vibration and noise in pumps 1

YOUR PARTNER IN CONDITION MONITORING

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VIBRATION AND NOISE IN PUMPS 5/26/2005

Introduction: Although certain amount of noise is to be expected from centrifugal pumps. Unusually high noise levels in excesses of 100db particularly high frequencies are the indicator of potential failures. The occurrence of significant noise levels indicates that sufficient energy exists to be potential cause of vibrations. Noise in pumping system can be generated by ---

• The mechanical motions of pump components. • The liquid motion in the pump and piping system.

The component motion noise and liquid motion noise can be transmitted to environment.


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SOURCES OF PUMP NOISE. 1.Mechanical noise sources. Common mechanical sources that may produce noise and vibrations. Because of pressure variations that are generated in liquid or air .

• Impeller rubbing • Seal rubbing • Defective or damaged bearing • Vibrating pipe walls • Unbalanced rotor

Improper installation of couplings often causes mechanical noise at two times pump speed harmonic---misalignment If pump speed is near or passes through the critical speed noise can be generated.

1. By high vibrations resulting from unbalance – • Rubbing of bearings • Rubbing of seals • Rubbing of impeller.

Symptom of rubbing : it may be characterized by high pitched sequel.

2. windage noise may be generated by • Motor fans • Shaft keys • Coupling bolts

3. damaged bearing noise : high frequency.


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LIQUID NOISE SOURCES These are the pressure fluctuations produced directly by liquid motion.

1. High velocity flow 2. Pulsations 3. Cavitations 4. Flashing 5. Water hammer 6. Flow separation 7. Impeller interaction with the pump cut water.

Pressure pulsations and flow modulations produce either a discrete or broadband frequency component. If generated frequencies excite any part of the structure or piping or pump =} then noise may be radiated into environment. Four types of pulsations

1. Discrete frequency components generated by the pump impeller such as vane pass frequencies plus its multiples.

2. Flow induced pulsation caused by turbulence such as flow past restriction and side branches in the piping system.

3. Broadband turbulent energy resulting from high flow velocities.

4. Intermittent bursts on broadband energy caused by cavitations, flashing water and water hammer.


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Causes of vibrations

1. Installation/maintenance

• Unbalance • Shaft-shaft misalignment • Seal rubs • Case distortion caused by piping load • Piping dynamic response to supports and restraints. • Support structural response to foundation/anchor

bolts/grout. • Improper assembly.

2. APPLICATION

• Operating off of design point • Improper speed/flow • In adequate NPSH • Entrained air

3. HYDRAULIC

• Interaction of pump (head-flow curve) with piping resonance

• Hydraulic instabilities. • Acoustic resonance (pressure pulsations) • Water hammer • Recalculation • Cavitations • Flow induced excitation (turbulences) • High flow velocities.


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4.Design/manufacturing

• Lateral critical speed • Tensional critical speed • Improper bearings • Improper seals • Rotor instability • Shaft misalignment in journals • Impeller resonance • Bearing/pedestal resonance •

Unbalance: Unbalance of rotating shaft can cause large transverse vibration at certain speeds known as critical speed that coincide with that natural or lateral frequency of shaft. Damage due to unbalance response may range from seal or bearing wipes to catastrophic failure of rotor. Excessive rotor unbalance can result from rotor bow. Unbalance of couplings, thermal distortion or loose parts After period of operation the pump rotor may become unbalanced by erosion, corrosion or wear. Unbalance could also be caused by non-uniform plating of the pumped product on to the pump impeller. In this instance cleaning the impeller could restore the balance. Erosion of the impeller by cavitations pr chemical reaction with the product may cause permanent unbalance requiring repair or replacement of the impeller. Wear of impeller or shaft caused by rubs will require repair or replacement. Another cause of unbalance can be occur if lubricated couplings have an uneven build up of grease or sludge.


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MISALIGNMENT: Angular misalignment between two shafts connected with a flexible coupling introduces a additional driving force that can produce tensional or lateral vibrations. The force in a typical industrial coupling is similar to universal joint. When a small angular misalignment occurs the velocity ratio across the coupling is not constant. If one shaft speed assumed constant the other shaft speed has faster rotational rate for part of the revolution and slower rotational rate for part of the revolution. This variation of rotating speed results in a second harmonic vibration component. PIPING AND STRUCTURE The pump relatively isolated from the piping. The weight and thermal loading on suction and discharge connections should be minimized. Static forces from piping may misalign the pump from its driver or for excessive loading the pump case may distorted and case or seal and bearing damage. Vibrations of piping or the support structure can be mechanically transferred to the pump. The piping and the structure should not have their resonant frequencies or multiples. The vibration transferred to the piping to the structure can be minimized by using a visco-elastic material.(belting material between the pipe and piping clamp.


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APPLICATION Improper application or changing conditions can result in variety of problems. Operation at high flow, low head conditions can cause vibration of rotor case, inadequate NPSH can result in cavitations that will cause noise and vibration BEARINGS General purpose and small pumps in processes plants generally have rolling element bearings. Noise and vibrations are commonly a result of bearing wear. As the rolling element or races wear, the worn surfaces or defects initially produce a noise and as wear increase vibration become noticeable and several high frequencies may occur that depend on the geometry of the bearing component and their relative rotational speeds. The frequency generally above the operating speed. Many ball bearing failure are due to contamination in the lubricant that have found their way into bearing SEALS The fluid dynamics of flow through seals have a dramatic effect on rotor dynamics. Hydrodynamic forces involved may contribute the stabilization of rotating machinery or make it unstable. Seals with large axial flow in the turbulent range such as in the water pumps tend to produce large stiffness and damping co-efficient that are beneficial to the rotor vibration and stability. Wear of the seals will increase the clearance and cause greater leakage and possibility to change the rotor dynamics characteristics of the seal resulting in increased vibrations. After the machine has been placed in operation. Common contaminants moister, dirt and other miscellaneous particles which when trapped inside the bearing may cause wear or permanently indent the balls and raceway under tremendous stresses generated by the operating load.


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The hydrodynamic bearing is superior to rolling element bearings for high speed application. The hydrodynamic bearing supports the rotor on a film of oil as it rotates. The geometry of hydrodynamic bearing and oil properties play a important role in controlling the lateral critical speeds and consequently the vibration characteristics of the pump HYDRAULIC EFFECTS Hydraulic effects and pulsations can result almost any frequency of vibration of the pump or piping from one per revolution to vane pass frequency and its harmonics. frequencies below running speed can be caused by acoustical resonance. Generally these effects are due to the impeller passing and discharge diffuser or some other discontinuity in the case any non-symmetry of these internals of the pump may produce an uneven pressure distribution that can result in forces applied to the rotor. TRANSIENTS Starting and stopping pumps with the attendant opening and closing of the valves are major causes of sever transients in piping system. The resulting pressure surge referred to as water hammer, can apply sudden impact force to the pump and its internals and the piping. Sever water hammer has caused cracks in concrete structures to which the pipe was anchored. Rapid closer of conventional valves used in feed water lines can cause severe water hammer. Increasing the closer time of the valve can reduce the severity of surge pressure. CAVITATIONS AND FLASHING. For many liquid-pumping systems it is common to have some degree of flashing and cavitations associated with the pump or with pressure control valves in piping system. A high flow rate produces more severe cavitations because of greater flow losses through restriction.


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Cavitations produces high local pressure that may be transmitted directly to the pump or piping and may also be transmitted through fluid to other area of piping. Cavitations are one of the most commonly occurring and damaging problem in liquid pump systems. The term of cavitations refers the formation and subsequent collapse of vapor bubbles in a liquid caused by dynamic pressure variation near the vapor pressure. Cavitations can produce noise, vibrations, loss of head and capacity as well as severe erosion of impeller and casing surfaces. Before the pressure of the liquid flowing through a pump is increased, the liquid may experience a pressure drop inside the pump case. This is due to part of acceleration of liquid into eye of the impeller and flow separation from impeller inlet vanes. If flow is an excess of design or the incident vane angle is incorrect, high velocity, low pressure eddies may form. If liquid pressure is reduced to vaporization pressure the liquid will flash. Later in flow path the pressure will increase. The implosion, which flow causes, what is usually referred to as cavitations noise. The collapse of the vapor pockets, usually on the non-pressure side of the impeller vanes, causes severe damage. (Vane erosion) in addition to noise. When a centrifugal pump is operated at flows away from the point of best efficiency, the noise is often heard around the pump casing. The magnitude and noise may vary from pump to pump and are independent on the magnitude of the pump head generated, the ratio NPSH required to NPSH available, and amount by low deviates from ideal flow. Noise is often generated when the vane angles of the inlet guides, impeller and diffuser are incorrect for the actual flow rate. Observing the complex wave or dynamic pressure variation using an oscilloscope and a pressure transducer can best recognize cavitations. The pressure waveform will be non-sinusoidal with sharp maximum peaks (spikes) and rounded minimum peaks occurring at vapor pressure. As the pressure drops it cannot reduce a vacuum less than vapor pressure. Cavitations like noise can also be heard at flows less than design, even then available inlet NPSH is an excess of pump required NPSH, This has been puzzling problem.


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Noise of a very low, random frequency but very high intensity results from backflow at the impeller eye or at the impeller discharge or both. Every centrifugal pump has this recirculation under certain condition of flow reduction operation in recirculation condition can be damaging to the pressure side of the inlet and or discharge impeller vanes(also to casing vanes). Recirculation is evidenced by an increase in loudness of a banging type, random noise, and an increase in suction and or discharge pressure pulsation as flow is decreased sound level measured at the casing of an 8000hp pump and near the suction piping during cavitations produced a wide band shock that excited may be high frequency., however in this case the vane passing frequency (number of impeller vanes times revolution/sec and multiplies are predominant). Cavitations noise of this type usually produces very high frequency best described as “ crackling “ Flashing is particularly common in hot water systems (feed water systems) when the hot, pressurized water experiences a decrease in pressure through restrictions (flow control valve). This restriction of pressure allows the liquid to suddenly vaporize or flash which results in noise similar to cavitations. To avoid flashing after restriction sufficient backpressure should be provided. Alternatively, the restriction could be located at the end of line so that flashing energy can dissipate in to layer volume. FLOW TURBULENCE Pump generated dynamic pressure sources include turbulence (vortices or wakes) produced in the clearance space between impeller tips and stationary diffuser or volute lips. Dynamic pressure fluctuations or pulsations produced in this manner can cause impeller vibrations or can result in shaft vibrations as the pressure peals impinge on impeller. Flow past an restriction or restriction in the piping may produce turbulence or flow induced pulsations. These pulsations may produce both noise and vibration over a wide frequency band. The frequencies are related to the flow velocity and geometry of the obstruction. These pulsations may cause resonant interaction with other parts of the acoustic piping system. Most of these unstable flow


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patterns are produced by shearing at the boundary between a high velocity and low velocity region in a fluid field. Typical example of this type of turbulence includes flow around obstruction or past dead water regions (closed water bypass line) or bi-directional flow. The shearing action produce vortices or eddies that are converted to pressure per turbulences at the pipe wall that may result in localized vibration excitation of the piping or pump components. The acoustic natural response modes of piping systems and the location of the turbulence have strong influence on the frequency and amplifications. This vortex shedding experimental measurements have shown that vortex flow is more severe when a system is acoustic resonance coincides with the generation frequency of the source. The vortices produce broadband turbulent energy centered around frequency.


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vibration and noise in pumps 1

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