A Study and Analyze of an Ejector in Steam Power Plant

Transcription

1 Volume-4, Issue-3, June-2014, ISSN No.: International Journal of Engineering and Management Research Available at: Page Number: A Study and Analyze of an Ejector in Steam Power Plant Krishan Pranav 1, Hemant Kumar Ranjan 2, M.P. Singh 3 1,2 M. Tech scholar, Mechanical Department, Jagannath University, INDIA 3 Associate Professor, Mechanical Department, Jagannath University, INDIA ABSTRACT Now a day s Energy consumption is very important for our future. This paper discusses about the use of Ejector in steam power plant to save the heat. The effect of process parameter on vacuum is calculated.from this investigation it is found that the vacuum continue produces to easily maintained. It was observed that the water is smoothly flow in pipe line to followed the path. Keyword- Ejector, Nozzle, Steam, Vacuum I. INTRODUCTION Steam-Jet Ejectors are pumping devices used in general to evacuate fluids from one process step and deliver them to another. A simple example is the removal of air from a closed vessel and discharging it to the atmosphere. The theory behind the Steam Jet Ejector involves converting high-pressure motive steam into a high velocity jet in a converging diverging nozzle. Steam jet Ejectors are based on the ejector-venturi principal and operate by passing motive steam through an expanding nozzle. The nozzle provides controlled expansion of the motive steam to convert pressure in to velocity which creates a vacuum within the body chamber to draw in and entrain gases or vapors. The motive steam and suction gas are then completely mixed and then passed through the diffuser or tail, where the gases velocity is converted in to sufficient pressure to meet the predetermined discharge pressure. Gas and vapors molecules surrounding the high velocity jet are entrained, or swept up, under the principle of viscous drag. The continuous entrainment of molecules near jet, and the jet itself drives the pumping effect and creates the vacuum. Further down past the nozzle tip the two constituents (motive steam and suction load) are mixed violently in the converging section of the venture and recompressed in the straight and diverging section. Upon discharge the stream is at an intermediate pressurehigher than the inlet suction pressure but lower than the motive steam pressure and traveling at a velocity much lower than that of the jet. In order to obtain lower absolute pressure for a given load, higher jet velocities (meaning more steam and thus large components) are required. Eventually, as the required vacuum level deepens, the single stage Ejector becomes too large and uses too much steam to be economically attractive. II. EJECTORS TYPES OF EJECTORS On basis of the motive fluid Liquid Jet Ejectors In a liquid Jet Ejector, the motive fluid is a noncompressible liquid (generally water) with no heat energy hence with a single stage centrifugal pump we cannot increase its pressure much. As a result motive fluid velocity at the nozzle tip is very less and it affects its non-condensable load handling capacity drastically Steam Jet Ejectors In a steam jet Ejector, the motive fluid is a jet of high pressure and low velocity compressible steam which exists from the nozzle at the designed suction pressure and supersonic velocity thus entraining the vapors in to the suction chamber. Due to the supersonic velocity of the steam at the tip of the nozzle, its non-condensable load handling capacity is much higher than liquid jet ejectors from the nozzle at the designed suction pressure and supersonic velocity thus entraining the vapor in to the suction chamber. Due to the supersonic velocity of the steam at the tip of the nozzle, its non-condensable load handling capacity is much higher than liquid jet ejectors Combination of Both 195

2 In this type of ejector system, both Liquid jet and Steam jet Ejectors are employed simultaneously to produce low absolute pressure. The suction of Liquid Jet Ejector is connected to the discharge of last stage of the steam jet ejector. On basis of number of nozzles Single Nozzle Ejector : The uniqueness of Shail Vac s multiple nozzle ejector is in its design and performance. A multiple nozzle ejector usually has one nozzle on centre while remaining of the nozzles equally spaced peripherally around it. In most of the cases, multi nozzle ejectors designed for the same conditions usually reduce steam consumption by 10% to 15% as compared to Single Nozzle Ejector. The advantage of multiple nozzle ejectors is its much higher efficiency compared to single nozzle ejectors. It is also considerably shorter in length than an equally rated single nozzle ejector. III. STEAM JET VACUUM SYSTEMS Ejectors range from Single up to Six Stage units, and can be either Condensing or Non-Condensing types. The number of Ejector stages required are usually determined by the economy of the ejectors and the level of vacuum required. The operating range for each stage of Vacuum Ejector can be seen below, also for reference 1 BarA = 760 mm HgA. 1 st Stage : 810mm HgA - 30mm HgA 2nd Stage : 130mm HgA - 3 mm HgA 3rd Stage : 25mm HgA - 0.8mm HgA 4th Stage : 4mm HgA - 75 microns HgA 5th Stage : 0.4mm HgA - 10 microns HgA 6th Stage : 0.1mm HgA - 3 microns HgA Single Stage Ejectors Single stage Vacuum Ejectors generally covers vacuum ranges from 30mm of Hg A up to atmospheric pressure. To maximize performance eight different designs are available with each ejector being optimized to operate in a specific vacuum range. This allows the motive steam consumption to be kept at a minimum for the selected ejector, and also ensures that operation will be stable. All single stage ejectors are designed to discharge either at or slightly above atmospheric pressure. Sizes range from 1 Inch to 6 Inch, however large size are available if required. Standard materials of construction are carbon steel or stainless steel, both of which are fitted with a stainless steel nozzle. IV. MULTI-STAGE STEAM JET EJECTORS Staging of ejectors becomes necessary for economical operation as the absolute suction pressure decreases. Based upon the use of auxiliary equipment, two and three-stage ejectors can either be condensing or noncondensing types. Four, five, and six-stage units can also be non-condensing, but are usually condensing types Two-Stage Steam Jet Ejector Staging of Ejectors is required for more economical operation when the required absolute vacuum level is reduced. Two stage Vacuum Ejectors generally cover vacuum ranges between 3mm HgA to 130mm HgA, however depending up on actual operating conditions a Single Stage may be more economical if at the upper limit of the operational envelope, or a Three Stage Ejector System if conditions are at the lower end. In operation a two stage system consist of a primary High Vacuum (HV) Ejector and a secondary Low Vacuum (LV) Ejector. Initially the LV ejector is operated to pull vacuum down from the starting pressure to an intermediate pressure. Once this pressure is reached the HV ejector is then operated in conjunction with the LV ejector to finally pull vacuum to the required pressure. Two stage systems can also be either Condensing or Non-condensing types. Condensers can be used as precondensers, inter-condensers, and after-condensers, all of which help to reduce the gas load being passed on to the next ejector stage. This helps to reduce motive steam consumption and also allows smaller ejectors to be used within the system. Depending up on the application Noncondensing systems can also be used, however this can be less efficient than Condensing Types as each ejector must entrain the full gas load from the previous stage. This can lead to ejectors becoming large and also increases motive steam consumption. Non-condensing types are usually used where it is not feasible to install condensers, or where service is intermittent, making operating costs a secondary consideration. 196

3 V. APPLICATIONS Five Two-Stage Steam Jet Ejectors equipped with precondensers installed on the roof of a chemical plant. (Fig. A) Two-Stage Non-Condensing Ejectors permits operation of the pump from a central control panel. (Fig. B) VI. THREE-STAGE STEAM JET EJECTOR Three Stage Ejectors Three stage Vacuum Ejectors generally cover vacuum ranges between 0.8mm HgA to 25mm HgA, however depending up on actual operating conditions a Two Stage Ejector system may be more economical if at the upper limit of the operational envelope, or a Four Stage Ejector system if conditions are at the lower end. In operation a Three Stage system consist of a primary Booster, a secondary High Vacuum (HV) Ejector, and a tertiary Low Vacuum (LV) Ejector. As per the Two Stage System, initially the LV ejector is operated to pull vacuum down from the starting pressure to an intermediate pressure. Once this pressure is reached the HV ejector is then operated in conjunction with the LV ejector to pull vacuum to the lower intermediate pressure. Finally the Booster is operated (in conjunction with the HV & LV Ejectors) pull vacuum to the required pressure. Three stage systems are also usually of the Condensing type. Again as per the Two Stage system, condensers can be used as pre-condensers, intercondensers, and after-condensers in order to reduce the gas load being passed on to the next ejector stage. Depending up on the application Non-condensing systems can also be used however this is less efficient than Condensing Types as each ejector must entrain the full gas load from the previous stage. 197

4 Three-Stage Ejectors are recommended for applications where a two-stage unit will not provide low enough suction pressure economically. Applicable range is from 26 mm Hg. abs. to 0.8 mm Hg. abs. but economics might dictate use of a Two-Stage Ejector at the upper part of the range and a Four-Stage Ejector at the lower end. Three-Stage Condensing Steam Jet Ejectors consist of a booster ejector, a booster condenser, and a Two-Stage Ejector consisting of a high-vacuum ejector, inter condenser, and low vacuum ejector. In some applications another condenser (after-condenser) can be used at the low vacuum ejector discharge. VII. APPLICATIONS Typical Three-Stage Steam Jet Ejector used in processing vegetable oils. Three-Stage Steam Jet Ejector. Three-Stage Steam Jet Ejector with Booster. Four, Five and Six Stage Steam Jet Ejector units are used for applications where required suction pressures are beyond the range of the ejectors previously described. Generally, suction pressure ranges are as follows: Four-Stage Ejectors 4 mm Hg. abs. to 75 Five-Stage Ejectors 0.4 mm Hg. abs. to 10 Six-Stage Ejectors 100 to 3 Four-stage units consists of (1) a primary booster ejector; (2) a secondary booster ejector; (3) a high vacuum ejector; (4) a low vacuum ejector; and (5) usually two condensers one after the secondary booster ejector and the other between the high vacuum and low vacuum ejectors. The condenser between the high and low vacuum ejectors is sometimes omitted, depending upon application requirements. Direct contact or surface condensers, arranged barometrically or at ground level, can be used. The four-stage is similar to the three-stage unit except that another booster ejector is added. In the four-stage, the primary booster is steam-jacketed to prevent build-up of ice on the diffuser internal bore. The five and six-stage units are similar in appearance to the four-stage ejector except that additional booster ejectors are added. While four, five, and six-stage ejectors are usually condensing types for reasons of efficiency and operating economy, it is possible to employ noncondensing types. VII. APPLICATIONS Typical Five Stage Steam Jet Ejector. Steel degassing. Chemical distillation. VIII. MAINTENANCE Four, Five and Six Stage Steam Jet Ejector These systems are similar to Three Stage Systems, however they include additional boosters which are equipped with Steam Jackets to prevent ice forming within the ejectors. These systems are usually of the Condensing type to increase efficiency and reduce motive steam consumption Vacuum equipment maintenance is relatively straight forward, but can vastly improve reliability and performance if approached systematically. Some users by with swapping out parts in periods of dire emergency: performance slowly erodes over a period of time and nothing is done until the unit ceases to effectively operate altogether. At that point spare parts are installed at correct the problem, (if the problem is in fact known and the correct spare parts are in the plan s inventory), and the unit is returned to service. The period of degraded performance leading upto failure and the downtime itself, however, can result in higher operating costs from longer cycle times, increased steam usage and production outages. For this reason, a systematic preventative maintenance (PM) program is highly recommended. 198

5 An effective PM program begins with documentation. 4. Malfunctioning of condensate traps: - Serial numbers, parts lists, performance curves, operating Confirm that the condensate trap provided at the bottom of histories and maintenance procedures should be kept up to the after condenser is functioning properly. Please ensure date by the maintenance or engineering departments for that the traps are designed to operate under vacuum and quick reference. preferably of Thermodynamic type. An effective PM program also included periodic 5. Air Blockage: - Remove air from the inspection and tests, yearly shut downs are perfect channel shell of inter and after condensers by crack opportunities to perform such tests, but an Ejector or opening the valves provided on top of channel shell. barometric Condenser can be disassembled in minutes for 6. Clogged Steam Nozzles: - Clogged steam a thorough visual inspection. A typical PM procedure nozzles with a new installation, pipe scale or dirt from the should include the following: steam lines can clog the EVACTOR steam nozzles, even Disassemble and inspect gaskets for evidence of leaks. when a strainer is use. Remove the nozzles for inspection Check for and document regions of corrosion and wear. and clean thoroughly with emery cloth, particularly the Check nozzle bores and throat diameters against original orifices. Note that additives to the boiler water have been specifications. known to clog nozzles. The deposits collect on the nozzle Clean all internal surfaces (nozzles, heads, throats, walls, and can be easily missed during inspection. diffusers) thoroughly. 7. Varying water pressure: - Especially in the Reassemble with new gaskets and required spare parts, smaller shelf and tube inter and after condensers, changing avoiding misalignment of the components. water pressure can cause trouble either by flooding or Blow down steam strainers and inspect traps. starving the unit. Any surging in the Intercooler water Record what spare parts were taken from inventory and pressure will usually reflect in the performance of the order replacements. complete two stages unit. 8. Rota meter / Flow meter: - Normally, Inspect the condensers for evidence of fouling and outlet vapors, which are coming out of the after condenser corrosion and clean thoroughly. should be vent out through a bypass line of the Rota meter Calibrate the vacuum, pressure and temperature gauges and the Rota meter has to be used only during flow and replace if necessary. measurement. Test the unit in-line against the performance curve with 9. Loop Height: - Please ensure that the either a blanked suction or Piccolo Tube. required loop height is available for condensate removal. If the same is insufficient then backpressure can result. Always use calibrated vacuum gauges or mercury manometers and use the correction factor for the REFERENCES barometric pressure prevailing at site. A properly stocked inventory of spare parts in necessary [1] Performance of an Air-Air Ejector: An Attempt at regardless of the extent of the PM program. At least on Numerical Modelling TASK QUARTERLY 7 No 3 nozzle for each ejector stage should be kept in stock. (2003), Division of Mechanics and Machinery If the desired vacuum is not obtained, it is found after Construction, Oczapowskiego 11, inspection due at one of the following causes: [2] HIGHER EFFICIENCY TARGETING IN A 1. Low Steam Pressure: - An Ejector will STEAM POWER PLANT BY USING PINCH operate with a higher than specified steam pressure but not TECHNOLOGY by Fazlollah ESKANDARI and with a lower pressure. Therefore, never operate the system Masoud BEHZAD, U.P.B. Sci. Bull., Series D, Vol. 71, below the minimum motive steam pressure specified in the Iss. 4, 2009 ISSN data sheet / P & ID. Excessive superheat will have the save [3] AN EXPERIMENTAL INVESTIGATION OF AN effect as low pressure. EJECTOR OPERATING WITH R245FA By David 2. High Back Pressure: - If a stage has a SCOTT, Zine AIDOUN and Mohamed OUZZANE, higher backpressure than it was designed for (see the International Seminar on ejector/jet-pump technology and system datasheet), a loss in vacuum will result. Check for application Louvain-la-Neuve, Belgium Paper No. 21 restrictions or obstructions in the exhaust lines, such as accumulate condensate. Also check water flow and temperatures. 3. Excessive Air Leakage: - Inspect the assembly for opening split pipes, missing gaskets, leaky valves, etc. Hydro-test with water to locate leaks, Make sure that the drain lines are sealed in the condensate loop. An air leak will tend to overload the inter and after Condensers creating a high backpressure condition. 199 Copyright Vandana Publications. All Rights Reserved.