Urea Reactor Liner Leakage (A Case Study)

Urea Reactor Liner Leakage (A Case Study)

(A Case Study)

Prem Baboo

Sr. Manager (Prod) National Fertilizers Ltd. India

Sr. Advisor & Expert for www.ureaknowhow.com

Abstract – This paper covers background information on the 30 years old revamped plant; problems experienced at the plant after many years of continuous operation, e.g., leaks in the SS 316 L (mod) lining due to the combined action of erosion and corrosion; basis of the revamp work on the design information provided by M/S. Saipem a leader in urea production technology. The urea reactor of Urea line 1 of 11 unit recently suffered liner leakage from weep hole Number 34 on dated 28/11/217.This was the second chance of liner leakage 1stleakage was occurred on dated 28th Jan 2006.The line-1 Plant was commissioned in 1988.A total number of 95 weep hole provided in 11/21 Reactors. In any segments four number weep hole are interconnected. This paper intended how to detect leakage and repairing of the weld joint and further precaution to prevent the corrosion/erosion.

Key Words – Urea Reactor, Weep hole, liner, Detection, Helium.

INTRODUCTION

National Fertilizers Limited, a Govt. of India Undertaking, was incorporated on 23rd August 1974. It is the second largest producer of nitrogenous fertilizer in the country and has four operating fertilizer units located at Nangal, Bhatinda, Panipat and Vijaipur with a total installed capacity of 32.083 lakh tones Urea.NFL, a Schedule A & a Mini Ratna (Category-I) CompanyThe Vijaipur unit, which is an ISO 9001:2000 & 14001 certified, comprises of two streams.The Vijaipur have two ammonia plant M/S. Haldor Topsoe Technology, Denmark capacity 1750 & 1864 TPD for Line-I & line-II respectively and four urea plant of M/S. Saipem ammonia stripping process, Italy. The Line-I plant installed in 1988 and that of line II in 1997.The capacity of

Urea-I urea II is 3030 & 3231 TPD respectively. The raw material used includes natural gas, water and power. Three Numbers Captive power plant of capacity 17 X 3 MW are used in this complex. Both the plants have consistently achieved high levels of capacity utilization.

REACTORS DETAIL

The all four reactors(R-1) of Urea line 1 &2 plants are 40 meters Height (tan to tan) and ID of the reactor is 2.04 meter. The liner of all reactors has SS 316 L (mod) while shell of line 1 & line 2 are differing. The line -1 both reactors are coil layered vessel while 31 unit has multi layered and that of 41 is mono block reactor. Detail as following tableNo-1.The passivation of reactor liner is done by air which is introduced in CO2 compressor in 1st stage. The all four Urea reactor have 15 number of high efficiency sieve trays. These have been provided to prevent the escape of gaseous CO2, which must react with NH3 in the lower portion of the reactor. These trays also help in preventing the internal recycling of the reaction products with higher specific gravity products formed in the upper part of the reactor. Reactor shell is made of CS with a 7mm liner of SS316L (mod) on the inner surface. Liner of the reactor is cladded to the CS shell and longitudinal seams and circumferential seams are differing in line-1and line-2 urea reactors. For detecting any leakage in the liner a weep hole monitoring system has been provided. The line-1 urea reactors have 95 numbers of weep holes. And Urea line-2 of 31-Urea reactors has 77 numbers weep holes and 41 reactorhas 98 numbers weep holes. In the weep hole monitoring system there are two loops of weep holes covering the entire reactor. Data are following in Table No.- 1, 2 & 3.

Table No.-1

Table No. 2

Table No. 3

Fig. No. 1

REACTORS LEAKAGE DETECTION AND REPAIRING

Reactor Liner Leakage on 28/01/2006.

In Urea Line-I and 2 plant all the weep holes are connected to conductivity meter based monitoring system. on dated 28/01/2006 the leakage alarm appeared in weep hole number 23 of 11 stream.The weep hole was checked physically and vapour analysed in laboratory found ammonia vapour and CO2.Immedially shut down was taken and leakage detected with instrument air and soap solution the leakage easily detected. For find out the leakage the liner plate behind the weep hole was pressurized with instrument air at te pressure of 0.5 kg/cm2 and other three interconnected weep holes were plugged. Soap solution applied on opposite side of liner. The pin hole leakage easily detected. The pin hole repaired by welding. The repaired portion was passivate with 10 % HNO3 solution and rinse with Demineralized water. The repaired portion was further checked with instrument air and soap solution no leakage was found. The shutdown duration was only 69hrs. Feed cut to feed in.

Reactor Liner Leakage on 28/11/2017.

On dated 28/11/2017, a weep hole leakage alarm from Weep Hole No. 34 was appeared on lectrotek system. Immediately checked the weep hole No.-34 and vapour analysed in laboratory found ammonia & CO2 vapour and leakage was confirmed. Immediately shut down was taken at 11.15 hrs on dated 28/11/2017.draining of the H.P loop started at 11.15 hrs. For air purging 11 PT-05 A/B cover open for air purging. Man entry permitted at 18.15 hrs on dated 29/11/2017 after confirming ammonia contents less than 25 ppm and Oxygen contents more than 20 %. Four Numbers man ways were opened for detecting leakage the leakage above 5th tray. So the 4 man ways opened. first of all, leakage detection tried with instrument air & soap solution. For detecting the leakage, the instrument air introduced at the pressure of 0.5 kg/cm2and other three interconnected weep hole plugged so that the pressure retains inside the liner and shell. Soap solution applied on liner for detecting the leakage. Four hour tried to leakage but success did not get. After that our further step to detect the leakage with ammonia and phenolphthalein indicator. But leakage could not have detected. Timing tabulated in Table No-4.

As the weep hole leakage was not clear, the exact location the leak could not be found out with Soap solution and ammonia test.Finally, the DP check and number of the spot, pin point found. All the circumferential Seam and longitudinal seam opposite of weep hole No. 34 between 4th& 5th trays welding done. After that ammonia is introduced at the pressure of 0.4 kg/cm2in weep hole No. 34 and hold up for 10 hrs. and liner and all welding seam checked by phenolphthalein indicator. No leakage was found and man hole cover boxed up. Start-up activities started and feed in done on dated 01/12/2017 at 11.15 hrs. Total time taken for this shut down was 72 hrs (3 days) feed cut to feed in. Typical lifetime of the 316L Urea Grade protective layer of a urea reactor is 20-30 years depending upon passivation and plant load number of the tripping of the plant process variations etc. Typical lifetime of a urea plant is 40-50 years. Every urea reactor will finally operate close to the end of lifetime conditions of the protective layer meaning that at a certain moment a leak in the protective layer is nearly unavoidable.

WEEP HOLE MONITORING

Urea line-1 weep hole monitoring with lectrotek system (Pune). In Urea line-2 Masibus weep hole monitoring system is incorporated. The Micro genie scan is a microprocessor based scanner which has been specially designed for resistance input to monitor reactor leakage easy and very simple in installation low pricing equipments and easy in maintenance.

Table No. 4

Fig. No. 2

This programme containing non-volatile memory for setup data, 4 duals and 4 digit display and LED bar for information RS -232

C serial port

The instrument is housed in a ½ DIN extruded Aluminum rack suitable for control room mounting. A small but highly functional Keyboard consisting of 4 membrane keys is provided as the as the instrument operator interface electrical and I/O temperature are instruments rear Via Screw type terminals. The electronics is located on modularly designed board for easy field trouble shooting and maintenance. All cards are plug in type and segregated by their function Viz including power supply CPU, A/D display.

All the weep holes have to be checked by passing instrument air every week/fortnight for any leakage and blockage & the pressure of the testing medium should not be more than 0.5 kg/cm2

SAFETY RISKS OF UREA HIGH PRESSURE EQUIPMENT

1. High pressures

2. High Temperatures

3. Various kinds of corrosion phenomena (inside and outside)

4. Crystallization risks.

5. Large volumes

6. Release of toxic ammonia in case of a leak

Integrity of carbon steel pressure bearing wall can be threatened by:

1. Carbamate corrosion due to damage of protective layer with (corrosion rate 1000 mm/year):

2. An early and reliable detection is a must.

3. Stress corrosion cracking behind loose liner when water and contaminants are present.

4. Stress corrosion cracking from outside when water and contaminants are present

Fig No.-3

Other methods of Leak detection

Numer of methods are available in market as following.

1. Instrument air & soap solution

2. Ammonia with phenolphthalein indicator.

3. Helium leak detection.

4. Vacuum testing method.

5. Ultrasonic flaw detection

6. DP Test method.

PRESSURE VESSEL

Pressure vessels are leak proof containers which contain media under pressure and temperature May be of any shape milk bottles, shaving cream, tires, tanks, reactors etc. The term pressure vessel referred to those reservoirs or containers, which are subjected to internal or external pressures.For higher operating pressures and higher temperature, new technologies have been developed to handle the present day specialized requirements. Multilayer Pressure Vessels have extended the art of pressure vessel construction and presented the process designer with a reliable piece of equipment useful in a wide range of operating conditions for the problems generated by the urea processes.

Classification of Pressure Vessel

High Pressure Vessel

Mono wall

Layered Vessel

Multi wall

Coil Layered

Multi Layered

Normally uneven corrosion in reactor occurs due to poor feed rate and quality, N/C ratio; high inert; chloride and sulphides and also iron precipitated water. Lack of passivation. 316 L Urea Grade needs a continuous oxygen supply to assure a passive laye r builds up. Sulphides cause stress cracking in stainless steel. Chlorides are somewhat corrosive to stainless steel. The combination is not good. It sounds like you might be getting a lot of impurities in there from somewhere. Very small quantities of these things (other than halides) are not likely to be very much of a problem, but you need to have an analysis made of the sludge you are getting to determine what all you have in there. Then it will be easier to judge how bad the situation is. With this kind of mix (if you are getting all of those things) you can get a lot of side reactions. If you have that many anions, you will need to get cautions from something. It is possible for it to find them in the stainless steel. Passivation air must not be less than 0.3% in compressor house in third stage. Following points to avoid corrosion/erosion.

1. Chloride in passivation air-seldom present in atmosphere.

2. N/C ratio should be proper.(In M/S. Saipem C/N ratio is 3.5 and in M/S. Stami is 2.8

3. Prevent iron precipitated water.

4. Check CO2 purity – 99.3%, Hydrogen and Iron present in carbon dioxide.

5. Oil content in liquid ammonia feed, as few grades of oil contains sulphur.

6. Holding of solution in shut down must be less than 48 hrs. However M/S. Saipem recommended 72 hrs. Initially, but after some cases of corrosion/erosion was observed further recommended for 48 hrs.

7. Ammonia liquid feed in reactor must be more than 132.6 0C(critical temp. of ammonia)

8. During holding of solution liquid ammonia to be given to reactor for 10 minutes and during start up ammonia feed 10 minutes prior to carbon dioxide feed.

9. Pay attention to depletion of oxygen due to dead spots.

Fig. No. 4

Fig. No. 5

CONCLUSION

My experience says that reactors always leak from the weld joints of liner initially. However, if the leak is not attended and carbamate comes in contact with shell. The damages are far beyond imagination. Weld joints are always more vulnerable to fail. The probability and consequence are

Fig. No. 6

combined to produce an estimation of risk. The results show that the lining leakage is the most severe risk and the SCC of layer shells risk is the second. Keep weep holes always open and inspect for leaks periodically. Urea reactors should be shut down immediately when a leakage is detected. Don't use vapour to detect the leakage of urea reactor vessel, in

case it should be used, the vapour quality should be ensured. The risk of highly corrosive urea-carbamate solution leaking through these liners always exists in this equipment which might eventually damage the carbon steel walls. The places having notches like corners of support cleats or crevices in the weld joints are more prone for SCC.The reactor liner leakage is the serious phenomenon. As and when urea reactor liner leakage started shut down must be taken as soon as possible otherwise carbon steel corrosion rate is high. The subject of pressure vessel technology is central to industries as a whole. The balance of safety with a sound economic approach is a major consideration to the designer,

manufacturer and operator. The legislation and standard must be such that these requirements are et.

LEGENDS

CS-carbon steel, TPD-ton per hour, MW-Megawatt. SS-stainless steel. SCC stress corrosion cracking, DP- Dye penetrating Test. Circ.-Circumferences, Long.- longitudinal. HP-High Pressure, L.S -low pressure Steam, K.W-High Pressure flushing water. H.W-about Medium pressure flushing water.ID-inner dia. P.W.H.T.-Post weld heat treatment. T.S. -Tensile strength