Back to Basics: What Makes a Good Pump?

Everyone is familiar with pumps, but how many people really think about how much depends on this ubiquitous invention? The scope of pump applications is wide: distribution and circulation of water in water supply and heat supply systems, irrigation in agriculture, in the oil industry, in fire extinguishing systems, etc.

A pump is a hydraulic machine designed to move fluid and impart energy to it. A schematic diagram of a simple pumping unit is presented below.

Figure 1 Pumping Unit Diagram
Figure 1: Pumping Unit Diagram
1 – intake valve; 2 – suction pipeline; 3 – vacuum gauge; 4 – pump; 5 – manometer; 6 – check valve; 7 – gate valve; 8 – pressure pipeline

Positive Displacement and Dynamic Pumps

According to the principle of operation, pumps can be divided into two main groups: positive displacement and dynamic. In positive displacement pumps, a certain volume of the pumped liquid is cut off and moved from the inlet to the pressure head, where additional energy is supplied to it. In pumps with dynamic action, the increase in energy occurs due to the interaction of the liquid with a rotating working body.

The most widely used pumps are centrifugal pumps which are of the dynamic type. The principle of centrifugal pumps uses a rotating impeller to create a vacuum in order to move the fluid. The impeller rotates within the housing and reduces pressure at the inlet. This motion then drives fluid to the outside of the pump’s housing, which increases the pressure.

These pumps benefit from a simple design and lower maintenance requirements and costs. This makes them suited to applications where the pump is used often or continuously run.

Figure 2 Centrifugal Pump
Figure 2a: Centrifugal Pump
Centrifugal Pump Designed using AxSTREAM
Figure 2b: Centrifugal Pump Designed using AxSTREAM

In most cases, the pumps are electrically driven, but if the pump is of high power and high speed, then these pumps are driven by steam turbines.

The main parameters of pumps are flow rate, head, rotation frequency, power consumption, efficiency, and cavitation reserve. Definitions for these terms are below.

  • Pump supply flow is the amount of fluid transported per unit of time.
  • Head is the height to which the liquid rises due to the energy received. This is the ratio of the total energy of the pumped liquid to the magnitude of the gravity force. The pump must create this head to cover the existing pressure difference at the outlet and inlet and overcome the losses occurring during transportation. Fig. 1 shows the total hydraulic head H, suction losses hls, and pressure losses hld, geometric discharge head Hgd and geometric suction head Hgs.
  • Rotation frequency is the number of revolutions per unit of time.
  • Power consumption is what is consumed by the unit’s electric motor.
  • Efficiency is the ratio of the net power to the power on the pump shaft.
  • Cavitation is the process of bubble formation in liquid media, followed by their collapse, accompanied by noise and vibration. In this case, the surface of the flow path elements is destroyed. To avoid this phenomenon, it is necessary to carefully select a pump with a sufficient cavitation margin. That is, with such a suction lift at which the pump would work without changing the technical characteristics.

For pumps of a dynamic type, the pump performance is very important. The characteristics are a map of the dependence of the main parameters of the pump on the flow and head at a certain viscosity and density of the liquid and the size of the impeller. On these characteristics, two main types of work can be distinguished: economic, where the maximum efficiency takes place and nominal, which will provide a given range of pump operation.

Figure 3 - Simplified View
Figure 3: Simplified View of the Pump Characteristics
H, Q, N, – head, flow, power, efficiency

The correct selection of the pump will guarantee safety, continuity, and economy.

Jet Pumps

A special group of pumps are the jet pumps (ejectors/elevators) and are mainly used for suction of air or water. These pumps are simple and reliable. For thermal power plants, steam jet ejectors are mainly used to create a vacuum in the condensers of the steam turbine plants.

In a jet pump (ejector), energy is transferred by mixing two streams entering the working chamber at different speeds. The working medium of the ejector can be air, steam or any other gas. The main condition for the correct operation of the apparatus must be that the working medium at the entrance to the jet pump had a higher pressure (active flow) than the moving medium (passive flow).

Figure 4: Water Jet Pump
Figure: 4 Water Jet Pump
1 – pipe into which the working stream is supplied; 2 – nozzle; 3 – receiving chamber; 4 – confuser; 5 – cylindrical section; 6 – diffuser; 7 – pressure pipe line; 8 – pipe into which the pumped liquid is supplied

Consider the principle of operation of a water jet pump. The working medium is supplied to the nozzle, where its potential energy is converted into kinetic energy. Coming out of the nozzle at high speed, the working stream entrains the medium that is in the receiving chamber. Mixing of streams and equalization of their speeds occurs. In the diffuser part, the flow rate decreases and the pressure increases.

For a jet pump pumping an incompressible liquid, the following basic parameters are introduced: injection coefficient, effective head, relative head, working fluid head, efficiency. Definitions for these terms are below.

  • The injection coefficient is the ratio of the volumetric flow rates of the passive and active flows.
  • Effective head is the ratio of the pressure difference at the outlet of the pump and the pressure of the medium being sucked into the product of density and the acceleration of gravity.
  • The relative head of the ejector expresses the ratio of the effective head to the sum of the useful head and the head of the working fluid.
  • Efficiency has the same definition as for other pumps.

 

The main loss in any jet apparatus is the loss of mixing energy for streams with different speeds. These losses are proportional to the square of the difference in flow rates at the beginning of mixing. By increasing the velocity of the injected flow at the entrance to the mixing chamber, these losses can be reduced.

Pumps – the Secret to Efficient Power Generation

Pumps are very important for a power plant. They are widely used in all areas of the power plant such as the feed pump, circulating water pump, condensate pump, oil pump, and many others. Pumps affect the safety of the power plant and play an important role in energy saving.

For example, it is very convenient to use AxCYCLE™ software when modeling thermal circuits of steam turbine cycles. AxCYCLE™ software is a conceptual tool for the thermodynamic simulation and heat balance calculation of different energy conversion systems. With the help of the program various thermal circuits were simulated (Figures 5 and 6).

Figure 5 400 MW Steam Turbine (P = 218 bar, 7 Extractions, district heating 49 MW), simulation in AxCYCLE™ program
Figure 5 400 MW Steam Turbine (P = 218 bar, 7 Extractions, district heating 49 MW), simulation in AxCYCLE™ program
Figure 6 500 MW Steam Turbine (
Figure 6 500 MW Steam Turbine (P = 34.3 MPa, 10 Extractions, End Seals, Ejector, TurboPump), simulation in AxCYCLE™ program.

In Figure 6, one can see one pump being used with an electrical drive and one pump with mechanical (shaft) drive. Except for the source of power, both pumps are thermodynamically similar. The main characteristics of the pumps, such as pump efficiency, relative mass flow ratio, pressure ratio, rotational speed, can be set manually or they can be calculated by the AxCYCLE™ program.

AxCYCLE™ software provides 3 types of pump component:

  • Pump without map – model for design study.
  • Pump with internal map – features a pump efficiency and pressure ratio as a function of mass flow rate. The map’s polynomial function used to calculate the efficiency is obtained as a 2nd order polynomial equation using 3 points for which the mass flow rate and corresponding efficiency and pressure ratio values are specified by the user. The actual value of the mass flow is inputted for the 2nd point while for the 1st and 3rd points a ratio compared to the 2nd point is used instead (see figure below).
  • Pump with external map – allows the calculation of pump performance using the external efficiency map (dependence on mass flow rate, rotation speed and pressure ratio).

 

Figure 7 Pump with Internal Map Characteristics
Figure 7 Pump with Internal Map Characteristics

An ejector element can also be used. The program has a component model of ejectors that simulates pump/mix suction flow with a motive flow via the jet effect.

Figure 8 Models of Ejectors in AxCYCLE
Figure 8 Models of Ejectors in AxCYCLE

In Figure 8, we have modeled a scenario where the working and sucked flow are the same fluid, and the model of the ejector works with ideal gases.

The AxCYCLE™ software does not need to know the dimensions of the installation, it is enough to know the parameters of the incoming fluid.

Over time, the design features of the pumping equipment are improved, but the theoretical principles of the installation remain unchanged. With the help of available software products, you can quickly and easily evaluate the operation of equipment for different modes and evaluate overall performance. Learn more here

References

  1. L.A. Richter, D.P. Elizarov, V.M. Lavygin Auxiliary equipment of thermal power plants
  2. V. Zhabo, Uvarovб Hydraulics and pumps
  3. I. Durnov, Pumps, fans, compressors