Reasons and Countermeasures for Low Rate of Passivity of Pressure Transmitter
The temperature-compensated and linear test of the LSIII series pressure transmitters is an important step in the production process. The equipment simulates instruments up to +85 °C and a minimum operating temperature of -40 °C. Continuous pressure supply, continuous automatic operation, linearization compensation of instrument parameters, equipment does not allow manual intervention during a test operation cycle, and pauses in the middle.
Since the LSIII series pressure transmitters were put into production, the temperature qualification rate has remained at around 63%, which is far from the 95% specified by the company's technical indicators. The pass rate is 63%, which is too low, and the number of repeated tests is increased to 2~3 times, which causes labor, production materials, equipment and shifts to increase, and production costs increase. For the LSIII series pressure transmitters produced in a certain month, the data was classified and statistically unqualified, and a questionnaire was made.
Low causal leakage rate high causal analysis
It can be seen from Table 1 that ": low temperature leakage" accounts for 78% of all unqualified items, so it is concluded that to increase the temperature pass rate, the maximum unqualified item, that is, "low temperature leakage" must be eliminated.
Pressure transmitter temperature once failed project survey
2.1 Cause analysis
In response to the problem of “high temperature leakage rate”, 13 influencing factors were found from the six aspects of “human, machine, material, method, ring and measurement”, among which “end factor” was 9 and “end factor reason confirmation table” ".
End factor confirmation table
2.2 Reason confirmation
Reason 1: The verification of the O-ring size of the device detection interface is unreasonable: It is found through observation and observation that after the pressure transmitter is clamped in the fixture, the sealing condition is normal, but after the temperature in the test box is running below 0 °C, due to At low temperature shrinkage, the size of the seal ring changes, resulting in a reduction in the pressure of the seal ring at the interface, insufficient compression deformation, and the failure of the seal eventually causes "low temperature leakage".
Reason 2: Verification of improper selection of equipment detection interface O-ring: The lowest temperature that the pressure transmitter will experience during the temperature test is - 40 ° C, and the temperature will be nearly 40 min. The group found that the leakage alarm during the constant temperature process at low temperature is better. Many, often after one cycle, even one cycle is not over, multiple instruments and equipment interface seals leak, tested the low temperature resistance of 10 O-rings at -40 °C, and found that the seals aging at low temperatures, Poor resistance to low temperature.
3.1 Solution One implementation
Increase the O-ring cross-sectional diameter to change the original size of the O-ring to absorb and offset the shrinkage caused by the shrinkage of the sealing ring under low temperature environment, to ensure that it is fully deformed under pressure and meet the requirements of low temperature sealing. After testing: the cross-sectional diameter increased from §¶2 to §¶3 to reduce the leakage rate. If the cross-sectional diameter continues to increase, the leakage situation does not improve significantly. Therefore, an O-ring with a cross-sectional diameter of §¶3 is selected. However, when the diameter of the cross section of the seal ring is increased, the weight is increased, and the O-ring is easily detached, which causes inconvenience to the installation. In order to increase the contact surface between the O-ring and the sealing groove, a sealing ring with a square cross section and a thickness and a width of 3 mm is adopted by an optimized design.
3.2 Solution 2 Implementation
Through the equipment spare parts table, the main component of the original O-ring material is nitrile rubber, and on this basis, the rubber grades with more suitable and low temperature resistance are compared. After searching for the data, N7009AA nitrile rubber with good low temperature resistance was selected as the raw material, processed according to the design drawings, external processing, and put into production.
In order to verify the actual effect, a one-month trial production was carried out on the modified O-ring. The temperature test was carried out on 200 pressure transmitters, and the temperature pass rate was 98%, which achieved good results. In order to confirm the final success of the implementation of the countermeasures, a three-month consolidation of production was carried out, and the pass rate was consistently above 95%.