Urea Plant Equipments Inspection

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Urea Plant Equipments Inspection

Prem Baboo

DGM (Production & Process)

Abstract:- There are many equipments in urea plant which have very important for holding time, partition plate passing, weep holes monitoring, medium pressure absorber, high pressure carbamate condenser, etc.Startup of medium pressure absorber, is one of the most difficult operations of chemical processes in urea plants. In the last few years we have accomplished both theoretical studies and experimental verifications. Perfect sealing of trays by special quality PTFE (Gore-Tax)As a result, significant reduction of startup time period can be achieved by implementing the developed perfect sealing and checking of holding with recommended standard rules. This paper summarizes our recent results from these studies and make standard. Passing rate of partition plate holding check of ferrules. Weep hole monitoring and checking of the leakage through liner in HP vessels. Absorber columns, and trays, can now be designed with a high degree of reliability. Absorber columns have been widely used in the past to separate mixtures of liquids into individual components like ammonia and carbamate mixture.

Key Words:- Medium pressure absorber, holding time standard, trays, gasket, stripper, HP carbamate condenser. Packing material. ferrules, liner. Weep holes, stripper, Reactor.

PROCESS DESCRIPTION

The urea process is characterized by a urea synthesis loop with a reactor operating at about 140-175 kg/cm2 (g) (for stripping process) with ammonia to carbon dioxide molar ratio at urea reactor inlet of 2.8 – 3.9. This allows a CO2 conversion into urea of 60 – 68% in the reactor itself, also perforated trays which prevent back-flow maintained plug flow and favour gas absorption by the liquid. Different type of HET vortex mixture with booster are used now a day to increase the conversion rather than vessel size and pressure. Different types of HET are developed like super cup etc. There are two kinds of chemical reactions at the same time in the urea reactor:

2NH3 + CO2 NH2-COO-NH4 + 136230 kJ/kmol of carbamate (at 1.03 kg/cm2; 25°C); NH2-COO-NH4 NH2-CO-NH2 + H2O – 17575 kJ/kmol of urea (at 1.03 kg/cm2; 25°C)

The First reaction is strongly exothermic and the second one is weakly endothermic and occurs in the liquid phase at low speed. Downstream the urea synthesis the decomposition (and relevant recovery) of unconverted chemical reagents is carried out in three subsequent steps: High Pressure Decomposition in H.P. stripper; Medium Pressure Decomposition in M.P. Decomposer and, finally, Low Pressure Decomposition in L.P. Decomposer. The decomposition reaction is the reverse reaction of the first one above showed, viz.: NH2-COO-NH4 2NH3 + CO2 (- Heat) and, as can be inferred from the equation, it is promoted by reducing pressure and adding heat. The urea reactor effluent solution enters the stripper, under slightly lower pressure than the urea reactor, where a fair part of the unconverted carbamate is decomposed, due to the stripping action of either NH3orCO2, so that the overall yield of the H.P. synthesis loop referred to CO2 is as high as 80-85% (on molar basis). Ammonia and carbon dioxide vapours from the stripper top, after mixing with the carbamate recycle solution from M.P. section, are condensed at the same pressure level of the stripper, in the H.P. carbamate condenser; thus producing the LS steam which is used in downstream sections.

After separating the inert gases which are passed to M.P. section, the carbamate solution is finally recycled to the reactor bottom by means of a liquid/liquid ejector, which exploits H.P. ammonia feed to reactor as motive fluid. This ejector and the kettle-type carbamate condenser above mentioned, allow a horizontal layout, which is one of the main features of urea process. Downstream of the stripper residual carbamate and ammonia are recovered in two recycle stages operating at about 17.5 kg/cm2

  1. (M.P. section) and 3.7 kg/cm2 (g) (L.P. section) respectively. Ammonia and carbon dioxide vapours coming from carbamate decomposition are condensed and recycled to H.P. section. The solution leaving the L.P. section arrives to the concentration section where process condensate is removed in order to reach a concentration of about 96 97% which is required to feed granulator. Urea Sections are characterized by the following main process steps: a) Urea synthesis and NH3, CO2 recovery at high pressure; b) Urea purification and NH3, and carbamate recovery at medium and low pressures; c) Urea concentration; d) Waste water treatment. Urea solution production unit is also provided with the following: Auxiliary installation; f) Steam networks; g) Condensate Recovery & Flushing networks.Even if the inspections was made before the first start up, For the inspection of the urea reactor, stripper Carbamate condenser and Carbamate Separator All inspectors were used rubber gasket and a wood cover in order to avoid possible damaging on the gasket seating area of manhole.

    The weep holes are provided for the following Equipments:

    1. Urea reactor

    2. Stripper

    3. Carbamate condenser and

    4. Carbamate separator

Weep holes are safe installation for protecting the above equipment in case of leakages from the lining welds, avoiding that process fluids could come in contact to the carbon steel material of equipment body. We had checked prior to start-up of the plant.

CRITICAL EQUIPMENTS FOR PARTITION PLATE, TRAYS & FERRULES

Following equipments are very sensitive for passing

  1. Medium Pressure Absorber

  2. High Pressure Carbamate Condenser

  3. Stripper ferrules

  4. MP & LP decomposers ferrules

MEDIUM PRESSURE ABSORBER

Most important is medium pressure absorber which is the heart of the urea plant because the ammonia is separated from carbamate mixture. The excess ammonia feed in the reactor separated by this vessel. The gaseous stream, leaving the top of the stripper, after mixing with the recovered solution from the bottom of medium pressure absorber, enters the carbamate condenser, Except for incondensable gases, vapours are condensed and recycled to the urea reactor, by means of carbamate ejector. Condensing vapours at high pressure and temperature allows the production of saturated steam at 3.5 kg/cm2 (g) in the carbamate condenser. Incondensable gases, consisting of inert gases (passivation air plus inerts with CO2 from B.L.) and containing a little quantity of NH3 and CO2, come out from the top of the carbamate separator and flow directly into the bottom of the medium pressure decomposer.NH3-CO2-H2O mixture, partially condensed coming from Medium pressure condenser, enters in the bottom of the absorber. Gases consisting of NH3, CO2, H2O and inerts, rising from the bottom, are absorbed by cold liquid ammonia reflux sent to the top tray of the absorber. CO2 and H2O vapours will condense forming ammonium carbamate which falls back to the bottom. Absorption heat is removed by the evaporation of NH3 coming from ammonia booster pumps, thus a current of inert gases saturated with ammonia and containing just a few ppm of CO2 leaves the top of the rectification section at an operating temperature of about 44 °C. The bottom level and temperature must be kept constant at about 40 % and 79 °C respectively. In order to avoid that a temperature increasing on the trays of the column can result in CO2 carry over (with consequent failure of all the system due to plugs formation on top of this equipment); one thermocouple has ben provided for each tray, in order to check constantly the operating value. In the event that the trays are clogged with carbonate, they can be washed with water to dissolve the solid by HW injection. The trays segments are perfectly tightened by gore tax Teflon type.

Fig.-1

The proper holding time had checked by taking water level and measured by scale some of the reading taken and tabulated as table number-1The level of carbamate solution in the absorber can be observed through the sight glasses installed in the lower part and it is controlled by the level controller which re-injects L.P. carbonate solution into the M.P. absorber feed upstream of MP condenser by means of pumps carbonate solution pump, the level of the MP absorber fluctuates in time of the start-up because the density variation when plant stable than it working normally. In that case level can be seen by the sight glass installed in the vessel and five number glasses are provided in the vessel also you can help by thermocouple placed at different location of the vessel and you can judge by temperature,e.g. bottom temperature is more (75-800C) if the trays temperature goes high then it is either the indication of high level or Ammonia reflux is low. Thermocouple mentioned in figure-1 and also sight glasses.

Fig-2

INTERNAL INSPECTION: AND HOLDING CHECK

  1. Opened the manholes of the top and bottom of the column.

  2. After the installation of each tray, before closing the man-hole have a visual check of the bottom tray.

  3. After the complete installation of each tray (closure of relevant manhole), verified the conformity of the bubble cap tray with the mechanical drawing and perform the sealing test according to below instructions.

  4. Ensured that weir height and down comer clearance are as per drawing.

  5. Checked that each thermo well is at minimum distance from the relevant tray.

  6. Verified that liquid inlet pipe is welded and well supported.

  7. The branches of the solution distributor must be well fixed to the relevant supports.

  8. Check if the vortex breaker is positioned into the solution outlet pipe and if it has enough space from the bottom.

  9. Verified that the orientation of the nozzles of the column is according to the Mechanical drawing.

Start to install the manhole on the first tray (from bottom) and using adequate gaskets, make a level of 75 mm with demineralised water above the tray. If a level of 50 mm minimum holds for 40 minute, the test can be considered positive; Means the level was down 37.5 mm in one hour then the trays are perfect otherwise it is necessary to improve the sealing and repeat again the test. Proceed to perform the same test also for the other trays. After cleaning the top of the column, box it up.

Sr. No.

Time in Min

1st Tray, Level in mm

2nd tray, Level in mm

3rd Tray, Level in mm

4th Tray, Level in mm

5th Tray, Level in mm

Remarks

1

00.00

75.00

75.00

75.00

75.00

75.00

All are ok

2

15.00

75.00

74.00

75.00

74.00

73.50

All are ok

3

30.00

74.00

73.50

74.00

73.50

73.00

All are ok

4

45 .00

73.50

73.00

73.50

73.00

72.50

All are ok

5

60.00

72.00

72.50

73.00

72.50

72.00

All are ok

Table No.-1

INSPECTION OF WEEP HOLES OF HP EQUIPMENTS

In order to monitor H.P. equipment the visual inspections were performed as described in the following procedure. Checked of weep holes installed in the high pressure equipment: reactor, stripper, carbamate condenser, carbamate separator; inspections and periodical actions was performed.

The weep holes are installed for protection and safety of the equipment and therefore they are made in a way that the process fluids do not come in contact with the carbon steel of equipment body in case leakages occur. In case of leakages through said welds the process fluids will come out of the weep holes and in this case the phenomenon is evident to the operating personnel of the plant who, at every shift, visually inspect this equipment. To perform this inspection, holes neither must be plugged nor obstructed by foul matters and therefore it is suggested to inspect and clean them at least every two/three months when the plant is running, flushing them by the LS steam hose one at a time. During this operation, with the equipment drawing in hand, verified that steam is flowing in the ducts and that it outcomes from the holes or from the holes opposite to the inlet holes whichare designed to be communicating. If this does not occur steam flushing was carried out until obstruction is completely removed. During the flushing, the steam pressure was monitored and always kept not higher that 0.5 kg/cm2 of the equipment internal pressure to avoid lining collapse. If leakage does occur from a weep hole, it will most likely start as a gas stream with a little liquid dripping out of the tube. If a leakage is discovered, increase the frequency of checking the leak to once per hour watching for changes in the flow and plan a repair procedure within a short time. The leak stopping could be an indication that product has crystallized in the hole and that corrosion of the carbon steel shell could happen.

CARBAMATE CONDENSER

The stripper overhead vapors are mixed with carbamate solution from M.P. section and then condensed in a kettle-type exchanger. The heat duty is used to produce saturated LS steam on HP condenser shell side. Operation – Stripper vapors mixed in the carbamate mixer with carbamate solution coming from HP carbamate pump enter the carbamate condenser where except for incondensable gases, the streamis condensed and sent to carbamate separator. On shell side of this equipment, the condensation heat is used to produce saturated steam at 3.5 kg/cm2 g, 147°C which is distributed to the various LS pressure steam users. Number of advancement in HP carbamate condenser has been done like segmental partition plate now connected to above hemisphere part of the dome. Advantages of this no dead zone and all part can be passivated and easily tighten no chance of passing but you cannot check passing rate. Second is the solution entry changed from man hole to above part of the dome as shown in the figure-3.

Fig-3

CARBAMATE CONDENSER LINER CHECK

Weep holes Testing

The gap was pressurized between the lining and base material through weep holes to a pressure with Helium and pressure must not higher than 0.5 kg/cm2g. A helium mass spectrometer is an instrument commonly used to detect and locate small leaks. It typically uses a vacuum chamber in which a sealed container filled with helium is placed. Helium leaks out of the container, and the rate of the leak is detected by a mass spectrometer. As shown in the figure-4. The calibrated sniffer was used and maintained distance from object was 3 mm. The sniffer scanning was determined by moving the sniffer over the calibrated leak at a rate of no more than 1 to 2 cm/sec or in such a way that the detected signal is equal to, at least, 70% of the value reached during the determination of the system´s response time. Holding time was maintained about 30 minutes minimum.

We had examined all area using detector proe or sniffer connected to the instrument through flexible tubing. The pressure inside component shall be hold for 10 minutes minimum. Interchangeability of lining sector was assured checking weep holes of linings adjacent to the sector under test. We were using integral testing procedures is called the integral (sample under pressure) method, with the chamber under investigation being placed in a sealed container. The chamber is pressured up with helium at the pressure of 0.5 kg/cm2g and the container is connected to the leak detector. In case of a leak, a sample of gas from within the container is drawn off and passes through a mass spectrometer where any increase (over the background reading) in helium levels is recorded. We had checked the all weep hole one by one and probe located accordingly. As shown in the figure-

  1. No leak was observed.

    Fig-4

    HP CARBAMATE PARTITION PLATE PASSING DETECTION

    For detection of carbamate partition plate passing with Gore-Tex Teflon the KW injection was given in bottom of the HP carbamate drain line and filled with water of half tube bundle when the water come from first row of upper part dome tube then stopped the kw injection. The water was used DM water (less than 50 ppm Chloride), The 20 litre bucket used to measure the leakage water quantity as shown in the figure number 5. First time checked found leakage about in 15 min was 40 litres. So the again tried the tightening by Gore-Tex Teflon now measure slightly was improved the result it was 35 lit in 15 min and third trial was very successful result tabulate in table number-2.As shown in the figure-5. The recommended limit by licenser is 100 litres per hour in segmental partition plate. After the leakage checked the minimum passing plug open and removed.

    Fig-5

    Sr. No

    Time in min

    Water measure quantity in lit

    Rate in lit/hr

    remarks

    FIRST TRIAL

    1

    15

    40

    160

    Out of recommendation limit

    2

    30

    80

    160

    SECOND TRIAL

    3

    15

    35

    140

    Out of recommendation limit

    4

    30

    70

    140

    THIRD TRIAL

    5

    15

    10

    40

    The passing is within the recommended limit, OK

    6

    30

    20

    40

    Table -2

    Fig-6

    WRONG GASKET FOUND IN CARBAMTE SOLUTION OUTLET

    During the inspection carbamate condenser tube side solution outlet nozzle found wrong gasket and checked by ferrite analyser it was carbon steel (CS). Also confirmed by PMI (projected Material Identification). This gasket was removed and original Urea 310MoLN inserted and shown in the figure- 7. Detail material analysis is as follows. –

    Chemical Analysis Urea 310MoLN

    C

    Cr

    Ni

    Mo

    Mn

    Si

    S

    N

    <0.02

    25

    22

    2.1

    2.0

    0.4

    0.01(max)

    0.12

    2 RE-69

    C

    Cr

    Ni

    Mo

    Mn

    Si

    S

    P

    N(max)

    < 0.02

    25

    22

    2

    1.5

    0.4

    0.015

    0.02

    0.15

    Table -3

    Fig-7 UREA REACTOR

    The reactor has a volume designed to allow the residence time necessary for carbamate conversion into Urea and water. Reactor is equipped with 18 sieve trays.ID of the reactor is 2.55 meters and height tan to tan is 47 meters. The liner thickness is 5 mm and 2RE-69 SS (25/22/2).The shell is made up of carbon steel to hold the pressure. HET is function is to prevent escape of gaseous CO2 and prevent internal recycle of products with higher specific gravity from top part toward the bottom and maintaining plug flow. For a correct operation, the reagent rates to the reactor are kept as constant as possible; this will ensure that reactor temperature, molar ratio, conversion ratio and production remain constant. In particular bottom and top temperature during reactor normal conditions are 175 -179 °C and 188-190 °C respectively. Reactor must be heated up to ~150°C with steam before pressurization with ammonia vapours as per start-up procedure. During heating, quick changes in reactor temperature must be avoided to prevent possible ruptures in lining welds. Leak Detection System is provided on the reactor shell to detect possible leakages in the lining. In case of any leakage is detected the unit must be immediately shut-down. To protect the reactor in case a sudden pressure increase occurs, an automatic interlock system has been provided to stop fluids (ammonia plus carbamate and carbon Dioxide) inlet.

    VISUAL INSPECTION

    Verified the conformity as per process specifications and also for checking if some internal parts suffer damages during the transportation, we had checked foreign material (deposit of rags, tools, traps, weirs, etc..) was left inside or if it is present any fabrication defect. We had checked an external inspection, especially for checking the presence of weep holes on the equipment of the H.P. section

    WEEP HOLES CHECKING

    Weep holes are safe installation for protecting the above equipment in case of leakages from the lining welds, avoiding that process fluids could come in contact to the carbon steel material of equipment body. Prior to start-up of the plant, we had verified whether all weep holes are visible, approachable and internal grooves free numbering was done as per drawing. In order to verified that internal circuit of all the weep holes is free. The weep holes map supplied by the equipment manufacturer was

    used for the identification. Once the location is completed we proceed to put an air hose connection in correspondence to one weep hole and start blowing through the opposite(communicating) one at a pressure not more than 0.5 kg/cm2g with help of regulator and instrument air is used for weep holes checking and detecting the path. As shown in the figure-8.

    There are many different leak detection systems available in the industry. First of all, the design philosophy of a leak detection system of the different licensors and manufacturers differs. A leak detection system in general is designed to detect leaks in the liner. Second the visible part of the leak detection system differs also from one plant to the other: some plants flush with air, others with carbon dioxide, again others with steam and some even wait until fumes exit the leak detection tubes, some use as an indicator of a leak a bottle with phenolphthalein, others an ammonia detector and again other measure the conductivity and vacuum detecting system. It is not important what kind of system one is using but it is important that one is using a system. Best practice is to appoint one person to be responsible for the system plus shift responsibility for frequent checking.

    Fig-8

    STRIPPER

    The stripper is a vertical in tube falling film decomposer in which the liquid, distributed on the heating surface as a film, flows by gravity to the bottom. In practice, it is a vertical shell-and-tube exchanger with the heating medium on the shell side, and an upper head on tube side sheet specially designed to permit the uniform distribution of urea and carbamate solution as shown i the figure-11. In fact, each tube has an insert-type distributor (ferrule) designed to put the feed uniformly around the tube wall in film form. The stripper receives the process solution from the reactor. The process stream is heated by steam to decompose the carbamate into NH3 and CO2 vapours and strip excess ammonia from the solution. The urea solution leaves the bottom of the stripper and flows to the M.P. decomposer. The stripped NH3 and CO2 vapours flow from the top of the stripper to the carbamate condenser.

    The holes of the ferrule act as orifices and their diameter and liquid head control the flow rate. As the liquid film flows, it is heated and decomposition of carbamate and surface evaporation occur. The carbon dioxide content of the solution is reduced by the stripping action of the ammonia as it boils out of the solution. Generated vapours (essentially ammonia and carbon dioxide) are removed by flowing to the top of the tube. The carbamate decomposition heat is supplied by means of condensing saturated steam at 219 °C.

    The gaseous stream, leaving the top of the stripper, after mixing with the recovered solution from the bottom of medium pressure absorber enters the carbamate condenser, Except for incondensable gases, vapours are condensed and recycled to the urea reactor by means of carbamate ejector.The temperature of the urea solution leaving the stripper is kept at 204°C (max.) by varying the steam pressure values at the stripper shell that normally has to be kept at about 22.2 kg/cm2 g.

    INTERNALS INSPECTION AND FERRULES HOLDING CHECK

    Removed hold down grid and Moving the ferrules by hands, checked if all are well fixed into the tubes and ensured for looseness. Performed spot check of ferrules holes in order to detect if some metal shaving is present due to the manufacture in workshop. Checked the following points.

    1. Uniformly machined.

    2. Checked each ferrule is below the grid and not displaced by it.

    3. Checked the tube sheet is free from dust.

    4. Checked the hold down grid is well fixed and all the stay bolts are tightened.

    5. Checked defect of construction on the shell courses of the lining at the top and bottom of the equipment.

    6. Checked the thoroughness of the passivation air distributor header at the bottom separator

    7. Checked in the bottom the lens gaskets and the outlet solution special pipe 8.

HOLDING TIME CHECKED FOR FERRULES

For holding check the DM (less than 50 ppm chlorides) water is taken by pump about 200 litres DM water is required. Filled the water up to tangential holes the level was checked by scale and time noted and every 5 minutes the reading of water level was taken. Stripper ferrules holding check was done by filling of DM water found satisfactory, approximate level was come down 4 mm in 15 minutes. The passing within the limit as per standard it is 60 mm in one hour. Total height from tube sheet to ferrules tangential holes is 65 mm including 5 mm tube end (dead level), all the internals checked found satisfactory. The reading is given as follows table number-4 and shown in the figure-9 & 10.

Sr.No.

Time

Level

Remarks

1

10.50

65 mm

Ok

2

11.05

61 mm

OK

3

11.20

57 mm

OK

4

11.35

53mm

OK

Table-4

The all ferrules checked for looseness found ok.

Fig -9

Fig-10

Similarly, the ferrules of MP and LP decomposer were checked by visually by hand these all were found loosed. All the ferrules were removed and Teflon tap was applied and fitted by hand pressure and slightly by mallet (wooden hammer). Now no need of holding check with water because they all are fitted well and internals of MP and LP decomposers fitted and man hole cover boxed up. Generally the ferrules checked by hand looseness no need of water holding check if any doubt you can check by water filling and holding time check, the holding time must be below than standard. The standard for both MP and LP are 60 mm per hour.

Fig -11

CONCLUSION

HP loop is the heart of the urea plant having high pressure and temperature with highly corrosive carbamate. A leak detection system with a short response time is important to avoid the formation of carbamate behind the liner. To wait until one sees fumes from the leak detection tubes is in my view not a proper leak detection system. Assure yourself that all leak detection holes are in operation, be aware that multi-layer carbon steel vessels have ventilation holes which are not to be confused with the leak detection system. An active vacuum based leak detection system may be installed for weep holes monitoring system however the conductivity based weep holes monitoring is also the best technique.But as and when leakage observed the shutdown must be taken to attend the leakage otherwise it doesnt take time for an accident. The carbamate corrosion due to damage of protective layer with higher corrosion rate. Stress corrosion cracking behind loose liner when water and contaminants are present. Every urea reactor will finally operate close to the end of life time conditions of protective layer meaning that at a certain moment a leak in the protective layer is nearly unavoidable.

REFERENCES

  1. Leak Detection Systems in Urea Plants UreaKnowHow.cow Mechanical Paper April 2009

  2. Urea Reactor Liner Leakage (A Case Study) by Prem Baboo, International Journal of Engineering Research & Technology (IJERT) Vol. 6 Issue 12,

    December 2017

  3. Helium Leak Detection in High Pressure Urea Reactor by Ali Al Siyabi& Ankit Niranjan Oman India Fertilizer Company Oman ,2015 Asian Nitrogen + syn gas.

  4. Fundamentals of leak detection by Hans Rottländer Walter Umrath Gerhard Voss,Editor: Leybold GmbH Cat. No. 199 79_VA.02.

Legends-HP-High Pressure, M.P-medium pressure, L.P. Low pressure. M/H-man hole, DM demineralized, ID- inner diameter, KW-very high pressure (160 kg/cm2), HET-high Efficiency trays.

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