Leakage Detection Problems on Bottom Tubesheets of Vertically Positioned HP Exchangers: An Experience With a Urea Stripper Available to Purchase

The exchanger under consideration is a bimetallic urea stripper, a high pressure heat exchanger used for stripping urea from carbamate solution at urea fertilizer plants. Its typical orientation with respect to ground is vertical, medium pressure steam being used as the shell side fluid. The paper provides an account of problems related to detecting tube leakage(s) at the bottom tube sheet of a urea stripper, primarily because of its positioning and presence of shell side condensate. This caused water to be retained in the tube to tubesheet annular spaces, hence forming a water seal and creating a barrier for the pressurization gas preventing it to pass through the leakage path towards the tube side. In order to save valuable time during a plant shutdown, a new technique of carrying out a helium-ammonia leak test was indigenously developed, tested and applied. However, despite the application of well developed and time-tested techniques of helium and ammonia testing, leakage location could not be ascertained and the tests remained largely inconclusive causing unnecessary delay and anxiety. The only real clue during inspection was a few droplets emerging from one of the tubes and the health of remaining 2387 tubes could not be reliably established. The paper opens with a brief discussion outlining the urea synthesis process along with the technical data specific to the urea stripper under discussion. The focus of the paper, however, remains at sharing with the reader; problems experienced during its leak testing and further recommend possible measures to tackle these problems in order to achieve improved testing effectiveness.