.?.lag 1948
mentation Electrical devices for liquid level control are simple, adaptable, and relatively inexpensive.
three preceding issues, we have described several of the more widely used types of equipment for measurement and control of liquid level. These included air bubbling systems, installations using pressure transmitters, and both constant displacement and variable displacement float type instruments. Most variables can be measured by electrical means. Liquid level is no exception. There are a variety of electrical methods for measurement and control of liquid level. The simplest electrical devices for indicating or controlling liquid level are those in which the liquid makes a n electrical circuit through a n insulated conductor or probe when it reaches the probe or breaks the circuit when it falls below it. Trimount, Faratron, B/W, Warrick, and Photoswitch liquid level controls are of this type. These devices can, of course, be used only with liquids that have some degree of electrical conductivity, as the liquid is used t o complete a n electrical circuit. Simple alternating current relays can be used for liquids having resistivities up t o 20,000 ohm-em. Most aqueous solutions of electrolytes have resistivities below this value. For liquids that have higher specific resistivities, vacuum tube operated relays must be used. These can be used for liquids having resistivities up t o 20 megohm-em. Liquids such as alcohol and distilled water have sufficient conductivity t o operate such relays. The simplest applications of relay type liquid level instruments are those in which a n alarm signal is t o
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be actuated when the level becomes dangerously high or low. For these applications relays having singlepole single-throw contacts can be used. Normally open contacts are required for high level alarm and normally closed contacts for low level alarm. Maintaining level between fixe4 limit8
Another common use for relay type liquid leveE instruments is t o maintain the liquid level in a vessel between two fixed limits. A slightly more complex contact arrangement on the relay is required for this purpose. A typical circuit for this type of control is shown in Figure 1. In this circuit a transformer, T,steps the line voltage down to a few volts t o operate the relay, R. Probes U and L are located a t the two levels between which the liquid is to be maintained. A double-pole singlethrow relay with normally open contacts is used. When the tank is empty, the relay contacts are open, and the pump, P , is off. As liquid flows into t h e tank, the control does not act until the liquid reaches the upper probe, U. Then the relay closes. Holding contacts, H , connect the two probes together, and t h e control contacts, C, start the pump, usually through a n auxiliary relay. Because the upper and lower probes are now conbected through the holding contacts of t h e relay, the motor runs until the level is pumped below the lower probe, L, breaking the circuit through t h e relay coil and allowing its contacts to open. The cycle then repeats. This type of operation is known as pumpdown operation. If the C contacts of t h e relay were normally closed, and the pump arranged t o pump into the tank, t h e pump would start when the level fell below probe L and stop when it reached probe U , giving pump-up operation. Figure 1 shows a separate transformer and relay. Some instruments on the market use transformer relays which combine the functions of these two units. The probes for use with instruments must be well insulated-particularly those for use with vacuum tube relays. Figure 1. Liquid Level Control, P u m p - u p Operation 89 A
Some commercial probes are constructed very much like a spark plug except that the central electrode is longer and heavier. Figure 2 shows a cutaway photograph of a typical double probe unit. It will be noted that the insulators are covered with a weatherproof housing-a necessary feature in high resistance electrical circuit,s. In addition t o straight level control, relay type level instruments are useful for interface level applications where one liquid is a conductor and the second is a nonconductor. One is often asked whether sparks of sufficient intensity t o ignite explosive or inflammable gas mixtures might result when the liquid makes or breaks contact with the probes in a circuit of this kind. Many relays of this type require only a few tenths of a watt to operate. There is evidence t o indicate that in spite of this low power, ignition can occur under certain Figure 2. Cutas p e c i a l c o n d i t i o n s . Vacuum way View of Phototube relays require only a few switch Probe for Liquid Level t h o u s a n d t h s of a watt t o Control operate. There seems t o be little likelihood that sparking - ' in the probe circuit of this st,yle of relay could cause ignition.
OLDBURY ELECTRO.CHEM ICAL COMPANY 0
AMORPHOUS PHOSPHORUS
PHOSPHORUS SE§QU I S U L P H I D E 0
Capacitance-sperated indicators
Manufactured t o strict
Various other methods for measuring or controlling liquid levels electrically have been suggested. One instrument made by the Wheelco Instruments Co. used the change in capacitance produced when the liquid level rose between the plates of a condenser t o detune an oscillating circuit. The resulting change in plate current operated a control relay. I n this unit, the condenser plates were clamped to opposite sides of a gage glass. This type of device would operate with nonconducting liquids with dielectric constants as low as 1.8. However, i t is no longer available. Recently, a capacitance-operated liquid level indicator designed primarily t o indicate the quantity of fuel in aircraft fuel tanks has been described by D. B. Pearson of the General Electric Co. This unit uses a condenser consisting of two concentric tubes in the fuel tank as part of a n oscillating circuit. Variation in plate current of the oscillator serves t o indicate the emount of fuel in the tank. The statement is made that tests have shown that sufficient energy is not available to cause an explosion if the concentric cylinder condenser in the fuel tank should accidentally be shorted.
specifications for the Match Trade
Plant and Main Office: N I A G A R A FALLS, NEW Y O R R
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New Tork Office:
19 RECTOR ST., NEW
YORK
6, N.
Y.
99 A
May 1948
91 A
INDUSTRIAL AND ENGINEERING CHEMISTRY
Taylor’s
ALL 316 STAINLESS STEEL Aneroid Manometer HERE
it is-the famous Taylor Aneroid Manometer-now available in all 316 stainless steel to handle tough corrosives like nitric acid, acetic acid, ‘sulphuric acid, phosphoric acid, caustics. Measures flow, level, or specific gravity. Has all the advantages of the previous model which is still standard for ordinary jobs. The Taylor Aneroid Manometer has been thoroughly field-tested in wartime chemical and petroleum plants where it earned a proud record for cutting maintenance costs t o a minimum, No mercury t o replace. No stuffing box. No internal pivots to wear out. Low volumetric displacement for full range differential. Differential pressure is measured by a bellows and transmitted through a torque tube. Here are a few money-saving advantages you get with the new all 316 Stainless Steel Aneroid Manometer:
N o sealing fluids or blowback systems needed because corrosive fluids can be directly introduced into the manometer. Entire .instrument is tough and dependable for long
service.
Easy to clean-no etched surfaces-no
pockets.
Range changes easily made by changing torque tube with another of a different range.
Available ranges, 20”, 50”, loo”, and 200” of water differential. Working pressure, 300 psi. Ask your Taylor Field Engineer or write Taylor Instrument Companies, Rochester, N. Y., or Toronto, Canada. Instruments for indicating, recording and controlling temperature, pressure, humidity, flow arid liquid level.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 40, No. 5
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& to.,Inc.
\ 6249
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81 Maiden Lane, New York 7, N. Y. 444 W. Grand Ave., Chicago 10, Ill. 605 Third St., San Francisco 7, Cal.
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1948
LEARN
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AT THE START THE COMMERCIAL POSSIBILITIES OF YOUR CONTEMPLATED PRODUCT! a
BUFLOVAK
invites you to submit to its extensive Research and Testing Laboratories your processing problems, so that you can learn, a t the start, by actual test, the commercial possibilities of the project, with data on production cost, capacity, and characteristics of the finished product. Such a test will benefit you greatly. Results are definite, because each test is conducted on a semi-plant scale, with equipment which gives accurate indexes of full production results. Buflovalr has solved 6,000 processing problems for companies to date, involving drying, evaporation, extraction, impregnation, solvent recovery, crystallization and food processing. All tests are confidential. We invite you to submit your processing problems to us today. Full details will be supplied upon request.
CHEMICAL and FOOD PROCESSING EQU PMENT m
Division o f Blaw-Knox Co. 1549 FILLMORE AVE.,
BUFFALO 11, N. Y .
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DRYERS .
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EVAPORATORS e
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SOLVENT RECOVERY AND . .DlSTl LLATION EQU I PM ENT *
BUFLOVAK EQUIPMENT
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PROCESSING KETTLES CHEMICAL CASTINGS
These Durco Valves give you MAXIMUM CORROSION RESISTANCE. Made of Durco's
high silicon irons, Duriron and Durichlor, they have permanent resisrance to every common corrosive. Unexcelled for sulfuric acid. Excellent for nitric, hydrochloric, acetic and hundreds of other corrosive solutions. WIDE RANGE OF APPLICATION. These valves are the most popular types for corrosive service. Almost any problem of corrosive flow control can be solved with one of these designs. Check, angle, plunger-release, three-way, float and relief valves are also available. W r i t e , wire or phone for the bulletins listed, or ask for a Durco sales engineer t o call,
This Duriran tap-lubricated plug valve has handled nitric acid and acid lead acetate far aver 18 years.
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THE DURIRON CO., INC., DAYTON '1, OHIO Branch ORices i n Principal Cities 61 -GM
