Machine Types and Critical Components

Drivers, Speed Modifiers, and Driven Machines: Process machinery is typically composed of a group of sub-elements that convert one type of energy into another until it is finally transferred into a useable form of fluid power within a process. Here is a simple flow chart showing how power flows through a machine train.

Energy (in) -> Driver -> Speed Modifier -> Driven Machine -> Process Fluid Power (out)

Machine train sub-elements are normally interconnected using flexible components called couplings. Figure 11 illustrates a simple machine train comprised of an electric motor directly coupled to a centrifugal pump.

Energy, such as electrical power, steam power, or fuel gas, is first converted into rotational output power. The speed of the driver output shaft may be increased or decreased by a speed modifier, i.e. gearbox or pulleys, depending on the requirement of process machine being driven. Finally, the output speed from the speed modifier powers the driven machine that produces fluid power in the process. Table 11 contains common designs for driven machines, drivers, speed modifiers, and combination machines.

Driven Process Machines

The purpose of a driven process machine is to deliver a given process fluid, at a given flow and pressure, to specific points in a process. Driven machines receive the power input from a driver or speed modifier and convert it into fluid power at the process machine's discharge flange. All driven process machines are composed of an input shaft, a casing to contain the process fluid, a suction nozzle for input flow, a discharge nozzle for output flow, bearings to support the rotor (or rotors), and one or two end seals to prevent process leakage into the atmosphere.

There are many different designs employed to convert rotary power into fluid power.

Process machines that move and compress gases are called compressors or fans and process machines that move liquids are called pumps. There are too many designs employed in the process industry for us to cover them all adequately here Instead of covering all design types, this chapter will concentrate on centrifugal and positive displacement pumps Compressors will be covered in Chapter 5.

Centrifugal pumps

Centrifugal pumps are one of the most common types used in industry. Figure 12a shows a generic, single stage centrifugal pump and Figure 12b illustrates a multistage centrifugal pump. These pumps can utilize either open or closed impellers and may have single or multiple stage designs. Centrifugal pumps utilize Bernoulli's principle to develop pressure (see Bernoulli's Principle Explained below) by first increasing the fluid velocity inside an impeller and then decreasing the fluid velocity in the discharge nozzle. These pumps consist of a shaft with bearings for support and an impeller as well as a pump casing. To prevent leakage from the pump casing to the atmosphere, most pumps employ packing, single or dual mechanical shaft seals.

Centrifugal pump performance is typically presented graphically with a series of curves similar to the group shown in Figure 1.3. The manufacturer usually provides curves that describe how flow, differential head, net positive head required, and efficiency change with pump flow.

Useful centrifugal pump facts:

• The suction or inlet nozzle to the pump is always bigger than the discharge nozzle.

• If a centrifugal pump has more than one impeller inside of it is called a multistage pump. If it has, for example five impellers in it, then it is a five stage pump.

Series and Parallel Operation

Centrifugal pumps may be operated in a series or parallel (see Figure 14) configuration. When operating pumps in series, the pressure is increased across each pump, but the flow through each pump is identical (minus any minor flow losses due to leakage). When operating in parallel, the pressure rise on each pump is identical, but the total flow is increased. However, the overall flow is not doubled with two pumps operating in parallel because of "system head" or pressure. The easiest way to understand system head is to remember that the discharge pipe size stays the same diameter and therefore tends to restrict the higher flow generated by two pumps operating in parallel. This bottleneck effect means that two pumps operating in parallel will always deliver less than twice the flow that one pump can deliver.

Bernoulli's Principle Explained

There are three physical forms of a fluid energy: Elevation energy, pressure energy, and velocity energy. The higher a liquid is stored, like water in a water tower, the greater its potential energy. The greater a fluid stream's pressure, the greater it's potential to do work. Similarly, the greater the velocity of a stream of fluid, the higher its capability to do work. As a fluid flows down a pipe, ditch, or river, there is a constant interaction between these three forms of energy.

The interplay of the three forms of fluid energy in a flowing stream is governed by Bernoulli's principle. Originally formulated in 1738 by the Swiss mathematician and physicist Daniel Bernoulli, it states that the total energy in a steadily flowing fluid system is a constant along the flow path. An increase in the fluid's speed must therefore be matched by a decrease in its pressure, i.e., energy is always conserved in a fluid stream.

• This principle explains why a moving stream of liquid or gas exerts less pressure than if it were at rest. Bernoulli's Equation can be used to approximate flow parameters in water, air, or any fluid stream that has very low viscosity as long as the fluid is assumed to have these qualities: fluid flows smoothly fluid flows without any swirls (which are called "eddies") fluid flows everywhere through the pipe (which means there is no "flow separation") fluid has the same density everywhere (it is "incompressible" like water)

In basic terms, the Bernoulli principle states that:

• At a constant velocity, if the elevation of a fluid stream increases, the pressure in the stream will decrease.

• At a constant velocity, if the elevation of a fluid stream decreases, the pressure in the stream will increase.

• At a constant elevation, if the velocity of a fluid stream increases, the pressure in the stream will decrease.

• At a constant elevation, if the velocity of a fluid stream decreases, the pressure in the stream will increase.

The last rule is the reason centrifugal pumps and...