Key operating points of diaphragm pumps and process improvements

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1 　Buffer tank 　　 To ensure a stable pressure and flow rate while the pump is in operation, buffer tanks are installed on both the inlet and outlet pipes of the pump. The buffer tank must be pre-charged with nitrogen to a certain pressure; this pre-charging pressure should, in principle, not be lower than 30% of the maximum operating pressure nor higher than 80% of the minimum operating pressure. Although a higher priming pressure can provide better cushioning, a decrease in pressure leads to pulsations that frequently strike the bottom of the tank, causing damage to the metal components inside. 　　When the operating pressure range is wide, it is sufficient to pre-charge the pressure to 30%–35% of the maximum operating pressure. The pre-charge pressures specified by diaphragm pump manufacturers are as follows: the pre-charge pressure for the inlet buffer tank should be 80%–90% of the inlet pressure, while the pre-charge pressure for the outlet buffer tank should be 60%–65% of the outlet pressure. During actual operation, significant vibration in the pump’s inlet pipe indicated that the pre-charge pressure of the inlet buffer tank was not appropriate. Based on the actual conditions, the pre-charge pressure of the inlet buffer tank was reduced to 60%–65% of the inlet pressure, and the vibration was immediately eliminated, allowing the pump to return to normal operation. 　　During operation, due to leaks and other factors, the pressure in the buffer tank gradually decreases. When this pressure reaches a certain level, the pump operates unevenly and produces abnormal noises. When operating at low pressures for an extended period, the diaphragm is subject to large fluctuations, which can lead to fatigue in the diaphragm and reduce its service life. To overcome this situation, it is necessary to pump air into the buffer tank during operation. Based on the experience gained during the pump’s commissioning and normal operation, it was decided to inject pressure into the buffer tank while the pump was in operation; the stabilization of the pressure gauge pointer served as an indicator that the pressurization had been successful. Several tests yielded excellent results, ensuring the long-term safe operation of the pump. 　　2 　Exhaust 　　1) Water injection for exhaust 　　Water injection into the slurry chamber for exhaust serves two purposes: ① Exhaust at the liquid phase. The exhaust of the slurry chamber is achieved by connecting a flushing water control valve on the inlet pipeline and a drain valve on the outlet pipe; ② Automatic addition of the propulsion fluid. During initial oil filling, the introduction of water will force the diaphragm to move toward the back of the diaphragm chamber, that is, to the automatic filling position. To obtain sufficient hydraulic force to push the diaphragm to this position, the opening degree of the drain valve on the outlet pipeline can be controlled to achieve automatic filling. The most critical aspect of water injection for exhaust is pressure control. The pressure specified by the pump manufacturers is 013 MPa, but in actual operation it is generally maintained between 0128 and 0135 MPa. Either too high or too low a pressure can result in either too much or too little oil being injected into the propulsion fluid chamber, which in turn can cause damage to the diaphragm during operation. 　　2) Oil injection and exhaust The purpose is to inject oil into the propulsion fluid chamber. The filling process is carried out continuously and automatically; during filling, the exhaust valve must remain open until no air is left after the propellant fluid has been discharged. At this point, the control lever has been moved away from the trigger position, and within one minute, the filling process stops automatically. At this point, the hydraulic chamber is filled, but for the piston, the diaphragm has not yet reached the proper position; filling the chamber will be accomplished by turning the crank. The manufacturer recommends using manual cranking to fill the fluid chamber, but this method is not only time-consuming and labor-intensive but also makes it difficult to achieve an optimal level of filling in the propulsion fluid chamber. Therefore, a pump started at low load is used as an alternative to manual cranking for filling the propulsion fluid chamber, with a low-load starting value of 10% being chosen. 　　3) Exhausting air during normal operation 　　During normal operation of the pump, due to dissolved gases in the propellant fluid, seal leaks, and other factors, more and more air accumulates in the propellant fluid chamber, which can damage the diaphragm. It is therefore necessary to exhaust the air from the propellant fluid chamber on a regular basis. 　　3 　Propellant Control System 　　As shown in Figure 1, under normal operating conditions, the magnet on the control rod moves along with the control rod between triggers ZSH and ZSL; at this time, the triggers are not activated. 　　 　　If there is too little propellant in the propellant chamber, the magnet will move to the trigger ZSL, which then sends a pulse signal to the PLC control box. The PLC will output a signal to open the fuel injection valves HV1, 2, or 3, allowing the propellant fluid to enter the propellant chamber. 　　If there is too much propellant fluid in the propellant chamber, the trigger ZSH also sends a pulse signal to the PLC control box; at this point, the oil discharge valves HV5, 6, or 7 open, and the propellant fluid is discharged from the propellant chamber. 　　If there is a fault in the propellant system and the filling or discharge valve remains open continuously for more than 3 minutes, the PLC will output a fault signal. 　　The injection and discharge of the above-mentioned propellant fluid are carried out automatically. However, during operation of the pump, the automatic injection and discharge process became uncontrolled due to issues such as triggers or internal components. To carry out repairs, it was necessary to shut down the pump. In order to avoid disruptions to production, after careful consideration, it was decided to use manual injection and discharge of oil. Due to a lack of experience, during the first attempt at oil injection, the vent valve on the propellant fluid chamber was not opened, resulting in an excessive amount of propellant fluid being filled in and causing damage to the diaphragm. Based on summarizing the lessons from previous failures, when refilling oil manually once again, open the exhaust valve to use the stroke pressure discharged from the chamber as an indication that it is fully filled. Several trials were successful, ensuring safe and continuous production. 　　4 　Start-up and shutdown loads 　Diaphragm pumps are driven by variable-frequency motors. Excessive or insufficient start-up and shutdown loads, large fluctuations in load within short periods of time, or operation at low loads for extended periods can all have an impact on the frequency converter. To minimize such situations, it is stipulated that the start-up and shutdown loads should be kept around 30 %. Additionally, controls are built into the instrumentation to ensure that any changes in load must occur over a certain period of time, thereby enabling effective maintenance of the frequency converter from a process perspective. 　　5 　Instrument air and lubricating oil pressure 　Both the instrument air pressure and the lubricating oil pressure are interlocked with the main motor. The filling and draining valves for the propulsion fluid are air-actuated valves, and a certain air pressure must be maintained at all times; the minimum operating pressure for these valves is not less than 0.4 MPa. Therefore, the instrument air pressure is generally kept above 0.15 MPa. The supply of air to these valves is filtered and atomized with oil. 　　While the pump is in operation, the lubricating oil pressure should not be lower than the pressure interlock value (0115 MPa), and the pressure difference across the lubricating oil outlet filter should not be too high. The steel ball inside the lubricating oil flow gauge should be in its upper position; when adjusting this flow rate, the adjustment range should not be too large, to prevent a sudden increase in flow rate from causing a rapid drop in pressure and triggering an interlock shutdown. 　　Through these process improvements and a thorough understanding of the key operating procedures, a stable operational environment is maintained for the pump at all times. Over the past year or so, there has been only one instance of shutdown due to electrical issues, enabling long-term, safe, and efficient operation.