Abradable Seal Test Rig for Quantifying Abradable Material Performance During Labyrinth Seal Rubs in Centrifugal Compressors: Design and Test Results

Abstract

In centrifugal compressors, the shaft seal clearance is critical to equipment efficiency and performance. Applications employing very small seal clearances must rely on abradable materials in the stator (housing) to prevent damage to rotating components in the case of rubs. An interaction (or rub) between the rotating seal geometry and stationary seal material can occur while traversing critical speeds or during upset conditions or transients outside the normal, designed operating range of the compressor such as thermal transients or excessive vibrations. The standard abradable seal material for compressor applications is mica-filled PTFE, but alternative materials are required in applications with elevated temperatures, elevated pressures, or harsh environments where chemical resistance is necessary. The behavior of these various abradable materials during a rub event is not well understood and needs to be further quantified to allow for proper material selection for each compressor design. In order to determine suitable alternative materials, a rub simulation test rig was developed to quantify the material wear characteristics and thermal stability of various seal samples. The test rig was designed with a 7.24-inch (184-mm) diameter rotating section with representative labyrinth seal teeth. The rotating shaft was coupled with a variable speed motor capable of operating up to 24,000 rpm, allowing for surface speeds up to 754 ft/s. A 180-degree abradable seal section was mounted concentric to the rotating shaft, which may be forced into the rotating seal teeth by a linear actuator to simulate the rub. The test rig was designed to accommodate various interaction rates, initially considering 0.1 and 1 mil/s (2.54 and 25.4 μm/s) to simulate a rub caused by thermal growth and a rub caused by vibration or bow, respectively. Furthermore, the test rig was equipped with an ambient heater and pressure enclosure to evaluate seal materials at elevated temperatures and with various gas compositions. This paper presents the design and capabilities of the abradable seal test rig as well as the initial rub test results for Fluorosint 500.