Pump Performance Curve – Rotary Pumps

Pump Characteristic Curve

Unlike centrifugal pump, Rotary pump do not generate pressure. Instead it works against the back pressure of a given system. The characteristic of the curve is very dependent on the liquid properties especially the viscosity of the liquid. The speed of the pump at a given viscosity will also play a role in the forming the pump curve.

Positive Displacement Pump Curve

Figure 10. A positive displacement pump performance curve

Figure 10 illustrates the characteristic curve for a rotary type positive displacement pump.

Common Problems in Centrifugal Pumps

Cavitation

Cavitation is a serious operational-type problem that takes place where there is insufficient suction pressure to meet the NPSH requirements of the pump.

“Cavitation” implies a dynamic process of formation of bubbles inside the liquid, their growth and subsequent collapse (implosion) as the liquid flows through the pump. Cavitation results in loss of volumetric efficiency. Cavitation also causes material erosion in the pump components (mainly the impeller) due to the forces released in collapsing the gas or vapor bubbles and this can result in complete failure of the pump.

Cavitation causes 4 main problems:

  1. reduces the pump total head and flow rate
  2. reduces the pump overall efficiency
  3. damages impeller and casing walls
  4. creates noise and vibration issues
Figure 8. Cavitation erosion in an impeller (Photo taken from Fluid Handling Pro website)

Pump Performance Curve – Centrifugal Pumps

Pump Characteristic Curve

Pump characteristic curve (or pump performance curve) determines the range of flows and energy produced for a fixed speed, size, design and suction conditions of the pump. The head / pressure (energy) produced by a pump must equal to or exceed the net head / pressure (energy) required by the system so that the pump is able to move a liquid from a lower pressure level to a higher pressure level. In addition, the net pressure (energy) available on the suction side of the pump system must be greater than the liquid vapor pressure to assure that liquid enters the pump without potential deterioration of performance or mechanical damage.

Centrifugal Pump Curve

Figure 9. A typical centrifugal pump performance curve

Figure 9 illustrates the characteristic curve for a centrifugal pump. A centrifugal pump performance characteristics are outlined for a ranged of capacity and head produced for fixed impeller geometry and a variety of impeller diameters.

The limits of the centrifugal pump curve

The centrifugal pump curve has minimum allowable flow and flow exceeding end of curve (maximum flow) limits which can result in significant mechanical damage to the pump if not avoided. If the rated flow is operated below the minimum allowable flow, flow recirculation can damage a pump while at the rated flow exceeding the end of curve, excessive NPSHr, horsepower and choke flow can cause mechanical damage to impellers, casing, shaft, bearings and seals.

Centrifugal Pumps

Centrifugal Pumps

Centrifugal Pumps are by far the most widely used of fluid handling pump types due to its design simplicity, durability, versatility and economics. More than 85% of all the installed pumps in any typical factories/plants are centrifugal pumps. The distinctive features and unique operating characteristics of centrifugal pumps will be further discussed below.

A centrifugal pump is made up of two main components: 

  • A rotating element consists of impeller and shaft; 
  • A stationary element including casing, stuffing box and bearing housing.

The impeller vanes are exploited to impart energy to the fluid through centrifugal force.

Figure 3. Centrifugal Pump

Figure 3 illustrates a simplified diagram how a typical centrifugal pump operates. In a centrifugal pump, fluid is introduced through the casing inlet to the eye of the impeller where it is picked up by the impeller vanes. As the impeller rotates, the rotary motion of the impeller imparts  a rotating motion to the fluid, forcing it to the outer periphery of the impeller vanes where it is collected in the pump volute. The volute is the portion of the pump casing that expands in cross-sectional area as it wraps around the pump casing. The casing volute collects the fluid discharged from the edge of the impeller at high velocity and gradually reduces the fluid velocity by increasing the flow area. This converts the velocity energy to pressure energy prior to discharging the fluid to the system.

Type of Pumps and Definitions

Positive Displacement Pumps

A Positive Displacement pump is also known as the power pump and it operates by continuously deliver liquid flow but independent of the pressure in the discharge piping  system. The discharge piping system produces resistance to this liquid flow, thereby generating pressure in the discharge piping system. Unlike centrifugal pump, it does not develop pressure but rather produces a flow of liquid.

Positive Displacement pumps can be further divided based on the mode of displacement:

  1. Reciprocating Pumps – works in a transitory dynamic condition called the pumping cycle via linear movement of the plunger or piston. A reciprocating pump makes up of liquid end and power end.
  2. Rotary Pumps – the displacement is by rotating action of gears, screws or vanes in a fixed casing/ chamber. Rotary pumps do not generate pulsation. The inherent high efficiency of a rotary pump makes its design very suitable to handle high viscosity liquid.
Unlike kinetic pumps, all positive displacement pumps can theoretically move the same volume of fluid at a given speed (rpm) no matter how the discharge piping system is setup. Thus, all positive displacement pumps are flow or volume devices. 
Figure 2. Screw Pump Cutout (Credit to CC BY-SA)

Dynamic Pumps

A Dynamic pump is distinguished by the way it operates. It uses a centrifugal force of the rotating impeller to impart kinetic energy to the fluid, moving the fluid from one level of pressure to a higher level of pressure. 

There are several varieties of dynamic pumps and some are classified as follow:

  1. Centrifugal Pumps are the most widely used fluid handling devices in the refining, petrochemical and general industrial applications. Typically, more than 85% of the pumps installed in an industry are centrifugal pumps.
  2. Regenerative Pumps are also sometimes called peripheral pumps. This type of pumps in general produces high head and low flow. It can also handle a certain amount of air or gas in the liquid as long as sufficient liquid remains in the pump casing to seal the clearance between the suction and discharge passages.
  3. Vortex Pumps or recess impeller pumps are designed to handle liquid with solids without clogging the casing passage.
  4. Special Effect Pumps are particularly used for specialized conditions in an industrial service.
All kinetic pumps deliver variable volume of fluid for a constant differential head at a specific flow condition by using rotating impellers or blades to increase fluid velocity. All kinetic pumps are relatively sensitive to pumping liquid properties and to system changes as well.

Let’s Talk About Pumps

What are pumps?

In general, regardless of the type of  pump, it is defined as a mechanical device or equipment that is used to:

  • transfer or move liquid from one point to the other by imparting kinetic and potential energy to the liquid.
  • built up pressure or pressurization purpose.

Pumps can be further classified into two main categories, according to their basic operating principle as detailed below:

  1. By means of dynamic (or kinetic) – in which energy is continuously added to increase the fluid velocities within the pump. Fluid pressure is increased when the fluid velocity is subsequently decreased.
  2. By means of positive displacement – in which energy is periodically added to the fluid by direct application of force to one or more moveable boundaries of any fluid-containing volumes, resulting an increase in pressure up to the value required to move the fluid through ports or valves in the discharge point. 

Figure 1 shows the Types of Pumps we discussed above.

Figure 1. Types of Pumps

Dynamic pumps can be further sub-classified into several varieties of centrifugal, regenerative and other special-effect pumps.

Displacement pumps are essentially classified into rotary and reciprocating types. Each of these classifications can then be further subdivided into several types of variations depending on the nature of service it is designed for.

Figure 1 above also presents an outline summary of the significant classifications and sub-classifications within these categories.