACITY CONTAINED BYONE TON OF STEEL VESSEL

Tons of N HUper ton/steel

Capacity rangemetric tons

Pressure storage bullets@23°C

Refrigerated storage spheres@0°C

Refrigerated storage tanks@ -33°C

2.75 up to 270

10. 450 to 2750

41 to 45 4500 to 45,000

TABLE 2. AMMONIA STORAGE TANK DESIGN.PRESSURE—TEMPERATURE—COMPRESSOR TYPE

Pressure Temperature Refrigerationbar °C compressor

Pressure storageSpheresRefrigerated storage

18.253.8 to 5.15

1.117

Ambient-1 to +2

-33

NoneSingle-stageTwo-stage

181

TABLE 3. REFRIGERATED AMMONIA STORAGE TANK.OPERATING TEMPERATURE

EquilibriumTypicalLocation

Sea Level (Tampa, Fla.)Southwest (Abliene, Texas)

@ above sea levelNorthwest Plains (Rapid City, S.D.)

@ above sea levelWestern Plains (Grand Junction, Colo.)

@ above sea levelMountain States (Cheyenne, Wy.)

@ above sea level

Atm. press,mm Hg

760718

708

636

605

Oper,temp., °C

-33°-34.5°

-35.5°

-36.7°

-37.8°

Storage tanktemp., °C

at

-32.5-33.8

-35

-36

-37.2

The first atmospheric storage tank was built in theUnited States in 1957, at Savannah, Ga. This tank has a ca-pacity of 7,275 tons and is a vertical cylindrical tank with acone roof and flat bottom. In the years since 1957, the de-sign and size of these atmospheric storage tanks haschanged, but this tank, and most of the other tanks builtsince then, are still in service. In both the United Statesand Canada, there are now six 36,360-ton tanks and three45,455-ton tanks in service. New 54,545-ton capacity tanksare being considered. The most popular size in recentyears, however, has been the 27,275-ton tank, with more ofthis size range now in service than any other size built inthe last ten years. The total number now exceeds 100 orone-third of all refrigerated tanks.

The accumulated storage capacity of the refrigeratedammonia tanks in the United States and Canada is illus-trated in Figure 1. This indicated that the capacity has in-creased from the early start in 1941 up to about 3.7 milliontons in 1970. Since 1970, this capacity continued to in-crease to almost 4.5 M tons in 1975. This latest survey in1983 indicated that the total now exceeds 5.45 M tons.

The number of tanks during this same period has in-creased from 150 in 1965, 265 in 1975 and now above 319tanks in 1983. Since these surveys were made on anunofficial basis, these numbers may be low by as much as10%.

M M _TOM£ Vj'iIM

SURVEY OBJECTIVESData collected from ammonia storage terminal operators

during 1983 are compared to the previous survey pub-lished five years ago and presented to the A.I.C.h.E. Am-monia Conference. This latest questionnaire was directedto three major objectives:

• Collect and tabulate design data on existing termin-als.

• Compare major terminal equipment features.• Identify typical operating and maintenance problems

as reported by terminal operators.Operators contributing to the survey were assured that

their identity would not be released so their problemscould be reported on a confidential basis. Data on approx-imately 50% of the refrigerated tanks were contributed tothe survey for a total of over 300 tanks. The accumulatedammonia storage capacity in refrigerated tanks now ex-ceeds six million tons. Only 14% of the owners operate50% of the tanks and locations.

The major problem reported in these terminals contin-ues to be refrigerated tank insulation failure. As thesetanks age, the tank base electric heating system failure isalso beginning to be a serious problem.

The industry has continued to have an outstandingsafety record considering the large number and variety ofinstallations in service.

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Figure 2. Refrigerated anhydrous-ammonia storage tankst—growth in number of tanks.

SURVEY QUESTIONNAIRE — 1983

The 1983 survey questionnaire sent out to the ammonia-terminal operators included ten separate groups of ques-tions Qbvering various areas of general interest:

1. General Data for Comparison of the Terminal Fea-tures at Each Site.

2. Terminal Layout and Local Conditions at the Site.3. Terminal Operating Crews and Emergency Help.4. Storage Tank Insulation Systems plus Under-Tank

Heating Systems.5. Retention Dike or Pits for Spill Protection.6. Ammonia Flares.7. Ammonia Refrigeration Compressors.8. Ammonia Condensers.9. Inert Gas Removal Units — Purgers.

10. Barometer Effect on Storage Tank Boil-Off Rates.

Only a few of the questionnaires returned this year con-tained data on all ten of the subjects included in the sur-vey. For this reason, statistics are reported as percent (%) ofthose terminals reporting data, not as % of the total.

General Data for Terminal Comparisons — USA and Canada

The first item of interest for most ammonia terminalowners or operators is how his installation compares withall the other facilities. This first part of the survey tries toanswer this question. Tables were prepared to summarizethe data received on terminal locations, tank size, numberof tanks. Since this section is primarily of a statistical na-ture, only notes on each table were considered appro-priate.

Table 4—Overall Summary of Refrigerated AmmoniaStorage

Table 5—-Number of Refrigerated Ammonia StorageTanks in Each Site

Table 6—Refrigerated Ammonia Storage TanksNumber of Tanks at Various Sites in Each State orProvince

Table 7—Typical Dimensions of Refrigerated StorageTanks

Table 8—Refrigerated Storage Tank Capacity vs. YearErected

Table 9—Refrigerated Storage Tank Fill Rate andSource of Supply

Table 10—Terminal Pump-Out Rates to VariousDestinations

Terminal Location Factors

Operators supplied a variety of information pertaining tothe actual local site for their ammonia storage terminals.This information is useful only for comparison of their siteto the average in the industry. These factors included:

TABLE 4. OVERALL SUMMARY OFREFRIGERATED AMMONIA STORAGE

Survey Total

Number of tanksNumber of ownersNumner of locationsTotal storage capacityAverage approximate

tank sizeRange in Tank Sizes:

SpheresTanks

Major Owner Statistics

31979

1985.715 M tons

18,000 tons

455 to 2,727 tons4550 to 45,350 tons

183

TanksOwnersLocations

Number

16611

100

% of total

52%14%50%

TABLE 5. NUMBER OP REFRIGERATED AMMONIA STORAGE TANKSOF EACH SIZE

Tank capacityin tons

455 to 2,725(spheres)

3,636 7,2729,070

22,41027,21031,745

18,14024,50029,93036,28045,35040,815

TotalUSA & Canada

Survey1978

number

19

21100

834

_1228

Survey1983

number

27

3612020

1056

_5319

NOTE: The difference of 90 between 1978 and 1983 does not represent the new tanksadded. These 1983 numbers were derived from two sources:1. Survey questionnaires returned In 1968, 1973, and 1983 .2. Current list of ammonia tanks erected by manufacturers, including Chicago

Bridge and Iron, Pittsburg Des Moines Steel, Graver, Brown Minneapolis.

• Terminal area• Access to the site (distance)• Distance to various neighbors• Utility supplies as well as power failures• Site drainage

Although most of the older terminals were built in ruralareas remote from urban housing, stores, schools, andother industrial facilities, this did not always insure theircontinued isolation. Table 11 summarizes data of this typecollected in 1983. Table 12 lists utility sources includingpower, fuel and water. The major comment from the opera-tors, in regard to the distance of the terminal to their closestneighbors, was that it should have been more.

Utility Supply to Terminals

Terminal Electrical Power. Almost all remote storage ter-minals are supplied by a local power company. The relia-bility of the power was measured based on the frequencyof power failures as follows:

TABLE 6. REFRIGERATED AMMONIA STORAGE TANKS

Number of Tanks in Each State/Province

State Sphere

AlaskaAlabamaArizonaArkansasCaliforniaFloridaGeorgiaIdahoIllinoisIndianaIowaKansasKentuckyLouisianaMaineMichiganMinnesotaMissouriMississippiNebraskaNew JerseyNew MexicoNew YorkNorth CarolinaNorth DakotaOhioOklahomaOregonPennsylvaniaTennessee

21

15

21

33211

4

2

32

33

12

264443

151

21

1211

22152

109

333

1632

10

323

21

Total1983

survey

2263

17751

391339113

2411988

241212345753

Total1978

survey

32932

32103362

20

1741

181313

43714

TexasVirginiaWest VirginiaWashingtonWyomingUSA TotalAlbertaBritish ColumbiaOntarioManitobaSaskatchewanNew BrunswickCanada TotalUSA & Canada Total

19431

827

11

351

136

20113

1321

51

18

140

21

994

11

6105

211

272162

2941236211

25319

212142

212934

16228

184

TABLE 7. TYPICAL DIMENSIONS OF REFRIGERATED STORAGETANKS

A. Tank Foundation—Ring Wall

Tank Side WallHeight, meters

19.8117.3717.0722.8627.1323.1619.5117.3720.4222.8621.3421.6430.7819.8119.20

36,281 54.56 19.81B. Tank Foundation—Piling and Concrete Pile Cap

TABLE 9. REFRIGERATED AMMONIA STORAGE TANKS Vs YEARERECTED

Number of Tanks

Capacity,tons

4,5356,8008,6209,070

13,60513,60513,60513,60516,32518,14020,41022,67527,21027,21027,210

Tank Diameter,meters

20.7327.4332.3127.1330.7833.8336.2739.6239.6238.7142.6745.1141.1550.2951.82

Tank Capacity in Tons

Capacity,tons

13,60520,41027,21029,93031,745

Tank Diameter,meters

32.3139.6239.6241.4539.62

45,350 56.3945,350 54.2545,350 44.20

C. Spheres Supported on Legs

Tank Side WallHeight, meters

252530.7832.6138.1026.8228.9632.0

Year

1941195619571958195919601961196219631964196519661967196819691970197119721973197419751976197719781979198019811982Total

4002720

11

2

5

4

1

4

36307260

1

3

5

5

5

12

3

907022675

2720029930

3174545351

— 2

5

14

35

43

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2

8

7

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24

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27 36 Ï4Ö ÏÖ5Years Overall Averages

Total no. tanks

YearTotal

12

5

15

24

43

72

44

8

11

10

41

21

4

9

§19

Capacity,tons

455907

1,4972,2762,4502,721

Sphere Diameter,meters

11.5814.6316.7618.5919.8120.42

1941 thru 19501951 thru 19601961 thru 19701971 thru 1980

1981/2Total

824

19187

310

319

NOTE: Tank height usually limited by soil loading and compressive strength of under-took insulation. Tank side-wall heights up to 20 m usually permitted on15,000/kg/m' soil bearing. Maximum height on foam glass insulation under tankusually limited to 34 meters.

TABLE 8. REFRIGERATED STORAGE TANK

Fill Rate and Source of Supply

Fill Rate, Tons/Hour

TABLE 10. TERMINAL PUMP-OUT RATES To VARIOUSDESTINATIONS

Pump-out rate, ton/hourRange

To destination Min. Max. Average Percent* of terminals

Range

Source of Supply

Plant FillRemote Terminals

Pipeline FillRail UnloadTrack FillBargeShip

Min.

11.8

2.722.722.72

90.7378

Max.

102.5

77.136.394.3

9071,814

Average

40

22.714.519.0

322825

Percent*of Terminal

40%60%24%34%17%26%9%

Fertilizer plantsPipelineRailroad loadingTruck load-outBargeShip

3.633.63

18.149.07

81.6390.70

150265218302454680

43.566.255.3

104245386

19%15%46%73%9%4%

*NOTE Total exceeds 100% since most of the terminals have more than one source offill (average 1,5 sources per terminal)

185

* Note: Total exceeds 100% since most of the terminals have more than one destinationfor product (average 1.9 destinations per terminal).

On the average — all terminals have 5.5 outages at 10V4minutes each per year.

— 90% terminals experience only 3.5outages at 6 minutes each per year.

Terminal Water Supply. Most of the older terminals usedwater cooled or evaporative water cooled condensers.Many of the newer terminals, however, now use air cooledcondensers with the result of requiring only sanitary

TABLE 11. TERMINAL SITE DATA TABLE 12. TERMINAL UTILITY SERVICES

(Percent of total terminals reported in Survey)

Table 11 A. Terminal Area—remote terminals only

Maximum range in area from4 hectares or less

12 hectares or lessover 12 hectares

33%72%28%

100%

Table 11B. Distance from site to highways, rail and rivers

HIGHWAYS—Average distance from site

152 meters or less 25%457 meters or less 60%914 meters or lessover 914 meters

75%25%

100%

RAIL—Average distance from site

122 meters or less244 meters or less366 meters or lessover 366 meters

33%68%

RIVERS—Average distance from site

305 meters or less 28%610 meters or less 42%915 meters or lessover 915 meters

81%19%

100%

58%42%

100%

Table 11 C. Distance to various neighbors from site.

Residences closest to terminals—

0.2 kilometers or less 10%0.4 kilometers 35%0.8 kilometers 30%1.2 kilometers 7%1.61 kilometers 5%3.22 kilometers 7%over 3.22 kilometers 6%

TOTAL 100%Stores (shopping centers, filling stations closest to

terminal)—0.4 kilometers or less 6%0.8 kilometers 10%1.2 kilometers 5%1.61 kilometers 18%3.22 kilometers 20%4.83 kilometers 15%6.44 kilometers 12%over 6.44 kilometers 14%

TOTAL 100%Schools, Hospitals, closest to terminal—

0.8 kilometers or less1.61 kilometers3.22 kilometers4.83 kilometers6.44 kilometers8.05 kilometersover 8.05 kilometers

6%20%17%15%18%10%14%

TOTAL 100%Industrial sites closest to terminals—

0.2 kilometers or less0.4 kilometers0.8 kilometers1.2 to 1.6 kilometers3.2 to 8.0 kilometersover 8 kilometers

TOTAL

16%20%21%16%15%12%

100%

(Percent of Total Terminals Reported in Survey)Table 12A. Source of Water Supply to Terminal— % of Total

City water supplyWell water supplyRiver waterLake waterWater re-useNOTE: Total %

terminals haveof supply.

34%

exceeds 100%, sincemore than one

38%severalsource

Table 12B. Water Drainage from Terminal— % of Total

Treatment pondsInside terminalLagoonsCity sewerBay

Directly to riversSubtotal

TOTALterminal drainage

13%3%2%2%

20%

Practically allwith 80% ending in a local river.

Table 12C. Terminal Fuel Supply—

100%by ditch,

% of Total

Natural gas 71%Propane or LPG 38%Multiple fuel (natural gas-propane) 12%Fuel oil 12%Steam 2%NOTE: Total %

terminalssource.

exceeds 100% since severalhave more than one fuel

Table 12D. Propane Fuel Storage Capacity— % of Total

186

68.0 M3 39%78.3 M3 42%

113.6 M3 19%

TOTAL 100%Average 81.5 M3 storage capacity at each terminalusing propane.

water. In feet, two terminals reported all water for the ter-minal was hauled in by truck.

Based on the questionnaire returns, the following datawere prepared:

Terminal General Operating Experience

The operating experience for remote ammonia storageterminals has been included in this part of the survey. Am-monia plants with refrigerated tanks are not included,since their operations are usually combined with theplants, and therefore would not be applicable to this re-port. In general, the information was reported in threecategories:A. Number of operators needed for different operations.B. Extent of the hours covered each dayC. Source of emergency help

The regular operating crew sizes as well as the tempo-rary help are summarized on Table 13. The next table,Table 14, summarizes the hours each day the terminal isstaffed. Both tables are broken down into two parts: thefirst lists data on a "one-tank" terminal, and the secondpart includes terminals with two or more tanks.

For single tank terminals the "standard" crew size isthree operators. This crew size increases to three or four onmulti-tank terminals. Temporary help is frequently usedduring part of the year. For example, during load-out oper-ations, about 2 extra helpers are added at each terminal.

TABLÉ 13. AMMONIA TERMINAL OPERATING CREWS

Regular Crew

Fill only

RangeLoad-out

RangeHold only

RangeTemporary Crews Added toHelp Regular Crew

Fill operation

Load-out operation

Hold only operation

One tankterminals

Number ofOperators

78% have3 or less2 to 1075% have3 or lessI to575% have3 or lessI tolO

10% have1 only90% have2 extraNone

Multiple tankterminalsNumber ofOperators

82% have4 or less2 to 975% have5 or less2 to 982% have4 or lessI to9

None

Average havetwo extraNone

TABLE 14. OPERATOR COVERAGE IN HOURS PER DAY AMMONIASTORAGE TERMINALS

Hours Per DayOne Tank Terminal

Fill operationLoad-out operationHold only operation

Two or More Tank Terminals

Fill & holdLoad-outHold only

TABLE 15. EMERGENCY HELP ON CALL FOR TERMINAL

% of all terminals

8hr 16 hr 24 hrPercent

541058

331455

8

7Percent

655

389035

618141

96%

39%

Local Fire and Police Dept. which may becity, county (sheriff), or state policeLocal plants in the area when available on areciprocal basisCivu defense organizations 54%Multi-terminal companies also arrange for back-up help in anemergency from the closest terminals.

The coverage at the terminal varies considerably but in

feneral, load-out operations require more coverage thanHing and holding operations. Between 80 and 90% of the

terminals provide a 24-hour day coverage during the load-out period.

Emergency help at remote terminals is usually pre-arranged on a stand-by basis with one or more of the fol-lowing local organizations:

— Police Departments— Fire Departments— Civil Defense— Nearby Industrial Plants

Storage Tank Insulation and Under-Tank Frost Protection Systems

One of the most common as well as the most expensiveproblems reported by terminal operators involves cold in-sulation failures. For purposes of discussion, the insula-tion systems have been separated into two parts. First, thedome and shell insulation system and second, the base orunder-tank insulation system. In this section the need andselection of under-tank heating systems are also included.Shell and Dome Insulation. Table 16 lists the types of shelland dome insulation now in service on various refrigeratedstorage tanks. The actual useful life of each type ofinsula-tion listed, varies considerably. This is probably due forthe most part to a combination of factors based on reportsfrom our wide range of terminal locations. Of the tankslisted in Table 16, about 93% of the double wall tanks haveexpanded perlite insulation. Since these insulations aresealed from the weather, failures are rare. The only prob-lem noted (and reported in to this conference in 1983) wascracking of the outside wall of double tanks caused bycompaction of the perlite. This resulted from expansionand contraction of the annular space as the tank is filledand emptied.

A relatively new feature reported in both single and dou-ble wall ammonia tank designs involves a suspended deckto support the top insulation. This design eliminates theneed on single wall tanks for external dome insulation,which is a problem to install as well as to maintain. On thedouble wall tanks, the suspended deck is more economicalto build than an internal dome. This feature was devel-oped originally for LNG tanks. Use of this design on am-monia storage however, is questionable in extremely coldclimates, since ambient temperatures below -33°C willresult in vapor condensation in the dome area above theinsulation. This could produce a partial vacuum in the tankwhich will cause air intake through the vacuum reliefvalve.

All of the major insulation failures reported have beenwith the composite insulations. The data are not completeenough to permit an analysis or comparison of types, butthe owners estimated expected life of the various types istabulated in Table 17.

TABLE 16. REFRIGERATED AMMONIA STORAGE TANK SHELL & ROOF INSULATION SYSTEMS

a. Double wall tank, with perlite, styrofoamor rock wool insulation

b. Composite Insulation SystemsStyrofoamFoamglassPolyurethaneFiberglass, cork, misc.

c. ProprietaryAlumiseal (reflective aluminum insulation)Thermacon (aluminum backed urethane panels)

1978Percent

35

8.521233

9.5

1983Percent

26

41733

173

187 100.0 100

TABLE 17. REFRIGERATED STORAGE TANKS. ESTIMATED RANGEOF SERVICE LIFE FOR VARIOUS TYPES OF INSULATION

Service Life in Years

StyrofoamFoam GlassPolyurethaneAlumisealThermacon

Minimum

554

15

toto .toto

No data

Maximum

152015

indefinite

The minimum range of life for composite insulation sys-tems occurs in areas subject to damage from hail storms.Mechanical damage of the weather seal on cold insulationswill result in failure by moisture penetration. Failureswere reported on polyurethane insulations by this morefrequently than on other types. Two owners with urethaneinsulations also reported damage by woodpeckers nestingin the wall insulation.Base Insulation and Undertank Heating Systems. Selec-tion of the under-tank insulation system requires consider-ation of tank loading, (a function primarily of the weight ofthe tank full of liquid) and the allowable heat leak throughthe insulation. Tanks supported above the ground on a pilecap usually are taller with maximum loading on the insula-tion. With free air circulation under the tank, no problemsare reported with frost build-up under the concrete. Fortanks supported on ring walls directly on the ground, theunder-tank insulations conductivity is a major factor. Theground below the tank must be protected against frostheaving. This protection is provided by a combination ofinsulation and heat source to maintain the ground under

TABLE 18. REFRIGERATED STORAGE TANK. UNDER-TANKINSULATION SYSTEMS

Type of Insulation

Perlite ConcreteWeathercrete or InsulationFoam Glass

1983Percent

271756

100

the tank at a temperature above freezing. The only excep-tions are in special areas where tanks are built on solid rockor "hard pan" that is impervious to water.

As the tanks age, frost accumulation under the tanksbuilt in the gound becomes a fairly common problem.Most tanks are provided with electrical heat under the tankinsulation. When this heating system fails, ice formationcan damage the insulation. Damage due to "frost heaving"of refrigerated storage tanks has been well documented inpapers presented to the A.I.C.h.E. ammonia conferences.This continues to be a major problem because of the inac-cessibility of inspection and repair.

Table 18 list the types of under-tank insulations re-ported in this survey and Table 19 lists the types of under-tank base-heating methods. No problems of any type havebeen reported for tanks erected on a pile cap with free aircirculation below the base. These include 37% of the tanksin the survey. The choice as to the type of foundation is al-most always based on allowable soil loading. Ring wallsupport with the center of the tank supported by theground is usually limited to about 9,763 kg per m2. Thisloading limits tank height to about 14.5 meters.

Below this allowable soil loading, the tanks are usuallybuilt on a concrete pile cap elevated above the ground. Apart of the extra cost in this design for the piling and pilecap is off-set by elimination of the need for under tankheating for under tank frost protection.

Electric heat systems are used on 88% of the tanks sup-ported on ring walls to protect the base from frost heaves.Failure of under-tank heating systems have resulted in se-rious damage to several tank foundations. If a leak occursin the tank bottom, the concrete ring wall and under-tankinsulation can be seriously damaged. Several papers havebeen presented to this conference in previous years de-scribing these problems. Reports this year indicate a widerange of actual or predicted service life for under-tankelectric heating systems. These range from 4 years up to 25years, with an average of about 13 years. The main prob-lem is water penetration into the conduit on electric heat-ing systems. Tanks located in dry areas have few problems.

Dikes or Retention Pits for Refrigerated Storage Tanks

The A.N.S.I. "Safety Requirements for the Storage andHandling of Ammonia" recommends that the area sur-rounding a refrigerated storage tank shall be provided with

TABLE 19. REFRIGERATED AMMONIA STORAGE TANKS. UNDER TANK INSULATION THICKNESS

Thickness inMinimum Maximum

Perlite ConcreteWeathercrete or InsulationFoam Glass

17.815.2510.2

38.120.320.3

Average

241914

TABLE 20. REFRIGERATED AMMONIA STORAGE TANKS.UNDER TANK FROST PROTECTION

Tank with Foundation Heat Systems

Electrical Resistance (Wire) HeatGlycol Circulation (Piping) HeatAir/Stream (Piping) Heat

Tank without Foundation Hearing

(on rock or on impervious soil)Tank Supported on Piling

With free air circulaton unde tankpile eap

1978Percent

7132

1983Percent

5641

22 37

TOTAL 188 100 100

drainage or be diked to prevent accidental discharge of liq-uid from spreading to uncontrolled areas. The require-ment for a new terminal is usually made based on localconditions and must be approved by the local zoning com-mission. As shown in Table 21, the survey indicated abouttwo thirds of the terminals have dikes. Only about five areconcrete dikes and several have a combination of earth andconcrete. Only two locations indicated "drainage ditchesto a remote pit."

The concrete dikes or walls are typically only 1.5 to1.8 m out from the tank wall. This arrangement results in ahigh wall, at least 75% of the tank height. In comparisonthe earth dikes are only about 25% of the tank height.Table 22 summarizes the data available on the height ofearth dikes, which range from l m to 6 m.

The capacity of the dike ranges between 100 and 200%of the tank capacity. Table 23 indicates the range of dikecapacities reported in the survey. "

The main purpose of a dike is, of course, to retain the liq-uid in the event of a spill. Four terminals actually reportedspills; two of these were overflow of the tank duringfilling. Only one of the tanks reporting a spill did not havea dike. This resulted from the pump at the refrigerated tankheaving from frost build-up. This frost build-up resulted ina suction line break causing the spill.

All except one terminal manager agreed that dikesshould be recommended. Only 15% of the operators have aplan for disposing of the ammonia liquid in the case of aspill.

For a single tank, the volume required includes the areawithin the tank. A free-board of 0.3 m above the liquid ca-pacity is usually provided for safety.

Ammonia Flares for Vapor Disposal

Ammonia vapor incineration offers a satisfactory methodfor temporary disposal of this vapor. The products of com-bustion of ammonia in air are water vapor and nitrogen.This system is normally only used as a stand-by method ofdisposing of ammonia vapor boil-off from the storage tanksduring emergencies. These emergencies may be causedby power failures or by refrigeration system equipmentbreakdowns. However, one terminal reported using theflare for disposal of ammonia boil-off vapor for over a yearwith no problems or complaints from the neighbors. Thistank was a double wall tank with a very low heat leak, so

TABLE 21. REFRIGERATED STORAGE TANK RETENTION DIKES

Percent of total

TABLE 23. CAPACITY OF REFRIGERATED STORAGE TANK DIKEDAREAS

Earth dikes combinationConcrete dikesDrain to holding pondNo Dike

Total

6331

33100%

TABLE 22. REFRIGERATED AMMONIA STORAGE TANKRETENTION DIKES

Earth Dike Heightin meters

USA & Canada1978

6%50%25%15%4%

1983

46%37%14%

up to 1.071.22 to 2.292.44 to 3.053.35 to 4.574.88 to 6.10

TOTAL 100% 100%Since almost all of the dikes are designed to hold100 percent of the tank contents, the overall areais quite extensive for the smaller dikes for thelarge capacity tanks.

% of Tank Capacity

100-120%125-150%175-200%

% of TotalNo. Terminals

70%22%

8%100%

the normal vent rate was probably only in the range of 68 to113 kg per hour. The ammonia plant at the site was shutdown and no stand-by compressors were available.

Table 24 lists data on the size, location, and height of theflares. Most of these flares were purchased from compa-nies specializing in these items. In spite of this, however,many minor problems were reported, primarily with light-ing or keeping the pilots lit on windy days. Complaints ofincomplete combustion of the ammonia were usually asso-ciated with this same problem with the wind. Over-sizingthe flare can also be a problem in recent years since someof the newer terminals vent safety valve discharge into theflare header. This capacity is usually many times largerthan normal tank boil-off rates, which results in incom-plete combustion.

About 73% of ammonia storage terminals, either atplants or remote from the plant, have flares. Practically allof the newer terminals have flares (over 95%).

The source of vapor to the flare is primarily from a pres-sure control valve which vents ammonia vapor from the re-frigerated storage tank in cases of power failures (for exam-ple) rather than venting through the tank relief valve. Thissystem protects the relief valve and, even more important,permits disposal of the emergency vapor vent by incinera-tion. Only three out of 100 terminal locations in the USAreported having emergency electrical generators as thereason for not having flares. In addition to refrigeratedstorage tank boil-off vapor, about 20% also vent the inertgas from the purger to the flare.

Several of the terminals also reported using the flaresfor incinerating propane vapor when tank cars in propaneservice are being converted to ammonia service.Problems Reported with Flare. Although 99% of the ter-minals consider flares as a satisfactory disposal method for

TABLE 24. REFRIGERATED AMMONIA STORAGE TERMINALFLARES

Flare Location 1978

Top of Tank or Stair TowerOn Top of DikeBalance—Outside of Dike

(Remote from tank)

50%20%30%

1983

28%21%51%

1983

Flare Diam- 1978eter—in mm Total

35%28%26%11%

75100150200300

100%Flare Height, meters

Top ofTank

29%33%19%14%5%

TopDike

15%

100% 100%

1978

1983Aver.

35%26%27%

9%3%

100%

1983

189

Top of Dike (average)Range

Top of Tank or Stair Tower(average)

Range

7.6(4.6) to 9.15)

30.5

10.7(7.62 to 12.2)

29.9

(22.9 to 42.7) (21.3 to 39.6)

emergency venting of ammonia vapor, the followingoperating problems were reported by the managers:

1) Flares are hard to light and high winds blow out theflame.

2) Fuel (gas) must be added to ammonia being vented tocompletely destroy odor.

3) One terminal had flare plugged with carbamate.4) Flares with propane pilots must heat trace propane

vapor to avoid condensation in cold water.

Refrigeration Compressors — Used on Ammonia Refrigeration

Probably every type of compressor manufactured hasbeen used on ammonia refrigeration systems. The conven-tional commercial type refrigeration units were first ap-plied to ammonia terminals for the recovery of boil-offvapor from the storage tanks. These included both lubri-cated sliding vane, and reciprocating compressors. Thesecommercial reciprocating compressors are single actingvertical units, with the connecting rod and crankcase un-der ammonia pressure. As the terminal refrigeration re-quirements increased, larger process type reciprocatingcompressors similar to those used in the ammonia plantswere selected. One ammonia pipeline terminal, for exam-ple, has three 450 double acting horizontally opposedcompressors to handle filling and holding refrigeration fortwo 36,400-ton storage tanks.

After the development of the oil flooded, rotary screwcompressor in the commercial refrigeration industry,these units were applied to the ammonia refrigeration re-quirements in the ammonia terminals. The first pair (onefirst stage and one second stage) were installed in 1973 ona refrigerated storage terminal in Iowa. Over the last tenyears these have given excellent service according to theowner. As a result, this type machine is now standard forthis company for all their terminals and terminal additions.

At ammonia manufacturing plants, the ammonia storagetank refrigeration requirements are usually (but not al-ways) combined with the main plant refrigeration com-pressors. On the new single-train ammonia plants theseare centrifugal compressors. When compressors are notavailable for any reason, a small "holding only" com-pressor is usually used for the ammonia storage tank boil-off vapor recovery. No reports were received on remoteterminals using centrifugal compressors.

Table 25 lists the percentage of each type of compressorreported in the survey. Table 26 compares the annual re-

TABLE 26. MAINTENANCE DOWNTIME FOR VARIOUS TYPES OFAMMONIA REFRIGERATION COMPRESSOR

Table 26A Compressor Maintenance ofRefrigeration Grade

Refrigeration grade

Process grade

Oil flooded rotary screw

* Downtime for repair—not just standby.

Table 26B Compressor Power—HP

Reciprocating compressors:Refrigeration grade

Process grade

Oil flooded rotary screw

Downtime* indays per year

64% 3 days or less91% 5 days or less62% 5 days or less90% 10 days or less51% 3 days or less84% 5 days or less

Compressor sizebased on motor

rating

75 kW or less 53%112 kW or less 78%186 kW or less 100%150 kW or less 50%300 kW or less 72%450 kW or less 100%150 kW or less 27%300 kW or less 84%450 kW or less 100%

Table 26C Comparison of MaintenanceRequirements

On the basis of downtime per 75 kWrating

Refrigerant grade 186 kW—4 days 1.6 days/75 kWProcess grade 450 kW—8 days 1.3 days/75 kWScrew compressor 450 kW—4 days 0.7 days/75 kW

pair requirements for the three most common type com-pressors of various sizes.

These statistics indicate that the rotary screw compress-ors have the minimum actual maintenance taking into ac-count the motor size required for the service, the size of thecompressors being a major factor. In addition, the higherdischarge pressure on the process units and on rotaryscrew compressors also increases maintenance. This

TABLE 25. TYPES OF REFRIGERATION COMPRESSORS USED ON REMOTE TERMINALS FOR AMMONIA STORAGE

Type compressor

Refrigeration Grade

Reciprocating Compressorsuch as: Frick

VilterYork

Process Type

Reciprocating Compressorsuch as: Ingersoll Rand

WorthingtonCooper-BessemerPenjax

(Pennsylvania)Oil-Flooded Rotary Screw Compressors

Such as: Frick, SullairFreezing Equipment SalsLewis-Houden, Stahl, or

Rotary Sliding Vane

Such as: FullerFrick

Range, k W(motor)

7.5 to 75

67 to 450

1978percent

total

26%

51%

1983percent

total

35%

110 to 335

up to 75

190

16%

7%

26%

Nonereported

higher pressure usually results from the use of air-cooledcondensers in the newer terminals.

Ammonia Condensers for Refrigeration Systems

The selection of the ammonia condenser is affected bythe availability of water and water disposal at the site. Thisin turn affects the type of compressor used for the refrigera-tion system. Table 27 compares the normal operating con-ditions of the four most common types of condensers andthe typical operating conditions. Air cooled condensersare not usually recommended for use with conventionalrefrigeration type compressors since these compressorsare not designed for operation in excess of 17 bar. Air tem-peratures exceeding 33°C will exceed this safe operatingrange. This was one of the main reasons for applying pro-cess type compressors when air cooling was a necessity.Process type units are not limited by this pressure.

Problems Noted in 1983 Survey:

1. Well or river water exchangersTube side of these units usually requires cleaning pe-riodically, ranging between 3 months and 9 months.Several owners report corrosion results in re-tubing,every five to six years. About half of the owners re-port no problems at all with these units.

2. Cooling Tower Water ExchangersWaterside cleaning is required between one andthree years, with re-tubing on several every fiveyears. About 85% of the owners report no problems.

3. Evaporative CondensersProblems with these units involved:

a. Mechanical failures, pumps/fansb. Limited capacity in severalc. Winter ice problemsd. Water fouling required cleaning, varying from

one to four times per year(one every two weeks).

e. Replaced 25% in periods ranging from 7 to 18years.

4. Air Cooled CondensersProblems with these units involved:

a. Dirt in tubing fins 42%Clean 2 to 3 times per year

b. Limited capacity in summer 17%c. Mechanical failures—fans & motors 9%

Subtotal 68%Almost 1/3 reported no problems 32%

100%

Inert Gas Removal Using Purge«

Ammonia liquid delivered to refrigerated storage ter-minals from railcars or pipelines contains dissolved or en-trained inert gases. As the pressure is reduced when theliquid enters the refrigerated storage tanks, the inert gasseparates from the bulk of the liquid. The refrigerationcompressors receive the inert gas mixed with flash vaporand discharge the mixture to the condenser. The inert gasaccumulates in the condenser and receiver because of theliquid seal. This gas buildup raises the condensation pres-sure unless it is removed by venting. Ammonia can be eco-nomically separated from inert gas by refrigeration at ele-vated pressure or by adsorption in water.

Considerable variation of opinion has been expressedby operators as to the need for inert gas removal from re-frigerated storage systems. This, combined with the varia-tion in source of supply and transport methods, obviouslylimits the accuracy of the data in this part of the survey.Manual venting of inert gas from the condenser or receivercan result in a considerable loss of ammonia. This loss isnot easy to detect by measuring inventory because of thelarge quantities of liquid stored.

Table 29 summarizes the types of purges used in theUnited States and Canadian terminals. The proprietaryunits include package units normally specified for com-mercial, closed circuit refrigeration systems.

No major change has been reported between 1978 and1983, either in the type of unit supplied or in the amount ofinert gas observed. Ninety terminal locations reporting on

TABLE 27. TYPICAL AMMONIA CONDENSER OPERATION

Well water once-thru (at 20°C)Shell & tube exchangerEvaporative condenser (25°C, wet bulb)Cooling tower water (at 29°C)Shell & tube exchangerAir cooled (finned surface) condenser (32°C air)Air cooled (finned surface) condenser (38°C)

AmmoniaCondensingTemperature

25°C

32°C35°C

43°C49°C

CompressorDischarge

Pressure* (Gage)

1.14 MPa

1.34 MPa1.48 MPa

1.89 MPa2.10 MPa

*An allowance for condenser pressure drop and inert gas partial pressure has been included in these values.

TABLE 28. TYPES OF AMMONIA CONDENSERS IN SERVICE ON REMOTE REFRIGERATED AMMONIA TERMINALS

Water cooled condensersWell water or river waterCooling tower water

Evaporative condensersAir cooled condensers

1978 PercentBy Type

3822

1983 PercentBy Type

17%31%

Subtotal

TOTAL191

601426

100

4917%33V2

100

TABLE 29. REFRIGERATED AMMONIA TERMINAL INERT GASPURGERS

US & Canada

TABLE 30. TYPICAL PURGE GAS ANALYSIS(FROM PURGER)—PIPE LINE TERMINAL

Double pipe or shell and tube exchangersProprietary purger units (Armstrong

Model 370)Water absorption towerNone

TOTAL

1978 1983Survey Survey

57% 62%

19% 21%4% 4%

20% 13%

100% 100%

these units indicate that approximately 90% of the termin-als that filled their refrigerated tanks from either tank carsor ammonia plant pipe line have an inert gas purger sys-tem. However, only about half of the terminals filled frombarges or ships have purgers. This difference is due to thelower concentration of inert gas in liquid ammonia after re-frigeration for loading into the barges or ships.

Many of the terminals without purgers reported ventinginert gas from the refrigeration system condenser accumu-lators, (receivers). When this is done, the ammonia concen-tration will be many times higher than the inert gas ventedfrom the purgers. The Armstrong #370 purgers have a lim-ited capacity and are normally specified only for commer-cial closed circuit ammonia refrigeration systems.

Purger rates were only reported by two terminals at 4.5& 23 kg per hour. Table 30 includes a purged-gas-analysis(out of purger) furnished by one of the terminal operators.

The refrigerated type purgers work reasonably well, butmaintenance problems occur with the expander valves.These valves either plug up or the tube connecting thevalve diaphragm to the bulb corrodes. These tubes onstandard valves are usually made of copper.

Effect of the Barometer on Refrigerated StorageTank Boil-off Rate

Daily weather reports usually include the barometric

Eressure for the local city. This pressure is either rising toring on fair weather or falling to bring on stormy weather.

Component

Ammonia NH-)Methane CH4Nitrogen NzHydrogen H2Air (based on 02 content)

Vol. %

TOTAL

1450162

18

100

This change in atmospheric pressure affects the operationof a refrigerated ammonia storage tank operating at 105Kpa or less. A drop of pressure (mercury) as reported wouldin effect double the gauge pressure in a 105 Kpa tank.

Table 31 indicates the theoretical pressure changes for atank design pressure of 108 Kpa with a control range 0 to107 Kpa prior to venting ammonia vapor to the flare or forthe vacuum relief to bring in air. For a 108-Kpa barometerdrop, this means also cooling the ammonia liquid in thetank by 0.8°C. For a 108 Kpa rise, the reverse effect must becarried out, warming the liquid 0.5°.

Since some of the tanks in service today are designedwith pressures as low as 15-cm water column it is obviousthat control may be very difficult with sudden changes inthe barometer.

Rather than depend on weather reports, many of the ter-minal operators now have their own recording barometerswhich gives these barometric pressure trends on a contin-uous basis. During the tank-filling season, a rising barom-eter allows an increase in maximum fill rate. The oppositeis, of course, the case when the barometer falls. Most of thetime, this change is not excessive but when a storm ap-proaches, the drop may be as high as 2.16 Kpa in one day (4in. Hg). Table 32 lists the typical normal and the maximumbarometer changes reported in the survey. The majorproblem reported from sudden drops in the barometer wasventing of ammonia, either out the storage tank safetyvalves on top of the tank (or venting to the flare).

With a rising barometer, air may enter through the vac-uum relief on the tank if the vapor pressure falls below at-mospheric pressure. This is not normally a hazard, since

TABLE 31.1LEVEL

Tank Operation

Barometer

LowLowNormalHighHigh

PresskPa

(gage)

5.524.142.761.380

Temp°C

-33.6-33.3-32.8-32.5-32.2

AtmosphericPressure

mm Hg

709734760786812

Change from Normal

mm Hg

51.725.80

25.851.7

TABLE 32. BAROMETER — RATE OF CHANGE BASED ON REPORTS FROM 35 TERMINALS

Drop in 24-hour period in. Hgmm Hg

% in one hourRise in 24-hour period in. Hg

mm Hg% in one hour

Minimum

0.102.54

10%0.102.54

5%

Maximum

2.050.875%2.0

50.850%

Average

1.230.526%1.0

25.423.5%

192

DISCUSSION

HAROLD PHILLIPS, ICI: In thé United Kingdom, we donot have a regulation for periodic ammonia tank inspec-tion. We do inspect tanks, however, occasionally. Basedon the results of these inspections, stress cracking of thecarbon steel tank does not seem to appear in refrigeratedstorage tanks. With ammonia spheres operating at elevatedtemperatures (and pressures) stress cracking does occur.I would like to ask if any of the returns on this surveyreported stress corrosion and if periodic inspections ofammonia tanks were being carried out.

HALE: Thanks for your information on stress corrosion. Ihave had no reports of stress cracking in the refrigeratedstorage tanks operating at -33°C. However, this question

was not included on the survey. Actually, several yearsago, Alan Cracknell of ICI wrote me an excellent letterabout this problem of stress corrosion in pressure spheres.I gave a copy of this letterto an engineer friend responsiblefor three spheres that had been in service over 15 years.Based on this recommendation, the spheres were in-spected and quite a few small cracks were found. Onecrack was about 3-ft long and 1/8-in. wide. He appreciatedthe advice and considers an accident in the future wasavoided by this repair. In regard to the inspectionfrequency, I have not received any information on acutalcompany policies or new government regulations beingconsidered.

193