In the petrochemical industry, representative samples of liquid petroleum gas and other high vapor pressure gases and liquids are needed for process monitoring, quality control and regulatory compliance. However, in this industry, process fluids and gases are volatile or could be harmful to the operator and the environment, or they could be dangerously altered through a chemical reaction when exposed to the atmosphere – so exposure must be avoided. Because each sample collected may pose a physical or health hazard, and operators may take hundreds of samples per year, samples must be taken safely and correctly using the appropriate equipment and containers.
Low-emission samplers are designed to provide the greatest amount of protection for both operators and the environment, with no spillage and minimal release of volatile organic compounds (VOCs) – organic chemicals that have a high vapor pressure at ordinary room temperature. Safe sampling also requires using a metal sample cylinder for collection and transport. Generally, this cylinder is a stainless-steel pressure-rated cylinder with inlet and outlet isolation valves to isolate the sample within the cylinder, and a connection method for easy removal of the cylinder assembly from the sample panel. Additional options might also be included depending on the customer application.
Safe sample cylinder assemblies
Sample cylinder assemblies provide two main advantages:
- They allow for samples to be collected at or near the pressure of the process being sampled. This assures that the sample is representative – constituents that can change state with pressure reduction are not lost during sample collection. An example of this type of sample would be liquid petroleum gas or other two-phase fluids that could flash off sample constituents if the pressure were reduced during sampling.
- Since the sample is a pressurized liquid that has a high vapor pressure, the sample cylinder assembly provides for a safe method of collection and transport as long as the maximum fill density meets regulatory requirements. The maximum fill density is the volume of a sample cylinder assembly usually expressed as a percent of total capacity. Maximum fill density is regulated by various governmental authorities to ensure safe transport of the cylinder. Regulations sometimes vary, but most limit the maximum fill density of transport containers for safety to 80 percent liquid fill of the cylinder at 15°C.
The most common sample connection types include speed loop and single point. In a speed loop sampling connection with a venting provision, there are two connections to the product line. There must be different pressures between the connections to initiate flow of product through the loop and the sampler. Either a centralized vent or flare collection system or a local scrubbing system (charcoal canister) is used to collect and process off gases or vapors released during the sampling process to eliminate product release to the local environment and operators.
With a single point sampling connection with a venting provision, the sampler has a single connection to the product line and no product return line. Off gases or vapors released during the sampling process are handled either through a centralized vent or flare collection system or a local scrubbing system (charcoal canister), eliminating product release to the local environment and operators. With proper design and operation of these two connection types, release of vapors after the sample cylinder is removed from the sampler can be minimized to a very low level.
Another less preferable option is single point sampling connection without a venting provision, where off gases or vapors are released into the local environment. Because of these releases and the limited cost savings, use of this type of design has limited application.
Sample cylinder assemblies then need to be transported for analysis to ensure production output is stable, efficient and safe. Representative sampling helps ensure quality, protect plant assets, and keep operators, the public and the environment safe.
Learn more about our sampling solutions for the petrochemical industry here.
This article was originally published on our website.