Chemical & Processing
Oil & Gas
Pharma Biotech
Infrastructure & Design

Improving Reliability of Naphtha Condenser in Ammonia Plant
This paper covers the problems encountered in Naphtha condensers in Ammonia Plant, Bharuch, Gujarat, India, and the cost effective reliability measures taken by mechanical maintenance group to improve availability of condensers for operation.

Gujarat Narmada Valley Fertilizers & Chemicals Limited (GNFC) has single stream Ammonia and Urea plant having name plate capacity of 1350 MTPD and 1800 MTPD respectively. Ammonia plant is having Texaco gasification technology to produce crude gas from partial oxidation of fuel oil. Detail engineering of ammonia plant was carried out by M/s Linde AG. The plant capacity is enhanced by way of various modifications, debottlenecking and revamps over the last two decades. Subsequently Formic acid, Methanol, Nitro phosphate complex, Acetic acid, Aniline, TDI (Toluene Di Isocynate), Ethyl acetate plants were also added in the complex.

For achieving zero pollution and maintaining green field environment a carbon extraction unit is installed in the process streams to recycle the unburn carbon generated during partial oxidation of fuel oil. Naphtha acts as a carrier fluid to absorb the carbon from the carbonised water (Soot water) and transfer it to fresh fuel oil before feeding it to Gasification process. Naphtha is condensed and recycled back to system.

Naphtha Condenser
In the system of Carbon Extraction Unit, two Naphtha condensers namely E306A and E306B are installed to condense the Naphtha vapour and some quantity of water generated in stripping process. The Heat exchanger is having Floating head design. Naphtha vapour passes through the shell side of the heat exchanger.

Cooling water is fed from bottom to top on tube side. Condenser is having Stationary tube sheet and channel section (cooling water side) of A181 Gr 1 material. The Shell components as well as the floating head tube sheet are of SS321 material. Stationary tube sheet is having 12 mm thick weld overlay of SS material towards shell side which is remaining in contact with the Naphtha water.

Naphtha water being corrosive in nature SS321 metallurgy was selected and cooling water being non corrosive, CS (A181 Gr 1) was selected at design stage. Seamless tubes are having A213 Gr TP321 material.

Problems Encountered
The condenser is opened during every shut down of Ammonia plant for cleaning of tubes. Only channel section is opened for hydro jet cleaning. Apart from cleaning activity, inspection of the tube sheet, partition and channel is performed for any damages, pitting or corrosion. Pitting of stationary tube sheet is found during every inspection and it was monitored. During inspection in the year 2004 and 2005 shut downs the corrosion and pitting has reached to alarming level.
Pitting depth had reached to such an extent that the tube to tube sheets welds were detached from the tube sheet at number of places. During hydro test from shell side at 7 kg/cm2 pressure, seven tubes were found leaking from the tube to tube sheet weld joints. Leaky tubes were plugged. Attempts made to weld the joints but not successful. Cracks/defects were generating in weld during welding. As an alternative measure the tubes were expanded (extra expansion) to attend the leakage at 7 kg/cm2 pressure. Condenser put back into service.

The corrosion of tubesheet was attributed to the pitting corrosion due to chemistry of the cooling water in circulation.

Remedial Measures to Improve Reliability
To overcome the problem of tube sheet damage as explained above for long term remedial measures to improve reliability of the condensers. Two alternatives appeared during the process of repair plan.

a. Replacement of complete tube bundle with new one with stationary tube sheet of SS321 material.
b. Reconditioning of existing tube sheet by In-situ machining the corroded surface and re-welding and expansion of tube to tube sheet joint.

Second alternative was a cost effective and seemed to be quite practicable. Tube sheet surface was required to be machined up to at 6 mm depth. The design calculations were checked for strength values. Minimum required thickness was 40.4 mm where as 65 mm thickness was adopted including corrosion allowance of 3 mm. Hence, machining of tube sheet by max depth of 6 mm was quite within the acceptable values.

In-situ machining being a specialised job the agencies performing such jobs was contacted. After technical review of the various aspects related to machining activities, the job was arranged through M/s Gemini Power Hydraulics Limited, Mumbai.

Repair Activities at Site
a. Removal of channel section for making access to tube sheet. Cleaning of tubes and tube sheet surface.
b. Preparation of fixture for mounting Milling machine.
c. Leveling and centering of the machine on the tube sheet. Mounting plate was clamped on tube sheet using C-clamps. It was supported at the bottom by stoppers inserted in the tubes. (see Figure 3)
d. Machining of tube sheet face including tubes and welds to remove the pitting marks. Tube area was machined up to 6 mm depth and non-tube area was machined up to 4.5 mm depth.
e. Machining of par tition grooves and peripheral Gasket seating surface.
f. J-grooves were prepared around the tubes for tube to tube sheet welding. Dye Penetrate (DP) test of the entire tube sheet was done to ensure defect free surface.
g. Tube to tube sheet welding done using ER-309 filler wires. All the joints were checked by DP test and defects observed were attended. (see Figure 4)
h. Weld overlay was done on the non-tube area using E309-Cb-16 electrodes. Preheat temperature of 125oC was maintained during welding. DP test of the entire tube sheet done to ensure defect free surface. (see Figure 4)
i. Light expansion (3 per cent) of tubes was done.
j. Pneumatic test of the tubes and tube to tube sheet joints done at 4 kg/cm2 pressure. No defect observed.
k. Channel section boxed up and condenser released for operation.
a. Corrosion allowance of 3 mm was considered during design stage. But the practical results reveals increased rate of corrosion and pitting with regard to cooling water chemistry.
b. Innovative methods shall be explored to salvage the High value components. In this case, the cost of reconditioning of tube sheet with improved metallurgy is about 28 per cent of the complete new tube bundle. Condition of tube sheet was subsequently inspected and found quite satisfactory.