Experience in Identifying \"Abnormal Noise and Vibration\" in Chemical Plants

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This post was last edited by yinkuilin6868 on 2015-11-17 at 14:49. In chemical processing operations, an experienced operator often relies on their keen hearing to detect abnormal noises and vibrations, able to identify such noises among ordinary noises and take action to address them in order to prevent equipment damage. Common causes of noise generally include the following: 1. Corrosion of internal components of the valve ； 　　2. Pump evacuation ； 　　3. High-power pump operating at low load ； 　　4. Surge in centrifugal compressors ； 　　5. Rupture of heat exchanger diaphragm ； 　　6. Flashing of hydrocarbons in water ； 　　7. Control valve screeching ； 　　8. Water hammer occurs in the steam pipeline. I. Abnormal noises in rotating equipment 1. Compressors The noise generated by compressors consists mainly of the gas dynamic noise from the main unit and the mechanical noise from the auxiliary units. Generally, the noise level measured under normal conditions is 84–102 dB. 　　Once the compressor starts to surge, it will emit a deep, roaring noise at regular intervals every few seconds. At this point, the compressor is operating in an unstable state; the rotor slides back and forth between the bearings, thereby pressing on the thrust bearings. This horizontal movement of the rotor inevitably damages the compressor shaft seal and bearings. Each surge sound indicates another slip of the rotor between the bearings. The higher this surge noise, the greater the force acting on the rotor in the horizontal direction, and the greater the risk; it can lead to damage to the compressor, starting from mild surging and progressing to complete destruction of the compressor. Generally speaking, a machine operating at 3000 r/min is more resistant to surge than one operating at 8000 r/min. 　　Causes of surge and remedies: ① Excessively high discharge pressure. Empty the receiver of the compressor aftercooler to reduce back pressure, or open the cooling water valve feeding into the aftercooler. 　　②The pumping rate is low. The anti-surge valve is opened, which allows the gas being released to circulate back to the inlet side of the rear compressor. 　　③The temperature of the inhaled gas is high. Most units are equipped with a mechanism to inject a small amount of light liquid hydrocarbon upstream of the compressor’s suction inlet; the liquid evaporates and cools the hot air stream drawn into the compressor. It is also possible to request the upstream process to reduce the temperature of the gas entering the compressor. 　　 2. Pumps The noise generated by pump motors primarily comes from the motor itself; this noise consists of electromagnetic noise resulting from the motor’s own electromagnetic vibrations, aerodynamic noise caused by the rear fan, and mechanical noise. It is generally 83–105 dB. Abnormal noise and vibration in pumps are mainly caused by pump cavitation, excessive pump capacity, and air entrainment in the pump. 　　Pump cavitation: The vibrating sound produced by a centrifugal pump is caused by cavitation, which indicates that the pumping pressure is not sufficient to prevent the liquid inside the pump from vaporizing. Vibration occurs when these bubbles deform and burst; if cavitation continues, the pump’s shaft seal, bearing shells, and impeller will all be damaged. 　　The fastest way to prevent the pump from running dry is to throttle the pump’s outlet to reduce the flow rate, and then raise the liquid level in the tank that the pump is drawing from. Another option is to simply stop the pump; the best approach is to interlock the pump with the liquid level in the extraction tank. 　　Cavitation in pumps: When cavitation occurs, noise and vibration are generated. Under the repeated action of intense forces on certain areas of the impeller, the material surface suffers from fatigue, progressing from pitting to the formation of severe honeycomb-like voids, which damages the blades. Furthermore, when cavitation is severe, the formation of a large number of bubbles occupies part of the space in the liquid flow channel, resulting in a significant decrease in the pump’s flow rate, head, and efficiency. 　　Therefore, for the pump to operate properly, the minimum pressure at the inlet of the blades must be maintained above a certain critical value; this minimum pressure should be higher than the saturated vapor pressure of the liquid at the temperature of transport. After cavitation occurs during operation, the pump must be vented immediately. 　　Pump capacity is too high: Centrifugal pumps with large capacities often produce a low, dull sound during operation when their capacity is reduced, and this is mainly due to internal circulation within the impeller. If this continues for a long time, the rotor part will get damaged. The only way is to increase the outlet flow rate. 　　For any rotating equipment with excessive vibration, its operation should be stopped immediately before the pump is damaged. II. Abnormal noise and vibration in valves and pipelines The noise generated by valves and pipelines is mainly caused by the friction of pressurized gas against the pipelines, or by sudden pressure drops that lead to disturbances in the surrounding gas. 　　Abnormal noise in valves is mainly caused by the following reasons: ① Foreign objects have fallen onto the valve seat ； 　　②High-speed liquid damages the valve ； 　　③The switch valve is used as a control valve ； 　　④Gate valve leakage ； 　　⑤A vapor-liquid mixture flows through the valve body. 　　Control valve screeching: A control valve in good condition should be able to close properly. When the control valve is fully closed and a dull noise is heard, it indicates that there is a leak of liquid passing through the valve seat; if the pressure drop across the valve is significant, the noise can be quite loud. Debris such as a small stone, screw, or welding slag stuck on the valve can cause it to not close properly. 　　The presence of noise indicates that high-speed fluid is passing through the valve, and over time this will cause wear on the valve itself. If the valve cannot be removed for repair, the best solution is to reduce the pressure in the upstream pipeline. 　　If the normal operating position of the control valve is closed, noise is often generated, and the pressure drop across the valve is high; as a result, severe vibrations occur when the gas-liquid mixture passes through the control valve. Reducing the temperature of the fluid can help to reduce these vibrations. 　　Screaming of the switching valve: Being able to determine whether a switching valve is leaking by listening to the sound is a skill that those dealing with accidents should master. 　　By pressing your ear against the bottom of the valve, you can hear this sound of fluid leaking clearly; if there is no leakage, no noise will be heard. Normally, gate valves are not used for control purposes; if the valve is opened by more than 2–3 turns, its interior will be eroded by high-speed fluid. In this way, when the valve is closed, the valve body will leak. A pressure difference of 2.1 MPa is sufficient to allow fluid at such high flow rates to pass through an eroded valve. III. Vibration in steam and condensate pipelines Abnormal vibration in steam pipelines is caused by the rapid condensation of steam on cold metal, or by the intense shocks resulting from the mixing of steam and condensed water; this phenomenon is known as water hammer, and the vibration it causes generates very loud noises. 　　Therefore, when introducing steam, it is necessary first to warm up the steam piping and drain all water from the lines to prevent water hammer. In steam-heated reboilers, heat exchangers, etc., terrible water hammer can also occur when steam is introduced into the pipeline for the first time. 　　Note: Warm the pipe, drain water! IV. Heat Transfer Equipment It is common for rotating equipment to produce abnormal noises, but it can be confusing when static heat transfer equipment that is operating normally also emits unusual noises and vibrations. The reasons are generally as follows: ① Water hammer ； 　　②Gas leaks into the liquid ； 　　③Rupture of heat exchanger diaphragm ； 　　④Resonance, etc. 　　Water hammer in heat exchangers: In heat exchangers where fluid flows, air or other gases may be present inside them before they are put into use. It is necessary to vent these gases from the heat exchanger and replace them with liquid. If the gases are not removed, they will mix with the liquid as it flows, resulting in turbulence and the formation of many bubbles. When these bubbles burst, the liquid rushes in to fill the space left by the bubbles, generating a large amount of force. This causes the equipment to vibrate violently continuously until all the gases have been expelled. For a heated liquid, if the heating temperature reaches the fluid’s boiling point, it will also result in a gas-liquid mixed flow, thereby causing water hammer. 　　Leakage: When light hydrocarbon liquids leak into a low-pressure liquid, they rapidly vaporize into steam; this sudden expansion causes pressure fluctuations that lead to movement of the pipelines. Leakage of high-pressure gas into low-pressure gas can also cause vibration in equipment and pipelines. Therefore, upon detecting such vibration, the vent valve and drain of the heat exchanger should be opened immediately, and the liquid composition should be inspected or sampled for analysis in order to identify the leak. 　　Rupture of the heat exchanger diaphragm: One example is sufficient to illustrate this issue. A pipe connected to the crude oil heat exchanger began to vibrate; after several days of such vibration, the flow of crude oil in the pipe stopped suddenly. When the heat exchanger’s head was opened, it was found that the partition inside the pipe was damaged and had blocked the crude oil outlet. The design allowable pressure drop is 345 kPa, while the operating pressure drop is 483 kPa. Partially blocked pipes significantly increase the pressure drop of the crude oil, and the partition plates eventually break due to the increasing pressure. When it is detected that the heat exchanger tubes are vibrating, the pressure drop between the shell and the tube side should be checked. 　　In short, the causes of abnormal noise and vibration are diverse. Experienced operators and professionals can immediately determine the cause based on the sound and take action right away to prevent accidents; sometimes, however, comprehensive consideration and analysis are also required.