Damped Unbalance Response
Unfortunately, particularly for the vendors, the undamped analysis opened somewhat of a Pandora's box. While the calculated frequencies were not quite accurate, they were better than any previous analysis, once the actual bearing parameters were input. It should be mentioned in the course of the rotor analysis development, the art of tilting pad design moved from just carrying a load at speed to one of tailoring the arc of the pad by a design method called preload to achieve the stiffness and...
Maintenance
Most of the maintenance ideas presented in Chapter 5 are also applicable to the axial. Concentricity of the rotor clearances in the stator is again stressed. Rotor tip clearances allow a reasonable tolerance. A 10 clearance change is not greatly significant. When an axial is taken out of service, the blading should be carefully cleaned and inspected. If minor damage is noted, the blade should be repaired or, if the damage is significant, it should be replaced. In most cases, minor,...
Rotary Compressors
The rotary compressor portion of the positive displacement family is made up of several compressor configurations. The features these compressors have in common are 1. They impart energy to the gas being compressed by way of an input shaft moving a single or multiple rotating element. 2. They perform the compression in an intermittent mode. 3. They do not use inlet and discharge valves. The helica and spiral-lobe compressors are generally similar and use two intermeshing helical or spiral lobes...
Impeller Overspeed Test
Obviously this test applies only to the centrifugal compressor. The manufacturer normally performs this test whether specified or not. Since the user does have an interest in the test, some discussion of the parameters is in order. API 617 mandates the test, which includes the requirement to spin the impeller at a speed of 115 of the maximum continuous speed for a duration of one minute. The tests are performed in a special overspeed chamber, which is evacuated to a minimum level to minimize...
Conversion Factors
Abbreviations. A angstrom, atm standard atmosphere, 760 mm of Hg at 0 C, cal calorie gram , cm centimeter, deg degree, gal gallon, U.S. liquid, gm and g gram, gmole gram-mole, J joule, kcal kilocalorie, kg kilogram, kJ kilojoule, km kilometer, kW kilowatt, 1 liter, lb avoirdupois pound, m meter, mi mile U.S. mm millimeter, N newton, oz avoirdupois ounce, pmole pound mole, pt pint, rad radian, rev revolution, s second, ton short U.S. ton, V volt, W watt. Others are as usual. 1.9859 cal. 10.731...
Gear Design and Application
The special purpose gear unit should have a specified API 613 service factor of 1.4 as a minimum. The service factor used in the gear design should he selected to meet the requirements of API for the application. The gear unit must be tuned to the system in which it operates. The vendor with the train responsibility must analyze the system to be certain the gear unit will not transmit torsional or lateral vibrations to the rest of the system or be damaged by system excitations. If the gear unit...
References Rqu
1. Balje, O. E., Study on Design Criteria and Matching of Turbomachines, Part B Compressor and Pump Performance and Matching of Turbocom-ponents, ASME Paper No. 60-WA-231, ASME Transactions, Vol. 84, Journal of Engineering for Power, January 1962, p. 107. 2. Boyce, Meherwan P., Gas Turbine Engineering Handbook, Houston, TX Gulf Publishing Company, 1982. 3. Durham, F. P., Aircraft Jet Power Plates, Englewood Cliffs, NJ Prentice-Hall. Inc., 1951. 4. Lapina, R. P., Escalating Centrifugal...
Liquid Buffered Seals
The positive seals are positive in the sense that the process gas is completely controlled, and in most applications, can be designed to avoid the loss of any gas, if the process gas and the sealing fluid are compatible to permit safe separation. In any event, the gas taken from the process is orders of magnitude lower than is the case for the restrictive seal. The positive seals take on the form of a liquid film seal or a contact seal, also known as the mechanical seal. The buffer fluid aids...
Isothermal Compression
Isothermal compression is presented here to represent the upper limits of cooling and horsepower savings. It is the equivalent of an infinite number of intercoolers and is not achievable in the practical types of compressors described in this book. For an isothermal process, From this, a theoretical value for the power used by the compressor can be found. This equation is useful when evaluating the benefit of multiple intercoolers, because it establishes the theoretical power limit that can be...
Degassing Drum
If the oil from the drainers is not contaminated, it can be returned to the reservoir for immediate reuse. If this be the case, a degassing drum is recommended to remove the entrained gases prior to returning to the reservoir. For flammable gases, this practice helps eliminate the explosion hazard in the reservoir. If a suitable low pressure recovery system is available, the gases removed can also be recovered. The alternative is to go to the flare with hydrocarbons. Figure 8-14 is a diagram of...
Lube Oil Overhead Tank
An alternative to the coastdown pump approach is the overhead lube oil tank. It has the valuable feature of being motivated by gravity. If reasonably maintained, it should be one of the more reliable methods available, It will safely permit the coastdown of a compressor without losing the bearings due to lack of lubrication during power failure. Figure 8-12 shows two schemes for the tank arrangement. While both systems are in successful operation, the method at b requires less instrumentation...
Commissioning the Lube Oil System
When the lube system arrives in the field, it will normally see the compressor and the interconnecting compressor piping for the first time. When the system first arrives, a check should be made to ensure that all the parts have been received. The system should be kept closed as much as possible to maintain cleanliness, while the field piping is being made up. This is where the stainless begins to bring in a return on investment because it should only require keeping dirt out. The carbon steel...
Steam Turbines
One of the first questions the designer must answer concerns which type steam turbine should be used. The back pressure turbine is selected when process steam demands are greater than the steam required for process drivers such as large compressors. This type turbine is also selected when various steam levels are required by the process. The condensing turbine is selected when steam demand for process drivers is greater than the low-pressure process steam requirements. It is also selected when...
Tests
A certain number of the tests will have been designated as witness tests. One is usually the hydrostatic test, which does not require an engineer but can be witnessed by an inspector. As a cost-saving measure, if engineers from another project happen to be in the plant at the time of the hydrostatic test, they may be able to arrange to witness the test, because the duration of the test is normally not long. It may take a total of two hours. If a gas leak test is called for, the test can take...
Thrust Bearings
Centrifugal compressor impellers, with the exception of the open impeller, are thrust unbalanced. The machine also has a requirement for a location device to maintain axial clearances. For these reasons, all centrifugal compressors use some form of thrust bearing. API 617 recognizes the need for the compressor thrust design to take into account peripheral factors such as the coupling. Gear couplings can transmit thrust to the compressor because of tooth friction. The standard uses an arbitrary...
Hydrostatic Test
It is conventional to perform a hydrotest on all pressure-containing casings for all compressors. For the reciprocating compressor, where there is no casing, it is the cylinders that are tested individually, based on VA times the maximum working gas pressure, but not less than 20 psig. The 20 psig minimum is generally recommended for the rotary, positive-displacement casing as well as dynamic compressors. Reciprocating cylinder water jackets are tested at VA times specified working pressure for...
Rotor Dynamics
Lateral critical speeds are somewhat misnamed, since they are now thought of as a damped response to some form of rotor excitation. Classically, they got named when compressor or other rotating machine speed, whether starting or operating, happened to coincide with a responsive lateral resonance damped natural frequency . Since the rotor of the compressor included inherent unbalance, it would excite the rotor-bearing support system. Because this occurred long before the speed squared effect of...
References Ylc
1. Compressors and Exhausters, ASME PTC 10-1965, New York American Society of Mechanical Engineers, 1965. 2. Displacement Compressors, Vacuum Pumps and Blowers, ASME PTC 9-1970, ANSI PTC 9-1974, New York American Society of Mechanical Engineers, 1970. New York American National Standards Institute, 1974. 3. Measurement of Shaft Horsepower, ASME PTC 19.7-1961, New York American Society of Mechanical Engineers, 1961. 4. Benedict, M., Webb, G. B., and Rubin, L. C,, Chemical Engineering Progress,...
Theoretical Work
Theoretical work or compressor head is the heart and substance of compressor design. Some basic form of understanding must be devel oped even if involvement with compressors is less than that of design of the machine itself. Proper applications cannot be made if this understanding is absent. The following theoretical evaluations will be abbreviated as much as possible to reduce the length and still present the philosophy. For the reader with the ambition and desire, the presentation will be an...
Dynamic Compressors
In dynamic compressors, energy is transferred from a moving set of blades to the gas. The energy takes the form of velocity and pressure in the rotating element, with further pressure conversion taking place in the stationary elements. Because of the dynamic nature of these compressors, the density and molecular weight have an influence on the amount of pressure the compressor can generate. The dynamic compressors are further subdivided into three categories, based primarily on the direction of...
Mollier Charts
Another form in which gas properties are presented is found in plots of pressure, specific volume, temperature, entropy, and enthalpy. The most common form, the Mollier chart, plots enthalpy against entropy. A good example of this is the Mollier chart for steam. Gases are generally plotted as pressure against enthalpy P-h charts . These are also sometimes referred to as Mollier charts. The charts are readily available for a wide range of pure gases, particularly hydrocarbons and refrigerants....
Equations of State
Gases can be treated individually or as mixtures by the methods just outlined for most applications including evaluation of vendor proposals. More sophisticated equations of state can be used for real gas applications when large deviations from the perfect gas law are anticipated. For mixtures, more sophisticated mixing rules can be paired with the equation of state when required. For hydrocarbons, the most widely used equation of state is the Benedict-Webb-Rubin BWR equation 2 . For a gas...
Compression Methods
Compressors have numerous forms, the exact configuration being based on the application. For comparison, the different types of compressors can be subdivided into two broad groups based on compression mode. There are two basic modes intermittent and continuous. The intermittent mode of compression is cyclic in nature, in that a specific quantity of gas is ingested by the compressor, acted upon, and discharged, before the cycle is repeated. The continuous compression mode is one in which the gas...