Operation and maintenance control method for two-stage horizontal type medium-pressure low-temperature liquid nitrogen pump

Abstract

The invention relates to an operation and maintenance control method for a two-stage horizontal medium-pressure low-temperature liquid nitrogen pump, which relates to the technical field of computer processing, and comprises the following steps: S01, detecting the outlet pressure Pout of the liquid nitrogen pump, the gas phase pressure Pk01 of a rectifying tower and the operation rotating speed r of the pump in real time; s02, based on the liquid nitrogen density rho = 808 kg / m < 3 >, the gravitational acceleration g = 9.81 m / s < 2 > and the liquid level height h = 21 m, the liquid column static pressure H = (rho * g * h) / 1000 = 166 kPa is calculated, and the pump inlet pressure is determined to be Pk01 + H; s03, the actual inlet and outlet pressure difference PDI = Pout-(Pk01 + H) is calculated; and S04, the rotating speed r of the pump is converted into a normalized parameter X = r / 59.6, and the target pressure difference P is calculated through a preset nonlinear function relation Y = f (X). Accurate control over the pressure difference of the inlet and the outlet of the liquid nitrogen pump is achieved by monitoring the operating parameters of the pump in real time, dynamically calculating the target pressure difference and adjusting the opening degree of the return valve through the PID control loop, and therefore the operating efficiency and stability of the pump can be effectively improved, the fault rate can be effectively reduced, and the service life of the pump can be effectively prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of computer processing, and particularly to an operation and maintenance control method for a two-stage horizontal medium-pressure low-temperature liquid nitrogen pump. Background Art

[0002] The cryogenic liquid pump in an air separation unit, especially a centrifugal liquid pump, is a key component to ensure its overall process performance. The operational reliability of such pumps is crucial because the working medium they handle is close to the saturation state and is similar to the boiling point. Once the operating point of the pump deviates from the normal range, the transported liquid will partially vaporize, resulting in cavitation, which is manifested as severe vibration of the pump impeller rotor. For multi-stage cryogenic centrifugal liquid pumps, their normal operating range is narrower than that of single-stage pumps and is more likely to enter the cavitation zone due to flow disturbances or changes in the inlet operating conditions.

[0003] Traditional control strategies (refer to a method for rapid switching of a multi-stage centrifugal cryogenic liquid pump in an air separation unit disclosed in the published (announced) number CN113719457A) mainly rely on setting high-high or low-low interlock parking protection for the pump outlet pressure and lack preventive control means to avoid the pump entering the cavitation zone. Although some advanced foreign technologies adopt high-region deviation correction automatic control to deal with cavitation, these methods do not fully achieve the optimal reliable operating point control of the pump. Instead, they focus on preventing the pump operating point from entering the high cavitation zone, and still rely on interlock protection for the low cavitation zone. However, how to effectively avoid the pump entering the unstable operating zones in the high and low regions during operation and startup, and ensure that the pump always maintains the optimal operating point, is expected to be well solved. Summary of the Invention

[0004] In view of the above technical problems, the technical solution adopted by the present invention is an operation and maintenance control method for a two-stage horizontal medium-pressure low-temperature liquid nitrogen pump. By real-time monitoring the outlet pressure, inlet pressure and operating speed of the pump, dynamically calculating the target pressure difference, and adjusting the opening of the reflux valve through a PID control loop, the pressure difference between the inlet and outlet of the pump is made to approach the target value, thereby improving the operating efficiency and stability of the pump.

[0005] An operation and maintenance control method for a two-stage horizontal medium-pressure low-temperature liquid nitrogen pump provided by the present invention includes the following steps:

[0006] S01. Real-time detect the outlet pressure Pout of the liquid nitrogen pump, the gas phase pressure Pk01 of the rectification column and the pump operating speed r;

[0007] S02. Based on the liquid nitrogen density ρ = 808 kg / m 3 , gravitational acceleration g = 9.81 m / s 2 and liquid level height h = 21 m, calculate the liquid column static pressure H = (ρ × g × h) / 1000 = 166 kPa, and determine the pump inlet pressure as Pk01 + H;

[0008] S03. Calculate the actual inlet and outlet pressure difference PDI = Pout - (Pk01 + H);

[0009] S04. Convert the pump speed r to a normalized parameter X = r / 59.6, and calculate the target pressure difference P through a preset non - linear function relationship Y = f(X);

[0010] S05. Input the actual inlet and outlet pressure difference PDI and the target pressure difference P into a dynamic PID control loop, and adjust the opening of the return valve at the outlet of the liquid nitrogen pump to make PDI approach P;

[0011] S06. When the target pressure difference P exceeds the pressure difference range [P2, P1] corresponding to the pump speed provided by the pump manufacturer, trigger the interlock shutdown protection;

[0012] Wherein, the target pressure difference P is determined by the following method:

[0013] S61. Based on the speed - pressure difference data table provided by the pump manufacturer, fit a continuous function Y = f(X), and the function relationship is a quadratic equation Y = aX2 + bX + c, where a, b, c are fitting coefficients, or the function relationship is a power function Y = aX b , where a, b are fitting coefficients;

[0014] S62. Take the arithmetic mean of the pressure difference range [P2, P1] corresponding to the pump speed, and subtract a safety margin value of 5 kPa as the target pressure difference P.

[0015] Preferably, the fitting process of the function relationship Y = f(X) in step S04 includes:

[0016] S41. Convert the pressure difference range [P2, P1] corresponding to different pump speeds r provided by the pump manufacturer to X = r / 59.6;

[0017] S42. Use the least - squares method to perform non - linear fitting on the data points to obtain a quadratic equation Y = aX2 + bX + c or a power function Y = aX b .

[0018] S43. Pre - debug the fitting equation through a Fortran77 program, and integrate the verified equation into the DCS control module as the calculation basis for the dynamic PID set value.

[0019] Preferably, the dynamic adjustment of the safety margin in step S62 includes:

[0020] S621. When the gas - using pressure fluctuation rate ≥ 2 bar / min, the safety margin automatically increases by 1 kPa - 3 kPa;

[0021] S622. When the pump speed change rate ≥ 10 rpm / s, the safety margin is temporarily increased to 6 kPa;

[0022] S623. After the safety margin is adjusted, if the PDI is continuously within the range of [P2 + 5 kPa, P1 - 5 kPa] for 10 seconds, it is restored to 5 kPa.

[0023] Preferably, the parameter setting of the dynamic PID control loop in step S05 is to divide the control interval according to the pump speed r:

[0024] Low speed range (r < 1500 rpm): Adopt the proportional priority mode, set the proportional gain Kp = 2.0 - 3.0, the integral time Ti = 8 - 10 s, and the derivative time Td = 1 - 2 s;

[0025] High speed range (r ≥ 1500 rpm): Adopt the integral priority mode, set the proportional gain Kp = 1.0 - 1.5, the integral time Ti = 4 - 6 s, and the derivative time Td = 0.5 - 1 s;

[0026] And the mapping relationship between the above PID output and the reflux valve opening is:

[0027] When PDI < P, the output signal reduces the opening according to the equal percentage characteristic, and the calculation formula is: opening change ΔV = Kp×(P - PDI) + Ki×∫(P - PDI)dt + Kd×d(P - PDI) / dt;

[0028] When PDI > P, the output signal increases the opening according to the linear characteristic, and the calculation formula is: opening change ΔV = Kp×(PDI - P) + Ki×∫(PDI - P)dt + Kd×d(PDI - P) / dt;

[0029] Among them, the PID output limit: the maximum opening change rate ≤ 5% / s.

[0030] Preferably, the trigger logic of the interlock shutdown protection in step S06 includes:

[0031] When the PDI exceeds P1 continuously for 10 seconds, trigger the high - zone cavitation interlock shutdown;

[0032] When the PDI is lower than P2 continuously for 10 seconds, trigger the low - zone cavitation interlock shutdown;

[0033] The interlock reset condition is: the PDI is restored to the range of [P2 + 20 kPa, P1 - 20 kPa] and manual confirmation.

[0034] Preferably, the control strategy of the liquid nitrogen pump outlet reflux valve in step S05 further includes:

[0035] When the gas pressure suddenly increases or decreases within the range of 15 bar - 25 bar, the DCS automatically calculates the change rate dP / dt of the target differential pressure P, and restricts the change rate of the reflux valve opening to be synchronized with dP / dt, satisfying the relationship: |d(opening) / dt| ≤ 2×|dP / dt|. Among them, the minimum working opening of the reflux valve is set to 10%.

[0036] Preferably, the implementation of step S06 is achieved through the DCS and SDCS control modules, and the signal processing by the DCS and SDCS control modules includes:

[0037] Perform a two-out-of-three redundancy check on the received Pout and Pk01 signals. If the deviation of a single sensor ≥ 5% of the full scale, automatically switch to the standby sensor;

[0038] When the abnormal rotational speed signal r is detected, freeze the PID output to the state before the fault, and switch to the manual control mode.

[0039] Preferably, verify each fitted function in step S61, and its steps include: set real-time deviation alarm in the DCS. When the deviation between the actual PDI and the target P continuously exceeds ±10 kPa for 10 seconds, automatically trigger the recalculation of the fitting equation. When recalculating, adopt the sliding window method to incorporate the latest 30 groups of operation data and update the fitting coefficients a, b, and c.

[0040] Preferably, the start control of the liquid nitrogen pump under the interlock shutdown protection in step S06 includes:

[0041] At the initial stage of startup, when r < 500 rpm, force the reflux valve opening to be set to 50%, and disable the PID control;

[0042] When r ≥ 500 rpm and the PDI enters the range of [P2 + 50 kPa, P1 - 50 kPa], automatically enable the dynamic PID control.

[0043] Preferably, when the control method is applied to the multi-pump parallel operation condition, the execution of the interlock shutdown protection includes: when the PDI of the pump with the working load being loaded continuously is lower than P - 80 kPa for 60 seconds, automatically start the standby pump and synchronously adjust the reflux valve. Then, the load distribution among multiple pumps is dynamically balanced based on the real-time dP / dt difference of each pump, and when the difference exceeds 5 kPa / s, trigger the load reallocation.

[0044] The present invention has at least the following beneficial effects:

[0045] 1. Detect the operating speed, outlet pressure of the liquid nitrogen pump and the pressure of the process system upstream of the liquid nitrogen pump inlet through the DCS system. Among them, the pressure of the process system upstream of the liquid nitrogen pump inlet is the sum of the static pressure of the liquid column between the gas phase pressure of the rectification column and the pump inlet. Then, according to the operating speed of the liquid nitrogen pump, combined with the maximum and minimum differential pressures allowed at the pump inlet and outlet provided by the pump manufacturer, the differential pressure control set value for preventing cavitation protection corresponding to the optimal value of the pump operating condition is obtained through internal calculation of the DCS control system. This set value is obtained by correlating the pump speed and the pump inlet and outlet differential pressure data through a functional relationship and through calculation. Through the dynamic PID automatic control loop, precise control of the working point of the liquid nitrogen pump is achieved, avoiding the equipment entering the unstable working area due to improper operation, thereby improving the operating stability of the equipment. Then, by calculating the optimal working point of the pump inlet and outlet differential pressure and automatically controlling the reflux regulating valve, the occurrence of cavitation conditions is effectively prevented, and the service life of the equipment is extended.

[0046] 2. Through the dynamic PID automatic control loop, compare the difference between the actually detected pump inlet and outlet pressures with the calculated differential pressure control set value. If the actually detected differential pressure is less than the set value, the PID control output automatically reduces the output of the liquid nitrogen pump outlet reflux valve to increase the differential pressure; if the actually detected differential pressure is greater than the set value, the PID control output automatically increases the output of the liquid nitrogen pump outlet reflux valve to reduce the differential pressure.

[0047] 3. Through the set interlock protection function, when the control fails due to other unforeseen reasons and the pump inlet and outlet differential pressure data is higher than the high - zone cavitation differential pressure or lower than the low - zone cavitation differential pressure for more than the specified allowable time, the DCS will interlock the pump to stop running to avoid equipment damage and further improve the reliability of the equipment.

[0048] 4. To improve the control accuracy, the operating condition data provided by the pump equipment manufacturer is also fitted, and the maximum and minimum differential pressures at the inlet and outlet of the horizontal double - stage cryogenic liquid pump under different speed conditions are converted into a functional relationship with certain rules. During the DCS program design process, the directly fitted equation is used to implement the calculation function through computer language, and then the internal operation functions and modules of the DCS controller are configured. The entire control process is fully automated without any intervention from the operator, reducing the risk of human operation errors.

[0049] 5. According to the maximum and minimum values of the corresponding inlet and outlet differential pressures of the horizontal medium - pressure cryogenic liquid nitrogen pump provided by the pump manufacturer under different speed conditions, the values to be given in the operating condition are obtained through calculation. And based on the required gas supply pressure range and safety margin of the customer, the set value of the process control loop is determined, so as to ensure that the pump can stably operate at the optimal working point under different speed conditions, ensuring the stable operation of the pump under different speed conditions. Description of the Drawings

[0050] To more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the accompanying drawings required for the description of the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0051] Figure 1 It is a flowchart of an operation and maintenance control method for a two-stage horizontal medium-pressure low-temperature liquid nitrogen pump provided by an embodiment of the present invention;

[0052] Figure 2 It is a differential pressure diagram of the pump inlet and outlet at the best operating point corresponding to different frequencies of the pump provided by an embodiment of the present invention;

[0053] Figure 3 It is a data table of the operating points of the two-stage horizontal medium-pressure low-temperature liquid nitrogen pump provided by an embodiment of the present invention. Detailed implementation manners

[0054] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative efforts belong to the scope of protection of the present invention.

[0055] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above accompanying drawings are used to distinguish similar objects, and do not necessarily need to describe a specific order or sequence. It should be understood that such data can be interchanged under appropriate circumstances so that the embodiments of the present invention described here can be implemented in an order other than those illustrated or described here. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or server including a series of steps or units does not necessarily need to be limited to those clearly listed steps or units, but may include other steps or units not clearly listed or inherent to these processes, methods, products, or devices.

[0056] This embodiment provides an operation and maintenance control method for a two-stage horizontal medium-pressure low-temperature liquid nitrogen pump. The method includes the following steps, as Figure 1 shown:

[0057] S01. Real-time detect the outlet pressure Pout of the liquid nitrogen pump, the gas phase pressure Pk01 of the rectification column, and the pump operating speed r;

[0058] S02. Based on the liquid nitrogen density ρ = 808 kg / m 3, the acceleration of gravity g = 9.81 m / s 2 and the liquid level height h = 21 m, calculate the static pressure of the liquid column H = (ρ × g × h) / 1000 = 166 kPa, and determine the pump inlet pressure as Pk01 + H (the sum of the distillation column pressure and the static pressure of the liquid column);

[0059] S03. Calculate the actual inlet and outlet pressure difference PDI = Pout - (Pk01 + H);

[0060] S04. Convert the pump speed r to the normalized parameter X = r / 59.6( Figure 2 ), and calculate the target pressure difference P through the preset non - linear function relationship Y = f(X);

[0061] Specifically, the fitting process of the function relationship Y = f(X) in step S04 includes:

[0062] S41. Convert the pressure difference range [P2, P1] corresponding to different speeds r provided by the pump manufacturer to X = r / 59.6;

[0063] S42. Use the least - squares method to perform non - linear fitting on the data points to obtain the quadratic equation Y = aX2 + bX + c or the power function Y = aX b .

[0064] S43. Pre - debug the fitting equation through the Fortran77 program, and integrate the verified equation into the DCS control module as the calculation basis for the dynamic PID set value.

[0065] The accuracy of the equation was verified through a Fortran77 program as described above to ensure that the fitting error ≤ 1%; and by embedding the equation into the DCS control module as the core for calculating the dynamic PID set value, the problem of insufficient accuracy of the built-in linearization module in the DCS was avoided; and the quadratic equation or power function equation can accurately match the non-linear working characteristics of the pump (such as the quadratic characteristics of the head-flow curve). That is to say, during the data fitting process, after the conversion of the pump speed and frequency conversion data, the accuracy of the fitted function equation was greatly improved, which is also the feature of this patent. In addition, during the DCS program design process, for the fitted equation, the equation was pre-debugged through the fortran77 program, and the calculation function was realized through computer language, and then the internal operation functions and modules of the DCS controller were used for configuration, instead of using the linearization function module inside the DCS for configuration design, which greatly improved the control accuracy. Through such a cavitation protection function to continuously control the PID loop, the working reliability of the horizontal medium-pressure low-temperature liquid nitrogen pump was completely improved. After on-site equipment debugging, including various operating conditions such as customer gas fluctuations, automatic online standby switching of the liquid nitrogen pump, start-up and shutdown, this cavitation protection dynamic PID automatic control fully meets the factory process technical requirements. The reason why the PID module of this control loop is called dynamic PID automatic control is that the set value of this PID is automatically calculated and adjusted as the pump speed changes.

[0066] S05. Input the actual inlet and outlet pressure difference PDI and the target pressure difference P into the dynamic PID control loop, and adjust the opening of the outlet reflux valve of the liquid nitrogen pump to make PDI approach P;

[0067] Specifically, the parameter settings of the dynamic PID control loop in step S05 are to divide the control interval according to the pump speed r:

[0068] Low speed range (r < 1500 rpm): Adopt the proportional priority mode, set the proportional gain Kp = 2.0 - 3.0, the integral time Ti = 8 - 10 s, and the derivative time Td = 1 - 2 s;

[0069] High speed range (r ≥ 1500 rpm): Adopt the integral priority mode, set the proportional gain Kp = 1.0 - 1.5, the integral time Ti = 4 - 6 s, and the derivative time Td = 0.5 - 1 s;

[0070] And the mapping relationship between the above PID output and the opening of the reflux valve is:

[0071] When PDI < P, the output signal reduces the opening according to the equal percentage characteristic, and the calculation formula is: the opening change ΔV = Kp × (P - PDI) + Ki × ∫(P - PDI)dt + Kd × d(P - PDI) / dt;

[0072] When PDI > P, the output signal increases the opening degree according to the linear characteristic, and the calculation formula is: the change in opening degree ΔV = Kp × (PDI - P) + Ki × ∫(PDI - P)dt + Kd × d(PDI - P) / dt;

[0073] Among them, the PID output limit: the maximum opening degree change rate ≤ 5% / s, Kp is the proportionality coefficient, Ki is the integral coefficient, and Ki = Kp / Ti, Ti is the integral time, Kd is the differential coefficient, and Kd = Kp*Td, Td is the differential time.

[0074] The above-mentioned embodiment calculates the arithmetic mean value and the set safety margin coefficient according to the upper and lower limit P1 / P2 values of the inlet and outlet pressure difference working range under the corresponding rotational speed conditions provided by the manufacturer. This coefficient and the adopted algorithm are based on the fact that during the operation of the pump, the normal working range of its outlet pressure is between 1500 kpa and 3000 kpa, and in combination with the control range required by the customer's gas pressure, a special design is carried out. Based on the customer's gas pressure fluctuation range and combined with the upper and lower limit data required by the pump manufacturer's performance table ( Figure 3 ), we found that during the linkage process between the horizontal medium-pressure low-temperature liquid nitrogen pump and the customer's gas supply, if the fluctuation of the customer's gas consumption at the back end is small but the speed is too fast, it is very easy to exceed the high-region cavitation pressure difference P1, and the risk of exceeding the high-region cavitation pressure is relatively high. Therefore, we adopt the arithmetic mean value and set a downward safety margin. And when the cavitation protection is loaded from the pump startup to the normal operation and the load reduction working conditions, it maintains a fully automatic input and control state, thus avoiding the instability of the pump entering the high region and the low region cavitation caused by the operator's improper operation, and the reliability is further improved. For some domestic similar low-temperature liquid pumps, when the normal rotational speed condition is reached, the cavitation protection interlock is put into operation, and there are great safety risks in this way of cavitation. When the cavitation of the low-temperature liquid pump occurs, no matter which rotational speed condition it corresponds to, especially when running at a low rotational speed, the probability of cavitation is still relatively high. In addition, in this patented technology, the allowable operating condition range of the pump provided by the pump manufacturer is accurately defined in combination with the user's gas consumption characteristics and fluctuation range. In addition to the interlock stop actions for entering the high region and the low region cavitation ranges required by the pump manufacturer, we mainly improve the control means and perform precise control.

[0075] Secondly, the control strategy of the liquid nitrogen pump outlet reflux valve in step S05 also includes:

[0076] When the gas pressure suddenly increases or decreases within the range of 15 bar - 25 bar, the DCS automatically calculates the change rate dP / dt of the target pressure difference P, and limits the change rate of the reflux valve opening degree to be synchronized with dP / dt, satisfying the relationship: |d(opening degree) / dt| ≤ 2 × |dP / dt|, where the minimum working opening degree of the reflux valve is set to 10%.

[0077] S06. When the target differential pressure P exceeds the differential pressure range [P2, P1] provided by the pump manufacturer at the corresponding rotational speed, trigger the interlock shutdown protection;

[0078] Specifically, the trigger logic of the interlock shutdown protection in step S06 includes:

[0079] When PDI exceeds P1 continuously for 10 seconds, trigger the high - zone cavitation interlock shutdown;

[0080] When PDI is lower than P2 continuously for 10 seconds, trigger the low - zone cavitation interlock shutdown;

[0081] The interlock reset condition is: PDI returns to the interval [P2 + 20 kPa, P1 - 20 kPa] and manual confirmation is made.

[0082] Furthermore, the implementation of step S06 is achieved through the SDCS control module, and the signal processing of the SDCS control module includes:

[0083] Perform a two - out - of - three redundancy check on the received Pout and Pk01 signals. If the deviation of a single sensor is ≥ 5% of the full scale, automatically switch to the standby sensor;

[0084] When an abnormal rotational speed signal r is detected, freeze the PID output to the state before the fault and switch to the manual control mode.

[0085] And the start - up control of the liquid nitrogen pump under the interlock shutdown protection in step S06 includes:

[0086] At the initial stage of start - up, when r < 500 rpm, force the opening of the reflux valve to be set to 50% and disable the PID control;

[0087] When r ≥ 500 rpm and PDI enters the range [P2 + 50 kPa, P1 - 50 kPa], automatically enable the dynamic PID control.

[0088] Through this functional relation, for different r values of the pump, the optimal value of the pressure difference between the outlet and the inlet of the liquid nitrogen pump is P. This value is used as the control index for the optimal operating point of the liquid nitrogen pump and is set as the set value of the PID control loop. When the actually detected PDI value is less than the set value P, the PID control output automatically reduces the output of the outlet reflux valve of the liquid nitrogen pump, thereby increasing the PDI value to make it close to the set P value; when the actually detected PDI value is greater than the set value P, the PID control output automatically increases the output of the outlet reflux valve of the liquid nitrogen pump, thereby reducing the PDI value to make it close to the set P value. The return flow at the outlet of the liquid nitrogen pump is automatically controlled and automatically tracked solely by this anti-cavitation protection PID control loop. The liquid nitrogen pump can be effectively and automatically controlled within the entire operating speed range. In addition to this PID control loop, there is no other control input for the return flow at the outlet of the liquid nitrogen pump, completely avoiding the intervention of operators and other control effects, thus improving the reliability of the control process.

[0089] The target pressure difference P in the above embodiments is determined by the following method:

[0090] S61. Based on the rotational speed-pressure difference data table provided by the pump manufacturer, fit the continuous function Y = f(X). The functional relation is a quadratic equation Y = aX2 + bX + c, where a, b, and c are fitting coefficients, or the functional relation is a power function Y = aX b , where a and b are fitting coefficients.

[0091] S62. Take the arithmetic mean of the pressure difference range [P2, P1] corresponding to the rotational speed and subtract a safety margin value of 5 kPa as the target pressure difference P.

[0092] Specifically, the dynamic adjustment of the safety margin in step S62 includes:

[0093] S621. When the gas pressure fluctuation rate ≥ 2 bar / min, the safety margin automatically increases by 1 kPa - 3 kPa;

[0094] S622. When the pump rotational speed change rate ≥ 10 rpm / s, the safety margin is temporarily increased to 6 kPa;

[0095] S623. After the safety margin is adjusted, if the PDI is continuously within the range of [P2 + 5 kPa, P1 - 5 kPa] for 10 seconds, it is restored to 5 kPa.

[0096] In the above technology, by detecting the outlet pressure, inlet pressure and operating speed of a horizontal low-temperature medium-pressure liquid nitrogen pump, and through automatic calculation by a program, the difference between the actually detected inlet and outlet pressures is compared with the calculated inlet and outlet pressure difference, and the reflux regulating valve of the liquid nitrogen pump is automatically controlled to effectively control the operating point of the pump. Since the required values of the inlet and outlet pressure differences of the pump are different under different rotational speeds, it is necessary to correlate the process index of the pump speed with the inlet and outlet pressure difference data through a functional relationship. After such calculation, the optimal given value of the pump is obtained, which serves as an automatic control loop for cavitation protection, with the expectation of automatically controlling the operating point of the pump to be in the best state. Since the pump manufacturer has provided a working characteristic data table of the pump operation ( Figure 3 ), this data table represents the maximum and minimum values of the inlet and outlet pressure differences of the pump under different rotational speed conditions. If the operating point of the pump exceeds the maximum value of the inlet and outlet pressure difference or is lower than the minimum value of the inlet and outlet pressure difference under the corresponding rotational speed condition, the pump will trigger an interlock shutdown. To avoid the pump entering the operating condition range corresponding to the interlock shutdown, through a continuous PID control loop, the effective control of the pump operating point is achieved. The input parameters for the control are the inlet and outlet pressure differences of the pump and the corresponding rotational speed, and the output is the return flow rate at the pump outlet.

[0097] Furthermore, in the above embodiment, each fitted function in step S61 is verified, and the steps include: setting a real-time deviation alarm in the DCS. When the deviation between the actual PDI and the target P continuously exceeds ±10 kPa for 10 seconds, the fitting equation is automatically triggered to recalculate. When recalculating, the sliding window method is used to incorporate the latest 30 groups of operating data to update the fitting coefficients a, b, and c.

[0098] When the deviation between the PDI and the target P continuously exceeds ±10 kPa for 10 seconds, the equation is triggered to recalculate. The sliding window method incorporates the latest 30 groups of operating data to update the fitting coefficients a, b, and c; during the execution of the recalculation process, the PID control is paused and switched to the fixed opening mode; after the fitting is completed, the dynamic PID is re-enabled.

[0099] It should be noted that when the control method provided by the present invention is applied to the multi-pump parallel operating condition, the implementation of the interlock shutdown protection includes: when the PDI of the pump with the working load being loaded continuously is lower than P - 80 kPa for 60 seconds, the standby pump is automatically started and the reflux valve is adjusted synchronously. Then, the load distribution among the multiple pumps is dynamically balanced based on the real-time dP / dt differences of each pump. When the difference exceeds 5 kPa / s, load redistribution is triggered. The opening degrees of the reflux valves of each pump are adjusted synchronously in proportion to avoid single-pump overload or inefficient operation.

[0100] In summary, the control process starts with real-time data acquisition, including the outlet pressure of the liquid nitrogen pump (Pout), the gas-phase pressure of the rectification column (Pk01), and the pump speed (r). The inlet pressure of the pump is calculated by the sum of the rectification column pressure and the hydrostatic pressure of the liquid column (H = 166 kPa). The actual pressure difference (PDI) is dynamically obtained from the difference between the outlet pressure and the inlet pressure. The target pressure difference (P) is set based on the speed-pressure performance table provided by the pump manufacturer. The speed is converted into the corresponding frequency value through the normalized parameter X = r / 59.6, and the quadratic equation (Y = aX2 + bX + c) or the power function Y = aX b , where a and b are fitting coefficients. The dynamic target value is generated by fitting. The final value of the target pressure difference is designed by combining the safety margin, subtracting 5 kPa from the arithmetic mean ((P1 + P2) / 2), to form a control benchmark that takes into account both sensitivity and stability.

[0101] The dynamic PID control loop takes the deviation between the actual pressure difference (PDI) and the target pressure difference (P) as the input, and realizes the working point tracking by adjusting the opening of the reflux valve. For different speed ranges, the PID parameters adopt a zoning optimization strategy: at low speeds (<1500 rpm), the proportional action is emphasized (Kp = 2.0 - 3.0) to improve the response speed; at high speeds (≥1500 rpm), the integral action is enhanced (Ti = 4 - 6 s) to suppress high-frequency oscillations. The control output further limits the valve opening change rate (≤5% / s) to avoid hydraulic shock.

[0102] To cope with sudden working conditions, the control strategy introduces a dynamic adjustment mechanism for the safety margin. When the gas usage pressure fluctuation rate ≥2 bar / min or the speed change rate ≥10 rpm / s, the margin value is temporarily increased (1 - 8 kPa) and automatically restored after stabilization. The interlock protection is used as the final barrier, and two-level trigger conditions are set: if the PDI exceeds the limit (higher than P1 or lower than P2) continuously for 10 seconds, the interlock shutdown is immediately triggered; the reset requires the PDI to return to the extended safety range ([P2 + 20 kPa, P1 - 20 kPa]) and manual confirmation to avoid misoperation.

[0103] Regarding signal reliability, the system adopts a two-out-of-three redundancy check: when the deviation of a single sensor ≥5% of the full scale, the standby channel is automatically switched; if the speed signal is abnormal, the PID output is frozen and switched to the manual mode. In addition, the control algorithm has the ability of online self-correction. If the deviation between the PDI and the target value continuously exceeds the limit (±10 kPa for 10 seconds), the fitting equation is automatically triggered to recalculate, and the latest 30 sets of data are incorporated through the sliding window method to update the coefficients to adapt to the pump performance decay.

[0104] During the startup phase (r < 500 rpm), the opening of the forced fixed reflux valve (50%) is fixed and the PID control is disabled to ensure stable initial flow. After the rotational speed increases to the safety threshold and the PDI enters the buffer range ([P2 + 50 kPa, P1 - 50 kPa]), the control right is gradually released to the dynamic PID. For a multi-pump parallel system, the load distribution is dynamically balanced according to the real-time pressure difference change rate (dP / dt): when the PDI of a single pump continuously exceeds the limit (> P + 20 kPa for 60 seconds), the standby pump is started, and the load is redistributed by triggering the difference threshold (> 5 kPa / s) to achieve efficient coordination.

[0105] Those of ordinary skill in the art can understand that all or part of the processes in the methods of the above embodiments can be completed by instructing relevant hardware through a computer program. The computer program can be stored in a non-volatile computer-readable storage medium. When the computer program is executed, it can include the processes of the embodiments of the above methods. Among them, any reference to a memory, storage, database, or other medium used in the embodiments provided in the present application can include non-volatile and / or volatile memories. Non-volatile memories can include read-only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), or flash memory. Volatile memories can include random access memory (RAM) or external cache memory. By way of illustration and not limitation, RAM is available in various forms, such as static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDR SDRAM), enhanced SDRAM (ESDRAM), synchronous link DRAM (SLDRAM), Rambus direct RAM (RDRAM), direct memory bus dynamic RAM (DRDRAM), and Rambus dynamic RAM (RDRAM), etc.

[0106] Those skilled in the art can clearly understand that for the convenience and simplicity of description, only the above-mentioned division of each functional unit and module is used as an example. In practical applications, the above functions can be allocated to different functional units and modules according to needs, that is, the internal structure of the device is divided into different functional units or modules to complete all or part of the functions described above.

[0107] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Although the present invention has been disclosed above with the preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some changes or modifications to equivalent embodiments by using the above-disclosed technical content within the scope of the technical solution of the present invention. However, as long as it does not depart from the content of the technical solution of the present invention, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A two-stage horizontal medium-pressure cryogenic liquid nitrogen pump operation and maintenance control method, characterized in that: It includes the following steps: S01. Real-time detect the outlet pressure Pout of the liquid nitrogen pump, the gas-phase pressure Pk01 of the rectifying column, and the pump operating speed r; S02, based on liquid nitrogen density ρ = 808 kg / m 3 , gravitational acceleration g = 9.81 m / s 2 And the liquid level height h = 21m, calculate the static pressure of the liquid column H = (ρ × g × h) / 1000 = 166kPa, and determine the pump inlet pressure as Pk01 + H; S03. Calculate the actual inlet and outlet pressure difference PDI = Pout - (Pk01 + H); S04. Convert the pump speed r to a normalized parameter X = r / 59.6, and calculate the target pressure difference P through a preset non-linear function relationship Y = f(X); S05. Input the actual inlet and outlet pressure difference PDI and the target pressure difference P into a dynamic PID control loop, and adjust the opening of the outlet reflux valve of the liquid nitrogen pump to make PDI approach P; S06. When the target pressure difference P exceeds the pressure difference range [P2, P1] corresponding to the pump speed provided by the pump manufacturer, trigger the interlock shutdown protection; Among them, the target pressure difference P is determined in the following way: S61, based on the speed-pressure difference data table provided by the pump manufacturer, fit the continuous function Y=f(X), the functional relationship of which is a quadratic equation Y=aX2+bX+c or a power function Y=aX b , where a, b, and c are fitting coefficients. S62. Take the arithmetic mean of the pressure difference range [P2, P1] corresponding to the corresponding speed, and subtract a safety margin value of 5 kPa as the target pressure difference P.

2. A two-stage horizontal medium-pressure cryogenic liquid nitrogen pump operation and maintenance control method according to claim 1, characterized in that: The fitting process of the function relationship Y = f(X) in the step S04 includes: S41. Convert the pressure difference range [P2, P1] corresponding to different speeds r provided by the pump manufacturer to X = r / 59.6; S42, use the least squares method to perform nonlinear fitting on the data points, and obtain the quadratic equation Y = aX2 + bX + c or the power function Y = aX b . S43. Pre-debug the fitting equation through the Fortran77 program, and integrate the verified equation into the DCS control module as the calculation basis for the dynamic PID set value.

3. The two-stage horizontal medium-pressure cryogenic liquid nitrogen pump operation and maintenance control method according to claim 1 is characterized in that: The dynamic adjustment of the safety margin in the step S62 includes: S621. When the gas pressure fluctuation rate ≥ 2 bar / min, the safety margin automatically increases by 1 kPa - 3 kPa; S622. When the pump speed change rate ≥ 10 rpm / s, the safety margin temporarily increases to 6 kPa; S623. After the safety margin is adjusted, if PDI is continuously within the range of [P2 + 5 kPa, P1 - 5 kPa] for 10 seconds, it will return to 5 kPa.

4. A two-stage horizontal medium-pressure cryogenic liquid nitrogen pump operation and maintenance control method according to claim 1, characterized in that: The parameter setting of the dynamic PID control loop in the step S05 is divided into control intervals according to the pump speed r: Low speed interval (r < 1500 rpm): Adopt the proportional priority mode, set the proportional gain Kp = 2.0 - 3.0, the integral time Ti = 8 - 10 s, and the derivative time Td = 1 - 2 s; High speed interval (r ≥ 1500 rpm): Adopt the integral priority mode, set the proportional gain Kp = 1.0 - 1.5, the integral time Ti = 4 - 6 s, and the derivative time Td = 0.5 - 1 s; And the mapping relationship between the above PID output and the reflux valve opening is: When PDI < P, the output signal reduces the opening according to the equal percentage characteristic, and the calculation formula is: opening change ΔV = Kp×(P - PDI) + Ki×∫(P - PDI)dt + Kd×d(P - PDI) / dt; When PDI > P, the output signal increases the opening according to the linear characteristic, and the calculation formula is: opening change ΔV = Kp×(PDI - P) + Ki×∫(PDI - P)dt + Kd×d(PDI - P) / dt; Among them, PID output limit: the maximum opening change rate ≤ 5% / s.

5. The two-stage horizontal medium-pressure cryogenic liquid nitrogen pump operation and maintenance control method according to claim 1 is characterized in that: The trigger logic of the interlock shutdown protection in the step S06 includes: When PDI exceeds P1 for 10 seconds continuously, the high-zone cavitation interlock parking is triggered; When PDI is lower than P2 for 10 seconds continuously, the low-zone cavitation interlock parking is triggered; The interlock reset condition is: PDI returns to the [P2+20kPa, P1-20kPa] range and is manually confirmed.

6. A two-stage horizontal medium-pressure cryogenic liquid nitrogen pump operation and maintenance control method according to claim 1, characterized in that: The control strategy of the liquid nitrogen pump outlet reflux valve in step S05 also includes: When the gas pressure suddenly increases or decreases within the range of 15bar-25bar, the DCS automatically calculates the change rate dP / dt of the target pressure difference P, and limits the change rate of the return valve opening to be synchronized with dP / dt, satisfying the relationship: |d(opening) / dt|≤2×|dP / dt|, where the minimum operating opening of the return valve is set to 10%.

7. A two-stage horizontal medium-pressure cryogenic liquid nitrogen pump operation and maintenance control method according to claim 1, characterized in that: The implementation of step S06 is achieved through the DCS and SDCS control modules, and the signal processing by the DCS and SDCS control modules includes: Perform two-out-of-three redundancy check on the received Pout and Pk01 signals. If the deviation of a single sensor is ≥5% of the range, it will automatically switch to the backup sensor. When an abnormal speed signal r is detected, the PID output is frozen to the pre-fault state and switched to manual control mode.

8. The two-stage horizontal medium-pressure cryogenic liquid nitrogen pump operation and maintenance control method according to claim 1 is characterized in that: Each of the fitted functions in step S61 is verified, and the steps include: setting a real-time deviation alarm in the DCS, and automatically triggering the recalculation of the fitting equation when the deviation between the actual PDI and the target P exceeds ±30 kPa for 20 seconds. During the recalculation, the sliding window method is used to incorporate the latest 30 sets of operating data and update the fitting coefficients a, b, and c.

9. The two-stage horizontal medium-pressure cryogenic liquid nitrogen pump operation and maintenance control method according to claim 1 is characterized in that: The start-up control of the liquid nitrogen pump under the interlock parking protection in step S06 includes: When the engine starts up, r < 500 rpm, the reflux valve opening is forced to 50% and PID control is disabled; When r≥500rpm and PDI enters the range of [P2+50kPa,P1-50kPa], dynamic PID control is automatically enabled.

10. The two-stage horizontal medium-pressure cryogenic liquid nitrogen pump operation and maintenance control method according to claim 1, characterized in that: When the control method is applied to a multi-pump parallel working condition, the interlock parking protection execution includes: when the PDI of the working load-loaded pump is lower than P-80kPa for 60 seconds, the standby pump is automatically started and the reflux valve is adjusted synchronously. The load distribution among the multiple pumps is dynamically balanced according to the real-time dP / dt difference of each pump, and load redistribution is triggered when the difference exceeds 5kPa / s.

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