Enclosed flares and open flares both destroy waste gas through combustion, but they do it differently and the choice between them has significant consequences for permit compliance, site safety, and operational reliability. This comparison covers what each technology is, where each is the right choice, and what to ask suppliers about their capability to deliver both.
What Is an Open Flare?
An open flare burns waste gas at the exit of an elevated stack tip the combustion zone is open to atmosphere, the flame is visible, and thermal radiation from the flame radiates outward to surrounding equipment and personnel. Open flares are the standard design for high-volume industrial, refinery, and midstream gas plant applications. They handle large flow rates efficiently, are relatively simple to operate, and can accommodate the wide flow range variations typical of production upset and emergency relief scenarios. The primary design constraints for open flares are the thermal radiation exclusion zone (governed by API 521 calculations), tip velocity limits for smokeless operation, and the requirement for a continuously burning pilot.
What Is an Enclosed Flare?
An enclosed flare (enclosed combustor or vapor combustor) burns waste gas inside a refractory-lined combustion chamber. The chamber walls contain the flame, retain combustion heat for complete destruction of the waste gas stream, and exhaust products of combustion through a stack. No flame is visible at any point during normal operation. Thermal radiation is contained within the chamber walls. Noise output is significantly lower than an open-tip flare of equivalent capacity. Enclosed combustors are ground-level installations no elevated stack is required making them compact and low-profile for production site applications.
Side-by-Side Comparison
| Factor | Open Flare | Enclosed Flare |
|---|---|---|
| Visible flame | Yes, visible during operation | No, combustion fully contained |
| Thermal radiation | Radiates from elevated flame; setback required | Contained in chamber walls |
| Installation profile | Elevated stack (25 to 150+ ft) | Ground-level, low profile |
| Noise | Higher combustion roar | Significantly quieter |
| Capacity | Scalable to very high flow rates | Best suited to moderate flow rates (up to ~2 MMscfd typical) |
| DRE (typical) | 98%+ when operating within 60.18 parameters | 98%+ (EPA-certified models) |
| OOOOb compliance | Yes, with continuous monitoring | Yes, qualified control device with temperature monitoring |
| Visible emission permit | More difficult to satisfy at low flow | Easier, no visible flame by design |
| Low-BTU streams | Can destabilize open flame | Refractory chamber maintains combustion stability |
| Best applications | High-volume relief, refinery, gas plant | Tank battery, wellsite, VRU overflow, near populated areas |
When to Choose an Open Flare
Open flares are the right choice when waste gas volumes are large typically above 2 MMscfd where the chamber size required for an enclosed combustor becomes impractical. They are also preferred when the gas stream is high-BTU with good combustion stability across the flow range, the site has adequate setback for the thermal radiation exclusion zone required by API 521 calculations, and the permit does not specifically prohibit visible emissions. Refinery flare headers, midstream gas plant flare systems, and large emergency relief systems almost always use open-tip flares for these reasons.
When to Choose an Enclosed Flare
Enclosed flares are the right choice when: the air permit requires no visible emissions from the controlled source; the site is near a populated area or residential zone where visible flame operation would be operationally or community-relations problematic; the thermal radiation setback for an elevated flare cannot be met within the site’s available footprint; the waste gas stream has a lower or variable BTU content that makes open-tip combustion stability unreliable; or a state environmental agency specifically requires a combustor rather than a flare for the controlled source. In upstream oil and gas production, enclosed combustors serve as primary control devices for tank battery vapor and as VRU overflow combustors at wellsite and centralized production facilities.
Using Both Technologies on the Same Site
Many production and processing facilities use both enclosed and open flare technology in a layered control system. A vapor recovery unit (VRU) handles normal gas volumes, an enclosed combustor handles VRU overflow during peak production or maintenance periods, and an elevated open flare handles emergency relief volumes that exceed the enclosed combustor’s rated capacity. This layered approach maximizes recovery under normal conditions, provides no-visible-emission combustion for routine emissions control, and maintains the high-volume emergency relief capacity that only an open flare can economically provide.
HPS Capability: Open Flares and Enclosed Combustors
Hero Process Solutions manufactures both open flare systems and enclosed flare combustors at its Kellyville, Oklahoma facility. HPS open flare product lines include air-assist flares, sonic flares, gas-assist flares, low-flow combustors, and emergency and utility flares. The HPS enclosed combustor line includes 12 EPA-certified models ranging from small wellsite vapor control units to large-diameter models for midstream and industrial vapor streams.
Because HPS manufactures both technology types in-house, HPS engineers can help operators select the right combination for their specific vapor volume, BTU content, permit requirements, and site constraints rather than recommending what a single-product supplier happens to make. Contact HPS at (918) 941-2166 or through the contact page to discuss technology selection for your application.
