Pergamon Microelectron. Reliab., Vol. 36, No. 4, pp. 477-480, 1996 Copyright © 1996Elsevier ScienceLtd Printed in Great Britain. All rights reserved 0026-2714/96 $15.00 + .00 0026-2714(95)00100-X BEHAVIOURAL ANALYSIS OF A SHELL GASIFICATION AND CARBON RECOVERY PROCESS IN A UREA FERTILIZER PLANT S. KUMAR Mechanical Engineering Department, Regional Engineering College, Hamirpur, (H.P.), India D. KUMAR and N. P. MEHTA Mechanical Engineering Department, Regional Engineering College, Kurukshetra 132119, India (Received for publication 20 April 1995) Abstract--The paper discusses the behavioural analysis for a shell gasificationand carbon recovery process in a urea fertilizerplant. The system consists of three subsystems D, E and F arranged in series, with three possible states--good, reduced and failed. The subsystem D has three parallel operating units with two units in cold standby. Failure and repair rates for each subsystem are taken as constant. Formulation of the problem is carried out using simple probability consideration. An expressionfor steady-state availability is derived. Taking data from a medium sized plant, the effect of each working unit on the system availability is tabulated. The results are supplied to the plant personnel for improvement in designing the system and planning the process for minimum failure. INTRODUCTION A probabilistic analysis of the system under given operative conditions is helpful in the design modifica- tion for minimum failure of the system and thus to optimize the system workings [1]. A urea fertilizer plant consists of a large number of sub-systems interconnected in series/parallel or both. These sub-systems are subjected to random failure and can be brought back into service after repairs/ replacement. The failures of sub-systems and their components depend upon the operating conditions and repair policy used which are difficult to predict precisely. Behavioural analysis is a tool to economize operational parameters in order to ensure the maxi- mum possible level of system availability 12]. The ammonia production process comprises various sub-systems namely air separation, shell gasification and carbon recovery, desulphurization, CO shift conversion, decarbonation, nitrogen washing and ammonia synthesis processes including refrigeration process [3]. One of the most important functional parts of the ammonia plant is the shell gasification and carbon recovery process which is the subject of our present discussion. It consists of three sub-systems: the high pressure feed pump (D) (having five units-- three working and two in standby), (E) gasifier, waste heat boiler, gasification economizer, quench pipe and carbon separator and carbon scrubber (F). The system model for the plant has been developed on the basis of an actual study conducted in a medium sized urea fertilizer plant located near Kurukshetra. The failure time/repair time data in the plant were studied. Based on the model, behavioural analysis of the system is carried out. The findings of the paper have been discussed with the management of the plants with a view to appraise them of the potential benefits that would accrue through the implementa- tion of the analytical results. MODELLING The system comprises of three sub-systems in series: (i) Sub system (D~, i= I-5) having three working units in parallel and two in standby, used to raise the pressure of heavy oil from 4 to 189 kg cm- 2 Failure of any one unit forces one to run the system with standby units. The complete failure of the system occurs only when three pumps are in a failed state. (ii) Sub-system (Ej, j = 1-5), having five units in series each unit comprising three parallel working units. Failure of any one unit reduces the capacity of producing ammonia. Complete failure occurs only when two units fail simultaneously. (iii) Sub-system (F), used to remove carbon through washing and cooling process. Its failure causes complete failure of the system. ASSUMPTIONS • Failure and repair rates are constant and statistically independent; • a repaired unit is as good as new; • repairmen are always available; • each sub-system has a separate repair facility and there is no waiting time for repair in the sub-systems; • service includes repair and/or replacement of the units; • the standby units are of the same nature and capacity as the active units; 477