When selecting a centrifugal pump, it is important to consider a range of parameters to ensure an optimal result. This guideline outlines key considerations that should be taken into account during the technical bid evaluation process.
Technical Bid Evaluation of Centrifugal Pumps
NPSH Requirements
The offered model should be proven and should have been operating satisfactorily for a minimum of 1 year in at least one installation. Additionally, the pump should not be selected in case the difference between NPSHA is less than 0.5 meters. NPSH testing must be asked whether the liquid is at/or near its boiling point, the liquid is gas saturated, or wherever NPSHA is less than 3 m.
Performance Correction
Pump performance must be corrected for viscous fluids; the performance correction factor given by the vendor should be checked with respect to hydraulic institute standards. An equivalent parameter for water must be mentioned on the datasheet.
Low Speed Pumps
In case a low-speed pump is required, a higher-speed pump should not be accepted unless substantiated with plausible technical reasons and operating experience for a similar application.
Parallel Operation
The pumps required for parallel should have a minimum of 10-20% higher shut-off head than the operating head. Pump with flat and drooping characteristics towards shut-off should not be selected for parallel operation. In the case of a single pump operation, the pump minimum flow shall be greater than the flow corresponding to the differential head coinciding with the shut-off head in the drooping characteristics.
Indigenous Pump Vendors
In the case of any indigenous pump vendors offering the pump with an inducer, his experience, and technical backup should be checked.
Double Volute Pumps
Double volute pumps will be preferred for high-capacity pumps (more than 250 m3/hr) and specifically for applications where continuous part-load operation is envisaged.
Impeller Type
Impeller type should conform to that specified in the datasheet, generally open impellers will be preferred for liquid with suspended solids.
Flanges
Non-standard flanges i,e 1-1/4’’, 2-1/2’’,5’’,7’’, and 9’’ should be provided with companion flanges by the vendor with nuts, bolts, and gaskets.
Suction Specific Speed
In the case of large water pumps, suction-specific speed should be calculated at the BEP of the maximum diameter of the impeller. Generally, this should be below 12,000 (US units). Pumps with suction-specific speeds above 12,000 with inducers are only acceptable. Sundyne pumps with higher specific speeds are acceptable. The vendor should be asked for a reference list for higher specific suction speed pumps. In case the vendor has provided pumps with satisfactory results even with suction specific speed >12000, pumps should be accepted with at least 20% minimum continuous flow of BEP flow.
Flushing Piping
Flushing piping generally is in SS and in the vendor’s scope of supply. Flushing piping of size less than ½’’ shall not be accepted. Piping with a screwed and threaded connection is not accepted. All the flushing plan piping shall be welded and flanged and terminated with ANSI flanges.
Mechanical Seals
Spring, hardware, and secondary elastomeric of the mechanical seal should be compatible with liquid handled. In the case of Plan 32 dilution of pumped liquid with flushing, the liquid will take place. The rate of dilution should be obtained from the vendor and should be exceeding the limit specified flow if any.
Cooling/Heating Fluid
Cooling/heating fluid quantities must be obtained from vendors. In the case of jacketed pumps where casing, bearings housing, and/or stuffing boxes are required to be jacketed, it should be checked whether the vendor has supplied a pump of similar capacity in jacked construction. Jacket design/test pressure should be specified in the inquiry.
Material of Construction
In the case of pumps with special metallurgy, it must be confirmed by the vendor if he has supplied an equivalent model in similar metallurgy. Any deviation in the material of construction should be brought to notice for acceptance or rejection. Hardness difference of 50 BHN should be maintained between wear rings. In the case of Austenitic steels, hard-faced wear rings should be asked for. Heavy-duty base plates wherever absolutely necessary should be taken for pumps conforming to API 610 requirements.
Motor Selection
Wherever pumps are expected to start on open discharge valve, motor KW selection shall be as per end of curve KW required. Also, GD2 value starting torque and speed-torque characteristics of the pumps should be asked for all motors rating of pumps starting to open discharge.
Inspection and Testing
Inspection and testing, documentation requirements should be checked according to project specification. The technical recommendations should be clear and without any open points. Reasons for acceptability/non-acceptability should be clearly indicated.
Vertical Pumps
In the case of vertical pumps, the installation length of pumps to be asked.
Interconnecting Piping
Design and engineering for Interconnecting piping between Fluid Coupling, Flushing Plan, Seal Plan, and Cooler. Piping Interface flange connections shall be as per ASME B16.5. Confirm nozzle allowable loads shall be 2 times the loads as indicated in Table 5 “External nozzle forces and moments” of API 610.