AB Progetti S.r.l. Phone Fax

Transcription

1 AB Progetti S.r.l. Phone Fax

2 STEAM JET EJECTORS AND VACUUM SYSTEMS

3 TABLE OF CONTENT 1 GENERAL DESCRIPTION PAG SINGLE STAGE EJECTOR PAG VACUUM SYSTEMS PAG EJECTOR FEATURES PAG MATERIALS PAG PERFORMANCE FACTORS PAG MOTIVE STEAM PRESSURE PAG MOTIVE STEAM TEMPERATURE PAG SUCTION PRESSURE PAG SUCTION CAPACITY PAG SUCTION PRESSURE VERSUS CAPACITY RELATIONSHIP PAG DISCHARGE PRESSURE PAG COOLING WATER PAG NUMBER OF STAGES AND MIXED SYSTEMS PAG HOGGING EJECTORS PAG NOISE PAG INSTALLATION PAG. 21 Pag.2

4 1. GENERAL DESCRIPTION 1.1 SINGLE STAGE EJECTOR The ejectors are pumps that mix or compress many differents fluids. Ejectors have no moving parts and operate by the action of one high pressure stream entraining air and other vapors (or liquids) at a lower pressure into the moving stream and than by removing them from the process system at an intermediate pressure higher than the suction pressure but many times lower than the motive steam pressure. The main components of the ejector are: - The nozzle - The suction and mixing chamber - The diffuser FIG. 1 illustrates the ejector's components with pressure and velocity diagrams related to the position along the ejector's axis. FIG. 2 illustrates the thermodynamic transformations on steam/water entropy enthalpy diagram. Since the steam jet ejector is the unit most commonly used for many process applications, it will be discussed in the greatest detail. Fig. 1 Pressures-velocities diagram With reference to fig. 2 we can follow the transformations operated by an ejector that entrains water vapor from the P3 pressure and discharges to the P5 pressure and operates with a motive steam pressure of P1. During operation the high pressure steam enters the steam chest and expands passing through the steam nozzle where the pressure decreases while the velocity encreases, then leaving the nozzle at the nozzle exit the motive steam reaches the maximum velocity and the minimum pressure (point 2). With compressible fluids the velocity is usually many times the local sound velocity. Air, gas or vapors, or liquid mixture enter the ejector through the suction nozzle, passing into the suction chamber. Here the water vapor or air or other mixture are entrained by and into the high velocity steam. Fig. 2 Enthalpy - entropy diagram This new mixture enters the inlet portion of the diffuser (convergent), passes through the diffuser throat (center narrow portion) and exits through the outlet end of the diffuser. In the diffuser the velocity head of the mixture is converted back to a pressure higher than the suction pressure. The expansion of motive steam in the nozzle, the mixing of motive fluid with the entrained fluid and the conversion from velocity energy to pressure energy of the mixture has an influence on the global efficiency of the process. Pag.3

5 From the above it can be understood how, not whitstanding the machine building and its operating simplicity, up till now it has not been possible to acheive a complete theoretical analysis of the laws describing physical phenomena. This is due to the extremely high number of parameters both fluidodynamic and thermodynamic involved in the process. Therefore for the ejector's design we use only an approximate theory supported by many experimental data. 1.2 VACUUM SYSTEMS A single ejector element is limited at the sametime by the ratio between discharge and suction pressure (dependent from the motive steam pressure and the back pressure) and the maximum load capacity. Ejectors similar to those shown in Fig. 3 are utilized in series or in parallel, with or without intermediate condensers, to operate in a very large range of conditions and at lower absolute suction pressures than a single stage unit. Fig. 4 suggests a few of the many type of ejector arrangements that are used in industry. The condensers may be direct contact or surface type. The Fig. 7 illustrates the section of a two stage system with inter and aftercondenser of surface type. The steam condensate can be re-used from this installation. Fig.3 Ejector section The Fig. 4-3 illustrates a three stage system where the discharge of the steam and non condensables from third stage is exausted in the atmosphere, while the Fig. 8 shows the section of a two stage system with direct contact condensers, where the steam coming from the second stage is condensed into the aftercondenser and, essentially, only non condensables leave the vent of the aftercondenser. Precondensers are recommended for any ejector system when the pressure conditions and coolant temperature will allow condensation of vapors, thus reducing the required design and operating load on the ejectors. This is usually the operating situation of a distillation system under vacuum. Pag.4

6 Fig. 4 Some possible arrangement of steam jet ejectors The over head or the last stage of multistage distilling units vapors are condensed in a unit designed to operate at a pressure little lower than the operating pressure of the top column or last stage, (the difference is only due to the pressure drop in the interconnecting piping). Then only the non condensables and vapor remaining after condensation pass to the ejector system. 4-1 Single stage ejector stages ejector system with intercondenser between 2nd and 3rd stage stages ejector system with intercondensers (3rd stage to the atmosphere) stages ejector syatem wth intercondenser (2nd stage to the atmosphere) stages ejector system (1st stage of non condensing type) stages ejector system (non condensing type) stages ejector system (1st and 2nd stages of non condensing type) Pag.5

7 Intercondensers are used to condense the steam from a preceeding ejector stage, thus reducing the inlet quantity of vapor mixture to the following stage. This is a means of increasing steam economy. Aftercondensers operate at atmospheric pressure. They do not affect the steam economy or ejector performance, normally they are used because of the following reasons: - They avoid the nuisance of exhausting steam to the atmosphere - In case of surface type aftercondensers they allow steam to be recovered - In case of direct contact type they can absorb odors and corrosive vapors. The selection between surface or direct contact type condensers is related to the advantage of one type in the respect to the other. The direct contact type condensers, usually barometric, have the following advantages: - Lower purchase cost - Less space requirements - They operate condensation at a temperature that is lower than the one in the surface type because cooling water and mixture temperatures are closer. - Lower maintenance cost. Fouling does not affect the performance of the condenser - When it is the barometric type, the discharge generally is a safety device against over pressure. The surface condensers have the following advantages: - The process and cooling fluids are not mixed together, the condensed steam can be recovered and cooling water is not contaminated by process fluids. - This type of condensers may be less expensive during operation if environmental problems are to be taken into account. 1.3 EJECTOR FEATURES Ejectors have the following features which make them good choices for continuously producing economical vacuum conditions: 1. They handle wet, dry or corrosive vapor mixtures. 2. They develop any reasonable vacuum needed for industrial operations. 3. All sizes are available to match any small or large capacity requirements. 4. Their efficiency is reasonable-too-good. 5. They have no moving parts, hence maintenance is low, and operation is fairly constant when corrosion is not a factor. 6. Quiet operation. 7. Stable operation within design range. 8. Installation costs relatively low when compared to mechanical vacuum pumps. Space requirements small. 9. Simple operation and maintenance. 1.4 MATERIALS Since the ejector is basically simple in construction it is available in many materials suitable for handling high corrosive vapors. Standard materials include iron, and stainless steels, bronzes, high nickel alloys. Other construction materials include porcelain, synthetic resins. Nozzles are generally made of S.S. or monel, condensers are sometimes made of the same materials. Pag.6

8 2. PERFORMANCE FACTORS 2.1 MOTIVE STEAM PRESSURE Motive steam design pressure must be selected at the same level as the lowest expected pressure at the ejector steam nozzle. The unit will not operate stably on steam pressures below the design pressure. It is recommended to keep as design value the minimum expected line pressure at ejector nozzle minus 0.5 BARa.This design basis allows for stable operation under minor pressure fluctuations. An increase in steam pressure over design will not increase vapor handling capacity for the usual "fixed capacity" ejector. Increased pressure usually decreases capacity due to the extra steam in the diffuser. The best ejector steam economy is attained when the steam nozzle and the diffuser are proportioned for a specified performance. This is the reason for the difficulty in keeping so-called standard ejectors in stock and expecting to have the equivalent of a custom designed unit. The "throttling type" ejector has a family of performance curves depending upon the motive steam pressure. This type has a lower compression ratio across the ejector than the fixed-type. The fixed-type unit is of the most concerned in this presentation. For a given ejector, an increase in steam pressure over the design value will increase the steam flow through the nozzle in direct proportion to the increase in absolute steam pressure. The higher the actual design pressure of an ejector the lower the steam consumption. This is more pronounced on one-and two-stage ejectors. When this pressure is above about 30 BARA the decrease in steam requirements will be neglegible. As the absolute suction pressure decreases, the advantages of high pressure steam becomes fewer. Fig. 6 Effect of motive steam pressure In the case of ejectors discharging to the atmosphere, steam pressures below 3 BARA at the ejector's nozzle are generally uneconomical. In the units that are designed for very small suction capacity the use of high pressure steam is not a good choice because of the difficulty of nozzle throat machining. In these units the motive steam consumption is increased to obtain a nozzle throat diameter over 1 mm. If the discharge pressure is lower, as in multistage units, steam pressure at the inlet can be lower. Single stage ejectors designed for pressures below 0.25 BARA cannot operate efficiently on motive steam pressures below 3 BARA. The first stages of a multistage system can be designed to operate with motive steam pressures below 1.5 BARA. This is possible because of the low suction pressure and it is convenient when motive steam from process (at low cost) is available. Pag.7

9 To insure stable operations steam pressure must be over a minimum value. This minimum is called the motive steam pickup pressure and it differs from one ejector to the other. To find the motive steam pickup pressure, after the motive steam pressure is reduced until the ejector breaks, the pressure is increased: the pressure at which the ejector becomes stable operating is the pickup pressure. Figure 6 indicates qualitatively the above and shows the effect of extra steam in the diffuser. As pressure is reduced along line e-c-a, the operation is stable until point a is reached. At this point the ejector capacity falls off rapidly along line a-b. As steam pressure is increased, stable operation is not resumed until point d is reached and the capacity rises along line d-c. With further increases it rises along c-e. This is the stable region. Operation in region c-a is unstable and the least drop in pressure can cause the system to lose vacuum. The relative location of points c and a can be controlled to some extent by ejector design; and the points may not even exist for ejectors with low ratios of compression. If the motive steam pressure is expected to be not stable the use of a steam pressure controller is recommended. 2.2 MOTIVE STEAM TEMPERATURE A few degrees of superheat are recommended (from 3 up to 8 DEG. C), but if superheated steam is to be used, its effect must be considered in the ejector design. A high degree of superheat is of no advantage because the increase in available energy is offset by the decrease in steam density. With surface condensers a high degree of superheat causes an increase of surfaces, and therefore an increase in cost. Wet steam erodes the ejector nozzle and interferes with performance by clogging the nozzle with water droplets. The effect on performance is significant and it is usually reflected in fluctuating vacuum. When only wet steam is available it is absolutely necessary to utilize an efficient water separator before the steam enters the nozzles. Pag.8

10 Fig. 7 Two stage ejector system with surface inter and after condensers section 2.3 SUCTION PRESSURE Suction pressure is determined by the requirements of the process. Pressure drops due to the piping connecting the installation to the inlet flange of the ejector system shall be taken into accont. Then the suction pressure for ejector system design shall be: operating absolute pressure of the system less pressure drops. Because the motive steam consumption of an ejector system increases exponentially as the suction pressure decreases, care should be taken to avoid as far as possible pressure drops. The length of the interconnecting piping should be reduced to the minimum value, the presence of bellows or other devices that cause pressure drops should be avoided. The piping diameter shall be larger or at least equal to the ejector system inlet diameter. For the same reason excessive margin on suction pressure specification should be avoided. Pag.9

11 Fig. 8 Two stage ejector with barometric inter and after condensers section For design purposes it is necessary to use absolute pressure. In plant operation pressures are used as "vacuum". It is important to eliminate confusion before making a proper performance analysis. 2.4 SUCTION CAPACITY The capacity of an ejector is expressed as kilograms per hour total of non condensables plus condensables to the inlet flange of the unit. For single stage the total flow rate and the mean molecular weight of the mixture may be specified, for multistage units the total capacity must be separated into kg/h of each non condensable with the molecular weight of each element, and Kg/h of condensables with the physical property of each component. In some cases the condensation curves of the mixture at different pressures may be necessary. Few vacuum systems are completely airtight, although some may have extremely low leakage rates. For the ideal system the only load for the ejector is the non-condensables of the process (absorbed gases, air, etc.) plus the saturated vapor pressure equivalent of the process fluid. Practice has proven what allowance must be made for air leakage. Pag.10

12 In an under vacuum system the following fluids are to be removed: - Air leakage from the atmosphere - Process released air - Process released non condensables - The saturated vapors of the process fluids Air leakage occurs at piping connections (flanges, screwed fittings, valves), stuffing boxes, mechanical equipment seals, etc... Whenever possible a system should be tested to determine air leakage. For new design and in situations where tests cannot be made, the recommended values of the heat exchange institute should be followed. These are minimum safe values but a very tight system will show better performances. HEI give these values for ejectors serving surface condensers and the maximum air leakage values for commercially tight process systems which do not enclose any agitator equipment. For design purposes the ejector is usually purchased to operate on a load at about twice these values. For systems with agitators and ordinary shaft seals, the system leakage should be increased by 2 kilograms of air per hour per agitator. If special seals are used this value may be reduced to 0.5. The greater the number of rotating shafts which must be sealed to the outside atmosphere, the more likely there will be the possibilities for increased leakage. An alternate design for air inleakage used by some process engineers is H.E.I. curves for commercially tight systems plus the summation obtained by examining the process system using the factors in Table 1. This method is considered to be conservative, however, as in general the incremental cost may be a very small one between a unit hardly large enough and one with ample capacity to take surges in air leakage. Pag.11

13 TABLE 1 TYPE FITTING AIR INLEAKAGE Screwed connections in sizes up to 2 inches 0.05 Screwed connections in sizes above 2 inches 0.1 Flanged connections in size up to 6 inches 0.25 Flanged connections in sizes 6 inches to 24 inches including manholes 0.4 Flanged connections in size 24 inches to 6 feet 0.55 Flanged connections in size above 6 feet 1.0 Packed valves up to 1/2" steam diameter 0.25 Packed valves above 1/2" steam diameter 0.5 Lubricated plug valves 0.05 Petcocks 0.1 Sight glasses 0.5 Gage glasses including gage cocks 1.0 Liquid scaled stuffing box for shaft of agitators, pumps, etc.per inch shaft diameter 0.05 Ordinary stuffing box, per inch of diameter Safety valves and vacuum breakers, per inch of nominal size 0.5 ESTIMATED AIR INLEAKAGE FOR VARIOUS TYPE OF FITTINGS [Kg/h] Pag.12

14 Since the determination of air inleakage involves considerable knowledge of vacuum systems and judgment, no empirical method can be expected to yield exact and correct values. Most manufacturers use one of the methods presented here, together with a factor to account for the basic type of plant, maintenance practices, operational tecniques of the production personnel, and other related items. Thus, for a tight and efficient plant, the leakage values of H.E.I. may sometimes be reduced to of the values read, while for a sloppy, looserun plant the values might be multiplied by 1.5 or 2 or the alternate method using table 1 may be checked or even multiplied by 1.5 or 2. It is necessary to point out that the lower the operating pressure of the plant the higher must be the attention in the seal type selection in order to avoid as far as possible air inleakage. The above to increase the steam economy. When ejectors pull non-condensables and other vapors from a direct contact water condenser (barometric, low level jet, deareator) there is also a release of dissolved gases, usually air, from water. This air must be added to the other known load of the ejector. Figure 16 presents the data for the amount of air that can be expected to be released when cooling water is sprayed or otherwise injected into open type barometric or similar equipment. Due to the uncertainty in the air inleakage determination, generally the capacity is overdesigned, this means that steam consumption may be higher than the necessary. Pag.13

15 Fig. 11 Performance curves in different operating conditions Pag.14

16 Because the steam consumption of the ejector system is independent from the flow rate of the entrained fluids, but is only dependent from nozzle throat, the steam consumption is always constant. For this reason, when the air inleakage is difficult to extimate, it may be convenient to have for each stage two ejectors in parallel designed for 1/3 and 2/3 of the maximum expected load. These ejectors shall be isolated by valves on the motive steam inlet and on the suction. At the first start up of the plant, if the mixture to handle is less than the maximum expected, one of the two ejectors will be isolated. The steam and water economy so realised during only one year operation is often greater than the extra costs due to the double ejectors. Fig. 13 Load capacity-suction pressure Pag.15

17 Fig. 14 System variation by varying motive steam pressure and back-pressure Fig. 15 Effect of high discharge pressure Fig. 16 Released air from water Pag.16

18 2.5 SUCTION PRESSURE VERSUS CAPACITY RELATIONSHIP SINGLE STAGE EJECTOR Fig. 11 shows the variation in suction pressure caused by the variation of entrained fluids flow rate in the hypothesis that all the other parameters are unchanged MULTISTAGE EJECTOR SYSTEMS Figure 13 illustrates three different multistage ejector designs, 1, 2, and 3, which indicate that design 1 is quite sensitive to changes in load above the design point. Designs 2 or 3 are less sensitive. The curve extending towards point 4 shows the primary or first stage capacity when all the vapor is condensed in the inter-condenser; or otherwise it shows the performance of the system when it handles air or an air vapour mixture, in this case it shows the over capacity of the secondary jet to take off all the non-condensables. The curve labeled 1 indicates performance at overload when air-handling capacity of the secondary stage is limited. This condition arises as a result of design for steam economy. If the capacity of the secondary jets is larger, the performance along curve 2 or 3 can be expected. When the secondary jet capacity is limited as curves 1, 2, or 3 indicate, a capacity increase brings a rise in suction pressure when the load increase is mainly air or non - condensables. Pag.17

19 Fig. 18 Vacuum range guide The increase in pressure is lower when load increase is due to condensables. This emphasizes the importance in sizing the secondary jets for ample non-condensable capacity, and the importance of specifying the range and variety of expected conditions which may confront the system. Once a system has been evacuated to normal operating conditions, it is possible for capacity to fall to almost zero when the only requirement is air in-leakage or small quantities of dissolved gases. Under these conditions, it is important to specify an ejector system capable of stable operation down to zero load or "shut-off" capacity. The curves of figure 13 represents such a system. Pag.18

20 2.6 DISCHARGE PRESSURE The performance of an ejector is a function of backpressure. Most manufacturers design atmospheric discharge ejector for a over pressure of 0.04 to 0.08 Bara in order to insure proper performance. The pressure drop through any discharge piping and aftercooler must be taken into consideration. Discharge piping should not have pockets for condensation. Figure 15 indicates the effect of increasing the single-stage ejector backpressure for various suction pressures. Figure 14 illustrates the effect of motive steam pressure on a system taking into account discharge pressure variations. When this pressure cannot be increased, the nozzle may be redesigned to operate at the higher backpressure. 2.7 COOLING WATER The ejector systems with surface or barometric condensers shall be designed to operate with the maximum expected cooling water temperature. River or sea water or water cooled in a cooling tower are generally utilized for this service. All the above types of waters have temperatures varying with the seasons. Motive steam consumption and cooling water flow rate required by an ejector system is strongly dependent on cooling water temperature. If the entrained fluids are condensables this has a greater influence. In some cases motive steam consumption can be double when operating with a cooling water at 30 C instead of 15 C. Because the vacuum system is designed to operate with the higher expected coolant temperature, when this temperature is lower, steam consumption can be reduced (within some limitations) by reducing motive steam pressure. Cooling water flow rate must be held constant, reduction can cause a lower vacuum. The maximum water temperature at the outlet of a vacuum system is a function of the type of water. This is mainly due to the presence in the water of carbonates, then in this case, above 40 C, the scaling may become a problem. 2.8 NUMBER OF STAGES AND MIXED SYSTEMS The fig. 18 is a guide for the number of stage selection. It is advisable to try to accomplish the specific operation with as few ejectors as possible, since this leads to the most stable operation and lowest first cost in the majority of cases. By keeping into account suction pressure, discharge pressure, cooling water and steam cost, we can perform an analysis in order to find the most economical system. In some case it is convenient to use as a last stage a water ring pump. This is a way to reduce energy costs. The selection between a system with only ejectors or a mixed system must be analyzed for any particular application because many parameters have influence on the energy and cost saving. 2.9 HOGGING EJECTORS The emptying time i.e. the time necessary to make the vacuum in a system starting from atmospheric pressure is related to: - System volume - Ejector capacity - Final pressure - Air inleakage from atmosphere and non condensables released by process fluids Pag.19

21 Usually in the continuous under vacuum operating systems the emptying time is not a problem and the vacuum is reached by using the operating ejector system. But if necessary an evacuation or hogging ejector is used to remove air from a system on start up. Its capacity is set to bring the system pressure down to near operating condition before the continuous operating ejector system takes over. Usually the steam consumption of this type of ejector is considerably high but this is not a problem because their operation is limited to a maximum of 1 or 2 hours NOISE The ejectors directly exausting into the atmosphere without a final condenser produce a high noise level. This noise can reach 130/140 db(a) at 1 m of distance. A silencer can reduce this noise to 85 db(a) at 1 m of distance. The noise arising from a condenser equipped vacuum system can reach 90/95 db(a) at 1 m of distance. But noise prediction is impossible because it is also dependent on the installation. (i.e. a concrete support structure is better from this point of view than a steel support structure). After installation, if the noise is over the maximum specified, the ejector system can be insulated by rock wool and sometimes by reducing the induced vibrations to the support structure by means of antivibration supports. Pag.20

22 3. INSTALLATION When erecting a steam ejector unit it is essential to ensure that the condensers, when of the direct contact type, are in the true vertical position (check with spirit level). Fig. 20 Recommended installation of motive steam pipe A distance of at least 1 m should be allowed above the upper part of the unit towards the roof or ceiling to avoid difficulties in any possible dismantling. Furthermore the unit should always be freely accessible. It is important to allow enough space for removal of the water distributor parts of the condensers and the changing of steam ejector driving nozzles. If large diameter steam pipelines are used up to the individual ejectors then a short section of pipe should be inserted to facilitate the removal of the driving nozzle when necessary (see Fig. 20). This is also valid for the removal of water distributor parts in direct contact condensers (see Fig. 21). When connecting the steam pipeline to the ejector itself ensure that the gaskets are in the correct place. Pag.21

23 Fig. 21 Cooling water pipe All pipelines connected to the unit must be clearly arranged without any stress. The pipelines must have at least the same inside diameter as the connection on the unit. Gaskets must not under any circumstance restrict the cross section of the pipe. Wherever possible, the steam ejector unit should be installed at a such height that the condensate formed will flow from the condensers (which are under vacuum) by its own weight. This kind of installation is called barometric and has the advantage that a water extraction pump is not necessary. Barometric installation requires a height of at least 11 m between the cooling water outlet flange of the direct contact condenser and the water level of the barometric warm water collector (see Fig. 23). The barometric legs should lead vertically down to the warm water collector. If this is not quite possible, any deviation should be gradual and not exceed an angle of 45. Bends should be given the largest possible radius. Fig. 22 Hotwell design Where a unit comprises several direct contact condensers, each must have its own separate barometric leg to the warm water collector, because coupling up of these legs leads to trouble (see Fig. 23). The barometric legs must discharge below the level in the warm water collector. The depth of immersion should be at least 400 mm (Fig.22). The cooling water consumption of the ejector unit should be used to establish the size of the warm water collector. As a guide value: volume of the warm water collector = at least double the volume in the barometric legs. During operation, liquid will be drawn up into each barometric leg to a height corresponding to the pressure difference between the condenser vacuum and atmospheric pressure. The distance between the end of the barometric legs and the bottom of the collector should be 200 mm. Moreover it is advisable to have the warm water collector fitted with a drain cock at the lowest point to allow complete emptying and cleaning. The barometric warm water collector must not be made airtight or welded up, and if a lid is fitted, it has to be provided with a sufficiently large vent connection and also this lid should be easily removable for inspection purposes. At short distance above the collector, each barometric leg should be fitted with a flange joint so that it can be blanked off easily if it is necessary to make a pressure test. In order to adjust the separate condensers for the most economic water consumption, the cooling water outlet temperature must be measured. Thermometers should be fitted to the outlet pipes for this purpose and care must be taken to avoid any restriction in these lines. Pag.22

24 Fig. 23 Barometric condensers installation Fig. 24 Thermometer installation Fig. 24 shows a practical thermometer pocket for this purpose which can be filled with a corresponding fluid for better temperature response. If there is not sufficient height for barometric installation then a non-barometric installation is necessary, using a self priming water extraction pump and a sealed intermediate collector. For this type of installation a certain vertical height is still necessary as well as a small suction head for the self priming pump. A minimum of 2 m (7 ft) is required between the cooling water outlet of the condenser and the pump suction connection, for this arrangement. Special instructions for non-barometric installation are supplied if required. Pag.23

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