Q&A on the Operation Techniques of Centrifugal Pumps (II)

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11. What are the advantages and disadvantages of mechanical seals compared to packing seals? Answer: The main advantages of mechanical seals are as follows: 1) They basically do not leak, or the leakage amount is minimal. According to statistics, the leakage rate of packing seals is 100 times that of mechanical seals. The leakage standards set by the Petroleum Ministry stipulate that for mechanical seals used in transporting heavy oil, the leakage rate should be ≤5 drops per minute ； When transporting light oil, the leakage rate is ≯10 drops per minute. 2) The friction coefficient is low, so the power consumption is also low; the frictional power of a mechanical seal is only 10–50% of that of a packing seal. 3) Under normal conditions, mechanical seals are made of high-quality materials that are not prone to damage, which reduces the amount of maintenance required. At the same time, it eliminates the wear on the shaft sleeve caused by packing seals. 4) Can be used for the sealing of various high-parameter pumps. It is suitable for transporting liquids under conditions of high temperature, high pressure, high vacuum, or those that are toxic and corrosive, and it provides excellent sealing performance. The disadvantages of mechanical seals are: 1) They have a complex structure, making them difficult to manufacture and maintain. 2) The cost is high, so it is not very economical. 12. Mechanical seals of centrifugal pumps are prone to leakage during startup; what measures can be taken to address this? Answer: When a centrifugal pump is started, leakage of the mechanical seal can occur due to insufficient exhaust, inadequate preheating of the pump body, and other reasons. If the leakage is caused by insufficient exhaust, which leads to pump evacuation and results in the separation of the rotating and stationary seal rings of the mechanical seal, the pump must be stopped for shaft turning to allow exhaust, or the pump’s outlet valve should be slightly opened at startup. Insufficient preheating of the pump body can easily lead to leaks at the static sealing points of the mechanical seal system; such issues generally disappear on their own after the pump has been operating normally for some time. If leakage still occurs in the mechanical seal despite taking the above measures, maintenance should be considered if the operating conditions permit it. 11. Under what circumstances should flushing oil be injected into the mechanical seal of a centrifugal pump? What is the function of flushing oil? What are the requirements for flushing oil? Answer: For centrifugal pumps that transport toxic, highly corrosive, or expensive fluids, the sealing requirements are very strict. Flushing oil must be injected when the medium being transported contains solid particles, or when a tandem mechanical seal or a double-end face mechanical seal is used. The main functions of flushing oil are as follows: flushing, cooling, lubrication, and sealing. 1) Flush out impurities between the friction of the rotating ring and the stationary ring. 2) Prevent the high-temperature oil pumped by the pump from entering the sealing chamber, and remove the heat generated by friction between the rotating ring and the stationary ring during operation, thereby reducing the temperature of the sealing elements. Extend the service life of the seal. 3) Maintaining a layer of liquid film between the sealed end faces serves as a lubricant. 4) Prevent high-temperature, toxic, corrosive, flammable, and valuable media from leaking out of the pump; prevent media containing particles from entering the sealing chamber and wearing out the sealing surfaces; and prevent air outside the pump from entering the pump, thereby providing a sealing effect. The pressure of the flushing oil should be 0.05~0.2 Mpa higher than the pressure of the medium to be sealed. The flushing oil must be a clean, particle-free liquid that does not evaporate easily, is non-corrosive, and does not affect product quality. The flushing oil system typically includes a storage tank as well as auxiliary equipment that serves functions such as pressure balancing, filtering, and cooling. 12. What are the different types of rinsing? Answer: Flushing can be divided into three forms: self-flushing, external flushing, and cyclic flushing. 1) Due to the high viscosity and density of the medium pumped by the pump itself, a cleaner medium is introduced from the outside into the pump, which serves as external flushing. 2) The medium delivered by the pump itself is forced into the seal chamber through a small hole in the mechanical seal gland, where it mixes with the medium inside the pump; this is known as self-flushing. 3) Circulating flushing refers to the process in which flushing oil brought in from the outside enters the sealing box, is led out through a dedicated pipe, cooled and filtered, and then returns to the sealing box. 13. Briefly describe the cavitation mechanism and its hazards in centrifugal pumps. Answer: When liquid flows through the pump impeller, the pressure distribution within the flow channel is determined by the characteristics of the flow, such as the shape of the blades and the sudden changes in direction of the liquid flow. Certain local low-pressure areas exist on the non-working surface near the inlet of the blade; when the pressure of the liquid flow in these low-pressure areas drops to the saturated vapor pressure corresponding to the temperature of the liquid, the liquid begins to vaporize and bubbles are formed ； As the bubbles flow with the liquid stream through the channel, they burst instantaneously when they reach areas with higher pressure. At the moment the bubble collapses, the liquid surrounding it rushes into the cavity formed by its collapse, accompanied by localized high temperatures and high-pressure water hammer effects; this is the mechanism behind cavitation. Cavitation causes significant damage to pumps, mainly in the following aspects: 1) A sudden decline in the pump’s performance. When cavitation occurs in a pump, the energy transfer between the impeller and the liquid is disrupted. The flow channels are not only blocked by bubbles but also experience increased flow losses; in severe cases, the fluid flow within the pump is interrupted and the pump can no longer function. 2) The pump generates vibration and noise. 3) In addition to mechanical damage to the surface of the pump’s flow-through components, if the gas released during liquid vaporization is corrosive, certain chemical damage may also occur (although the former type of damage is the primary one). In severe cases, the surface of the impeller (especially near the blade inlets) becomes honeycombed or spongy. 14. What is the relationship between the density of the fluid inside a centrifugal pump and the pump’s volumetric flow rate, head, and efficiency? Answer: The difference in medium density has no relation to the pump’s volumetric flow rate, head, or efficiency, but it does have a significant impact on the pump’s weight flow rate, pressure, and shaft power. This is because at a certain rotational speed, the centrifugal force acting on the liquid is proportional to the density of the liquid; in other words, the pressure generated by this centrifugal force is also proportional to the density. However, the head is expressed in the form of H = P/υ. Therefore, the effect of liquid density is eliminated; the pump’s head-flow curve remains unchanged, but the shaft power will vary depending on the liquid density. When a pump is used for water circulation, since the density of water is greater than that of the actual fluid, the shaft power required by the pump increases. It is important to control the flow rate in such cases to prevent the motor from being overloaded. 15. Which characteristics of a centrifugal pump can be represented from its characteristic curve graph? Answer: By analyzing the various curves in the graph, it can be seen from the flow rate–head (Q–H) curve that as the flow rate increases, the head decreases, but by a very small amount. This indicates that when the flow rate remains constant, the pressure inside the pump stays stable. When the flow rate changes, the operating pressure of the pump fluctuates little; however, to ensure sufficient pressure inside the pump, the discharge volume cannot be increased arbitrarily. From the flow-power (Q-N) curve, it can be seen that the relationship between flow and power is as follows: power consumption increases as the flow rate increases, and it is at its lowest when the flow rate is zero (with the pump outlet valve fully closed). Therefore, when starting a centrifugal pump, the outlet valve must be closed; otherwise, the high power consumption often leads to electrode tripping or damage to the electrodes, as well as increased mechanical wear. From the flow-efficiency (Q–η) curve, it can be seen that the efficiency curve has a peak value, which is known as the optimal operating point. Operating near this peak is the most economical and reasonable approach. Therefore, the flow rate, head, and power corresponding to the maximum efficiency are important for selecting and using pumps. When selecting a pump, the appropriate pump should be chosen based on the optimal operating point indicated on its performance curve. 16. What is the effect of changes in liquid viscosity on pump performance? Answer: When a centrifugal pump transports fluids with a viscosity higher than that of water (such as crude oil, lubricating oil, sulfuric acid, etc.) compared to water, the performance parameters change as follows: 1) The pump’s flow rate decreases. As the viscosity of the liquid increases, the inhibitory effect of the tangential viscous force gradually spreads to the fluid flow between the blades, reducing the fluid velocity inside the impeller and thereby decreasing the pump’s flow rate. 2) The pump’s head is reduced. As the viscosity of the liquid increases, the energy required to overcome viscous friction rises, thereby reducing the head generated by the pump. 3) The shaft power of the pump increases. When the density of the liquid differs little from that of water, the increase in power loss (disc loss) is mainly due to the friction between the rear cover plate of the impeller and the liquid while transporting mucus. Furthermore, the increased hydraulic loss due to friction between the liquid and the cover plate also leads to an increase in shaft power. 4) The efficiency of the pump decreases. Although the increased liquid viscosity reduces fluid leakage and thus improves the volumetric efficiency of the pump, the increase in hydraulic losses and cover losses leads to a decrease in the pump’s hydraulic efficiency and mechanical efficiency, resulting in a lower overall efficiency of the pump. 5) The allowable net positive suction head required for the pump increases, as the dynamic pressure drop from the pump inlet to the impeller inlet increases with rising liquid viscosity, thereby increasing the pump’s allowable net positive suction head. In summary, when transporting viscous liquids, the characteristics of the pump change significantly. Therefore, for oils with too high viscosity, which have poor fluidity, they are not suitable for transportation using centrifugal pumps; generally, when the viscosity is greater than 650 centipoise, reciprocating pumps or gear pumps should be used. 17. What precautions should be taken when starting a centrifugal pump? Answer: The following precautions should be observed when starting a centrifugal pump: 1) Under no circumstances should a centrifugal pump be operated without liquid, to prevent damage to its components. 2) Centrifugal hot oil pumps must be preheated to avoid accidents caused by excessive temperature differences between cold and hot fluids. 3) After the centrifugal pump is started, it is not allowed to run for an extended period of time with the outlet valve closed; the operating time should be less than 1–2 minutes. 4) The flow rate of a centrifugal pump must never be adjusted using the inlet valve, to prevent cavitation. 18. What is the difference between a cold oil pump and a hot oil pump? Answer: 1. A pump used at temperatures below 200°C is a cold oil pump, while one used at temperatures above 200°C is a hot oil pump. 2. Ordinary hot oil pump sealing mechanisms are filled with seal oil, whereas cold oil pumps are not. 3. The pump body of the hot oil pump is of vertical segmented type, while that of the cold oil pump is of horizontal split-type. Some hot oil pumps have a structure to prevent the movement of the pump’s centerline. 4. The clearance of the ring at the hot oil pump is larger, while it is smaller for the cold oil pump. 5. Heat oil pumps are usually made of carbon steel or alloy steel, while cold oil pumps can be made of cast iron. 6. The hot oil pump needs to be preheated before starting, while this is not necessary for the cold oil pump. 7. The supports, bearing housings, and mechanical seals of the hot oil pump all require water cooling, whereas this is not necessary for the cold oil pump. 8. The model of hot oil pumps is generally indicated by the letter R, while cold oil pumps are indicated by the letter J. 19. Why can’t a cold oil pump pump hot oil? Answer: The reasons why a cold oil pump cannot handle hot oil are as follows: 1. The materials used for the components of a cold oil pump and a hot oil pump are different; for example, the impeller of a cold oil pump is made of cast iron, while that of a hot oil pump is made of cast steel or alloy steel. Cast iron parts cannot operate at high temperatures. 2. The clearance of the mouth ring in a cold oil pump is small, while in a hot oil pump this clearance is larger due to expansion. If hot oil is pumped using a cold oil pump, the clearance of the mouth ring decreases at high temperatures, which can lead to wear on the impeller and the pump casing. 3. The bearing housing, packing box, mechanical seal, and supports of the hot oil pump are all equipped with cooling mechanisms to prevent overheating in these areas, improve the operating conditions of the components, and ensure that the supports do not deform excessively, thus preventing the pump and motor from running out of alignment. Cold oil pumps do not have these mechanisms, so they cannot transport hot oil. 20. Why is it necessary to preheat the backup hot oil pump before starting it? What issues should be considered when preheating? Answer: If the backup hot oil pump is not preheated before startup, during an emergency start, hot oil will flow rapidly into the pump, resulting in uneven heating of its components. Due to the different amounts of thermal expansion among these components, it is easy for leaks to occur in the pump’s seals (such as the pump cover, connection fittings for auxiliary pipelines, and flanges on the inlet and outlet pipelines) ； In severe cases, it can cause the rotating components within the pump to collide with the stationary components, leading to accidents. When preheating, pay attention to the following points: 1) Ensure that the preheating process is carried out correctly ； 2) Preheating is carried out at a rate of 50°C per hour; in emergency situations, the preheating speed can be increased by taking certain measures (such as using steam to blow on the pump body to assist with preheating). However, too high a speed can cause the pump body to heat up rapidly, leading to leaks at various joints and bending or jamming of the rotor. 3) The rotor should be rotated during preheating, usually once every 30–45 minutes, to prevent bending of the main shaft and to facilitate the removal of gas from within the pump. 4) The bearing housings, pump bases, and sealed cooling water must all be opened to protect the shafts and bearings.