Constant pressure cylinders vs traditional cylinders: how to make the right choice for your liquid mixtures?

What are constant pressure cylinders?

Constant pressure (or piston) cylinders are used for liquid hydrocarbon mixtures and provide the highest levels of accuracy. This is achieved by eliminating the headspace in the cylinder, ensuring components with different vapour pressure do not change phase disproportionately and lead to inaccuracy in the certified values.

Liquefied gases in these cylinders are great for maintaining constant pressure and consistent concentrations throughout their life as the liquid and gaseous phases are physically separated.

What are traditional cylinders?

Liquid withdrawal overpressure cylinders are traditional cylinders that are filled with a liquid mixture that has an inert overpressure gas added to the headspace. A dip tube allows for liquid withdrawal, while the overpressure gas provides motive force and ensures that there is no vaporisation in the sample pipework.

BOC cylinders include a dual port valve connected to gas headspace, to allow for simultaneous connection of overpressure gas for pressurisation. This allows the application of constant overpressure over the life of the cylinder.

To assist in the decision between piston and traditional cylinders, BOC conducted several simulations to determine the effects of hydrocarbon mixtures in contact with overpressure gas including helium, hydrogen, and nitrogen. The following key findings summarise the results:

Component concentration in liquid phase: When liquid is withdrawn from a traditional cylinder and the mixture is used, the concentration of lighter components in the liquid phase reduces whilst the concentration of heavier components increases. For constant pressure cylinders, the composition stays constant throughout the life of the cylinder.

Effect of pressure and temperature: Without top-up of the overpressure during use, the cylinder pressure will decrease over the life of the cylinder.

Overpressure gas concentration in the liquid phase: If the overpressure gas is not topped up over the life of the cylinder, the liquid phase concentration decreases. This effect is not seen if the overpressure gas is topped up during the use of the cylinder as it constantly keeps the overpressure gas dissolved in the liquid phase.

The findings answer key questions for laboratory managers and maintenance teams using constant pressure cylinders or traditional cylinders and demonstrate the benefits and drawbacks of both.

All these effects are important to consider, both when choosing between a constant pressure and a traditional cylinder, and when deciding how to use a traditional cylinder. BOC can assist with this by providing detailed composition use charts for your mixture of choice upon request.

To discuss your requirements with one of our technical specialists, contact BOC today.

1800 658 278
scientific@boc.com
www.boc.com.au

For more information on the key findings, please click here.