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Using Industrial Hydraulics | Applications of Computer-Aided Manufacturing |
AMAZON multi-meters discounts AMAZON oscilloscope discounts __6.1 Ejectors Jet pumps or injectors operate by converting the potential energy of a motive fluid to kinetic energy of the driving jet thus reducing the static pressure and moving the pumped liquid directly without the assistance of moving mechanical components. The motive medium can be gas, air for example, steam or a liquid, e.g. water. There are different combinations of motive-pumped media. The most usual are water/steam air, water/steam water, air-water, water-water. Jet pumps working with gases as the pumped medium are called ejectors. By applying the momentum equation, equation 3.7, in Section 3, to a control volume, which surrounds the medium in the mixing tube: Certain characteristics of jet pumps can be learned by studying equation 1.3. In order that the pressure P6, exiting the mixing tube, should be greater than P4 the velocity leaving the jet nozzle c4 must be considerably higher than ce and c5. Considerable mixing losses are therefore also unavoidable. Pressure increase is greatest when p = 0. The shear stresses at the wall of the mixing tube have a tendency to reduce the pressure increase. A diffuser is mounted after the mixing tube where velocity is reduced and static pressure increases. The relationship between the pumped mass flow p and the motive fluid mass flow p is called the flow coefficient and is designated: The pressure relationship for a jet pump can be defined further as" total pressure increase of the pumped media Z ~~_ total pressure increase of the motive media. Using these designations, jet pump efficiency can be expressed as: Jet pump losses consist of flow losses in the nozzle, intake chamber, mixing tube and diffuser. The mixing losses constituting the greatest loss. Mixing losses are primarily dependent upon the area relationship: a = motive fluid nozzle outlet area = (;)2 Equl.7 mixing tube cross-sectional area For each combination of pressure relationship z and flow relationship q there is an optimum area relationship a, see Fig. 110. Other important design parameters are the distance between the mouth of the nozzle and the start of the mixing tube; the length of the mixing tube and the angle of the diffuser. The lowest pressure in a liquid jet pump occurs in the upstream section of the mixing tube. If the lowest pressure reaches the liquid's vapor pressure then cavitation will occur. The cavitation number is defined as: If, for a given jet pump, p02 is reduced at the same time as P01 and P03 is adjusted so that the pressure relationship z is maintained constant, the cavitation number s will reduce without initially changing the value of q and h. Further reduction of P02 causes successively more cavitation in the mixing tube and a rapid reduction in efficiency. This value of cavitation number is designated •k, see Fig. 111. Fig. 110 Performance curve for a jet pump Fig. 111 Cavitation in liquid jet pumps Another way of illustrating the onset of cavitation in a liquid jet pump is that at constant motive pressure and back-pressure, P01 and P03 respectively, P02 reduces. The pump's optimum operating point will then follow down the performance curve, z increases, q and s reduce. For 0 > ~k the pump operates without any effects of cavitation. At 0 = ok the performance is reduced in relation to the cavitation free operation. Prolonged operation where cavitation is present can cause damage to the mixing tube and the diffuser. Jet pumps have certain fundamental advantages --No moving parts --No lubrication requirements --No sealing problems --Self-priming, can evacuate the suction line --Non-electrical, no temperature or sparking problems The most obvious disadvantage is the low efficiency. Maximum efficiency being 25 to 30%. But with small flows the power absorbed will be small therefore low efficiency may be of no consequence. Steam jet pumps are normally used to achieve low pressure at the inlet side. For inlet pressures of down to 103 Pa absolute, water steam is generally used as the motive medium. For even lower pressures, down to hundredths Pa, oil steam is used as the motive medium. Practical examples include de-airing condensers, evacuating flammable gases and liquid transportation coupled with simultaneous heating requirements. Compressed air is often available and is the most usual motive medium for gas jet pumps. Some common practical examples of liquid jet pumps, usually using water as the motive medium, are illustrated in Fig. 112. Jet pumps are especially useful if the various combinations of motive and pumped fluid can be mixed simultaneously. Examples of this are steam jet pumps used in ventilation systems where simultaneous humidifying is required and liquid jet pumps for transporting liquids which require simultaneous dilution. An interesting application is where the use of normal refrigerants is prohibited or a green policy is imposed. Here, water is the refrigerant and water is the motive fluid. Water temperatures down to 5 degrees could be produced without difficulty. Leakage would be easily observed, without any hazards, and topping up would be simple. Deep well pumps with ejector Deep well pumps with ejectors complement the previously described automatic water supply packages, in Part. 3.2, when the level of the water in a well or primarily in a borehole, lies more than 5 to 7 meters below the pump. An ejector, or jet pump is placed below the surface of the water in the borehole, which is supplied with motive water from a pump located at ground level. By means of the ejector, the water from the borehole together with the motive water is transported to the pump. It follows that it’s necessary to have two hoses or pipes, as shown in Fig. 113, between the pump and the ejector. Discharge water is taken from a separate outlet on the pump. For multi-stage pumps this outlet is located approximately in the middle of the pump at a suitable pressure. By using an ejector the level of the water can be more than 100 meters below the pump. Inasmuch as the water supply to the borehole is small, dry running of the pump system can be avoided if the ejector is equipped with a suction pipe which is approximately 10 m long. A self-regulating effect is thus created due to cavitation in the ejector. The pump system is primed prior to initial start by filling a small reservoir on the pump. During normal operation, start and stop is performed automatically by the pressure in an accumulator. Fig. 112 Liquid jet pumps practical examples Fig. 113 Deep well pump with ejector Fig. 114 Barrel-emptying pumps for fluids and viscous liquids Motive water Ejector Suction pipe L. (water intake) Diffuser for pressure conversion To pump Ejector nozzle Suction pipe (water intake) __6.2 Barrel-emptying pumps (powered and hand pumps) Barrel-emptying pumps are designed for emptying small containers and tanks. They consist either of a motor section or immersion tube and a pump section. See Fig. 114. The pump section is located in the lower end of the immersion tube and is driven by the motor via an extended shaft. The shaft is protected by a sealed column. The pumped liquid flows between the column and the extended shaft to the pump's outlet at the motor end. For obvious reasons, a barrel-emptying pump should be lightweight and easy to transport to the next container to be emptied. These pumps are manufactured with a variety of immersion tube lengths, between 210 and 1200 mm. They can be made from many materials having good resistance to chemical corrosion. Pumps can be fitted with low voltage, totally enclosed electric motors, motors for use in potentially hazardous atmospheres and air motors. The pump design for fluid liquids is of the centrifugal or mixed-flow type whilst screw or progressing cavity pump types are used for viscous liquids. Pumps are capable of flows up to 14.5 m^3/h and heads of 25m. Semi-rotary hand pumps and rotary hand pumps When power of any type is not available muscle-power must be used. Semi-rotary hand pumps are similar to vane pumps but with valves. One set of valves are in the vanes, the others in the casing. On the filling stroke, the valve in the vane opens to allow liquid to change sides. When the vane is moved in the opposite direction, the liquid is squeezed in the casing opening the casing valve. Pumps normally have two chambers providing a suction and discharge stroke for each action. Semi-rotary hand pumps have good suction lift capabilities and work well with fairly viscous liquids. As well as barrel-emptying applications, these pumps are often used for priming lube and fuel oil systems on large engines, pumps, compressors and other equipment. __6.3 Hydraulic-ram pumps The principle of a hydraulic-ram is illustrated in Fig. 115. The supply line must be of a certain length, usually 5 to 20 m. The air vessel is located 0.5 to 3 m below the water surface and pumps water up to a higher level, usually a tank for fresh water supply. If the waste valve suddenly closes, the mass of water in the supply pipe is subjected to rapid deceleration which causes a rise in pressure at the waste valve and delivery valve. The delivery valve opens and water is forced up into the air vessel. When the water in the supply line momentarily changes direction, a partial vacuum is created at the waste valve causing it to open, whereupon water begins to flow in the supply line again. The action of water flowing through the waste valve causes it to close again and the cycle is repeated. The following applies for the hydraulic ram: = Motive water velocity in supply line (m/s) = Delivered velocity in delivery line (m/s) = Supply head(m) = Delivery head(m) = Efficiency (non-dimensional) Fig. 115 Diagrammatic arrangement of the hydraulic-ram pump Fig. 116 Efficiency of hydraulic-ram pump Fig. 117 Hydraulic-ram pump. Papa Pump Ltd. The efficiency depends upon the quotient z, Hp/Hd according to Fig. 116. By means of comparison, it may be noted that for large scale water delivery via a water turbine, electric grid system and water pump, efficiencies of up to about 50% can be obtained, whereas for domestic use via electric grid and water pump the efficiency is usually less than 10%. The hydraulic ram has long been used for transporting fresh water for domestic and agricultural purposes. There are installations which have been in use for more than 50 years. Hydraulic rams are sold in many sizes for supply water flows Qd from 60 I/h to 9 m^3/h with total delivery heads of up to about 40 m. Fig. 117 shows a section through a typical pump. __6.4 Air-lift pumps The air-lift pump consists of a riser tube immersed in the pumped liquid, a compressed-air line and a pressure chamber where air forces the liquid, via small holes, into the riser tube, as shown in Fig. 118. The air/liquid mixture is lighter than the surrounding liquid and therefore rises up the tube. The flow of liquid up the riser tube increases as the flow of air is increased up to a certain maximum value, after which it begins to decrease. The air supply must be compressed to a pressure which is equivalent to the immersed depth plus the losses in the piping and inlet holes. The product of the work done during compression and the air mass flow, neglecting the efficiency of the compressor, is equal to the power input to the pump P ~ input. Fig. 118 Air-lift pump Fig. 119 Examples of maximum efficiency and air requirement for air-lift pump Pump efficiency neglects compressor efficiency The pump efficiency n ~ Liquid --H q =Equ 11 input is greatly dependent upon the immersed depth in relation to the delivery head, Fig. 119. The quantity of air required in kg per cubic meter of pumped liquid, designated L, is greatly affected by the immersed depth. The advantages of the air-lift pump are: --Simple construction, no moving parts --No sealing problems --Small risk of blockage --Not sensitive to temperature The disadvantages are: --Low efficiency --Immersed depth requirements, S/H >1 --Compressed air can be expensive Air-lift pumps are used for pumping sludge, contaminated liquid, large particles, sugar beets, and also hot or corrosive fluids. __6.5 Contraction pumps A contraction pump consists of a special rubber hose which is reinforced in such a way that when it’s stretched by means of applying an axial force, the diametral contraction is such that the volume is reduced. Pumping effect is achieved by fitting non-return valves to each end of the hose, suction and discharge valves. Suction is achieved as a result of the hose returning to its original length and volume as a result of releasing the axial force. Efficiency is very good since the only losses are valve losses and hysteresis losses in the rubber, which can be reduced to a minimum within the pump's most suitable operating range. Typical values are 95% to 98%. Applications include hand operated or motor driven deep well pumps for well depths of 30 to 100 m and borehole diameters of 50 to 150 mm. Pump performance depends upon the type of hose. Normally hose stretch is 10% of its length and the transported volume is 5 to 20% of the internal hose volume. For hand operated pumps the flow volume is within the region of 5 to 20 I/min depending upon the depth of the well. A particular advantage is the simplicity of installation and removal and the single tube, which functions both as a pull-rod and a delivery pipe, resulting in considerable weight and cost savings. __6.6 Macerators Some solids handling pumps include a macerator in the pump inlet. A macerator is a type of solids grinding or cutting machine which ensures solids entering the pump are below a certain size. Macerators cannot normally be used for hard, strong solids such as gravel, copper ore or tailings. They are most often used for long stringy solids as found in pulp, sewage and rubbish handling. The macerator ensures that solids entering the pump will pass right through the pump without fouling. Some macerators may be capable of handling soft coal. Extra power must be available to cover the macerator's requirements. __7 Useful references Fire & Explosion ATEX Regulations Part 1. The Regulations implement a European Directive called, the ATEX Directive No. 1992/92, concerned with the risks from fire and explosion arising from flammable substances stored or used in the workplace. The Regulations apply from 1st July 2003 to new workplaces or workplaces which undergo modifications, extensions or restructuring after July 2003. Existing workplaces must comply by 30th June 2006, but it should be noted that the requirements of this legislation are replicated in other legislation without such a time provision and reference should be made to the Safety, Health and Welfare at Work (General Application) Regulations the Safety, Health and Welfare at Work (Chemical Agents) Regulations, 2001. ATEX Manufacturers Directive 94/9/EC. This directive defines each of the following product groups for use in potentially explosive atmospheres: Electrical and non-electrical equipment, Electrical and non-electrical protective systems, Electrical and non-electrical components, Electrical and non-electrical safety devices, It places responsibilities on the manufacturer of these products. ATEX User Directive 99/92/EC. This directive is concerned with the health and safety of workers with relation to potentially explosive atmospheres. It places responsibilities on an employer. Dangerous Substances and Explosive Atmospheres Regulations (DSEAR). From July 2006 employers must have completed the risk assessment, classification and documentation of their workplace and personnel. ISO 2858 End-suction centrifugal pumps (rating 16 bar) Designation, nominal duty point and dimensions. ISO 3069 End-suction centrifugal pumps Dimensions of cavities for mechanical seals and for soft packing. ISO 2858End-suction centrifugal pumps (rating 16 bar) Designation, nominal duty point and dimensions. ASME/ANSI B73.1 Specification for Horizontal End Suction Centrifugal Pumps for Chemical Process. ISO 13709 Centrifugal pumps for petroleum, petrochemical and natural gas industries. ANSI/API STD 610 Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries. ISO 13709 Centrifugal pumps for petroleum, petrochemical and natural gas industries. ASME/ANSI B73.2 Specifications for Vertical In-Line Centrifugal Pumps for Chemical Process. ISO 21049 Pumps Shaft sealing systems for centrifugal and rotary pumps. API 682 Shaft Sealing Systems for Centrifugal and Rotary Pumps. |