Centrifugal Pump Analysis: Performance, Cavitation, and System Variables

Posted on Jan 6, 2025 in Design and Engineering

Centrifugal Pump Fundamentals

A high fluid head that can be elevated by a pump is called…

The power required for a pump depends on what variable? Vapor pressure.

The discharge pressure in a reciprocating pump piston depends only on the action of the pump system.

The main function of a centrifugal pump casing is to convert the rotor’s speed into useful pressure energy.

The ratio of energy transfer between a fluid pump and the energy supplied is called: pump efficiency.

In general, using centrifugal pumps, through the curves, it can be noted that: the height gauge decreases with increased flow.

In reciprocating pumps, the suction and discharge valves are located in the casing or on the piston, depending on the manufacturer.

To generate flow, a centrifugal pump combines centrifugal force with mechanical impulse.

Case Study: Pump System Analysis

Statement: The system presented in Figure 1 is equipped with an NB-40-200 pump, whose operational curves are presented in Figure 2 (curves trace to water at 30 degrees Celsius). This pump is pumping water at 70 ºC (absolute vaporization pressure indicated in Table 1). The desired system flow rate is 56 m3/h. Considering that L1 = 7 m (Figure 1), and the atmospheric pressure is 10 CSF, and pressure losses are in the suction stretch, is this pump cavitating under the described conditions? Why? A cavitating pump because the absolute vapor pressure of the pumped fluid (at the pumping temperature – 70 degrees Celsius) is greater than the absolute suction pressure (NPSHd) of the pump. This will provide the necessary condition for fluid vaporization.

Statement: A centrifugal pump NB-100-250 is operating at an angular speed of 1750 rpm with a 211 mm rotor. The curve is presented in the figure. The installed system has a function given by the curve H = Q / 5. Knowing the operating system for projects with:

  • Flow rate of at least 140 m3/h
  • Pressure of at least 16 mca
  • NPSHd of 2 mca

It can be said that the pump will cavitate when this system operates.

False because the pump curve does not cross the system curve (211 mm rotor) at the point where the flow rate Q = 100 m3/h. At this point, the NPSHr of the pump is less than 2 mca (NPSHd value).

Statement: A centrifugal pump NB-100-250 is operating at an angular speed of 1750 rpm with a 211 mm rotor. The curve is presented in the figure. The installed system has a function given by the curve H = Q / 5. Knowing the operating system for projects with:

  • Flow rate of at least 140 m3/h
  • Pressure of at least 16 mca
  • NPSHd of 2 mca

It is said that the pump will operate at its maximum efficiency. False. The system curve crosses the pump curve (211 mm rotor) at the point where the flow rate Q = 100 m3/h. At this point, the pump efficiency is 69%.

Statement: A centrifugal pump NB-100-250 is operating at an angular speed of 1750 rpm with a 211 mm rotor. The curve is presented in the figure. The installed system has a function given by the curve H = Q / 5. Knowing the operating system for projects with:

  • Flow rate of at least 140 m3/h
  • Pressure of at least 16 mca
  • NPSHd of 2 mca

It can be said that a 10 kW motor is sufficient (with margin) for this pump. True. The system curve crosses the pump curve (211 mm rotor) at the point where the flow rate Q = 100 m3/h. At this point, the power required for the motor is around 8 kW.

Statement: A centrifugal pump NB-100-250 is operating at an angular speed of 1750 rpm with a 211 mm rotor. The curve is presented in the figure. The installed system has a function given by the curve H = Q / 5. Knowing the operating system for projects with:

  • Flow rate of at least 140 m3/h
  • Pressure of at least 16 mca
  • NPSHd of 2 mca

It can be said that the highest pressure will occur when the discharge valve is completely closed. False. The highest pressure in the pump will occur at a flow rate of 40 m3/h.

Statement: A centrifugal pump NB-100-250 is operating at an angular speed of 1750 rpm with a 211 mm rotor. The curve is presented in the figure. The installed system has a function given by the curve H = Q / 5. Knowing the operating system for projects with:

  • Flow rate of at least 140 m3/h
  • Pressure of at least 16 mca
  • NPSHd of 2 mca

It is said that the pump meets both the system’s flow rate and pressure requirements. False. The pump meets the pressure requirement, as it operates with a system pressure of 20 mca at the discharge (the system requires 16 mca). However, the pump does not meet the system’s flow rate demand, as it provides 100 m3/h, and the system demands 140 m3/h.

Understanding Cavitation

Define and describe the process of cavitation occurrence in a centrifugal pump. Cavitation is a physical phenomenon that occurs in centrifugal pumps when the pressure of the fluid being sucked is reduced, reaching values equal to or below its vapor pressure (liquid <> vapor). This forms bubbles (steam) that, as they are displaced by the rotor, reach pressures higher than the vapor pressure again (vapor <> liquid), causing them to implode. This phenomenon occurs inside the pump when the NPSHd (system) is less than the NPSHr (pump). Cavitation causes noise, damage, and reduces the performance of hydraulic pumps.

NPSH and System Variables

List at least two variables that do not influence the NPSHd (available) value of a system and explain why these variables do not influence the NPSHd value.

Variables that do not influence the NPSHd value are those not related to the suction system coupled to the pump. Therefore, changes to these variables would not alter the pressure available at the pump suction. These variables are:

  • Pv – Vapor Pressure of the pumped liquid: This is a property of the fluid and not of the suction system coupled to the pump.
  • Pdischarge – This variable will alter the operating point of the pump, therefore it influences NPSHr.
  • Rotation: 1640 rpm – This variable will also alter the operating point of the pump, therefore it influences NPSHr.

NPSH Definitions

Define the NPSHr (required) of a centrifugal pump and explain how this variable relates to the NPSHd (available) in a system. The acronym stands for Net Positive Suction Head.

  • NPSHd (available): The absolute pressure existing at the pump suction (rotor inlet). It must be higher than the vapor pressure of the pumped fluid, and its value depends on the characteristics of the system and the fluid.
  • NPSHr (required): The minimum absolute pressure required at the pump suction (rotor inlet), which, under pumping conditions, must be higher than the vapor pressure of the pumped fluid to avoid cavitation. This depends on the pump’s characteristics and must be provided by the manufacturer (along with the pump curve).