X 0J Internationale Fachmesse Foire internationale International u für die kerntechnische des industries Nuclear D Industrie nucléaires Industries Fair nuclex 72 16-21 October 1972 >a Basel/Switzerland o CH-4021 Basel/Schweiz o Telephon 061-32 38 50 Telex 62 6Ö5 fairs basel

Technical Meeting No. 7/6

The Characteristics of Electromagnetic Pumps

R.C. Werner Mine Safety Appliances Company 1

THE CHARACTERISTICS OF ELECTROMAGNETIC PUMPS R. C. Werner

INTRODUCTION Electromagnetic pumps are useful devices t& pump liquid metals. Since they do not require seals or moving parts, as mechanical pumps do, they help solve the problems associated with high temperature and possible corrosion with materials required other than just to contain the fluid. The pumps have had quite wide acceptance for handling sodium and NaK (a liquid alloy of sodium and potassium) associated with experimental systems and the liquid-metal-cooled fast breeder reactor program. This paper discusses electromagnetic pumps with regard, first, to differences and similarities to electric motors; second, to performance considerations; and third, to safety and reliability features. CHARACTERISTICS AS COMPARED TO ELECTRIC MOTORS An electromagnetic pump is a device which utilizes the interaction of a magnet field or fields and an armature current or currents to produce a useful force on an elec- tric conducting fluid just as in a.n electric motor except the fluid motion is obtained directly rather than by a rotating mechanical motion. The basic and most significant difference of an electromagnetic pump to that of an electric motor is the replacement of the -carrying part of the mechanical rotating armature with a fluid. First, the fluid must be confined in a greatly increased magnetic air gap and, second, the fluid cannot be divided into several individual electrical conductors. This requires stationary parts in the armature and limits the armature1 windings to an equivalent of one-half or one turn. If the electromagnetic pump is of the conduction type where the armature current is supplied from the out- side and must be conducted through the wails confining the fluid, then large currents at very low must be 2

supplied whether the pump is operated on AC or DC. For DC-operated pumps, the power supply is located away from the pump an« d losses are occurred in the connecting lines due to the large current. For AC-operated pumps, the power converter may be located right at the pump to save line losses but the losses are replaced by large inductive cir- culating currents due to the poor power factor so that in the overall comparison both types of pumps are nearly equal in small sizes. Another method of reducing these large lead line current losses is to induce the operating right in the fluid itself. This requires a moving field either produced by mechanical motion or by the several methods normally used with fixed windings? i.e., shaded poles, split phases or multiphase field windings. Here the arma- ture currents may still have to pass through the walls confining the fluid or in special designs limited only to the fluid. Thus/ the may be steady, alternating or moving and may be produced by permanent or electric magnets and, when moving, by shading the poles or by using some form of split or multiphase . The armature current may also be steady, alterna- ting or moving and may be produced by an outside electrical supply or by induction near or within the pump through use of alternating or moving magnetic fields. PERFORMANCE CHARACTERISTICS ««••nHnrtmMHMMBBnaMHnan^aflaMaMnn . The output of an electromagnetic pump is measured in terms of the flow rate and the developed head of the fluid. Both of these terms vary, depending on the charac- teristics of the system in which the pump is installed. The electromagnetic pump is really only a pressure-generating device. The flow rate will change as the power level to the pump is changed until the developed head just balances the pressure drop of the system at the same flow rate. 3

In general, larger flow rates are handled by pumps with larger cross-sectional areas for the fluid, with an involved balance between the width, height, velocity and back EMF generated to meet the specifications. Higher pressures require smaller cross-sectional areas and longer pumping sections or relooping the flow stream through the pump two or more times in separated piping to meet the specifications. Most pumps will then have their highest pressure at shut-off and will develop decreasingly less pressure as flow rate is permitted to increase. The slope will depend upon the loss of pressure due to hydraulic losses within the pump and the amount of back EMF generated which hinders the flow of armature current as the flow velocity in- creases . Other factors of consideration in the design of an electromagnetic pump include the power supply, temperature of the fluid/ physical and electrical properties of the fluid, materials of construction and their physical and electrical properties at temperature and, finally, the atmospheric conditions surrounding the pump. Efficiency can normally be increased by building the pump larger but the designer is soon limited in a practical way by the pri-ce of the pump and the cost of interest rates on his investment. There is, however, no physical limitation as to the size to which an electro- magnetic pump can be designed and built. r s' For stationary field pumps, whether operated on either AC or DC, the maximum velocity of the fluid has no limit except to overcome the hydraulic frictional losses and the generator back-EMF. Pumps with velocities as high as 70 fps have been built and operated. However, for moving magnetic fields, whether by actual physical motion or by use of multiphase arrange- ments, the maximum velocity has the additional limit of the velocity of the field. Therefore electromagnetic 4

pumps of this type will have a rather definite maximum flow rate; i.e., a little slower velocity than the magnetic field. Maximum efficiency will generally occur in the 35 to 45 fps velocity range depending on the shape of the performance curve desired. Pressure developed will be in the order of 2 to 3 psi per inch of length in the active pumping section. In some applications of moving-field electro- magnetic pumps where there is the possibility of blocked flow under full power conditions, it may be required that the amount of increase in pressure must be limited so as not to over stress the pressure-containing walls of the piping and equipment. In such cases a more flat type of pump performance curve is desired which requires that the operating point be chosen at a larger differential between the velocities of the fluid and the magnetic field. This will also dictate some additional loss in efficiency. If the electromagnetic pump is to be located in a hostile environment such as relatively high-temperature or high-radiation areas, then the DC-operated conduction- type pumps have some advantages in that, because of the very low voltage required for operation, the electrical in- sulation requirements become minimal. In fact, stainless steel supports have been used for the electrical insula- tion . For moving-field pumps the air gap is wider with more use of thermal insulation to protect the field wind- ing and the best grade of electrical insulation is used in the windings themselves. Generally the pump is designed in such a way that the windings can be replaced easily and remotely when required. SAFETY AND RELIABILITY CHARACTERISTICS Electromagnetic pumps can be designed and manu- factured to any safety and/or code requirements. However when the confinement walls for the liquid metal are made thicker, there is a resultant drop in efficiency or the 5

pump must be made larger. Generally one chooses a design of minimum size and near maximum efficiency that will meet all of the safety and code requirements. The walls confining the liquid metal are generally thinner than the piping walls but thicker than the bellows seals used in liquid-metal valves. The wall can be de- signed to the ASME Section III (Nuclear Power Plant Components) of the Boiler and Pressure Vessel Code. The nozzle connections can handle some stress load due to the piping but are generally kept to a minimum. There are, however, a few precautions that must be taken for good, safe and reliable operation of electro- magnetic pumps. First and foremost is that the desired effect of the pump is to work on the liquid metal. If the pump should be started up empty or run dry during opera- tion, or should a gas bubble move into the working area of the pump, the armature current applied or induced will be confined to the wall where much heat will be produced with little or no cooling. This problem is more severe with conduction-type pumps where most of the electrical resist- ance for the armature current is in the leads. The loss of moving fluid and the resultant back EMF will have only a minor effect on reducing the armature current which will continue to flow, heating the pump walls to the melting point. This error in operation can easily be prevented by continually monitoring the liquiâ-metal flow, the out- put pressure of the pump and/or measuring the temperature of the pumping section walls.- This problém is not so severe in linear induction pumps where the length of the armature connectors is very small and the loss of the fluid greatly reduces the arma- ture current and permits rather long operation times without damaging the pump. It is generally wise still to continue to monitor the flow, pressure output and tempera- ture of the pumping section. There will also be a change in the phase relationship between the applied voltage and 6

current if the pump runs dry, and this could also be used to monitor the operation of the pump. •Secondly, and not so severe, is the danger of the blacking, of total flow, Although this will raise the out- put pressure of the pump, the pump and system should be designed so that this increased pressure is compatible. Again monitoring the items mentioned above will apply and can be used to prevent actual damage to the pump. Thirdly, the inlet pressure to an electromagnetic pump must be sufficient to prevent cavitation. Just up- stream to the active pumping section, a portion of the armature current will enter the liquid metal at an angle that is not perpendicular to the wall. This current will tend to pump the liquid metal away from the wall and, if the static pressure is not sufficient, will form a vapor bubble. The bubble will collapse under AC operation when the current reaches zero or, under DC operation when the bubble flows downstream to a region of higher pressure. These bubbles when formed have been known to collapse under low vapor pressure conditions with such force as to damage the pumping section by actually removing metal. This condition must therefore be prevented by having sufficient static pressure at the inlet to the pump to over-balance the vapor pressure of the fluid and pumping action at this point* Fourthly, the monitoring of select temperature locations and comparison of pump performance with earlier tests can be used to detect any detrimental deterioration during the life of the pump. In conclusion, it can be noted that electromagnetic pumps can be designed, built and operated to any general set of specifications. When good practice is applied to design, construction and operation, the pumps can provide both safe and reliable service.