0
Modified Bourdon Gage
0 0
For Measuring Pressure in Polymer Processing Equipment
0 0
JAMES F. CARLEY Polychemicals Departmenf, E. I. du P o d de Nemours 6 Co., Inc., Wilmington, Del. \
I
the lower tip of the tube. Assemble the nipple, the adjusting sleeve, and the adjusting bolt, leaving out the closing screw. The bolt should be about five turns into the sleeve. Screw the transmission tube about one turn into the tee and connect the grease gun to the top of the tee. Cover the side hole with the thumb while pumping grease into the tee until it leaks around the top of the transmission tube. Screw the nipple-sleeve-bolt assembly tightly into the tee and force in grease until it is extruding steadily from the closing-screw hole in the bolt. Detach the grease gun, turn the closing screw all the way in, and turn in the adjusting bolt until the level of the grease is even with the upper end of the tee. Now screw the Bourdon gage into the tee while backing off the adjusting bolt t o relieve the gage pressure. 1
N THE processing of polymers, it is often important to know
the static pressure in dies, extruders, injection-molding machines, etc. I n experimental work, especially, i t is necessary to know melt pressures. For one reason or another, the usual means of measuring pressure are difficult or impossible to use for measurements of melt pressure. These difficulties are due mainly to four requisites of the situation: (1) The sensitive element must be small; (2) the gage must not be affected by temperature or must be automatically compensated for temperature changes; (3) the sensitive element often must be flush with the inside wall of the equipment; and (4)the sensitive element and any transmission lines should not fill with polymer. An instrument fulfilling some of these needs was used by Gilmore and Spencer ( 1j in their studies of injection molding, The author has constructed similar instruments and found they did not meet requirements (2) and (4)under a wide range of operating conditions. This paper presents a simpler, more compact instrument, a modified Bourdon gage designed to meet the above requirements. It is cheap, easy to make, and easy to maintain, and its range can be changed quickly.
All the joints should be tight, but no pipe dope need be used as the threads are tightly sealed by the grease. The gage is now ready to install. Installing the Gage At the point where the gage is to be installed, a hole is cut and tapped to mate the tip of the transmission tube. The short conical section ensures a tight seating and prevents leakage of polymer into the threads. Before the gage is screwed into the hole, the threads of the transmission tube are coated with an oil suspension of graphite or other temperatme-resistant lubricant. I
Construction of Gage
The gage consists of three parts: (1) a standard Bourdontube gage like thosp commonly used with pressure regulators, changed slightly, ( 2 ) a transmission tube and plunger (Figure l), and ( 3 ) an adjusting sleeve to compensate for large changes in temperature. The assembled gage is shown in Figure 2, installed in the barrel of an extruder. The tubing in the gage, including the Bourdon tube, is filled with a pressure-transmitting grease, I n his work, the author has used Dovr Corning high vacuum grease (a silicone lubricant) because of its thermal stability and low temperature coefficient of viscosity. The Bourdon tube is modified in two ways to facilitate filling it with grease: (1) The throttling orifice in the neck of the tube, if there is one, is unscrewed and discarded; and (2) in the end of the tube, a small hole is drilled through the connecting link, penetrating into the free space inside the tube. Through this hole the air can escape as grease is pumped into the bottom of the tube with a grease gun. The hole is tapped for a I-mm. screw, which is screwed into the hole after the tube has been filled (Figure 3).
?
Assembling the Gage
In assembling the instrument it is important that all the parts be completely filled with grease and that no air be trapped in the system. The instructions for filling the system that follow are particularly designed to accomplish this end. First attach a carefully filled grease gun t o the Bourdon tube and force in grease until it is extruding steadily from the small hole at the end. Detach the gun and turn the 1-mm. screw into the hole. Next slip the plunger into the transmission tube and attach the grease gun to the top, Force the grease into the annulus through the slots in the plunger until grease appears a t
Figure 1.
858
Transmission Tube
(left) and
Plunger (right)
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1953
If the' adjusting sleeve interferes with installation, cloFe nipples and an elbow can be used t o reduce the over-all radius of the gage. If the cold gage is screwed into a hot die or machine barrel, the grease in the transmission tube will expand much more rapidly than the metal as they heat. The adjusting bolt should be backed off t o relieve this pressure; otherwise, grease will slowly esca e through the annulus between the plunger and tube. If tKe temperature rises rapidly, the grease cannot escape fast enough, and the up er pressure limit of the gage may be exceeded unless the bolt is {acked off. Once the gage has heated up, small rises in the operating temperature (10' C., say) will cause no trouble, except that about 0.02 cc. of grease will be lost.
BOURODN GAGE STEEL TEE y--AOJUSTlNQ
(2000 SLEEVE
859
reading a t values above 5% of maximum (2). Very inexpensive gages, such as the 0 to 3000 pounds-per-square-inch type usually supplied for nitrogen cylinders, have larger errors. Accuracy may be only within 3y0 of maximum. At lower pressures relative errors are even larger. These Bourdon tubes also exhibit hysteresis, even after repeated loading, so that the precision may not be within 2y0 of maximum. Greater accuracy and precision can be found in costlier gages. The speed of response depends strongly on the viscosity of the grease between the plunger and tube wall. This in turn is dependent on the temperature a t the tip. One gage was installed in the die of an extruder 2 inches in diameter where the melt temperature was near 200" C. The slight cyclic variations in pressure caused by eccentricity of the screw in the barrel were easily followed by one of these gages a t screw speeds up t o 60 r.p.m. If the gage were t o be used regularly a t much lower temperatures, say below 100' C., it should be filled with a less viscous grease. Except for the pipe tee and nipple, the auxiliary parts were designed to withstand internal pressures up to 10,000 pounds per square inch with a working stress of 15,000 pounds per square inch, Pipe fittings can be obtained for various working pressures. The author has used gages for pressure ranges from 0 t o 60 to 0 to 3000 pounds per square inch. The Bourdon gage may be changed very quickly if a different pressure range is needed. It is believed that the gage can be used to measure the much higher pressures developed in inj ection-moldingmachines, without increaiGng the size of the tip. A few of these gages have seen intermittent service for several months on plastics extruders and on a melt spinning machine with good results.
CLOSING SCREW STEEL NIPPLE ( 2 0 0 0 PS.1.) TRANSMISSION
TUBE
,/
yi"-BOLT 2 0 N F HEX HO.
EXTRUDER BARREL
I"- 4 0 NC ROUND
OHJ.FILLISTER
EXTRUDER SCREW
Figure 9.
Assembled Gage in an Extruder
ADJUSTING SLEEVE DETAIL MTL.: STEEL
ADJUSTING BOLT DETAIL MTL.: STEEL
CLOSING S C R E MTL.: STEEL OR B R A S S .
If the unit in which the gage is mounted is shut down and allowed t o cool, the gage ought t o be taken out. Immediately
after it is removed, a small pressure is applied with the adjusting bolt until the plunger is all the way down and a little grease has leaked past it. The tip is wiped and the gage can be set aside t o cool.
Discussion The assembling procedure is meant to expel air that might be trapped in corners. If air bubbles were left in the system, they would be greatly compressed when the gage was operating a t high pressure. The plunger travel would therefore be much greater and the risk of fouling in the clearance would increase. When Bourdon gages with 2.5-inch dials are used, the greatest plunger travel should be less than 0.1 inch. This number is based on measurements of the change in the volume of a Bourdon tube with rising pressure. The pressure is transmitted by the grease, while the plunger serves to keep grease and polymer apart. Because the grease is free to escape past the plunger, changes in temperature do not affect the pressure measurement for more than a few minutes. Several of the assembled gages were calibrated with a dead-weight tester; the errors were the same as they had been before the gages were filled. The accuracy of these instruments, if they are in good condition, is supposed to be within 1% of the maximum
?'"mm'L'U''-
OAR SOLDERED INTO END OF GAGE TUBE
DETAIL ON TIP OF BOURDON TUBE
Figure 3.
Modification
of Bourdon Gage
The plunger clearance may eventually become fouled with polymer, so that the response is sluggish or the plunger sticks. It can be conveniently cleaned with a rod that is designed for cleaning a 0.22-caliber pistol. Cleaning rod and patches can be bought from any sporting goods supplier. Appropriate solvents should be used. To remove badly caked or difficultly soluble films, the residual grease is first cleaned out of the tube, and then the polymer is burned out in a salt bath or with a blowtorch. Several manufacturers make adapters for Bourdon gages which transform the tube movement into a mechanical or electrical
INDUSTRIAL AND ENGINEERING CHEMISTRY
860
signal. The signal can he picked up by an appropriate recording instrument. Thus, the grease-filled gage may be readily converter to a recording instrument. Conclusion
The grease-filled pressure gage is a cheap, sensitive, and accurate means of measuring polymer pressures. I t is virtually unaffected by changes in temperature, its range may be changed quickly, and its maintenance is simp1e’ It can be wherever there is room for a half-inch hole, it, can be placed very close to moving parts, and it occupies no volume inside the proeessing equipment. The gage is easily adapted for continuous recording of pressure.
Vol. 45, No 4
Acknowledgment
The author is indebtcd to many of his fellow workers for suggestions for improving the gage, and to H. G. Ryan, Jr., ‘who drew thP figures. literature Cited (1) Gilmore, G. D., and Spencer, K. S., Modern Plastics, 27, 143 ff. (1950). (2) Rhodes, T , J , , “Industrial Instrumentsfor Measurement and Control,” 1st ed., p. 31, S e w York, McGraw-Hill Book Go., 1941. ACCEPTEDXovember 2 2 , 1952.
RECEIVED f o r review .Sugust 19, 1952.
8
c e e
icrofiltratio Irnpregnated
e EDWARD
D. KANE
Cuno Engineering C o r p . , Meriden, Conn.
F
ILTRATIOY, the separation of solids from a liquid suspending medium by the passage of the fluid through a membrane or barrier, has long been recognized as a chemical enginering unit operation. The chemical engineer today, faced with a problem of solid separation, has a great many methods a t his disposal. A recent summary of the field was presented by hIiller (10). As the first step toward the selection of filtration equipment for a particular application, the engineer must consider the following important factors ( 4 ) : Maximum allowable percentage of suspended solids in the filtrate Physical characteristics of the suspended solids to be removed (size, shape, and nature) Amount of material to be removed These considerations apply broadly to the entire field of filtration. This paper, however, is concerned only with the narrower field of clarification-Le., the removal of a small amount (less than 2%, generally) of a micronic-size contaminant, to obtain a filtrate that is essentially clear. If the solid suspended matter iE not a semicolloidal material which under a pressure will distend and “slime” over a media, or a true colloidal material which exhibits a Brownian motion, the use of a clarifying element without a filter aid can be considered. Examples of these easily filterable solids are sand. silt, pigments, and automotive sludges. In fact, any rigid material present in rather small amounts and in a particle size range below the commercially available screen or cloth sizes (approximately 50 microns) can be filtered with these clarifying units. Screens or cloths can be used to filter in the micronic range with the use of filter aids, but large and expensive equipment is required. The author has recently completed a literature survey in connection with micronic filtration for the Ordnance Department of the Army, and, a t that time, the lack of emphasis on the use of the inexpensive types of micronic filters was noted. The purpose of this paper is to point out the uses of these filters and, in particular, to discuss in detail the application of a woolen fiber resinimpregnated type of cartridge. Types of Filter Elements. In the field of micronic filtration or clarification with inexpensive throw-away types of filter elements,
there are three general classes. Surface or edge-type filters are generally paper or resin-treated paper. They will handle particle sizes dom-n to the range of 1 micron or thereabouts. They operate very much like ordinary filter paper in that once a particle of contaminant has lodged on the surface, the useful life of that portion of the filter has ended. To increase the filter life to an economic length of service, the manufacturers have resorted to pleating, stacking a number of paper disks, or spiral winding of the paper to form a tube, the thickness of which is equal to the width of the ribbon wound. Another type consists of the assortment of powdered metal or porous ceramic media, These materials are made to filter to controllable limits of particle sizes of an assortment of materials suitable for most chemical applications. The last type are called depth-type filters and are either randompacked material or rigid, oriented structures. The randompacked filters would, it is believed, be unsuitable for fluid clarifi-
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