How to Boost Natural Gas Vapour Recovery

Oil and natural gas producers have always been diligent about recovering as much of their oilfield product as possible. The product alone has considerable value, so any product losses impact the bottom line. But that’s not the only reason these producers have made product recovery a priority, especially in recent years.

In the USA, the Environmental Protection Agency (EPA) recently issued a final rule designed to sharply reduce methane and similar emissions from oil and natural gas operations, including from existing sources across the United States. The final rule includes new standards to reduce methane and volatile organic compounds from new, modified and reconstructed sources. It also includes emissions guidelines that set procedures for states to follow to limit methane from existing sources.

Environmental regulations have continuously become more stringent over the decades and this recent action from the EPA follows that same pattern. Oil and natural gas producers, as has been the case over the last 10 to 20 years, must be cognisant not only of the value of their product but also of mitigating its environmental impact. One of the ways to do this comes through an operation’s vapour-recovery capabilities.

The Challenge

Keeping gas vapours contained presents its share of challenges. Crude oil storage tanks, for example, have always been prone to losing gas vapours. Even with various door hatches, gaskets and low-pressure pop-off vent valves – all designed to control vapour loss – vapours might still seep through.

The product alone has considerable value, so any product losses impact the bottom line. But that’s not the only reason these producers have made product recovery a priority

In the past, such tanks were not subject to environmental regulations, resulting in many of these tanks being poorly maintained. Failure to properly maintain crude oil storage tanks could result in vapour releases. Vapour releases might also occur when a tank’s pressure relief valves open to release excess vapour pressure.

According to the EPA, new rules require that vapour releases now be contained, flared or reclaimed. Several types of compressors tend to be popular for the reclaiming process. Rotary vane and rotary screw compressors, as well as reciprocating piston compressors, are the standouts. An operator will typically select one of these three options depending on the overall operating conditions (i.e., required flow rate and discharge pressure).

While many of these compressor technologies excel in these functions, the traditional lubricated piston compressors and some of the rotary designs might experience leaking. Additionally, small amounts of condensate (condensed liquids) might form in those compressors, which can cause internal problems. For example, condensate collects in the crankcases on some reciprocating compressors and contaminates the lubricating oils, forcing frequent oil changes. The condensate can also, in many cases, cause significant power-frame damage to bearings and shafts. The damage can compound to the point where the entire compressor must be replaced.

The Solution

Operators, however, have technology to counter the problems created by condensate while also minimising leaks. The design of an oil-free reciprocating gas compressor allows the technology’s distance piece to collect and control the condensate, keeping it out of the critical power-frame area where the damage typically occurs. The oil-free design also has two sets of rod seals that contain the gas at the distance piece, reducing the amount of gas vapours that can escape to the environment through the crankcase, which is typical in most styles of reciprocating machines.

The distance-piece design feature of these compressors is critical because harmful condensate can form inside the compressor due to the wide mix of hydrocarbons that can often be found in the gas stream. When the gas is compressed, heavier hydrocarbons and small amounts of water vapour can form. In compressors without a distance piece, the condensate leaks directly into the crankcase, potentially damaging the quality of the lubricating oil and leading to significant bearing damage or a complete failure of the power frame.

Oil-free reciprocating gas compressors are a sustainable option... with the ability to efficiently recover gas from a variety of storage tanks, leading to a higher amount of saleable end-product, while simultaneously helping protect the atmosphere

Oil-free reciprocating gas compressors help control condensate that may form inside the compressor. Their design provides a means to control small amounts of condensate by collecting the condensate in an area of the compressor. Easy operator access to this compartment allows the user to empty the distance-piece compartment regularly. The distance piece can also be vented to a safe disposal area when available. This design gives the distance piece the ability to provide an effective means of controlling condensate.

Oil-free reciprocating gas compressors were designed to handle hasardous, toxic or flammable gases. Their oil-free design provides truly unique features that were at one time considered unneeded and unimportant in the natural gas market. These products are environmentally sensitive and offer superior vapour-recovery mechanics and sealing technology. Specifically, oil-free reciprocating gas compressors:

• Do not add emissions to the gas being handled or recovered
• Maximise control of gas leakage from the compressor due to the compressor’s full distance piece compartment and piston-rod seals. The two sets of rod seals create a compartment between the process gas and the lubricated crankcase, providing leakage control and minimising any leakage to the crankcase or the atmosphere.
• Feature ductile-iron construction of all pressure parts results in greater resistance to harmful thermal and mechanical shock
• Have high-efficiency PEEK valves specially designed from a self-lubricating material that provides superior sealing characteristics, high efficiency and durability
• Contain a heavy-duty, precision-ground crankshaft featuring roller bearings and integral counterweights to help ensure smooth, quiet operation
• Have an epoxy coated crankcase to protect against corrosive environments
• Possess the ability to cover a wide range of inlet and discharge pressure conditions that generally exceed other technologies’ capabilities.

All of these design advances mean that oil-free gas compressors are a safe and effective method to move and recover gas vapours. In today’s highly regulated tank vapour-recovery applications, these important features help deliver peace of mind to the oilfield operator and producer.

While single-compartment distance-piece designs will generally satisfy most design requirements, Blackmer also offers triple-packed (two-compartment distance-piece models) for applications where customers might be especially focused on leakage control. These proven designs have been used in industrial and chemical plant applications for many years.

Single-stage models, which are recommended for use on low to moderate compression ratio applications, typically are available in several sizes with piston displacements up to 125 cubic feet per minute (212 m³/hr) at working pressures up to 1,000 psia (69 bara). The two-stage models, which are available for higher compression-ratio requirements, tend to offer piston displacement up to 53.7 cubic feet per minute (91.2 m³/hr) at a working pressure up to 615 psia (42.4 bara).

Conclusion

Process system designers and environmental compliance managers at oil and gas companies—and at others businesses around the country—are always looking for ways to continue incorporating improved vapour-control technology that helps minimise vapours and VOCs that are released to the atmosphere at tank battery locations.

Oil-free reciprocating gas compressor designs provide a future-focused and effective method to mitigate vapour emissions. Oil-free reciprocating gas compressors are a sustainable option that provide operators with the ability to efficiently recover gas from a variety of storage tanks, leading to a higher amount of saleable end-product, while simultaneously helping protect the atmosphere.

Ron Crouch is product management director for Blackmer Gear.