Session: 05-11 Turbine Facility Sensors & Diagnostics

Paper Number: 124365

124365 - Dry Gas Seal Condition Monitoring With Embedded Acoustic Emissions Sensors 

Dry gas seals (DGS) are commonly used in turbomachinery applications when handling gases. These include centrifugal compressors, expanders, or turbines. Historically DGS have been monitored via leakage and pressure measurements taken outside of the compressor. In recent years John Crane, a leading manufacturer of DGS, has developed a monitoring system with sensors embedded in the seal itself. The system allows seal designers, equipment manufacturers and machinery operators to monitor the condition of the seal during service via direct seal measurements as opposed to the traditional downstream or upstream measurements. The system consists of inboard and outboard temperature sensors, a liquid presence sensor and an acoustic emissions (AE) sensor.

AE sensors, when embedded into a DGS, measure high frequency elastic waves propagated through the outboard retainer of the DGS. AE sensors use a frequency range greater than 50kHz, thereby ignoring lower frequencies that are associated with normal compressor operation. Focusing on these high frequency vibrations with the AE sensors, provides an exciting opportunity to measure any contact events between the rotating and stationary rings of the DGS.

Although the DGS are designed to be non-contacting, there are known operating conditions such as startup, shutdown, slow roll, turning gear, ratcheting and wind milling where contact occurs, as well as process upsets which may lead to unexpected contact during normal operation. All these contact events can cause some damage to the DGS, the amount of damage varies depends on many factors including seal face materials, duration of contact and compressor operating conditions. Measuring the frequency and duration of these contact events can allow seal manufacturers and operators to verify the health of the DGS, extend seal life by modifying operating procedures and even predict the remaining seal life.

The paper will demonstrate the benefits of embedding the AE sensor in the DGS. It will show rig test data comparing an AE signal from an embedded sensor to a sensor mounted externally to the test barrel, but still only 200mm from the seal faces.

DGS use grooves on the rotating face to generate a gas film between the sealing faces. This gas film is designed to be just a few microns thick under all operating conditions. During compressor startups the rotating and stationary faces of the DGS rub against each other. They do this until the shaft speed is sufficient for the grooves to generate enough hydrodynamic lift to separate the seal faces. This paper includes examples of startups from both rig and field testing. It also includes rig test data showing how the startups change as the seal faces become worn. Measuring the AE signal during a startup is a good way of verifying the seal is functioning as intended.

This paper includes data collected from multiple in-service compressors. The data shows the AE signals for a variety of different compressor operating states. It is demonstrated that the AE signal from all these operating states, can be easily distinguished from the AE signal generated by seal face contact events.

Presenting Author: Christopher Palmer John Crane

Presenting Author Biography: Chris Palmer is a lead engineer within the Gas Seal R&D department at John Crane. Chris graduated from Bath
University in 2006 with a degree in Mechanical Engineering. Since then, Chris has worked in several different
industries, including defense, renewable energy and most recently mechanical seals. Chris joined John Crane in
2014 and has worked on a wide variety of projects within the realm of new product design and development.
Chris is the R&D technical lead for the John Crane DGS condition monitoring product.

Authors:

Christopher Palmer John Crane
Colleen Fritz John Crane Inc.
Chris Kotar ExxonMobil