Block 1: DGS Function, arrangements, types and major parts, separation seals
This block describes fundamentals of gas lubricated mechanical seals for compressors or respectively dry gas seals. It covers the working principle, physical effects, main parts and used materials, typical arrangements and balance of supply and leakage flow. Furthermore the different types and function principles of barrier seals as well as their main seal matching will be described. The subjects of this block are the basics for the subsequent blocks and enables people who are not much acquainted with DGS technology a quick access.
Block 2: Seal Management System for DGS
Safe and long operation of the DGS is one of the key factors for increasing the MTBF respectively ensuring an uninterrupted operation of the compressor unit.
The Seal Management System can be understood as an integral part of the shaft sealing solution for centrifugal compressors. It consists of several modules (primary seal gas module, flare module, secondary seal gas module and separation gas module) providing the required seal gas flow and pressure to the DGS.
The block describes the operation philosophy of the SMS with focus on DGS in tandem arrangement with internal labyrinth. Each module will be described in its function and design:
– primary seal gas module: ensures clean and dry gas at the requested pressure, flow and temperature,
– primary vent module: monitoring of the process and atmospheric side seal,
– secondary seal gas module: ensures clean gas to the aspheric side seal,
– separation gas module: provides the needed flow or pressure of clean separation gas to the barrier seals.
Block 3: Typical challenges & potential solutions I
a) Contamination of Dry Gas Seal, during operation and pressurized hold
The reliability of gas seals is largely dependent on having a continuous supply of clean and dry seal gas. In a dynamic mode the gas supply systems takes process gas from a higher pressure level in the compressor, conditions it and provides it to the gas seal to create the ideal environment for the gas seal. This ensures the gas seal is provided with effective protection against contaminated process gas.
Compressor gas seals are very robust sealing devices, but need to be operated in a dry and clean environment. The leading root cause of gas seal failures is contamination. Dry gas seals are being contaminated if the gas reaching the seal is not conditioned properly or the clean seal gas flow towards the seal is inefficient holding back the untreated process gas. The second scenario is very common in modes having a lack of seal gas flow, which suggests no means to produce seal gas flow is available, such as a high pressure gas source or booster for the seal gas supply. This is where it pays to have a reliable, clean gas supply. Without seal gas flow, sufficient seal gas cannot be provided to the gas seal and results in the gas seal being contaminated.
This session will discuss the challenges with contamination resulting from poor process gas quality, condensation and pressurized hold conditions. This paper will describe contamination to the gas seal by process gas, during commissioning, by particle and by liquids, which are caused by inadequate seal gas supply. Then it will focus on different methods of providing seal gas flow during transient conditions. And finally, it will discuss solutions to ensure a reliable, clean gas flow to the seal at all relevant conditions.
b) Low speed operation – DGS in contact operation
The general layout-target of DGS in the design phase is achievement of complete lift-off of the seal faces at all possible operation conditions. Contact free operation is the key factor for unlimited life time. However there are specific conditions where lift-off is impossible. In this block this typical conditions are described (e.g. coast down, turning). On the basis of the expected duration of contact mode operation adequate design measures and material selections are illustrated.
c) Separation seal reliability and efficiency
The separation seal has a major function within the centrifugal compressor shaft sealing solution. It prevents bearing oil contamination of the main seal, hence the reliability of the DGS depends on the reliability of the barrier seal. Furthermore it prevents the bearing brackets to be contaminated with process gas.
Several designs of the barrier seals applies in the oil & gas industry. Well known as:
– double labyrinth,
– static or dynamic carbon segmented rings,
– but also the coaxial barrier seal design.
The block is showing typical issues with known barrier seal solutions by examples. It explains the root cause and present solutions ensuring a reliable and efficient operation of barrier seals for centrifugal compressors.
d) Low Pressure applications considering DGS in tandem arrangement
DGS in tandem arrangement allows additional safety in the unlikely case of malfunction of the primary seal by using the atmospheric side seal as backup seal. However using tandem seals in low pressure applications (sealing pressure near to or below the primary vent pressure) request needs special remark on the monitoring philosophy and operation mode of the DGS. This part of the block is showing solutions on this subject.
Block 4: Practical workshops
– Get in touch with a dry gas seal
Block 5: Do a root cause analyses in teams
Daniel Goebel is Head of Application Engineering, Sales Support and Service Compressor Seals and Systems at EagleBurgmann, located in Wolfratshausen / Germany. He has more than 13 years professional experience in all aspects of compressor gas seals and seal support systems for compressors like application engineering, product management, onsite support and troubleshooting. Daniel Goebel holds a degree in industrial engineering and management of the Munich University of Applied Science and started with EagleBurgmann back in 2003 as application engineer.