Monday, February 5, 2018

Impact of Design Defect on Steady Operation of Alumina Refinery

         Hi Friends,

We are well aware that alumina production is a continuous hydro-metallurgical process. The steady operation of plant and equipment results in remarkable plant performance. At times, minor design defects causes severe operational problems in Alumina refinery. Unsteady operation of Alumina refinery directly impacts the efficiency of the plant. Thus, all out efforts are made to minimize the design defects during implementation / modification stages of plant / equipment / facilities. 

In earlier posts, we have highlighted a few design defects and the methodology to overcome those limitations as well. In present post, we will discuss in brief a few additional design defects which have been experienced by the author in the operative plants in India and abroad. A few design defects are briefly outlined in subsequent paragraphs-

1.       Bauxite slurry heating system: 
Generally, heating of ground bauxite slurry at around 100 degree centigrade is carried out either during pre-desilication or before feeding to pre-desilication tanks. The heating of slurry is carried out to attain the desilication temperature by following four methods-
·       Direct injection of steam to pre-desilication tanks,
·       Indirect heating using Double pipe heat exchangers,
·       Indirect heating using spiral heat exchangers and
·       Indirect heating using Shell & tube heat exchangers.
Direct injection to bauxite slurry increases the dilution to process and hence never considered. Double pipe heat exchanger for bauxite slurry heating is not recommended because of acute scaling problems for the reasons already explained in earlier post. Last two options stated above are recommended among which the last one i.e. Shell & tube heat exchanger is the preferred choice because of lower capital investment as well as ease in maintenance.

2.       Schedule of pipes for slurry handling: 
Slurry generated in red area of Alumina refinery has high abrasiveness because of presence of silica which causes erosion of pipelines. Thus, pipelines of adequate thickness and schedule 80 should be installed.  Schedule-40 pipes should always be avoided as it will require frequent replacement.

3.       Selection of MoC for pumps, valves and fittings: 
As such carbon steel is the best suited material of construction (MoC) for Alumina refinery having alkaline environment. But presence of silica causes fast erosion in stationary as well as rotating equipment. Thus, for handling abrasive slurry in red area, equipment / components made of carbon steel as well as Nicast are never used. For such critical service conditions, Nihard / high nickel steel / high chrome steel is the right choice. Never compromise on MoC of impeller and casing of slurry pumps, connected valves and inlet Tees of slurry flash tanks particularly for handling Bauxite slurry. Ensure the right MoC for these critical components for steady operation of Alumina refinery.

4.       Minimum shell thickness of Digesters:
Digesters are the pressure vessels installed for dissolution of alumina present in bauxite in caustic liquor. The digestion vessels are designed on the basis of its recommended operating conditions. The shell thickness is always based on the pressure but shell thickness is always kept more than 20 mm to take care of any eventualities because of instrument failure and operational lapses as well.

5.       Provision of non-return valve in steam lines to process vessels: 
Alkalinity in boiler feed water is controlled below 0.01 gpl Na2CO3. Though direct injection of steam to process are always avoided but a few designers keep provisions. In such situation, non-return valve in steam line near process area is a must as sudden drop in boiler pressure for any reason may case backing of alkaline liquid to steam lines which may develop stress corrosion ultimately causing sudden failure of steam line within short duration. It is essential for safeguarding the equipment as well as safety of operating personnel in the plant.

6.       Provision of flanges in pipelines: 
Sometimes, flanges are not provided in slurry pipelines to minimize capital investment in project stage. But, it cause severe operational problems because of difficulties in cleaning of welded lines. Thus flanges must be provided at the interval of 6m to 10m depending of practical experience operating personnel.

7.       Adequate capacity of Evaporator Hot well: 
Tail pipe of Barometric condenser is kept submerged to a depth of about 0.50 meter in water of Hot well of evaporation unit. The Hot well is designed to maintain adequate seal water for circuit. The volume of Hot well is always kept more than five times the submerged volume of leg of Barometric condenser. If water volume in Hot well is inadequate, the vacuum seal will be broken and air drawn through tail pipe will drastically affect the performance of evaporation unit.

8.       Minimum plate thickness of tail pipe of Barometric condenser: 
Generally, the vacuum requirement for efficient evaporation system is 685 mm of mercury. Thus, the minimum acceptable plate thickness for the tail pipe is 6 mm. Otherwise, sudden increase in vacuum during start up with negligible load of non-condensable may cause rupture of tail pipe of Barometric condenser. This problem was faced in a newly constructed Alumina refinery which could be rectified by carrying out modification for re-startup.

9.       RPM of impellers of slurry pumps: 
RPM of impeller is the prime criteria for selecting the pumps for Alumina refinery. In Alumina refinery, impeller rpm for handling bauxite residue as well as hydrate is relatively kept at lowest possible level so as to avoid breaking of the flocs and particles. This issue has already been discussed at length in earlier posts.

These are just a few examples of design defects of Alumina refinery. There are many such design defects observed in Alumina refinery which badly affects the continuous operation of Alumina refinery and thereby by huge monetary losses for the company. Hence, role of Owner’s consultant becomes very important to avoid such design defects during engineering state of execution of the plant. We would like to have your comments / remarks for further improvement.

You may like the article on "Systematic approach for design of Slurry mixing agitator" published earlier.
Best regards.

Rajendra Kunwar
Principal Consultant-Engineering
CETI Gurugram, Haryana, India.

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