Optimising compressor dry gas seal line design in FEED stage

A proactive approach to engineering excellence is presented by a strategic initiative to enhance the design of compressor dry gas seal lines within FEED deliverables.

Rajib Talukder
Saudi Aramco

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Article Summary

Leveraging a rich pool of expertise from pre-commissioning and commissioning phases, this initiative is an effort to elevate front-end engineering design (FEED) processes. It highlights the importance of a proactive approach to design and standardisation, ensuring engineering solutions are robust and efficient from the start.

The narrative explores the process of revisiting and refining FEED deliverables, leveraging the author’s experience in spearheading a FEED standardisation initiative within a distinguished engineering firm. This initiative aimed to streamline FEED deliverables by integrating the extensive knowledge and expertise of the engineering community.

Optimising compressor dry gas seal (DGS) line design during the FEED stage is crucial for several reasons:
• Reliability and performance: Proper design optimisation ensures that the DGS system operates reliably and efficiently. This is essential for maintaining the performance of the compressor and avoiding unplanned downtime.
• Safety: A well-designed DGS system enhances safety by minimising the risk of seal failure and gas leakage. Safety is a critical consideration in the oil and gas industry, and a flawed design could lead to hazardous situations.
• Environmental compliance: Efficient DGS systems help in minimising fugitive emissions, contributing to environmental compliance. Reducing leaks and emissions is essential for meeting environmental regulations and sustainability goals.
• Operational costs: Optimisation in the FEED stage can lead to cost savings during the operational phase. A well-designed system may require less maintenance, resulting in reduced operational and maintenance costs over the lifespan of the compressor.
• Energy efficiency: Properly designed DGS systems contribute to the overall energy efficiency of the compressor. This can lead to lower energy consumption and reduced operating costs.
• Long-term asset integrity: The design decisions made during the FEED stage have a long-term impact on the integrity of the compressor and associated equipment. A well-optimised design helps in preserving the integrity of assets, extending their operational life.
• Compliance with standards and codes: Adhering to industry standards and codes is essential for ensuring the reliability and safety of the compressor system. Design optimisation in the FEED stage ensures that the system aligns with relevant standards and codes.
• Early issue identification: Identifying and addressing potential design issues early in the project life cycle is more cost-effective than making corrections during the detailed engineering or operational phases. FEED stage optimisation allows for early issue identification and resolution.
• Ease of operation: A well-optimised design makes the compressor system easier to operate and maintain. This is important for the personnel managing the equipment, as it reduces the complexity of operation and maintenance procedures.
In summary, optimising compressor DGS line design during the FEED stage is critical for achieving reliability, safety, environmental compliance, and cost effectiveness throughout the lifecycle of the compressor system. It allows for proactive decision-making and sets the foundation for a well-performing and efficient facility.

Key insights were gained from the collective experience in pre-commissioning and commissioning, particularly in enhancing compressor deliverables through modifications in FEED documentation. Two primary recommendations emerged:

P&ID note enhancement
It was proposed to include a specific note on the compressor’s piping and instrumentation diagram (P&ID) concerning the seal gas line. This note would emphasise the importance of minimising pockets, ensuring proper slopes, and providing adequate heat tracing and insulation. The goal is to maintain temperatures above the dew point, thereby pre-empting the need for modifications during the engineering, procurement, and construction (EPC) phase, which could lead to change orders.

Compressor datasheet improvement
Another recommendation focused on the compressor datasheet, suggesting the inclusion of precise dew point values or graphs. This contrasts with the more generic directive of maintaining temperatures ‘20°C above the dew point’.Providing EPC contractors with specific, actionable information from the outset is intended to facilitate a smoother project execution.

However, these suggestions were perceived as exceeding the traditional scope of FEED, which typically deals with pipeline sizes of 4in and larger. The consideration of details for a DGS line, which is only 1in in diameter, was viewed as overly granular for the FEED stage. The argument was that detailing such specifics in the datasheet might unduly prolong FEED activities, with these aspects being more appropriately addressed during the EPC phase.

While reviewing the EPC deliverables for a diesel hydrotreater (DHT) recycle gas (RG) compressor, the author, drawing upon extensive experience in project oversight, identified critical areas of improvement. This analysis aimed to ensure that the design and operational integrity of the compressor system met the highest standards of efficiency and reliability. The following sections detail specific observations made during the review, an analytical process undertaken to understand the implications of these findings, and mitigation measures adopted to address these gaps effectively.

Dry gas seal line arrangement
Findings: The review of EPC deliverables for a DHT recycle gas compressor revealed significant gaps in the DGS line arrangement:
• The 1in seal gas line lacked tracing and insulation, a deviation from the vendor’s P&ID, which showed tracing and insulation post-seal gas filters.
• EPC P&ID documentation failed to specify the need for sloping the 1in line back towards the tapping point.
• Additionally, the EPC contractor’s approach to dew point temperature, set at 70°C based on compressor suction from an RG gas scrubber treated with lean amine solution, was identified as a key area of concern.

Analysis: The analysis commenced with the FEED contractor’s recommendation, anchored in API-692 guidelines, which was added to the compressor mechanical datasheet. This recommendation emphasised the need for the seal gas line to be traced and insulated, potentially incorporating a heater to maintain the temperature at least 20°C above the dew point of the seal gas. The EPC contractor was then tasked with carrying out detailed dew point calculations at the compressor discharge, from where dry seal gas tapping is taken.

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