Many times we come across the term pump suction specific speed(Nsss) during design and pump selection stages. If you wonder why this term is used quite often in pump selection then this article will help you to understand it.
Suction Specific Speed
It is a dimensionless number and index relating flow, Net Positive Suction Head required (NPSHr), and speed for pumps. The suction specific speed of the pump must always be calculated for the conditions corresponding to the capacity at Best Efficiency Point (BEP) of the pump at maximum impeller diameter. (Refer below the figure for suction specific speed formula)
In the case of the double suction impeller which actually has two inlets. Hence the flow is divided by two. Refer below figure for formula.
Importance of Suction specific speed
The following are some of the points to highlight the importance of suction specific speed
- The pump suction specific speed will provide an assessment of a pump’s susceptibility to internal recirculation.
- It acts as a tool for comparing cavitation and predicting NPSHr of the pumps. For a given specific speed, the lower the value of Nsss, safer the pump against cavitations.
- Suction specific speed is used to limit the pump speed. Increase in pump speed without proper suction condition can result in abnormal wear and possible failure for excessive vibration, noise and cavitation damage.
Reason of Limit the Suction Specific Speed
As we seen from the above formula, the suction specific speed value is increased with decreased in NPSHr. The figure shows how the NPSHr curve varies with respect to Pump Suction Specific Speed.
Normally the NPSHr value can be decreased by increasing the eye diameter of the impeller. Thereby the inlet velocity of the pumping liquid decreases and thus the NPSH required is reduced at the same pump speed. Due to the increased in eye diameter, the peripheral velocity at impeller eye is increased. This will cause some portion of the inlet liquid will flow back to the impeller. This is called as suction recirculation (refer below figure).
During suction 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 (Nsss).
Suction Specific Speed Limitation
Till date, there is no standard limit for suction specific speed available in International standards. On the other hand, each end user(purchaser) having their own standard to limit the suction specific speed based on their experience. In general, the maximum acceptable limit of suction specific speed is
Nsss (Metric unit) = 10000 m3/hr,m,rpm
Nsss (US unit) = 8500 gpm,ft,rpm
Low energy pumps:
The pump whose Nsss value is less 8000(metric unit) is called as low energy pump.
High energy pumps:
The pump whose Nsss value is greater than 13,000(metric unit) are called as high energy pump. While selecting high energy pumps, both purchaser and pump manufacturer shall jointly adopt an NPSH test procedure for measuring vibration and abnormal noise at the pump operating range. So that vibration and abnormal noise produced due to cavitation will be examined and corrective action to be taken.