Reliability Improvement of MP Decomposer

Reliability Improvement of MP Decomposer

Introduction
The intent of this article is to discuss the experience for the reliability improvement of one of the important exchanger in Urea plant i.e. Medium Pressure decomposer (MPD).
Function of MPD is to decompose unconverted carbamate into carbon-dioxide and ammonia increasing the urea solution concentration. MPD has a unique design, being divided in to two shells with single tube bundle. The upper shell heating media is boosted LS at 6.5 kg/cm2 whereas the remaining heat is provided by saturated medium pressure condensate at 22.0 kg/cm2, in the bottom exchanger. An intermediate tube sheet separates the two shells and heating media.
Intermediate tube sheet leakage was first experienced during the hydraulic test of exchanger shell side. This passing of MP condensate from lower shell to upper shell is posing a threat to the tubes with reference to the impingement of condensate at increased velocities.
Study/ Modification Description
To improve the reliability of MPD following options were considered:

1. Complete by-passing of Exchanger-B shell side:

Completely by-passing of lower exchanger was considered, however due to considerable decrease in heat duty and reduction in plant load, the option was dropped.

1. Provision of MP Steam in Exchanger - A:   

Providing MS on the shell side of Exchanger - A would decrease the pressure difference across the intermediate tube sheet. However due to unavailability of surplus MS, the option of letting-down KS to MS was not economically justified.

1. Re-rolling of MPD Tubes:

Re-rolling of MPD tubes was also a possible solution (confirmed by OEM).
CTI (experts in tubes re-rolling) declined to perform in-situ tubes re-rolling job in view of specific conditions of MPD, as the enlargement of tube sheet holes is most likely eccentric and there would be thickness loss after 12 years of operation therefore danger of tube failure with expansion, either with hydraulic pressure or mechanical re-rolling remains present.

Therefore, tubes re-rolling option was dropped owing to the associated risk of damage to the tubes.

Provision of flashed steam at ~6.5 kg/cm2g to Exchanger - B:

The option was to flash MP condensate from 22.0 to 6.5 kg/cm2g. This flashed steam is to be introduced in Exchanger - B (In place of MP condensate). This will reduce pressure difference across the intermediate tube sheet and thus the by-passing problem.

Conclusion
Provision of flashed steam at 6.5 kg/cm2g to Exchanger - B being the only feasible solution to improve the reliability of MPD against the possible threat of tube leakage was carried out.