Liquefied gases are commonly used in many industrial processes. These include food freezing, freeze grinding, rubber de-flashing, environmental testing, metallising, biological storage, shrink fitting and many other applications where reduced temperatures processing is required.

In most installations the liquid is stored in a bulk tank and transferred to the process equipment by pipeline. Because of the extreme low temperature of the gas, the flow in the pipe is inevitably two phase, i.e. gas and liquid.

A S Scientific Products Ltd. has continuously developed the phase separation equipment described on this site. Please select the links at the end of the page for more detailed information on the following products; Gas Vents, and Pressure Reducing System.

Equal Pressure
Some cryogenic equipment has been designed making the assumption that pure liquid can be made available directly at the equipment. In practice, however, it is often necessary to site a bulk storage tank some distance from the equipment, and even the most efficient line will supply a mixture of gas and liquid. Typically, this takes the form of gas slugs interspersed with liquids.

In sensitive cryogenic equipment the gas slugs lead to a variation in the cooling medium and in some circumstances, the equipment fails to function correctly. The two most vulnerable types of equipment are those where:

• The process is control time rather than temperature (for example, in some de-flashing equipment);
• Rates of cooling have been established by relying heavily on latent heat of vaporization of liquid nitrogen (for example, in environmental testing equipment where a 200°C/minute cooling rate is often demanded).

A standard phase separator (equal pressure type) can be used to overcome these problems, see the diagram below.

Equal Pressure Diagram

Pressure Reducing - from 20 psi tank pressure to head pressure
A standard phase separator (equal pressure type) can be configured so that liquid is fed directly to the float valve from the bulk storage tank (at 20 psi maximum pressure). This produces a substore of liquid at atmospheric pressure from which liquid can be drawn. A large exhaust vent is used to limit the pressure rise when the phase separator is filling. When the phase separator is full, the float valve automatically shuts off the supply.

This system helps overcome the problems associated with equipment which requires liquid to be delivered at a pressure of only a few psi, but where the user would like to have the convenience of bulk storage and a piped supply.

Typical applications:

• Filling of cold traps where splashing is not required.
• Where a specified low pressure must not be exceeded.

Pressure Dropping (typically to 10 bar to 1 bar)
Often there is a requirement to have the liquid supply pressure reduced as well as removing gas from the liquid. A pressure dropping phase separator achieves this by using an electronic level control system and a valve capable of withstanding the high supply pressure. High pressure liquid is flashed down to the pressure set by relief valve, and liquid can then be made available from the phase separator at the relief valve pressuring setting.

Consideration should be given to the loss of liquid nitrogen likely to be incurred when producing low pressure liquid in this way.

Typical applications:

• Decanting of small quantities of liquid from high-pressure liquid storage tanks primarily for use in gas supply application.

For more detailed information, see Gas Vents and Pressure Reducing System