Identify abnormal noises and vibrations in the equipment by relying on hearing

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In chemical production operations, experienced workers often rely on their hearing to detect abnormal noises and vibrations from equipment, and they address these issues in order to prevent damage to the critical equipment in chemical plants. The causes of noise are as follows: ► Corrosion of internal valve components ； ► Pump evacuation ； ► Low-load operation of high-power pumps ； ► Surge in centrifugal compressors ； ► Heat exchanger partition rupture ； ► Noise of check valves ； ► The screech of controlled felling ； ► Water hammer occurred in the steam pipeline. I. Abnormal noises in rotating equipment 1. Compressors The noises generated by compressors can be categorized as those of centrifugal compressors and piston compressors; they are primarily composed of the gas dynamic noises from the main unit and the mechanical noises from the auxiliary units. Generally, the noise level measured under normal conditions ranges from 84 to 102 decibels. ► Noise from centrifugal compressors: When a centrifugal compressor experiences surging, it emits a deep, roaring noise at regular intervals of a few seconds. At this point, the compressor is operating in an unstable state; the rotor slides back and forth between the bearings. This horizontal movement of the rotor relative to the thrust bearing inevitably damages the compressor’s shaft seal. Each instance of surge indicates another slip of the rotor between the bearings; the louder the sound of such surge, the greater the horizontal force acting on the rotor, and the higher the risk, which can lead to complete destruction of the compressor, starting from mild surges. Generally speaking, a machine operating at 3000 r/min is better at preventing surge than one operating at 8000 r/min. Causes of surging and corrective measures: A. Excessive discharge pressure – Drain the receiver of the compressor aftercooler to reduce the pressure, or open the cooling water valve leading to the aftercooler. B. Low exhaust rate: Open the anti-surge valve to allow the discharged gas to return to the inlet side of the rear compressor. C. High intake gas temperature: Most units are equipped with facilities for injecting a small amount of light liquid hydrocarbons upstream of the compressor’s intake. The evaporation of this liquid cools the hot gas stream entering the compressor; alternatively, it is also possible to require upstream processes to lower the temperature of the gas before it enters the compressor. ► Noise in piston compressors: The noise and vibration associated with piston compressors are mainly due to mechanical factors. Additionally, improper disposal of waste during operation can lead to oil and water entering the cylinders, which also causes noise. A. If the valve disc and spring of a valve break, it will cause changes in the pressure and temperature at the inlet and outlet of the compressor; moreover, when listening to a damaged valve with a stethoscope, a hissing sound of air leakage can be heard. B. If the clearances of the rotating parts of the compressor are not appropriate, such as when the clearances between the crosshead, the large bearing shells, and the small bearing shells are too large or too small, the compressor will emit a thudding sound during operation. C. If some mechanical debris falls into the compressor’s cylinders, or if the gap between the piston and the cylinder head is too small, the compressor will produce a metallic knocking sound while it is running. When such a sound is heard, the machine should be stopped immediately for inspection and repair. Otherwise, major equipment damage accidents will occur. D. Due to untimely removal of process wastewater, oil and water entering the cylinder can cause liquid slugging; the sound associated with liquid slugging is also a ‘thudding’ noise. In such cases, it is necessary to enhance wastewater removal, and if liquid slugging is severe, the machine must be stopped for maintenance. In some chemical plants, production accidents have occurred in which gas valves broke and fell into the cylinder, destroying the piston and cylinder head. Therefore, in production practice, we can determine whether the compressor is operating properly by listening to the different sounds it produces during compression. 2. Pumps: The noise generated by pump motors mainly comes from the motor itself. This noise consists of electromagnetic noise resulting from the electromagnetic vibrations within the motor, aerodynamic noise caused by the fan at the back of the motor, and mechanical noise. Generally, it’s 83–105 decibels. Abnormal noise and vibration in pumps are mainly caused by pump cavitation, excessive pump capacity, and air entrainment in the pump. ► Pump evacuation: The vibrating sound produced by the centrifugal pump is caused by evacuation, 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 this evacuation 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, then raise the liquid level in the pump’s suction tank; another option is to simply stop the pump. The best approach is to install interlocking between the pump and the liquid level in the suction tank. ► Cavitation in pumps: The noise and vibrations that occur when cavitation takes place result from the repeated action of intense forces on certain areas of the impeller; this causes fatigue in the material’s surface, leading from pitting to the formation of severe honeycomb-like voids, which ultimately damages the impeller. Additionally, when cavitation is severe, the generation of a large number of bubbles occupies part of the liquid flow path, resulting in a significant decrease in the pump’s flow head and efficiency. Therefore, for the pump to operate properly, the minimum pressure at the inlet of the impeller must be maintained above a certain critical value; this minimum pressure should be greater than the saturated vapor pressure of the liquid at the temperature of transfer. In actual operation, after cavitation occurs, the pump must be vented immediately to expel the trapped gas. ► The pump capacity is too high. Pumps with a large capacity often produce a low, dull noise during operation when their capacity is reduced; this is mainly due to internal circulation within the impeller. Over time, this can cause damage to the impeller, and the only solution 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 gases under pressure against the pipelines, or by disturbances in the surrounding gases resulting from sudden pressure drops and evacuations. The noise generated by valves is mainly due to the following reasons: 1. Noise caused by the vibration of check valves ； 2. Foreign objects have fallen onto the valve seat ； 3. High-speed liquid damages the valve ； 4. The selector valve is used as a control valve ； 5. Leakage in the gate valve. ► The noise generated by the vibration of check valves comes mainly from lift-type check valves. Check valves are generally installed at the outlets of compressors and pumps, with the purpose of preventing high-pressure gas and liquid from flowing back into the system when the compressors and pumps are shut down. The gas discharged by the compressor is pulsating, which causes the valve element of the check valve at the outlet to move irregularly within the valve body. This results in significant noise, and over time the guide sleeve of the check valve wears out; in severe cases, the guide sleeve may even break, severely affecting production. In production practice, a swing check valve is used at the compressor outlet, which can basically overcome the noise caused by the pulsations in the compressor airflow. Some chemical plants have successful cases of modifying the outlet check valves of shift gas compressors, thereby completely eliminating vibration and noise from those valves. Another situation that can cause noise in check valves is when there is a large pressure difference before and after the valve; this also occurs in actual production, as is the case in the operation of pressure swing adsorption for hydrogen production. When the pressure difference before and after the check valve exceeds 0.5 kg, the check valve used in hydrogen production vibrates violently and produces a loud clashing sound of metal against metal. During each maintenance session for the hydrogen check valve, it is found that the guide sleeve of the check valve is severely worn, which significantly affects the long-term operation of the plant. Replacing the check valve with a programmable valve can basically solve this problem. ► Screeching of control valves: The image shows a water pump control valve. A control valve in good condition should be able to close properly; however, if it emits a dull screeching sound when fully closed, it indicates that there is leakage as liquid passes through the valve seat. If the pressure drop across the valve is high, the noise can be very loud. A small stone, screw, weld spatter, or similar object stuck on the valve can prevent it from closing properly. Noise indicates that high-speed fluid is flowing through the valve; over time this can cause wear on the valve body. If it is not possible to remove the valve 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, it often makes noise; moreover, the pressure drop across the valve is quite large. Vibration occurs when the gas-liquid mixture passes through the control valve and is throttled. Reducing the temperature of the fluid can minimize this vibration.