Methanol shielded pump (seek experts! ）

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This post was last edited by yuchenchf on 2017-5-18 08:35. Can a shutdown of the shielded pump used for transporting flammable and explosive materials lead to fires or explosions? I. Case scenario: In the methanol cracking hydrogen production unit of a petrochemical company, the methanol feed pump is a canned motor pump manufactured by Dalian Empire. The process flow is as follows: methanol raw material tank → outlet pipeline → outlet valve → pipeline → canned motor pump → feed to the hydrogen production unit. During the operation of the methanol cracking hydrogen production unit, the feed pump, namely canned motor pump P-7201A, operated smoothly throughout. Before the unit was started up, the entire system had been properly purged, and there were no leaks in the entire feed system. At 14:00 on March 20, the outlet valve of the methanol feed tank failed to open (or the outlet valve closed as a result of an extremely low liquid level in the feed tank), which caused no methanol to reach the inlet of the shielded pump; as a result, the pump continued to run idly. Technical parameters of the shielded pump: Equipment name, specification model, medium to be transported, flow rate in m3/h, pressure in Mpa, temperature in °C, head in m. For the raw material feeding shielded pump F21-317H4M-0506T1-BV, the medium is methanol, with flow rate of 301.0, pressure of 0.4, temperature of 100 °C, and head of 100 m. II. Analysis, conclusions, and preventive measures: As can be seen from the above example, the entire feeding system was properly purged before the plant started operation, and there were no leaks in the entire system. The methanol feed pump that was in operation suddenly stopped due to a failure of the outlet valve on the upstream methanol storage tank; as a result, there was no flow of material at the inlet of the canned motor pump, causing it to operate idly. Therefore, for a shielded pump used to transport flammable and explosive materials, will the absence of material at the inlet cause a fire or explosion? To address this issue, an analysis is conducted in conjunction with this case. 1. Working principle and structural characteristics of shielded pumps (1) Working principle of shielded pumps A shielded pump is a sealless pump; both the pump and the drive motor are enclosed within a pressure vessel filled with the medium to be pumped. This pressure vessel has only static seals, and an electrical winding set is used to generate a rotating magnetic field that drives the rotor. This design eliminates the rotating shaft sealing mechanism found in traditional centrifugal pumps, thereby achieving complete leaklessness. A canned motor pump combines the pump and the motor together; the rotor of the motor and the impeller of the pump are mounted on the same shaft. A shielding sleeve is used to separate the rotor from the stator of the motor. The rotor rotates within the medium being transported, with power being transmitted to it through the magnetic field of the stator. (2) Structure of the shielded pump: The shielded pump is a special type of centrifugal pump, mainly consisting of a pump body, impeller, stator, rotor, front and rear guide bearings, shaft sleeves, and a thrust disk. The motor and pump are integrated into one unit; the surfaces of the stator and rotor are enclosed by non-magnetic thin-walled materials to isolate them from the process medium. The rotor is supported by front and rear guide bearings and is immersed in the medium being transported. An impeller is mounted at the rotor shaft end, creating a seal-less structure to achieve leak-free transportation. Figure 1: Cross-sectional view of the structure of a shielded pump. Figure 2: Assembly diagram of the structure of a shielded pump. 2. Conditions necessary for a substance to catch fire and explode (1) Conditions required for a fire to occur A fire is defined as a combustion that gets out of control in terms of time or space. Combustion, on the other hand, refers to an exothermic reaction that occurs when a combustible material reacts with an oxidizer, usually accompanied by flames, light emission, and/or smoke. For a substance to burn, it must meet the necessary and sufficient conditions for combustion. Prerequisite: The presence of fuel, an oxidizer, and a source of ignition at the same time ； Sufficient conditions for combustion: (1) a certain concentration of combustible material ； (2) A certain oxygen content: (3) A certain ignition energy ； (4) Unchecked chain reaction. (2) Conditions necessary for a substance to explode: Combustible substances (combustible gases, vapors, and dusts) must be uniformly mixed with air (or oxygen) within a certain concentration range to form a premixed gas; an explosion will occur only when this mixture comes into contact with a source of ignition. This concentration range is known as the explosive limit. An explosive mixture can explode only when its concentration is within the explosive range; it will neither explode nor catch fire when the concentration is below the lower explosive limit ； It will not explode at levels above the upper explosion limit, but it can catch fire. 3. Properties of methanol: Methanol is a volatile, colorless, flammable, and toxic organic compound with a molecular weight of 32.04 and a boiling point of 64.7°C. Methanol is flammable, and its vapors can form explosive mixtures with air. It can catch fire and explode in the presence of open flames or high heat. It undergoes a chemical reaction or causes combustion when in contact with oxidizers. 4. Case analysis: Regarding the topic of this question, an analysis is carried out as follows based on this case: (1) From the principle of shielded pumps and their structural diagrams (Figure 1 and Figure 2), it can be seen that shielded pumps do not have any rotating shaft sealing devices; in other words, there are no dynamic seals, only static seals, and there are only 5 such static seal points. Static sealing points of the shielded pump: bearing housing at the sealing point, lower bearing housing of the motor assembly, inlet flange and outlet flange of the pump casing, and exhaust connection – total of 11111. (2) Based on an analysis of the operating conditions of the feed system, it can be seen that the system had been properly purged before switching on, and the entire system was leak-free; this rules out the possibility of air (or oxidized gases) from other system components entering the system. There are no combustible substances in the system. (3) Methanol is highly volatile; therefore, when the outlet valve of the methanol feed tank fails to close, there is no material at the inlet of the shielded pump, but there is still a large amount of volatile methanol gas present, and the system pipes contain flammable gases (volatile methanol gas). (4) When the feed to the shielded pump is interrupted, the pump operates idle, which can cause wear and damage to its bearings; if not detected in time, this can lead to damage to the stator and rotor shielding sleeves of the pump, as well as electrical leakage due to contact between the motor stator and the medium. Therefore, allowing a shielded pump to run idle for an extended period can cause damage to the bearings and shaft sleeves, and the pump as well as the motor will heat up. In such operating conditions, there are sources of ignition such as high temperatures and electric sparks. (5) Even under the most severe conditions of damage to the stator and rotor shielding sleeves, the shielded pump is designed to prevent damage to its own structure; as long as the 5 static sealing points remain intact and leak-free, the pump can meet the requirements of being leak-free. Fire and explosion conditions: Flammable gas (volatile methanol vapor) + oxidizing agent (the system is properly purged, with no leaks, and there is no oxidizing agent such as air or oxygen) + ignition source (high-temperature heat source or electric spark). 5. Conclusion: Assuming that all other system components besides the shielded pump are in good condition, without any leaks, that the system has been properly purged before operation, and that the shielded pump itself has not suffered severe damage, the lack of oxidizing agents (air or oxygen) within the system means that the necessary and sufficient conditions for combustion are not met ； Furthermore, there are no leaks in the equipment and facilities of the entire system, and the surrounding environment lacks flammable materials; thus, the necessary and sufficient conditions for gas combustion are not met. Therefore, for shielded pumps used to transport flammable and explosive materials, during operation, a failure of the upstream valve resulting in no material reaching the inlet (a material shortage) will not cause fires or explosions. Precautionary measures: Although shielded pumps are leak-free pumps used for transporting flammable, explosive, and toxic substances, if the various safety measures are not properly implemented and failures in these pumps go unnoticed, it can lead to an escalation of the fault, resulting in substance leaks and thereby causing fires or explosions. Therefore, we should take the following safety measures: 6. Preventive measures: (1) Before starting up the plant, it is necessary to strictly follow the plant startup plan and ensure that the systems used for transporting flammable and explosive materials have been properly purged. (2) Check whether all the equipment and facilities in the entire system are in good condition and free of leaks. (3) Strengthen equipment inspections to detect faults promptly. (4) Strengthen the monitoring of key equipment such as shielded pumps used for transporting flammable and explosive media; safety alarms or interlock devices can be added in accordance with design specifications. (5) Conduct regular maintenance and inspection of the equipment.