PRESSURIZED MAGNETIC FLOAT LEVEL INDICATOR FOR RECIPIENTS CONTAINING LIQUIDS AT HIGH PRESSURE AND TEMPERATURE

Abstract: The pressurized magnetic float level indicator for recipients containing liquids at high pressure and temperature of the invention comprises a tubular body with a vertical axis which is closed at its ends and communicates at the top and bottom with a recipient containing liquid whose level is to be indicated, within which there is a float (5) provided with a magnetic element (11) designed to orientate a plurality of rotary magnetic dipoles aligned in parallel with the tubular body, the float (5) being provided with inner rings (10) and being pressurized with an evaporating mixture having a stream pressure which varies with variations in the internal temperature of the float (5) along a curve corresponding to the variation of the pressure with the temperature of the liquid contained in the recipient and in the tubular body externally to the float (5), this stream pressure within the float at least partially balancing out the pressure outside it.

Description: The present invention relates to a pressurized magnetic float level indicator for recipients containing liquids at high pressure and/or temperature. In order to detect the level of the liquid contained in closed devices such as tanks, boilers and the like, use is advantageously made of magnetic level indicators having a float which moves axially in a tubular body communicating at its ends with the recipient whose level is to be indicated, which float bears a permanent magnet designed to orientate a plurality of magnetic indicator dipoles, whose position shows the level in the recipient. A level indicator of this type is described in Italian Patent Application No. 24279 A/81 in the name of the applicants, to which reference should be made. This level indicator is particularly reliable and secure in most cases, since it in particular contains no transparent components which are components of lower overall strength and which often make it difficult to see the existing level, since the unit is sealed from the outside in all the cases; in some however, which are particularly problematic due to the high temperature and pressure of the liquid contained in the recipient whose level is to be indicated, this type of indicator may also be brought into a critical condition. The float bearing the magnet must be able to operate in these conditions as well without its structure, which has to be very light to enable it to float, being degraded by the external pressure and the high temperature. An example of such situations is provided by the cylindrical bodies of steam generator boilers, for instance in thermoelectric plants, which operate with water at a temperature which may reach 370°, corresponding to an equilibrium pressure of 220 bar, together with a water density of some 0.5 g/cm³. In these conditions, the structural resistance of a float which has to be very light in view of the density of the water, is not enough to prevent its degradation as a result of instability due to external pressure; it is possible to use pressurized floats, i.e. floats which have an internal pressure which partially balances out the external pressure in working conditions, but even these floats are not suitable for extreme conditions, since they must be pressurized to a level which is too high at ambient temperature if they are to oppose the external pressure at the working temperature efficiently. It is also possible to use, within the float, a quantity of an evaporating liquid, whose vapor pressure increases rapidly with temperature, for example the same as the external liquid, but this internal liquid must be used in extremely small quantities so that it is not detrimental to the ability to float; these have the further problem that during the heat transients, when the temperature within the float is different from that outside it, there is no longer an equilibrium between the internal and external pressures which may lead, in these conditions, to the degradation of the float structure. An example of this exploitation is given into DE 21 32 439 that shows a float indicator pressurized with internal pressure balancing external pressure, such equilibrium condition being obtained by a pressurized fluid that is the same fluid of which the level is to be measured. These equilibrium conditions are the only suitable and safe working at speed, while no guarantee of equilibrium is given during the transition thus during a fast external pressurization the internal pressure is late and the float can break, as well during a fast depressurization the internal pressure is again late and the float explodes. This leads, therefore, to the problem of providing a float for magnetic level indicators which can withstand very harsh working conditions without degradation taking place during the transient phases. These results are achieved by the present invention which provides a pressurized magnetic float level indicator for recipients containing liquids at high pressure and temperature comprising a tubular body with a vertical axis which is closed at its end and communicates at the top and bottom with a recipient containing liquid whose level is to be indicated, within which tubular body there is a float provided with a magnetic element designed to orientate a plurality of rotary magnetic dipoles aligned in parallel with the tubular body, said float is provided with inner reinforcing rings and is pressurized by an evaporating mixture at least formed by a water soluble gas and helium, the vapor pressure of said mixture within the float varying in response to variations in the temperature and pressure of the liquid in said recipient to at least partially balance out the pressure outside the float. More particularly the float has a cylindrical body with a vertical axis, with semi-spherical end caps, said inner reinforcing rings being disposed fairly closely to one another, a magnetic dipole being supported to rotate freely about a vertical axis between a pair of adjacent inner rings. According to the present invention said mixture can also contain a quantity of the same liquid contained in the recipient to which the indicator is connected and when the operation in pressurized water at high temperature are carried the evaporating mixture contained in the float is formed by a quantity of water, containing in solution a water soluble gas which can be separated, and Helium, all of which is kept at a pressure greater than atmospheric pressure at ambient temperature, more particularly the gas which can be separated being formed by ammonia in a preferred ratio from 1/200 to 1/20 approximately of the internal volume of water containing a gas which can be separated in solution. In a preferred exploitation and for operation in pressurized water at high temperature, helium is present in the float at a pressure at ambient temperature of between 20 and 70 bar. Further details are given in the following detailed description made with reference to the accompanying drawings, in which: Fig. 1 is an overall view, partly in section, of a level indicator provided with a float of the invention; Fig. 2 is an axial section through the float of the invention; Fig. 3 is a pressure/temperature diagram of the operating conditions of the float; As Fig. 1 shows, a magnetic level indicator, used with a closed pressurized recipient 1, formed for instance by the cylindrical body of a boiler for steam production, is formed by a tubular element with a vertical axis 2 which is closed at its ends and communicates through relative members 3, provided with interception valves 4 which are open during operation, with the recipient 1. Within the tubular element 2, where the liquid level is identical to that in the recipient 1 communicating therewith, there is a float 5 provided internally along the line at which it floats in the liquid with a permanent magnet. In parallel with the tubular element 2 there is a plurality of small cylinders 6 with horizontal axes supported in a rotary manner in relative housings 7 and provided with inner magnetic members whose axes are transverse to their axis of rotation. These small cylinders are oriented by the action of the magnet contained in the float 5 and by means of the different colouration on their surface indicate the position of the float and, consequently, the level of the liquid in the tubular element 2 and the recipient 1. For use in recipients operating in particularly harsh conditions, i.e. at high pressure and high temperature, and in particular in the case of cylindrical bodies of boilers, in which the liquid whose level is to be detected is water which has, moreover, a fairly low density in these conditions, the float must also have particularly good properties of strength and lightness. The structure of the float of the invention, designed to withstand, by means of its internal pressurization, the conditions which may occur in the plant, is shown in Fig. 2. The float comprises a cylindrical body 8 closed at its ends by semi-spherical caps 9 and is provided with inner reinforcing rings 10. The magnet 11 is disposed at the floating level of the unit between two inner rings 10 and is supported to rotate about the longitudinal axis of the float, transverse to its axis of magnetization, on two plates 12 connected by welding to the rings 10. In order to allow the achievement of welds which guarantee full structural rigidity between the cylindrical body and the rings 10, the body 8 is constructed from adjacent cylindrical segments, each of the height between two consecutive rings, welded together and to the rings with relative weld beads 13. A tube 14 enables the interior of the float to be supplied with liquid and pressurized gas, the float then being closed before use to maintain the internal pressurization conditions desired. The float, so that it is light enough to float in a liquid whose density may have, in the case of pressurized water at 350-360°C for example, a value of approximately 0.5 g/cm³, and ensuring at the same time that it withstands the external operating pressure, is advantageously made of titanium. The mechanical structure formed in this way is not, however, sufficient to withstand the pressures which occur in all temperature conditions and the interior of the float must be pressurized to counterbalance the external pressure. The external pressure acting on the float may cause one of its walls, which are not very thick, to buckle and thus cause the degradation of the structure as a result of elastic instability, even at loads which are substantially lower than the values admissible for the material if subjected to internal pressure; strength is therefore ensured within the float by a pressurized gaseous mixture which offsets or reduces the pressure imbalance on its walls. As shown in Fig. 3, the presence of the liquid contained in the recipient in which the float is to be used, i.e. water in the case in question, varies in accordance with the curve 15 with variations in the temperature; the pressure within the float must therefore oppose this external pressure, maintaining the float walls at overall stress values, taking account of both the internal pressure and the external pressure, which are substantially no greater than the safety values at each temperature for the material being used. In order to balance the external pressure at least partially a light pressurized gas such as helium can be introduced into the float; with increasing temperature, however, the internal pressure in the float follows a course of the type shown by curve 16 of Fig. 3 which increases in a substantially linear manner in accordance with the laws of gases. As can be seen, beyond a certain temperature value the internal pressure, initially greater than the external pressure, becomes substantially lower than the external pressure thereby exposing the float to the risk of degradation by buckling, while it is not possible to increase the initial pressurization pressure of the float beyond a certain limit so as not to subject the walls of the float, in the case of low external pressure values, to excessive stresses. It is therefore provided to use within the float a small quantity of a mixture comprising an evaporating liquid and a gas dissolved therein which are provided overall with a curve of vapor pressure variation as a function of temperature which is close to that of the liquid in which the float is immersed, i.e. water in the case in question, without the quantity of mixture introduced to obtain an adequate compensation having to be such as to increase the weight of the float excessively. The mixture is formed, for operation in pressurized water, as in the case of cylindrical boiler bodies, by a quantity of water in which a quantity of ammonia is dissolved. The overall quantity of the liquid mixture is advantageously from 1/200 to 1/20 of the internal volume, with ammonia at a concentration of 20 to 50%; the pressurization is achieved with helium at a pressure of between 20 and 70 bar at ambient temperature. For instance, for a float with a volume of approximately 1 litre and a weight of 500 grams, designed to operate in pressurized water, up to a temperature of 360-380°C, the overall amount of liquid mixture may be between 5 and 50 cc formed by a mixture of ammonia in water at the above-mentioned concentration. This mixture is designed to ensure, during transmission from ambient temperature and pressure, i.e. approximately 20° and 1 bar, to the maximum working temperature and pressure, a rapid initial increase in the internal pressure due to the pressure of ammonia above the solution up to its complete separation; from then on there is a progressive increase in pressure largely due to the steam pressure of the water which is substantially similar to the pressure variation with the temperature of the external liquid, until the working pressure and temperature are reached, in accordance with curve 17 of Fig. 3 which is of an experimental nature. As the Figure shows, the pressure provided by the mixture contained in the float does not increase beyond a certain limit and has a course similar to that of the external liquid as shown by curve 15. The value δ existing between the external pressure acting on the float and the internal pressure in the float itself, which represents the absolute value of the external pressure acting on the float, is such that it can be withstood by the reinforced float structure. During cooling of the unit temperature within the float remains higher than the external temperature as a result of the limited thermal conductivity of the material forming the walls of the float. This means that the internal pressure in the float is higher than its value at equilibrium and follows a curve 18 similar to curve 17, but translated substantially to the left, i.e. towards lower temperature values. When this phenomenon occurs, however, the curve 18 must be kept below the curve 19 which shows the internal pressure at which the material yields in the presence of external pressure varying in accordance with curve 15 at the various temperatures: i.e. the temperature, and thus the internal pressure in the float, must be prevented from remaining too high with respect to the external pressure, as a result of the thermal cooling transient, since this would be dangerous for the structural integrity of the float itself. Curve 20 of Fig. 3 shows the pressure valve in the float corresponding to a yielding stress in the material, calculated theoretically, in the absence of external pressure. As Fig. 3 shows, a slight intersection between curve 18 and curve 19 may be admissible, provided that it is of limited size: in these conditions there will be a slight permanent deformation of the material as a result of the internal pressure which is, however, of controlled size and causes the float to assume a shape which is more resistant without, however, giving rise to cracks and the like; this stress is in any case a result of internal pressure and does not therefore lead to problems of instability and buckling. The mixture of the invention thus makes it possible to pressurize the interior of the float so that when the temperature increases its internal pressure increases with a course similar to that of the external pressure, thereby maintaining the walls of the float balanced or in any case at least partially compensating the pressure imbalance thereon without there being additional stresses due to delays in bringing the internal conditions in line with the external environment. Although the float has been described in detail with reference to the use of ammonia in solution in water, use could be made of a different substance which is highly soluble in water at ambient temperature and which thus separates rapidly with increasing temperature giving rise to the desired increase in pressure in accordance with a law which ensures in all conditions a compensation of the external pressure maintaining the walls of the float in a resistant condition. Although the float has been described with reference to its use in water at high temperature and pressure, it can also be used with different liquids; the evaporating mixture which it contains should be selected for this purpose to ensure the desired law of internal pressure variation.

Claims: Pressurized magnetic float level indicator for recipients (1) containing liquids at high pressure and temperature comprising a tubular body (2) with a vertical axis which is closed at its end and communicates at the top and bottom with a recipient (1) containing liquid whose level is to be indicated, within which tubular body there is a float (5) provided with a magnetic element (11) designed to orientate a plurality of rotary magnetic dipoles aligned in parallel with the tubular body (2) characterized in that the float (5) is provided with inner reinforcing rings (10) and is pressurized by an evaporating mixture at least formed by a water soluble gas and helium, the vapor pressure of said mixture within the float (5) varying in response to variations in the temperature and pressure of the liquid in said recipient to at least partially balance the pressure outside the float. Pressurized magnetic float level indicator for recipients (1) containing liquids at high pressure and temperature as claimed in claim 1, characterized in that the float (5) has a cylindrical body (8) with a vertical axis, with semi-spherical end caps and said inner reinforcing rings (10) are disposed fairly closely to one another, a magnetic dipole (11) being supported to rotate freely about a vertical axis between a pair of adjacent inner rings (10). Pressurized magnetic float level indicator for recipients (1) containing liquids at high pressure and temperature according to claim 1, characterized in that said mixture also contains a quantity of the liquid contained in the recipient (1) to which the indicator is connected. Pressurized magnetic float level indicator for recipients containing liquids at high pressure and temperature as claimed in claim 3 characterized in that for operation in pressurized water at high temperature the evaporating mixture contained in the float (5) is formed by a quantity of water, containing in solution a water soluble gas, which can be separated, and Helium, all of which is kept at a pressure greater than atmospheric pressure at ambient temperature. Pressurized magnetic float level indicator for recipients containing liquids at high pressure and temperature as claimed in claim 4, characterized in that for operation in pressurized water at high temperature the gas which can be separated is formed by ammonia. Pressurized magnetic float level indicator for recipients containing liquids at high pressure and temperature as claimed in claim 5, characterized in that for operation in pressurized water at high temperature, the float (5) contains from 1/200 to 1/20 approximately of the internal volume of water containing ammonia. Pressurized magnetic float level indicator for recipients containing liquids at high pressure and temperature as claimed in claim 6, characterized in that for operation in pressurized water at high temperature, the float (5) contains a solution of approximately 20 to 50% of ammonia in water. Pressurized magnetic float level indicator for recipients containing liquids at high pressure and temperature as claimed in claim 4, characterized in that for operation in pressurized water at high temperature, helium is present in the float (5) at a pressure at ambient temperature of between 20 and 70 bar.

Inventors: BONETTI, GIULIO
Applicants: CESARE BONETTI S.P.A.
Application year: 1988
Main classification: G01F23
IPC codes: G01F23