Choosing a right pump for any chemical process plant is very crucial in engineering because it is going to decide the faith of plant. By going wrong in selection process may not only put plant in danger but may also lead in irreparable damages and the company has to pay for it heavily. Hence, pump is the heart of any chemical process plant and so pump selection for chemical process plant has to be done carefully.
There are various types of pumps available in the market, which can be used for different applications and hence, it is very necessary that we choose right type of pump for a particular process. As far as flue gas desulphurisation(FGD) is concerned, most of the pumps are of centrifugal and piston plunger types. Some of the important factors for proper selection of pump in such cases are type of liquid, liquid composition, type of solids in the liquid, pump capacity, pump head and the suction head available.
Nowadays, caustic-based and lime-based FGD are more used compared to others. One of the foremost reasons behind this is, Caustic-based systems are much cleaner and easy to handle compared to lime-based system due to this pump selection in caustic-based system is much simpler than lime-based system.
RULES FOR THE SELECTION OF LIME-BASED SYSTEM:
MATERIALS: The two types of limes used in FGD operation are hydrated lime and limestone. Limestone is more abrasive than hydrated lime, so the selection of material for the construction (MOC) of casing and impeller is the crucial factor. Hard MOC is usually preferred over other kinds of MOC. Materials having hardness of 600 to 650 BHN is also commonly used MOC for limestone-based application. For hydrated lime application, Stainless steel may be right choice having up to 10-12% solid concentration. If solids are hard then MOC of pump must be soft. This is because due to soft quality of MOC of a pump, solids are bounced back and there is less wear of pump components. In soft construction of pump, rubber lining to casing and impeller is one of the options. Non-metallic pumps are also an option, but sticking to mechanical design criteria is the best option as far as international standard for such pumps is concerned.
TYPE OF IMPELLER: In FGD, the re-circulation liquid has 9 to 10% concentration of lime and it is in the form of solids in the re-circulation liquid so pumps must have Impeller of semi-open construction.
1. Operations are smoother and non-clogged.
2. Less wears and tears of impeller.
1. operation is less efficient.
2. Prone to more vibration.
SELECTION OF RPM FOR PUMPS :
Selection of RPM is one of the most important factors, which decides its operation. But, at the same time operating cost and maintenance cost are also two factors, which cant be neglected in FGD projects.
Basically, there are two rpms for pumps
1. low rpm
2. high rpm
Low rpm pumps require high initial investment but lasts for a longer time.
ADVANTAGE OF LOW RPM PUMPS:
1. Long product life.
2. Minimum vibration.
3. Reduced Downtime.
DISADVANTAGE OF LOW RPM PUMPS:
1. High initial settlement cost.
2. Heavier construction.
High rpm pumps are also very effective but again construction and maintenance cost is high.
SYSTEM RESISTANCE CURVE WITH RESPECT TO PERFORMANCE CURVE.
Consideration for system resistance curve with respect to performance curve is very important as it determines vibration, impeller wear, noisy operation, frequent bearing failure, etc. Therefore, it is very important for proper working following care is taken:
1. Flow rate at BEP should be more than normal operating flow of the pump. This kind of technique also ensures smoother operation of pump.
2. The diameter of impeller should not be less than 105% of minimum impeller diameter of selected pump. This ensures flexibility in adjusting head of pumps by trimming the impeller.
There are two types of pump available viz., horizontal back pull out type and vertical glandless pumps. Among the two, horizontal back pull out type is commonly used while vertical glandless pumps are better option available in the market for reducing operational and maintenance cost of pumps.
1. Eliminates operational and maintenance cost of mechanical seals
2. Strong in construction.
3. Available in different MOC.
4. Initial cost is less compared to horizontal pumps.
1. Mounting of pumps is more difficult compared to horizontal pumps
2. It creates complexity in re-circulation tank design.
3. To avoid cavitation, extra precaution is required.
SELECTION OF SEALING FOR PUMPS.
Appropriate selection of sealing ensures good operation and maintenance. Correct selection of seals for pumps may reduce operation and maintenance cost by 20-30%. Basically, there are two types of seals viz. Mechanical seal and cartridge seal.
For acidic/liquid with solids, mechanical seals are necessary. But at the same time maintenance and high replacement costs are its big problem
Cartridge-type mechanical seals are useful to reduce downtime of pumps. In addition to it, Gland and stuffing box setting is also more popular and easy to maintain for water application and clear non-hazardous liquid.
CONCLUSION: THE ABOVE-MENTIONED METHODS WILL NOT ONLY HELP TO CHOOSE RIGHT PUMP, BUT MAY ALSO INCREASE EFFIENCY AND REDUCE OPERATIONAL PROBLEMS OF PUMPS IF PROPERLY APPLIED.