Case Study on Performance Impact in Compressor Dry Gas Seals During Transient Condition
Authors: Manikandan Arumugampillai , Moorthy Subramanian 
Published: November 2017

Case study on performance impact in compressor dry gas seals during transient condition

Centrifugal compressors are used to pressurize the process gas in oil and gas plants. It is considered as one of the most critical equipment in the plant and needs highly safe and reliable operation. This case study represents the performance impact and failure of dry gas seals (DGS) while the unit was in transient condition

DGS systems are specially designed to seal the process gas in centrifugal compressor. The seal gas is extracted from the discharge of same compressor and supplied to DGS after filtration with necessary control (Pressure or flow) based on the requirement in the seal cavity. Ingression of foreign material into the running gap of the seals leads to degrade sealing performance and eventual failure of the seal. After sealing the process gas, a very small amount of the sealing gas passes through the primary seal faces which are connected to the flare.

This case study describes the detail of performance impact and failure of Dry Gas Seal (DGS). In a process plant the compressor was used for High Pressure Synthesis Gas Compressor – Ammonia. During the restart of the compressor unit we found high leakage in primary vent on the drive DGS. The same seal was working normal for the period of 2 years before the shutdown. During the shutdown period the unit was kept in barring to avoid the shaft bowing or any struck in DGS. All running parameters before the shutdown was normal.

High primary vent leakage on the drive end indicated the abnormality of DGS and did not permit the unit for startup. Initial investigation was performed on the primary vent pressure transmitter and flow transmitters, seal gas filter, seal gas inlet line, functionality of the Pressure Control Valve (PCV). But all of them were found normal so it was decided to disassemble the DGS cartridge from the compressor During the disassembly, the DGS cartridge was found difficult to remove out. Active thrust bearing, thrust collar and journal bearings were found normal. No major abnormality was found on the bearing pads. Tertiary seal was found normal.

DGS cartridge outer area was found with high amount of black powder deposits externally. Seal chamber area was found with lot of black powder deposits in the primary vent are. Based on the finding it was decided to perform internal inspection on the DGS cartridge.

During disassembly below were the findings ➢Primary / inboard side seal faces were found with contact marks➢Primary / inboard side seals were found struck with no axial movement➢Primary / inboard side stationary seal face was found damaged at anti-rotation pin area.➢Primary / inboard side rotary seal ring was found heavy scoring marks and sealing grooves worn out➢Primary / inboard side rotary seal ring tensile spring under the rotary seal was found damaged➢Secondary / outboard side seal faces were found normal

Chemical analysis was carried out on the black powder samples collected during the disassembly of DGS. Analysis report indicated the presence of Nickel (Ni) and Flourine (F)

It is confirmed that the entry of debris in between the primary seal faces caused the struck and relative movement resulted the damage of tensile spring on rotary seal ring.

Root cause of high torque between the seal faces were analysed and found to be due to the absence of positive seal gas pressure. During transient condition entry of process gas from compressor casing became condensate and contaminated the dry gas seal faces as there was no external source or seal gas booster available in this seal gas system

Site team was advised to introduce the seal gas booster with integrated logic control to provide filtered gas to dry gas seals through the seal gas filter

This case study is providing the trouble shooting experience and useful information for all the gas compressors incorporated with dry gas system without seal gas booster or external supply in oil, gas and petrochemical industry to have more reliability.