Pump Specific Speed and Suction Specific Speed
Suction specific speed is one of the critical items that should be considered in pump selection phase (basically in technical evaluation before purchase order). Most of client specifications indicate limitation of suction specific speed values in pump selection criteria. When talking about suction specific speed concept, it is better also to talk about specific speed concept to know their differences
Fig-1 Specific Speed vs Impeller Design
A. Specific speed
Specific speed is a non-dimensional value that is a function of pump speed, flow and head
Ns = RPM (Q^0.5) / H^3/4
Ns= specific speed
RPM = pump speed
Q = pump flow
H = pump head
Note: For double suction impellers, Q = Q/2, where Q, and H are taken at best efficiency point (BEP)
Specific speed is like a type number classification, the value is used to determine the type of pump. For example, radial type impeller, mixed flow impeller type or axial flow impeller type. Suction specific speed is different, it is related to low velocity stall or suction recirculation with regard to cavitation.
Specific speed is used to optimize stage efficiency for a given value of flow and head.
In pump design, specific speed is used to optimize the following design parameters:
- Impeller discharge flow velocity
- Impeller tip speed
- Impeller inlet and discharge blade angles
- Discharge throat velocity
B. Suction Specific speed
S (suction specific speed) is determined by the same equation used for specific speed Ns but substitutes NPSHR for Head. S considers the inlet of the impeller and is related to the impeller inlet velocity. The relationship for S is:
S = RPM (Q^0.5) / NPSHR^3/4
S= specific speed
RPM = pump speed
Q = pump flow
H = pump head
Note: For double suction impellers, Q = Q/2, where Q, NPSHR are taken at best efficiency point (BEP)
NPSHR is related to the pressure drop from the inlet flange to the impeller. The higher the NPSHR, the greater the pressures drop. The lower the NPSHR, the less the pressures drop. From the equation above, we can show the relationships between NPSHR, S, inlet velocity, inlet pressure drop and the probability of flow separation. For high suction specific speed will effect on low NPSHR, low inlet velocity and low inlet passage pressure drop, this make high probability of flow separation. So, flow separation will occur for high specific speeds resulting from low inlet velocity.
Then the question is at what flow the disturbance and resulting cavitation happen. This is not easy to answer, since unstable flow range is a function of the impeller inlet design and inlet velocity. In general recirculation is as a function of suction specific speed, so the onset flow of recirculation increases with increasing suction specific speed. It means that the higher the value of suction specific, the sooner the pump will cavitate when operating at flows below its best efficiency point. The term “minimum stable continuous flow” is the flow at which the onset of suction recirculation can begin.
Theoretically, increasing the eye diameter decreases the inlet velocity and thus reduces the NPSHR. Reduction in inlet velocity causes NPSHR to drop as flow moves to the left of pump performance curve and when the speed is reduced. It is no problem as long as flow remains at or near best efficiency point. If flow moves to far to the left of its best efficiency point, the increased peripheral velocity distorts the flow into the inlet and the flow back out of the impeller, then suction recirculation occurs. During this recirculation, intense vortices arise and cause low pressure areas that will lead to cavitation and severe pressure pulsations. The effect of impeller eye diameter on potential suction recirculation can be evaluated using Suction Specific Speed (S).
For centrifugal pump, suction specific speed can range from about 5000 to over 20000 (in rpm speed, cubic meters per hour capacity and meters Head). Some international standard recommend suction specific speed is under 11000 in order to maintain a reasonable range of flows without the potential for suction recirculation
