Oil and natural gas producers have always had an economic rationale to capture or recover as much product as possible in the oilfield. Any vapors that were vented to the atmosphere or flared off during the production process were no longer saleable, or not worth the time and labor needed to reclaim them. Now, an even more compelling reason for producers to optimize their operation’s vapor-recovery capabilities exists.
In August 2011, the U.S. Environmental Protection Agency (EPA) introduced New Source Performance Standard 40 CFR, Part 60, Subpart OOOO, or the “Quad O” regulation. In October 2012, Quad O became law and began affecting oil and gas storage facilities, natural gas processing plants and gas wells across the country.
Quad O establishes emission standards and compliance schedules for the control of volatile organic compounds (VOC) and sulfur dioxide (SO2) emissions from storage tanks in tank batteries that are used to temporarily hold liquids produced during oil and natural gas production.
More specifically, Quad O includes a requirement that VOC emissions be limited to less than 6 tons per year (tpy)—a daily equivalent of 33 pounds—from storage tanks that contain crude oil, condensate, produced water and other unrefined petroleum liquids.
Per the edicts of the Quad O regulation, storage tanks were divided into two distinct groups:
- Group 1—Any storage vessel constructed, reconstructed or modified after Aug. 23, 2011, but before April 12, 2013
- Group 2—Any storage vessel constructed, reconstructed or modified after April 12, 2013, or within 60 days after startup, whichever is later
On April 15, 2014, all Group 2 storage tanks must be fully compliant with Quad O, while all Group 1 tanks have until April 15, 2015, to attain Quad O compliance.
This has led to a rush by oil and natural gas producers to upgrade their vapor-recovery abilities so that their tank batteries will emit less than the allowable 6 tpy of VOCs and SO2.
This article will demonstrate how a specific type of compressor technology—oil-free reciprocating gas compression—can be used to maximize the performance of vapor-recovery units at storage facilities, all with the ultimate goal of satisfying the emission limits of the EPA’s Quad O regulation.
Crude oil storage tanks often have issues with gas vapors escaping, even though most tanks have various types of door hatches, gaskets and low-pressure pop-off vent valves that have been designed to control and limit the amount of escaped product vapors.
Without any strong environmental regulations restricting this venting, many tanks were, and are, in poor shape, with faulty hatches and safety valves that leak badly. In some cases, operators do not even close the hatches, or the tanks have faulty gaskets that allow vapors to leak uncontrollably. In many instances, releases would occur simply because of the buildup of vapor pressure in the tanks, which would cause the tank’s relief valves to open and release the excess vapor pressure.
The Quad O mandate requires that 95 percent of vapor releases now be contained, flared or reclaimed. Several types of compressors have been popular for the reclaiming process. Rotary vane, rotary screw and reciprocating piston compressors are all used. Depending on the overall operating conditions (required flow rate and discharge pressure) a particular compressor technology would be selected.
However, oil-free reciprocating gas compressors are ideal for applications in which discharge pressures are around 50 psig (3.4 barg), and in some extreme cases can be more than 500 psig (34.4 barg). Typical discharge pressures from crude oil storage tanks are usually less than 200 psig (13.8 barg).
The challenge for the compression equipment is that traditional lubricated piston compressors and some rotary designs do not have leak-tight sealing or the ability to handle small amounts of condensed liquids (condensate) that can form in the compressor. This condensate collects in the crankcases on some reciprocating compressors and contaminates lubricating oils, causing extremely frequent oil changes and, in many cases, significant power-frame damage to the bearings and shafts. In many cases, frequent complete compressor replacement is considered “acceptable” when the damage is considerable.
Oil-free reciprocating gas compressor technology with a distance-piece design is a unique feature that allows the condensate to be collected and controlled at the compressor’s distance piece, keeping it out of the critical power-frame area in which damage can occur. The added benefit of the oil-free design is that it has two sets of rod seals that seal the gas at the distance piece, reducing the amount of gas vapors that can escape to the environment through the crankcase. This is typical in most styles of reciprocating machines.
Recognizing the untapped potential for an improved vapor-recovery technology in the oilfield, a Michigan pump manufacturer provides compressor products that cover a broad range of markets, including oilfield vapor-recovery applications.