Particle monitoring in high-pressure gas lines

In today’s high-technology manufacturing processes, there are a diverse number of procedures demanding minimal levels of particle contamination. With continuous monitoring improvements, the minimum thresholds are always being challenged for an environment to become “more clean”.

Standard clean practices include particle monitoring to manage and reduce particle contamination. These practices require particle monitoring at multiple levels to eliminate existing particles, restrict the introduction of new particles, and prevent the generation of new particles. The benefits gained not only enhance product yields, through reduction of particle contamination, but also may detect and prevent catastrophes.

In both pharmaceutical and semiconductor manufacturing operations, clean gases are frequently used both as process materials and for manufacturing operations. These gases can be inert, flammable, or toxic and are normally transported at pressures ranging from just above ambient up to a few hundred pounds per square inch (psig). These gases have the common requirements of purity and assuredness of particulate cleanliness.

The optimal method to achieve these requirements is through particle content monitoring. Very clean gases are required for many manufacturing processes where particle measurements at levels of 1-10 particles per cubic foot (STP) and size ranges of 0.1 or 0.2 µm may be required. Particle Measuring Systems (PMS) offers several systems capable of sensing particles at 0.1 µm, which are subject to pressure levels of several thousand psig. Additionally, PMS systems utilise non-intrusive monitoring and are capable of handling various inert, toxic, and flammable gases.

When air samples are removed from a compressed gas line, the particle content must not be affected. Unfortunately, sample acquisition and handling will frequently result in particle loss, with the loss becoming greater as the particle size increases above one or two micrometers. Particle loss occurs from disturbances in the airflow and is commonly seen when sampling devices such as nozzles, valves, sample lines, etc. are used.

Because of the inherent inertia of particulate material, the particle trajectory in the sampling system departs from the airflow path, resulting in deposition of particles on the inner wall of the sampling device. For this reason, either isokinetic sampling or nonintrusive measurement is required. If the sample monitoring system operates only at ambient pressure, requiring reduction of line pressure after the sample has been obtained, then there is no way of avoiding loss of larger particles during the pressure change process. In this situation, measurements are restricted to particles less than 2.0-3.0 µm in diameter. When monitoring toxic or flammable gases, leakage or venting to the environment cannot be permitted. Nonintrusive observation or positive retention of side-stream sample lines is required. There are no comprehensive standards in print at this time.

A preliminary draft has been prepared by ASTM, F01.10.07 Standard Practice For Determination Of Particulate Contamination In Compressed Gases Using An Optical Automatic Particle Counter, as a methods document. The standards SEMI has published discuss particle specifications for specific clean gases. These standards discuss maximum particle concentrations at threshold size, verification methods, and particle measurement ranging from 0.02µm to 0.2µm. This application note will indicate how some of the measurement problems can be handled by applying procedures, indicated in both drafts, with standard PMS instruments.

For measurement of particles at line pressure with maximum size sensitivity, the PMS HPGP series or the CGS series probes can be used. The HPGP series works in conjunction with a PDS-PA, which is a microprocessor based data system. The PDS-PA will work with up to two HPGP probes. For sampling at higher pressure, the CGS series offers several probes to select from. The probes work with CGS-DAS, which is a data acquisition system.

For measurement of particles in a compressed gas line with a standard ambient pressure particle counter, a PMS pressure diffuser can be used to reduce the gas line pressure to ambient levels before drawing samples into the counter. Remember, isokinetic sampling must be set up in the compressed gas line and particles larger than 2.0µm or so in diameter will probably be lost in the flow control system.

When installing a sample probe, losses will be minimised by installing a straight sample tube into an elbow in the high pressure line so that the sample tube faces into the flow, rather than installing a curved pitot tube type of sample line. The pressure diffuser can then be connected to the sample probe exhaust.

DO NOT USE THIS METHOD WITH HAZARDOUS GASES.

For measurement of particles as small as 0.1µm, a PMS pressure diffuser and an appropriate atmospheric pressure PMS particle counter will operate at the desired size sensitivity. For side-stream sample measurement at pressure, use the appropriate HPGP or CGS series.