The Water Hammer Protection of Long-distance Pressure Sewage Pipeline Engineering (Part Two)
4. The calculation of the water hammer protection
4.1 Check valves
A check valve is a valve that automatically opens and closes the valve disc by relying on the flow of the medium itself to prevent the medium from flowing back. When a check valve is not set, the flow drops sharply after stopping the pump. The flow reverses at 6.8 seconds, and the reverse flow reaches the maximum value at 13 seconds, which is -2130m3/h and is 1.18 times that of steady-state operation. It finally stabilizes at -980m3/h. The reverse time of the pump exceeds 120 seconds, which does not comply with the relevant regulations of the Design Codes of Pumping Stations in GB 50265-2010. When a check valve is set, the flow rate drops rapidly after the pump is stopped. At 7.5 seconds, the flow rate reaches zero and no reversal occurs. When a check valve is not set, the pressure head reaches the highest value of 0.829MPa after stopping the pump for 4 seconds, and then decreases to a stable value.
When a check valve is set, the water pump does not reverse due to the check valve, but the pressure at the check valve is higher after the pump is stopped, up to 1.85MPa; the pressure changes more drastically, which shows that although the reverse problem of the water pump can be solved only by using ordinary check valves, it will increase the water hammer's pressure and cannot effectively solve the water hammer. In view of the above situation, consider the use of a two-stage slow-closing check valve to prevent the pressure from being too high or the pump's reverse flow rate from being too great. After the pumping station is stopped, the water flow changes from the forward flow to the reverse flow. In order to reduce the water hammer caused by closing the valve, the two-stage closing method is adopted to gradually close the check valve. The two-stage buffer time can be set reasonably, and it is usually fast closed, accounting for about 90% of the whole closing time. Then, it is slowly closed, accounting for about 10% of the whole closing time. An interception effect is generated and achieves the purpose of reducing dynamic water pressure. When the slow closing time is 50 seconds, the minimum pressure of the water hammer behind the valve is 0.825MPa. The starting reversal time gradually decreases with the increase of the slow closing time, and the maximum reversing speed is not affected by the slow closing time. Considering multiple factors, 50 seconds is set for the slow closing time, that is, a two-stage slow closing check valve with a nominal diameter DN1200 is selected. Fast close the check valve for 5 seconds, accounting for 90% of the whole closing time, and then slowly close the check valve for 50 seconds, accounting for 10% of the whole closing time.
4.2 Air valves
The air valve has the function of automatically opening or closing. When the water hammer in the pressure pipe produces negative pressure, the air valve opens to allow air to enter the pipe. When the temperature or pressure changes and the air in the pipe needs to be released, the air valve can also release the air in the pipe, ensuring the safe operation of the entire pipeline system. The calculation formula of air intake flow is as follows:
（3）
In the formula, C is the chezy coefficient, which is 130m/s; S is the slope of the pipeline, which is 1.3‰; D is the inner diameter of the pipeline, and the pipeline of DN1200 is used in this project. After calculation, the air intake flow of the air valve is 1600L/s. The calculation formula of the air discharge of the air valve is as follows:
Q discharge=20Q water (4)
In the formula, Q water is the water flow rate of the pipeline, with a value of 1.5m3/s. The air discharge of the air valve is 30L/s based on equation (4). After calculation, when the pressure value is 1.4m, the suction flow corresponding to the suction port's diameter of 150mm is 1926L/s, which meets the requirement for the exhaust volume of 1600L/s; when the pressure value is 138m, the air discharge corresponding to the exhaust port diameter of 6.3mm is 64L/s, which meets the requirement for an exhaust volume of 30L/s.
4.3 Two-stage slow-closing check valves combined with gas injection micro-exhaust valves
The two-stage slow-closing check valve and gas injection micro-exhaust valve were used in the pipeline of the project, and the simulation calculation of the water hammer was carried out. The results showed that under the combined protection of the check valve and air valve, whether it is the left pipeline or the right pipeline, the positive pressure of the water hammer is significantly reduced. The maximum water head line of the water hammer is almost the same as the steady-state water head line when the pump is stopped. At the same time, the negative pressure of the water hammer is effectively controlled. The whole pipeline mostly has the positive pressure, and the minimum negative pressure is controlled at within -2m, which meets the design requirements.
5. Conclusion
(1) Through the simulation analysis and calculation of the water hammer for the stopped pump without protection, it is found that under steady-state operating conditions, there is no negative pressure in the pipeline and the pipeline's operation is safe and stable. Under unprotected conditions, the maximum pressure head of the entire pipeline is 6 to 7 times the rated pressure of the pump outlet. If the positive pressure of the pipeline is too high, the pipeline may burst. The minimum pressure head exceeds the range of -3m, and the negative pressure value is high, which easily makes the water hammer occur and is not conducive to the safe and stable operation of the pipeline.
4.1 Check valves
A check valve is a valve that automatically opens and closes the valve disc by relying on the flow of the medium itself to prevent the medium from flowing back. When a check valve is not set, the flow drops sharply after stopping the pump. The flow reverses at 6.8 seconds, and the reverse flow reaches the maximum value at 13 seconds, which is -2130m3/h and is 1.18 times that of steady-state operation. It finally stabilizes at -980m3/h. The reverse time of the pump exceeds 120 seconds, which does not comply with the relevant regulations of the Design Codes of Pumping Stations in GB 50265-2010. When a check valve is set, the flow rate drops rapidly after the pump is stopped. At 7.5 seconds, the flow rate reaches zero and no reversal occurs. When a check valve is not set, the pressure head reaches the highest value of 0.829MPa after stopping the pump for 4 seconds, and then decreases to a stable value.
When a check valve is set, the water pump does not reverse due to the check valve, but the pressure at the check valve is higher after the pump is stopped, up to 1.85MPa; the pressure changes more drastically, which shows that although the reverse problem of the water pump can be solved only by using ordinary check valves, it will increase the water hammer's pressure and cannot effectively solve the water hammer. In view of the above situation, consider the use of a two-stage slow-closing check valve to prevent the pressure from being too high or the pump's reverse flow rate from being too great. After the pumping station is stopped, the water flow changes from the forward flow to the reverse flow. In order to reduce the water hammer caused by closing the valve, the two-stage closing method is adopted to gradually close the check valve. The two-stage buffer time can be set reasonably, and it is usually fast closed, accounting for about 90% of the whole closing time. Then, it is slowly closed, accounting for about 10% of the whole closing time. An interception effect is generated and achieves the purpose of reducing dynamic water pressure. When the slow closing time is 50 seconds, the minimum pressure of the water hammer behind the valve is 0.825MPa. The starting reversal time gradually decreases with the increase of the slow closing time, and the maximum reversing speed is not affected by the slow closing time. Considering multiple factors, 50 seconds is set for the slow closing time, that is, a two-stage slow closing check valve with a nominal diameter DN1200 is selected. Fast close the check valve for 5 seconds, accounting for 90% of the whole closing time, and then slowly close the check valve for 50 seconds, accounting for 10% of the whole closing time.
4.2 Air valves
The air valve has the function of automatically opening or closing. When the water hammer in the pressure pipe produces negative pressure, the air valve opens to allow air to enter the pipe. When the temperature or pressure changes and the air in the pipe needs to be released, the air valve can also release the air in the pipe, ensuring the safe operation of the entire pipeline system. The calculation formula of air intake flow is as follows:
（3）In the formula, C is the chezy coefficient, which is 130m/s; S is the slope of the pipeline, which is 1.3‰; D is the inner diameter of the pipeline, and the pipeline of DN1200 is used in this project. After calculation, the air intake flow of the air valve is 1600L/s. The calculation formula of the air discharge of the air valve is as follows:
Q discharge=20Q water (4)
In the formula, Q water is the water flow rate of the pipeline, with a value of 1.5m3/s. The air discharge of the air valve is 30L/s based on equation (4). After calculation, when the pressure value is 1.4m, the suction flow corresponding to the suction port's diameter of 150mm is 1926L/s, which meets the requirement for the exhaust volume of 1600L/s; when the pressure value is 138m, the air discharge corresponding to the exhaust port diameter of 6.3mm is 64L/s, which meets the requirement for an exhaust volume of 30L/s.
4.3 Two-stage slow-closing check valves combined with gas injection micro-exhaust valves
The two-stage slow-closing check valve and gas injection micro-exhaust valve were used in the pipeline of the project, and the simulation calculation of the water hammer was carried out. The results showed that under the combined protection of the check valve and air valve, whether it is the left pipeline or the right pipeline, the positive pressure of the water hammer is significantly reduced. The maximum water head line of the water hammer is almost the same as the steady-state water head line when the pump is stopped. At the same time, the negative pressure of the water hammer is effectively controlled. The whole pipeline mostly has the positive pressure, and the minimum negative pressure is controlled at within -2m, which meets the design requirements.
5. Conclusion
(1) Through the simulation analysis and calculation of the water hammer for the stopped pump without protection, it is found that under steady-state operating conditions, there is no negative pressure in the pipeline and the pipeline's operation is safe and stable. Under unprotected conditions, the maximum pressure head of the entire pipeline is 6 to 7 times the rated pressure of the pump outlet. If the positive pressure of the pipeline is too high, the pipeline may burst. The minimum pressure head exceeds the range of -3m, and the negative pressure value is high, which easily makes the water hammer occur and is not conducive to the safe and stable operation of the pipeline.
(2) Through the analysis and calculation of the water hammer protection with a check valve, it is found that an ordinary check valve can prevent the pump from reversing, but it will increase the water hammer's pressure and cannot effectively solve the water hammer. A two-stage slow-closing check valve is adopted, that is, fast closing for 5 seconds, accounting for 90% of the whole closing, slow closing for 50 seconds, accounting for 10% of the whole closing, which can prevent excessive reverse flow and reduce the water hammer's pressure behind the valve.
(3) Speaking of the calculation of the intake and exhaust volume of the air, the air valve is reasonably selected, and the air injection micro-exhaust valve with an intake port diameter of 150 mm and an exhaust port diameter of 6.3 mm is adopted, which can meet requirements for the air intake flow of 1600L/s and the air exhaust volume of 30L/s. The reasonable selection and distribution of the pipeline can better prevent the water hammer.
(4) A combined protection method of the two-stage slow closing check valve and gas injection micro-exhaust valve was selected and applied to the project. The results showed that the water hammer protection measures not only significantly reduced the positive pressure of the water hammer, but also eliminated the negative pressure of the water hammer, which played a good role in protecting the pipeline.