In urea plant Medium pressure Inerts gases recover HRU burner in Captive Power Plant. The inserts containing useful fuel in the form of Hydrogen & Methane about 40-45% volume percentage of total inerts. This inerts also containing Oxygen because the passivation air is given in carbon Dioxide compressor suction line for passivation of Reactor vessel and all downstream Stainless Steel vessel. In the presence of Oxygen there are chances of explosive mixture of Hydrogen and Oxygen in exit of MP section final vent line, to overcome this dangerous situation natural gases are added in the MP section before MP condenser so that the range of explosive can be avoided. These gases Hydrogen & Methane come with carbon Dioxide gas from Ammonia plant, carbon Dioxide about 0.5-0.7 % and Methane about 0.08 to0.1%. But we have seen after implementation of this scheme, frequent leakage was observed from inlet flange as well as heavy erosion was noticed in the inside surface of vapour inlet line and flange. Erosion was noticed from NG injection point and extended downstream up to nozzle flange in elliptical pattern. Material of construction of the process piping is A312 TP -316 L (Cr-18%, Ni-12%, Mo-2.0%, N-0.2%, and C-0.03%) which is compliable as per basic licenser. Hence frequent thinning of the pipe wall nearing injection point is due to sudden expansion of natural gas. In order to minimize pipe wall erosion phenomenon near NG injection point an alternative arrangement of NG injection extending the 1” NG line up to center of the vapour inlet line has been implemented and location of injection also changed as fig.-3.

1. See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/291321637
A CASE STUDY OF THINNING OF NG (Natural
Gas) INJECTION LINE IN MP SECTION BEFORE
MP CONDENSER IN UREA PLANT
TECHNICAL REPORT · JANUARY 2016
DOI: 10.13140/RG.2.1.2096.7445
2 AUTHORS, INCLUDING:
Prem Baboo
National Fertilizers Ltd.,India
26 PUBLICATIONS 0 CITATIONS
SEE PROFILE
Available from: Prem Baboo
Retrieved on: 20 January 2016

2. A CASE STUDY
OF
THINNING OF NG (Natural Gas) INJECTION LINE IN MP
SECTION BEFORE MP CONDENSER IN UREA PLANT
Author
Prem Baboo
Sr. Manager (Prod)
National fertilizers Ltd. Vijaipur, India
prem.baboo@nfl.co.in
+919425735974
Sr. Advisor & Expert, www.ureaknowhow.com
Fellow of Institution of Engineers (India)

3. ABSTRACT
In urea plant Medium pressure Inerts gases recover HRU burner in Captive Power Plant. The inserts
containing useful fuel in the form of Hydrogen & Methane about 40-45% volume percentage of total
inerts. This inerts also containing Oxygen because the passivation air is given in carbon Dioxide
compressor suction line for passivation of Reactor vessel and all downstream Stainless Steel vessel.
In the presence of Oxygen there are chances of explosive mixture of Hydrogen and Oxygen in exit of
MP section final vent line, to overcome this dangerous situation natural gases are added in the MP
section before MP condenser so that the range of explosive can be avoided. These gases Hydrogen &
Methane come with carbon Dioxide gas from Ammonia plant, carbon Dioxide about 0.5-0.7 % and
Methane about 0.08 to0.1%. But we have seen after implementation of this scheme, frequent leakage
was observed from inlet flange as well as heavy erosion was noticed in the inside surface of vapour
inlet line and flange. Erosion was noticed from NG injection point and extended downstream up to
nozzle flange in elliptical pattern. Material of construction of the process piping is A312 TP -316 L
(Cr-18%, Ni-12%, Mo-2.0%, N-0.2%, and C-0.03%) which is compliable as per basic licenser. Hence
frequent thinning of the pipe wall nearing injection point is due to sudden expansion of natural gas. In
order to minimize pipe wall erosion phenomenon near NG injection point an alternative arrangement
of NG injection extending the 1” NG line up to center of the vapour inlet line has been implemented
and location of injection also changed as fig.-3.
INTRODUCTION
The Vijaipur Unit of National Fertilizers limited Vijaipur, India is the first gas based plant on HBJ
(Hazira-Bijapur-Jagadishpur) gas pipeline. Commercial production of Line-I group of plants was
declared in July 1988 and that of Line-II in March 1997. Line-I Ammonia capacity 1750 MTPD and
line-II Ammonia is 1864 MTPD. Two streams of Urea of 3030 & 3231 MTPD capacity. Thus the
overall urea capacity is 6261 MTPD. Both the Ammonia Plants are based on the steam reforming of
natural gas and naphtha with technology supplied by Haldor Topsoe, Denmark. Whereas feedstock for
Line-I Plant is natural gas, the Line-II plants have been designed to process natural gas and naphtha to
a maximum limit of 50% as feedstock. Urea plants are based on the well-known total recycle
ammonia stripping process of M/s. Saipem, Italy. Now the urea line-II plant producing 3500 MTPD
urea and line-I producing 3100 MTPD. The Vijaipur unit have 450 TPD carbon Dioxide Recovery
(CDR) plant to reduced greenhouse gases. The plant have on line stack monitoring system for NOx,
SOx and also planning to installed on line dust monitoring system for Prilling Tower dust emission to
control spm from Prilling tower. The Vijaipur unit have three No GTG for producing power 17X3
MW.GTG I & 2 have 40 bar steam HRU and GTG-3 have 100 bar steam generation HRU. The plant
has Urea bio hydrolyser for treatment of urea waste. The bio hydrolyser system has Hafnia LV
bacteria to convert urea waste. 2000 ppm urea can convert into 2 ppm urea in 6-7 hrs residence times.
DETAIL STUDY AND REASON OF THIS CAUSE
This energy saving scheme was approved on dated 21/10/2011,subsequently scheme was
implemented in August 2012 in 31/41 units of urea line-II.As part of the scheme NG injection
through 1” along with flow transmitter and control valve is given at the vapour inlet line of
Medium pressure condense(E-7) approximately 500 mm upstream of inlet line.
Soon after the commissioning leakage was observed from the inlet flange and it was sealed
on line by Furnaniting on dated 25/08/2012.Inlet nozzle flange were opened during April
2013 shut down of urea-II for inspection and normalizing of the joint. On inspection of the

4. flanges heavy erosion was noticed in the inside surface of the pipe a flanges. The erosion
started from the NG injection point and spread downstream up the nozzle flange in elliptical
region as shown in the figure No. 1. No erosion found outside the elliptical region. The
thickness of the pipe reduced up to 4.0 mm from the original pipe 12.00 mm. As a remedial
action inlet pipe from NG injection point and inlet nozzle connecting flanges were replaced in
the April 2013 shut down.
Fig. No. -1(Erosion inside the pipe)
Inlet line of 31 E-7 developed leakage through a small hole on the parent pipe of inlet nozzle
on 13/09/2014 again. On examination of the leakage point drastic reduction in the inlet
nozzle pipe thickness was noticed. Thickness had even reduced to 2 to 3 mm around the
leaking point, from the original value of 12 mm on 03/01/2015 again. Leakage reoccurred on
the weld joint of the pipe flange again downstream of NG injection point. Pipe thickness
reduction up to 3-4 mm was measured at the region adjacent to leak point. Thickness
measurement at various point was done, which depicted the similar elliptical region with
reduced thickness, downstream of NG injection point.

5. Material of construction of the pipe is A312 TP 316 L which compatible with service. The
pipe had been in use since the plant commissioning. The reduction was observed only after
the start of NG injection with the commissioning of Inerts off gas(C-3 off gas) utilization
scheme. The scheme has been implemented in all the urea units11/21/31/41 in the identical
manner, however the problem appeared and reoccurred in 31unit only, 41 unit also but less
erosion observed 11/21 were also inspected 2-3 mm of reduction observed. Hence it needs to
be analyzed from operation point of view.
MODIFICATION OF LINE INJECTION LOCATION AND INTERNAL
It had been decided in brain storming meeting the NG injection line enlarged and discharged
at center line of the pipe and vertical line of the pipe rather than horizontal as per figure No.2
& 3 so that the contact between pipe wall can be reduced aim to delivered the NG at the
centre of the main pipe, thus avoiding any dynamic action of the flowing fluid at the centre
on the inside surface of the pipe, and new idea implemented in May 2015 shut down. Till
date no leakage observed and checked by NDT no erosion or thickness reduction observed.
The location of NG injection point also changed horizontal line to vertical line as shown in
the figure No 3.
Fig. 2 (Modified NG inlet injection at the centre of the pipe)

6. Fig. 3 (Modified NG injection location)
WHY NG INJECTION IN MP CONDENSER LINE?
For passivation of the high pressure vessel like reactor, stripper, carbamate condenser and all
downstream vessels the air is introduced in carbon Dioxide in compressor 1st
stage. The carbon
Dioxide also have Hydrogen & Methane these gases are inerts for urea reaction and all inerts are
vented out through medium pressure section these gases have useful fuel so it recovers in heat
recovery unit in captive power plant. But drawback of these gases some time makes explosive mixture
with hydrogen and Oxygen. To avoid this explosive mixture some NG is added to medium pressure
section so that the explosive range can be disturbed as per figure No.4

7. Fig. No-4 (triangular diagram with introduction of natural gases.)
If the NG will not add to the system the Hydrogen & oxygen explosive range will fall in explosive
range triangle as figure No. 5.The ‘C’ point lies in explosive triangle area which is dangerous
condition and this ratio must be revised by adding more NG in inlet line of medium pressure
condenser. Hydrogen gas is very flammable and produced explosive mixtures with air and oxygen.
The explosion of the mixture of hydrogen and oxygen is quite loud. the explosive limits of hydrogen
in air range from about 18 -- 60 % the flammable limits are from 4 -- 75 %, in oxygen the limit of
flammability goes all the way from 4% to 95% . The mixtures may be ignited by spark, heat or
sunlight. The hydrogen auto ignition temperature the temperature of spontaneous ignition in air, is
500 °C Pure hydrogen-oxygen flames emit ultraviolet light and with high oxygen mix are nearly
invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine
compared to the highly visible plume of a Space Shuttle Solid Rocket Booster. The detection of a
burning hydrogen leak may require a flame detector; such leaks can be very dangerous.

8. Fig. No.5
The detail flow diagram of medium pressure section is shown in the figure No. 6, brief
description of the process- The Ammonia and carbon Dioxide feed in plug flow types reactor.
The pressure of reactor is 160 bar and top temperature of reactor is 1900
C and that of bottom
temperature is 1720
C.The process is M/S Saipem process, Italy. The urea formed in exit of
reactor is about 35% by weight. The excess reactant of the process is Ammonia and limiting
is carbon Dioxide. The CO2 pressurized with compressor and ammonia pressurized with
Ammonia feed pump. After Reactor the Urea stream feed to stripper for unwanted Carbamate
decomposition and two another decomposer medium pressure and low pressure, MP
decomposer have 18 bar pressure and that of LP is 4 bar pressure. For achieve urea
concentration 99.7 % three numbers of vacuum stages are there 1st
stage Pre concentrator
Pressure 0.34 ata and 2nd
vacuum stage having 0.33 ata and third stage 0.03 ata pressure.
Waste water section for treated urea generated waste having distillation tower and hydrolyser
for breakup of urea into ammonia & carbon Dioxide gases with steam hydrolysis.

9. Fig. No.6
ADVANTAGES OF THIS SCHEME AS A ENVIRONMENTALY &
ECONOMICALLY
If methane is allowed to vent into the air before being used from MP section vent for instance it
absorbs the sun’s heat, warming the atmosphere. For this reason, it’s considered a greenhouse gas,
like carbon dioxide. It is also one of the green house gases like carbon Dioxide. Yet methane deserves
more attention than it's received so far because, as you note, it's arguably more deleterious to the
environment than the widely feared CO2. The Environmental Protection Agency uses a statistic
called Global Warming Potential (GWP) to assess the threat posed by various greenhouse gases. GWP
measures how much heat one molecule of a gas will trap relative to a molecule of carbon dioxide.
Methane has a GWP of 21 as show in the figure No. 7, which means it's 21 times more effective at
preventing infrared radiation from escaping the planet. So, although methane emissions may be
relatively piddling, they're definitely a cause for concern. (Their one saving grace is an atmospheric
lifetime of just 12 years, versus between 50 and 200 years for carbon dioxide.) So the environment
point of view this gas recovery is most important.

10. Fig No. 7
In view of advantages the 40-45 % useful fuel containing this off gases. The detail calculation is
shown in the table No. 1.
Table No. 1

11. CONCLUSION
The reader will decide, what is happening in this phenomenon? What is the probable cause behind
erosion? The comments are most welcome! If you required more data then I will be provided
immediately. Our idea there is two phase (liquid & vapour) in the inlet of NG line. The fluid stream
contains Ammonia, Carbon Dioxide, ammonium Carbamate & water Content in liquid & vapour
phase. The fluid stream is having temperature 1120
-1150
C and NG temperature 30-400
C. There is
quenching phenomenon because NG pressure about 35 bar and system pressure 18 bar also
temperature deviations. The inlet connections of the NG to the Ammonia/CO2/water vapour line were
reviewed and re-routed to avoid erosion to it due to bi phased hammering.
Any way this scheme is most beneficial about environment point of view and also economically. We
have seen in table No. 1, USD 858578 saving per annum per stream, including line-I plant it is
approximately double of this figure which was venting before 1912.Now we are recovering dangerous
pollutant like methane and ammonia.
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