Steamy duties for polymer seal

Edited by the Engineeringtalk editorial team Dec 16, 2003

A revolutionary guided wave radar probe developed by Magnetrol International uses a Victrex Peek polymer seal component to reliably seals out high-temperature steam.

A revolutionary guided wave radar (GWR) probe has been developed by Magnetrol International to meet the unique challenges posed by level measurement in saturated-steam environments.

The probe has been designed with a Victrex Peek polymer seal component, which reliably seals out high-temperature steam.

A 30% glass-fibre-reinforced grade of Victrex Peek polymer is used for a critical part of the process seal contained in the probe.

Victrex Peek polymer is renowned for its excellent hydrolytic stability, together with its outstanding chemical resistance and mechanical performance in extreme heat and pressure conditions.

This makes it ideally suited to withstand the harshest operating conditions of saturated steam applications.

Designed for applications such as boilers, de-aerators and feed water heaters typically found in the power industry, Magnetrol's liquid level control consists of a two-wire, 4 to 20mA transmitter and steam probe feedthrough.

Bob Botwinski, Guided Wave Radar Product Manager explains: "In GWR technology, the transmitter sends a high frequency signal down the probe to measure the water level.

Since the probe is used in high-temperature steam, resistance to that environment is essential".

Following unsuccessful trials with a feed-through seal design based on ceramic insulators brazed to metal components, Magnetrol turned to Victrex Peek polymer, which has a proven track record of long-term service in steam.

Exposure to steam requires a high continuous service temperature combined with hydrolysis resistance and the Victrex Peek polymer components continue to perform successfully at temperatures up to 300C.

The feedthrough design requires that the pressure load on the seal assembly be concentrated on a relatively small area of Victrex Peek polymer at the upper end of the probe head.

The high compressive strength of the polymer, even at elevated temperatures, is critical to being able to conservatively rate the probe at 110bar service pressure at 318C.

The seal components are completely enclosed in stainless steel and all the parts are exposed to high temperature.

The close match in thermal expansion coefficients between Victrex Peek polymer and stainless steel minimises induced stresses due to differences in thermal expansion rates.

In addition to forming a seal, the probe head assembly must also pass the electrical signal used to measure the water level.

For this reason, a certain range of impedance must be maintained through the probe.

Impedance is a function of the ratio of the inner and outer conductors, and the dielectric constant of the insulator between them.

The higher the dielectric constant of the insulator material, the larger the outer conductor must be to achieve a particular impedance.

The relatively low dielectric constant of Victrex Peek polymer permits the dimensions of the probe head to be within design limits.

The probe prototype was produced using extruded Victrex Peek polymer rod.

This eliminated the expense and risk of building a mould before the design was proven.

Now that the design is successful, the Victrex Peek polymer parts are cost-effectively injection moulded with consistent processability and precision.