(55bt) Zero Purge Gas Emission for New Compression Stations Designed for Natural Gas Pipeline Transmission

1. Introduction

The use of ventstacks at natural gas facilities (compression stations and processing units) are commonly associated to the use of purge gas. Purge gas is used as a industry recommended practice by API-521 which consists in a continuous upward gas flow inside the ventstack tower. The constant gas flow guarantees positive pressure inside the ventstack preventing atmospheric air inlet. Purge gas flow practice is recommended by API in order to avoid explosive mixture air-methane inside the chimney and it also mitigates any possible risk of explosion.

The practice of using purge gas have been acknowledged and accepted by the brazillian and north american industry for many years. However, its use put a constant natural gas stream into the atmosphere during the entire facility operation.

Such practices as purge gas must be reevaluated and optimized for future and existing facilities. In order to do so, this paper aims to present a proposal to eliminate the use of purge gas for natural gas compression stations designed for natural gas transmission pipeline system during its normal operation.

2. Theme Development

API-521 states that gases and pressure relief vapors can be discharged directly into the atmosphere if the environmental regulatory bodies allows such discharges. High levels of hydrocarbon vapors when discharged into the atmosphere can create an explosive mixture with air downstream of the ventstack oulet. The mixture of hydrocarbons and air at the top of the ventstack chimney can be ignited by lightening during tropical storms, for example.

The main concern regarding closed safe relief systems is the flame propagation downwards the ventstack chimney which could lead to a fire incident inside the depressurization header and possible explosion. The pressure and density difference between air and natural gas can allow air inlet into the ventstack creating an air/gas flammable concentration gradient with flashback potencial.

Purge gas continuous emission at optimal and calculated flowrates is a API-521 recommended procedure to mitigate explovise mixture into the depressurization system. However, purge gas as practiced in the current industry is commonly overdesigned which results in a high gas emission rates that can be responsible for at least half of the compression station atmospheric gas emissions.

• Compression station closed despressurization system

Figure 1 shows one of the most used facility configuration for natural gas compression station for gas transmission pipeline. The schematic shows the compression station interface with the main pipeline grid using a valve arrangement to allows gas flow into the facility. The compression station layout shown in Figure 1 has two coalescer filters (A/B), three compressors (A/B/C) and three gas coolers (A/B/C). The fuel gas systems has three filters (A/B and C) and two gas heaters (A/B).

The depressurization system is also represented in Figure 1 and it is composed by the shutdown valves (SDVs), blowdown valves (BDV 001/002/003/004), pressure safety valves (PSVs 001/002/003/004/005/006), a depressurization header and the ventstack (A). The PSVs open during the occurency of a process instability relifing the system when the pressure goes above the normal operation setpoint trying to bring the system back to normal pressure operation without stopping the gas flow. If the pressure continues to rise and hit the setpoint of the SDVs, the gas flow is immediately interrupted and all BDVs opens resulting into the entire facility blowdown.

Figure 1 – Compression station commonly used closed depressurization system and facility layout.

Source: produced by the author.

The closed depressurization system represented by Figure 1 is one of the most common currently used systems in the natural gas industry. Such configuration allows all the facility gas contribution (process instabilities and blowdown) to be directed to a remote and safe location where the gas can be discharged to the atmosphere. The top of the ventstack is open to atmospheric air and for this particular reason continuous purge gas flow is required to avoid air inlet into the vent chimney and header. Continuos purge gas flow for this particular engineering design is a safety requirement and its widely adopted by the industry.

• Compression station open depressurization system.

Following the same main idea of the schematic presented at Figure 1 it is possible to modify the engineering design of the depressurization system for the compression station by segregating the process instabilities gas contribution (PSVs) from the facility blowdown (BDVs). The segregation of BDVs allows the ventstacks to be open to the atmosphere but without the possibility of explosive mixture formation. BDVs valves will only open under emergency situations or predictable maintenance. Figure 2 shows the proposed configuration for the open depressurization system. The system has six BDVs (001/002/003/004A/B/C), two ventstacks (A/B) and a nitrogen system (Figure 3) to prevent ignition and flame propagation while depressurization occurs.

Figure 2 – Compression station open depressurization proposal.

Figure 3 – Depressurization system PFD.

Source: Produce by the author.

The schematic in Figure 2 shows the segregation of all the compression station BDVs that composes the emergency depressurization system. The BDVs will always be closed during normal operation and in the event of a ESD or predictable maintenance which requires the entire facility blowdown, the valves will open and the gas contribution of all BDVs will be directed to the ventstacks. In this new configuration the ventstack will be open to atmospheric air without the possibility of air-gas explosive mixture formation and therefore, conntinous purge gas flow will no longer be required.

The PSVs gas contribution is significantly lower than the BDVs because process instabilities around the facility is not intended to happen during normal operation. The PSVs will only open when a system disturbance occurs and pressure rises above the normal operation setpoint. If the disturbance is corrected, the valve will close again a few moments later when the system hits normal pressure without any damage to the process facility. For this configuration is absolutely mandatory that the PSVs triggering setpoints must be lower than the SDVs and BDVs in order to avoid safety devices triggering when small process disturbance occurs.

The low gas contribution generated by the PSVs’ triggering can be directed to the atmosphere along the entire facility operational area considering a safe hight for discharge. Therefore is no longer necessary to collect the PSVs gas contribution into the despressurization header and each compression station system will have its own PSV discharged locally into the operational area. For the safety of the plant is absolutely mandatory that all equipment, instrumentation and wiring must be atex protected alongside all the facility operational area.

The combined configuration shown in Figure 2 and 3 no longer requires the use of a continues purge gas flow into the de depressurization system. The BDVs will be always closed during normal operation and they will be placed as close as possible to the ventstack chimney, therefore it will no longer be possible to occur any explosive mixture nor up neither downstream the valves. The PSVs will be strategically and safely placed throughout the facility operational area and its gas discharges will be direted to the atmosphere at a safety hight that can be defined by a gas dispersion CFD simulation.

• Potential CH₄ emission mitigation

For a conventional brazillian compression station it is reasonable to assume 0,025% up to 0,05% purge gas and other types of fugitive contribution and very small gas leakage. Considering a transportation capacity of 10.10⁶ m³/day (ten million cubic meter per day) at 20ºC and 1atm.

Table 1 shows a roughfully estimation of CH₄ emission of a natural gas compression station placed in in Brazil which adopts constantly purge gas emission as recommended by API.

Table 1 – Estimated CH₄ purge gas emission in a natural gas compression station in Brazil

System	Compression station (m³/day)	Percentage (% per day)
Transportation capacity	10,000,000.00	100
Purge gas	2,500 up to 5,000	0.025 up to 0.05

Source: Produced by the author

The total amount of CH₄ emission per year in a single facility represents amore than 1,007,181 cars per 100 km driven in a single year. Purge gas represents the majority of the total facility emission and its elimination can significantly reduce the plant carbon emission. (1 car = 25 kg CO₂/100 km driven and 1 kg CH₄ = 21 kg CO₂ eq.)

3. Final Remarks

The brazillian industry of naural gas pipeline transmission is very tradicional and it has more than 30 yearso of rubust and reliable operation. However, sustainability, climate change, carbon emissions and netzero operation were not a key concern by the time they were designed and constructed. Therefore, it is absolutely necessary to reevaluate some concepts and technical requirements in order to meet the sustainability goals and achieve the netzero future that society has been currently persuing.

The engineering design proposal presented in this paper is one of the first steps towards natural gas transimission decarbonization. The design proposal is simple, accurate, safe, reliable and sustainable for new compression station designs. This new concept allows the elimination of a continuous methane stream into the atmosphere without the neeed of any other auxiliar equipment nor use of inert gas.