Empirical studies indicates that safe operating ranges from Best Efficiency Points (BEP) are more narrow at higher Suction Specific Speeds. The specific speed (ns) is a characteristic coefficient derived from the similarity conditions which allows a comparison of impellers of various pump sizes. Which suction specific speeds are acceptable?Īs a rule of thumb the Specific Suction Speed should be below 9000 (calculated with US gpm) to avoid cavitation and unstable and unreliable operations. It should not be confused with specific speed (ns) and is always defined as a positive figure. Figure 4.3 Pressure and volumetric flow rate transients using the standard pump curve data. The rotating frequency of the pump shaft therefore characterises a pump’s rotational speed (n). There is a conversion factor of 51.6 between the two versions.

In such cases it should be made clear what unit system is involved - for example Ns 4000 (gpm units) or Ns 77.5 (metric units). The unit used for rotational speed is s–1 (rev/s) pump speed is generally given in min–1 (rpm). Note: Specific speeds using this definition are often presented as 'dimensionless' numbers - but the value returned depends on the unit system used. The specific speed of a turbine is given by the manufacturer (along with other ratings) and will always refer to the point of maximum efficiency. Equation 1 gives the value for the pump specific speed, H is the pump total head, N the speed of the impeller and Q the flow rate. The specific speed value for a turbine is the speed of a geometrically similar turbine which would produce unit power (one kilowatt) under unit head (one meter). A large value of D/d reduces the rpm as well as the mechanical efficiency of the wheel. The Pelton wheels with a single jet operate in the specific speed range of 4-16, and therefore the ratio D/d lies between 6 to 26 as given by the Eq. The specific speed is useful to compare the. What is specific speed of Pelton turbine? It is defined as the speed of a similar turbine working under a head of 1 m to produce a power output of 1 kW. It is an important factor used in the design and selection of pumps and impellers. Specific Speed is a dimensionless rating of pump discharge performance derived from an equation involving shaft speed, flow rate and differential head at a pump’s Best Efficiency Point. Water - Density Viscosity Specific Weight, Dynamic and Kinematic Viscosity of Water in Imperial Units (BG units).

Specific speed ns in english units