Sampling systems can be found in processing plants, refineries, various industrial plants and, of course, in the chemical industry. They are primarily used for quality and process control and to verify the performance of analytical instruments. The decisive factor is that the sample must be representative at the time of sampling and must also comply with the specified conditions as far as possible during analysis.
There are numerous options for configuring sampling systems in which gas or liquids are taken in sealed cylinders. Perhaps the most efficient design is a closed system in which the sample circulates continuously through the cylinder during sampling. A closed system takes samples from a pressurised process and transports them back into the process at a lower pressure – usually at a point upstream of the pump. Since this design turns the sampling system into an extension of the processing system, the need for flushing can be reduced or even eliminated.
Cylindrical sampling systems can be used for gas and liquid samples, but they differ in design. The flow path must be different for liquids and gases in order to flush out phase-shifted media from the cylinder. Gases should flow from top to bottom in the cylinder to push any liquid/condensate out of the sample cylinder during filling – and to ensure that no liquid accumulates in the cylinder and interferes with laboratory analysis. Liquids, on the other hand, should be filled from bottom to top to displace the vapour space and ensure that the cylinder is always full.
Liquid Applications And A Continuous Flow
Pure liquid sampling systems take liquids in non-pressurized bottles. These are drawn directly from the process and the containers are then transported further without the risk of spillage or evaporation. Such a system can be used in many liquid applications where the process fluid does not fractionate or evaporate. It is critical to ensure that the sample remains representative. This precaution allows the use of inexpensive glass laboratory bottles for samples – with the added benefit of providing immediate visual feedback on the quality of the sample stream.
Continuous flow is often useful for sampling, for example when a sample requires constant movement (e.g. to prevent freezing), or when the line to the sampling point is very long. The sample flows through a bypass loop into the sampling system, ensuring that the sampled fluid remains representative of the process – precisely because it does not stay in the lines for long.
If the fluid sample is under high pressure or hazardous, a fixed volume system should be selected. In such a system, the sample first flows into a metal cylinder and is then gently pressed into the sampling bottle by an after flow gas at low pressure. This prevents unintentional overfilling.
The article is an abridged reproduction of an article by Matt Dixon, senior principal development engineer at Swagelok, and was published in cav 12/2019.