How do I calculate specific speed?

Specific speed is calculated from the following formula, using data from these curves at the pump's best efficiency point (bep.):

As an example we will make a calculation of Ns in both metric and U.S. units :

1. Q= 110 L/sec. or 396 M3/ hour or 1744 G.P.M.
2. H = 95 meters or 312 feet.
3. Speed = 1450 rpm.

Also question is, what is meant by specific speed?

Specific speed is an index used to predict desired pump or turbine performance. i.e. it predicts the general shape of a pumps impeller. It is this impeller's "shape" that predicts its flow and head characteristics so that the designer can then select a pump or turbine most appropriate for a particular application.

Also Know, what is specific speed of a turbine? specific speed. – Specific speed of a turbine is defined as the speed of a geometrically similar turbine which produce a unit power when working under a unit head. – By geometrically similar means that turbine is identical in shape, dimensions, blade angles and gate opening etc. it is denoted by the symbol Ns.

Similarly, it is asked, what is specific speed of a centrifugal pump?

Specific speed is defined as “the speed of an ideal pump geometrically similar to the actual pump, which when running at this speed will raise a unit of volume, in a unit of time through a unit of head”. The performance of a centrifugal pump is expressed in terms of pump speed, total head, and required flow.

Is specific speed dimensionless?

Specific Speed Definition The specific speed (NS) of an impeller or pump is a dimensionless index number used to relate the hydraulic performance of a centrifugal pump to the shape and physical proportions of its impeller.

Related Question Answers

What is the unit of specific speed?

Specific speed is the speed (in RPM) a geometrically similar impeller would need to operate in order to deliver 1 L/s of fluid at a head of 1 m (US units : 1 USgal/min at a head of 1 ft).

Which turbine has highest specific speed?

Pelton turbine

What is suction specific speed?

A. Suction Specific Speed is an index that describes the characteristics of the suction side of the impeller. It is calculated using the NPSHr in feet, speed (N) in rpm, and flow rate (Q) in gpm at the pump's Best Efficiency Point and maximum impeller diameter.

What is pump speed?

Pump Specific Speed, N_S is a method of characterising a pump duty by head, flow rate and rotational speed. Pump specific speed may be used to determine an appropriate pump design for a given application when choosing between axial, radial or multistage centrifugal designs or positive displacement pumps.

Which pump is more efficient?

centrifugal pumps

How do you calculate suction speed?

Suction specific speed

1. Q = Capacity in GPM with largest impeller at the BEP.
2. NPSH = Net positive suction head required to prevent cavitation. Remember that this number is for 68°F. (20°C.) fresh water.
3. To raise a number to the 3/4 power, cube the number and then take the square root twice.

What does it mean when a pump is cavitating?

Simply defined, cavitation is the formation of bubbles or cavities in liquid, developed in areas of relatively low pressure around an impeller. The imploding or collapsing of these bubbles trigger intense shockwaves inside the pump, causing significant damage to the impeller and/or the pump housing.

What is the difference between pump and turbine?

A pump absorbs useful energy and converts it to kinetic energy and gives it to a fluid stream. The turbine does the exact opposite as it absorbs energy from a fluid stream and converts it to work. A pump increases the energy of the fluid stream whereas a turbine decreases the energy.

What is the significance of specific speed of centrifugal pump?

Specific speed is defined as “the speed of an ideal pump geometrically similar to the actual pump, which when running at this speed will raise a unit of volume, in a unit of time through a unit of head”. The performance of a centrifugal pump is expressed in terms of pump speed, total head, and required flow.

What is pump efficiency?

Pump efficiency is defined as the ratio of water horsepower output from the pump to the shaft horsepower input for the pump. Water horsepower is determined by the flow rate and pressure delivered from the pump. The efficiency of a particular pump is estimated by determining two values.

What will happen when we increase the RPM of a centrifugal pump?

The pump discharge/flow and the pressure/head increases with increase in rpm. i.e. shaft speed, keeping the impeller diameter constant. Consequently, power requirement also increases manifold.

What is NPSH of a centrifugal pump?

The quantity used to determine if the pressure of the liquid being pumped is adequate to avoid cavitation is the net positive suction head (NPSH). The net positive suction head available (NPSHA) is the difference between the pressure at the suction of the pump and the saturation pressure for the liquid being pumped.

What is minimum starting speed of centrifugal pump?

To get the minimum starting speed of a centrifugal pump, it is notice that if the pressure rise in the Impeller is more than or equal to manometric head (Hm) then the centrifugal pump will start to deliver water. If the impeller is rotating, the water in contact with the Impeller is also rotating.

Why priming is used in centrifugal pump?

So impeller CANNOT impart enough energy to air to go out of casing and suck water so priming is compulsory for in case of centrifugal pump. In case of reciprocating pump, it can push out all air by itself as it has suction and delivery valves to displace “fluid” positively.

Which turbine has highest flow rate?

When well designed, a Francis turbine can capture 90%–95% of the energy in the water. The Francis design has been used with head heights of from 3 to 600 m but it delivers its best performance between 100 and 300 m. Flow rate is often the limiting factor for a given head.

Which turbine has low specific speed?

The Formula of Specific speed for turbine is as: Above formula implies that for higher head the specific speed gets lower. Thus Pelton turbine with higher head has lower specific speed than Francis and Kaplan turbines which are meant to operate on lower head than the Pelton Turbine.

What is impulse turbine?

An impulse turbine is a turbine that is driven by high velocity jets of water or steam from a nozzle directed on to vanes or buckets attached to a wheel. The resulting impulse (as described by Newton's second law of motion) spins the turbine and removes kinetic energy from the fluid flow.

What is runaway speed of turbine?

The runaway speed of a water turbine is its speed at full flow, and no shaft load. The turbine will be designed to survive the mechanical forces of this speed. The manufacturer will supply the runaway speed rating.

What is RPM in pump?

Rotational speed (also called speed, or speed of rotation) can be quantified as the number of revolutions a rotating system makes within a defined period of time. The unit used for rotational speed is s^–1 (rev/s); pump speed is generally given in min^–1 (rpm).

What is the specific speed of Kaplan turbine?

250 to 1000 rpm

Why does pump efficiency increase with speed?

Pump Performance Pump efficiency also increases with pump rotational speed, especially high speeds. The increase is not as pronounced at 3,600 rpm and below. Specific speed also affects head-capacity curve shape. Intermediate speeds produce curves between the extremes.

What is the formula of velocity?

Velocity Formula. The velocity is the time rate of change of displacement. If 'S' is the displacement of an object in some time 'T', then the velocity is equal to, v = S/T. The units of velocity are m/s or km/hr.

How is the specific speed of the pump related to the rotation speed?

Pump Performance Pump efficiency also increases with pump rotational speed, especially high speeds. The increase is not as pronounced at 3,600 rpm and below. Specific speed also affects head-capacity curve shape. Low specific speeds (500 to 1,500) produce flat curves.

What is priming in centrifugal pump?

Priming is the process in which the impeller of a centrifugal pump will get fully submerged in liquid without any air trap inside. This is especially required when there is the first startup. But it is advisable to start the pump only after priming.