Understanding The Head Discharge Curve Of Screw Pumps

Understanding a screw pump's head discharge curve is essential for selecting the right pump and optimising its performance. The curve illustrates the relationship between the pump's head (maximum height it can pump liquid against gravity) and flow rate, revealing its ability to produce flow under specific pressure conditions. This information is critical for matching the pump to the application's requirements, ensuring efficient operation, and preventing issues like cavitation and pump damage. The head discharge curve also helps determine the required pump speed and power, with higher flow rates and pressures demanding greater speed and energy.

Pump head and shut-off head

The concept of "head" in pumps is one of the most misunderstood physical characteristics. It is related to pressure, specifically, the maximum height that a pump can achieve pumping against gravity. This is dependent on the pump's efficiency, and the higher the liquid in the tank, the higher the pump will be able to pump the water.

The "total head" is a more useful measure and is calculated as the difference between the liquid level in the suction tank and the head in the vertical discharge pipe. This number is independent of the level of liquid in the suction tank.

The "shut-off head" is the amount of head a pump can produce at zero flow. It is the maximum head achieved by the pump when the discharge is kept at zero. As the flow rate increases, friction is introduced into the system, reducing the total head. The amount of head lost due to friction is called "friction head" or "friction loss".

The relationship between total head, shut-off head, discharge head, and suction head can be seen in pump performance curves. These curves help users select the right pump for their specific needs by plotting head versus flow rate. The required flow rate and total head will intersect at a certain point on the pump's performance curve, allowing for the determination of whether the pump will be appropriate for the required application.

Understanding Pump Curves

Pump curves, also known as pump selection curves, pump characteristic curves, efficiency curves, or pump performance curves, are graphical representations of the head generated by a specific pump model at various flow rates. They are used to determine a pump's ability to produce flow under different conditions, such as pressure and flow rate.

The head and flow curve is the most commonly used curve to describe pump performance. It plots the pump total head on the y-axis and the flow rate on the x-axis. This allows for the performance of the pump to be shown independently of the density of the fluid pumped.

Other important curves include the pump efficiency curve, which shows the pump efficiency at various flow rates, and the pump input power curve, which shows the amount of input power required for different flow rates.

Understanding pump head and shut-off head is crucial for selecting the right pump for a specific application. By referring to pump performance curves, users can make informed decisions about the suitability of a particular pump based on its ability to generate flow under various conditions.

Pump performance curve

A pump performance curve is a graphical representation of the head generated by a specific pump model at rates of flow from zero to maximum at a given operating speed. The curve shows the pump's performance and range. The head and flow curve is the most commonly used curve to describe pump performance.

The performance curve shows the volume of fluids a pump can transfer under various pressure conditions. The curve shows the pump's ability to produce flow under the conditions that affect pump performance. The curve also shows the pump's ability to produce flow under specific operating conditions, giving you a window into your options so you're not locked into just a few choices during the selection process.

The pump performance curve is also called a pump selection curve, pump characteristic curve, efficiency curve, or pump performance curve. It gives you the information you need to determine a pump's ability to produce flow under the conditions that affect pump performance.

The curve shows the two performance factors on the X,Y axis so you can see the volume of fluids a pump can transfer under various pressure conditions. The pump performance curve shows the pump characteristics and performance metrics based on head (pressure) produced by the pump and water flow through the pump. Flow rates depend on pump speed, impeller diameter, and head.

The head is the height to which a pump can raise water straight up. Water creates pressure or resistance at predictable rates, so we can calculate pump head as the differential pressure that a pump has to overcome to raise the water. The pump performance curve can be used to determine the required power to operate the pump.

Pump efficiency

> ηp = Pw / Pp

Where ηp is the pump efficiency, Pw is the pump output power (power imparted to the liquid), and Pp is the pump input power.

The pump efficiency curve illustrates the pump's efficiency at various flow rates, with the flow rate where efficiency is maximised known as the pump's Best Efficiency Point (BEP). This point is crucial as it indicates the flow rate and head at which the pump operates with maximum efficiency at a given speed and impeller diameter. Operating at the BEP has several advantages, including lower vibration and noise levels, minimal recirculation within the impeller, and shockless entry of the fluid into the impeller.

The pump input power curve is another important factor in pump selection. It illustrates the amount of input power required for the pump to operate at different flow rates. This information is vital for properly selecting a driver for the pump. The equation for pump input power is:

> Pp = Q * H * s / (3960 * ηp)

Where Q is the flow rate, H is the total head, s is the specific gravity, and ηp is the pump efficiency.

When selecting a pump, it is essential to consider the viscosity and temperature of the fluid, as well as the voltage and frequency. The pump curve will indicate the flow rate and pressure the pump can handle, helping to ensure efficient performance and prevent issues such as pump damage, poor performance, and unnecessary energy consumption.

Additionally, understanding the Total Dynamic Head (TDH) is crucial. The TDH comprises the static head (difference in height between the pump and discharge point), suction lift (difference in height between the fluid and pump inlet), and friction loss (energy losses as the fluid flows through pipes, valves, etc.).

In conclusion, by considering pump efficiency, input power requirements, fluid properties, and the TDH, one can make an informed decision when selecting a pump for a specific application, ensuring optimal performance and efficiency.

Pump input power

The pump input power curve is a crucial aspect of understanding a screw pump's performance and selecting the right pump for specific applications. This curve illustrates the amount of input power required by the pump for different flow rates. The input power can be calculated using the equation:

> Pp = {{Q · H · s} / {3960 · ηp}}

Where:

• Pp = pump input power (in brake horsepower)
• Q = flow rate (in gallons per minute or gallons per hour)
• H = pump total head (the difference between discharge head and suction head)
• S = specific gravity of the fluid
• Ηp = pump efficiency

The pump input power curve is essential for properly selecting a driver for the pump. It helps determine the amount of power the pump needs to operate effectively at various flow rates.

When selecting a pump, it is important to consider not only the current demand but also future requirements. Sizing the pump for performance variables rather than peak efficiency is a common practice to ensure the pump can meet changing needs.

Additionally, the pump input power curve can be used to troubleshoot issues with existing pump systems or to size equipment for new projects. By understanding the relationship between flow rate, pump speed, and input power, engineers can make informed decisions about pump selection and system design.

Moreover, the input power required by the pump is influenced by the viscosity of the fluid being pumped. Higher viscosities can impact the allowable pump speed and may require a larger pump to accommodate the desired flow rate. Therefore, when considering pump input power, it is crucial to take into account the specific fluid being pumped and its viscosity.

In conclusion, the pump input power curve is a vital tool for selecting the right pump, ensuring efficient operation, and troubleshooting performance issues. By understanding the input power requirements at different flow rates, engineers can make informed decisions to optimize their pumping systems.

Net Positive Suction Head (NPSH)

NPSH is defined as the total head of fluid at the centre line of the impeller, less the fluid's vapour pressure. The purpose of this measurement is to identify and avoid operating conditions that lead to the vaporisation of the fluid as it enters the pump – a condition known as flashing. If the fluid's pressure is below its vapour pressure, bubbles will form and collapse in a damaging process called cavitation, which can cause significant wear and metal fatigue on impellers and pump cases.

NPSH is normally considered in two forms: NPSH-R (NPSH Required) and NPSH-A (NPSH Available). NPSH-R is a pump property, quoted by pump manufacturers as a minimum suction pressure that must be exceeded for the pump to operate correctly and minimise flashing and cavitation. NPSH-A is a system property, calculated from the suction-side system configuration, and is the suction-side pressure less the vapour pressure of the pumped fluid at that point.

To avoid cavitation, NPSH-A must exceed the pump's NPSH-R rating. A safety margin of 0.5 to 1m is usually required to account for factors such as the pump's operating environment, changes in weather, and increases in friction losses.

The NPSH margin describes the safety factor by which NPSH-A must exceed NPSH-R to avoid cavitation. It can be quoted as a ratio or as the difference between the two values. As a general rule, NPSH-R should be less than NPSH-A by at least 5 feet or 10% of NPSH-A, whichever is larger.

NPSH is a very important part of a pumping system. If NPSH-A is not higher than the pump's NPSH-R, the pump will not perform properly and there is a risk of cavitation, which can damage the pump and shut down operations.

Frequently asked questions