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Estimating the Specific Speed of a Pump or Turbine

	Applies To
	Product(s):	HAMMER
	Version(s):	08.11.xx.xx, 10.XX.XX.XX
	Area:	Modeling
	Original Author:	Jesse Dringoli, Bentley Technical Support Group

Problem

What is the equation for estimating the specific speed of a pump or turbine?

Background

The specific speed is a function of the impeller geometry and streamlines, and essentially defines the shape of the four-quadrant representation of the pump. HAMMER needs this additional information because of the possibility of reverse flow and speed during a transient simulation. Each Specific Speed corresponds to an internal curve whose values are a percentage of the "rated" flow, head and speed. The "rated" conditions are from the initial conditions calculation - the pump's initial head, flow and speed. So, it is important that the pump be operating at or near its best efficiency point during the initial conditions, so as not to skew the four-quadrant representation.

Solution

The Specific Speed can be estimated with the following equations if it is not available from the manufacturer:

Pumps

Where:

N_s is specific speed (rpm)

N is pump rotational speed (rpm)

Q is flow rate (m³/s or gpm) at the point of best efficiency

H is total head (m or ft) per stage at the point of best efficiency

Turbines

In US units n is in rpm, P is in hp, and H is in ft.
In SI units n is in rpm, P is in kW, and H is in m.

For turbines, there are three different specific speeds available to choose from: “SI=115, US=30,” “SI=170, US=45,” and “SI=230, US=60.” Starting with HAMMER CONNECT Edition Update 3 (version 10.03.02.75, you can display the operating range for the turbine specific speeds. You will see this range in the turbine properties or FlexTables.

Note: In a case where you need to have a specific four-quadrant curve not represented by the choices above, it is possible to create a custom four-quadrant curve. Please see the Help topic “Pump and Turbine Characteristics in Bentley HAMMER” for details and the article in the "see also" below. Note that the data that you enter for custom quadrant curves is unitless - it represents the shape of the curve, which is applied to the initial head drop and flow across the turbine to construct the characteristics curve (with units) used during the simulation.

For more, see Help topic "Specific Speed"