Rod Packing

A packing is required on double-acting cylinders to provide a barrier to leakage past the rod where it passes through the crank end cylinder closure. The same arrangement is needed at the head end if a tail rod or tan dem cylinder is used. The packing may consist of a number of rings of packing material and may include a lantern ring (see Figure 3-14). The lantern ring provides a space into which a gas or liquid may be injected to aid in the sealing process. If cooling of the packing is...

Conducting the Test

The problem with field testing is getting the test gear in place. If the process has been suitably defined, with the computer terminal turned on to the local favorite equation of state, the testing may begin. If the bodies are available to take the data, an all-manual set up works well. If getting to the point to read the data is a problem, then a transducer should be used. It should be good quality and calibrated before and after the test run. Automation by way of a data logger does help keep...

Sizing

The displaced volume of the sliding vane compressor can be calculated if certain geometric data are available. Unfortunately the vendor catalog data generally will not be very useful in establishing the geometry because frame designations are not directly related to any convenient measurement. The vendor's literature does give the expected capacity for air at various pressures, which include the displaced volume and the vol- Figure 4-22. Pressure-volume diagram of the compression cycle of a...

Mounting Plates

Either soleplates or baseplates should be furnished for most of the compressor types. Reciprocating compressors are sometimes directly mounted to the foundation and the frame grouted. Most other compressors are mounted on an intermediate surface in the form of soleplates or a structural steel base. The user must make a number of decisions, beginning with the basic decision of sole or baseplate. This decision must be revealed to the vendor in the specification. If a baseplate is chosen, the...

Basic Design

At first glance, it may appear unnecessary to specify design parameters. This might appear to be the vendor's responsibility. While the vendor does take care of the basic design, there are optional areas where he must receive instructions (by way of the specification) to be able to complete the design. The data sheet is used to convey the process design parameters, capacity, temperature, pressure, and gas data. The verbage can explain or better define any of the specifics. For example, if gas...

Blades

While the process engineer does not need to understand the aerodynamic theory that makes the axial compressor perform, a few basics may be helpful. Acquaintance with the blading nomenclature and the fundamental velocity triangle may also keep the terms used around these compressors from seeming like a foreign language. The airfoil in most axials is of the NACA type, generally a 65 series. A typical designation is 65-(18) 10. The 65 defines the profile shape. The 18 represents a lift coefficient...

Stagger

For a given degree of reaction and value of Va or flow rate, a choice can be made of the stagger angle or setting of the blades. From the consideration of two defining equations Figure 6-6. An axial compressor controlled diffusion airfoil. (Courtesy of Elliott Company Figure 6-6. An axial compressor controlled diffusion airfoil. (Courtesy of Elliott Company For fixed values of R and Va, a selection of a value of a2 (corresponding to a value of stagger angle) fixes a limiting value of (X from...

Shaft End Seals

In controlling gas leakage, shaft end seals are either restrictive or positive in nature. The labyrinth seal is one form of restrictive seal. The reasoning for the labyrinth end seal is generally the same as discussed in the interstage seal section. A procedure for the calculation of restrictive seal leakage is given in Appendix D. Another common form of restrictive seal is the carbon ring seal (see Figure 5-49). This seal consists of a series of carbon rings, using either solid or segmented...

Enclosures

The enclosure selected affects cost substantially. Corrosive and hazardous atmospheres encountered dictate the protection needed. Today's standard motor enclosure for indoor applications is the open, drip-proof enclosure for induction and high-speed synchronous motors. For large motors, open, drip-proof construction is available up to about 2Q,0(> 0 hp and is used for squirrel-cage, synchronous, and wound-rotor motors. For larger motors, the next degree of protection is weather-protected....

Weight Flow Control with Variable Stator Vanes

A turbine-driven axial compressor is used to supply air to a blast furnace. The furnace requires a certain constant weight flow, depending upon the size of furnace and the size of the charge. An example of a possible arrangement is shown in Figure 8-40(c) and can be described as follows 1. The compensated flow transmitter determines the process flow. It converts this quantity to a signal that is proportional to the process flow and sends it to the flow controller. The transmitter could be a...

Casings

Most casings on both flooded and dry compressors are cast, normally of grey cast iron. API 619 5 limits the use of cast iron by specifying steel for services in excess of 400 psig, discharge temperatures in excess of 500 F, and for flammable or toxic gases. While rare, austenitic and high nickel casings have been furnished. On dry compressors, the casing Figure 4-13. A rotor set for an oil-free helical-lobe compressor. (Courtesy ofA-C Compressor Corporator ) normally includes a water jacket....

Common Features

Rotary compressors as a group make up the balance of the positive displacement machines. This group of compressors has several features in common despite differences in construction. Probably the most important feature is the lack of valves as used on the reciprocating compressor. The rotary is lighter in weight than the reciprocator and does not exhibit the shaking forces of the reciprocating compressor, making the foundation requirements less rigorous. Even though rotary compressors are...

Enclosure Applications

Motor enclosure types for compressor service under shelters are b. Totally enclosed inert gas or instrument air-filled b. Modified drip-proof (air intake at bottom) The force-ventilated type is preferred when a Division 1 classification is necessary. The standard drip-proof enclosure is preferred in Division 2 or in safe locations. For an outdoor installation, serious consideration should be given to using NEMA Type 2 weather-protected enclosures rather than the completely enclosed types. In...

Arrangement

The single stage can be arranged, as has been discussed in the previous paragraphs, in the overhung style. Figure 5-4 shows a schematic of the compressor. Note that the flow enters axially and exits in a tangential direction. For a comprehensive discussion, it should be mentioned that the overhung style is, on very rare occasions, constructed in the multistage form, usually overhanging no more than two impellers. The overhung compressor is generally more competitively priced than the...

Torsional Damping and Resilient Coupling

Many believe that couplings are only torque transmitters between different pieces of equipment with a secondary function of handling ever present misalignment. While couplings are typically sized and chosen based on the before mentioned requirements, they can take on another role, that of a torsional fixer Once the driver and driven equipment have been chosen and it is deter mined that none of the items will be subject to any lateral vibration problems, the system torsional analysis is...

Rod Drop Monitor

Rod drop monitoring is a monitoring system used on horizontal reciprocating compressors. Most horizontal reciprocating compressors use wear bands also known as rider rings to support the piston in its cylinder The wear bands are generally made of a plastic material which in time abrades and requires replacement. Should the wear go unchecked, contact with the cylinder wall by the piston will occur with the potential for serious damage to the cylinder and the piston. Also the packing may be...

Curve Shape

Figure 1-3 presented a general form performance curve for each of the compressors. The centrifugal compressor exhibited a relatively flat curve compared to the other machines. Flat is defined as a relatively low head rise for a volume change. Translated to pressure terms, it means a relatively low pressure change for a given volume change. It is important to understand some of the basics that contribute to the curve shape. Figure 5-24 shows that if the flow is reduced for the radial wheel, a...

Mechanical Design

The centrifugal compressor is composed of a casing containing a rotating element, rotor, which is supported by a set of bearings. For most multistage compressors, shaft end seals are located in-board of the bearings. Figure 5-28. Chart for minimum straight inlet piping. Use this chart and the given factors in conjunction with Figure 5-29. Courtesy of Elliott Company Figure 5-28. Chart for minimum straight inlet piping. Use this chart and the given factors in conjunction with Figure 5-29....

Anti Surge Control

Surge is part of the inherent operating characteristics of centrifugal and axial compressors. It is probably one of the most misunderstood of all the characteristics of these compressors. Regard for surge seems to vary from complacency to terror based sometimes on fact and experience and other times on rumor. While surge was described in Chapters 5 and 6 for the centrifugal and axial compressor, a quick review to introduce anti surge control is included here. As flow is reduced in an axial or...

Cylinder and Packing Lubrication

Lubricated cylinders use a separate mechanical lubricator to force feed, in metered droplet form, a very precise amount of lubricant to specified points. This minimizes the amount of lubricant in the cylinder and allows a lubricant most compatible with the gas to be selected without compromising the frame lubrication system. Lubricant is fed to a point or points on the cylinder to service the piston rings and the packing when required. In a few cases, as in air compressors, the packing is...

Transfer Valves

For the twin arrangements, a two-way, six-ported, continuous-flow valve will be required. If it is a tapered plug type, a means of lifting the plug will be required. The purpose of the tapered plug valve is to provide a tight metal-to-metal seat to prevent or at least minimize leakage on reseating after a transfer. Cylindrical bore valves of the two-way, six-port type valves are available. These valves use O-rings to act as seals to prevent leakage. Another arrangement, particularly well suited...

Pressure Control Valves

For positive displacement pumps, a bypass-type control valve should be furnished to set the primary lube system pressure. The valve should be able to maintain system pressure during pump startup and pump transfers, which includes relieving the capacity of one pump, while both are running. The valve should provide stable, constant pressure during these transients. Flow turndown of 8 to 1 is not unusual. Multiple valves in parallel should be used if a single valve is not suitable. The valve...

Shop Balance Machine

This is where most balancing of rotors takes place. The machines range from heavy, direct-driven to the light, belt-driven type. There are numerous shop-made versions to balance a particular type of rotor, especially for the high speed, very light rotors. The machines spin the rotors at a speed considerably less than operating speed. The force generated by the unbalanced rotor, recalling the simple force equation is measured. Because of the range of rotor weights, it is not difficult to see why...

History

While a form of helical-rotor compressor was invented in Germany in 1878 1 , the helical-lobe compressor as used today is credited to Alf Lysholm, the chief engineer of Svenska Rotor Maskiner AB (SRM). Mr. Lysholm conceived the idea in 1934 as part of gas turbine development at SRM. The original compressor was an oil-free design using timing gears to synchronize the rotors. Three male and four female rotor lobes were used, and a steeper helix angle permitted higher built-in compression ratios...

Pistons and Rods

Dresser Rand Reciprocating Compressor

The lowly piston, one of the more simple items, has one of the most important functions of the entire compressor. The piston must translate the energy from the crankshaft to the gas in the cylinder. The piston is Figure 3-7. Cylinder with clearance pocket. (Courtesy of Dresser-Hand) Figure 3-7. Cylinder with clearance pocket. (Courtesy of Dresser-Hand) equipped with a set of sliding seals referred to as piston rings. Rings are made of a material that must be reasonably compliant for sealing,...

Reciprocating Shaking Forces

Reciprocating compressors include linear translational motion as a part of their normal operation. A single cylinder compressor will have an inherent set of shaking forces as part of its operating characteristics. The forces are resolved to reaction forces on the main bearings, where they transmit through the frame and the foundation. The purpose here will be to show the reader the basics of how these forces are generated and a general philosophy on how they can be minimized by balancing or by...

First Law of Thermodynamics

The first law of thermodynamics states that energy cannot be created or destroyed, although it may be changed from one form to another. Stated in equation form, it is written as follows If the change in energy to the system is expanded, then AU change in internal energy APE change in potential energy AKE change in kinetic energy If the work term Wt is expanded to breakdown shaft work done to or from the system and the work done by the system, then W( W + (pvAm)out - (pvAm)in (2.37) W shaft work...

Second Law of Thermodynamics

The second law of thermodynamics was actually postulated by Carnot prior to the development of the first law. The original statements made concerning the second law were negative they said what would not happen. The second law states that heat will not flow, in itself, from cold to hot. While no mathematical relationships come directly from the second law, a set of equations can be developed by adding a few assumptions for use in compressor analysis. For a reversible process, entropy, s, can be...

Variable Frequency Drives

A relatively new innovation for use in electric motor compressor drives is the variable frequency power source. Fundamentally, the power source converts an existing three-phase source into DC then uses an inverter to convert back to a variable frequency supply. Thyristors or transistors are used to switch the output at the required frequency. Figure 7-6. An integral geared compressor driven by a motor with an open drip-proof enclosure. The enclosure also includes 85dba sound attenuation....

Out

Diagram of a single-stage overhung type centrifugal compressor. A less common form of the single stage is shown in Figure 5-5. In this form, the impeller is located between two bearings, as is the multistage. This type of compressor is sometimes referred to as a beam type single stage. The flow enters and leaves in a tangential direction with the nozzles located in the horizontal plane. The between-bearing single stage is found most commonly in pipe line booster service where the...

Capacity Control

The reciprocating compressor is a fixed displacement compressor in ils basic configuration however, several methods are used to overcome this limitation to permit running at multiple operating points. In the discussion on cylinders, mention was made of clearance pockets. By use of the clearance pockets, the cylinder capacity can be lowered (see Equation 3.5). If the pocket is connected directly to the clearance area, the clearance term e can be increased. Increasing the clearance reduces the...

Synchronous Motors

Synchronous motors have definite advantages in some applications. They are the obvious choice to drive large, low-speed reciprocating compressors and similar equipment requiring motor speeds below 600 rpm. They are also useful on many large, high-speed drives. Typical applications of this type are geared, high-speed (above 3,600 rpm), centrifugal compressor drives of several thousand horsepower. A rule of thumb that was used in the past for constant speed applications was to consider the...

Dry Gas Seal System Control

The control system should be kept as simple as possible to maintain reliability. For tandem seals, the system control for the seal gas supply has traditionally been a differential pressure control, using a direct-operated control. The differential pressure usually was set at 10 to 25 psid. The flow using this control is generally much higher than needed. The differential pressure control can be modified to measure the pressure drop across an orifice and the control converted to a volume...

Pulsation Control

The intermittent personality of the reciprocating compressor becomes evident when the subject of pulsations is broached. Because discharge flow is interrupted while the piston is on the suction stroke, pressure pulses are superimposed on the discharge system's mean pressure. At the suction side of the system, the same type of interruption is going on, causing the suction pressure to take on a non-steady component. The frequency of the pulses is constant when the speed is constant, which is the...

Continuous vs Periodic Monitoring

There are two types of condition monitoring continuous and periodic. Continuous monitoring, as its name implies, examines measurements taken on a continuous basis. Periodic monitoring is based on measurements taken at regular time intervals. With the advent of computerized real-time systems, the distinction between continuous and periodic condition monitoring must be modified. Though technically periodic, a scanning system operating fast enough to protect against catastrophic failure is...

Rotor Dynamics Verification

The API mechanical standards for the rotary and centrifugal compressor have a test specified for proof of rotor insensitivity. This would normally be the test invoked at this point to ultimately prove the rotor. The fifth edition of API 617 expanded on this test and changed the acceptance criteria from those based on amplification factor to an acceptance level based on internal seal clearances. The results from these tests will have additional value if they are used to verify the mathematical...

Compression Cycle

Figure 5-17 is a section of a typical multistage compressor, which should aid the reader in following the flow path through the machine. Figure 5-17. Flow path through typical stages on a multistage unit. (Courtesy of Elliott Company Figure 5-17. Flow path through typical stages on a multistage unit. (Courtesy of Elliott Company Gas enters the impeller from one of several sources. In the case of the first impeller of a multistage, the flow has moved through an inlet nozzle and is collected in a...

Objectives of Centrifugal Compressor Mechanical Tests

It is a good idea for the user to have a witness to check that someone has inspected and verified operation of all safety and warning instrumen tation used in the shop. Granted, the compressor is in the vendor's shop and repairs are on the manufacturing account, but why risk damage and late shipment just because the witness was bashful While he's at it, have him ask if the oil is being filtered to a level of at least 10 microns. It probably isn't feasible to open the filter and look, but at...

Balance Piston

Balance Piston Arrangements

U is desirable to have additional axial-load control on the multistage compressor. A balance piston, also referred to as the balance drum, can be located at the discharge end see Figure 5-46 . The balance piston consists of a rotating element that has a specified diameter and an extended rim for sealing. The area adjacent to the balance piston opposite the last stage location is vented, normally to suction pressure. The differential pressure across the balance piston acts on the balance piston...

Suction Drums

All types of compressors will benefit if suction drums are part of the inlet system design. However, as a practical consideration, it would be difficult to justify a drum on small spared compressors, particularly the off-the-shelf' type. If the compressor is in critical service, even when spared and or replacement cost is deemed significant, suction drums should be considered. The purpose of the drum is to exclude liquid from entering the compressor while operating. Ideally, the drum should be...

Lubrication Systems

Compressor Block Diagram

Lubrication is a fundamental requirement for all compressors with the exception of those equipped with an alternative form of bearing such as the magnetic bearing. If it is a tiny unit, the lubricant may be sealed into the rolling element bearings by the bearing manufacturer. In process service, lubrication of bearings takes on a more elaborate form. Some of the smaller units will probably use an attached oiler or an oil mist system. Because this affects only the smaller units, this section...

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...

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

Enthalpy Chart Conversion

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...

Liquid Buffered Seals

Liquid Film Seal

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...

Degassing Drum

Balancing Drum Centrifugal Compressor

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...

Thrust Bearings

Thrust Collar Centrifugal Compressor

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

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

Pinionie 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...

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

Reciprocating Pump Performance Graphs

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...