HANDLING AND USES OF THE ALKALI METALS

Sodium Handling Equipment J. F. C A G E , JR.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: January 1, 1957 | doi: 10.1021/ba-1957-0019.ch007

Atomic Power Equipment Department, General Electric Co., Schenectady, N.Y.

As an indirect result of various Atomic Energy Commission p r o g r a m s , n e w t e c h n i q u e s and equipment have been developed for handling liquid metals. M a n y of these are available commercially to industrial users of sodium. The equipment includes electromagnetic pumps, magnetic flowmeters, pressure t r a n s m i t t e r s , and equipment for determining and controlling the oxide content of sodium systems characterized by being completely leakless and having no moving parts.

The use of s o d i u m a n d s o d i u m - p o t a s s i u m alloys as reactor coolants has been the subject of a substantial amount of research a n d development. T h i s has b e e n c a r r i e d out b y v a r i o u s laboratories of the A t o m i c E n e r g y C o m m i s s i o n a n d s u b c o n tractors, i n conjunction w i t h n u c l e a r plants s u c h as t h e e x p e r i m e n t a l breeder reactor a n d the prototype for the S e a w o l f p o w e r plant. A b y - p r o d u c t of this w o r k has been the development of n e w techniques a n d equipment f o r h a n d l i n g l i q u i d metals. A p p l i c a t i o n of l i q u i d metals to n u c l e a r plants requires e x t r e m e l y h i g h s t a n dards. E q u i p m e n t must be dependable, m a i n t e n a n c e - f r e e , a n d capable of o p e r a t i o n at elevated temperatures. P u r i t y of the coolant must be m a i n t a i n e d at h i g h levels; p i p i n g systems a n d their components must b e almost perfectly sealed. S p e c i a l e q u i p m e n t w a s developed to move, measure, a n d m a i n t a i n the l i q u i d metals used i n this service. I n general, this e q u i p m e n t m u s t be completely sealed a n d incapable of c o n t a m i n a t i n g the fluid i t handles. S i m i l a r e q u i p m e n t is b e c o m i n g available c o m m e r c i a l l y o n a n i n c r e a s i n g l y b r o a d scale, a n d is finding application i n m a n y n o n n u c l e a r processes i n w h i c h l i q u i d metals are used. A l t h o u g h l i q u i d metals are used i n the atomic field, u s u a l l y o n l y as heat transfer fluids, the special features i n e q u i p m e n t developed f o r this service c a n be advantageously a p p l i e d i n processes i n w h i c h l i q u i d metals are used i n stoichiometric quantities.

Electromagnetic Pumps Because of the f a v o r a b l e electrical c o n d u c t i v i t y of l i q u i d metals i n t h e i r m o l t e n states, s o d i u m a n d s o d i u m - p o t a s s i u m a r e r e a d i l y p u m p e d b y e l e c t r o magnetic p u m p s . T h e s e are, of course, totally sealed a n d , h a v i n g n o shaft seals or stuffing boxes, h a v e h i g h d e p e n d a b i l i t y a n d f r e e d o m f r o m maintenance. E l e c tromagnetic p u m p s utilize the w e l l - k n o w n motor p r i n c i p l e — " a conductor, c a r r y i n g a n electric c u r r e n t , a n d located i n a magnetic field, experiences a force, this force b e i n g m u t u a l l y p e r p e n d i c u l a r to the magnetic field a n d the d i r e c t i o n of c u r r e n t flow." I n electromagnetic p u m p s , t h e conductor is the l i q u i d m e t a l , enclosed i n a r e l a t i v e l y t h i n - w a l l e d pipe o r duct, a n d the force is manifested as a pressure rise i n the p u m p . A n u m b e r of different arrangements, p r i n c i p l e s , a n d structures to produce the orthogonal c u r r e n t a n d field h a v e been studied a n d 60

In HANDLING AND USES OF THE ALKALI METALS; Advances in Chemistry; American Chemical Society: Washington, DC, 1957.

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utilized, w i t h a r e s u l t i n g n u m b e r of different types of electromagnetic p u m p s . T w o of these types h a v e been successful i n n u m e r o u s different applications, a n d therefore are of general interest.


T h e first of these is the direct c u r r e n t conduction type ( F i g u r e 1). C u r r e n t flowing i n the exciting w i n d i n g s , 1, magnetizes the pole faces, 2, a r e t u r n p a t h for the flux b e i n g p r o v i d e d i n the i r o n structure, 3. D i r e c t c u r r e n t f r o m a separate source flows t h r o u g h the fluid i n the duct, 4, brought i n t h r o u g h bus bars connecti n g to the terminals, 5. T h e r e s u l t i n g force, indicated b y the arrows, causes a pressure rise i n the duct. O p e r a t i o n of the p u m p is s i m i l a r to that of a direct c u r r e n t motor i n m a n y respects. T h i s type of p u m p has the highest potential efficiency, a n d is least sensitive to variations i n fluid resistivity a n d elevated fluid temperatures. P u m p s of this type r e q u i r e special, l o w v o l t a g e - h i g h c u r r e n t p o w e r supplies, the expense of w h i c h has l i m i t e d their application. T h e alternating current, conduction type ( F i g u r e 2 ) , also has a n e x c i t i n g w i n d i n g , 1, pole faces, 2, a magnetic circuit, 3, a duct, 4, a n d terminals, 5. P u m p i n g action is p r o d u c e d as indicated b y the arrows. T h e h i g h c u r r e n t is obtained b y transformer action f r o m a single t u r n w i n d i n g , 6, coupled to the p r i m a r y w i n d i n g a n d connected to the electrical terminals b y a large conductor, 7. P u m p s of this type operate f r o m conventional alternating c u r r e n t p o w e r sources. T h i s p u m p has made it i m p r a c t i c a l for use i n applications i n w h i c h the r e q u i r e d p u m p i n g power is higher t h a n about 1 h p . A h i g h l y desirable characteristic of electromagnetic p u m p s is their ease of control. R e d u c i n g the voltage a p p l i e d to the alternating c u r r e n t type, or the c u r r e n t s u p p l i e d to the direct c u r r e n t type, produces a r e d u c t i o n i n the pressure developed i n each. T h e corresponding change i n flow is d e t e r m i n e d b y the

Figure 1. Direct current conduction electromagnetic

pump


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Figure 2.

Alternating current conduction electromagnetic

pump

h y d r a u l i c characteristics of the system i n w h i c h they are e m p l o y e d . C o n t i n u o u s operation c a n be obtained at almost a n y flow f r o m zero to f u l l p u m p r a t i n g .

Magnetic Flowmeters J u s t as the direct c u r r e n t motor has its counterpart i n the electromagnetic p u m p , the generator has its counterpart i n the magnetic flowmeter. T h e voltage generated is used to measure flow rather t h a n as a source of p o w e r ; so, m o r e strictly speaking, the magnetic flowmeter is analogous to a tachometer generator. M o r e i m p o r t a n t t h a n its analogy is its application. T h e magnetic flowmeter has p r o v e d to be so w e l l suited to m e a s u r i n g the flow of l i q u i d metals that it has been used i n this service almost to the complete e x c l u s i o n of flowmeters of other types. A magnetic field ( F i g u r e 3) created b y permanent magnet pole pieces, 1, passes t h r o u g h the pipe c a r r y i n g the fluid, 2. M o t i o n of the fluid i n the field generates a voltage i n the fluid w h i c h is d i r e c t l y p r o p o r t i o n a l to the velocity. V o l t a g e is m e a s u r e d t h r o u g h connections, 3, m a d e directly to the e x t e r n a l pipe w a l l . V o l t a g e generated i n the field causes a c u r r e n t to circulate i n the w a l l s of the pipe a n d i n the fluid at field boundaries. T h e voltage drop p r o d u c e d b y this c u r r e n t reduces the measurable voltage f r o m the m a x i m u m theoretical. T h i s loss, however, is p r o p o r t i o n a l to the flow, so that the voltage m e a s u r e d at the pipe w a l l is still d i r e c t l y p r o p o r t i o n a l to flow, p r o v i d e d that this voltage is m e a s u r e d w i t h a n instrument of v e r y h i g h i n t e r n a l impedance. A s the voltage b e i n g m e a s u r e d across the f u l l pipe diameter is the s u m of the i n c r e m e n t a l voltages a l o n g the diameter, it follows that this voltage is p r o p o r t i o n a l to the average fluid v e l o c i t y p e r p e n d i c u l a r to the diameter. C o n s e q u e n t l y , magnetic flowmeters i n h e r e n t l y correct for variations i n velocity profiles i n the plane of this axis, a n d are, i n general, r e l a t i v e l y insensitive to a p p r o a c h conditions. F u r t h e r m o r e , n o r e d u c t i o n of pipe diameter is r e q u i r e d , a n d the r e s u l t i n g pressure drop i n the fluid is substantially eliminated. E n e r g y dissipated i n the c i r c u l a t i n g currents produces a pressure drop, but this is negligible. A c c u r a c i e s a p p r o a c h i n g that of the i n d i c a t i n g or r e c o r d i n g instrumentation are obtained u s i n g indirect c a l i b r a t i n g methods.



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Figure 3.

Magnetic flowmeter

Pressure Instrumentation A n u m b e r of p r e s s u r e - m e a s u r i n g systems h a v e been used i n l i q u i d metals w i t h v a r y i n g degrees of success. T h e tendency of oxides a n d insolubles to migrate to cold zones, plus the difficulty i n k e e p i n g l o n g lines at u n i f o r m temperatures, m a k e the direct a p p l i c a t i o n of B o u r d o n - t y p e pressure gages difficult. T h e seal-pot technique has been w i d e l y a n d successfully used. It consists of a closed chamber, h a l f filled w i t h l i q u i d m e t a l a n d connected b y a s m a l l line to the point i n the p i p i n g at w h i c h the pressure is b e i n g measured. T h e pressure of the inert gas i n the u p p e r h a l f of the vessel is adjusted to balance the l i q u i d m e t a l pressure, so that the l e v e l i n the c h a m b e r is constant. T h e gas pressure is then measured b y conventional means. P r o p e r operation requires that the c h a m b e r operate above the oxide saturation temperature of the l i q u i d m e t a l to p r e v e n t concentration of oxides i n the chamber, a n d that p r o v i s i o n be made to p r e v e n t p l u g g i n g of the gas lines b y condensed a n d solidified s o d i u m . Better dependability has b e e n obtained u s i n g pressure t r a n s m i t t i n g e q u i p ment of the type u s i n g a pneumatic pressure to balance the force across a m e t a l d i a p h r a g m . T h e a p p l i c a t i o n of this type pressure transmitter to h i g h - t e m p e r a t u r e l i q u i d m e t a l systems has r e q u i r e d the development of d i a p h r a g m w e l d i n g a n d stabilizing techniques, a n d special design features to p r o v i d e temperature c o m pensation.

Oxide Measurement T h e presence of oxides i n s o d i u m a n d s o d i u m - p o t a s s i u m systems m a y h a v e a n u m b e r of undesirable effects: O x y g e n m a y be a c o n t a m i n a t i n g foreign element i n processes u s i n g s o d i u m ; s o d i u m a n d potassium oxides, as solids, are capable of p l u g g i n g pipes a n d otherwise p r o v i d i n g m e c h a n i c a l interference; at elevated temperatures, the presence of o x y g e n i n the s o d i u m has a m a r k e d effect o n the corrosion rate of a n u m b e r of materials. T h i s i m p o r t a n t parameter is understood a n d controlled o n l y w h e n accurately measured. E q u i p m e n t for t a k i n g samples for quantitative c h e m i c a l analyses is b y n a t u r e complicated because of the a l k a l i metals' affinity for o x y g e n . I n addition, the effect of temperature o n the s o l u b i l i t y of s o d i u m oxide i n s o d i u m makes s a m p l i n g for oxide p a r t i c u l a r l y difficult. T h i s effect is s h o w n i n F i g u r e 4. O x i d e solubility v a r i a t i o n w i t h temperature is used to advantage i n the p l u g g i n g indicator, a device for m e a s u r i n g the oxide content of l i q u i d s o d i u m a n d s o d i u m - p o t a s s i u m systems o n a semicontinuous basis without r e m o v i n g samples ( F i g u r e 5 ) . A p o r t i o n of the m a i n flow stream is d i v e r t e d t h r o u g h a bypass loop containing a cooling jacket, a n orifice plate, a n d a magnetic flowmeter. I n o p e r a tion, the temperature of the bypass stream is successively r e d u c e d b y increasing


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TEMPERATURE °F. Figure 4. Solubility of sodium oxide in sodium and sodium-potassium

FROM PROCESS STREAM

Figure 5.

Plugging indicator schematic


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the coolant flow u n t i l it falls slightly below the v a l u e corresponding to saturation of the l i q u i d w i t h the oxide. A t this point, solid oxides precipitate out, p l u g g i n g the orifice a n d r e d u c i n g the flow i n the bypass loop. T h i s is indicated o n the flowmeter. B y n o t i n g the temperature at w h i c h this occurs, the saturation l i m i t , a n d hence, the oxide content c a n be accurately d e t e r m i n e d . F l o w is forced t h r o u g h the p l u g g i n g indicator b y connecting it across a source of pressure — a p u m p — or across a pressure drop — such as a p a r t i a l l y closed v a l v e . T h i s pressure differential must be kept constant w h i l e a p l u g g i n g r e a d i n g is b e i n g taken, u s u a l l y a p e r i o d of a f e w minutes. A i r is u s u a l l y used as the coolant; " s h o p " h i g h - p r e s s u r e air c a n be used, the cooling rate b e i n g regulated b y the c o n t r o l v a l v e . A f t e r the p l u g g i n g temperature has been determined, the coolant is v a l v e d off, a n d the oxides w h i c h have a c c u m u l a t e d o n the p l u g g i n g plate are redissolved. C a l i b r a t i o n of the p l u g g i n g indicators is difficult because it appears to be m o r e accurate t h a n a n y other means of oxide determination. A l t h o u g h it is usable over a w i d e range of temperatures, it cannot be operated accurately i n systems containing quantities of other insolubles. T y p i c a l equipment is s h o w n i n F i g u r e 6.

Figure 6.

Plugging indicator

T h e equipment s h o w n is a n a i r - c o o l e d p l u g g i n g indicator i n the process of assembly. T h e flowmeter, inlets, outlets, a n d cooling jacket are apparent. T h e w i r e w r a p p i n g is h e a t i n g cable to p r o v i d e p r e h e a t i n g of the u n i t a n d to p r e v e n t p r e m a t u r e precipitation of the oxides.

Cold Trap T h e effect of temperature o n the s o l u b i l i t y of i m p u r i t i e s is u t i l i z e d to m a i n t a i n p u r i t y i n l i q u i d m e t a l systems. T h e process, c o m m o n l y c a l l e d cold t r a p p i n g , uses a cooled p i p e section i n w h i c h oxides c a n precipitate out a n d b e r e m o v e d f r o m the m a i n b o d y of the l i q u i d m e t a l . T h i s e q u i p m e n t is s h o w n schematically i n F i g u r e 7. L i k e the p l u g g i n g indicator, the bypass c o l d trap makes use of the pressure rise o r d r o p i n the m a i n system to p r o v i d e the operating pressure differential. T h e bypass stream passes d o w n t h r o u g h a cooling section w h e r e the stream temperature is r e d u c e d b e l o w the saturation temperature. O x i d e s precipitate out, a n d a r e r e m o v e d i n the filter section. A s s u m i n g 100% effective filtration, the r e t u r n i n g stream has a n oxide content c o r r e s p o n d i n g to the stream's lowest temperature. T h i s has been closely approached u s i n g stainless steel w o o l , or stainless steel mesh, i n the filter sections. T h e amount of oxide r e m o v a l is a


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FROM PROCESS STREAM

COOLANT

ISOLATING VALVE

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0

Figure 7.

R

Bypass cold trap schematic

function of the difference between the r e t u r n stream a n d the m a i n stream t e m peratures. A regenerative type of heat exchanger c a n be inserted between the c o l d trap a n d the m a i n stream to p r o v i d e a m a x i m u m temperature differential w i t h a m i n i m u m of heat loss. T h e efficiency of the cold trap i n r e m o v i n g oxides has been demonstrated to be h i g h ; i n addition, other insolubles are r e m o v e d b y the filter. In large systems i n t e r m i t t e n t l y open to the a i r , i n t e r m i t t e n t l y operating cold traps h a v e been adequate to m a i n t a i n l o w oxide levels. I n this service a n isolating v a l v e is r e q u i r e d to p r e v e n t r e t u r n of the t r a p p e d oxides to the m a i n system w h e n the cooling is shut off.

References (1) (2) (3)

B a r n e s , A. H., Nucleonics 11, N o . 1, 16-21 (1953). J a c k s o n , C. B., "Liquid Metals H a n d b o o k , S o d i u m ( N a K ) S u p p l e m e n t , " T . I . D . 5227, U. S. A t o m i c E n e r g y C o m m i s s i o n , W a s h i n g t o n 25, D . C., 1955. L y o n , R. N., "Liquid Metals H a n d b o o k , " N A V E X O S P-733 ( R e v . ) , U . S. G o v e r n m e n t P r i n t i n g Office, W a s h i n g t o n 25, D . C . , 1952.


HANDLING AND USES OF THE ALKALI METALS

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