Compressors and Modern Process Applications - Hardcover

Bloch, Heinz P.; Godse, Arvind

Hardcover

ISBN 10: 047172792X ISBN 13: 9780471727927

Publisher: Wiley-Interscience, 2006

Synopsis

A modern reference to the principles, operation, and applications of the most important compressor types

Thoroughly addressing process-related information and a wider variety of the major compressor types of interest to process plants, Compressors and Modern Process Applications uniquely covers the systematic linkage of fluid processing machinery to the processes they serve.

This book is a highly practical resource for professionals responsible for purchasing, servicing, or operating compressors. It describes the main features of over 300 petrochemical and refining schematics and associated process descriptions involving compressors and expanders in modern industry.

The organized presentation of this reference covers first the basics of compressors and what they are, and then progresses to important operational and process issues. It then explains the underlying principles, operating modes, selection issues, and major hardware elements for compressors. Topics include double-acting positive displacement compressors, rotary positive displacement compressors, understanding centrifugal process gas compressors, power transmission and advanced bearing technology, centrifugal compressor performance, gas processing and turbo-expander applications, and compressors typically found in petroleum refining and other petrochemical processes.

Suitable for plant operation personnel, machinery engineering specialists, process engineers,as well as undergraduate students of this subject, this book's special features include:

• Flow schematics of modern process units and processes used in gas transport, gas conditioning, petrochemical manufacture, and petroleum refining
• Listings of licensors for each process on the flow schematics
• Identification of each process flow schematic of compressors, cryogenic, and hot gas expanders at their respective locations
• Important overview of surge control, estimating compressor performance, applications for air separation and gas processing plants, petroleum refinery issues, and important criteria that govern compressor selection and application

Placing hundreds of associated process flow schematics at the fingertips of professionals and students, author and industry expert Heinz Bloch facilitates comprehension of the workings of various petrochemical, oil refining, and product upgrading processes that are served by compressors.

About this title

About the Author

HEINZ P. BLOCH is a consulting engineer. He resides in Colorado and is a licensed professional engineer in New Jersey. He served over twenty years in various machinery engineering capacities at Exxon and retired from his position as Exxon Chemical's Regional Machinery Specialist in the United States. Mr. Bloch has authored twenty full-length books and more than 670 papers and articles, conducted over 500 technical courses on all six continents, helped found Texas A&M's International Pump Users Symposium, and served as the Reliability Editor for Hydrocarbon Processing magazine. Contributor Arvind Godse is a chartered Mechanical Engineer with prior exposure to Rotating Machinery and association with leading contractor organizations. Among these are Engineers India, Kuwait National Petroleum, Qatar Petroleum, and PDO / TEBODIN Oman.

From the Back Cover

A modern reference to the principles, operation, and applications of the most important compressor types

Thoroughly addressing process-related information and a wider variety of the major compressor types of interest to process plants, Compressors and Modern Process Applications uniquely covers the systematic linkage of fluid processing machinery to the processes they serve.

This book is a highly practical resource for professionals responsible for purchasing, servicing, or operating compressors. It describes the main features of over 300 petrochemical and refining schematics and associated process descriptions involving compressors and expanders in modern industry.

The organized presentation of this reference covers first the basics of compressors and what they are, and then progresses to important operational and process issues. It then explains the underlying principles, operating modes, selection issues, and major hardware elements for compressors. Topics include double-acting positive displacement compressors, rotary positive displacement compressors, understanding centrifugal process gas compressors, power transmission and advanced bearing technology, centrifugal compressor performance, gas processing and turbo-expander applications, and compressors typically found in petroleum refining and other petrochemical processes.

Suitable for plant operation personnel, machinery engineering specialists, process engineers,as well as undergraduate students of this subject, this book's special features include:

• Flow schematics of modern process units and processes used in gas transport, gas conditioning, petrochemical manufacture, and petroleum refining
• Listings of licensors for each process on the flow schematics
• Identification of each process flow schematic of compressors, cryogenic, and hot gas expanders at their respective locations
• Important overview of surge control, estimating compressor performance, applications for air separation and gas processing plants, petroleum refinery issues, and important criteria that govern compressor selection and application

Placing hundreds of associated process flow schematics at the fingertips of professionals and students, author and industry expert Heinz Bloch facilitates comprehension of the workings of various petrochemical, oil refining, and product upgrading processes that are served by compressors.

From the Inside Flap

The aim of this book is to impart an understanding of critically important compressor parameters. Without getting lost in too much mathematical treatment, this text highlights the various interactions of importance in gas compression. It is profusely illustrated so as to give size, scale, and configurational details of compressors and their principal components.

Excerpt. © Reprinted by permission. All rights reserved.

Compressors and Modern Process Applications

By Heinz P. Bloch Arvind Godse

John Wiley & Sons

Copyright © 2006 Heinz P. Bloch
All right reserved.
ISBN: 978-0-471-72792-7

Chapter One

Positive-Displacement Compressors

Although a fair number of types of large compressors can be found in modern process plants, they can, certainly for our purposes, be separated into positive-displacement and dynamic machines. In the positive-displacement category, reciprocating compressors and twin-helical screw machines are of primary interest. Reciprocating compressors are certainly the "older" of the two. It's on that basis that we will provide an overview of reciprocating compressors before moving on to other process compressor types of interest.

1.1 RECIPROCATING COMPRESSORS

Some refining and gas-processing facilities (e.g., hydrocracking) are best served by piston-type reciprocating compressors. Accordingly, an overview of these machines is an important complement to this text.

Reciprocating compressors (see Figs. 1-1, 1-2, and 1-5), are the oldest and most widely used process compressor type. They are manufactured in a variety of different configurations, including vertically oriented and skid-mounted ("packaged") (see Fig.1-2). Their application ranges are quite wide and include both lubricated and nonlubricated cylinder models, as shown in the coverage chart of Fig. 1-3. So-called secondary compressors often use plain plungers instead of the more typical pistons shown in Fig. 1-4.

An outstanding reference document, API Standard 618, describes many recommended features of reciprocating-process gas compressors. Fig. 1-5 shows a typical compressor cross section and the customary nomenclature. Other important components are also depicted on this cross-sectional view.

Compression is produced by the forced reduction of gas volume by the movement of a piston or plunger in a cylinder. Suction and discharge valves (see Figs. 1-6 and 1-7), are spring-loaded and work automatically from pressure differentials generated between the compressor cylinder and piping by the moving piston.

Reciprocating compressors are manufactured as both air-cooled and water-cooled models. Since we are discussing the compressor as related to petrochemical and oil refining processes, we shall concern ourselves only with the cooled, liquid-jacketed type. This type compresses gas on the forward stroke and also on the reverse stroke of the piston.

Reciprocating compressors have a wide range of applications. Crankshaft speeds may range from 125 to 1,000 revolutions per minute. Piston speeds range from 500 to 950 feet per minute, the majority being 700 to 850 feet per minute. The nominal gas velocity is usually in the range of 4,500 to 8,000 feet per minute, and operational discharge pressures may vary from vacuum to 60,000 pounds per square inch (psi).

1.2 MAJOR COMPONENTS DESCRIBED

1.2.1 Crankcase

The crankcase (Fig. 1-8) is a U-shaped cast iron or fabricated steel frame. The top is left open for installation of the crankshaft. To prevent the top from opening and closing as a result of the forces of the throws, it is held together with torqued bolts and spacers or, alternatively, keyed spacers. These are placed directly above the main bearings. The main bearings, spaced between each throw, have removable top covers for ease of assembly and ready removal of the babbitted bearingliner shells. The keyed spacers are, therefore, preferred since their removal is easiest for access to bearing covers. Main bearings, however, in the types of compressors used in petrochemical plants, are so overdesigned for stress that they seldom require removal for rebabbitting.

1.2.2 Crankshaft

The crankshaft (see Fig. 1-8) is the heart of the machine, and usually the most expensive component. Each throw is forged and counterweights are bolted on to balance the reciprocating mass of the crosshead and piston. If the crankcase moves on the foundation, it will cause the throw to open and close through each revolution, resulting in fatigue and breakage. For this reason, the dimension at the open end of the throw must be taken periodically while barring the crankshaft through 360 degrees. This procedure is called "taking crankshaft deflections" and is recommended as an annual check.

1.2.3 Connecting Rod

The connecting rod (Fig. 1-9) is provided with pretorqued bolts fastening cap to body at the crank end. The split is shimmed and shims can be removed as wear progresses. The wrist pin is free-floating and held in place with caps in the crosshead, which allows the connecting rod to find its own center.

1.2.4 Crosshead

The crosshead (Figs. 1-9 and 1-10) runs between two guides with about one mil per inch (0.001" per inch) of diameter clearance. It is often weighted so that the mass inertia of all reciprocating parts is sufficient to reverse the stress on the wrist pin, even when one end of the piston is under pressure. If this is not done, the wrist pin will wipe all the oil from the side under stress and will bind or "get stuck."

1.2.5 Lubrication

Lubrication of the frame is accomplished either by a pump driven from the crank end or by a separately mounted pump. The pump takes oil from the crankcase sump and pumps it through a cooler and filter, usually 25 micron, then through piping to the main bearings. The crankshaft has holes drilled from the main bearing surface through to the connecting-rod bearing face. From here, the oil passes up through a hole in the connecting rod to the wrist pin and from there, through holes to the crosshead sliding faces. Oil scraper rings in the frame end prevent oil leakage out along the piston rod. Because of this tortuous passage of oil, prelubrication is required before start-up. This is accomplished with an auxiliary lube pump.

Crankcase oil heaters are specified for outdoor compressors to keep the oil at required viscosity and to prevent condensation with resultant corrosion. Oil, however, is a poor conductor and local overheating and carbonization have occurred with these heaters. Therefore, when using crankcase heaters while a compressor is not in operation, the auxiliary lube pump should be continuously run.

1.2.6 Cylinder Materials

Up to 1,000 psi, cylinders are normally made of cast iron. Above this working pressure, materials are cast steel or forged steel, at the manufacturer's discretion. Nodular iron castings are sometimes specified in preference to cast iron.

API-618 specifies that all cylinders must have replaceable liners. These are usually of cast iron because of its lubricating and bearing qualities. Liners should be honed to a finish of 10 to 20 microinches.

1.2.7 Cylinder Sizing

Cooling jackets, lubricant, and packing and ring material limit cylinder temperatures. A compressed gas discharge temperature of 275F is set as ideal maximum, with 375F as the absolute limit of operating temperatures. Using a common thermodynamic equation with the above limits, the design ratio per cylinder can be set. The design compression and tension load on the piston rod must not be exceeded and, therefore, rod load must be checked on each cylinder application.

Rod load is defined as follows:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where

R.L. = rod load in compression in pounds

[A.sub.HE] = cylinder area at head end in any one cylinder

[A.sub.CE] = cylinder area at crank end, usually [A.sub.CE] = [A.sub.HE] - area of rod

[P.sub.2] = discharge pressure, psia

[P.sub.1] = suction pressure, psia

The limiting rod loads are set by the manufacturer. Some manufacturers require lower values on the rod in tension than compression. In this case, the above limits are checked by reversing [A.sub.HE] and [A.sub.CE]. Note, however, that the supplemental specifications of reliability-focused user companies often impose a limit on the maximum allowable stress acting on the net thread root area of piston rods. This limit is largely based on field experience and relates to rod attachment limitations rather than stress failures of the rod itself.

Compressors below 500 psig, such as air compressors, are usually sized by temperature considerations. Rod load usually becomes the limiting factor in applications above these pressures.

1.2.8 Cylinder Cooling

In process compression, higher pressures are normal. Therefore, compression ratios are low. Low compression ratios give low temperature rises. Thermosiphon cylinder cooling is, therefore, specified, with a discharge temperature limit of about 200F. Thermosiphon cooling consists of filling the jackets with an appropriate liquid such as water or a suitable radiator fluid and letting the heat radiate from the outer cylinder walls. The purpose of filling the jackets is to obtain an even heat distribution throughout the cylinder.

Above 200F, coolant circulation is applied. Raw water is to be avoided because it leaves deposits in the cooling jackets that are extremely difficult to clean out. A closed system is specified, therefore, which consists of a reservoir, circulating pumps, and heat exchanger.

The old saying "cooler is better" is really incorrect here. Overcooling of the cylinder to a temperature below the dew point of the compressed gas must be avoided to prevent cylinder corrosion, or liquid build-up that results in a slug of liquid. Therefore, the coolant must be bypassed around the exchanger under temperature control. It is also recommended that a thermostat and heater be mounted on the reservoir and the coolant circulated when the compressor is stopped on standby, to maintain the cylinders at a temperature above the gas dew point.

The coolant is usually 50% ethylene glycol and water, to prevent freezing. This has a lower specific heat than water. Therefore, the manufacturer must always be asked to size heat exchangers on this basis, even if the initial coolant is intended to be only water.

Compressor valves are the most critical part of a compressor; they generally require the most maintenance of any part. They are sensitive both to liquids and solids in the gas stream, causing plate and spring breakages. When the valve lifts, it can strike the guard and rebound to the seat several times in one stroke. This is called valve flutter and leads to breakage of valve plates. Light-molecular-weight gases such as hydrogen cause this problem mostly, and it is controlled in part by restricting the lift of the valve plate, thus controlling valve velocity.

API valve velocity is specified as:

V = D x 144/A

where

V = average velocity in feet/minute

D = cylinder displacement in cubic feet/minute

A = total inlet valve area per cylinder, calculated by valve lift times valve opening periphery, times the number of suction valves per cylinder in square inches

Compressor manufacturers sometimes object to the above specification, since it gives a valve velocity for double-acting cylinders one half the value compared to equivalent single-acting cylinders. Therefore, manufacturers' data on double-acting cylinders often indicate a valve velocity double the API valve velocity and care must be taken to find out the basis upon which valve velocity is given. For heavier molecular weight gases (~20), API valve velocities of about 3,580 fpm are selected, and for lighter molecular weight gases (MW = 7), 7,000 fpm.

Manufacturers often use interchangeable suction and discharge valves. This can lead to putting valves in the wrong port, which can result in massive valve breakage or broken rods or cylinders. Reliability-focused users specify that valves must not be interchangeable. However, this feature can be lost or broken off, so correct valve placement should always be checked.

1.2.9 Pistons

Pistons (Fig. 1-9) are usually cast iron and are often hollow, to reduce weight. This space can fill with gas and is an explosive hazard when removing the piston from the rod. One should, therefore, specify that an easily removable plug must be supplied to vent this space before handling. Larger pistons are made of aluminum to reduce weight. These pistons have large clearance in the bore to allow for thermal expansion, on the order of 20 mils per inch of diameter. Rider rings are often supplied on cast iron pistons and are necessary on all aluminum pistons. These, or the whole piston, are rotated 90 about once per year to reduce wear. Bearing loads of pistons and rider rings are based on a unit stress calculated from half the rod-plus-piston weight in pounds, divided by the diameter of the piston or rider ring, times the width in square inches. Normal limits are 5 psi for Teflon(r), 12 psi for cast iron, 14 psi for bronze, and 22 psi for Allen metal. Teflon compression rings are specified and are useful up to 500 psi [DELTA]P. Above these loadings, copper-bearing material or Babbitt metal are used for wearing rings and bronze for compression rings. Teflon compression rings are often offered with steel expander rings underneath. These should be avoided because when the compression rings wear, the expander rings can score the cylinder. Designs of pistons and rings are available that will hold the compression ring out against the cylinder without expander rings.

1.2.10 Piston Rod

The piston rod (Fig. 1-10) mounts into the crosshead, and must be locked, either by a locknut or a pin, to prevent backing off. The rod is adjusted in the crosshead to equalize the end clearance of the piston in the cylinder. This is checked by barring over the machine, crushing a piece of soft lead, and measuring the remaining thickness. This is called the bump clearance.

The rod must be hardened where it passes through the packing. Some rods are chrome plated, but problems have occurred with them, especially on high-pressure machines with a high heat buildup. This can easily cause spider web cracks in the chrome which, in turn, can flake off and destroy the packing. The best arrangement is to purchase a flame-hardened rod. As wear occurs, the rod could be plated with tungsten carbide, which should last the life of the machine. High-pressure machines often have the rod extended through the piston and out the cylinder head to balance the pressure load (rod load) on the piston. Such an extended-through rod is called a tail rod. Tail rods have been known to break off and be ejected from the cylinder like a missile. Reliability-focused users specify that all tail rods must be housed in a capture box strong enough to contain the tail rod should breakage occur.

1.2.11 Packing

Compressor packing (Fig.1-11) is made up of two rings in pairs, mounted in steel or cast iron cups with the open end of the cups facing away from the pressure. The cups are bolted together and have vent, oil, and drain holes drilled in them where required. These must be correctly aligned each time the packing is opened. If Teflon is specified for packing for pressures above 500 psi, generally an additional metallic backup ring is used to prevent the Teflon from extruding out of the cups.

Compressor manufacturers supply a distance piece (Fig. 1-5) between the cylinder and the crankcase for access to the packing. This is usually good for a three- or four-cup set. In process applications, however, especially above 2,000 psi, packing sets run to 8 or 18 cups or more. Extra long distance pieces must be specified in order that sufficient space is available to dismantle the cups and change the packing rings. If any doubt exists, the extra long distance piece should be specified, as the extra cost is minor. It is generally not possible to change to a longer distance piece after the machine is built.

1.2.12 Gaskets

Cylinder end covers, valve covers, and valves are gasketed to the cylinder. Metallic gaskets are specified. Manufacturers often offer nonmetallic gaskets, which have led to considerable trouble with leaks, especially on low-molecular-weight gases.

Often, the gasket seats must be lapped to obtain a good seal. Soft iron or metallic V-ring sets have given the best service. O-rings confined four ways on valve cover plugs have also given good service, but should never be used where they can be crushed when pulling down the cover. Plug holes must be chamfered to prevent cutting of O-rings upon entry.

(Continues...)

Excerpted from Compressors and Modern Process Applicationsby Heinz P. Bloch Arvind Godse Copyright © 2006 by Heinz P. Bloch. Excerpted by permission.
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Edition details

• Publisher: Wiley-Interscience
• Publication date: 2006
• Language: English
• ISBN 10: 047172792X
• ISBN 13: 9780471727927
• Binding: Hardcover
• Edition number: 1
• Number of pages: 352