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Overview of Rotating Machinery

By Robert X. Perez

Rotating machines power our production facilities by safely transporting a wide variety of liquids, gases, and solids from one point in the process to another. Those of you who have worked in a production site for any length of time know that process machines come in a wide range of designs and sizes. It soon becomes apparent that not all process machines are created equal; some are more critical than others; some are small and some are very large; some spin fast and some turn relatively slow. The great diversity in their construction and application can be daunting to those new to the industry and a challenge to the veteran.

Drivers, Speed Modifiers, and Driven Machines:

Process machinery refers to mechanical assemblies, or trains, designed to transport fluids within chemical processing facilities. These trains are 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.

Figure 1.1 An electric motor coupled directly to a six-throw, reciprocating compressor.

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

Machine train sub-elements are normally interconnected using flexible components called couplings (Figures 1.3 and 1.4). Coupling designs range from elastomeric to metallic disc pack designs. Figure 1.2 illustrates a simple machine train comprised of an electric motor directly coupled to a centrifugal pump.

Available energy, usually in the form of electrical power, steam power, or fuel gas, is first converted into rotational output power. Sometimes, the speed of the driver output shaft must 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 power from the speed modifier drives the driven machine that delivers fluid power to the process.

Figure 1.2 An electric motor coupled to a centrifugal pump.

Figure 1.3 Elastomeric couplings.

Figure 1.4 Disc pack type couplings.

Table 1.1 contains common design options for driven machines, drivers, speed modifiers, and combination machines. There are numerous combinations of driven machines, drivers, and speed modifiers. In some cases, process machines can be made up of a combination of a driver and a process driver (Figure 1.2). For example, a turbo expander can have an expansion wheel and a compressor impeller on the same shaft, enabling both components to be contained in a single housing.

Here are a few real examples of machinery trains you can find operating in process settings:

In Figure 1.1, electricity is used to power an electric motor coupled to a reciprocating compressor. The energy flows as follows:

Table 1.1 Common types of process machinery elements.

Driven machines Drivers Speed modifiers Combination machines Centrifugal compressors Electric motors Gearboxes Turbo expanders (Figure 1.5) Reciprocating compressors Steam turbines Hydraulic speed modifiers Turbochargers Rotary compressors Gas turbines Sheaves and belts Centrifugal pumps Reciprocating engines Reciprocating pumps Rotary pumps Fans

Figure 1.5 A turbo-expander consists of a compressor and expander on the same shaft.

Electrical Power (in) rotating magnetic fields drive electric motor electric motor powers reciprocating compressor compressor cylinders generate high pressure gas (out)

Figure 1.6 A natural gas fired engine (on the right) drives a reciprocating compressor (on the left).

Figure 1.7 An electric motor driving a forces draft (FD) fan.

Figure 1.8 An electric motor drives a gearbox, which in turn drives an axial compressor.

In Figure 1.6, natural gas is used to run a reciprocating engine, which in turn powers a reciprocating compressor.

Fuel Gas (in) Engine power is produced by internal combustion of the fuel gas engine drives reciprocating compressor compressor cylinders generate high pressure gas (out)

In Figure 1.7, electricity is used to run an electric motor that is directly coupled to a fan.

Electrical Power (in) rotating magnetic fields drive electric motor electric motor powers fan fan forces air through the system (out)

In Figure 1.8, an electric motor drives an axial compressor. To drive the compressor at the proper speed, a speed increaser is required. Here the input shaft, driven by the motor, turns the low-speed bull gear. The bull gear drives a high-speed pinion gear, which is coupled directly to the axial compressor.

Unspared versus Spared Machine Trains

Machine trains are considered either spared or unspared, i.e., critical. If a process train has an installed "twin" machine train that's fully functional, but idle, for process redundancy, it is considered spared. However, if a process train does not have an installed "twin" machine train for redundancy, it is said to be a critical machinery train. A machine train can be considered critical if a single mechanical failure will result in a process slowdown or outage. For this reason, unspared process machines tend to receive more scrutiny with regards to condition monitoring and surveillance.

Most processing facilities categorize machinery into several criticality classes. For example, a site may use levels A, B, C, and D criticality classifications and define them as follows:

• Level A train outrages result in:
  • complete unit or plant outages or
  • could result in significant safety or environmental events.

• Level B train outages result in:
  • reduced processing capacity or
  • major repair costs.
• Level C train outages have no or minor process consequences but may result in moderate repair costs.
• Level D train outage have no process consequences and minor repair costs

Level A (Critical) machine trains should be the most reliable machines in the plant by design. They need to be able to run continuously and safely between process outages. Predictive and preventative maintenance programs should be thoughtfully designed and meticulously maintained to ensure reliable performance.

Driven-Process Machines

The ultimate 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. Most 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 (see Figure 1.8).

Critical Machine Components

The most important machine components are rotor support bearings and end seals. Bearings support all forces acting on the rotor, while allowing it to rotate freely with minimal friction. Most bearings are either rolling element bearings (Figure 1.10) or fluid film bearings (Figure 1.11). Bearing reliability is closely tied to making the proper bearing selection, selecting the right lubricate supply system, and maintaining lubrication cleanliness. End seals are highly engineered components that limit process leakage rates to acceptable levels. There are wet seals (Figure 1.12), which require a liquid to lubricate the seal faces and dry mechanical seals, which use the process gas or an external buffer gas to lubricate the seal faces. Figure 1.13 shows a dry gas seal used in process compressors.

Figure 1.9 This is a cross section of a centrifugal gas compressor. Notice that the rotor is supported by bearings and that gas containment seals are located at the two ends of the compressor casing.

Figure 1.10 Rolling element bearings is a bearing that carries a load by placing rolling elements (such as balls or rollers) between two concentric, grooved rings called races. The relative motion of the races causes the rolling elements to roll with very little rolling resistance and with little sliding.

Figure 1.11 A journal bearing consists of a smooth shaft rotating inside a cylindrical bearing. A thin layer of lubricant separates the journal from the bearing. Journal bearings are used to reduce friction in continuously rotating shafts.

Those...