Modern Methods of Gas and Condensate Recovery during Pigging Operation

1 Importance of Gas Recovery during Pigging operations

It is estimated that just from methane emissions, 21,400 Mcf of gas can be recovered annually lost during pigging operations (EPA, 2005). Gas accumulation and treatment plants are installed downstream of gathering systems (GA-SYS), where the liquid entrained in hydrocarbon-rich gases is prone to condense. Pigging of GA-SYS is often accomplished by using round-shaped or bullet-like pigs to strip out the condensed liquid. This mitigates the pressure drop in GA-SYS that consequently improves gas flow rate and augments the pipeline efficiency. The liquid thus collected is temporally stored at GA-SYS pressure and subsequently stored in a tank at low pressure. Pigging of such liquids is carried out before the gas is administered into the treatment plant compressors (TPCs). The lighter hydrocarbon gasses (CH₄, for example) are disengaged from the stored liquid by flashing and venting (Davidson, 2002).

1.1 Methane Emissions

A vapour recovery compressor (VRC) compresses flash vapours (methane) from the pigged liquid storage tank. The operating time and design flow rate of a VRC determines CH₄ emission savings (MES).

1.2 Economic Analysis

The table below illustrates economic analysis and environmental benefits associated with MES as an example. In this case, a GA-SYS that operates at a 200‒300 PSI gauge pressure is pigged at a frequency of 30‒40 times per annum. The pigging operation produces approximately 477 m³ of liquid condensate in a year. An electric compressor costing US$ 24,000 constitutes a small vapour recovery system (VRS) with an operating and maintenance cost higher than US$ 1000 per year (EPA, 2005).

The number of pigging sessions in GA-SYS strongly depends on the composition of the gas, ambient temperature, geographical location of GA-SYS, and weather conditions. The composition of entrained gasses (CH₄, C₂H₆, C₃H₈, and C₄H₁₀, for instance) in condensed hydrocarbon liquid, the energy content and gas value, gas value, and the volume of collected liquid altogether define the economic feasibility of gas recovery by the pigging operation.

2 System for Gas Recovery during pigging

The gas released into the environment results in a significant revenue loss. Therefore, there is a dire need to recover the gases. As aforementioned, installation of a VRS can be made to recover the vented gas. The recovery of the desired flashing gas is achieved by taking advantage of the pressure drop between high- and low-pressure liquid storage tanks. It alleviates the undesired gas emission and generates more revenue with the sale of recovered gas. For this purpose, a gas- or electric-engine driven VRC in addition to a low-pressure liquid pump and a vapour/liquid flash vessel are installed depending upon temperature/pressure conditions and the desired system design. Such a set-up necessitates several on-site pieces of equipment such as a pig launcher/receiver, flash vessels for vapour/liquid separation, and high- and low-pressure liquid storage vessels. The VRC for vapour compression and subsequent condensation is designed keeping in view the weather conditions on-site across the year. At constant pressure, for example, the pigging operation results in an augmented amount of the liquid condensate in winter that requires conduct pigging more frequently than the summer season. Therefore, the most suitable gathering lines are the ones that can recover a huge volume of liquid at a moderate to high (150-300 PSIG or higher) pressure while undergoing more frequent pigging operations. The frequent removal of condensed liquids from upstream gathering lines is mandatory for a successful recovery of gas and liquid condensate at a gas gathering and treatment unit (Zadakbar, Vatani, & Karimpour, 2008).

Such vapour recovery units (VRUs) are designed and installed to comply with the emission standards set by the Environmental Protection Agency (EPA) and other regulations stated by the law enforcement authorities of the state. VRUs are aimed to mitigate air pollution and lessen fire hazards through the recovery of valuable hydrocarbon gases.

2.1 PROCESS FLOW AND COMPONENTS

A typical VRU is shown in Figure 1 below. The main components of a VRU include a compressor, suction scrubber, lubricating section, liquid transfer pump, a gas bypass system, and a control panel. A brief sequential description of each of these components is presented as follows.

2.1.1 Suction Scrubber

The vapours are introduced into VRU via a suction scrubber. The purpose of this section is to eliminate any liquids that are condensed in the gas vent line. The liquid settles out in suction scrubbed and collected in the bottom. The removal of liquid ahead of the compressor is of paramount importance to avoid any damage that can potentially trigger due to dilution of compressor lubricant and consequent wear and tear of the mechanical parts of the compressor.

2.1.2 Liquid Transfer Pump

The liquid transfer pump turns ON automatically when a liquid level sensor transmits the signal to the level controller after a certain liquid level set point in the scrubber is reached. An outlet pipeline connected on the backside of the scrubber serves the purpose of liquid transfer into the storage tank. The hydrocarbon gases accumulated on top of the liquid bed in the suction scrubber are introduced into the compressor for subsequent compression and injection into gas GA-SYS.

2.1.3 Compressor

The key component of a VRU is a single- or multi-stage compressor unit. The choice of a single- or multi-stage compressor depends on desired pressure conditions. Generally, two types of compressors (Rotary and Reciprocating) are used for this purpose.

Single-stage rotary compressors are used when a lower discharge pressure (up to 50 psig) is required. For higher discharge pressure, 125 psig, for example, the two-stage rotary compressor is used.

For a discharge pressure of 75 psig and above, a single-stage reciprocating compressor is used. For a discharge pressure over and above 125 psig, a multi-stage reciprocating compressor is employed. The situations where two-stage compressors are deemed necessary, a second liquid scrubber is installed at the outlet of the first compressor. This arrangement is made to obstruct the introduction of any liquid that is condensed during the compression session of the first compressor. Liquid slugs, if allowed in the second compressor, can damage mechanical parts of the compressor.

2.2 Liquid Ring Compressors for Vapor Recovery

Ideally, the most suitable compressors used in a VRU are liquid ring compressors (LRCs) preferred over any other compressor due to their ability to process dirty, explosive, and corrosive gasses. They exhibit intrinsically safer operation even during the processing of gasses with variable composition and higher H₂S and H₂O contents. Because of their simple and robust design, LRCs can handle the salt/organic materials’ carryover (occasionally, though) without having any detrimental impact on the overall process. In addition, a little maintenance requirement compared to other compressors makes it an ideal choice for VRU applications, particularly when a reliable and robust process for vapour recovery is sought (Yazdani, Asadi, Dehaghani, & Kazempoor, 2020).

2.3 Cooling System for Compressor and Vapors

Cooling of the compressor housing and the compressed vapours is important for streamlining the operation of this section. Multiple cooling systems are available in a VRU based on the type of compressor used and design conditions. For instance, in the case of reciprocating compressors, the cooling fins on the compressor housing act as a source of heat transfer between compressed vapours and the environment. Like cooling fins outside a motorbike engine, the cooling fins outside the compressor housing increase the surface area exponentially, increasing the heat transfer rate. However, frequent cleaning is required to effectively utilise the cooling fins to enhance the heat transfer coefficient. This arrangement nullifies any chances of compressor overheating.

On the other hand, in the case of rotary compressors, two strategies are in place for compressor and vapour cooling. The first one encompasses the cooling phenomenon by passing the hot water through cooling coils radiator equipped with cooling fins. This arrangement is akin to an automobile radiator. Hot water dissipates heat to the environment through coil walls and cooling fins. The convective rate of heat transfer can be further increased by wafting the cooling fins with a fan. The second strategy covers the cooling of hot compressed vapours by passing them through an aerial cooler consisting of a series of tubes equipped with cooling fins to increase heat transfer rate by extended surface area. Both of these methods have their merits and demerits. However, care must be taken while using a water-based cooling system. Potential discrepancies in the case of a water-based cooling system can be avoided by:

• Maintaining an optimal level of coolant.
• Keeping hoses and seals of the assembly in good condition.
• Adding anti-freeze agents in the winter season.
• Using anti-corrosion materials.
• Making arrangements to avoid dirt and rust.
• Keeping the assembly free of any salt- or scale-forming minerals.

2.4 Gas By-Pass System

The vapour pressure inside storage tanks, in this case, is generally measured in inches of H₂O. The compressor’s automatic ON/OFF operation is linked with a set-point of vapour pressure inside the storage tanks. The continuous operation of the compressor is not feasible concerning maintenance and use of electric power. Therefore, it is desired to arrange the ON/OFF operation of the compressor as and when required. Such a system is termed a gas bypass system. The compressor turns ON at a vapour pressure of nearly 2.0 inches of H₂O and turns OFF when the pressure drops to about 1.0 inches of H₂O. As soon as the pressure increases beyond 2.0 inches of H₂O, the gas bypass system activates. A partial fraction of compressed gas vapours is recirculated back into the scrubber and compressor via a bypass valve associated piping that essentially constitutes the gas bypass system. The rest of the vapours are discharged to GA-SYS. The partial opening of the control pilot depends on the set point of vapour pressure inside the storage tank. If the pressure drops well below 1.0 inches of H₂O, the compressed gas is recirculated by fully opening the bypass valve to the compressor. The VRU shuts down automatically by a timing device if a continued dip in pressure beyond the set-point is experienced. In a nutshell, the gas bypass system maintains the vapour pressure at a set point for the smooth functioning of the plant.

3 Gas Condensate Recovery System for pigging

Before enhancements, the pig launcher and pig receiver loading operations witnessed an emission of only 0.02% of the total estimated volume. After enhancement, the system-wide emissions are reduced to as low as 84.7%. In addition, the pigging operation emitted only 0.003% of the total volume. Followings are some of the advantages associated with the use of this process.

• Cost-effective fabrication
• Installation ease
• Liquids mitigation on pig launcher/receiver sites
• Gas volume reduction for ease of release by the use of small pig barrels
• Increase in system efficacy by prevention of gas loss using high/low jumpers.
• Reduced emissions by portable flares.

STEP-I: The low-pressure gathering pipelines flow from good facilities to a compressor station is shown in Figure 4. The sinusoidal pipeline is just pigged at this stage, which means that pig is introduced into the system. However, the launcher valve is closed at this stage.

STEP-II: Over time, the condensed liquid starts accumulating in the sinusoidal line. At this stage, too, the launcher valve is still closed. The accumulated condensed liquid in the sinusoidal line is removed by pigging in the coming stages.

STEP-III: At this stage, the launcher valve is opened, and the bypass lines are closed. The pig is just launched to remove condensed liquid. The red and green colour coding illustrated on valves represents the closed and open valves, respectively.

STEP-IV: At this stage, it is illustrated that the pig reaches the first trough of the sinusoidal line by pushing the condensed liquid.

STEP-V: At this stage, the pig pushes the condensed liquid towards the receiver. Note that the bypass line valve at the receiver end is closed, whereas the receiver valves are open that guide the pushed liquid into the slug catcher/separator tank.

STEP-VI: At this stage, the pig pushes the liquid further into the slug catcher/separator through bypass and receiver lines. The condensate is led to the condensate storage tank and delivered to the sale depot via trucks.

STEP-VII: At this stage, the bypass is closed, so the pig is pushed into the receiver. The remaining liquid in the bypass line is pushed into the slug catcher by gas and pig.

Step-VIII: At this stage, the pig removal (from receiver bypass), isolation, and depressurisation are carried out. The vent is opened for depressurisation.

STEP-IX:  At this stage, the pig is removed from the barrel by opening the receiver end. The trapped liquid (1—2 gallons) is collected in the buckets.

STEP-X: At this stage, the vent valves are closed, and the launcher is prepared for the next cycle, that is, bypassing isolation and depressurisation.

STEP-XI: At this stage, the launcher is opened, and the pig is introduced for the next cycle operation while having the vent valves closed. The launcher valve is closed at this stage.

STEP-XII: At this stage, the system is ready for the next pigging operation. The launcher and receiver valves are closed. Vents are also closed.

4 Safety Measures during pigging and gas recovery

4.1 Hazard identification

Hazardous materials such as n-BTEX (benzene, toluene, ethylbenzene, and xylene), H₂S, and natural radioactive materials left as residues in pigging operation in the form of gases, solid waxes, or condensates offer a potential threat to human health when their intake exceeds the maximum allowable limits (1 ppm for benzene and 5 ppm for hydrogen sulfide for 8h time-weighted average).

4.2 Safe Access to the system

To offset the potential health and safety hazards, it is recommended that access should only be given to the authorised staff. The entry into the receiver by anyone should be restricted. Eye washing and emergency shower facility should be provided close to the workplace. Barriers and caution notices should be displayed properly.

4.3 Equipment and procedures

4.3.1 Control equipment

On-site provision must be made for the control equipment that includes (i) hydrocarbon detectors (fixed/portable), (ii) hydrogen sulfide detectors, (iii) benzene monitoring device, (iv) a fully enclosed and ventilated receiver for pig isolation. We must also have to ensure (i) displacement of vapours to a safe place or a VRS, (ii) provision of angled and covered drains to avoid residues’ accumulation and minimise vapours’ release.

4.3.2 Control procedures

• The vent and receiver MUST be depressurised before the door opening.
• Sufficient time (5 min) MUST be given after opening for the gases/mists to clear.
• The air MUST be monitored for hydrogen sulfide and benzene contents after venting.

4.3.3 Personal protective equipment (PPE)

• The working personnel MUST keep portable personal alarms with them on-site.
• Compatibility of PPE items MUST be ensured.

4.3.4 Respiratory protective equipment (RPE)

CE-marked respiratory protective equipment MUST be provided with a minimum protection factor assignment of 10 for vapours when the pig is removed and cleaned.

4.3.5 Other protective equipment

Type-III disposable hood-based coveralls and chemical resistant nitrile gloves MUST also be provided with plenty of them as a backup inventory stock.

4.4 Maintenance, examination, and testing

4.4.1 Checking and maintenance

The plant maintenance and equipment monitoring MUST be scheduled and properly followed. The charging and proper functioning of the portable personal gas detectors MUST also be ensured before any use.

4.4.2 Examination and testing – RPE

The respiratory protective equipment that is seldom used MUST be examined and tested at least once a month. The RPE that is frequently used should be tested at least after every three months.

4.4.3 Records

Record of all tests and examinations MUST be kept for at least 5 years.

4.5 Exposure monitoring

The correct level- and type of respiratory protective equipment should be used, and, for this purpose, the archived monitoring record can also be used. Otherwise, personal air monitoring should be carried out, and its results can define if the biological monitoring should be carried out or not. Was should be tested for NORM, for safe disposal.

4.5.1 Cleaning and housekeeping Areas

The pig cleaning should be accomplished at a dedicated space having a proper ventilation and drainage system. Low-pressure washing is used to clean the receiver bay to make it residual oils- and waxes-free. For this purpose, a full set of PPE, including slicker suit, RPE, boots, gloves, and goggles, MUST be provided.

4.5.2 Waste

The hazardous waste containers must be labelled clearly, and a UN number can also be included wherever required. The waste must be stored and disposed of safely.

4.5.3 Personal decontamination and skincare

The personnel involved must be instructed to properly clean their skin and wash their hands before every break. The following items must be provided to them for the purpose above.

1. Warm water
2. Nail brushes
3. Mild skin cleansers
4. Soft paper
5. Fabric towels
6. Pre-work skin creams (for dirt removal)
7. After-work creams (for skin oil removal)

The use of abrasive cleaners should be discouraged.

4.5.4 Health surveillance

Low-level health surveillance that includes dermatitis/skin checks should be conducted by well-trained and responsible staff.

4.6 Training and supervision

The managers and supervisors should be given the task of systematic training and supervision to the staff involved. They should be trained for the appropriate and timely use of the equipment, benzene monitor, and safe working procedures. Proactive response on alarms ringing and safe evacuation in case something goes wrong MUST also be part of training. In addition, the concerned staff should also be supervised to make sure they follow the safe work procedures. The general personnel and maintenance workers should know the potential hazards and risks involved.

5 References

Davidson, R. (2002). An introduction to pipeline pigging. Pigging Products and Services Association, 9.

EPA. (2005). Recover Gas from Pipeline Pigging Operations. Partner Reported Opportunities (PROs)for Reducing Methane Emissions.

Yazdani, E., Asadi, J., Dehaghani, Y. H., & Kazempoor, P. (2020). Flare gas recovery by liquid ring compressors-system design and simulation. Journal of Natural Gas Science and Engineering, 84, 103627.

Zadakbar, O., Vatani, A., & Karimpour, K. (2008). Flare gas recovery in oil and gas refineries. Oil & Gas Science and Technology-Revue de l’IFP, 63(6), 705–711.