How to adjust the pressure of hydraulic system

1、 How to conduct pressure tests on hydraulic systems?

A typical hydraulic system generally consists of a pressure oil supply system, a pressure regulation system/component, a flow regulation system/component, a direction regulation system/component, and a load execution system/component   (such as cylinders and motors). If feedback control classification is required based on the load technical parameters of the system, we refer to open-loop control hydraulic system as hydraulic transmission system and closed-loop control hydraulic system as hydraulic control system. In practical engineering, both simple and complex hydraulic systems generally include these four basic systems. Some hydraulic systems designed for special tasks are added on top of them, such as pressure holding systems, cooling systems (such as the filling valve cooling seen in closed circuits in construction machinery), boosting systems, synchronization systems, impact systems, reciprocating systems, and online monitoring systems for oil status, etc. Although hydraulic systems have various practical applications, there are some industry standards that need to be followed for their pressure testing and subsequent system debugging.
Firstly, it should be noted that the pressure test of the hydraulic system should be conducted after the pipeline flushing is qualified, the installation is completed, and the complete system is formed.
The pressure test steps are as follows:

1.1Air freight conversion test
1.1.1Prior to the system's idle operation test, a refueling cart or device with a filter should be used to add a working medium that meets the design requirements to the system's fuel tank. This type of refueling vehicle or device and working medium can be purchased in the market. I suggest it is best to purchase products from well-known brands in the industry,   and remember not to directly inject unfiltered working medium into the fuel tank. In addition, the filtration accuracy of the fuel truck should be at least equal to the maximum filtration accuracy of the system. The advantage of doing so is to prolong the actual use time of the filter element installed in the system, as replacing any components installed in the circuit of the hydraulic system is labor-intensive and time-consuming.

1.1.2Before conducting the air load test, remove the connecting pipe between the hydraulic pump's oil outlet and any possible oil leakage port. Rotate the coupling by hand according to the pump's specifications to allow the pump to absorb oil and the oil outlet to discharge until there are no bubbles in the oil. Then fill the drain port of the pump and/or motor (if equipped with a drain port) with the working medium that meets the filtration accuracy requirements, and finally connect the removed pump outlet pipe and drain pipe. Actual hydraulic systems may use gear pumps/motors, plunger pumps/motors, vane pumps/motors separately or in combination due to differences in customer, load technology requirements, and environmental requirements. In some special low-noise situations, medium and low-pressure screw pumps may even   beused.   Generally speaking, both plunger pumps and plunger motors have oil drain ports, so it is necessary to check whether the pumps and motors in the system have oil drain ports before air running. In addition, if a large amount of force is applied when rotating the pump coupling, or even if it cannot be rotated at all, it is necessary to check whether the pump inlet shut-off valve is open, whether the inlet filter accuracy is too high, whether the coaxial installation accuracy of the   pump input shaft and motor shaft exceeds the accuracy requirements, and whether the input shaft of the pump itself is too tight when the product is shipped. If it is found to be a problem with the pump itself, the pump should be reinstalled. If it is a coaxiality issue, the position accuracy of the pump shaft needs to be readjusted. As for other potential issues and solutions that may arise, they will not be elaborated further due to time constraints.

1.1.3 Air freight conversion test
Before conducting the air load test, a short-circuit transition plate should be used to isolate hydraulic actuators such as   cylinders or motors from the system circulation circuit. If high-precision and expensive components such as servo   valves and proportional valves are used in the system, they should also be isolated from the circuit together. The purpose of isolation is to allow the working medium to short-circuit and return directly to the oil tank through the main relief   valve without passing through these components. In addition, the adjustment screw/handle of the main overflow valve of the system should be tightened to the fullest extent, and all valves that may increase the resistance of the working medium, such as throttle valves/speed control valves, pressure reducing valves, directional valves, etc.,   that may   be installed in the system should be opened to the maximum, and the oil cylinder and motor should not be loaded.
After connecting the power supply, jog the motor to check if the power supply is connected incorrectly. If there is no wrong connection, continuously jog the motor and extend the starting time of the motor. If the pressure on the system pressure gauge rises rapidly during the start-up process, the main relief valve of the system should be immediately checked   for malfunction and resolved. After eliminating the pressure surge phenomenon, continue to operate the electric motor until the system runs smoothly. In addition, the debugging personnel should also pay attention to the pressure difference and oil temperature changes of the filter before and after the air running test. If the pressure difference is too large after the test, the filter element should be removed and flushed or replaced. If the oil temperature is found to be too high during the test, it is necessary to check whether the components, especially the overflow valve diameter, are selected too small, whether the pump is excessively worn due to pollution or other reasons, and whether other components including the pipeline diameter are too small. Due to various reasons for system oil temperature overheating, it will not be elaborated here.

1.2Pressure test
1.2.1After the above air transportation test is successful, the following steps of pressure testing can be carried out:
Firstly, determine the test pressure of the system
When the system working pressure is less than 16MPa, the test pressure is 1.5 times the working pressure;
When the system working pressure is higher than 16MPa, the test pressure is 1.25 times the working pressure.

1.2.2 Determine the method of adjusting the test pressure: The adjustment of the test pressure should be gradually increased, and each level should be stabilized for 2-3 minutes. After reaching the test pressure, the pressure value should be maintained for about 10 minutes, and then it should be reduced to the working pressure and maintained for 10 minutes. Here, it should be noted that the pressure level can roughly divide the experimental pressure by a number within the range of 5-10, and increase the pressure step by step. For example, if the experimental pressure is 315 bar and the divisor is selected as 10, the pressure levels will be 32 (rounded), 64, 96, 128, 160, 192... 315 bar, respectively.

1.2.3. During the process of adjusting the test pressure, it is necessary to carefully and comprehensively observe the following situations:
Check whether there is any leakage of working medium in the system pipeline welds and themselves, pipeline and component/accessory interfaces, component sealing surfaces, and short-circuit plate seals. Also, check whether there is oil   leakage on the surface of the hose and whether there is permanent deformation in the hard pipe itself. If there is any, immediately release the pressure of the main relief valve and interrupt the test until the fault is resolved before starting the pressure test from scratch until the test is qualified. After the occurrence of oil leakage in the weld seam during the experiment, the pipeline should be removed in a timely manner and the working medium should be removed before welding. After welding, the pipeline and weld seam should be cleaned again, and then the pipeline should be connected to the system to restart the pressure test.
During pressure testing, unrelated personnel should be avoided from entering, and open flame operations should not be carried out simultaneously within a 5-meter range of the testing area. No personnel are allowed to adjust system components arbitrarily during the testing process, and hammers or other objects should not be used to strike pipelines. All workers should try to stand in a safer place.

1.2.4. After the pressure test process is completed, the test record should be filled out in a timely manner.

2.How should the main system and sub circuit pressure regulation of the hydraulic system be carried out after completing the pressure test?

I am unable to provide a detailed explanation on this issue as it falls within the scope of formal system debugging, and the questioner has not provided a specific hydraulic schematic. In actual complex hydraulic systems, pressure regulating valves such as relief valves, balance valves, sequence valves, and pressure relays may be used, and there are too many types and specifications of these valves to be specifically described. In addition, there is also a problem with the adjustment sequence when adjusting these valves, so I must provide detailed action instructions for the specific working cycle of the system before I can reply. Of course, I also need to provide a brief answer to this question.
Firstly, it should be noted that before adjusting the pressure of the main and sub circuits, the short-circuit transition plate used for the idle running test should be removed, the system should be restored to its pre short-circuit state, and the   actuator should be connected to the load to establish the load pressure.

Secondly, before entering the pressure debugging, the pump station debugging should have been completed, and the inspection of the pump station should be carried out after two hours of operation at the working pressure of the pump station. The main inspection is to check whether the pump, accumulator (if any), oil tank accessories, etc. have oil leakage, whether the oil temperature is working normally, and whether there are abnormal vibrations and whistling in the pump station. If there are any, they need to be solved item by item.
Finally, it should be noted that the pressure debugging of the system generally follows the principle of "pump station debugging first, other circuit debugging later", and should basically follow the sequence of "main system working pressure adjustment first, sub circuit working pressure adjustment later". During pressure debugging, the set values between the main system relief valve and the sub circuit relief valve, as well as between each relief valve in the sub circuit, should not cause vibration in the system. Of course, the actuator needs to be loaded when adjusting the system's working pressure.

Finally, it should be noted that when designing and debugging hydraulic systems, a detailed reading of each component used in the system should be conducted, not only to understand its performance, but also to know the position and function of each interface. In addition, it is more important to fully understand and value the important instructions of the components, understand various technical requirements and environmental conditions. In a word, the better you understand and master the components, the better your system design will be, and the smoother the debugging will be. This is particularly important for complex hydraulic systems. A good system aims to achieve the goals of simplicity, ease of operation, cost-effectiveness, ease of maintenance, reliability, and safety with as few components as possible.