Rotodynamic pump is a generic term which classifies radial-vane, Francis-vane, mixed-flow and axial flow pumps. Very often, this term is incorrectly interchanged with the word centrifugal pump. Strictly speaking, only radial-vane pumps are classified as centrifugal pumps.
Specific speed of pump:
- It is a dimensionless number that describes the shape of a rotodynamic pump impeller.
- Specific speed of most refinery pumps ranges from xxx to xxxx.
- Pumps with high head low capacity occupy the range xxx to xxxx, whereas low head high capacity pumps have a specific speed of xxxxx or larger.
- Pump efficiencies start to drop drastically at specific speeds below xxxx. Additionally, smaller pump capacities exhibit lower efficiencies than higher capacities at all specific speed.
- The major use of specific speed number is to help us specify pumps that are more efficient. Maximum pump efficiency is obtained in the specific speed range of xxxx.
- Pump affinity laws assume that pump impeller shrouds are parallel, this is only true for radial-vane pump.
- Pumps with low to medium specific speeds, b.h.p curves decrease with reduction with Q. Thus, operating such pumps at lower than design flow will not overload the driver. For pumps with high specific speed, b.h.p. curves increase with a reduction in Q, operating such pumps at lower than design flow may overload the driver, and the min. permissible flow is dictated by the selected driver horsepower.
Suction Specific Speed (SSS) of pump:
- It is calculated for a pump at its B.E.P. with the max. diameter impeller and provides an assessment of a pump’s susceptibility to internal recirculation.
- SSS formula is for single suction pumps. If the pump is a double suction design, then BEP flow rate for the calculation is simply split in half. For multistage pumps, it is the first stage flow and NPSHr.
- All centrifugal pumps are subject to internal recirculation in the suction and discharge areas of the impeller at certain flows below its B.E.P.. In general, internal recirculation at the pump suction is the most frequent cause of field problems.
- The capacity at which suction recirculation occurs is directly related to its suction specific speed (SSS). The higher the SSS, the closer the beginning of recirculation will be to the B.E.P.. Operating a pump at flows below the recirculation capacity leads to high vibration/noise, premature impeller erosion, and premature bearing failure.
- Pump recirculation can cause surging and cavitation even when the NPSHA exceeds the pump manufacturer’s published NPSHr by a considerable margin.
- Impeller patterns with SSS greater than about xxxxx will require a minimum flow of xx% of B.E.P..
- Most pumps cannot be operated below xx % of B.E.P. flow without incurring internal recirculation.
- In general, minimum continuous stable flow increases as SSS increases
Preferred Operating Region of Pump:
- Some pump vendors are not aware of the phenomena of internal recirculation and may not even heard of the term minimum continuous stable flow. They actually select pumps based on the recommendation given by pump manufacturer’s software.
Pump Construction:
- Cast steel is often required in refinery or high pressure service because it is more resistant to fracture than cast iron when sprayed with water during a fire.
- Cast iron pumps are never provided with raised face flanges. Therefore, steel flanges at suction or discharge piping should be of flat-face and not raised-face type. Full-face gaskets must be used with cast iron pumps.
- When NPSHA is small, double suction pumps may be considered in lieu of single suction pumps. However, double suction pumps are more vulnerable to recirculation than single suction ones and may require minimum flows in the B.E.P. range of xx%.
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