Causes of vibration of the fan impeller. Consequences of untimely balancing of impellers of smoke exhausters

Increased vibration of the fan is one of its main "troubles", causing premature failure of components, parts, impeller, blades, bearings, couplings, destruction of the foundation and the fan itself as a whole.

Causes of fan vibration:

• shaft imbalance;
• misalignment of the drive;
• wear or damage to bearings;
• defects in the electromagnetic part of the drive (electric motor);
• gear defects (if there is an intermediate gearbox);
• influence of aerohydrodynamic forces;
• resonant phenomena, etc.

The fan vibration level most accurately reflects the current technical condition of the fan, the quality of its assembly and installation. In other words, by controlling the vibration level of the fan, it is possible to identify all the above-mentioned flaws and take timely measures to eliminate them, ensuring trouble-free operation of the fan.

The method for measuring the vibration of industrial fans with a power of up to 300 kW is regulated, and more powerful - GOST ISO 10816-3. In this article, we will consider industrial fans with a power of up to 300 kW and a method for monitoring their vibration state in order to determine some basic level vibrations and tendencies of its change.

First of all, we note that all industrial fans with a power of up to 300 kW are classified according to the level of permissible vibration and unbalance into the BV category (see Table 1):

In accordance with the requirements of GOST 31350-2007 (ISO 14694:2003), vibration measurements are carried out on bearings in directions perpendicular to the axis of rotation of the shaft. Recommended measurement points are shown in fig. one.


a) for a horizontal axial fan


b) for horizontal centrifugal single inlet fan

c) for horizontal double inlet radial fan

d) for vertical axial fan

Figure 1. Points and directions of fan vibration measurements

Measurements of absolute vibration on bearing supports are made using BALTECH VP-3410 vibrometers (VibroPoint series) with inertial type contact sensors - piezoaccelerometers (acceleration sensors). When carrying out measurements, one should clearly observe the standard requirements for the reliability of fastening, installation direction, and the absence of a significant effect of the mass and dimensions of the sensor on the measurement results. In general, a total measurement uncertainty within ± 10% of the measured parameter is allowed. BALTECH vibration meters are universal and allow, depending on the requirements of the fan manufacturer, to measure three vibration parameters (vibration displacement, vibration velocity or vibration acceleration).

Permissible vibration limits for fans during operation are given in Table 2. It should be noted that due to the mass and rigidity of the support system at the place of operation, these values ​​are slightly higher than the vibration values ​​during factory tests.

Table 2. Vibration limit values ​​during fan operation.

All new fans must meet the “Commissioning” level. As parts are used and worn out, the fan vibration level inevitably increases, and when the “Warning” level is reached, it is necessary to investigate the causes of increased vibration and take measures to eliminate them. The operation of the fan in this state should be limited in time until repair work is carried out.

When the "Stop" level is reached, the fan must be immediately stopped and measures taken to eliminate sources of critical vibration levels. Failure to do so may result in serious damage leading to the destruction of the fan. In general, based on the statistics of operation of fan equipment, it is considered necessary to take measures to eliminate sources increased vibration when its level exceeds the base value by 1.6 times or 4 dB.

When monitoring fan vibration, it is important to pay special attention to the abrupt change in the vibration level over time. A jump in vibration is a clear indication of the occurrence of some kind of malfunction, and in this case it is necessary to inspect the fan and eliminate the detected shortcomings.

In some cases, the displacement of the shaft relative to the bearing housing is additionally measured using non-contact vibration sensors - induction, eddy current, etc. Table 3 shows the permissible values ​​​​of the shaft displacement, which should be understood only as recommended - in fact, these values ​​\u200b\u200bmay be different in depending on the type and dimensions of the plain bearing, the magnitude and direction of the load, etc.

Table 3. Maximum displacement of the shaft inside the bearing

Vibration control and vibration monitoring of fans is most conveniently carried out using a portable portable device "PROTON-Balance-II". Its main advantage over simple vibrometers is the ability to balance fans in their own supports in accordance with the requirements of GOST 31350-2007 (ISO 14694:2003), as well as temperature control of bearing assemblies and fan speed control.

To learn how to measure the vibration of fans and gain skills in working with the vibrometer-balancer "PROTON-Balance-II" and other vibrometers of the company "BALTECH", it is recommended to take the course TOR-103 "Fundamentals of vibration diagnostics. Vibration fans GOST » in training center advanced training of our company in St. Petersburg, in Astana or in Lübeck (Germany).

Causes of damage to draft machines

The causes of damage to draft machines during operation can be mechanical, electrical and aerodynamic.

The mechanical reasons are:

Imbalance of the impeller as a result of wear or deposits of ash (dust) on the blades;
- wear of the elements of the coupling: loosening of the fit of the impeller bushing on the shaft or loosening of the impeller braces;
- weakening of the foundation bolts (in the absence of lock nuts and unreliable locks against unscrewing the nuts) or insufficient rigidity supporting structures machines;
- weakening of the tightening of the anchor bolts of the bearing housings due to the installation of uncalibrated gaskets under them during alignment;
- unsatisfactory alignment of the rotors of the electric motor and draft machine;
- excessive heating and deformation of the shaft due to increased temperature flue gases.

The reason for the electrical character is a large non-uniformity of the air gap between the rotor and the stator of the electric motor.

The reason for the aerodynamic nature is a different performance on the sides of smoke exhausters with double suction, which can occur when one-sided skidding of the air heater with ash or improper adjustment of dampers and guide vanes.

In the suction pockets and volutes of draft machines transporting a dusty environment, the shells, as well as the suction funnels of the volutes, are subject to the greatest abrasive wear. The flat sides of the volutes and pockets wear out to a lesser extent. On axial smoke exhausters of boilers, the body armor wears out most intensively at the locations of guide vanes and impellers. The intensity of wear increases with an increase in the flow rate and the concentration of coal dust or ash particles in it.

Causes of vibration of draft machines

The main causes of vibration of smoke exhausters and fans can be:

a) unsatisfactory balancing of the rotor after repair or imbalance during operation as a result of uneven wear and damage to the blades near the impeller or damage to the bearings;
b) incorrect alignment of the shafts of machines with an electric motor or their misalignment due to wear of the coupling, weakening of the supporting structure of the bearings, deformation of the linings under them, when many thin uncalibrated gaskets are left after alignment, etc.;
c) increased or uneven heating of the smoke exhauster rotor, which caused shaft deflection or deformation of the impeller;
d) unilateral drift of air heater ash, etc.

Vibration increases when the natural vibrations of the machine and the supporting structures coincide (resonance), as well as when the structures are not sufficiently rigid and the foundation bolts are loosened. The resulting vibration can lead to a weakening bolted connections and clutch fingers, keys, heating and accelerated wear of bearings, breakage of bolts for fastening bearing housings, beds and destruction of the foundation and machine.

Prevention and elimination of vibration of draft machines requires comprehensive measures.

During the acceptance and delivery of the shift, they listen to smoke exhausters and fans in operation, check the absence of vibration, abnormal noise, the serviceability of the attachment to the foundation of the machine and the electric motor, the temperature of their bearings, and the operation of the coupling. The same check is made when walking around the equipment during a shift. When defects are found that threaten an emergency stop, they inform the shift supervisor to take the necessary measures and strengthen the supervision of the machine.
Vibrations of rotating mechanisms are eliminated by balancing and centering them with an electric drive. Before balancing, the necessary repair of the rotor and bearings of the machine is carried out.

Causes of bearing damage

In draft machines, rolling and sliding bearings are used. For plain bearings, inserts of two designs are used: self-aligning with a ball bearing and with a cylindrical (rigid) bearing surface for fitting the insert into the housing.

Bearing damage may be due to oversight of personnel, defects in their manufacture, unsatisfactory repair and assembly, and especially poor lubrication and cooling.
Abnormal operation of bearings is identified by an increase in temperature (above 650 ° C) and a characteristic noise or knock in the housing.

The main reasons for the temperature increase in bearings are:

Contamination, insufficient amount or leakage of grease from bearings, mismatch of the lubricant to the operating conditions of draft machines (too thick or thin oil), excessive filling of rolling bearings with grease;
- the absence of axial clearances in the bearing housing necessary to compensate for the thermal elongation of the shaft;
- small landing radial clearance of the bearing;
-small working radial clearance of the bearing;
- sticking of the lubrication ring in plain bearings at very high level oil that prevents free rotation of the ring, or damage to the ring;
- wear and damage of rolling bearings:
paths and rolling elements crumble,
cracked bearing rings
the inner ring of the bearing is loose on the shaft,
crushing and breakage of rollers, separators, which is sometimes accompanied by a knock in the bearing;
- violation of the cooling of bearings with water cooling;
- unbalance of the impeller and vibration, which sharply worsen the load conditions of the bearings.

Rolling bearings become unsuitable for further work due to corrosion, abrasive and fatigue wear, and destruction of cages. Rapid bearing wear occurs in the presence of a negative or zero working radial clearance due to the temperature difference between the shaft and the housing, incorrectly selected initial radial clearance or incorrectly selected and performed fit of the bearing on the shaft or in the housing, etc.

During installation or repair of draft machines, bearings should not be used if they have:

Cracks on rings, separators and rolling elements;
- nicks, dents and peeling on the tracks and rolling elements;
- chips on the rings, working sides of the rings and rolling elements;
- separators with destroyed by welding and riveting, with unacceptable sagging and uneven spacing of windows;
- discoloration on rings or rolling elements;
- longitudinal flats on rollers;
- excessively large gap or tight rotation;
- residual magnetism.

If these defects are found, the bearings should be replaced with new ones.

To ensure that the rolling bearings are not damaged during disassembly, the following requirements must be observed:

The force must be transmitted through the ring;
- axial force must coincide with the axis of the shaft or housing;
- impacts on the bearing are strictly prohibited, they should be passed through a soft metal drift.

Apply press, thermal and impact methods of mounting and dismounting of bearings. If necessary, these methods can be used in combination.

When disassembling bearing supports, control:

Condition and dimensions of the housing and shaft seating surfaces;
- the quality of the bearing installation,
- alignment of the housing relative to the shaft;
- radial clearance and axial play,
- condition of rolling elements, separators and rings;
- lightness and lack of noise during rotation.

The greatest losses occur when placing a turn in the immediate vicinity of the outlet of the machine. A diffuser should be installed directly behind the outlet of the machine to reduce pressure losses. When the diffuser opening angle is greater than 200, the diffuser axis must be deflected in the direction of rotation of the impeller so that the angle between the extension of the machine shell and the outer side of the diffuser is about 100. When the opening angle is less than 200, the diffuser should be made symmetrical or with the outer side, which is a continuation of the machine shell . The deviation of the diffuser axis in the opposite direction leads to an increase in its resistance. In a plane perpendicular to the plane of the impeller, the diffuser is symmetrical.

Causes of damage to impellers and casings of smoke exhausters

The main type of damage to impellers and casings for smokers is abrasive wear during transportation of a dusty environment due to high speeds and high concentration entrainment (ash) in flue gases. The main disk and blades wear out most intensively in the places of their welding. Abrasive wear of impellers with forward-curved blades is much greater than that of impellers with backward-curved blades. During the operation of draft machines, corrosion wear of the impellers is also observed during the combustion of sulphurous fuel oil in the furnace.
Wear zones of sheet blades must be hardfaced. The wear of the blades and disks of the rotors of smoke exhausters depends on the type of fuel burned and the quality of the operation of the ash collectors. Poor operation of ash collectors leads to their intensive wear, reduces strength and can cause unbalance and vibrations of machines, and wear of casings leads to leaks, dusting and traction deterioration.

Reducing the intensity of erosive wear of parts is achieved by limiting the maximum speed of the rotor of the machine. For smoke exhausters, the rotational speed is taken to be about 700 rpm, but not more than 980.

Operational methods to reduce wear are: operation with a minimum excess of air in the furnace, elimination of air suction in the furnace and gas ducts, and measures to reduce losses from mechanical underburning of fuel. This reduces flue gas velocities and the concentration of ash and entrainment in them.

Reasons for the decline in the performance of draft machines

The performance of the fan deteriorates when the impeller blades deviate from the design angles and when their manufacture is defective. It must be taken into account. that when surfacing with hard alloys or strengthening the blades by welding linings in order to lengthen their service life, a deterioration in the characteristics of the smoke exhauster may occur: excessive wear and improper anti-wear armor of the smoke exhauster body (reduction of flow sections, increase in internal resistances) leads to the same consequences. Defects in the gas-air path include leaks, cold air suction through the blower hatches and places where they are embedded in the lining, manholes in the boiler lining. idle burners, passages of permanent blowing devices through the boiler lining and tail heating surfaces, peepers in the combustion chamber and pilot holes for burners, etc. As a result, the volume of flue gases and, accordingly, the resistance of the path increase. Gas resistance also increases when the tract is contaminated with focal residues and when the mutual arrangement of the superheater and economizer coils is disturbed (sagging, interlacing, etc.). The reason for the sudden increase in resistance may be a break or jamming in the closed position of the damper or the guide apparatus of the smoke exhauster.

The occurrence of leaks in the gas path near the smoke exhauster (open manhole, damaged explosive valve, etc.) leads to a decrease in vacuum in front of the smoke exhauster and an increase in its performance. The resistance of the tract to the place of leakage drops, since the smoke exhauster works to a greater extent to suck air from these places, where the resistance is much less than in the main tract, and the amount of flue gases taken from it from the tract decreases.

The performance of the machine deteriorates with an increased flow of gases through the gaps between the inlet pipe and the impeller. Normally, the diameter of the pipe in the clear should be 1-1.5% less than the diameter of the inlet to the impeller; axial and radial clearances between the edge of the pipe and the entrance to the wheel should not exceed 5 mm; the displacement of the axes of their holes should not be more than 2-3 mm.

In operation, it is necessary to promptly eliminate leaks in the places where the shafts pass and near the housings due to their wear, in the gaskets of the connectors, etc.
In the presence of a bypass duct of a smoke exhauster (forward running) with a loose damper, a reverse flow of ejected flue gases into the suction pipe of the smoke exhauster is possible in it.

Recirculation of flue gases is also possible when two exhausters are installed on the boiler: through the left exhauster - to another working one. With the parallel operation of two smoke exhausters (two fans), it is necessary to ensure that their load is the same all the time, which is controlled by the readings of the ammeters of the electric motors.

In the event of a decrease in productivity and pressure during the operation of draft machines, the following should be checked:

Direction of rotation of the fan (smoke exhauster);
- the condition of the impeller blades (wear and accuracy of surfacing or lining installation);
- according to the template - the correct installation of the blades in accordance with their design position and angles of entry and exit (for new impellers or after replacing the blades);
- compliance with the working drawings of the configuration of the volute and the walls of the body, the tongue and the gaps between the confuser; accuracy of installation and completeness of opening of dampers before and after the fan (smoke exhauster);
- rarefaction in front of the smoke exhauster, pressure after it and pressure after the blower fan and compare with the previous one;
- tightness in the places where the shafts of the machine pass, if a leak is detected in them and in the air duct, eliminate it;
- the density of the air heater.

To ensure trouble-free and reliable operation of fans and smoke exhausters, it is necessary:
- systematically monitor the lubrication and temperature of the bearings, prevent contamination of lubricating oils;
- fill rolling bearings with grease for no more than 0.75, and at high speeds of the draft mechanism - no more than 0.5 of the volume of the bearing housing in order to avoid heating them. The oil level should be at the center of the lower roller or ball when filling the rolling bearings with oil. The oil bath of ring lubricated bearings should be filled up to the red line on the oil sight glass indicating normal oil level. In order to remove excess oil when the housing is overfilled above the permissible level, the bearing housing must be equipped with a drain tube;
- to provide continuous water cooling of bearings of smoke exhausters;
- to be able to control the discharge of water cooling the bearings must be carried out through open pipes and drain funnels.

When disassembling and assembling plain bearings, replacing parts, the following operations are repeatedly controlled:
a) checking the centering of the housing in relation to the shaft and the tightness of the lower half-liner;
b) measurement of the upper, side gaps of the liner and the tightness of the liner by the housing cover;
c) the condition of the babbit surface of the liner filling (determined by tapping with a brass hammer, the sound must be clear). total area peeling is allowed no more than 15% in the absence of cracks in the places of peeling. Peeling is not allowed in the area of ​​the stubborn collar. The difference in diameters over different sections of the insert is no more than 0.03 mm. In the bearing shells on the working surface, the absence of gaps, scratches, nicks, shells, porosity, foreign inclusions is checked. The ellipticity of the lubrication rings is allowed no more than 0.1 mm, and the non-concentricity at the split points - no more than 0.05 mm.

Service personnel should:
- monitor the instruments so that the temperature of the exhaust gases does not exceed the calculated one;
- carry out scheduled inspection and maintenance of smoke exhausters and fans with oil change and bearing flushing, if necessary, elimination of leaks, checking the correctness and ease of opening of gates and guide vanes, their serviceability, etc.;
- close the suction openings of the blower fans with nets;
- make a thorough acceptance of spare parts arriving for replacement during the overhaul and current repairs draft machines (bearings, shafts, impellers, etc.);
- to test draft machines after installation and overhaul, as well as the acceptance of individual units during installation (foundations, support frames, etc.);
- do not allow acceptance into operation of machines with bearing vibration of 0.16 mm at a speed of 750 rpm, 0.13 mm at 1000 rpm and 0.1 mm at 1500 rpm.

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Rice. 6.7 (I - good; P - satisfactory TS; W - unsatisfactory).

The given norms refer to measurements in octave bands, in which f o falls. When measured in 1/3 octave, these norms should be reduced by 1.2 times.

6.7. Centrifugal separators

The assessment of the vehicle is carried out according to the correctness of their functioning, in particular, the performance, the degree of fuel purification, starting characteristics and operation of the controls. The presence of faults is determined by the level of shock impulses, vibration, by inspection and non-destructive testing.

Quality their work is evaluated by the water content in the fuel and oil (up to 0.01%) and the content of mechanical impurities (metal particles no more than 1-3 microns, carbon particles no more than 3-5 microns). The optimal viscosity of the oil product during separation is 13-16 cSt, and the maximum viscosity is 40 cSt. The maximum water content in treated fuel and oil is achieved when the separator is controlled at 65-40% of the nominal capacity.

Control by the power consumed by the separator (strength of current) during start-up and operation, as well as the start-up time, it allows you to determine the TC of the separator drive (brake, worm gear) and the quality of the drum self-cleaning. With a good TS, the starting time should be less than 7 minutes, with a satisfactory one - (7-12) minutes. and unsatisfactory - more than 12 minutes.

With a good TC, the load current on the separator motor should be in the range (14.5 - 16.5 A), unsatisfactory - more than 45 A (for example, for the MARCH 209 separator).

Examination The TC of the separator can be carried out by opening and closing the drum. Here the following are possible situations, for example, with unsatisfactory TS;

The drum does not close when water is supplied to form a hydraulic seal, it does not flow out of the separated water pipe after 10-15 s;

The drum does not open, the drum is not cleaned at the appropriate position of the mechanism control valve;

The drum remains open (or opens) when the mechanism control valve is switched to the position corresponding to separation.

The condition of the upper bearing located in the damper device is assessed by measuring the level of shock pulses on the separator housing carrying the damper device. The degree of TS is determined by establishing a relative change in the level of impulses from a known good TS. Its increase by 2 times indicates that the bearing has reached the limit value. The condition of the vertical shaft lower bearing is monitored at a point located on the bearing housing.

The condition of mounted gear pumps is monitored by the level of shock pulses on the pump housing. It should be borne in mind that the level of shock pulses on the pump housing increases when operating on good fuel.

The vibration level of the separator according to the vibration velocity is determined at the frequencies of the drive (f pr) and the drum (f bar). Depending on the vehicle, it may prevail at one of these frequencies. The levels of vibration velocity depending on the power for various categories of TC separators are shown in fig. 6.8. .

Separator vibration standards

Rice. 6.8. (I - good TS; P - satisfactory; III - unsatisfactory).

The given vibration velocity levels refer to the main elements of the separator (horizontal and vertical drives), the separator drive electric motor and mounted pumps. The norms refer to measurements in octave bands, which fall f pr and f bar. When measured in 1/3 octave, these norms should be reduced by 1.2 times.

The level of the TS of the separator can also be determined during their inspection by measuring the units (for example, determining the position of the pressure and control disk in height, the junction of the locking ring by marks, the position in height, the beating of the upper part of the drum shaft, the gap in the seal of the movable bottom of the drum) and checking condition of all seals. Inspection of the worm gear and brake is usually combined with cleaning and disassembly of the separator drum.

Non-destructive testing of the drum and its shaft in the area of ​​the drum landing and the threaded connection on the shaft of the drum fastening nut is carried out during the next survey.

6.8. Piston compressors

Their vehicle can be judged on correct functioning, in particular performance and parameters compressed air. The presence of faults is determined by the level of shock impulses, vibration, temperature of parts, as well as during inspection and in the process of non-destructive testing.

As basic characteristics of reciprocating compressors, it is recommended to use the relative reduction in performance.

σV \u003d [(V ref - V ks) / V ref ] * 100% , (6.4)

where V ref - nominal capacity; m 3 / h

V ks \u003d 163 * 10 3 - compressor performance during control; m 3 /h;

V δ is the volume of the air retainer filled during the control, m 3 ;

P 1 , P 2 - air pressure in the air retainer, respectively, at the beginning and end of the control MPa;

T 2 - temperature of the surface of the air guard, K;
Θ - time of pressure increase in the air receiver from the value of P 1 to P 2 , min.

Norms relative performance degradation for three TC categories are: I - (good) -< 25 %; П (удовлетво­рительное) - (25-40)%; Ш (неудовлетворительное) - >40 %.

Another way to assess the TC of compressors is to monitor the level of vibration. It is measured in the vertical plane on the cylinder heads (on the compressor axis) and in the horizontal plane on the upper edges of the cylinder block (on the cylinder axis).

Level vibration velocity, measured in the horizontal plane at the main crankshaft speed, makes it possible to judge the state of fastening and clearances in the frame bearings, and at frequencies 2f 0 and 4f 0 - about the clearances between the piston and the bushing, as well as the condition of the rings. Similar measurements made in the vertical plane at the same frequencies make it possible to estimate the size of the gaps in the head and crank bearings. It should be noted that vibration associated with head bearing failures can occur at a frequency of 500 to 1000 Hz.

Typical vibration spectra of compressors are shown in fig. 6.9..

Noise and vibration control When installing fans, it is necessary to fulfill certain requirements common to different types of these machines. When installing fans of other designs, it is very important to carefully center the geometric axes of the fan and motor shafts if they are connected using couplings. In the presence of a belt drive, it is necessary to carefully control the installation of the fan and motor pulleys in the same plane, the degree of tension of the belts and their integrity. The suction and exhaust ports of the fans are not...

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Installation of fans. Noise and vibration control

When installing fans, it is necessary to fulfill certain requirements common to different types of these machines. Before installation, it is necessary to check the compliance of the fans and electric motors intended for installation with the project data. Particular attention should be paid to the direction of rotation of the impellers, to ensure the required clearances between rotating and stationary parts, to check the condition of the bearings (no damage, dirt, lubrication).

The easiest installationelectric fans(design 1, see lecture 9). When installing fans of other designs, it is very important to carefully center the geometric axes of the fan and motor shafts if they are connected using couplings. In the presence of a belt drive, it is necessary to carefully control the installation of the fan and motor pulleys in the same plane, the degree of tension of the belts, and their integrity.

Shafts at radial fans must be strictly horizontal, roof fan shafts strictly vertical.

Motor housings must be grounded, couplings and belt drives protected. Suction and exhaust openings of fans not connected to air ducts must be protected with meshes.

indicator good quality fan mounting is to minimize vibration. vibrations these are oscillatory movements of structural elements under the action of periodic perturbing forces. The distance between the extreme positions of the oscillating elements is called vibration displacement. The speed of movement of points of vibrating bodies varies according to a harmonic law. RMS speed value is normalized for fans ( v  6.7 mm/s).

If the installation is carried out correctly, then the cause of vibrations isunbalanced rotating massesdue to uneven distribution of material around the circumference of the impeller (due to uneven welds, the presence of shells, uneven wear of the blades, etc.). If the wheel is narrow, then the centrifugal forces caused by the imbalance R , can be considered located in the same plane (Fig. 11.1). In the case of wide wheels (the width of the wheel is more than 30% of its outer diameter), a couple of forces (centrifugal) may appear, periodically changing their direction (with each revolution), and therefore also causing vibrations. This so-calleddynamic imbalance(as opposed to static).

Rice. 11.1 Static (a) and dynamic (b) 11.2 Static balancing

unbalance of the impeller

When static imbalance, to eliminate it, static balancing is used. To do this, the impeller fixed on the shaft is placed on balancing prisms (Fig. 11.2), installed strictly horizontally. In this case, the impeller will tend to take a position in which the center of unbalanced masses is in the lowest position. The balancing weight, the value of which is determined experimentally (by several attempts), must be installed in the upper position and, in the end, be securely welded to the rear surface of the impeller.

Dynamic imbalance with a non-rotating rotor (impeller) does not manifest itself in any way. Therefore, manufacturers must dynamically balance all fans. It is performed on special machines with the rotation of the rotor on flexible supports.

Thus, the fight against vibrations begins with balancing the impellers. Another way to reduce fan vibrations is to install them onvibration-isolating bases. In the simplest cases, rubber gaskets can be used. However, special springs are more effective. vibration isolators , which can be supplied complete with fans by manufacturers.

In order to reduce the transmission of vibrations from the supercharger through the air ducts, the latter must be connected to the fan usingsoft (flexible) inserts, which are cuffs made of rubberized fabric or tarpaulin 150-200 mm long.

Both vibration isolators and flexible connectors do not affect the magnitude of the supercharger vibration, they only serve to localize it, i.e. they do not allow it to spread from the supercharger (where it originates) to the building structures on which the supercharger is installed, and to the air duct (pipeline) system.

Vibrations of the structural elements of fans are one of the sources of noise generated by these machines. Noise is defined as sounds that are perceived negatively by a person and are harmful to health. Fan noise caused by vibrations is calledmechanical noise(this also includes noise from the bearings of the electric motor and the impeller). Therefore, the main way to combat mechanical noise is to reduce fan vibrations.

Another major component of fan noiseaerodynamic noise. In general, noises are all sorts of unwanted sounds that irritate a person. Sound is quantified sound pressure, but when normalizing noise and in noise attenuation calculations, the relative value noise level in dB (decibels) is used. The sound power level is also measured. In general, noise is a collection of sounds of different frequencies. The maximum noise level occurs at the fundamental frequency:

f=nz/60 , Hz;

where n rotation speed, rpm, z number of impeller blades.

Noise characteristicfan is usually called a set of values ​​of sound power levels of aerodynamic noise in octave frequency bands (i.e. at frequencies of 65, 125, 250, 500, 1000, 2000 Hz (noise spectrum)), as well as the dependence of the sound power level on flow.

For most blowers, the minimum level of aerodynamic noise corresponds to the nominal operating mode of the blower (or is close to it).

Installation of pumps. The phenomenon of cavitation. suction height.

The requirements for the installation of blowers in terms of eliminating vibrations and noise fully apply to the installation of pumps, however, when talking about the installation of pumps, it is necessary to keep in mind some features of their operation. The simplest circuit pump installation is shown in fig. 12.1. Water through the inlet valve 1 enters the suction pipeline and then into the pump, and then through the check valve 2 and the gate valve 3 into the pressure pipeline; the pumping unit is equipped with a vacuum gauge 4 and a pressure gauge 5.

Rice. 12.1 Schematic pumping unit

Since, in the absence of water in the suction pipeline and the pump, when the latter is started up, the vacuum in the inlet pipe is far from sufficient to raise the water to the level of the suction branch, the pump and the suction pipeline must be filled with water. For this purpose, branch 6 is closed with a plug.

When installing large pumps (with an inlet diameter of more than 250 mm), the pump is filled using a special vacuum pump, which creates a deep vacuum when working in air, sufficient to lift water from a receiving well.

In conventional structures centrifugal pumps the lowest pressure occurs near the inlet to the blade system on the concave side of the blades, where the relative velocity reaches its maximum value, and the pressure reaches its minimum. If in this area the pressure drops to the value of the saturation vapor pressure at a given temperature, then a phenomenon occurs called cavitation.

The essence of cavitation is the boiling up of liquid in the area reduced pressure and in the subsequent condensation of vapor bubbles when the boiling liquid moves into the region high blood pressure. At the moment of bubble closure, a point sharp impact occurs and the pressure at these points reaches a very large value (several megapascals). If the bubbles at this moment are near the surface of the blade, then the impact falls on this surface and causes local destruction of the metal. This is the so-called pitting - a lot of small shells (as in smallpox).

Moreover, not only mechanical destruction of the surfaces of the blades (erosion) occurs, but also the processes of electrochemical corrosion are intensified (for impellers made of ferrous metals cast iron and non-alloyed steels.

It should be noted that materials such as brass and bronze resist the harmful effects of cavitation much better, but these materials are very expensive, so the manufacture of pump impellers from brass or bronze must be appropriately justified.

But cavitation is harmful not only because it destroys the metal, but also because the efficiency decreases sharply in the cavitation mode. and other parameters of the pump. The operation of the pump in this mode is accompanied by significant noise and vibrations.

The operation of the pump during the initial stage of cavitation is undesirable, but allowed. With developed cavitation (formation of caverns - separation zones), the operation of the pump is unacceptable.

The main measure against cavitation in pumps is to maintain this suction head H sun (Fig. 12.1), in which cavitation does not occur. This suction height is called acceptable.

Let P 1 and c 1 - pressure and absolute flow velocity in front of the impeller. R a is the pressure on the free surface of the liquid, H - pressure loss in the suction pipeline, then the Bernoulli equation:

from here

However, when flowing around the blade, on its concave side, the local relative velocity may be even greater than in the inlet pipe w 1 (w 1 - relative speed in the section, where the absolute is equal to from 1 )

(12.1)

where  - cavitation coefficient equal to:

The condition for the absence of cavitation is P 1 >P t ,

where P t - saturated vapor pressure of the transported liquid, which depends on the properties of the liquid, its temperature, atmospheric pressure.

Let's call cavitation reservethe excess of the total head of the liquid over the head corresponding to the pressure of saturated vapors.

Determining from the last expression and substituting in 12.1, we get:

The value of the cavitation reserve can be determined from cavitation test data published by manufacturers.

displacement blowers

13.1 PISTON PUMPS

On fig. 13.1 shows a diagram of the simplest piston pump (see lecture 1) of one-sided suction driven through a crank mechanism. The transfer of energy to the fluid flow occurs due to the periodic increase and decrease in the volume of the cylinder cavity from the side of the valve box. In this case, the specified cavity communicates either with the suction side (with an increase in volume), or with the discharge side (with a decrease in volume), by opening one of the valves; the other valve is then closed.

Rice. 13.1 Diagram of a piston pump 13.2 Indicator diagram

single acting piston pump

The change in pressure in this cavity is described by the so-called indicator diagram. When the piston moves from the extreme left position to the right, a vacuum is created in the cylinder R p , the liquid is entrained behind the piston. When the piston moves from right to left, the pressure increases to a value R naked , and the liquid is pushed into the discharge pipeline.

The area of ​​the indicator diagram (Fig. 13.2), measured in Nm/m 2 , represents the work of the piston in two strokes, referred to 1 m 2 its surface.

At the beginning of suction and at the beginning of non-discharge, pressure fluctuations occur due to the influence of the inertia of the valves and their “sticking” to the contact surfaces (saddle).

The displacement of a piston pump is determined by the size of the cylinder and the number of strokes of the piston. For single acting pumps (Fig. 13.1):

where: n number of double piston strokes per minute; D piston diameter, m; S - piston stroke, m;  about volumetric efficiency

Volumetric efficiency takes into account that part of the liquid is lost through leaks, and part is lost through valves that do not close instantly. It is determined during pump testing and is usually o = 0.7-0.97.

Let us assume that the length of the crank R much less than the length of the connecting rod, i.e. R/L  0 .

Moving from the left extreme position to the right, the piston travels a path

x=R-Rcos  , where  - angle of rotation of the crank.

Then the piston speed

Where (13.1)

Piston acceleration:

Obviously, the suction of fluid into the valve box and the injection from it are extremely uneven. This causes the occurrence of inertial forces that disrupt the normal operation of the pump. If both parts of expression (13.1) are multiplied by the piston areaD2/4 , we get the corresponding pattern for the feed (Fig. 13.3)

Therefore, the liquid will move unevenly throughout the pipeline system, which can lead to fatigue failure of their elements.

Rice. 13.3 Displacement curve of a piston pump 13.4 Piston delivery schedule

single acting double acting pump

One way to equalize the flow is to use double-acting pumps (fig. 13.5), in which two suction strokes and two discharge strokes occur per revolution of the drive shaft (fig. 13.4).

Another way to increase the uniformity of the feed is to use air caps (Fig. 13.4). The air contained in the cap serves as an elastic medium that equalizes the speed of the fluid.

Full work piston per double stroke

And power, kW.

Rice. 13.5 Diagram of a piston pump

double acting with air cap

This is the so-called indicator power indicator diagram area. Real Power N more than the indicator by the value of mechanical friction losses, which is determined by the value of mechanical efficiency.

13.2 RECIPROCATING COMPRESSORS

According to its principle of operation, based on the displacement of the working medium by the piston, the piston compressor resembles a piston pump. However, the working process of a reciprocating compressor has significant differences related to the compressibility of the working medium.

On fig. 13.6 shows a diagram and an indicator diagram of a single-acting reciprocating compressor. On the diagram(v) the abscissa shows the volume under the piston in the cylinder, which uniquely depends on the position of the piston.

Moving from the right extreme position (point 1) to the left, the piston compresses the gas in the cylinder cavity. The suction valve is closed during the entire compression process. The discharge valve is closed until the pressure difference between the cylinder and the discharge pipe overcomes the resistance of the spring. The discharge valve then opens (point 2) and the piston forces the gas into the discharge pipeline up to point 3 (leftmost position of the piston). Then the piston begins to move to the right, first with the suction valve closed, then (point 4) it opens and the gas enters the cylinder.

Rice. 13.6 Schematic and indicator diagram 13.7 Diagram of a gear pump

reciprocating compressor

Thus line 1-2 corresponds to the compression process. In a reciprocating compressor, the following are theoretically possible:

Polytropic process (curve 1-2 in Fig. 13.6).

Adiabatic process (curve 1-2).

Isothermal process (curve 1-2).

The course of the compression process depends on the heat exchange between the gas in the cylinder and environment. Reciprocating compressors are usually made with a water-cooled cylinder. In this case, the process of contraction and expansion are polytropic (with polytropic exponents n

It is impossible to push all the gas out of the cylinder, because the piston can not come close to the cover. Therefore, part of the gas remains in the cylinder. The volume occupied by this gas is called the volume of harmful space. This leads to a decrease in the amount of gas sucked in. V Sun . The ratio of this volume to the working volume of the cylinder V p , is called the volumetric coefficient o \u003d V sun / V p.

Theoretical displacement of a reciprocating compressor

Valid feed Q \u003d  about Q t.

The work of the compressor is spent not only on compressing the gas, but also on overcoming frictional resistance.

A=A hell +A tr .

The ratio A hell / A \u003d  hell is called adiabatic efficiency. if we proceed from a more economical isothermal cycle, then we get the so-called isothermal efficiency. from \u003d A from / A, A \u003d A from + A tr.

If work A multiply by mass feed G , then we get the compressor power:

N i =AG indicator power;

N hell =A hell G with an adiabatic compression process;

N of =A of G in an isothermal compression process.

Compressor shaft power N in more than the indicator by the value of friction losses, which is taken into account by the mechanical efficiency: m \u003d N i / N in.

Then the total efficiency compressor =  from  m.

13.3.1 GEAR PUMPS

The scheme of gear pumps is shown in fig. 13.7.

Pinched gears 1, 2 are placed in housing 3. When the wheels rotate in the direction indicated by the arrows, the liquid flows from the suction cavity 4 into the cavity between the teeth and moves into the pressure cavity 5. Here, when the teeth enter the pinch, the liquid is displaced from the cavity .

The minute flow of a gear pump is approximately equal to:

Q \u003d  A (D g -A) in  o,

where: A - center-to-center distance (Fig. 13.7); D g - head circumference diameter; in - width of gears; n - frequency of rotation of the rotor, rpm; about volumetric efficiency, which is in the range of 0.7...0.95.

13.3.2 Vane pumps

The simplest diagram of a vane pump is shown in fig. 13.8. An eccentrically located rotor 2 rotates in housing 1. Plates 3 move in radial grooves made in the rotor. Section of the inner surface of the housing av and cd , as well as the plates separate the suction cavity 4 from the discharge cavity 5. Due to the presence of eccentricity e , when the rotor rotates, the liquid is transferred from cavity 4 to cavity 5.

Rice. 13.8 Diagram of a vane pump 13.9 Scheme of a liquid ring vacuum pump

If the eccentricity is made constant, then the average pump flow is:

Q=f a lzn  o ,

where f a - the area of ​​the space between the plates, when it runs along an arc aw; l - rotor width; n - frequency of rotation, rpm; about - volumetric efficiency; z number of plates.

Vane pumps are used to create pressures up to 5 MPa.

13.3.3 WATER RING VACUUM PUMPS

Pumps of this type are used to suck air and create a vacuum. The device of such a pump is shown in Fig. 13.9. In a cylindrical body 1 with covers 2 and 3, a rotor 4 with blades 5 is eccentrically located. When the rotor rotates, water partially filling the body is thrown to its periphery, forming an annular volume. In this case, the volumes located between the blades change depending on their position. Therefore, air is sucked in through the crescent-shaped hole 7, which communicates with the pipe 6. In the left side (in Fig. 13.9), where the volume decreases, air is forced out through the hole 8 and pipe 9.

In the ideal case (in the absence of a gap between the blades and the housing), the vacuum pump can create a pressure in the suction pipe equal to the vapor saturation pressure. At a temperature T \u003d 293 K, it will be equal to 2.38 kPa.

Theoretical feed:

where D 2 and D 1 outer and inner diameters of the impeller, m; a minimum immersion of the blade in the water ring, m; z - number of blades; b blade width; l blade radial length; s blade thickness, m; n speed, rpm; about volumetric efficiency

jet blowers

Jet superchargers are widely used as elevators at the input of heating networks into buildings (to ensure mixing and circulation of water), as well as ejectors in exhaust ventilation systems of explosive premises, as injectors in refrigeration plants and in other cases.

Rice. 14.1 Water jet elevator 14.2 Ventilation ejector

Jet superchargers consist of a nozzle 1 (Fig. 14.1 and 14.2), where the ejecting fluid is supplied; mixing chamber 2, where the ejecting and ejected liquids and diffuser 3 are mixed. The ejecting liquid supplied to the nozzle exits it at high speed, forming a jet that captures the ejected liquid in the mixing chamber. In the mixing chamber, there is a partial equalization of the velocity field and an increase in static pressure. This rise continues in the diffuser.

To supply air to the nozzle, high-pressure fans (low-pressure ejectors) are used, or air is used from a pneumatic network (high-pressure ejectors).

The main parameters characterizing the operation of a jet supercharger are the mass flow rates of the ejector G 1 \u003d  1 Q 1 and ejected liquid G 2 \u003d  2 Q 2 ; full pressure ejector P 1 and ejected P 2 liquids at the inlet to the supercharger; mixture pressure at the outlet of the supercharger P3.

As the characteristics of the jet blower (Fig. 14.3), dependencies are built on the degree of pressure increase P c /  P p from the mixing ratio u=G 2 /G 1 . Here  P c \u003d P 3 -P 2,  P p \u003d P 1 -P 2.

For calculations, the equation of momentum is used:

C 1 G 1 +  2 c 2 G 2 +  3 c 3 (G 1 + G 2 )=F 3 (P k1 -P k2 ) ,

where c 1 ; c 2 ; c 3 velocity at the exit from the nozzle, at the inlet to the mixing chamber and at the exit from it;

F3 cross-sectional area of ​​the mixing chamber;

 2 and  3 coefficients taking into account the non-uniformity of the velocity field;

Pk1 and Pk2 pressure at the inlet and outlet of the mixing chamber.

efficiency jet supercharger can be determined by the formula:

This value for jet blowers does not exceed 0.35.

draft machines

smoke exhausters - flue gases are transported through the boiler ducts and the chimney and, together with the latter, overcome the resistance of this path and the ash removal system.

Blow fansoperate on outdoor air, supplying it through a system of air ducts and an air heater into the combustion chamber.

Both smoke exhausters and blowers have impellers with backward curved blades. The designations of smoke exhausters contain the letters DN (exhaust fan with backward curved blades) and the numbers impeller diameter in decimeters. For example, DN-15 a smoke exhauster with backward curved blades and an impeller diameter of 1500 mm. In the designation of draft fans VDN (blower fan with backward curved blades) and also the diameter in decimeters.

Draft draft machines develop high pressures: smoke exhausters up to 9000 Pa, blowers up to 5000 Pa.

The main operational features of smoke exhausters are the ability to work at high temperatures (up to 400 C) and with a high dust (ash) content - up to 2 g / m 3 . In this regard, smoke exhausters are often used in gas dust cleaning systems.

An obligatory element of smoke exhausters and draft fans is a guide vane. By constructing the characteristics of this smoke exhauster at different installation angles of the guide vane and highlighting the areas of economical operation on them (  0.9  max ), get a certain area zone of economical operation (Fig. 15.1), which are used to select a smoke exhauster (similar to the summary characteristics of general industrial fans). A summary graph for blow fans is shown in Fig. 15.2. When choosing the standard size of a draft machine, it is necessary to strive to ensure that the operating point is as close as possible to the maximum efficiency mode, which is indicated on individual characteristics (in industrial catalogs).

Rice. 15.1 Smoke exhauster design

Factory characteristics of smoke exhausters are given in catalogs for gas temperature t har \u003d 100  C. When selecting a smoke exhauster, it is necessary to bring the characteristics to the actual design temperature t . Then the reduced pressure

Smoke exhausters are used in the presence of ash collecting equipment, residual dust content should not exceed 2 g/m 3 . When selecting smoke exhausters from the catalog, safety factors are introduced:

Q to \u003d 1.1Q; P to \u003d 1.2P.

In smoke exhausters, impellers with backward curved blades are used. In practice, the following sizes are used in boiler rooms: DN-9; ten; 11.2; 12.5; fifteen; 17; 19; 21; 22 single suction and DN22 2; DN24  2; DN26 2 Double suction.

The main units of smoke exhausters are (Fig. 15.1): impeller 1, "snail" 2, running gear 3, inlet pipe 4 and guide vane 5.

The impeller includes an "impeller", i.e. blades and discs connected by welding and a hub mounted on a shaft. The running gear consists of a shaft, rolling bearings located in a common housing and an elastic coupling. Bearing lubrication crankcase lubrication (oil located in the housing cavities). To cool the oil, a coil is installed in the bearing housing, through which cooling water circulates.

The guide apparatus has 8 rotary vanes connected by a lever system with a rotary ring.

Two-speed electric motors can be used to regulate smoke exhausters and draft fans.

LITERATURE

Main:

1. Polyakov V.V., Skvortsov L.S. Pumps and fans. M. Stroyizdat, 1990, 336 p.

Auxiliary:

2. Sherstyuk A.N. Pumps, fans, compressors. M. “Higher School”, 1972, 338 p.

3. Kalinushkin M.P. Pumps and fans: Proc. allowance for universities on special. "Heat and gas supply and ventilation", 6th ed., Revised. And add.-M.: Higher school, 1987.-176 p.

Methodical literature:

4. Guidelines for laboratory work on the course "Hydraulic and aerodynamic machines". Makeevka, 1999.