It is introduced that the test media specified in the relevant standards for low leakage test of valves are methane and helium, and in the actual test process, there is the phenomenon that valve manufacturers use helium for test evaluation in advance and then send the third party for methane low leakage type test. The low leakage test was designed to investigate the numerical relationship between helium and methane for a packing seal structure under the same environmental conditions, and to provide reference data for users who need to compare the parameters.
Currently, the laws and regulations related to volatile organic gas (VOC) emissions have been released and implemented, and industrial valves are required to meet higher requirements for low leakage performance in various systems and conditions of use. The American Petroleum Institute (API) standard has taken the lead in implementing low leakage requirements for ventili as mandatory requirements, and the main standards for testing are API 624, API 641, API 622 and so on. In domestic valve industry practice, the more commonly used standard is ISO 15848, the main difference between the two is that the API series standards use methane as the test gas, while the ISO standard provides that helium or methane can be used (in the actual test most of the use of helium), but in 2015 before the revision of the standard, because the leak rate measurement unit requirements for methane does not have operability, the actual more Helium was used as the test medium. As a major greenhouse gas, methane is more informative for assessing greenhouse gas emissions because it has a smaller molecular volume than other greenhouse gases.
There are safety risks associated with using methane for high and low temperature cycling tests. If helium can be used to evaluate the seal design for compliance with the API series standards, it can be used for self-inspection during manufacturing to reduce the time and cost of third-party testing. According to ISO15848 standard, helium or methane is used as the test medium for the seal class, there is no reciprocity, and the standard does not specify whether there is direct test data to prove it. In this paper, we designed a test device and measurement equipment using helium or methane as the test medium, and collected multiple sets of data to supplement the gaps in the relevant test data.
2. Experimental Design
2.1 Test setup
The test setup is based on the low leakage test tooling specified in API 622-2011, and the applicable graphite packing ring size is Φ254mm×Φ381mm. The test setup is designed with a separate leakage measurement port at the packing gland, and the leak detection equipment probe is connected to the measurement port during the measurement (Figure 1).
Figure.1 Test tooling
2.2 Measuring equipment
Two sets of testing equipment were used for methane measurement: vodik flame ionization detector, model TVA-2020 (Thermofisher, USA), with a measurement accuracy of ±1×10-4% (v/v). The zero point and 0.012% range point were calibrated before measurement. A helium mass spectrometer leak detector, model L300i (Leybold, Germany), with a minimum leak detection rate of ≤5E-12 Pa-L/s was used for helium measurements, calibrated with a built-in TL7 standard leak.
Table.1 Data acquisition sequence for single group packing leakage
|Serial number||Test pressure/MPa||Torque/Nm||Test medium|
2.3 Data Acquisition
The graphite packing for the test was taken from six groups, which were taken from imported products, joint venture products (domestic production) and domestic products. Each group of packing was assembled according to the technical regulations and requirements, and then tested according to the order in Table 1, and the leakage amounts of methane and helium were measured under the same conditions, and the measured values were in ppmv, and each measurement was repeated 10 times with an interval of 6~10 s. The test did not consider the effect of heating or cooling on the leakage amounts of the two test media, and the test was conducted at room temperature.
Table.2 Comparative data of methane and helium leakage (average values)
|Methane leakage LM (1 x10-4%, v/v)||Helium leakage LH (1 x10-4%, v/v)||Ratio (LH/LM)|
According to the long-term test observation, even if the inner cavity of the tooling is depressurized, the fugitive leakage at the packing can still be measured steadily for up to 30 min without any significant decrease. Therefore, after switching the test medium, a vacuum pump must be used to do suction on the measurement port to suck the residual medium at the packing before retesting, otherwise there is a greater impact on the measurement value.
2.4 Data collation
The data obtained from the comparative measurements of methane and helium leakage were averaged over 10 measurements as the measurement results. The relevant data were compiled as shown in (Table 2).
Based on the data in Table 2, the data were compiled as shown in Figure 2.
Figure.2 Helium Leakage to Methane Leakage Ratio
According to the analysis of the test results, the ratio of helium leakage to methane leakage tends to be 4 times higher with increasing leakage volume, which is the reciprocal of the molecular weight ratio (16 for methane and 4 for helium), under the same conditions at room temperature, excluding the measurement error at low methane concentration. Whether there is a relationship between the fugacity of the gas and the molecular weight of the gas needs to be investigated by increasing the number of data collected under different test conditions (temperature, pressure, comparison medium, etc.).