CHAPTER 1
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 centrifugal compressors are able to convert a high velocity flow from a rotating impeller into high pressure.
We can use the example of how an airplane wing works to better understand Bernoulli's principle. Because of the wing's shape, air moving over the top of the wing must travel farther and therefore faster than the air traveling across the bottom of the wing. This difference in air velocities results in a lower pressure across the top of the wing than under the wing, resulting in a net upward force that can lift an airplane. An easy way to remember Bernoulli's principle is as follows: If the fluid's velocity increases the pressure decreases; and if the fluid's velocity decreases the pressure increases. These two relationships are inversely proportional (Figure 1.5)
Troubleshooting Centrifugal Pumps
Here are some useful relationships to remember when dealing with centrifugal pumps:
• If the pressure on the discharge pressure gauge is increasing, the flow is likely decreasing.
• If the flow is increasing, the horsepower required by the pump is increasing, which should be reflected by increasing amps or kilowatts.
• If the fluid viscosity is increasing or getting thicker, the discharge pressure will fall and the horsepower required by the pump will increase.
• If the flow is increasing, the net positive suction head required (NPSHR) will need to increase to prevent cavitation (see definition below).
Cavitation is caused by either 1) operating a pump too close to the boiling point of the liquid at its suction or 2) by trying to pump more than a pump is designed to handle. Pump designers use a term called net positive suction head (NPSH) to determine whether there is enough suction pressure to prevent cavitation. Typical net positive suction head requirements can be seen in Figure 1.3 (Centrifugal Pump Curves), where they are depicted as dotted vertical lines, labeled: 2', 25', 3', 35', 4', 45' 2' means that at this particular flow, two feet of liquid suction head is required to prevent cavitation, 25' means that at this particular flow, two and a half feet of liquid suction head is required to prevent cavitation, and so forth.
As an operator, you simply need to remember that the higher the flow rate out of a pump the greater the suction pressure must be to insure that cavitation does not take place An operator is in the position to control some parameters that will affect NPSHA. For example, if the pump is taking suction from a tower or tank, either the liquid level or the pressure can be increased as a means of stopping or reducing pump cavitation. Reducing the pump flow can have a positive effect on the NPSHA as well and may stop cavitation
Responding to Pump Problems
When you detect a pump problem and decide to act, always list the symptom or symptoms that were noted on the work request, such as not enough pressure or flow, a seal is leaking, the drive motor is tripping off line, there is high vibration on the pump, etc. Do not list what should be done about the suspected problem on the work request. Comments such as: fix the pump, change the pump, replace the pump, etc are not helpful to the maintenance planner. Comments such as: Seized, vibrating, low pressure, tripping off line, etc are helpful. Always, allow the maintenance crew to investigate the situation and perform what they think are the necessary corrections, adjustments or repairs.
Problems that overhauling equipment will not solve
• Plugged suction strainers.
• Plugged discharge strainers.
• Air leaking into a suction flange.
• Check valve on standby pump leaking.
• Pump turning backward.
• Wrong pump speed.
• Bypass valve open requiring too much flow.
• Blockage downstream of the pump.
• Too much flow being required from the pump.
• All of these should be checked before writing a work request for repair.
Positive displacement pumps
Positive displacement pumps (see Figures 16 and 17), unlike centrifugal pumps, theoretically can produce the same flow at a given speed (RPM) regardless of the discharge pressure. Thus, positive displacement pumps are constant flow machines. However, a slight increase in internal leakage as the pressure increases prevents a truly constant flow rate. As the pump is turned faster, more liquid is ejected from the pump There are only two ways to increase output where a positive displacement pump is used—turn it faster or buy a bigger pump.
In this type of pump, flow on the discharge side of the pump must never be stopped because discharge pressure will continue to build until something is damaged. There are many types of positive displacement pumps, but regardless of how a particular unit is configured, the results are the same. As the pump turns, the liquid inside it is trapped and must have a place to go. Positive displacement pumps have the same size inlet and outlet. On many of these pumps the suction and discharge can be changed by turning the pump in the opposite direction. Without a functional relief device on the discharge of the pump, catastrophic damage could occur.
Positive displacement pumps also have performance curves (see Figure 18 above), but they tend to look quite different than centrifugal pump curves.
Useful relationships to remember when dealing with positive displacement pumps include:
• If the discharge pressure is rising, the required horsepower is also rising.
• If the pump is required to deliver more flow (e.g.—gallons per minute, barrels per day, etc.), then the speed of the pump must be increased or a larger pump is required.
• If the viscosity of the fluid is increasing, then the horsepower required to pump the fluid is also increasing.
• If the pump flow is increasing, then the net positive suction head required to prevent cavitation must also increase.
These basic relationships can help to troubleshoot a problematic positive displacement pump. When you detect a pump problem and decide to act, always list the symptom or symptoms that were noted on the work request, such as 'not enough pressure or flow', 'the outboard seal is not sealing', 'there is high vibration on the pump', etc. Allow maintenance to evaluate the situation and determine the proper course of action
Drivers
A driver is any machine that takes electrical, steam, or fluid energy and converts it into rotary power that can be used to drive a process machine. The key attributes of a driver are reliability and efficiency. Various types of drivers are employed depending on the requirements of the driven machine and the available energy sources. For example, if electrical power is available and a constant speed is required for a driven machine, then an electrical motor can be used. If steam is available and variable speed is required to drive a process machine, then a steam turbine can be utilized. Steam turbines can be very efficient by driving a piece of equipment such as a pump and reducing steam pressure at the same time, eliminating the need for a steam let down or control valve.
If there is a difference between the design speed of the driver and its driven machine, a speed increaser or decreaser may be required to properly couple them. Gearboxes and a system of sheaves, belts, or chains can be used to transmit the required power and the required rotational speed. Size, cost, efficiency, and reliability are all factors an engineer considers when selecting the type of speed converter to be used.
Electric Motors
Electric motors are the most common type of rotating equipment driver in most process facilities. They are used not only to drive pumps but also to drive many other types of equipment found in industry. Figure 1.9a below shows a totally enclosed, fan-cooled (TEFC), electric motor, which consists of two major components (see Figure 19b)—A rotating element, called the rotor, and a stationary housing encircling the rotor, called the stator. A stator is composed of a set of stationary electrical windings that generate a rotating magnetic field created by the three phase electric power connected to them.
Three phase power, commonly used throughout industry, is composed of three circuit conductors carrying three alternating currents (of the same frequency but 120 degrees out of phase). From an operations point of view, three phase power means that if any two leads connecting the motor are moved to a new mounting location externally at the connection to the power source, the direction of the motor will be reversed. Remember that, because many types of equipment must turn in a specific direction to perform properly, it is imperative that the direction of rotation of a newly wired electric motor be checked before installing the coupling between the driver and the driven machines.
As a general rule, if you touch a motor and it is too hot to keep your hand on it, it is likely that the motor is running heavily loaded. The fins on the outside of a motor are to aid with cooling and to stiffen the stator. These should be kept clean and clear of debris and insulation. Quite simply, the hotter a motor runs, the shorter its life will be.
There are many factors in determining what is too hot, such as the class of insulation and the load on that motor at a given time. The best analysis is done by knowing what normal temperatures are and by responding if a significant change in the operating temperatures is noticed. Additional information, such as the significance of the change, may mean getting help from an acknowledged professional.
Normally, a motor will not fail immediately due to higher operating temperatures. The effects of high temperature operation are cumulative and manifest themselves over time, resulting in a shortened service life. Large motors with an enclosure for placement outdoors need to be checked to ensure the screens or filters that protect the windings in the stator stay clean. If they become clogged, the temperatures may go up and the motor life may be significantly reduced.
