Fault Analysis of a Hydraulic Oil Cooling System

Fault Analysis of a Hydraulic Oil Cooling System

At present, rotary drilling RIGS adopt a separate variable cooling system. The design objective is that when the oil temperature of the hydraulic system is lower than the set system oil temperature, the cooling system will not work. When the system oil temperature exceeds the set value, the speed of the cooling system's operation is directly proportional to the oil temperature, thereby achieving the purpose of system energy conservation.

However, the rotary drilling rig experienced the following malfunction: In the winter when the ambient temperature was only a few degrees, the hydraulic fan was found to be rotating upon startup, which was inconsistent with the design concept. It is certain that there are design or installation defects.

The schematic diagram of the heat dissipation system is as follows:

The control principle is as follows: A thermosensitive relief valve is installed on the return oil line of the hydraulic system to sense the temperature of the cooling medium and provide a control pressure to the control port (x port) of the variable mechanism of the variable pump. The variable pump supplies the motor with the corresponding flow rate according to the size of the control pressure. This enables the fan to rotate faster when the oil temperature is high and slower when the temperature is low. Thus achieving the goal of energy conservation.

The working principle of the thermosensitive relief valve is shown in the right figure. It mainly consists of a housing, a thermocouple (1), a valve seat and a valve cone. The thermocouple extends in response to temperature changes and pre-compresses springs 2 and 3 through the spring seats. If the temperature of the thermocouple is lower than the set temperature, the pre-applied compressive force of spring 2 is eliminated, and spring 3 conducts P and T to drain the oil.

The working principle of a variable pump is that the pump's displacement varies with the pressure at the control port X. When the pressure at port X is 0, the pump has the minimum displacement.

Based on the above analysis, it is known that the principle is fine. The parts are imported components, and there is no problem when they are tested separately.

A pressure gauge was connected at M1 to measure the pressure. It was found that when the gauge value was 2 to 3bar, the cooling motor had already started to rotate. When the engine speed was high, the motor could rotate at full speed at the designed speed, with basically no speed change phenomenon. The oil supply volume of the pump can be determined by the rotational speed of the quantitative motor. The crux of the problem lies in that the variable pump has started to supply oil at full displacement before the thermal relief valve provides pressure.

I checked the information about the pump but found no relevant explanations. The reason for this is that the pressure at the pump outlet (port B) is too low, causing the pressure in the control oil circuit to be extremely low. Therefore, regardless of the pressure at port X, the variable pump always has the maximum displacement, which is the same principle as the two-point variable control pump having the maximum displacement at low pressure. Add a relief valve at port B and adjust the pressure to enable the pump to operate normally. The measured pressure value is approximately 13bar. From the graph, it can be seen that the relationship between the pump and the pressure at port X should be corrected to the solid line in the graph.

A back pressure valve of the same pressure was modified to replace the relief valve and installed on the system. The heat dissipation system worked normally and was consistent with the designed operation. After adding a back pressure valve to other vehicles with the same configuration, this fault could all be eliminated.