I
W. M. MILLER Research and Development Department, Tidewater -Oil Co., Martinez, Calif.
A Wet Test Meter for l o w Flows, High Pressures Accuw.m
Small flows of gases can now be meus-
ured accurately in the luborutory at pressures up to
7000 p.s.i.g.
This
can be done with
a recently developed small compact high pressure wet test meter
MEASURExfEST of small flo\vs of gases under pressure is a problem \vhich has plagued pilot plant operators for a number of years. Several years aqo. a small-scale semiautomatic catalytic desulfurization pilot plant was constructed in this laboratorv. Rotameters could not be relied on to measure accurately the small flow of incoming high pressure hydrogen makeup gas. For example, calculations of the hydrogen consumed in desulfurizing a cracked naphtha to the same sulfur level in two different tests varied at times as much as 100 standard cubic feet per barrel (SCF bbl.) of charge. One of the objects in running this pilot plant was to obtain accurate hydrogen consumption data on various naphthas and gas oils for use in forecasting refinery runs and in designing neu commercial desulfurization units. Since the reliability of h! drogen consumption data was questionable, the accuracy of the measurement of incoming hydrogen needed to be improved. ,411 manufacturers of wet test meters in the United States and England. were contacted to see if they wo;ld build a smali high pressure wet test meter to fit our need. \\'hen this pioved to be fruitless. the literature was searched and the problem was discussed with research men in other companies. T h e following are some of the methods being used to measure small flolvs of gases at high pressure.
Hypodermic Needie Orifice Meter Several research companies use this type of metering for low flow rates of both liquids and gases a t high pressure. The orifice meter is made by silver soldering the desired size hypodermic needle in the center of a plate and mounting the plate in a pipe union. T h e pressure differential across the orifice is measured by a differential pressure cell and recorded by a conventional recording flow meter. An integrator used in connection with the flow meter increases the accuracy of the measurement. IVhen used for metering gases, a filter and a drier are used ahead of the orifice to remove dirt and moisture. The opening in these hypodermic orifice meters is so small that any foreign matter will cause inaccurate readings. Coiled Capillary Flow Meter This method employs a differential pressure measurement across a coiled capillary tube to meter iotv flows of gases at high pressure (,5). The coiled capillary tube used for measuring hydrogen flow in the Esso Computer Operated Pilot Plant ( I ) has a range of 5 to 30 SCF per hour at 450 p.s.i.g. The tube is ' 8 inch O.D. by 0.055 inch I.D. and is coiled in a 5 , inch diameter coil 8 feet long. The differential pressure is sensed by a pressure transducer which develops a n a.c. voltage that is proportional to the flow. This type of flow meter has the advantage of having a wider range than hypodermic needle orifice meters.
Some Methods Used to Measure Small Flows of Gases at High Pressures Hypodermic needle orifice meter Coiled capillary flow meter b Measuring gas flow b y pressure drop b Thermal electric flow meter b Conventional wet test meter in a high pressure case Magnetic drive high pressure wet test meter b Direct drive high pressure wet test meter b
VOL. 53. NO. 2
FEBRUARY 1961
127
I
Schematic design showing cross sectional view of the working parts of the meter
Measuring Gas Flow by Pressure Drop
In this method the gas to be measured is withdrawn k o m a high pressure storage vessel of knoiin volume. The volume of gas leaving the storage vessel is calculated from the pressure drop in the vessel. The rate of flow can be determined by reading the pressure and temperature of the gas in the storage vessel periodically and referring to a calibration curve. The gas flow can be adjusted manually to maintain a desired rate or be regulated to maintain a desired pressure. If the high pressure vessel is not large enough to store all the gas needed during a test, additional make-up gas can be added to the vessel. When gas is added to the storage vessel?
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pressure and temperature readings are taken before and after the addition. Thermal Electric Flow Meter
One type of this kind of meter (Laub Electro-Caloric FloLvmeter, Industrial Development Laboratories, Pasadena, Calif.) uses a ‘,‘r-inch pipe about 1 foot long equipped with a heater and two resistance thermometers. The heater Ivound around the pipe slightly downstream from the center heats the gas as it passes through the pipe. The two resistance thermometers wound around the pipe a t either end measure the differential temperature ol the gas flowing through the pipe. For calibration purposes. the wattage input to the heater can, be adjusted by a Powerstat. Difference
INDUSTRIAL AND ENGINEERING CHEMISTRY
in temperature varies with the mass rate of flow of the gas and is indicated on a microammeter which is connected to the resistance thermometers through a 1Vheatstone bridge. Conventional Wet Test Meter in a High Pressure Case
A conventional atmospheric pressui e wet test meter can be used to meter gases under high pressure if it is enclosed in a high pressure case and the pressure inside and outside of the meter equalized. iVhen this method is used. the dial pointers on the meter inside the case are usually read through a small glass window in the high pressure case. .4 variation of this method (2) uses similar equipment but transmits the rotation of the meter to an external
WET TEST METER counter by electrical means. This is accomplished by a 50- to 100-tooth disk attached to the meter shaft and two inductance coils all located inside the meter case. T h e position of the teeth with respect to the coils changes the inductance of the coils. \Vires brought out through the case connect the coils to a n external bridge circuit Lvhich operates the relay counter system. Magnetic Drive High Pressure Wet Test Meter
In this type oC meter there is no physical connection between the rotor inside the high pressure body and the dial mechanism on the outside of the meter. .A magnetic coupling on the rotor shaft drives the dial gear train. California Research Corp.'s version of this meter uses a standard l , 90th cubic foot per revolution !vet test meter rotor enclosed in a case made of two &inch extra heavy lvelding pipe caps bolted rigidly together. I n the center of one cap is a 1' 4 inch stainless steel (nonmagnetic) pipe about 4 inches long a n d closed at the outer end. Four Alnico magnets 5 8 inch \vide by 2 inches long are mounted at right angles to each other on the rotor shaft inside the 1' '4inch pipe. Similar magnets surrounding the pipe make up the magnetic coupling which drives the dial gear train.
Direct Drive High Pressure Wet lest Meter
I n this type of meter the rotor inside the high pressure case drives the external dial gear train by a shaft passing through a pressure-tight bearing in the meter body. A low friction ball bearing, "0" ring bearing used in commercial orifice meters works satisfactorily in this application because it is leak proof u p to 5000 p.s.i.g. California Research Corp. has made several meters of this type. They use a commercially available I-liter atmospheric \vet test meter rotor and enclose it in a case made of t\vo %inch extra heavy Lvelding pipe caps bolted rigidly together. Their meters arc also equipped with Jerguson high pressure gage glasses to read the water level in the meter i d ) . O f all the above methods, a direct drive high pressure wet test meter best suited our needs. \Vith permission of the California Research Corp. to use their design a smaller version of their direct drive \vet test meter was built (page 128). T h e first step \vas to size the rotor so that it xvould make several revolutions in measuring 2 to 3 SCF hr. of hydrogen at 1000 p.s.i.g. This is the rate of hydrogen normally used in our tests. T h e rotor should make more than one revolution and prererably several in meas-
uring this amount of hydrogen to minimize the measuring errors caused by slight variations of volume in the different compartments of the rotor. T h e first rotor was a '/~-scale copy of a ' l o cubic foot per revolution standard wet test meter rotor. T h e American Meter Co.: San Francisco, Calif., furnished a 10 cubic foot rotor in a n unassembled form to use as a model. T h e dimensions of the ','s-scale rotor were 4'; 2 inches in diameter by 1';s inches wide. Tests made Lvith this rotor indicated that it was too large for our purpose so another was made of the same diameter but reduced in width to 1 inch. This one proved to be satisfactory and is the one now in use. The graph below shows the number of standard cubic feet of hydrogen metered a t various pressures ~8s.revolutions of the meter. T h e body of the meter was made as small as possible for compactness and strength. It consists of two parts. T h e back was made b) welding a 5-inch extra heavy irelding cap to a 13; 1 6 inches thick by 7'3/16-inch 0 . D . flange. T h e front cover plate is 11/j8 inches thick at the bolt circle and 115: 61 inches thick where it fits inside the flange. Fourteen '!?-inch cap screws are used to bolt the front cover to the flange portion of the back. Seoprene ''0'' ring located in a li's-inch recess in the flange
',
900 950
1 . a
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-
-
_
_
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.O STANDARD CLEIC - 3
CORRECTU) TO
e?,
These curves show the standard cubic feet of hydrogen per revolution of the meter when it i s being metered at 850 to 1000 pounds of pressure VOL. 53, NO. 2
FEBRUARY 1961
129
This i s the meter after the final changes were made in design
is used as a gasket. T h e meter has been tested hydraulically to 2000 p.s.i.g. but may- stand higher pressures. The meter was not equipped with a water sight glass because of the danger of breaking and because visual adjustment of the water level in wet test meters does not assure accurate gas measurement. T h e volume of water in the meter is about 455 ml. and the level is about &,'8 inch above the rotor shaft. To adjust the level, water is added or removed through a small tube inserted in the thermowell connection a t the top of the meter. Tests were made with the water level 3.2, 4.8, and 6.4 ml. below the correct level. This work indicated that the water level can drop 3 ml. without affecting the accuracy of the meter more than 0.47,. After finally solving the mechanical problems of this meter, the meter was run night and day at 1000 p.s.i.g. for a month without losing more than 10 ml. of water. The meter is calibrated under pressure once a week. T h e calibration is done by reducing the pressure of the hydrogen leaving the meter and measuring it in a standardized atmospheric wet test meter. A correction factor is applied to readings made during the following week and about once a month water is added to adjust the level. In assembling the meter for the first time, the rotor was connected to the dial mechanism by a straight shaft This shaft passed through a pressure tight bearing in the meter body. One end of the shaft was geared to the dial mechanism and the other end connected to the rotor by a loosely fitted spline to keep it from binding. T h e pressure-
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tight bearing was the type used in commercial orifice meters rvhich has a built in thrust bearing and uses an "0'' ring to make it leak proof. When the meter was put in operation it worked satisfactorily for 250 hours and then stopped. I t was thought that the pressure tight bearing was defective so another bearing of the same type was installed. This time the meter only ran 150 hours before it stopped. As the meter operated after being allowed to rest for a few hours under no pressure, the trouble was believed to be due to a gradual swelling of the "0"ring when the meter was under pressure. A pressure-tight bearing of the type used in commercial orifice meters which has a grease seal instead of a n ''0" ring was tried. T h e shaft in this type of bearing is free-floating and requires an external thrust bearing. Attempts to make the meter bvork successfully with a nylon thrust bearing on the shaft next to the rotor and with a jeweled thrust bearing on the end of the shaft outside the meter both failed. Instead of the "0"ring causing the meter to stop in this case it was probably an increase in friction resulting from the grease being gradually packed tighter around the shaft by the pressure of the water in the meter. The rotor is so small and its movement so slow that the slightest increase in friction causes the rotor to stop. All these attempts showed that more torque had to be supplied to the shaft to overcome the friction and allow the meter to operate successfully. The solution of this problem proved to be simple. By installing a set of gears inside the meter and relocating the shaft, the shaft could be made to turn one tenth as fast
INDUSTRIAL AND ENGINEERING CHEMISTRY
as the rotor and thus increase the torque on the shaft. I t was necessary to make several changes in the meter to try out this idea. These changes included: the installation of a six-tooth driving gear on the rotor shaft and a small graphite bearing to support the shaft; the relocation of the original pressure-tight bearing to allow a 60-tooth gear on the bearing shaft tu mesh with the 6-tooth gear on the rotor shaft; rebuilding of the dial mechanism to permit the large pointer on the dial to make one revolution for each revolution of the rotor. Changes were also made to guard against accidental circumstances. These were: the installation of a 0.040-inch orifice in the gas outlet line to prevent rapid depressuring of the meter which might rupture the rotor; and the installation of a spring loaded ball check in the gas inlet line. This was to prevent water from backing out this line should the pressure to the meter be reduced suddenly. T h e meter after the last changes in design is shown at left. Moving the gears inside the meter body evidently supplied the necessary torque because at the date of writing this article, the meter has operated for more than 2000 hours without any trouble. Use of this meter now makes it possible to determine hydrogen consumption data with a reliability of =t10 standard cubic feet per barrel of charge in this desulfurization pilot plant. Future plans call for automating this meter so that readings can be taken a t night when no one is in attendance at the desulfurizing pilot plant. In this way automatic weight balances can be used on this unit at night as has been done for over a year on our catalytic reforming pilot plant ( 3 ) . Acknowledgment
T h e author wishes to thank L. A. Bisso, for the development of this meter, and E. J. Ammer. literature Cited (1) Balekjian, G., Hines, C. K., Boycks, E. C., Measurement and Control in a Computer Operated Pilot Plant, ISA Preprint K5-4d-SO. ( 2 ) Dewey, Philip W. (to Standard Oil Co. of Indiana), U. S. Patent 2,702,897 (February 22, 1955). (3) Miller, W. M., "Guiding Refinery Operations with Small Semiautomatic Pilot Plants," presented at 24th midyear meeting, .4.P.I., New York, May 1959. (4) Muth, J. E., California Research Coru., Richmond. Calif., private commuGication. (5) Powell, H. N., Browne, \V. G., Rev. Sci. Irish. 28, 138-41 (February 1957).
RECEIVED for review October 18, 1960 .ACCEPTED November 29, 1960 Some features of this meter are covered by patent application.
