Rotodynamic pump is a generic term to broadly classifies radial-vane, Francis-vane, mixed-flow and axial flow pumps. Very often, rotodynamic pump is often 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. The higher the pump specific speed, the larger the deviation of actual results from affinity law calculations,
- 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 & Relationship with Internal Recirculation:
- 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
- Allowable Operating Window of Pump:
– The allowable operating window of pump decrease as SSS increases.
- NPSH Margin
- NPSHR of a pump is the NPSHA that will cause TDH of pump to be reduced by 3%. This head drop is due to flow blockage by cavitation vapor in the impeller eye. Therefore, NPSH margin is required to prevent onset of cavitation (i.e. incipient reduction in TDH).
- Pump with low suction energy can operate with little NPSH margin. Low suction energy pumps typically do not experience cavitation damage, whereas very high suction energy pumps are typically susceptible to cavitation damage.
- Pump Construction & Modification:
- 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%.
- Pump impellers with a minimum of xx degree vane overlap are typically low suction energy pumps. Impellers with little or no vane overlap have high or very high suction energy respectively.
- Pumps with low SSS have larger impeller OD and smaller impeller eye, whereas pumps with high SSS have smaller impeller OD and larger impeller eye.
- Typically, not more than xx% of impeller diameter is removed by machining when trimming is called for. High clearance between impeller and volute tongue will affect pump efficiency, cause pressure fluctuation and flashing.
- Impeller trimming will shift BEP to the left but it will not affect the minimum flow at which suction recirculation will commence.
- Impeller diameter trimming should be limited to about xx% of calculated values due to the larger discrepancy between Affinity Law calculations and actual results for higher specific pumps.
- Sharpening impeller blades on the underside to increase outlet angle will obtain up to x % more head near the BEP.
- Vertical Turbine Pump (VTP)
- Shaft size and bearing spacing of short set VTP (i.e. < approx. 15m) should be selected such that its 1st critical speed is at least xx% above the maximum operating speed. Typically, intermediate column length and bearing spacing shall not exceed x ft for pump speed above xxxx rpm, and x ft for pump speed up to xxxx rpm.
- Empirical formula for 1st critical speed:
Vcrit = A . d / L2 (rpm)
where A = Empirical coefficient (xx)
L = bearing distance in m (max. bearing distance should be limited to x m)
d = shaft diameter in mm
3. If pump takes suction from sea, distance from discharge head centerline to minimum low water level
D = H + W + S
where H = Height of discharge head centerline above Admiralty chart datum
W = Estimated wave height (e.g. sea state 1 = 0.3m, sea state 2 = 0.9m, etc.)
S = Larger of V or NPSHr
V = Submergence to prevent air suck into suction bell due to vortex (inch)
= D + (0.574 Q / D1.5) (where D=suction bell dia. (inch), Q=flowrate(gpm))
4. Clamp type coupling is preferred to threaded line shaft coupling for better line shaft straightness.
5. Allowable vibration limits with speed above 600rpm, pumping fluid free of solids:
i. Power rating below 200kW
– Preferred operating region: _ mm/s (rms)
– Allowable operating region: _ mm/s (rms)
ii. Power rating above 200kW
– Preferred operating region: _ mm/s (rms)
– Allowable operating region: _ mm/s (rms)
6. Recommended max. tip speed of VTP impeller:
i. Dirty water: _m/s
ii. Slurries up to 25% solids conc. by weight and mean solid size of 200 microns: _ m/s
iii. Slurries with higher conc. of solids and much larger solids size: _ m/s