SPECIFIC SPEED S072
Specific speed is a term used to describe the geometry (shape) of a pump impeller. People responsible for the selection of a correct size pump can use this specific speed information to:
- Select the shape of the pump curve.
- Determine the efficiency of the pump.
- Anticipate motor overloading problems.
- Predict net positive suction head required (NPSHR) numbers.
- Select the lowest cost pump for their application.
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, efficiency and required flow. This information is available from the pump manufacturer’s published curves. Specific speed is calculated from the following formula, using data from these published pump curves at the pump’s best efficiency point (BEP):
- Q = Capacity in GPM (Largest impeller at the BEP). For a double suction pump use one half the capacity.
- H = Total differential head, developed by the largest impeller at the BEP,( in feet)
- To raise a number to the 3/4 power. cube the number and then take the square root twice
The following chart gives you a graphic picture of the impeller shape represented by this number:
- Note: dividing the US units by 51.64 will yield the SI units
The major use of the specific speed number is to help you specify pumps that are more efficient.
- The maximum pump efficiency is obtained in the specific speed range of 2000 to 3000.
- Pumps for high head low capacity occupy the range 500 to 1000. While low head, high capacity pumps may have a specific speed of 15,000 or larger.
- For a given head and capacity the good news is that the pump having the highest specific speed, that will meet the requirements, probably will be the smallest size and the least expensive. The bad news it that the pump will run at the highest speed where abrasive wear and cavitation damage become a problem.
- Efficiencies start dropping drastically at specific speeds below 1000. Also smaller capacities exhibit lower efficiencies than higher capacities at all specific speeds.
- In propeller and other high specific speed impellers (axial flow) it is not practical to use a volute casing. Instead, the impeller is enclosed in a pipe like casing.
- The lower the specific speed number, the higher the power loss you get with wear ring clearance.
- Impellers with low specific speed numbers (1,000) are narrow compared to the outside diameter
- Impellers with high specific speed numbers (10,000) are often called propellers, and are wide compared to their outside diameters.
Pumps are traditionally divided into three types: radial flow, mixed flow, and axial flow. When you look at the above chart you can see there is a gradual change from the radial flow impeller, which develops pressure principally by the action of centrifugal force, to the axial flow impeller, which develops most of its head by the propelling or lifting action of the vanes on the liquid.
In the specific speed range of approximately 1000 to 6000, double suction impeller are used as frequently as the single suction impellers.
If you substitute other units for flow and head the numerical value of Ns will vary. The speed is always given in revolutions per minute (rpm.). Here is how to alter the Specific Speed number (Ns) if you use other units for capacity and head:
United States ……. Q = gpm, and H = feet, divide the NS by 1.63
British ……………….Q = Imp. Gpm, and H = feet, divide the NS by 1.5
Metric ……………….Q = m3/hour and H = meters, divide the NS by 1.9
As an example we will make a calculation of NS in both metric and U.S. units:
- Q = 110 l/sec. or 396 m3/ hour or 1744 gpm.
- H = 95 meters or 312 feet
- Speed = 1450 rpm.
If the above results were describing an actual application, we would notice that it was a low specific speed, radial flow pump, meaning it would be a large pump with a low efficiency.
Going to 2900 rpm. or higher would increase the Ns to 1000 or more, meaning a smaller pump with a much higher efficiency but this higher rpm would have other possible consequences :
- The higher efficiency would allow you to use a less powerful driver that would reduce your operating costs.
- A smaller pump makes associated hardware cheaper. For instance, a smaller diameter shaft means a lower cost mechanical seal and lower cost bearings.
- Cavitation could become a problem as the increase in speed means an increase in the net positive suction head required (NPSHR).
- If you are pumping an abrasive fluid, abrasive wear and erosion will increase with increasing speed.
- Many single mechanical seals have problems passing fugitive emission standards at the higher pump speeds.
- High heat is a major cause of bearing failure. The higher pump speeds contribute to the problem.
The following diagram illustrates the relationship between specific speed (Ns) and pump efficiency. In general, the efficiency increases as Ns increases.
Specific speed also relates to the shape of the individual pump curve as it describes head, capacity, power consumption and efficiency.
In the above diagram you will note that
- The steepness of the head-capacity curve increases as specific speed increases.
- At low specific speed power consumption is lowest at shut off and rises as flow increases. This means that the motor could be over loaded at the higher flow rates unless this was considered at the time of purchase.
- At medium specific speed the power curve peaks at approximately the best efficiency point. This is a non-overloading feature meaning that the pump can work safely over most of the fluid range with a motor speed to meet the best efficiency point (BEP) requirement.
- High specific speed pumps have a falling power curve with maximum power occurring at minimum flow. These pumps should never be started with the discharge valve shut. If throttling is required a motor of greater power will be necessary.
- As a rule of thumb, lower specific speeds produce flatter curves, while higher specific speeds prouce steeper ones
Here is another curve to show you the relationship between specific speed, capacity and horsepower requirements:
Keep in mind that efficiency and power consumption were calculated at the best efficiency point (BEP). In practice most pumps operate in a throttled condition because the pump was oversized at the time it was purchased. Lower specific speed pumps may have lower efficiency at the best efficiency point, but at the same time will have lower power consumption at reduced flow than many of the higher specific speed designs.
The result is that it might prove to be more economical to select a lower specific speed design if the pump had to operate over a broad range of capacity.
The clearance between the impeller and the tongue of the volute has a bearing on efficiency, pressure pulsations and cavitation. For high efficiency you would want a small clearance, but this produces larger pressure pulsations and the increased flow in this area can reduce the fluid pressure enough to cause flashing of the product and a type of cavitation known as The vane passing syndrome.
For impellers up to fourteen inches in diameter (355 mm) this clearance should be a minimum of four percent of the impeller diameter. If you are using greater than fourteen-inch diameter impellers the clearance should be at least six percent of the impeller diameter. Also remember that as this clearance increases the impeller experiences some slippage. That is the major reason that we do not like to remove more than ten percent of the impeller diameter when trimming is called for.