ML003739219

Regulatory Guide Rg 5.48 Design Considerations Systems for Measuring Mass of Liquids

ML003739219
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Issue date:	02/28/1975
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RG-5.48
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February 1975 U.S. NUCLEAR REGULATORY COMMISSION REGULATORY GUIDE REGULATORY GUIDE 5.48 DESIGN CONSIDERATIONS SYSTEMS FOR MEASURING THE MASS OF LIQUIDS A. INTRODUCTION makes possible more accurate measurement of the solution bulk. It also improves the ease of achieving Section 70.58, "Fundamental Nuclear Material accurate bulk measurements and obtaining repre Controls," of 10 CFR Part 70, "Special Nuclear sentative samples. The determination of chemical and Material," requires that the quantity of special nuclear isotopic concentrations in samples by traditional material (SNM) on inventory be known on the basis of analytical techniques is independent of vessel design, and measurement at the beginning and end of each material chemical and isotopic assay are not discussed in this balance interval. Section 70.51 of 10 CFR Part 70 guide.

requires that all SNM added to and removed from processing during that interval be measured. Section 1. Bulk Measurement of Liquid 70.58 also requires that all random and systematic errors associated with such measurements be controlled so that a. Measurement of Weight the limit of error of material unaccounted for (LEMUF) at the end of any material balance interval does not Determination of solution bulk by weighing is exceed the amount specified in the regulation. advantageous because this makes it unnecessary to measure the temperature and specific gravity of the This regulatory guide pertains to design considera solution, eliminating random and systematic errors tions for methods of measuring the mass of liquid inherent in measuring temperature and specific- gravity contained in a vessel and identifies those design consider and avoiding the possibility of mistakes in making ations which the NRC staff considers to be adequate for temperature corrections. Another advantage of deter minimizing the error associated with that measurement. mining solution bulk by weight is that there is no Equipment and procedures for obtaining liquid samples requirement for dimensional uniformity within the are the subject of another regulatory guide, which is vessel to achieve calibration linearity.

currently in preparation.

A disadvantage of weighing is that rigid piping B. DISCUSSION connected to the vessel can produce mechanical stresses due to deflection and thermal expansion. These stresses Measurement of the SNM content of a vessel can affect the indicated weight. Weighing systems of the containing a homogeneous liquid solution consists of null-balance type. which restore the vessel to a reference three principal operations: (1) measurement of liquid position, can minimize the effect of stress caused by mass (either by weighing directly or by measuring the deflection of the vessel with increasing weight. However.

volume, temperature, and specific gravity); (2) obtaining such systems do not compegsate for thermal expansion.

a representative sample of the vessel contents: and (3) Where disconnects are permissible and the number of assay of the sample to determine the SNM concentration connections is small enough to make disconnection in terms of unit weight or unit volume. feasible, the vessel may be disconnected from the piping during a weight determination to eliminate this effect.

This guide deals with measurement activities that Otherwise, temperatures are held constant or an addi are affected by vessel design. Proper attention to design tional correction is included.

USNRC REGULATORY GUIDES Copies of published guides may be obtained by request indicating the div,sioný desired to the U S Nuclear Regilatots Conmmission, Washington D C 20555 Regulatory Guides ate ,ss-ed to describe ard -k availble Iothe ibl Atterrtiii Duret tot if Standatds Developiment Coninients and suggestions for methods acceptable to the NRC staff of ,npNeooenting spe-,f pi1s of the iiprovetreits n these guides are enicouraged aird should lie sent to the Commissi ons tegulatiors to delineale techniques used t)y the stiff ini Secretary of Ite CoirtittisSioi U S Nuclear Regulaltory Contmssion evaluating specific problems or postulated ai cidents or to prov-de quiditaie to Washirglto* D C 20555 Attenrtion Docketi q iarid Sirsi cc Section1 appliiants Regulatory Guides are riot substitutes fot tequlatioins and ( Or l ......

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In vessels equipped with heating-cooling jackets, level can be measured by any of several devices: purged errors in solution weight measurement can occur if the dip tubes, sight glass, or other types of level sensors.

weight of fluid in the jacket varies. This is more likely to be a problem when steam rather than a liquid heat transfer medium is used for heating. If a liquid heat Pairs of pneumatic dip tubes are used in high transfer medium is used, the problem may be less severe in a vessel with internal coils than in a vessel with an radiation fields because of their simplicity. The opening of one of the tubes is submerged, and the opening of the other is in the vapor space above the liquid. The pressure I

external jacket.

difference between the two tubes while air is passed Weighing a vessel containing SNM can be done by through the tubes at the same rate is proportional to the mounting the vessel on an industrial platform scale or by product of the height of liquid above the opening of the placing hydraulic load cells under the vessel supports. submerged tube and the density of the liquid. This Industrial weighbeam balances and pendulum balances pressure difference is sometimes called the "weight have capacities of tens of thousands of kilograms and factor." For vessels having a uniform cross section, the accuracies of 0.1%.i12 weight factor is directly related to the mass of liquid Under carefully controlled conditions, an accuracy better than 0.1% can be above the submerged tube opening.

achieved; 14,000-kg cylinders of uranium hexafluoride are routinely weighed with an accuracy of 0.01%. Specific gravity can be measured by using a pair of Pendulum balances indicate weight without requiring the dip tubes, the ends of which are submerged to different manipulation of standard weights and thus are more but known depths. However, the results are usually less readily adapted for remote reading and automatic reliable for accountability purposes than can be obtained printout. by measuring the specific gravity of a representative sample withdrawn for chemical and isotopic analysis.

Industrial use of weighbeam or pendulum balances The liquid level in a vessel is the quotient of the weight has been largely limited to applications where the vessel factor, expressed in suitable units, divided by the is accessible for maintenance and adjustment. Where specific gravity of the liquid. When properly designed radiation levels preclude such access, direct weighing can and installed, 4 a pair of purged dip tubes can measure be accomplished by supporting the vessel on hydraulic liquid level with a precision of 0.6% of full scale.

load cells or by applying electric resistance-wire strain Accuracies of 0.1% to 0.2% can be achieved in a gages to the vessel supports. calibrated tank under carefully controlled conditions.

Hydraulic load cells have a capacity of about 25,000 A sight glass gives a positive reading of liquid level, kg. 2 An accuracy of 0.1% can be achieved. 3 The load on but is subject to breakage (which could result in the all the supports can be easily combined and can be read at any convenient remote location. The readings are sensitive to temperature, but substantial temperature spread of contamination). Sensing of liquid level can be done by other means, such as hydrometer-type floats or ultrasonic ranging. The resultant accuracy is generally L

correction is possible. not adequate for accountability measurements, however.

The feasibility of using hydraulic load cells to In the design of vessels for volume measurement, measure the weight of process feed was studied at the the systematic error associated with the existence of a Idaho Chemical Processing Plant. 3 Because the installa liquid heel below the lowest limit of the measuring tion was on an existing vessel that was not adapted to device can be a serious problem. Heel uncertainty is not the purpose, the tests were inconclusive. However, important for throughput measurements, where dif valuable insight was gained into desirable design features ferences in liquid level are measured and the heel can be for such an installation. accounted for in the measurements. For measurements of absolute quantities during an inventory, however, the Electric strain gages are highly sensitive and have a error introduced by the heel volume can constitute much capacity of 200,000 kg. Strain-gage readings are in of the measurement error. The complexity of volumetric fluenced by temperature, but this effect can be readily calibration is greater when the vessel cross section varies compensated for. Their accuracy approaches that for with height, as may result if there are components hydraulic load cells, but they are expensive and rela (impellers, baffles, heat-cooling coils, etc.) within the tively fragile; hence, they are not normally used for the vessel. On the other hand, effective mixing to obtain weighing of vessels containing SNM. homogeneity generally requires some obstructions such as spargers or impellers and baffles. Hence, the designer

b. Measurement of Volume, Temperature, and seeks the trade-off features that are the most desirable Specific Gravity for each application.

Bulk measurement of volume is usually accom Other drawbacks to determining bulk contents by plished indirectly: the liquid level is measured directly: measuring volume are the complications introduced by then a prior calibration of liquid level vs. volume is temperature changes and the errors contributed in applied. Removals or additions to process can be L

making the necessary temperature corrections. Most accurately measured by filling a precisely calibrated tank vessels expand at elevated temperatures. although slab volume to overflowing and then transferring the known tanks may decrease in- volume with increasing tempera increment of volume to the process. Alternatively. liquid ture if the sides are constrained to buckle inward.

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Not only does the vessel volume change with C. REGULATORY POSITION temperature, but the contained solution also expands when heated. Therefore, each liquid-level measurement This section provides guidance for vessel design to has associated with it the temperature at which the facilitate the accurate measurement of contained SNM.

measurement is made. Furthermore, laboratory determinations of specific gravity and SNM concentra 1. The following guidelines are of general tion are frequently made at temperatures other than applicability to the design of vessels in which those at which the bulk volume was measured and at accountability measurements are to be made.

which aliquots were iaken. Therefore, the temperatures at which aliquots are taken should be recorded. Unless a. The vessel design should take into account the uncertainties associated with temperature measure the elements of random and systematic error con ments are held within tolerable bounds and all tempera tributed by each component of the bulk measurement ture corrections are properly made, the accuracy of the and sampling operations and should minimize the overall bulk volume measurement will be poor. measurement error resulting therefrom.

2. Vessel Design Features Pertainent to Obtaining a b. The vessel design should consider the Representative Sample* means for standardizing and calibrating each measure ment operation* and should provide for continuing

a. Mixing of Vessel Contents quality assurance of the measurement system during plant operation.

To ensure a liquid solution having uniform compo sition, vessel contents can be mixed by gas spargers, by c. To arrive at the overall vessel design, the mechanical impellers, or by external recirculation designer should integrate the bulk measurement and (pumping the solution out and back in). sampling operations with all process requirements and with any additional constraints imposed by considera Gas spargers are relatively ineffective, lengthy sparg tions of safety and criticality prevention.

ing being required to bring about uniformity of the tank contents. They also increase the load on the offgas d. Piping connected to measurement vessels filters. However, they are inexpensive and require little should be designed to drain reproducibly and, if or no maintenance. Spargers are more effective in tall, possible, to drain quickly.

narrow tanks than in short, wide ones. The optimum gas flow rate is about 0.75 m 3 /sec (1 .3 cfm) per square foot e. The design should require that only with of tank cross section.4 the measurement vessel and its contents at rest should measurements be made.

Mechanical agitators (motor-driven impellers) pro vide positive mixing action and are widely used where 2. The following guidelines apply to vessels whose maintenance is possible. bulk content is determined by weighing the vessel and its contents.

b. Obstacles to Effective Mixing

a. The tank should be installed in such a Precise measurement requires that the composition manner that no variable extraneous loads are imposed as and temperature of the solution to be measured be a result of mechanical or thermal stresses in attached completely uniform before the solution is sampled. piping. Systems having essentially zero deflection (null Unfortunately for this purpose, vessels containing balance) should be considered as a means for reducing solutions of SNM are often, for criticality prevention, stress effects from connecting piping. Even with a made in the shape of slender cylinders, thin slabs, or thin null-balance system, the vessels should be protected annuli shapes that are not suitable for effective mixing. from thermal and mechanical forces. Care should be Cylindrical vessels whose axes are horizontal or nearly so taken to minimize rigidity of piping to weighing vessels, are especially poorly adapted to efficient mixing. If a 5 as by the use of properly supported flexible metal lines slab or cylinder does not have adequate storage capacity or small-diameter piping with reasonably long horizontal for a given volume of liquid and if" several such slabs or runs. To minimize thermal stresses, the design should cylinders are manifolded in parallel, the problem is provide for constant or reproducible temperatures in the further aggravated. When several such vessels are connecting piping insofar as possible.

manifolded in parallel, uniformity of the contents of all of the vessels is difficult to achieve. b. Convolutions in flexible metal lines and any bends, incorporated to minimize piping rigidity, Boron-containing raschig rings used as tank filters to should be oriented so that the piping drains freely.

prevent criticality are another impediment to effective mixing.

• See, when published, a regulatory guide (in preparation).

"Measurement control Program for Special Nuclear Material Accounting." See also ANSI Standards N15.18, "Mass

• Recommended procedures and equipment for taking ('alibration Teciniques for Nuclear Materials Control." and liquid samples are contained in another regulatory guide (in NI15.19, "Volume Calibration Techniques for Nuclear Materials preparation) in this series. Control" (both in prcparation).

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c. Mechanical linkages between the weighing circumference is uniform with cylinder length.

device and readout mechanism should be minimized, and Anomalous circumference variations in cylindrical measurements should be recorded by means of remote vessels should not exceed 0.05 percent between printout, particularly if the alternative is a series of calibration points.

levers to transfer the reading from the vessel to the measuring location. In any case, digital printout is recommended in order to avoid parallax errors, (4) The vessel interior should be as free as misreading, and recording blunders. If an integrated possible of coils, baffles, and other objects that interfere computerized inventory system is used, the weight with the linearity of the calibration.

should be recorded by the computer.

(5) When mixing baffles or other internal

d. Any mechanical linkages used in the load devices are essential to the design, such baffles or devices transmission system should be lubricated and protected should have a uniform cross section throughout the from corrosion or the introduction of dust. range where measurements are normally made. Two or three such constant-area regions are permissible in a

e. If a heating-cooling jacket is needed on the vessel, provided the transition areas are outside normal solution tank, the piping should be designed so that operating regions.

either the jacket can be filled to a constant volume or it can be drained completely to eliminate errors caused by varying weights of fluid in the jacket. c. Provision should be made for periodic in situ recalibration of vessels by the addition of precisely

f. The weighing system should be designed so known quantities of liquid. 6 that initial calibration and periodic recalibration can be done in place. This requires that the system have the d. For vessels in which absolute quantities are capability of adding precisely known weights to the measured, provision should be made for keeping the heel vessel over the entire measurement range. volume constant, and the heel volume should be kept small. To the extent feasible, the heel volume ordinarily

g. Since vessels mounted on weighing systems should not exceed 1% of the nominal volume of liquid are not supported against shocks or earthquakes, measured.

appropriate use should be made of staybars or limit stops. e. Whether mixing is done by sparging or by an impeller, measurement of the liquid level should be

3. The following guidelines apply to vessels whose done when the vessel contents are quiescent in order to bulk content is determined by measuring volume, temperature, and specific gravity.

a. The vessel should be calibrated by liquid get an accurate reading. During measurement, recircu lating samplers should be turned off and the gas flow to the dip tubes should be the minimum practical.

L Ldditions in such a manner that the total error of the f. If pneumatic dip tubes are used for bulk volume calibration at any liquid level is within the determining liquid level, care should be taken in accuracy appropriate to its overall contribution to the selecting the diameter of the dip tubes, the length of gas limit of error of material unaccounted for. (Greater supply lines, the shape of the open end (bubbler orifice),

accuracy in calibration is normally required for vessels the fixing of the dip tubes in the tank, and the materials containing feed or product solutions than for vessels that of construction; also, equalization of flow rates in any contain waste solutions.) The calibration should include pair of dip tubes 7 should be provided.

a sufficient number of liquid levels to bracket closely all discontinuities or fluctuations in cross-sectional area. The dip tubes should be installed so that the opening of one of them is located above the highest level

b. The following features in the design of a reached by the liquid and any foam formed on top of vessel enhance the capability of obtaining overall the liquid. The lower dip tube should be located as low measurement errors that are acceptable: as practicable.

(I) The vessel should be a right circular The lower tube should be rigid and should be cylinder, with the principal axis oriented vertically. supported to resist bending. The openings at the ends of Vessels having a slab configuration should be avoided the tubes and the pneumatic supply lines should be unless special provisions are made for ensuring designed so that pressure drop between tube ends and uniformity of cross section with height. the point where pressure is measured does not exceed about 3 pascals (0.01 in. H 2 0) for the flow rated used.

(2) The ratio of vessel height to horizontal Pneumatic supply lines should be leak-tight.

cross-sectional area should be such that the product of the cross-sectional area and the smallest increment of To provide a means for measuring the specific liquid level that can be measured does not exceed the gravity of the liquid, an additional dip tube may be required volume sensitivity. installed with its open end in the liquid a fixed distance above the lower tube. This measurement can be used to (3) For cylindrical vessels fabricated of confirm any specific gravity determinations on a sample flat stock, care should be taken to ensure that the of the liquid contents.

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The type of readout, manometer or transducer, of liquid in each vessel, and each vessel in the bank should be evaluated for each dip-tube application. A should be capable of being sampled independently.

manometer gives a more direct measurement. Also, a precision manometer read by a careful, trained operator 4. The following guidelines apply to design gives better accuracy than can be obtained with considerations pertaining to mixing of vessel contents to transducers routinely associated with process instru ensure the obtaining of a representative sample.

mentation. However, a precision pressure transducer can give readings that are just as accurate -without the need a. A single gas sparger may be used for vessels for a trained operator. Furthermore, the transducer up to 3 m (10 ft) in diameter; larger vessels should have readings can be transmitted directly to a data collection more than one sparger.

system, thereby avoiding reading errors entirely.

However, transducers should be calibrated frequently. b. Use of raschig rings in vessels in which For either type of instrument, the design should provide accountability measurements are to be made should be for in-place calibration. avoided wherever possible. Other means of preventing criticality should be considered for such vessels. If If an integrated computerized inventory system raschig rings are used, precautions should be taken to is used, the differential pressures and the equivalent ensure thorough mixing and completeness of transfer.

solution bulk should be recorded by the computer. Because raschig rings tend to compact during use, the designer should consider the need for more frequent

g. The weight factor should not be used calibration of vessels filled with raschig rings.

without temperature correction unless it can be demonstrated that any change in specific gravity is c. If mixing is done by external recirculation exactly offset by a change in liquid depth. This and if the recirculating pump and piping are not condition will occur only if the vessel cross section is dedicated to the measurement vessel, care should be constant with changing temperature or if the changes exercised that the solution to be measured is completely encountered in vessel volume or temperature are small returned to the measuring vessel and that none escapes enough that thermal expansion of the vessel walls can be elsewhere in the process. In any case, the returning neglected. solution should be distributed in the vessel volume for example, by spraying.

h. If liquid level is measured by means of a sight glass, care should be taken to ensure that the D. IMPLEMENTATION density of the liquid in the sight glass is the same as the density of the liquid within the vessel. The scale This section provides information to applicants and attached to the sight glass should be of noncorrosive licensees regarding the NRC staff's plans for using this material and should have a low coefficient of thermal regulatory guide.

expansion; provision should be made for its calibration.

The designer should also ensure that the sight glass will Except in those cases in which the applicant measure all liquid levels to be encountered during its use. proposes an alternative method for complying with Protection should be provided against breakage of the specified portions of the Commission's regulations, the sight glass. method described herein will be used in the evaluation of submittals in connection with special nucleal niaticial

i. If the use of several thin slabs or long license, operating license. or construction permit applica cylinders in parallel is necessitated by criticality tions docketed alter June 1, 1975.

considerations. provision should be made for ensuring the homogeneity of the contents of all of the vessels. If an applicant whose application for a spccial Alternatively, provsion should be made for ensuring nuclear material license, art operating license. om a that solution cannot leak from one vessel to another and construction permiut is docketed on or before Joune I.

that solution intended for a given vessel cannot leak 1975, wishes to use this regulatory guide in developing (e.g., through a valve) to another vessel in the bank. submittals for applications, the pertinent portions of the Provision should also be made for measuring the volume application will be evaluated on the basis of tius guide.

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REFERENCES

1. D. M. Considine and S. D. Ross, "Handbook of 4. J. T. Long, "Engineering for Nuclear Fuel Applied Instrumentation," pp. 5-41 to 5-54, Reprocessing," pp. 330-332, Gordon and Breach, I McGraw-Hill, New York, 1964. New York, 1967.

5. J. E. Harrell, "Mixing and Sampling Enriched U-235

2. D. M Considine, ed., "Process Instruments and Fluids in Cylindrical Storage Containers," USAEC Controls Handbook," pp. 7-8 to 7-23, McGraw-Hill, Report Y-1561, January 17, 1967.

New York, 1957.

6. C. G. Rodden, ed., "Selected Measurement Methods for Plutonium and Uranium in the Nuclear Fuel

3. F. M. Groth and F. 0. Cartan, "Evaluation of Cycle," USAEC Report TID-7029, 2d edition, Instrumentation for Nuclear Fuels Reprocessing pp. 61-65, 1972.

Plant Input Weight Measurements," USAEC Report ICP-1014, July 1972. 7. J. T. Long, op. cit., pp. 733-736.

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