ChicagoBridge& Iron

CompanyCBT-5393

REFRIGERATED AMMONIA STORAGE TANKSINSULATION SYSTEMS, DIKING AND PURGING

By: Jay M. ShahSupervising Design Engineer

Chicago Bridge & Iron CompanyOak Brook, Illinois 60521

For Presentation at The26th Symposium on Safety

In Ammonia Plant andRelated Facilities.Montreal, CanadaOctober 5-9,1981

R1FRIGERATID AMMOMA STORAGE TANKS

INSULATION SYSTEMS, DffilWG AND PUHGING

Bys Jay M. Shah

Supervising Design Engineer

Chicago Bridge & fron CompanyOak Brook, ülinois 60521

For Presentation at The26th Sympc«ium on SafetyIn Ammonia Plants andRelated Facilities.

Montrealj CanadaOctober 5-9, 1981

ABSTRACT

Various designs of refrigerated ammonia storage tanks and insulation systems that are

commonly used are presented. Some design considerations for secondary containmentsystems for ammonia facilities are provided. A procedure is given for placing an ammoniatank in service and also for taking a tank out of service for inspection, repairs ormodifications.

REFRIGERATED STORAGE TANKS

Large flat bottom tanks are designed for near atmospheric pressure, generally l to 2 psig

(6895 to 13790 pascal). Tank walls are cylindrical, roofs are either spherical or ellipsoidal

and bottoms are normally flat or slightly cone shaped. The tank rests upon some form ofload hearing insulation which transmits the weight of the contents to the foundation andground beneath the tank. Anchorage for flat bottom tanks must hold the shell down

against uplift forces from wind, earthquake and internal pressure acting under the roof.Anchorage must also permit the vessel to move radially in response to temperature

changes. A system of anchor bolts or straps attached to the tank shell embedded inconcrete is normally used. The weight of concrete must be greater than the uplift forces.

Ammonia tanks are generally designed in accordance with API Standard 620 Appendix R.This standard is prepared specifically for low temperature tanks suitable for the storagetemperature of ammonia at -27<>F (-33<>C) and even lower temperatures for the storage ofother refrigerated products. The material of construction is normally high manganese

carbon steel which retains its notch ductility at low temperature. These materials arelisted in API 620 Appendix R and include A537 and A516 Grades 60 and 70.

INSULATION SYSTEMS

Flat bottom tanks can be of two types, different primarily in the type of insulation system

used. A conventional doublé wall tank (essentially tank within a tank), Figure l, has the

insulation distributed in the annular space. Loose fill perlite is the primary insulationused in the shell and roof of a doublé wall tank. This insulation system requires a dryinterstitial gas to protect the insulation. The outer tank is usually designed for about 2

inches (50 mm) of water pressure. The outer tank normally is not designed for holdingliquid ammonia.

An economical modification to the conventional doublé wall insulation system is thesuspended deck insulation concept whieh CBI introduced in 1966. Figure 2 shows thisconcept* This insulation system eliminates the inner tank roof. The prineiple is similar tothe suspended ceiling in a building where insulation limits the heat transfer to or from thespace below.

A widely accepted and eeonomieal insulation system is a single wall tank with externalinsulation as shown in Figure 3. The vessel wall is insulated with rigid polyurethanefoamed-in-place between the shell and an aluminum jacket. The inside face of the

corrugated aluminum sheet is primed with a coating designed to enhance adhesion

between the jacket and the foam. The exterior surf ace is coated with an alkyd orpolyester enameL The suspended deck type insulation is also used for single wall tank

because it provides maintenance free inexpensive roof insulation. Externally appliedïnsulation on curved roof surfaces can be used but will require maintenance and issusceptible to damage from natura! phenomena sueh as high winds etc.

Single wall insulation can be repaired while the tank is in service. The damaged portion is

cut out, a new piece of aluminum vapor barrier is fastened in place, and new foam isexpanded into the patch area through a small filling hole. Block urethane patehes and

hand spray techniques may also be used.

Single wall insulation systems should be inspected at least once a year. Inspection and

maintenance for single wall insulation system consists of inspecting the jacket seams andthe jacket to foundation connection to assure that all fasteners are snug and the jointsclosed. Open areas should be recaulked and pulled tight.

DIKING SYSTEM

The American National Standard, "Safety Requirements for the Storage and Handling of

Anhydrous Ammonia", (ANSI K61.1-1972) stipulates that dikes or drainage be provided toprevent accidentally discharged liquid from spreading to uncontrolled areas.

Secondary containment should be provided to satisfy specific project needs for each

project. A seeondary containment system should be selected which will meet the needsfor limiting the hazards, but which also considers operator safety, plant operation andconvenience of inspection and maintenance. Design of the seeondary containment system

will be influenced by such characteristics of the site as its dimensions, topography, soil

conditions, ground water conditions and the use and occupation of adjacent and nearbyproperties and waterways.

The following four types of secondary containment systems are commonly used:

1. Remote impounding basins serving one or more tanks.

2. Earth dike or low wall impounding system surrounding one or more tanks.

3. Non-integrated, high wall impoundment around a single tank. The tank andcontainment systems are structurally independent.

4. Integrated high wall impoundment incorporated into a single tank. The tank andcontainment system are not structurally independent.

The remote impounding basin utilizes the natural topography augmented as necessary byexcavation and diking. Dikes, and channels around the tanks direct the flow to the

impounding basin with a minimum exposure to other tanks and surrounding facilities.

Earth diking surrounding one or more storage tanks is the most common type of secondary

containment system used (Figure 4). Dike heights usually range between three to twelvef eet. The clearance from dike to tank provides for access to the tank and space to locate

equipment near the tank. The floor of the impounding area is sloped or contoured todirect spilied product away from the tank to a sump or low area. The dikes are normally

eonstrueted of compacted earth but reinforced concrete can also be used or combinedwith earth.

Non-integrated high impounding walls structurally independent of the tank may be of low

temperature steel, reinforced concrete or prestressed concrete construction (Figure 5).With the wall close to the tank, it is important that the structural independance of tankand wall including independence of foundation support should be maintained. If pumps are

located outside the wall, the wall is penetrated by pump suction line and the pipe forms

part of the secondary liquid boundary. In such a case a remotely controlled shutoff valveshould be provided inside the dike and also outside the dike wall. Also a low dike

encompassing the pipe and the pump should be provided as well as other safety features to

minimize the potential hazards.

An integrated containment concept (Figure 6) has the appearance of doublé wallconstruction. Both inner and outer tank walls and bottoms are made of low temperaturematerials. The primary and secondary containments are supported by the samefoundation. Piping penetration through the primary and secondary containers is necessaryif pumps are located outside the tank. Foundation failure and pipe break are credible

events. To contain product spilied as a result of these events, a supplemental dikesurrounding the integrated containment system is desirable.

PROCEDURE TO PLACE AMMONIA TANK IN SERVICE

The suggested procedure described here is very generalized in nature. There are many

details and considerations which vary with each individual project and should beconsidered for placing ammonia tank in or out of service. The following procedure

assumes that the tank is at ambient temperature and contains air.

(1) Nitrogen Purge

Air and ammonia gas mixed in the ratio of 15.5 to 26.6 percent ammonia in air create aflammable mixture. The hazard of fire or explosion can be reduced in a tank being purged

with ammonia by decreasing the oxygen content to a safe concentration before admittingthe ammonia. The recommended purging end point is 12.5% oxygen by volume (this

includes a 20% saf ety factor) when nitrogen is used as a purging gas. (D

Nitrogen purge gas normally enters the tank at the bottom and flows up through the

suspended deck vents and into the vapor space in the dome. The purge gas is exhaustedthrough the relief vents and through the dome vents.

During intial purge the flow of N2 in the tank should be kept at a reduced rate tominimize the mixing. In actual practice displacement purge is difficult to achieve and

some mixing takes place. Experience indicates that nitrogen required to purge the

ammonia tank is about l i times the tank volume.

(2) Ammonia Purge

For the ammonia purge of the tank all venting should be done through a flare, if available,

otherwise exhaust gas should be vented at a safe location. Purging is normallyaceomplished by flowing ammonia vapor into the top of the storage tank. The warmammonia vapor being lighter than nitrogen, will form a pocket in the top of the tank. The

purge is intended to slowly piston the nitrogen out through the bottom nozzle and out tothe flare or vent stack. Since pistoning out nitrogen is quite effective, purging shouldcontinue until vapor composition in the tank is higher than 95% ammonia by volume.

(3) Tank Cooldown

Prior to establishing a high fill rate of cold ammonia liquid, the tank should be cooleddown gradually to avoid high thermal stresses. Admission of cold liquid to a tank bottom

may cause buckling of the bottom due to thermal stresses between the local area which iscooled and the remainder of the bottom. The local buckling could cause deformation orpossible damage to the bottom plates. It is recommended that the cooldown nozzle forammonia be located at the top and the location of the nozzle be such that severe local

cooling does not occur. The cool-down nozzle should be provided with spray ring or splashcap for uniform liquid distribution. The cooldown line should be used for liquid service

during initial cooldown only. Cooldown should be considered complete when liquid startsaccumulating at the bottom.

PROCEDÜEE TO REMOVE AMMONIA TANK OUT OF SERVICE

The following procedure assumes that the tank contains cold liquid ammonia.

(1) Liquid Removal

As mueh liquid ammonia as possible should be removed by pumping out of the

storage tank. It may not be possible to completely empty the contents of the tankby pumping. Residual ammonia liquid can be removed by injecting warm ammoniagas to vaporize the remaining liquid in the tank.

(2) Tank Warm-Up

Warm ammonia gas should be introdueed into the bottom of the tank through the

pump suction or other line. The purge gas and vaporized ammonia is dischargedfrom the top of the tank through the vapor withdrawal line to the flare or to theammonia refrigeration or processing plant. Caution must be exercised inintroducing the warm gas so that the tank design pressure is not exceeded andexcessive temperature differences are not created in the tank bottom. The warmgas flow should continue until the tank bottom temperature is above 32<>F (0<>C).

(3) Nitrogen Purge

Dry nitrogen gas should be introduced into the bottom of the tank through pumpsuction line and discharge the effluent from the top of tank through the vapor

withdrawal line to flare or a safe vent point. Nitrogen purge should continue untilthe ammonia concentration is bèlow 15%. The vent nozzle on the dome should be

opened and purging should be continued.

(4) Air Purge

After the nitrogen purge has been completed and tank shell temperature is abovethe dew point of air, fans should be connected to the pump suction nozzle and relief

valve nozzle. Make sur e that the tank is isolated from any ammonia source byphysically disconnecting all process piping. The roof vents should be open wide and

roof manhole should be open part way. The fans should be turned on to force airinto the tank. The exhaust gas from the tank should be monitored to determineconcentration of ammonia and nitrogen in the air being exhausted. When it isdetermined that it is safe to enter the tank, a pressurized air source should be

connected to the purge vent on dome of the roof. The air source should be able toprovide one air change per hour in the dome area to remove a possible build-up ofgas.

(5) Inspection

After the inspection and/or repair tank should be placed in service following the

procedure described in previous section.

BÏBLIOGRAPHY

(1) "Purging Principles and Practice", American Gas Association, 1975.

(2) D. M. Morrison and J. J. Aarts, "Refrigerated Storage Tank Retainment Walls",Presented at the Safety Symposium for Ammonia Plants, Portland, Orgeon, August

17-20, 1980.

(3) C. C. Hanke Jr., I. V. LaFave, and L. F. Litzinger, "Purging LNG Tanks into and outof Service Considerations and Experience". AG A Distribution Conference, May 6-8,1974.

(4) "Low Temperature Storage", CBI Bulletin No. 8400.

INNER KOOF

LOOSi F1LLPERLITE

INSUtATION

. RESiliENT3LANKET

fNNIR TANK

OUTEB TANK

ANCHOiAGl

CONCSETitlNGWALL

NDATIONLOAD BEARING

INSULAÏIONHEATING COILS

IN SANOOUTE1 STIEL

BO' iOTfOM

Figöre 1The Doublé Wall Wlth The

, Corswentional Inner Tank

SUSPËNDEDi NSU LATINGDECK

LOOSE F1LLPERLITEINSULATION

EARTHEM-ADiKE

SELF-SUPPORTINGTANK ROOF STA1R AMD PLATFORM

INNER STEELTANK "GOLD"

OUTER STEELTANK"WARM"

R ESI LI ENTBLANKET

INNER STEELBOTTOD"GOLD"

ANCHORAGE

CONCRETERINGWALL FOUNDATION

HEATtNG.COILS

LOAD BE AR INGINSULATION

OUTER STEELBOTTOIVS

Figyre 2Doublé Wali Tank Wlth Sospended losylatiog Deck

SUSPENDEDSNSULATiNGDECK

POLYURETHANEINSULATtONBONDED TO METALJACKET AND TANK WALL

METAL JACKET

ANCHORAGE

EARTHEN _\DiKE

SELFSUPPORTINGTANK ROOF

STA! R PLATFORMAND PIPING TOWER

STEELTANKGOLD"

CONCRETERING WALL FOUNDATION

HEATINGCOILS

LOAD BEARINGINSULATiON

STEEL BOTTOM"COLD"

Figure 3Single Wal! Tank

4

SELF-SUPPORTINGTANK ROOF

STAIR PLATFORMAND PSPING TOWER

SUSPENDEDSNSULATINGDECK

Al R SPACE

PRESTRESSEDCONCRETEDIKE

POLYURETHANEINSULATIONBONDED TO METALJACKET AND TANK WALL

METAL JACKET

ANCHORAGE

CONCRETERINGWALL FOUNDATION

STEEL TANK"GOLD"

__HEATINGCOILS

LOAD BE AR ING STEEL BOTTOfVtSNSULATION "COLD"

Figure 5Singie WaS! Tank With Concrete Dike

SUSPENDEDINSULATJNGDECK

POLYURETHANE SNSULATIONBONDED TO METAL JACKETAND TANK WALL

SELF-SUPPORTINGTANK ROOF

\

STASR PLATFORMAND PiPING TOWER

INNER STEELANK "COLD"

OUTER STEELTANK "COLD"

VAPOR SPACE

METAL JACKET

ANCHORAGEINNER STEELBOTTOM

CONCRETESEPARATION

SLAB

EARTHENJ CONCRETEDIKE RINGWALL FOUNDATION

HEATING.COILS

LOAD BEARSNGINSTALLATION

OUTER STEELBOTTOM "COLD"

Flgure 6Entegrated Low Temperature Tank