1. Introduction
The Daqing Petroleum Administration Water Supply Company's annual electricity costs account for approximately 30% of its total water supply costs, with electricity expenses in the water transmission and distribution环节 accounting for about 70% of the total. Therefore, optimizing the daily operation of the external transmission pumps to keep the electricity consumption per ton of water transmission and distribution within a reasonable and economical range is a key focus of energy conservation efforts. We have consistently explored, diligently researched, and adopted effective measures to address this issue.
2. Problem Statement
The company's Longhupao Water Intake Plant is located on the north bank of Longhupao Lake, with a daily water intake capacity of 500,000 cubic meters. During the initial design phase, considering medium- and long-term planning and the most unfavorable water volume and level conditions, two pump unit types were selected: one with a 1400KW/10KV motor and a 900HR intake pump (H62 meters, Q6360 cubic meters/hour); the other with a 900KW/10KW motor and a 700HR intake pump (H62 meters, Q3900 cubic meters/hour). The secondary pumping station of the water intake plant—the Central Water Diversion Plant—is designed to supply 500,000 cubic meters of water per day across its first and second phases. Due to oilfield production restrictions, water consumption in the external network has been declining year by year. Currently, the actual water delivery capacity of the water intake plant is only 250,000-300,000 cubic meters per day, and the total pump flow rate Q is only 104-125 million cubic meters per hour. The head loss of the water pipeline is 18-20 meters, while the pump's nameplate head is 62 meters. If the pump pressure is to drop to match the pipeline resistance, the pump will operate under overload. Therefore, the water intake plant can only control the pump pressure by adjusting the outlet gate to allow the pump to operate in the high-efficiency range. This operation not only increases the head loss of the gate and wastes a lot of electrical energy, but also, if the gate throttling adjustment is not appropriate, the pump can easily deviate from the high-efficiency range or operate under overload, leading to an increased pump failure rate and shortening the unit's service life. In the past two years, the pump has been repaired more than 10 times, costing more than 1 million yuan in maintenance fees. The Central Water Diversion Plant is a major production facility for the company and plays a crucial role in ensuring the water supply for the oilfield. Therefore, the technical transformation of the water pumps at the Longhupao Water Intake Plant is imperative.
3. Preliminary calculations and feasibility studies for the frequency converter retrofit scheme
Under normal circumstances, pumps operate under rated operating conditions, designed and optimized for optimal performance. Figure 1 shows a typical pump operating curve. AB is the pump's performance curve, matched with the rated system pressure curve OB. The rated flow rate at rated pressure is obtained at point B, where the pump has its highest efficiency. Closing the valve controls the flow rate. When the flow rate decreases, the pump operates at points P, Q, R, and S respectively. At this point, the pump needs to operate under a very high pressure differential, therefore the pump's energy output is much higher than the actual system requirements. This excess energy manifests as heat loss at the valve and is carried away by the liquid flow. Dividing the pump's output energy by its efficiency yields the total input energy to the pump. Adjusting the pump outlet valve opening to reduce the flow rate results in significant energy loss.
By using a frequency converter to regulate speed, the motor can drive the pump to operate at varying speeds. The pump's characteristic curve can be matched with the system's needs at any flow rate. The flow rate is proportional to the motor speed, and the resulting pressure difference is proportional to the square of the speed, as shown in Figure 2. After stepless frequency conversion speed regulation, countless pump characteristic curves AB-CD can be obtained. Any intersection point of the pipeline characteristic curve and the shaded band formed by ABDC can be used as the operating point to adapt to changes in the external network water volume. The flow rate corresponding to points P, Q, R, and S can be achieved at a relatively small head, with only a small energy loss. Its energy-saving effect is quite considerable.
For the same centrifugal pump, the following pattern applies:
Q1/Q2=n1/n2H1/H2=(n1/n2)2N1/N2=(n1/n2)3
Pumps #1, #3, and #6 at the Longhupao Water Intake Plant are all imported large pumps with motor speeds of 748 r/m, 745 r/m, and 750 r/m, respectively.
If pump #3 is adjusted to a speed of 500 r/m, its flow rate, head, and power parameters are as follows:
Q<sub>original</sub>/Q<sub>variable</sub> = n<sub>original</sub>/n<sub>variable</sub> = 748/500 = 1.496
Q = 4251.3 cubic meters/hour
H<sub>original</sub>/H<sub>variable</sub> = (n<sub>original</sub>/n<sub>variable</sub>)<sup>2</sup> = 1.4962
H_transformation = 28.59 meters
N<sub>original</sub>/N<sub>variable</sub> = (n<sub>original</sub>/n<sub>variable</sub>)<sup>3</sup> = 1.4963
N<sub>variable</sub> = 412.2 kW
If the speed is adjusted to 550 r/m, then Q = 4663 cubic meters/hour, H = 34 meters, and N = 547 kilowatts. The pump parameters after speed adjustment are basically suitable for the current operating conditions. Of course, the above calculations are based on the assumption that the system's pipeline characteristic curve remains unchanged. After installing the variable frequency speed control device, the pump outlet valve can be fully opened, and the system's pipeline characteristic curve will shift outward. Therefore, the pump operating parameters after speed adjustment are more likely to meet the requirements of the actual operating conditions. Thus, this scheme can be determined as feasible.
4. Energy saving estimation of variable frequency speed control scheme
The formula for calculating the energy consumption of a water pump is: P = (K × H × Q) / η, where K is the margin coefficient and η is the efficiency.
Assuming the actual pressure of the water pump is reduced from the current 5.8 kg/m³ to 2.5 kg/m³, while the flow rate remains unchanged, then P25/P58 = 25/58 = 43%, meaning the power consumption is theoretically reduced by 1 - 43% = 57%. At an electricity price of 0.41 yuan/kWh, a motor efficiency of 0.95, and an operating period of 330 days/year, the original annual electricity cost would be (1400KW × 24 × 330 × 0.41) / 0.95 = 4.78 million yuan.
After the variable frequency speed control reduces the speed to 2.5 kg, the annual electricity cost is: 478 × 43% = 2,055,400 yuan, and the annual electricity cost savings are 478 - 2,055,400 = 2,725,000 yuan. Initially, the energy-saving effect appears quite considerable.
5. Plan Determination
Considering the actual conditions of the central pump, and after careful calculation, demonstration, multi-faceted investigation, and comparison of numerous manufacturers, we decided to install a high-voltage, high-power frequency converter manufactured by Beijing Leadway Company. This frequency converter meets the pump's process requirements in all aspects, has high reliability, and offers short modification time and quick results.
The specific plan involves closing the connecting valve between Phase I and Phase II, allowing Phase II's pipeline network to operate independently. The frequency converter will drive Pump #6 in a one-to-one manner; in case of Pump #6 failure, it can switch to mains frequency operation. Switching between mains frequency and frequency converter is done manually via a bypass cabinet. Furthermore, the switching between mains frequency and frequency converter is interlocked by electrical and PLC logic, ensuring safe unit operation. Constant pressure control and closed-loop operation are implemented, with the Phase II pipeline pressure at 0.1 MPa. Automatic valve linkage control is employed, with cooling water flow detection and protection functions.
6. Trial operation status of the high-pressure variable frequency speed control device for pump #6 after modification.
On January 6, 2002, technicians from Beijing Leadway and relevant personnel from the water intake plant conducted a trial run of the variable frequency speed control device for pump #6. The trial run results are as follows:
Before trial operation, pumps #3 and #7 were operated in parallel, and the operating parameters are shown in the table below:
Pump pressure of #3 pump (MPa) Pump pressure of #7 pump (MPa) Phase I flow rate (m³/h) Phase II flow rate (m³/h) Phase I pipe pressure (MPa) Phase II pipe pressure (MPa) Phase I outlet valve opening % Phase II outlet valve opening % 0.57 0.57 5000 5000 0.17 0.16 12 12.6
To ensure the stable operation of the central water diversion plant during the trial run, we notified the substation and the central water diversion plant in advance through dispatch to prepare for the trial run. First, pump #7 was shut down, the manifold connecting valve between pumps #3 and #6 was closed, and the operation of pump #3 was adjusted. The opening of the first-phase outlet valve remained unchanged at 12%. The frequency converter of pump #6 was set to 37 Hz, and pump #6 was started. After the operation stabilized, the second-phase outlet valve was opened to 70%, and the operating status of the pump unit was observed at frequencies of 35 Hz, 33 Hz, 30 Hz, and 28 Hz.
Set frequency (Hz) Pump pressure of pump #6 (MPa) Pump flow rate of pump #6 (m³/h) Output current (A) 370.21650077350.19600071.5330.17560067300.14480052280.12435050
After trial operation, we set the frequency to 30 Hz for open-loop operation and assigned water plant staff to keep separate operation records for pump #6. After several days of operation, the frequency converter operated stably, demonstrated full functionality, and was easy to operate, fully meeting the requirements of the actual working conditions and achieving the expected results.
7. Energy efficiency test after renovation
The energy-saving effect must ultimately be verified by instruments. The most direct and reliable method is to use an electricity meter to measure and compare the energy consumption before and after the frequency conversion modification.
Pumps #3 and #6 were originally operating at power frequency. The calculations are based on the operating records from August 1st to 20th, 2001:
Total electricity consumption: 1,214,700 kWh;
Total water supply: 5,887,400 cubic meters;
Total unit consumption of the pumping station: 0.2064 kWh/cubic meter;
Calculations were performed using the operating records of pumps #3 and #6 at power frequency from July 8th to 19th, 2001:
Total electricity consumption: 667,500 kWh;
Total water supply: 3,115,200 cubic meters;
Total unit consumption of the pumping station: 0.2143 kWh/cubic meter;
The total unit consumption of the pumping station during 32 days of continuous operation of pumps #3 and #6 at industrial frequency is:
(0.2064+0.2143)/2=0.21 kWh/m³.
Currently, pump #3 is operating at fixed frequency, and pump #6 is operating at variable frequency (30Hz). The manifold valve is closed, and both pumps are operating independently. The calculation results are based on records from 19:00 on January 9th to 7:00 on January 14th, 2002.
Total electricity consumption: 154,200 kWh;
Total water supply: 1,064,700 cubic meters;
Total unit consumption of the pumping station: 0.14 kWh/cubic meter;
When the original pump #6 was running at the power frequency, its power consumption was 0.24 kWh/m³; now, when it is running at the variable frequency, its power consumption is 0.06 kWh/m³.
From 19:00 on January 9, 2002 to 7:00 on January 14, it is 4.5 days. Therefore, the average daily water volume is: 1064700/4.5 = 236600 cubic meters.
Annual electricity savings: 0.07 * 23.66 * 365 = 6.04 million watt-hours, equivalent to RMB 2.6878 million.
8. Improved water plant processes after frequency conversion upgrade
1) The high-voltage frequency converters produced by Beijing Leadway Power Equipment Co., Ltd. have a built-in Siemens S7-200 series PLC, which is very convenient for implementing external logic control on site. For example, the valve linkage function eliminates the need for operators to perform any valve operations when starting or stopping the pump, reducing workload and the risk of misoperation; the cooling water flow protection function ensures the safe and stable operation of the pump motor.
2) After the frequency conversion upgrade of pump #6, the motor speed, current and pump outlet pressure all decreased significantly, which significantly improved the operation of the motor and pump. This not only directly saved power consumption, but also reduced the impact on the motor and power grid due to the soft start function of the frequency converter, extended the service life of the equipment and reduced the maintenance cost of the equipment.
3) The frequency converter adopts pressure closed-loop control to ensure a water pressure of 0.1MPa in the pipeline network. Due to the smooth speed regulation and high control accuracy of the frequency converter, the pressure fluctuation range of the pipeline network is very small, which can fully meet the on-site water supply process requirements.
4) The high-voltage frequency converters produced by Leadway Company have complete and sensitive fault detection, diagnosis, alarm and trip functions to ensure the safe operation of motors and water pumps.
5) The monitoring functions of the host computer and the remote dial-up monitoring function create the necessary conditions for users to achieve centralized monitoring.
9. Conclusion
The successful implementation of the frequency conversion retrofit for the water pumps at the Longhupao Water Intake Plant has proven to be a success, yielding significant economic and social benefits. These benefits include: 1) saving substantial amounts of electricity and reducing water supply costs; 2) increasing the system's adjustable range and improving operational flexibility; 3) reducing pump motor speed, minimizing startup shock, mechanical friction, and vibration, and extending unit lifespan; 4) reducing pump operating noise and improving the working environment; and 5) intelligent linkage functions and comprehensive fault protection features, which both enhance work efficiency and system safety and reliability.
Therefore, high-pressure, high-power variable frequency speed control technology is a high-tech field that benefits the country and its people, with broad application prospects. Its application in water pump speed control or constant pressure water supply systems in the water supply industry has significant practical implications. This high-pressure, high-power variable frequency speed control device will undoubtedly become the preferred equipment for energy conservation and upgrading in the future water supply industry!
