Plant Preservation: Issues & Remedies/fileser… · plant re-start had to be delayed due to some issues faced related to plant preservation. Background Plant site of EFERT is divided

Plant Preservation: Issues & Remedies

The document enumerates the operational experience of Engro Fertilizers Limited, at one of the world scale Ammonia plant. Detailed analysis of the issues faced during plant re-startup after a shutdown of about 7 months are presented. Major portion of the paper encompasses the experiences, problems encountered and key lessons.

Asim Rasheed Qureshi Engro Fertilizers Limited Pakistan

Syed Ali Raza Sani Engro Fertilizers Limited Pakistan

Introduction

he Chevron Chemical Corporation Ammonia Plant was installed in 1967 in Pascagoula USA, being one of the largest ammonia plants in the world at

that time. In 1992, the ammonia plant was relocated to Pakistan by Engro Chemicals Pakistan Limited which was renamed later as Engro Fertilizers Limited (EFERT). Several de-bottlenecking projects entailed increase in plant capacity from 1360 MTPD to 1650 MTPD. Commissioning of EnVen (Plant-2) in 2011, being the world’s largest single train urea plant at that time, had made EFERT the fifth largest urea producing site of the world. After safe and smooth operation of Plant-1 of about 20 years, the plant was shut down for a period of 7 months, in October 2012, due to scarcity of natural gas in the country. In April 2013, natural gas supply was restored to the plant; however plant re-start had to be delayed due to some issues faced related to plant preservation.

Background

Plant site of EFERT is divided into two parts, Plant-1 (1650 MTD Ammonia & 2850 MTD UREA) and Plant-2 (2194 MTD Ammonia & 3835 MTD UREA). Plant-1 consists of two urea plants, two utilities unit and one ammonia plant fulfilling the raw material requirement of the two urea plants. Due to the gas curtailment issue prevailing in the country, it was decided to run Plant-2, being more energy efficient, at maximum load and to keep Plant-1 shutdown till further gas availability which was quite uncertain.

EFERT, being the pioneer in implementing vigorous safety systems had developed stringent procedures to preserve the site in mothballed condition. In pursuit of achieving world class safety standards, EFERT has aligned itself with DuPont safety system. The preservation procedures were made keeping in view the standard guidelines as well as plant’s operational experience.

T

2372014 AMMONIA TECHNICAL MANUAL

After Plant-1 was shut down, a slight delay in preservation occurred due to the uncertainty of gas resumption. Plant-1 was preserved keeping in view the EFERT’s preservation guidelines. Plant-1 was being preserved for such a long duration (7 months) for the first time after being commissioned in 1992. Steam and condensate headers were depressurized through low point drains. The reactors were properly blinded and kept under positive nitrogen pressure. Cooling water headers were preserved following a wet passivation methodology. Synthesis compressor and Refrigeration compressor trains were kept under positive nitrogen pressure and as per EFERT’s preservation guidelines, it was planned to manually rotate them at 180 degrees at a period of 15 days. The utility boilers were preserved using the wet lay-up procedure.

Despite of adhering to the developed standards and procedures, EFERT faced several issues at the time of startup. These issues are discussed below.

Figure 1. Process air compressor

Heavy corrosion in ammonia plant process air compressor piping

The process air compressor (KGT-2501) provides process air for the secondary reforming and a source of nitrogen for the ammonia synthesis process. It also provides plant and instrument air supply through its third and fourth stage’s suction. Figure 1 shows process air flow through the compressor. Refer to table 1 for air compressor (KGT-2501) parameters.

Observations

In order to prepare the machine for startup after a seven months long shutdown, ratcheting of the turbo train was initiated. However, it was unable to rotate the machine. Turbo train components were then dis-coupled in order to separate Gas Turbine / LP Case Compressor / HP Case Compressor. Upon taking manual rotation steps this time, GT rotor was successfully rotated. However, LP case could not be moved.

238 2014AMMONIA TECHNICAL MANUAL

The Air Machine’s rotor jamming problem resulted in approximately 72 hours delay in plant startup.

Table 1. Air machine Parameter

Root cause & lesson learnt

Table 2 shows sequence of events related to air compressor piping corrosion. The main reason for stuck up of the LP case was corrosion particles which fell into the LP case

S. # Date Event

1 April 11,

2013 Ammonia-2 Plant Startup activities initiated.

2 April 14,

2013

KGT-2501 (air Machine gas turbine) coupling was removed for slow-rolling; however the compressor did not rotate. Compressor side coupling was removed and attempts were made to rotate it but no success was achieved.

3 April 15,

2013

LP & HP casing of compressor were dis-coupled along with the gas turbine in order for the machine to rotate freely but the machine did not rotate.

4 April 16,

2013

LP & HP casings were removed along with suction & discharge piping and thorough cleaning of

all machine parts/rotor were performed.

5 April 17,

2013

The rotor jamming issue was resolved and machine was put on slow roll.

Table 2. K-2501 Sequence of events

From compressor discharge piping. Erosion marks on the impellers and inlet guide vanes (IGVs) are due to water ingress from 3rd stage knock out (KO) drum.

A rundown tank installation project was in progress, providing lube oil to the machine in case of lube oil failure. It was due to this project that lube oil connections for the rotor bearing had been disconnected. After the run down tank’s installation, flushing of lube oil circuit kept continued through flushing oil. Since the lube oil connections for the rotor bearings had been disconnected, the rotor couldn’t be manually barred over as per EFERT’s preservation guide lines which dictate that the gear boxes having no forced feed lubrication should be rotated 180 degrees on a 15 days period. The machine was not kept under positive nitrogen pressure as it was unforeseen that corrosion would take place after properly draining, depressurization and drying.

Due to uncertainty of gas restoration in order to start Plant-1 again, air compressor casing was not preserved with nitrogen and blinds were not installed at immediate flanges

The breath- in & breath-out phenomenon created ways for moisture deposition inside machine’s housing. Thus the process of corrosion started and resulted in flakes formation which became attached with the rotor clearance and moving parts, preventing the rotor from rotating during machine’s slow rolling and ratcheting. Figure 2 and 3 shows pictorial observations related to the LP and HP casing corrosion.

KGT-2501 Parameters

Load 17326 HP

(12.92 MW) Normal Speed 5100 RPM Axial Air Compressor Stages 16

Air Inlet Temperature 115 °F (46 °C)

Air Inlet Pressure 14.4 psi

(99.28 KPa) Turbine Exhaust Temperature

1000 °F (538 °C)

Turbine Exhaust Pressure 14.7 psi

(101.35 KPa)

Air Flow 598,000 lbs/hr (271248 kg/hr)


Figure 2. Corroded LP Casing

Figure 3. Corroded HP casing

Also the analysis of corrosion in discharge piping only indicated that operating the discharge piping at higher temperature (>100°C) for more than 45 years has set-in a phenomenon

known as microstructural degradation / carburization.

During the rotor jamming issue, attempts were made to freely rotate it by filling the compressor


casing with water while steam was crack opened in it but no positive results were obtained. LP & HP casings were then removed along with suction & discharge piping and thorough cleaning of all machine parts/rotor were performed. After cleaning the machine was boxed up and put on slow roll.

Heavy corrosion in CO2 absorber bottom & rich solution flash vessel

A flow diagram of ENGRO’s CATACARB facility is shown in figure 7. The solution itself consists of a mixture of potassium carbonate in water, to which an activator (DEA plus boric acid) and a corrosion inhibitor (vanadium) have been added. Vanadium is kept in the active, corrosion inhibiting pentavalent (V5), rather than in the inactive quadravalent (V4) state through the continuous injection of a small air flow into the circulating solution. A mechanical filter and an activated carbon drum are used to clean up the solution.

History of absorber (C-2519):

The absorber tower is a completely stress relieved carbon steel vessel. The only stainless steel parts are the gas distributor in the bottom and the wash trays in the top. The gas distributor nozzle is 304L stainless steel clad and so is the tower above the upper bed.

The bottom section contains two large beds. 24 and 12 feet of pall ring packing is loaded respectively in the two beds. The first two feet are occupied by SS-304 and the remaining volume is filled with carbon steel packing. The top section contains three smaller beds in which the top two feet of each bed is occupied with SS pall rings and the remaining volume is filled with carbon steel packing.

Frequent problem of dislodging and damage of trays had been significantly improved after 1998 modifications (replacement of bubble cap trays with pall ring packing beds). First vessel entry

was made in April 2004 thru three man ways (M-1, M-4, and M-6, refer to figure 4.) and

Figure 4. Absorber man ways for inspection

visual inspection revealed all internals intact. Some vanadium deposits were found on top trays which were removed. Mesh was found dislodged from the center and was re-clamped with SS wires. In turnaround 2005, top two trays were inspected from M-1 only without vessel entry. No deposits on trays, loose caps, nuts/bolts, unusual signs of breakage/corrosion/damage were observed. No inspection was done on other man ways. In turnaround 2009, vessel entry was made thru all man ways (M1, M2, M3, M4, M5 & M6). Wire mesh and support plate holes found choked (40-50%) in all man-ways, which were cleaned. Distributors, bubble caps trays, gratings, channels and support rings of beds were found in healthy condition. Detailed inspection of bottom gas risers/distributors was not done by installing scaffolding. However visual inspection from M6 was ok.

Observations

During routine inspection of Absorber (C-2519), while the plant was shut down, it was found that the vapor distributor and support plate were heavily corroded as shown in the figure 6. Heavy deposition of CATACARB was also found near M-2. Severe corrosion was also observed in rich solution flash vessel (HP flash vessel).


Figure 5. CATACARB depositions.

Figure 6. Vapor distributor and support plate damage


Due to gas curtailment, plant remained shut down for almost 7 months (Oct 2012 to Apr-2013). First startup was in April 2013; since then we had started/shut down the plant for 3 times. All three times the plant remained in operation for around 10 days at reduced load i.e. 80~100% versus design of 150%. These

frequent startup and shutdown jerks proved to be quite detrimental for the absorber due to back flow of process gas upon vent valve opening.

Although DEA / HCOOK (Potassium Formate) / Bicarbonate concentration was found in the normal range, increasing their concentration results in an increase in overall specific gravity. At specific density above 1.32Kg/l catacarb system becomes extremely corrosive. A low steam to gas ratio project was performed in year 2008 – 2009 for energy conservation of ammonia plant. It resulted in a gradual increase in specific density of the system to a maximum value of 1.34Kg/l. As a result of low steam to gas ration, DEA had to be increased in the system in order to limit the CO2 slip from absorber. With the increase in specific density as mentioned, the corrosion phenomenon had started in the system, with the absorber bottom bed being more prone to corrosion. Total iron in CATACARB solution (semi lean solution) showed an increasing trend from 2011-2012 (Figure 7). Increase in total iron is a direct indication of the corrosion phenomenon occurring in the system. A peak value of 178 ppm of iron was observed in the catacarb system against the maximum allowable limit of 100 ppm.

A 2” wash water line enters the top of absorber, providing wash water to the top trays which is used to remove catacarb solution mist, being carried over with the gas phase. The same line serves the purpose of flushing absorber while performing its preservation during plant shutdown. The line size proved to be inadequate as the bottom bed of absorber couldn’t get flushed properly leaving behind catacarb solution pockets that led to corrosion in absorber bed. The plant had never been preserved before for such a time period. It remained operational throughout after its commissioning and therefore the issue of inadequate wash water flow couldn’t get highlighted before.


Frequent startups/shutdowns & CATACARB solution pockets existing at the bottom of absorber (C2519) as a result of improper flushing, played a vital role in absorber’s bottom bed damage.

The packing bed was re-installed after inspection and washing whereas the bottom

support plate was replaced with the in-house modification. It was thus decided, after thorough inspection, to replace the complete bed with stainless steel and replace the in-house modified bottom support plate with the newly procured support plate.

Figure 7. Total iron in CATACARB solution.

Figure 8. Process diagram of CATACARB unit

0

20

40

60

80

100

120

140

160

180

200

Dec‐11 Feb‐12 Apr‐12 Jun‐12 Aug‐12 Sep‐12 Nov‐12 Jan‐13 Mar‐13May‐13 Jun‐13 Aug‐13 Oct‐13 Dec‐13

Total Iron Concentration (PPM)

TIME

Max. Allowable Limit


Thinning of Portions of Cooling Water Header

Water leakage was observed during routine visit of cooling tower from cooling water return header at Utility-1 Unit. During inspection of pipelines it was observed that the pipe has been severely corroded. The humid living conditions of these pipes are the perfect catalyst for the deterioration of pipes. Water in cooling towers comes from a variety of sources, and the source dictates the pH balance of the water. The damage caused by particulates becomes exasperating when the water loses its pH balance and becomes either too acidic or too alkaline. The imbalance of pH can cause either scale formation or corrosion.

The calcium or magnesium carbonate becomes deposited on pipes, creating scales. When scales build up, it can trap oxygen, causing corrosion.

Scale buildup may cause microbial growth between the scales and pipes. This can cause pipe corrosion called microbial induced corrosion (MIC) in addition to the already present scale problem.

The warm environment (not too hot/not too cold) is the perfect atmosphere for microbes to thrive. The microbial growth causes differential aeration. Without the ability to diffuse evenly, oxygen develops into a concentration of cells, causing the cell concentration to eat away at the pipe where it is trapped. Every 25-30 degree change in temperature doubles the corrosion rate.


Wet passivation of cooling water associated headers and dried depressurization was not carried out due to parallel activities around cooling tower during turnaround 2012 that resulted delayed cooling tower draining and initiated the process of active corrosion inside cooling water header.

Limitation of time for coming to plant operation further hindered the replacement of cooling water return (CWR) header. It was thus decided to install hot box-up on 150 ft long CWR header encapsulating the existing corroded and fragile metal.

Figure 9. Corroded CWR header

High iron pick-up from Utility-1 boilers

During plant shut down period, wet lay-up methodology was adopted for Utility-1 boilers preservation. According to this methodology, carbohydrazine batch was prepared in de-aerator with carbohydrazine concentration of around 1500-2000 ppm. As soon as the desired carbohydrazine concentration was reached, the batch was dosed into the steam drum of boilers via BFW header. Tri Sodium Phosphate (TSP) was added in the steam drum to maintain a pH


of 10 ~ 11. The carbohydrazine concentration in the steam drum was controlled at a value of 500-800 ppm.

The desired concentration of preservation chemicals were being monitored through weekly laboratory analysis of steam drum’s Continuous Blow down (CBD) sample. However as a cross check a sample of Intermittent Blow down (IBD) was sent for analysis and it showed high iron pick-up. Since the boiler was in a mothballed state, lack of thermo-syphon resulted in non-uniform phosphate distribution across the boiler, thereby creating ways for corrosion build-up. TSP kept on precipitating down without providing effective protection layer in the absence of thermo-syphon.

The wet lay-up methodology didn’t prove to be effective for the Utility boilers in mothballed state. As a result of this, a new approach of dry lay-up was adopted and a new nitrogen header was laid down to serve the purpose.

Dry lay-up strategy includes initially filling the boiler with hot condensate then draining it under nitrogen pressure until the boiler is completely filled with nitrogen. The boiler is then kept under positive nitrogen pressure.

Iron pick-up as high as 14.7 ppm was observed in the blow-down sample lab analysis against the maximum allowable limit of 2 ppm.

Figure 10. “A” type boiler at Utilities-1 plant

Conclusion

As a lesson-learnt from afore mentioned issues, detailed MOCs (Management of Change) protocols were conducted and guidelines were developed to avoid recurrence of such incidents in future. The preservation guidelines were revised which are described below in detail.

Literature Citation

Corrosion History of a Hot Pot CO2 Absorber. (AICHE:1987)

The Auxiliary Boiler Failures. (AICHE: 2002)

Original paper was presented in Nitrogen Syngas Symposium 2014.


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d fe

ed lu

bric

atio

n,

lube

oil

wil

l be

star

ted

for

5-10

min

utes

onc

e a

mon

th (

ensu

ring

oil

fl

owin

g fr

om th

eir

resp

ecti

ve o

utle

ts /

drai

ns).

Sm

all &

Med

ium

siz

e M

achi

ne's

G

ear

Box

-

15 D

ays

Gea

r bo

xes

havi

ng n

o fo

rced

fee

d lu

bric

atio

n sh

ould

be

rota

ted

180

degr

ees

on th

e 15

day

s fr

eque

ncy.

Sem

i lea

n so

luti

on r

ever

se p

umps

-

Mon

thly

F

or s

emi l

ean

solu

tion

rev

erse

pum

ps’

turb

ine

trai

ns (

excl

udin

g P

ump

&

PH

T),

CW

pum

ps &

BF

W p

umps

, 180

deg

rees

rot

atio

n is

sug

gest

ed o

n m

onth

ly b

asis

. Dur

ing

this

act

ivit

y lu

be o

il s

houl

d be

kep

t onl

ine.

Coo

ling

Wat

er P

umps

A

min

e C

arbo

xyla

te

Onc

e C

W p

umps

and

oth

er w

ater

pum

ps s

houl

d be

flu

shed

wit

h a

vola

tile

co

rros

ion

inhi

bito

r (a

min

e ca

rbox

ylat

e) to

pre

vent

cor

rosi

on.

Rec

ipro

cati

ng M

achi

nes

- O

nce

per

Mon

th

For

rec

ipro

cati

ng m

achi

nes,

lube

oil

sys

tem

to b

e en

ergi

zed

once

a m

onth

fo

r a

few

hou

rs


Figure 11. Sample checklist to monitor plant preservation conditions



Documents

Plant Preservation: Issues & Remedies/fileser… · plant re-start had to be delayed due to some issues faced related to plant preservation. Background Plant site of EFERT is divided