How to take gas samples

Since it is a as at normal temperature and pressure, liquefied gas samples can be injected into the gas phase after vaporization or directly into the liquid phase. Early standards used the method of sample injection after vaporization. Generally, the cylinder was turned upside down, a section of stainless steel coil was connected to the bottom, soaked in a water bath, and the quantitative loop injection port was connected to the back. Theoretically, this is equivalent to gas phase injection.

But here we will discuss the gasification process. There are actually many possible problems in this gasification process. The key problem is that light components gasify quickly and heavy components gasify slowly. As a result, the gas entering the quantitative loop cannot represent the true situation of liquefied gas.

Q: How to take gas samples?

A: To ensure that the vaporized gas is representative of the original liquid, the following points must be ensured:

1. The sample flows out of the sampling container in liquid state. For cylinders, the end without the overflow pipe should be pointed downward, and the liquid should be discharged from the bottom to the vaporization device.

2. The liquid discharged from the container must be completely, quickly and fully vaporized in the vaporization device. In order to control the flow rate of gas after gasification, the throttle valve at the outlet of the container is very critical, and it must be able to and easily adjust the flow rate.

3. During the entire gasification process, there must be no entry of pollutants, no dead volume, and no adsorption of components of concern in the sample.

4. Vaporization injection of liquefied gas samples

Early standards all used the method of vaporizing the liquefied gas sample and then injecting it. Generally, two methods can be used: water bath heating vaporization and flash evaporator vaporization.

(1) The specific operation of water bath heating and gasification sampling is as follows: turn the cylinder upside down, connect a section of stainless steel coil below, soak it in the water bath, and connect the quantitative loop injection port at the back.

(2) The specific operation of flash vaporization sampling is as follows: turn the cylinder upside down, connect the steel wire at the lower outlet to the inlet of the flash evaporator, and after heating and vaporization by the flash evaporator, connect the outlet of the flash evaporator to the quantitative loop injection port.

From opening the valve to stabilizing means the process from when the sample just enters the flash evaporator to when the sample reaches stable vaporization in the flash evaporator. From stabilization to valve closing, it means the process from when the sample is stable to closing the valve at the bottom of the cylinder. From closing the valve to stopping bubbling, it means the process from closing the lower valve of the sample cylinder to stopping bubbling in the bubbling bottle.

When the valve is just opened, after the sample enters the flash evaporator, the light components will be vaporized immediately, while the heavy components will need a certain period of time to heat up before they can be vaporized. Therefore, during this process, the content of the light components is higher than normal, and the heavy components are higher than normal. is low. After a period of time, as the sample continues to enter the flash evaporator and be vaporized, the heavy components accumulate in the flash evaporator to a certain extent, and the gasification of the light and heavy components reaches a stable level. After vaporization, the light and heavy components flow out of the flash evaporator. The amount is consistent with the amount of light and heavy components in the liquefied gas sample entering the flash evaporator. After closing the lower valve of the sample cylinder, the flash evaporator will no longer be replenished with liquefied gas samples, and the samples accumulated inside will continue to vaporize. The light components will vaporize cleanly quickly, while the heavy components will vaporize slowly and need to wait for a period of time. The gasification is clean, so the content of light components at this stage is lower than normal, while the content of heavy components is higher than normal.

To sum up, the correct injection time should be controlled between stabilization and valve closing as shown in the figure. Under this condition, other methods must be used to stop the bubble to ensure atmospheric balance and accurate injection volume.