by R. F. Wall Monsanto Chemical
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Co.
INSTRUMENTATION E
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W O R K B O O K
F E A T U R E
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A N e w Mass Flowmeter for Liquid Streams A new, high accuracy mass flowmeter may be helpful for many problems requiring accurate measurement of mass flow of liquid streams M
ETERING
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STREAM
FLOWS
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one of the most common measurements required for process control purposes. In many ways, flow has been one of the least satisfactory of process controls. T h e conventional orifice flowmeter is the most commonly used element for measurement and control of flow, and has many features that are definitely advantageous. It is very simple, easy to install and maintain, and it is unlikely that a more economical flowmetering device will ever be developed. However, the accuracy has long been subject to question, particularly if the stream composition, density, temperature, and other factors are subject to much variation, and the nonlinear characteristics of the orifice both detract from the accuracy and complicate the problem of using the orifice meter output as a signal for process control purposes. There are several modifications of the simple orifice plate that may provide some improvement in performance, and anotherform, the variable area flowmeter, is better for some jobs.
or the seller; flow measurements within a process that are used in determining inventories, conversions, or process yields and are therefore critical for cost accounting purposes; and metering of materials to those processes where the relative proportion of materials reacted or blended is especially critical. In many process control problems of this type, precision of flow measurement has long been a problem of considerable economic significance, particularly where the density or viscosity of the stream is subject to variation because of changes in composition, temperature, pressure, and other variables and where a true mass measurement is desired. A great deal of attention has been given improving the precision of flow measurements through compensation for these variables a n d / o r
Fischer-Porter Flowmeter
Forte Transducer
Pros and Cons of Orifice Meter
Despite its well known disadvantages, the orifice meter in its various forms is adequate for many and perhaps most applications, where flows are held reasonably constant at a level that need not be known with more than nominal accuracy—e.g., material transfer within a process, at a rate that may be varied either manually or automatically to fit the needs of the process and need not be known with absolute accuracy. However, many flowmetering problems require maximum precision of measurement : the measurement of liquid products for sale, in which an error costs either the buyer
the correction of the indicated flow from supplemental measurements, as of density. Turbine-type flowmeters have been a fairly recent development, and are considered highly accurate for volumetric flow. T h e most elaborate and precise device developed for measurement of mass flow is the true mass flowmeter based on the principal of the Coriolus force. This device is complex and therefore expensive, somewhat resembles a concrete mixer in size and general appearance, and thus is something of a problem to install in a process plant. Hence, it apparently has never attained very widespread use, despite the distinct advantage that a very precise mass flow measurement is made independent of stream density, composition, viscosity, or presence of suspended material.
Electromagnetic Pickup
Vorioble Force dement
Rotor
Fischer a n d Porter's new mass f l o w m e t e r has v e r y simple basic p r i n c i p l e ; is easily installed a n d m a i n t a i n e d ; can serve as transducer f o r i n d i c a t i n g , rec o r d i n g , a n d controlling elements; a n d is r e a s o n a b l y p r i c e d I/EC
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A new, high accuracy mass flowmeter has been announced recently by Fischer and Porter, combining in a single transmitter a turbine-type volumetric flowmeter and a force element to provide a density component, with the readout measurement of mass flow of a liquid in a process stream. Fischer and Porter's new mass flowmeter is shown at left—-the basic principle of operation is very simple. T h e variable force element produces a voltage signal proportional to the volume flow rate squared times the fluid density, (Q2p). The turbine flowmeter element produces a signal that is converted into a voltage directly proportional to volumetric flow rate, (Q). T h e electrical control unit of the meter divides the (Q2p) signal of the force element by the (Q) signal of the volumetric element to calculate the mass flow ORKBOOK
F E A T U R E S
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INSTRUMENTATION
I/EC rate as (Qp).
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Dimensionally, (Z,3/ TY (M/D)/{L /T) equals M/T, mass per unit time, and the meter readout is in terms of actual mass flow. T h e appropriate constants are applied in the control unit so that the pounds-per-hour flow readout is direct. This mass flow readout may be presented on a conventional indicator or recorder, or on the digital readout device shown—an excellent example of a modern trend in process instrumentation. An accuracy of ± 1 % of instantaneous mass flow value and a repeatability within ± 0 . 5 % of indicated flow are quoted for the meter. Ranges of up to 1,000,000 pounds per hour are available, with rangeability as high as 10 to 1. A wide variety of liquids can be handled. For maximum accuracy, the streams metered should be clean, nonfouling, noncorrosive for the stainless steel used in construction, contain no suspended solids, and have a viscosity no higher than about 400 centistokes. Higher viscosities or the presence of some suspended solids will adversely affect accuracy. Behind the basic simplicity of the instrument a great deal of attention has been given to perfecting and developing the design to obtain the high accuracy reported. T h e flowmetering elements are preceded by a carefully designed flow straightening device to eliminate effects of turbulence and irregularities in flow introduced by the piping preceding the flowmeter. T h e shape of the variable force element has been very carefully developed so that the flow coefficient is identical to that of the following turbine flowmeter. T h e bladed rotor of the volumetric flow element is of a magnetic stainless steel. As the rotor turns under the influence of the moving fluid the blades vary the reluctance of the flux path of a permanent magnet and induce pulses of current in a pickup coil in the magnetic circuit of a frequency proportional to the rate of flow. This frequency modulated signal is subsequently converted to a direct current voltage proportional to the volume rate of flow, and combined with the signal from the force element in a small electronic analog computer to provide the mass flow reading of the meter. Assembly is simple, and the flowmeter is 70 A
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A Workbook Feature
small and easily installed in process lines. I t has been designed with consideration of problems of disassembly for inspection and maintenance. Uses for N e w Mass Flowmeter
This new development by Fischer and Porter appears to make available a new mass flowmeter of high accuracy for liquid streams, and may be a considerable help to process control engineers faced with flow metering problems requiring high absolute accuracy. There are many of these, and this new flowmeter is already in use on a variety of flow measurement problems that predominantly follow predicted areas of application. There is considerable use in metering liquid petroleum products for sale. An accurate mass measure is provided independent of temperature or the differences in composition that occur with this type of material. T h e liquefied hydrocarbon gases are particularly subject to this type of error. It is used in accurately blending hydrocarbon stocks to produce fuels of specified characteristics. O n e of the most valuable areas of application will be in petroleum processing, for streams often vary considerably. Composition and the density changes that result have made the precise measurement of flows a difficult problem. Another particularly important general area of application for which Fischer and Porter's new mass flowmeter is receiving a lot of attention is the accurate measurement of process inventories and quantities of material transferred from one unit to another. These are necessary to determine conversions, yields, losses— i.e., just what the process is doing, how efficiently the various unitsareoperating, bottlenecks in the process, and detailed cost figures for the system. Only if this information is accurate and available can process be brought to optimum efficiency and profitability. It is usually assumed that operational control of a process does not require flow measurement and control of this high precision, but with experience—and more accurate process information—this may be proven wrong, and in many cases accurate flow control may be highly profitable in routine process operation. In batch processes, for blend-
INDUSTRIAL AND ENGINEERING CHEMISTRY
ing or for reaction, accurate metering of materials charged is often critically important for quality a n d / o r costs. While Fischer and Porter are understandably reluctant to recommend their flowmeter for liquid streams that are not clean, nonfouling, and of low viscosity, there are a lot of important flow measurement problems falling outside this ideal category, and users are inevitably going to see how far they can go. T h e Fischer and Porter mass flowmeter is in service metering crude oil flow with entirely satisfactory results, and crude oil is not considered a very clean material. Suspended solids may cause trouble, depending on the kind of solid, the amount, how. well suspended, and the tendency to stick to surfaces. Emulsions and other two-phase systems are also questionable; probably some will work and some will not. T h e Fischer and Porter mass flowmeter is even being used to measure gas stream flow, and may prove valuable for this type of measurement that is so difficult to make with high accuracy. Another interesting application is the measurement of liquid oxygen flow; density variations make conventional measurements inaccurate. Correction factors are available for viscous materials, and with their use this type of material can be handled. Users are apparently willing to try this new flowmeter on any stream that flows, and experience should soon define where it will and will not work. Fischer and Porter's new, high accuracy mass flowmeter is a modern instrumentation development meeting today's high standards of process control technology. It is easily installed and maintained, and can serve as a transducer for any of the usual indicating, recording, and controlling elements. Pricing is reasonable, permitting its use on any applicable flowmetering problem where accuracy is of value.
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