The Analysis and Maintenance of Failure Causes of Hydraulic Check Valves (Part One)

Abstract
The reliability of the hydraulic check valve on the pipeline of the circulating water system directly affects the safe operation of the circulating water pump. The hydraulic pressure of the hydraulic check valve at the outlet of the circulating water pump could not be maintained after inspection, and the oil pump was frequently started and stopped. The check valve could not be opened or closed. After the valve was opened, the motor and oil pump tank were overheated. In view of the above-mentioned failures, the analysis was conducted from the aspects of control principles, field maintenance implementation and function identification of hydraulic check valves to provide a technical guarantee for the safe operation of the hydraulic check valve in the later period.
 
In order to prevent the circulating water from flowing back when the circulating pump is started or stopped, a hydraulic check valve is specially installed on the pipeline. The hydraulic check valve can effectively protect the circulating pump from reversing due to the reverse flow of circulating water, and mainly prevent the following accidents from occurring: ①When the pump is stopped, the reversing happens due to the valve not being tightly closed, causing the impeller to move and shaft sleeve to be loose, and the dynamic and static part to rub and damage in severe cases. ②When the pump is reversed, the outlet valve should be closed quickly, and it is strictly forbidden to start under the reverse condition. Otherwise, it will not only cause an impact on the system and then the water hammer, the motor will also be burnt due to excessive starting torque.

When the system was checked before starting, the following faults were found for the hydraulic check valve: ①The electric connecting pressure gauge of the hydraulic station exceeded the limit and the oil pump could not be started. ②The pressure gauge was replaced; the oil pump could be started but the oil pressure could not be maintained; the oil pump frequently started and stopped. ③The motor and oil pump tank were overheated when the valve was opened. In order to eliminate the above faults, the following analysis was carried out.
 
1. Working principles of hydraulic check valves
1.1 Structural composition
The hydraulic check valve is mainly composed of the valve body, hydraulic station and drive control: ①The valve body is composed of the valve body, valve plate, valve shaft, and seal (Figure 1). ②The hydraulic station is composed of the oil tank, oil pipe, motor, oil pump, accumulator, globe valve, one-way valve, throttle valve, overflow valve, three-way electromagnetic reversing valve, electromagnetic reversing valve, lifting cylinder, locking cylinder, stroke indicator and stroke switch. ③ Drive control is composed of switches, control buttons, indicator light, relay, operation panels and heavy hammers. (Figure 2)
 

Figure 1 The structure of the valve body
 
1.2 Control principles
Local and remote PLC control can be realized through the "local or remote control" button.

(1) Hydraulic station control

Press the "The Starting of the Oil Pump" button, the operation indicator light of the oil pump will be on. When the oil pressure reaches the upper limit set by the electric connecting pressure gauge, the oil pump stops automatically.

 
Figure 2 The Working principle
 
(2) Energy storage control
When the internal pressure of the hydraulic station is lower than the lower limit set by the electric connecting pressure gauge, the solenoid valve of the accumulator is energized, and the oil pump automatically opens to store pressure in the accumulator. When the pressure reaches the upper limit pressure, the solenoid valve loses electricity and the oil pump automatically stops and stores energy.
 

2. The analysis and maintenance of failure causes of hydraulic check valves

2.1 The analysis of the failure
2.2 The analysis of the reasons 
(1) The oil pump started automatically but the oil pressure couldn't be maintained; the oil pump frequently started and stopped, and the check valve couldn't be opened or closed. The possible causes were: ①The hydraulic pipeline was not smooth, causing blocking or leaking to happen. ②The solenoid valve, reversing valve or three-way reversing valve of the hydraulic station was blocked. ③The manual pressure relief needle globe valve of the hydraulic station leaked. ④The hydraulic station lifted and targeted the oil cylinder, and it could not be targeted after receiving the signal of the opening position.
 

(2) When the valve was opened, the motor and oil pump tank were overheated. The possible reasons were: ①The amphenol connector was aging or damaged, which led to short-circuit connection, and caused the motor's temperature to rise due to friction and vibration after continuous working. ②The electric connecting pressure reached the rated value. The motor drove the oil pump to continue to run; friction and vibration generated heat, causing the temperature to rise.

 
(3) The valve body was jammed. The possible reason may be that the connecting shaft between the transmission and valve body was corroded, which caused the valve body to jam. In order to prevent the circulating water pump from reversing when the circulating water pump was started or stopped, and to avoid the serious consequences caused by the pump reversing, the safe, stable and reliable operation of the hydraulic check valve must be ensured. Therefore, a comprehensive investigation of the valve failure was required.