AUTOCLAVEWITH COMPLICATZID HEADCONSTRUCTION Inside diameter 5 feet. wall thickness: 6 inch&. Courtesy A . 0.Smith Co&oration
FACTORS IN HIGH-PRESSURE DESIGN
Preliminary estimates relative to the economics of the proposed industrial operation naturally follow the laboratory studies. If these show prospects of gross return that will provide adequate profit after covering operating expenses and carrying charges, the design and operation of a pilot plant may be advisable. The time available may be inadequate for complete investigation of all factors, even in a well-equipped and -staffed laboratory, and the laboratory report may show that needed data are lacking. Probably some of the information supplied by the laboratory will have been obtained through mathematical computations rather than actual tests, and, even if the experimental data are fairly complete, they may not be directly applicable to plant-size equipment. A new process should reach production dimensions by a series of steps, a small pilot plant being the first to follow the laboratory studies.
C. W. SEIBEL U. S. Bureau of Mines Helium Plant, Amarillo, Texas
D
ESIGNING equipment for use under high pressure is not outside the realm of the average engineer, but it is a highly specialized branch of engineering and should be directed by men with wide experience in that field. Mistakes not only are costly but may cause loss of life. Twenty years ago high-pressure equipment was small, and the upper pressure range was from 3000 t o 4000 pounds per square inch. Steels then available for pressure vessels did not encourage the use of temperatures above 500' F. Today plants are operating a t 15,000 pounds per square inch, gas is being handled in thousands of cubic feet per minute, and, even under high pressures, temperatures greater than 900" F. are common. The success of a project involving high pressures depends in no small measure on the thoroughness and accuracy of the preliminary investigations. The initial studies, usually made in a research laboratory, should provide experimental data covering the chemical and physical properties of materials entering and leaving the process, and the chemical reactions and changes of state involved in each step. The resulting report should list materials suitable for construction, describe the effect on these materials under plant conditions of the substances to be processed, and point out precautions to be taken to avoid use of unsuitable material or equipment, hazards to health, and similar dangers. A flow sheet based on laboratory experiments should be available, giving temperatures, pressures, volumes, yields, and by-products. The laboratory-scale apparatus for separation of helium shown in Figure 1, constructed in some of the earliest work of the Bureau of Mines relating to helium, is illustrative of a type of equipment that may be used in preliminary investigations to be followed by semi-commercial-scale or pilotplant tests.
Several volumes would be required to cover all the principles of design of highpressure equipment. The many uncertainties in present knowledge of the art make complete full-scale design by inexperienced engineers inadvisable. Operators who cannot afford to employ specialists in design of high-pressure equipment on their own staffs would do well to refer such problems to reputable concerns that have had wide experience in this phase of engineering. Young engineers may be called upon to handle certain features of high-pressure equipment design, and this article is addressed primarily to them in an effort to point out some of the factors which should be considered and checked even though the major part of the designing is to be done by others. 414
i I
I
In designing any plant it is important to realize that each part must be capable of performing its required duty. Next, the proposed design must be capable of fabrication. For example, if a riveted tank with convex ends is to be used, the design must provide an exit for the man who "bucks up" the rivets from the inside. Strict attention to small details is required for successful performance. Neglecting them where high-pressure equipment is involved may cause a disaster. Successful design of high-pressure apparatus requires more than the correct mathematical application of formulas. It necessitates practical experience possessed only by those who have specialized in that field. High-pressure equipment is expensive even in small dimensions, and the size of the first pilot unit must be considered from the standpoint of cost, the completeness of the laboratory data, and the number of steps likely to be required before full scale is reached. When the over-all size or output of the proposed pilot unit has been established, a preliminary flow sheet should be prepared showing as completely as possible the sizes of parts, temperatures, pressures, volumes, products, and related afactors.
Compressors Design of compressors to obtain the initial high pressure should be placed in the hands of a reliable concern specializing in such equipment. Reputable compressor manufacturers can be trusted with confidential data; therefore it is well to select a very reliable manufacturing concern and supply it with all the relevant data available. Perhaps the laboratory report warned against the use of certain metals or gasket materials. Possibly the fluid will be corrosive or erosive (9) and provisions must be made for replaceable cylinder liners o r other parts, The gases may be toxic or expensive, and hence leakage past the rods must be reduced to a minimum. As pressures increase perhaps provision must be made to trap out condensed vapors, including water. Special lubricants may be required. Some of these items will be mentioned in more detail later, but all such factors must be considered by the designer of the compressor. Even though the major features of compressor design are t o be placed in the hands of specialists, some preliminary investigations are necessary. Many factors that enter into the design of a satisfactory compressor have to do with the lubricatjion of the cylinders, pistons, rods, and valves.
I
than one stage. FigI ure 2 shows discharge temperatures for air I under one-stage - compression, a s s u m i n g that the intake air is a t a pressure of 14.7 pounds per s q u a r e inch absolute and a temperature of 60" F. The actual temperatures of discharge will be lower than those indicated on Figure 2 because no account has been taken of jacket cooling or radiation and air does not comply strictly with the laws of an ideal gas. Compressor oil with a flash test of 565" and fire test of 640" F., or perhaps higher, and with other requisite properties is available for oil-lubricated equipment. On the basis of Figure 2 air might be compressed to 144 pounds per square inch absolute before reaching a temperature corresponding to an oil flash point of 565" F. However, allowances must be made for changes in atmospheric temperature and other factors. In summer the suction temperatures probably will be nearer 80" F., carbon will deposit on valves, and the gaa may carry lint or dust particles: therefore such a discharge pressure is not warranted except possibly in very small machines. With a fixed inlet temperature, the discharge temperature will increase with an increase in the compression ratio Pd/Pi. To keep the discharge temperature within bounds, the compression must be performed in several stages and the gas must be cooled between stages. The compression ratio for any number of stages can be found from the formula: ~
!I
R (compression ratio)
=
(
, where n
=
number of com-
pressions or stages In the above problem where Pd = 4515 and Pi = 14.7, for n = 2, R = 17.5; for n = 3, R = 6.75; and for n = 4, R = 4.18. From Figure 2 the discharge temperature for R = 6.75 (three-stage compression giving a discharge pressure of 99 pounds per square inch absolute on the first stage) would be somewhat more than 430' F., and it would be much better to use a four-stage machine giving R = 4.18 (discharge pressure of 61 pounds per square inch absolute on the first stage) and a theoretical discharge temperature of (4.18)0*2B x 520" Abs. = 326" F. The actual discharge temperature would be a little lower.
Number of Cylinders When an ideal gas is compressed, its temperature increases according to the formula
where Pd,
-
ratio of specific heats CJC, discharge and initial temperatures (absolute) Pi = discharge and initial pressures (absolute) y
Td, Ti
=
Accordingly, if the proposed pilot plant requires air at a discharge pressure of 4515 pounds per square inch absolute and has a suction pressure of 14.7 pounds absolute a t 60" F., and the compression is to be performed in one cylinder, theoretically the final or discharge temperature will be 4515 x (460 + 60) = 2270+ (m) o'29
" F.
Obvioudy the air must be compressed in more
FIQURE1. A BUREAUOF MINESLABORATORY-SCAL~ APPARATUS FOR SEPARATINQ HELIUMFROM NATURAL GAS 415
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INDUSTRIAL AND ENGINEERING CHEMISTRY
FIGURE 2. DISCHARGE TEMPERATURE FOR AIR UNDER ONESTAGE COMPRESSION, ASSUMINGSUCTION OF 14.7 POUNDS PER SQUARE INCHABSOLUTEAT 60° F.
In making a final s e l e c t i o n of t h e number of stages, the following should be kept in mind: Theoretically the horsepower requirement for compressing a given quantity of gas to a fixed discharge pressure becomes less as the number of stages increases, but the over-all cost of the compressor equipment and intercoolers becomes greater. The necessity of balancing the two sides of a duplexdrive machine may require an even number of stages, necessitating a four-stage machine where the work otherwise could be done in three stages. Having determined the number of stages of compression, the compression ratios and volumes must be corrected for deviation from the laws of an ideal gas. Some gases are more compressible, others less compressible, than is indicated by PV = RT (4, 11, 18). At high pressures the deviation becomes appreciable, and serious errors would be made in calculating cylinder size if such corrections were neglected. As an example, the resultant volume of nitrogen, compressed isothermally a t 20" C. from 1 to 1000 atmospheres, is twice that which the ideal gas laws indicate (11). For such computations and the calculation of cylinder sizes reference is made to Ernst, Reed, and Edwards (1.2). The power requirements can be calculated or taken from curves in any one of several handbooks (7, SO). Choice of the compressor drive depends largely on the type of power available. Compressor manufacturers will supply any type of drive in standard use. Compressors usually are rated on piston displacement, and in estimating the actual volumetric capacity due account must be taken of clearances and slippage of gas past the rings and valves, as well as the inlet temperature and pressure. An increase of 5" F.in suction temperature will lower the gas delivered by about 1 per cent. At high altitudes the suction pressure, if barometric, will be less, and the output will be reduced accordingly.
In determining the final discharge pressure and volume, future requirements should be anticipated and allowance should be made for pressure drop in the piping, traps, dryers, scrubbers, and other passages between the compressor and the final point of delivery. Valves and rod packing probably will require as much attention as any other part of the compressor equipment and should be made easily accessible. Modern design does not require that piston rods on the high-pressure stages be packed. In fact, four-stage machines in which only the low-pressure rod is packed are on the market. Frequently rods are provided with a so-called lantern gland, in which the total number of packing rings is divided into at least two groups. This permits gas that has leaked past the initial set of rings to be piped off, leaving the last group of rings under low pressure. Leakage is thus reduced virtually to zero (28). Intercoolers are part of the compressor and should be designed by the manufacturer of that equipment. The bracing of intercooler coils is important. If dissimilar metals are used for the coils in open-tank type coolers, proper insulation must be provided to prevent electrolytic action. Also, some metals and alloys commonly used for tubes are not satisfactory in certain types of water. A brass return elbow that was used on iron pipe and damaged by dezincification is shown in Figure 3. Frequently it is advisable or necessary to vary the output of the compressor equipment. Such possibilities should not be overlooked when the order is placed. Many machines are built with a d j u s t a b l e clearance p o c k e t s i n t h e low s t a g e , providing one solution of the problem. Variable-speed d r i v e also is a possibility. Safety valves, drains, and gages should be installed on the discharge of each cylinder. The discharge from safety valves or drains should not be piped to the c o m p r e s s o r suction without removing the h e a t of compression. Hot compressed g a s entering the low-stage cy1 i n d e r might raise the inlet temperature and result in a dangerous increase in the discharge temperature. If gages are placed a t eye level, means should be taken to protect workmen from glass or metal that may fly if the gage tube bursts. Adequate scale traps and filters on the suction side often pay for themselves in a short time by lowering maintenance costs. The location of the compressors should be considered carefully with reference to accessibility for inspection and repair. Some compressors have been placed so close to a wall that the pistons could not be pulled without making a hole in the wall. Placing the center of the equipment under a building truss that is reinforced for use as a crane rail helps the maintenance crew.
Piping On leaving the compressor, the gas may have to be conducted some distance before it reaches the reaction equipment. The most economical pipe sizes and pressure drops can be calculated from formulas found in the handbooks (6,21).
The selection of pipe sizes depends largely on the allowable pressure drop between the compressor discharge and the reaction equipment. Most high-pressure piping is welded where possible, with only enough flanges to permit dismantling for repair. Whether overhead or underground lines should be used is debatable. Underground lines are preferable from the standpoint of safety, but they are not as easy to inspect, leaks are harder to find, and corrosion must be considered. Properly hung overhead lines, insulated against climatic conditions if necessary, have proved satisfactory, but generally it is not advisable to run them above a roadway. Expansion in the pipe line must be considered. I n high-pressure work this is usually provided for by bends, which should be located with care and anchored properly to prevent transmission of the thrust to other equipment (8). If threaded joints are used, care must be exercised in cutting the threads. When threads are cut by hand, it is often advisable to cut an oversize thread and make a finishing cut with a block die run on flush. The tendency for threads on copper pipe to tear while being cut can often be overcome by using carbon tetrachloride as a cutting lubricant. Tests to destruction have shown that welding side outlets to pipes or headers weakens them so that they may fail a t 75 per cent or less of the strength of the plain pipe. Such openings should be properly reinforced (14).
Materials for Construction As the demands for new or better materials for construction have arisen, they have been met in a large measure. The design engineer now has a t his command so many different steels and alloys that their proper selection has almost become a problem in itself. Hopkins (16) gives some general classifications of chromium steels. Where corrosion is not a factor, 2 to 5 per cent chromium steels are used. As straight chromium steels of 12 to 14 per cent chromium are heat-treatable, they are very useful where corrosion resistance is not a factor. The steels of 16 to 18 per cent and 23 to 27 per cent chromium have excellent corrosion-resistant properties against many substances, but they are not suited to heat treatment, and grain growth in and near welds may cause serious lowering of resistance to impact. Chromium steels have been improved in many respects by the addition of substances such as nickel, nitrogen, titanium, silicon, and molybdenum. For specific information about new steels and alloys, reference is made to a symposium in INDUSTRIAL AND ENGINEERING CHEMISTRY (17). Laboratory reports may aid in the selection of steels, but it must be remembered that impurities in the material to be
processed may affect the corrosive action m a t e r i a 11y . Also, the corrosion resista n c e of unstressed steel may differ from that of the same steel under the stress imposed in actual use. Electrolysis also must be considered; as Fenwick and Johnston ( I S ) pointed out, it is often unsafe to assume that two metals will behave in accordance with predictions based on the electromotive series of metals. Anything that changes the physical properties may increase electrolytic action; therefore pressure equipment should be heat-treated, and cold working should be done with caution. Before leaving the subject of steels, it may be said that properties other than high tensile strength should not be overlooked; ductility and resistance to shock are perhaps just as important.
Materials for Low Temperatures I n the past the low-temperature, high-pressure industries have avoided the use of steels because they lose ductility a t low temperatures and therefore have poor shock-resisting properties. Steels that retain much of their normal ductility at low temperatures are now on the market, but their actual use a t temperatures of the order of 300" F. below zero has not come to the writer's attention. Copper, Tobin or naval bronze, nickel, and Monel metal have been found suitable for low-temperature conditions. Some of the high-tensile-strength bronzes, such as manganese and aluminum bronze, can be used if soldering is not required. Tin-lead alloys may penetrate between the grains of manganese bronze and cause failure under low stress (IO). Aluminum bronze does not tin well. Table I gives the properties of several metals a t room and liquid-air temperatures, and shows clearly that low temperature decreases the ductility of the older types of steels, although it greatly increases their tensile strength. Apparently tin-lead solders show no tendency to go to the gray modification a t the temperature of liquid air, and they can be used safely. Properties of other materials are given elsewhere (3, 6).
Pressure Vessels Numerous factors enter into the design of large and, in many instances, even small pressure vessels. For reasons expressed in following paragraphs, the writer believes that
OF LIQUID-AIRTEMPERATURE ON TENSILE PROPBIRTIES TABLEI. EFFECT
OF COMMERCIAL
METALS"
--Yield Material
Point-Tensile Strength---Elongation -Reduction of AreaIn liquid In liquid In liquid In liquid airb At 20' C. airb At Z O O C. airb At 200 c. At 20° C. airb Lb. per 89. in. Lb. per sq. in. % % % %
Tin alloys: Cast-tin 1-in. chill bar on end (Sn 99.8%) 1,020 3 500 2.550 5,600 81.3 1.8 93.1 3.6 Cast-solder 4-in. chill bar on end (Sn 50%. Pb 50%) 5,070 11:200 6,590 20,300 68.5 8.8 80.1 9.0 Co per base alloy: 8old-rolled naval brass 1-in. round (Cu 60%, Zn 39%, 81,230 47.5 50.8 52.1 48.7 39,850 57,000 29,000 Sn 1%) Nickel alloy: Hot-rolled Monel metal 1-in. round (Ni 68% Cu 27%. 91,500 135,400 71,100 45.5 53.0 66.2 68.0 44,400 Fe -t Mn 4- C Si0.5%'0) Co per: ??old-rolled copper (hard) 1-in. round (Cu 99.8%) 43,400 50,400 45,800 53,000 16.0 19.0 55.9 65.4 Steels : Carbon steel (C 0.45%) 1.25-in. round, 1500° F. water quench l l O O o F. air cooled 76,900 150,200 104,400 160,400 25.0 9.8 61.3 9.4 Nickel s t h (C 0.25$$, Ni 3%) 1.25411. round, 1500' F. oil auench. l l O O ' air cooled 76,400 132.900 98,800 148.700 27.3 15.5 65.9 17.6 a Based on tests conducted for the Bureau of Mines at the Washington Navy Yard. b The specimens were immersed in boiling liquid air which had a temperature of about -183' C. when the tin alloys, copper base alloy, and nickel alloy were tested, and about -190° C. when the copper and steels were tested.
+
.
417
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INDUSTRIAL AND ENGINEEHING CHEMISTRY
418
design of pressure vessels should he left to engineers experienced in that work. For economical results a pressure vessel should be so designed that there are no points of stress concentration after fabrication. The factor of safety should be ample to wver the expected useful life despite corrosion and to take care of the maximum stress that the vessel will be called upan to stand. But just what is an ample factor of safety, and on what property of steel does it depend? According to Wood (31, 3$), factors of safety of 5 are used on the basis of the ultimate
The factor of safetyis providedfor in theselection of allowable fiber stress S. For welded construction, when the welding is done under proper conditions, present practice indicates that the weld can be considered as equal in strength to the rest of the sheet. X-ray analysis of the finished weld provides it safeguard (16). hssuming values and solving for wall thickness t required by each of the above formulas, the results shown in Table 11 are obtained. The relative weight 1does not remain with any one formula, as conditions of 8 / P and the diameter vary. In the example where the inside diameter is 30 inches, there is a relative weight difference of approxknately 20 per cent between the Barlow and Clavarino formulas. As its name indicates, the thin-wall formula should he used only for thinwalled vessels, and perhaps it does not belong with the other three in this discussion. If the extra thickness is not needed, excess weight resulting from the use of an inappropriate formula may increase the cost by an amount that exceeds the fee for designing the equipment. Wall thickness is not the only factor to be considered. Improper shape of the head may prevent full use of the strength of the cylinder. Openings in the walls or heads also affect the ultimate strength so that they should be reinforced properly (%?). TARLE IT. WALLT H I C K B EIUD ~ ~ ERELATIVE ~ Wrioa~s OBTAINED FROM Poiiaao~~s 1 TO 4 S = 5000 S = 0250 s = 15,ooo s 18 no0
Formuis 1.
The & o r
was wed on iron pzpc.
tensile strength, 2.5 on the proportional limit or yield point, and 1.5 if bawd on the creep limit. The creep limit may be defined as the maximum stress which if maintained at a given temperature for a definite time, iisiially 1000 hours, causes no appreciable creep. If the factor of safety is Lased on the creep limit, the actual inaxhum operating temperature must he determined with care, because the rate of flow or creep for a given stress may he doubled for a rise in temperature of 25O F. ($8). The creep of steel cannot be predicted from the behavior of steel a t room temperature, and the creep limit of steel of specific composition is not always obtainable. The following are recognized formulas for use in designing the pressure vessels: The Barlow formula. sometimes referred to as the simple iormula, S =- 1
p
P
2800
P = 490
pI
4,100
I.D.-O.SQBI~. I . I ) . - I I . ~ ~ II. ~D .. - 41".
I%ailow:
I . in.
0.382
Relative wt. 2 2. Thin wail: 1. in. 0.2u Relative s t . 1 3. tarn&: L. in. 0.287 Relative wt. 1.46 4. c1svsrino: 1. in. 0.822 Rslstivewt. 1.67
P
-- 1:mo
I.D.=~oI~.
0.5
0.75
1.37
0.45 1.W
I
1.22 1.06
0.47
0.05
1.27
1.3 1.13
0.41 1
0.7
1.15
1.22
1.14
1.48 0.54
1.39
1.19
1
A welded autoclave with a complicated head is shown in the photograph on page 414. Jasper gives interesting technical data on heads and openings and a selected bibliography on the behavior of steels a t elevated temperatures (19). Pressure vessels constructed of two different metals are finding some favor where corrosion is a serious factor (16, 87). In these vessels carbon steel is used to carry the stress, and an inner lining of alloy sheet gives protection from corrosion.
I - R
One for thin cylinders,
l'he Lain6 iormrlla developed about 100 years ago,
A modification of Clavarino's formula of 1880, S
p=
where S
1.3
+ 0.4 R2
1-RP
allowehle fiber stress, Ib./sq. in. internal presuure, Ib./sq. in. R = ri/ro, ratio of inner to outer radias L = wall thickness P
= =
4. EFFECTOF 'rEMPERATUEE SPREAD AND DIAMETER O F TUBES ( N = 200) O X LENOTA OF A N
FIGURE
CH*NOER
INSIDB INTER-
t
Pressure vessels may be seamless, gas- or electric-welded, hammer-welded, or riveted, and often combinations of these types of fabrication are used. Each method has its advantages in the classes of work to which it applies. Hammer-welded and riveted construction is limited to the lighter gages of metals, seamless vessels are limited to weights fixed by the maximum size of ingot that is produced, and welded vessels are limited in size by transportation facilities. r l - - - - - ' ," . "
I
"
"
/h
'/.-
#fY#bPT
I
'
.
'
I
"
"
I
"
"
I
I
3 / ,
These formulas have the disadvantage of requiring some cutand-try methods, but charts can be drawn to facilitate the computations. F o r example, Figure 4, for a 200-tube interchanger, shows how the length of tubes needed is affected by the temperature spread a t the warm end of the interchanger and by the diameter of the tubes; Figure 5, for h b e s of 0.18-inch inside diameter, shows how the number of tubes affects their length and the pressure drop through the interchanger. Allowance must be made for the increased pressure drop and decreased efficiency with time that are caused by accumulation of solid material on the walls of the tubes. Carbon dioxide is particularly troublesome in low-temperature operations because of its tendency to coat the tubes. One of several ways of applying the formulas is indicated by the following example, based on interchanger temperatures of -130" C. at the cold-end inlet, -110O C. a t the cold-end outlet, f 2 0 " C. a t the warm-end inlet, and +14" C. a t the warm-end outlet. For these conditions and other assumed values as indicated :
o f Tubes
T t,
FIQURE5. EFFECT OF NUMBER OF TUBESON LENGTHAND PRESSURE DROPOF AN INTERCHANGER WHEN THE INSIDE DIAMETER OF THE TUBESIs 0.18 INCH
c* = D
After fabrication, pressure vessels should be tested with hydrostatic pressure 50 per cent greater than the working pressure maintained for an adequate period. A similar test after erection is usually advisable. Some vessels used for the transportation of compressed gas must be tested a t a pressure that is five-thirds times the working pressure to meet the requirements of the Interstate Commerce Commission.
Special Equipment Some industrial concerns experienced in operations involving high pressures and temperatures have been giving attention recently to high-pressure low-temperature processes, such as cycles for separating still vapors into their constituents. Several inquiries received by the Bureau of Mines have indicated that the formulas given below, which have not appeared in print for about 12 years ( I % ) , may be helpful. These formulas were derived by engineers of the Bureau of Mines from the work of Nusselt (24). They were used in .designing interchanger equipment of the Bureau of Mines Helium Plant a t Amarillo, Texas, and have proved satisfackory. They are useful primarily for gas-to-gas interchange. The formula for length of tubes is
and the corresponding formula for pressure drop is AF =
where AP t,
T
D L N G C, p
[0.0001 + 0.0000002 X
= = = = = = = = =
pressure drop t,hrough tubes, kg./,s log mean temperature difference, preliminary calculations the geometric mean can be used in place of the log "mean) temperature rise or drop, C. inside diameter of tubes, meters length of tubes, meters number of tubes mean flow of gas, kg./sec. mean specific heat of gas, kg.-cal./kg. mean density of gas, kg./cubic meter
= 144" C. (change from -130" C.to f14" C.) = 11.66 (for tem erature differences of 20" C . end and 6" Cf at warm end)
G
0.28
a t cold
. ...
= 50-75-100.. , .etc. = 0.00457 = 0.0984 for nitrogen
Solve for L in feet
Seamless copper tubing is obtainable in 12-foot lengths, and Figure 5 indicates that, if the tubes are to be 12 feet long, a 200-tube interchanger will be advisable. The formula for AP shows that the pressure drop through the interchanger is 0.85 pound per square inch when the cold-end inlet pressure is 2 atmospheres absolute and p is 3.25. If more pressure drop is allowable, fewer tubes may be used. A somewhat different method of using the formulas is given by Ernst, Reed, and Edwards (12).
Flanges, Bolts, and Gaskets These three items go hand in hand when used for high pressure. Each depends on the others to make leak-tight joints, and joints that are not tight will jeopardize the satisfactory, economical, and safe operation of the plant. When high pressure is associated with high temperature, the problem may become very difficult. The design features have been covered by several excellent articles ( l a , 20, 25, 29). The dimensions adopted by the designers of pressure vessels for flanges attached to these vessels usually afford a basis for selecting similar flanges for pipe lines and other equipment. Satisfactory service of a gasket joint depends not only upon proper selection of the gasket material and suitable design of the flange but also upon the character of the machined surfaces, the care in installation, and the method of tightening the bolts. Gaskets, which usually are relatively inexpensive items, should be inspected carefully before they are installed, and gaskets that are defective should be discarded. In highpressure installations it is well to enclose the gasket, a t least partially, and not depend entirely upon friction to prevent the gasket from blowing out. Workmen should be cautioned to keep the machined faces of flanges free from nicks or tool marks, especially when removing gaskets from grooved recesses. 419
INDUSTRIAL AND ENGINEERING CHEMISTRY
420
Safety Valves and Rupture Disks Some form of safety device is essential on all high-pressure vessels. The device should have adequate capacity to handle the volume of fluid entering the container and should be placed in a position where the line between it and the pressure vessel can never become closed or restricted. There should be no fitting between the safety valve and the pressure vessel that could be used (inadvertently or othemise), to cut off or throttle the flow. Safety valves of ordinary commercial types are used in many installations, hut they have several objectionable features, especially in low-temperature work. Their performance may he unsatisfactory or uncertain in the presence of liquid or solid substances, and they seldom reseat to give a complete shut-aff after they have been blown a few
VOL. 29, NO. 4
be of a nature that will reveal any weakening of tbe pressure equipment before dangerous conditions result. The following precautions are aids to safe operation: Small lines, even to gages and recording instruments, should he avoided. Standard eight-inch imn pipe has a safety factor of a little more than 6 at a pressure of 2 0 N pounds per square inch, hut it does not have enough mechanical strength to support a 180-pund man. Sooner or later somebody will use a horizontal run for a ledder. Bolts should not he tightened nor should attemDts he made to repair leaks under presSure. The discharge from ssfety valves or rupture disks should he placed where workmen will never be in line with the opening. Valves should not he denended unon for an absolute shutoff: flanges with blank disks a& desirabie for such a purpose. If it is necessary to use one compressor for different aqes at various times, the piping should be disconnected to avoi8 the wssihilitv of numoine E mixture of eases. as a result of leakage &t a valve Or some &her reason. When a new plant is started, the men should know what is expected of them and what their responsihilities are. The operations should be stsrted from known conditions. All valves should 1
_
I
"._-__.
E'lonar: 6. DISTORTION OF A NIPPLI CARRYING A S.AFETY VALVE AS A ~ZEROLT OF .%N Orr, EXPLOSION IN A CONPRESSOR D~SCHAEQE times. Unless great care is used in the selection of materials for their construction, ordinary types of safety valves used under corrosive conditions may "freeze" and allow the pressure to exceed that for which th3 valve is set. T h k difficulty can be overcome to some extent by installing an oil seal between the reaction vessel and the seat of the safety valve ($2)' but this remedy is not entirely fool-proof. No safety valve can be expected to provide relief for pressure that builds up with the rapidity of an explosion. Figure 6 shows tbe condition of a close nipple after an explosion. This nipple was on the discharge piping of an air compressor, and a 2.5-imch safety valve set for 600 pounds per square inch was attached to its outer end. As tbe result of an oil explosion in the discharge line, the safcty valve was blown from the fitting, but not until the pressure had built up enough to distort the nipple. Rupture disks are used in place of safety valves on many pressure vessels. These disks may be of the bulging or shearing type, and both types have their limitations. The metal used for the disks must be of uniform composition and tbickness, and provision must be made to prevent the disk from clogging the line on the discharge side after it ruptures. With proper design, disks can be made to rupture within 2 per cent of the calculated pressure and retain tbat ability over long periods. Data on this subject have been given by the National Safety Council (e$)and by Boynum (1).
Safety Considering the large number of Iiigb-pressure installations now operating in tiiis country, their safety record is very good. Satisfactory safety conditions can he maintained only by continned vigila,nce and frequent periodic inspections. The possibilities of corrosion and erosion at high temperatures or high pressures must he kept in mind and inspections should
Proper ventilation of the plant should not be overlooked. If there are toxic or objectionable gases at any int in the cycle, K&S masks should he available for use if leaks gveloo. .- In handing oxygen under high pressure, care m-wt he exercised to avoid all traces of oil, and oil-lubricated Compressors must not he usedfor compressing oxygen. If machine work is to he done on equipment used with high-pressure oxy en, mechanics should be cautioned against the use of cutting o# or even greasy hands. Work by the Bureau of Mines has shorn the danger of spontaneous ignition of oils in oxygen even under pmsures of a hundred pounds or less and at temperatures 8 8 ow 2,s 120- c. (Z).
Literature Cited (1) Boynum. M. E., Chem. & M e t . E w . , 42, 260 (1935). (2) Brooks. S. E., Bur. Mines, Rept. Innaatiaations 2555 (1923) Chemical Engineer's Handbook, 1st cd., p. 720 (1934). Chem. d Met. Eny., 43, No. 10 (1936). Cmnmasred Air M a a (Phiilipsburg, N. J.), "Compressed Air Data," D. 68 (1916. Crane Go.; Cd&a 52. 697 (1936). Crane Co.. Heating, Piping. Air Conditioning. 9, 35 (1937). Dickenson. J . H. S.. J . I m t . Metals, 24, No. 2, 315 (1920). Diliey. J. R;, Ch& &- M e t ; &~ .a,38. 280 (1931). Ernst, F. A.. Reed, F. C., and Edwards, W.' L ., IND. E m . CKFAE.. 17, 775 (192%. (13) Fenwick. F.. and Johnston. .J.. Ibid.., 28.. 1374 11986) ~~, ~~, j14j Greens, T.W., Heatiw, Pipi&. Air Conditioning. 9, 92 (1937). (15) Hodge, J. C., J . Am. Welding Soc., Oct., 1930. (16) Hopkinu, R. K., IND.Exo. CEEX.. 28. 1386 (1936). (17) INDI ENQ.CWEM., 28, 1367-1416 (1936). (18) International Critics1 Tablea, Vol. 111, p. 3, New York. MoGraw-Hill Book Co., 1928. (19) Jasper, T . M.. A. 0. Smith Corp., Bull. 201-A (1930). (20) Jasper, T . M., Hentina, Pipiny, Air Conditioniw. 8. 605. 672 (1936); 9, 43, 109 (1937). (21) Kent, R. T.. Meehmioal Engineers' Handbook. 10th ea., p. 669, New York, John Wilw B Sons, 1923. (22) Natl. Safety Council, Pmnphlet 68 (1927). (23) Norton. F. H., "Creep of Steels st Eigh Temperature," 1st ed., N e w York, MoGraw-Hill Book Co., 1929. (24) Nusselt, W., Z.Ver. deut. rea., 53, No. 43 (1909). (25) Ssndstrom, C. 0.. Chem. &.Wet. Ew.,41, I30 (1934). (26) Smith Corp.. A. 0.. E d l . 218 (1936). (27) Ibid., 522 (1936). (28) Sommers, H. A,, Chem. g: .Wet. Eny.. 37, 574 (1930). (29) Thorn, F. C., IND. ENO. C~EM.. 28, 164 (1936). (30) Union Steam Pump Co., Union Enginwring Handbook, 3rd ed., p. 9, Battle Creek. Mioh., 1921. (31) Wood, 3. X., Chem. d Met. En& 36, 610 (1929). (32) Ibid., 36, 737 (1929).
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REcsrvl;o Meich 9, 1937. Published by permission of the Diieator. IT. S. B U I ~ Qof U Mines. (Not svbject to oopyiight.)
