major ammonia leak incident in urea plant/fileser… · ammonia receiver (v-105). later by using an...

2011 [1] AMMONIA TECHNICAL MANUAL

Major Ammonia leak incident in Urea plant

Oman India Fertiliser Company was commissioned in April-2005. The performance in the first five

commercial years of operation has been very good and all the envisaged project targets have been

accomplished. Meticulous Root Cause Analysis of the problems surfaced at various stages have been

analysed by the OMIFCO engineers in close association with the vendors and the consultants and

implementation of the identified corrective measures in the shortest possible time has been the key to

the achieved success of the company.

The paper highlights about a critical Ammonia leak incident that occurred in the Urea 21 plant and

how the situation was analysed to prevent re-occurrence of similar incidents. In chemical Industry,

Careful Root Cause Analysis of the problems is very important and complacency cannot be ensured

unless the remedies identified are convincing and consistent with the findings of the Root Cause

Analysis made.

S G Gedigeri

Oman India Fertiliser Company

Introduction

MAN INDIA FERTILIZER

COMPANY S.A.O.C. (OMIFCO) was

set up as a joint venture project under

the initiative of Government of Sultanate of

Oman and Government of India. OMIFCO is

owned 50% by Oman Oil Company, 25% by

Indian Farmers Fertilizer Co-Operative Ltd

(IFFCO) and 25% by Krishak Bharati Co-

Operative Ltd (KRIBHCO). OMIFCO was

registered in the Sultanate of Oman as a closed

joint stock company in the year 2000.

The Ammonia Urea complex comprises two

trains, each with a design capacity of 1750

MTPD Ammonia and 2530 MTPD granulated

Urea, along with all supporting Utilities. It is

designed to produce total 1.65 million tonnes of

granulated Urea and 0.25 million tonnes of

surplus liquid ammonia annually for export,

using natural gas. Storage facilities for Urea (2X

75000 MT) and Ammonia (2X30000 MT) as

well as jetty with ship loaders are part of the

project.

Photo-01: OMIFCO Complex-Sur, Oman

O

12011 AMMONIA TECHNICAL MANUAL


The complex has two service Boilers of capacity

2 X 70 MT/hr and two HRSG boilers of

capacity 2 X 110 MT/hr. Also the complex has

its own captive power plant with two 30 MW

Frame 6B Gas Turbine Generators and Import

power connectivity with the national grid for

backup power.

Description of the System:

OMIFCO owns and operates two Urea Plants

designed by M/S SAIPEM, Italy (Previously

Snamprogetti). The Urea manufacturing

technology used at OMIFCO is based on the

Ammonia Stripping Process.

As per the process steps involved for producing

Urea, feed Ammonia is fed to the reactor at

elevated pressure. In the Urea plant the liquid

Ammonia at about 15°.C (59°.F) and 23.5 Bar G

(340.84 psig) pressure is initially received in an

Ammonia receiver (V-105). Later by using an

Ammonia booster pump (P-105-A/B), liquid

Ammonia is pumped to the suction of the high

pressure Ammonia feed pumps (P-101-A/B).

The high pressure feed Ammonia pump

transfers liquid ammonia to the Urea reactor

through an ejector where the liquid Ammonia

acts as a propellant for the high pressure

Ammonium carbamate solution. (Figure-01)

Liquid Ammonia from

Ammonia Plant

Ammonia

Receiver-V-105Ammonia Booster

Pump P-105

High Pressure

Ammonia Pump P-101Recycle Ammonium

Carbamate solution

SCHEMATIC DIAGRAM FOR

THE INCIDENT LOCATION

CO2

To HP StripperFailure Location

Figure: 01

21-HV-1021

Location of the failed pipe:

The location of the failed weld joint was on the

pipe rack in the Urea-21 unit of the OMIFCO

complex. The pipeline going from the Ammonia

booster pumps common discharge to the

common suction of the high pressure Ammonia

feed pumps has several weld joints. One of the

weld joints nearer to the location where booster

pumps common discharge pipe line joins the

pipe rack had developed leakage during the

normal running of the plant and later failed

subsequently.

Incident Scenario of Ammonia Leakage:

In Urea-21 unit on 17th January 2008 at 04.40

hours the weld joint on the common suction line

of HP Ammonia feed pumps P-101 A/B had

failed and caused heavy Ammonia leak.

Immediately the plant emergency shutdown was

initiated and the affected system was isolated

and depressurized. Control valve 21-HV-1021,

located at the outlet of Ammonia receiver V-

105, on the common pipeline going to the

suction of P-105-A/B pumps was closed from

2 2011AMMONIA TECHNICAL MANUAL


the central control room to isolate large

Ammonia hold up equipment like Ammonia

receiver V-105, MP Absorber C-101, and

Ammonia condenser E-109 from the snapped

off pipeline. This had helped in significantly

reducing the Ammonia quantity leaked into the

Atmosphere. A Level-1 emergency was

declared and all the affected area was cordoned

off and cleared of any workforce present in that

vicinity. As all the working staff moved to a

safe location nobody had received any

Ammonia exposure.

OMIFCO’S HSE (Health, Safety &

Environment) team and the Production staff

controlled and confined the leakage to within

the plant battery limits by using their own fire

tender and the personnel protection appliances.

On scrutiny it was found that the elbow to pipe

spool piece of the line 8”-21-P57-53A-V on the

North-South pipe rack of Urea-21 plant had

snapped off. The plant was back on stream on

19th January 2008 at 10.50 hours after the repair

and rehabilitation of the failed pipe line [Photo-

05].

Pre failure history of the affected pipe:

One of the weld joints, on the elbow to pipe

spool piece of the line 8”-21-P57-53A-V on the

North-South pipe rack leaked at the 4 o’clock

position. In the first week of Sept. 2007 the

leaky joint was sealed on line by means of

clamp [Photo-02]. It again started leaking

through the clamp on 4th Jan 2008 and an on

line sealing compound was re-injected to stop

the leakage. However, on 17th Jan 2008 morning

hours this joint fully snapped off from the weld

joint leading to leakage of substantial amount of

ammonia [ Photos-03 &-04].

Photo-02: Weld joint with Sealing Clamp.

Photo-03: Counterparts of Off-centred pipe.

Photo-04: Side view of the snapped pipe.



Photo-05: Pipeline view after rehabilitation.

Detailed specification of the pipe:

Line Number: 8”-21-P57-53A-V

Service Class: 53A (Mean Pressure

Ammonia - Ammonia

(First & Second ST) –

Liquid Ammonia (With

Stress Relieving)

Pipe Material: A333 Gr.6 SMLS

(0.3 C% & 1% Mn.)

Normalized & tempered

Steel

Design: Line Size 8” (203.2 mm)

Sch. 20 (6.35 mm Thick)

Temp. & Pressure: (-) 45 O C (- 49 °F) and

43 bar, (623.66 psi)

Max Hydro Test 64.6 Bar. Abs (936.94 psi)

Pressure.

Corrosion Allowance: 1.27 mm (internal)

(0.05 Inch)

WPS: SPC.WL.ES.360

Welding Class: 3T

PWHT: Yes, (All Thickness

Require PWHT for this

Piping Class.)

As per ASME Sect-II Part ‘A and ASME Sec

IX for above grade material:

Investigation and observations:

This particular butt weld joint which had failed

circumferentially was in operation for a period

of 39 months, before its failure.

After the failure and after removing the on line

sealing clamp in the Visual examination the

following were observed:

•••• Complete snapping of the pipe at the failure

location from the weld joint.

•••• The weld joint failed circumferentially

through the butt weld only and not cracked

from the parent metal of elbow / pipe.

•••• Rust marks were seen in the area adjacent to

the weld of painted pipeline. (Refer Photos

03 & 04)

•••• ID surface of the pipe looked blackish

brown indicating a general form of

corrosion. (Photo: 03)

•••• Fractured surface was flat and in transverse

direction. At one location weld material had

peeled off.

•••• The counterparts of the failed joint displaced

off center.

•••• Circumferential corrosion damage is seen

near to the failure location where the paint

peeled off precisely at the region of clamp.

•••• Fracture occurred perpendicular to

longitudinal direction and in a single plane

from the weld / HAZ region.

•••• Fracture surface comprised of both fatigue

and brittle nature. (Photos: 06 & 07).

•••• Low magnification view confirms cracking

both from OD and ID and in the weld zone

near weld HAZ (Photo: 06)

•••• SEM (Scanning Electron Microscopy)

analysis confirmed that the cracking is a

typical fatigue damage starting both from ID

and OD and assisted by mild corrosive

atmosphere. (Photo: 06)

Failed Weld Joint



Photo-06: SEM Result showing Crack initiation &

propagation under the combined influence of

corrosion and fatigue.

• Crack movement was under the combined

influence of high cycle fatigue and corrosion

mode prevailed due to the marine

environment.

• Secondary cracks of corrosion fatigue were

also noticed on the fracture surfaces along

with corrosion products. (Photo:09)

• EDS (Energy Dispersive Spectrum) Analysis

confirmed that the corrosion damage on

fracture surface was due to salt containing

moisture from marine atmosphere. (Figure:

02)

• Overall microstructure of weld metal, HAZ

and parent metal was normal and acceptable

for the service. Pitting like corrosion damage

at OD surface was also seen. (Photos: 11 &

12)

Repair and rehabilitation carried out:

The pipe spool piece was cut from the other end

also which was welded to the downstream side

elbow. Both the elbows ends bevel edges were

prepared by means of in-situ grinding. A new

pipe spool piece was cut from the pipe of same

size and same LTCS material grade available in

warehouse.

The bevel edge preparation at both the ends of

the pipe spool piece was done as per the

approved WPS for proper fit up. After proper fit

up of both the elbow to pipe spool piece butt

weld joints for the root and fill for both the

joints was carried out by the TIG welding

process (GTAW) using 2.4 mm (0.094 Inch)

dia. filler wire ER 70S-6 as per the Welding

Procedure Specification (WPS) approved for the

given Pipe Service Class and Welding Class.

During the welding process an inter pass

temperature of 140 oC (284 °F) was maintained

by means of digital thermometer.

Dye Penetrant tests of both the root weld and

final weld joints were carried out. No defect was

observed.

Hardness of newly made 02 nos. elbows to pipe

weld joints was also checked on the weld, heat

affected zone (HAZ) and the parent metal in the

near vicinity and found to be within the

acceptable limit of 225 BHN.

Insitu Radiography of both the butt weld joint

was carried out using IR 192 source of four Ci

strength. No significant defect was observed.

Stress Relieving (PWHT) of both the weld

joints was carried out subsequently as per the

SR Cycle prescribed in the approved WPS as

mentioned below.

PWHT:

Heating: @ 150 °C (302 °F)/ Hr after

200 °C (392 °F)

Holding Time: 2.4 Minutes/mm of

thickness @ 600 to 630 °C

(1112 to 1166 °F)

(01 Hr Min. in this case)

Cooling: @ 150 °C (302 °F)/ Hr in air to a

temperature of 300 °C (572 °F).

Subsequently natural cooling in wrapped condition.



Hardness: Hardness measurement at the weld,

pipe and HAZ area after the PWHT was again

carried out and found to be within the

recommended limit of 225 BHN.

Non Destructive Tests carried out:

•••• Tensile test was carried out on the sample

and the results met the requirements of

ASTM A 333 Gr.6 material for tensile

properties.

•••• V notch charpy Impact test was carried out

on the sample drawn at -56.0°.C. (-68.8°F)

Impact test results met the requirement of

ASTM A 333 Gr.6 material.

•••• General hardness was measured at different

locations and results were found to be

matching with the requirements of ASTM A

333 Gr.6 material.

Causes of Failure:

•••• The pipe is exposed to sea water atmosphere

and the paint had peeled off at some places

near welding and corrosion has taken place.

•••• The failure had taken place on account of

fatigue cracking assisted by mild corrosion

initiated at weld and HAZ both from OD and

ID. Fatigue was due to cyclic load caused by

unexpected shutdowns, start-ups, pump

change overs and vibrations on the pipe.

• The prevailing marine atmosphere, the

vibration stresses from the pipe line while in

operation and the surface corrosion that acted

as stress concentration site, had resulted in

corrosion fatigue.

• Low Magnification Examination confirmed

multiple origins of the cracks all over the

circumference starting from both OD as well

as ID. (Photo: 07)

Photo-07: Low Magnification View at the fractured

Surface. Multiple origins observed from OD & ID

surfaces.

Photo-08: Low magnification (33X) shows multiple

origins in the form of ratchet marks.

Photo-09: Presence of corrosion products observed

at the cracked surface.



•••• Optical metallography indicated pitting like

damage on the OD surface of the pipe.

(Photo: 10)

Photo-10: Corrosion entering metal due to

pitting like surface damage at weld location

•••• Both SEM (Scanning Electron Microscopy)

Analysis and Optical Metallography

confirmed the failure to be on account of

high cycle fatigue arising out of system’s

cyclic operations and piping vibrations,

assisted by corrosion.

•••• The cracking has taken place in the weld and

HAZ zone as seen in WFMPI (Wet

Fluorescent Magnetic Particle Inspection)

and Optical Metallography. (Photos 11, 12,

and 13)

•••• All the cracks are filled with corrosion

products. Even the secondary cracks seen on

the fracture surface were having corrosion

fatigue nature. ( Photo-09)

•••• EDS (Energy Dispersive Spectrum) analysis

confirmed that the corrosion damage on

fracture surface was due to salt containing

moisture from marine environment. (Figure-

02)

Photo-11: Indicates that fracture has taken

place at the fusion zone and perhaps moved

into the weld.

Photo-12: Corrosion fatigue crack moving

precisely along the fusion zone of the weld.

Photo-13: WFMPI revealed Secondary

transverse hairline cracks at HAZ on the weld

ID surface.



Figure-02: Results of Energy Dispersive Spectrum.

Elements Percentage

Present Elements

Percentage

Present

Oxygen 37.22 Chlorine 1.07

Sodium 1.31 Calcium 0.87

Magnesium 0.87 Iron 57.74

Silicon 0.93

•••• Presence of Oxygen, Sodium, Magnesium,

Chlorine and Calcium confirms that the

corrosion is induced by the Salty marine

atmosphere.

•••• HAZ microstructure shows slight inclination

towards having some internal stresses which

is reflected from micro-hardness values.

•••• The surface corrosion in form of tiny pits has

provided favorable sites for stress

concentration to take place both from OD and

ID

•••• Micro Structure examination has also

confirmed corrosion fatigue cracking having

trans granular nature of propagation.

(Photo:14)

•••• Matrix has banded ferrite and pearlite with

mild branching nature. (Photo:14)

Photo-14: Corrosion Fatigue Cracking-showing

trans granular nature of propagation. Matrix is

banded ferrite and pearlite.

• Crack observed adjacent to fracture surface

on pipe indicating another origin of fatigue

crack.

• The humid salty marine atmosphere

provided the climate for corrosion to take

place.



•••• Pipe has been supported especially at elbow

joint where the change in the direction of the

Liquid Ammonia takes place.

•••• The prevailing vibrations on the pipe line

induced the fluctuating stresses.

Thus, naturally the most vulnerable site for

corrosion fatigue damage to take place is weld

and HAZ.

Micro Hardness Profile did not indicate any

severe abnormality. Values at HAZ and Weld

were slightly on higher side (Refer Figure-03).

Figure-03: Micro Hardness Profile Diagram.

Most Probable Cause of failure:

•••• This failure might be due to corrosion,

material defect and internal stresses owing to

welding joint.

•••• The stresses might have developed on

welding due to injection of sealing compound

and clamping

Corrective Measures taken:

•••• Flexibility and stress analysis of the pipe

line from Ammonia receiver (V-105) to the

Ammonia Booster pumps (P-105-A/B) and

from the discharge of the Ammonia booster

pumps to the suction of the HP Ammonia

Pumps (P-101A/B) was carried out

separately by a reputed local consultant and

a foreign consultant of International repute.

•••• The reports indicated no over stressing at

any point in the pipe line and no alarming

forces and moments are noticed at the

restrained points.

•••• Reports also indicated that stresses are

within the allowable limits and the failure

of pipe weld joint is not due to

overstressing.

•••• WFMPI (Wet Fluorescent Magnetic

Particle Inspection), testing and Insitu

metallography of all other weld joints on

the affected pipe line were examined and

those weld joints which did not pass all the

tests were repaired. Radiography was

carried out for checking the ID cracking.

•••• One new On-Off type control valve HV-

1013 has been installed on C-101 MP

Absorber outlet Ammonia vapours line in

both the Urea plants. This will facilitate

quick isolation of C-101 in the event of any

leakage (Figure-04).


C-101

MP Absorber

E-109

Ammonia Condenser

CW

P-101

H.P Ammonia feed Pump

To Urea

Reactor R-101Ammonia

Booster Pump

P-105

Condenate

LP Steam

P-102

Carbamate Recycle Pump

Ammonia

Receiver

E-111

Ammonia from B/L

V-105

C-105

HP Stripper Over head Vapors

From MP

Condneser

21-HIC-1013

Location of new ON/OFF

control valve

Carbamate

Condenser

V-101

Carbamate Separator

To MP Decomposer

or Flare

Cold

Condneate

To flare

Figure-04: NEW CONTROL VALVE AT THE VAPORS OUTLET OF C-101

•••• New on line Breathing Air sets with

extendable hose of 50 meters length have

been provided inside the plant field area to

facilitate quick isolation in the event of

Ammonia leaks.

•••• Care is being taken for the quality of the

painting on the pipe surface particularly

near weld joints to avert peeling of paint

and subsequent underneath corrosion

damage.

Conclusion:

•••• Corrosion-fatigue is the result of the

combined action of an alternating or cycling

stresses and a corrosive environment.

•••• On line sealing of leakages either from pipes

or weld joints should be avoided which

otherwise might cause inadvertent additional

stresses, to the component which has been

already under the influence of some parental

residual stresses.

•••• Contrary to a pure mechanical fatigue, there

is no fatigue limit load in corrosion-assisted

fatigue.

•••• Much lower failure stresses and much

shorter failure times can occur in a corrosive

environment compared to the situation

where the alternating stress is in a non-

corrosive environment.

•••• OMIFCO has decided not to do online

sealing for any developed leakages of

Ammonia pipe lines.


major ammonia leak incident in urea plant/fileser… · ammonia receiver (v-105). later by using an...

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