Summary of the Domestic Application of High-Temperature and High-Pressure Shielded Pumps in Hydrogenation Units
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The following article is shared from the internet: ------------------------------------------------------------------------- 【Abstract】 It describes the problems that occurred during one operating cycle of domestic shielded pumps in high-pressure hydrogenation units. Based on an analysis of the causes of these faults, corrective measures were proposed, resulting in significant benefits. 【Keywords】 Hydrogenation unit, shielded pump, localization, high temperature and pressure. I. Introduction A certain hydrogenation unit employs a two-stage series full-hydrogenation process, with the operating pressure at the inlet of the second reactor reaching as high as 15.65 MPa. This imposes high requirements on the performance of the feed pump for the second reactor; conventional centrifugal pumps with mechanical seals are unable to meet such pressure conditions, making shielded pumps an ideal choice. Under similar operating conditions, a domestic high-temperature and high-pressure canned pump was used for the first time as the feed pump in the second-stage reaction unit of a petrochemical company; it was capable of withstanding high-temperature and high-pressure operating conditions. The specific parameters of this pump are as follows: the medium is heavy paraffin oil, the inlet pressure is 15.65 MPa, the outlet pressure is 16.55 MPa, and the normal flow rate is 30 m3/h. II. Introduction to the shielded pump: This high-temperature and high-pressure shielded pump is of the high-temperature separation type, being a single-stage, single-suction centrifugal pump. The pump impeller is directly mounted at the shaft extension end of the motor rotor, and it is secured using a streamlined impeller cover. To reduce the heat transfer from the pump side to the motor side, a thermal insulation screen is installed between the pump and the motor. The high-temperature and high-pressure shielded pump is equipped with a separate circulation system, which consists of auxiliary systems such as an external heat exchanger, a rear liquid injection system, and a monitoring system, as shown in Figure 1.
Based on the temperature of the liquid being pumped, electric pumps can be roughly divided into two parts: a high-temperature pump head consisting of the pump body, impeller, and insulation shield, and a motor section with a lower temperature. The flow path of the circulating fluid in the pump head section is: pump inlet → impeller → pump outlet. The liquid circulation in the motor section is achieved through an auxiliary impeller mounted on the rotor: 1) auxiliary impeller → front bearing → gap between the two shield sleeves → rear bearing → heat exchanger → auxiliary impeller. 2) Auxiliary impeller → Front bearing → Auxiliary impeller. Shielded pumps are a type of sealless pump; they have fewer components compared to conventional process centrifugal pumps, and they are used in applications involving corrosion or toxic substances, accounting for over 30% of such cases. Proper use of shielded pumps generally only requires regular replacement of the bearings, and there are already over 75,000 hours of operation without any need for maintenance worldwide. In the near future, as their application scope expands, their economic advantages will become even more evident. III. Occurring faults and deficiencies During one production cycle of operation, the shielded pump experienced the following faults and deficiencies: 1) After maintenance and restart, leaks occurred at the pump body flange and the pump inlet flow meter orifice due to inadequate preheating ； 2) Reduced winding insulation during long-term pump operation leads to unexpected burnout and pump shutdown ； 3) The shaft displacement measurement element is inaccurate, preventing the pump’s own interlock system from being activated ； 4) During the operation of the shielded pump, it was also found that the manual air release process after starting the pump took a long time and was inconvenient to operate.
IV. Rectification Plan Based on the problems that occurred, the following rectification plan is formulated: 1) When the system is used for the first time or after the gaskets have been removed and replaced, all bolts must be tightened to the specified torque. The medium conveyed by the unit should be gradually raised to the prescribed temperature and pressure at a rate of ≤40°C. Disc springs should be installed at the flanges where temperature fluctuations are significant. It is necessary to raise the temperature slowly to ensure proper preheating; no leaks have occurred in this system since then. 2) There are two reasons for the winding to break down or burn out: one is that the winding operates at high temperatures for a long time, resulting in a decrease in its insulating properties ； Second, during the start-up and shutdown of the pump, water or steam enters the motor casing due to thermal expansion and contraction in the system. Going forward, the company will strictly switch the pump every two months and regularly measure the insulation resistance of the stator windings to ensure that its insulation resistance remains within normal levels. If conditions permit in the future, technical modifications can be carried out to add a vacuum pumping system or a nitrogen sealing system, thereby preventing air from entering the motor’s wiring system. Nitrogen is injected into the wiring chamber to maintain a slight positive pressure within the motor coil’s sealed system, thereby preventing moisture from the air from entering the electrical connections. This approach is derived from the sealing mechanisms used in pumps inside cryogenic tanks; it is explained briefly as follows: The pump used in cryogenic tanks is a type of sealless centrifugal pump (similar to a cartridge pump), and the sealing issue here relates to the connection between the external cables and those inside the tank. Cryogenic electrical connectors and instrument connectors are used for transmitting power and instrument signals, as shown in Figure 2
On one side of the through-joint, the low-temperature process gas is sealed off, while on the other side, the atmosphere is sealed off; N2 is introduced into the cavity in between, and both ends are sealed to achieve the effect of sealing the gas； The N2 pressure in the intermediate chamber is lower than the pressure of the process medium inside the pump, but higher than the ambient pressure. The intermediate chamber is equipped with pressure monitoring instruments as well as alarm/interlock* systems; any leakage on either side will be detected and trigger an alarm/interlock response. Note: The nitrogen sealing for through-joints is maintained via static pressure supplementation; the inlet and outlet valves for the nitrogen sealing of low-temperature through-joints are usually kept closed. The normal N2 pressure ranges from 0.1 to 0.5 MPa. A drop in pressure indicates a seal leak, and the valves should be opened manually to replenish the pressure ； In the case of a pressure increase alarm, apart from checking for abnormal N2 pressure, it indicates an internal leak in the seal; the pump is shut down via high-pressure interlock. 3) The pump shaft itself experiences thermal expansion and contraction; when the pump is started or stopped, or during heavy rainfall, the shaft displacement changes significantly. To address this issue, an umbrella-shaped shelter has been added, and the shaft displacement measurement device is activated only after 30 minutes of pump operation, once the temperature stops changing. 4) According to Article 5.1.6 of the \"SH∕T 3148-2016 Technical Specifications for Sealedless Centrifugal Pumps in the Petrochemical Industry\", heavy-duty sealedless centrifugal pumps shall be of self-ventilating type ； Sealless centrifugal pumps for medium and light loads should be of the self-ventilating type. For manually vented, seal-less centrifugal pumps, the seller shall hang a “warning” sign in a prominent position on the pump to remind the buyer to manually vent it before starting up and before and after maintenance. This device features a high-temperature separation-type shielded pump; there are no vent holes at the top of the pump or its pipelines. The vent valve is located above the cooler on the top of the pump. Since the medium contains H2, manual venting is required each time the pump is started, which takes a long time and is inconvenient to operate. The original manual valve has been replaced with an automatic vent valve, and the exhaust pipeline behind the valve leads into the container for collection and treatment, as shown in Figure 3.
On one side of the through-joint, the low-temperature process gas is sealed off, while on the other side, the atmosphere is sealed off; N2 is introduced into the cavity in between, and both ends are sealed to achieve the effect of sealing the gas； The N2 pressure in the intermediate chamber is lower than the pressure of the process medium inside the pump, but higher than the ambient pressure. The intermediate chamber is equipped with pressure monitoring instruments as well as alarm/interlock* systems; any leakage on either side will be detected and trigger an alarm/interlock response. Note: The nitrogen sealing for through-joints is maintained via static pressure supplementation; the inlet and outlet valves for the nitrogen sealing of low-temperature through-joints are usually kept closed. The normal N2 pressure ranges from 0.1 to 0.5 MPa. A drop in pressure indicates a seal leak, and the valves should be opened manually to replenish the pressure ； In the case of a pressure increase alarm, apart from checking for abnormal N2 pressure, it indicates an internal leak in the seal; the pump is shut down via high-pressure interlock. 3) The pump shaft itself experiences thermal expansion and contraction; when the pump is started or stopped, or during heavy rainfall, the shaft displacement changes significantly. To address this issue, an umbrella-shaped shelter has been added, and the shaft displacement measurement device is activated only after 30 minutes of pump operation, once the temperature stops changing. 4) According to Article 5.1.6 of the \"SH∕T 3148-2016 Technical Specifications for Sealedless Centrifugal Pumps in the Petrochemical Industry\", heavy-duty sealedless centrifugal pumps shall be of self-ventilating type ； Sealless centrifugal pumps for medium and light loads should be of the self-ventilating type. For manually vented, seal-less centrifugal pumps, the seller shall hang a “warning” sign in a prominent position on the pump to remind the buyer to manually vent it before starting up and before and after maintenance. This device features a high-temperature separation-type shielded pump; there are no vent holes at the top of the pump or its pipelines. The vent valve is located above the cooler on the top of the pump. Since the medium contains H2, manual venting is required each time the pump is started, which takes a long time and is inconvenient to operate. The original manual valve has been replaced with an automatic vent valve, and the exhaust pipeline behind the valve leads into the container for collection and treatment, as shown in Figure 3.
On one side of the through-joint, the low-temperature process gas is sealed off, while on the other side, the atmosphere is sealed off; N2 is introduced into the cavity in between, and both ends are sealed to achieve the effect of sealing the gas； The N2 pressure in the intermediate chamber is lower than the pressure of the process medium inside the pump, but higher than the ambient pressure. The intermediate chamber is equipped with pressure monitoring instruments as well as alarm/interlock* systems; any leakage on either side will be detected and trigger an alarm/interlock response. Note: The nitrogen sealing for through-joints is maintained via static pressure supplementation; the inlet and outlet valves for the nitrogen sealing of low-temperature through-joints are usually kept closed. The normal N2 pressure ranges from 0.1 to 0.5 MPa. A drop in pressure indicates a seal leak, and the valves should be opened manually to replenish the pressure ； In the case of a pressure increase alarm, apart from checking for abnormal N2 pressure, it indicates an internal leak in the seal; the pump is shut down via high-pressure interlock. 3) The pump shaft itself experiences thermal expansion and contraction; when the pump is started or stopped, or during heavy rainfall, the shaft displacement changes significantly. To address this issue, an umbrella-shaped shelter has been added, and the shaft displacement measurement device is activated only after 30 minutes of pump operation, once the temperature stops changing. 4) According to Article 5.1.6 of the \"SH∕T 3148-2016 Technical Specifications for Sealedless Centrifugal Pumps in the Petrochemical Industry\", heavy-duty sealedless centrifugal pumps shall be of self-ventilating type ； Sealless centrifugal pumps for medium and light loads should be of the self-ventilating type. For manually vented, seal-less centrifugal pumps, the seller shall hang a “warning” sign in a prominent position on the pump to remind the buyer to manually vent it before starting up and before and after maintenance. This device features a high-temperature separation-type shielded pump; there are no vent holes at the top of the pump or its pipelines. The vent valve is located above the cooler on the top of the pump. Since the medium contains H2, manual venting is required each time the pump is started, which takes a long time and is inconvenient to operate. The original manual valve has been replaced with an automatic vent valve, and the exhaust pipeline behind the valve leads into the container for collection and treatment, as shown in Figure 3.
On one side of the through-joint, the low-temperature process gas is sealed off, while on the other side, the atmosphere is sealed off; N2 is introduced into the cavity in between, and both ends are sealed to achieve the effect of sealing the gas； The N2 pressure in the intermediate chamber is lower than the pressure of the process medium inside the pump, but higher than the ambient pressure. The intermediate chamber is equipped with pressure monitoring instruments as well as alarm/interlock* systems; any leakage on either side will be detected and trigger an alarm/interlock response. Note: The nitrogen sealing for through-joints is maintained via static pressure supplementation; the inlet and outlet valves for the nitrogen sealing of low-temperature through-joints are usually kept closed. The normal N2 pressure ranges from 0.1 to 0.5 MPa. A drop in pressure indicates a seal leak, and the valves should be opened manually to replenish the pressure ； In the case of a pressure increase alarm, apart from checking for abnormal N2 pressure, it indicates an internal leak in the seal; the pump is shut down via high-pressure interlock. 3) The pump shaft itself experiences thermal expansion and contraction; when the pump is started or stopped, or during heavy rainfall, the shaft displacement changes significantly. To address this issue, an umbrella-shaped shelter has been added, and the shaft displacement measurement device is activated only after 30 minutes of pump operation, once the temperature stops changing. 4) According to Article 5.1.6 of the \"SH∕T 3148-2016 Technical Specifications for Sealedless Centrifugal Pumps in the Petrochemical Industry\", heavy-duty sealedless centrifugal pumps shall be of self-ventilating type ； Sealless centrifugal pumps for medium and light loads should be of the self-ventilating type. For manually vented, seal-less centrifugal pumps, the seller shall hang a “warning” sign in a prominent position on the pump to remind the buyer to manually vent it before starting up and before and after maintenance. This device features a high-temperature separation-type shielded pump; there are no vent holes at the top of the pump or its pipelines. The vent valve is located above the cooler on the top of the pump. Since the medium contains H2, manual venting is required each time the pump is started, which takes a long time and is inconvenient to operate. The original manual valve has been replaced with an automatic vent valve, and the exhaust pipeline behind the valve leads into the container for collection and treatment, as shown in Figure 3.