Positive Displacement Flowmeter JERRY MCAFEE, Universal Oil Products Company, Chicago, 111. tioiis may be used. The mercury reservoir can be a suitable length of pipe or any convenient vessel. The U-tube is connected by means of unions to valves B and C, which in turn join the line carrying the stream to be measured. Valve A is interposed between the point of connection of B and C. The crosses and plugs shown and valve D are provided for convenience in filling and cleaning. Before the gage glass is connected, its volumd between two convenient markings is determined. When the device is in operation the flow is normally in the direction shown in the sketch, with A , R,and C open. T o measure the flow rate, A is closed, forcing the oil (or other fluid being measured) into the mercury reservoir, the mercury into the gage glass, and oil out of the gage glass into the main line. Since as much oil is forced out of the system as enters, the net rate of flow in the main line remains unchanged. The measurement itself consists in determining the time required for the mercury level to rise from one calibration mark to the other, thus filling a known volume. When this measurement is completed, A is reopened, and the mercury seeks its own level. Here again, since oil is discharged from one le as rapidly as it enters the other, no interruption in the net i o w occurs.
IN
LABORATORY and pilot-pladt work, particularly when continuous processes are studied, it is often necessary to measure the rate of flow of small, sometimes pulsating or irregularly flowing streams which may vary in density or viscosity, In many such instances i t is inconvenient or impossible to use conventional orifice- or Ven’turi-type meters, rotameters, or calibrated tanks. Obviously, some form of positive displacement meter is desirable for such service; but unfortunately most commercially available meters of this type are either too large or have pressure, temperature, or other limitations which make them unsuitable.
Calculation of the flow rate from the observed time is simple. Thus, if the calibrated volume is V gallons, and the time required to fill i t is t minutes, the rate of flow, R, in gallons per minute, is V obtained by the equation R = 7. In one typical example, the calibrated volume was 0.046 gallon and the observed filling time ‘ION
was 30 seconds. The rate of flow was then
0.046
30 X
60 = 0.092
gallon per minute. While the design described is cheap, simple, and highly satisfactory, i t can be modified t o meet particular demands and utilize existing apparatus. The calibrated volume should be of such a size that the time required for a single determination is neither so long as t o be inconvenient nor so short as to be inaccurate, The author has found 30 t o 60 seconds satisfactory. More elaborate modifications of the basic design have been suggested (1) for applications where a n automatically determined rate or a record of total flow is desired. While the device was developed for, and to date has been limited to, the measurement of liquid streams varying from liquid propane to heavy recycle oil, i t can also be used for measuring gas streams by taking proper account of the compression of the gas on the upstream side of the meter, using a low-density sealing liquid, and limiting the variation in liquid level t o a low value.
H WUCLCLASS A problem often encountered in these laboratories which is thus complicated is the measurement of recycle or “combined feed” streams in continuous cracking pilot plants. Often these streams are hot and are subject to changes in temperature and composition in the course of a run. Usually they are transferred by a reciprocating pump, and always the continuity of their flow must be maintained. While alternately used surge tanks, pump stroke calculation, and calibrated orifices or rotameters can be employed with some success, all these methods have certain drawbacks in such use. The device described below (I) was developed especially for this service and has been in use about 2.5 years. It is also applicable t o many other flow-measurement problems, and has been found particularly useful in obtaining rapid, approximate measurements of flow .rates even when the quantity flowing over a long period is determined by more accurate means, such as calibrated charge tanks. Its chief advantages are simplicity of construction and operation; applicability over wide ranges of temperature, pressure, and stream composition; and a positive calibration which does not change with operating conditions or fluid properties.
LITERATURE CITED
(1) McAfee, Jerry, U. S. Patent 2,325,695 (Aug. 3, 1943). THISdevice was described by L. S. Kassel of the U.O.P. Research and Development Laboratoriee at the round table diecussion OD pilot-plant design, construction, and operation, Division of Petroleum Chemistry, 106th Meeting of the AMERICANCEEMICAL SocImr, Pittsburgh, Pa.
Course in Instrumental Methods of Analysis and Control An ESMWT course in instrumental methods of analysis and control will be given at the University of Southern California, LOB
Angeles, starting June 12. The course now being taught by Sidney W. Benson is to be expanded for the summer session to 7 hours of laboratory work and one hour of lecture each week for 15 weeks, devoted to study of the applications of physical properties of chemical substances to the analysis and control of chemical processes,
The diagram shows the psrticular design of this flowmeter which has been most general y used in the author’s laboratories. It consists essentially of a U-tube, one leg of which is a gage glass, the other a reservoir for mercury, water, or other liquid heavier than and immiscible with the flowing stream. A Jerguson “Reflex” type glass has been used most commonly because it allows operation at high temperatures and pressures, but any glass satisfactory for the prevailing condi-
and including laboratory work with basic instruments such as pressure gages, flowmeters, rotameters, thermocouples, potentiometric circuits, colorimeters, spectrophotometers, and electronic relays.
303
