Case study – Dry Gas Seal Boosters’ Failure and Impact on Centrifugal Compressor Auxiliary System
Seal gas boosters are used as part of dry gas seal's sealing system in centrifugal compressors. 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. Process gas leakage from the compressor are sealed by Dry Sas Seals (DGS). Inorder to have proper functionality of DGS positive clean gas supply is required which is provided by seal gas booster. This case study represents the repeated failure of seal gas boosters and potential impact of dry gas seal system
DGS systems are specially designed to seal the process gas leakage 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. Providing the positive pressure and flow is most important to dry gas seals. Usually process gas is extracted from discharge and supplied as seal gas after filtration. But during start up and shutdown condition compressor positive pressure will not be available to dry gas seal. Absence of positive supply to dry gas seal gas contaminate the seals. To avoid this issue seal gas boosters are used so that boosters can provide positive supply during start-up, shutdown and even during slow run of compressors. Mainly seal gas boosters are divided into pneumatic or electromagnetic type. They are normally divided into two types such as magnetic transmission type which will be driven by electric motor. Other one will be pneumatic type which will be driven by mechanical transmission.
Magnetic type / Electro magnetic type is used in case of sour gas service and pneumatic type is used in case of non-sour application. Booster is used to provide conditioned seal gas flow and not just to increase the pressure. In case of high pressure requirement external gas souce can be used.
Site is configured with 44 motor driven centrifugal compressor trains. Most of seal gas boosters were tested in supplier facility before shipment. But consecutive failures such as high vibration, noise, low differential pressure and reverse flow were observed during the startup of the first 4 boosters. The boosters are motor driven having magnetic type piston to develop the required flow. Initial checks were performed as per the trouble shooting guide but the root cause could not be identified as the problems were different case by case.
During the first unit commissioning, electric motor which drives the booster got overload trip. Entire loop was checked thoroughly and no abnormalities were found. Initially it was understood could be caused by some foreign particles between the pistion and cylinder. After disassembly, heavy scoring was found but no foreign particles were found. Rootcause analysis revealed that the wear was caused by excessive sliding force. The forces were caused by wrong size of bush. It was communicated to the supplier and replaced by new one which sorted out the issue.
During second unit commissioning, seal gas booster was not developing the required pressure and flow so it was decided to perform the internal inspection on seal gas booster. After disassembly, the piston was found struck and not having free movement inside the cylinder. This was resulting in low differential pressure across the seal gas booster. It was also noticed with excessive wear was found on the magnetic piston. Sand particles were found on internal which suggested that the preservation procedures were not followed properly, as indicated in supplier manual.
During third unit commissioning, similar low seal gas flow was noticed across the seal gas booster. After several hours of operation also the pressure did not get developed across the booster. Booster was developing negative pressure instead of positive pressure. Entire loop was checked and no blockages were found. Non-return valve located at the inlet of the compressor was removed out and function of the valve was completely struck and the valve in discharge was not working properly. They were repaired and during next re-start the unit was found working normal.
During fourth unit commissioning, seal gas booster was not deliverying the required pressure and flow. All inlet and outlet lines were checked and no abnormalities found. Internal inspection on booster confirmed the wear between the piston and cylinder. It was caused by magnetic misalignment between the magnetic driver and cylinder. The components were replaced and repositioned as per manufacturer instruction and restarted. After the re-start unit was found working normal.
All corrective actions were carried out on these units and preventive measures were taken to avoid similar problem in other boosters. All storage and preservation procedures were followed as per site quality check list. Site team also verified the downstream lines and seal gas filters to ensure the foreign materials or damaged component materials were not carried forward and affected the seal gas filter cartridge.
This case study is providing the trouble shooting experience and useful information for all the gas compressors incorporated with dry gas system with seal gas booster in oil, gas and petrochemical industry to have more reliability.