Cache the Flash
The USEPA and state environmental regulatory agencies are in the process of requiring more and
more air emission controls for oil and gas (O&G) operations. Regulations such as NSPS OOOO
and Colorado’s Rule 7 affect flashing losses from storage vessels (tanks). Also, the USEPA is
currently analyzing the greenhouse gas (GHG) data reported by O&G for more emission sources to
target for further control. We expect the need to meet more stringent standards for VOC and GHG
(methane) emissions will continue in the near future.
Innovative companies wanting a proactive way to meet their sustainability goals, comply with air
quality standards and permits are implementing end-of-pipe controls and adding process changes
as ways to reduce emissions. End-of-pipe controls typically include vapor recovery systems, flares
and enclosed combustors. Ideas for your company to implement or recommend are given below to
increase production and decrease emissions using some low to no cost process changes.
Flashing Losses
Processes that involve crude oil and condensate undergoing pressure drops result in the
separation of natural gas from the oil fraction. This release of natural gas is commonly referred to
as “Flash Gas". Figure 1 shows that at each stage of separation (i.e., pressure drop), flash gas is
liberated and must be sent back to the system (fuel gas, pipeline) or controlled (vapor recovery,
combustor).
To maximize production and increase profits, operators ensure that natural gas from each stage of
separation is recovered and routed back to the system. Of course, making this happen is
dependent on the operating pressure of the sales/gathering pipeline and compression equipment
available to inject the produced natural gas from each stage of separation into the sales or
gathering line.
Very important to note is that at each stage of separation, the flash gas BTU per standard cubic
foot (BTU/scf) tends to increases as the pressure drops. So flash gas from the storage tank will
have a higher BTU/scf than flash gas from the low pressure (LP) separator. Flash gas from
storage tanks can range from 1500 to 2500+ BTU/scf; natural gas from LP separators would be in
the 1100 to 1300+ BTU/scf range.
If a company is paid on a BTU content of the natural gas then recovering the rich flash gas from
storage tanks will increase profits.
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Figure 1. Flashing Losses
In consideration of this, some process changes that can help maximize the recovery of flash gas
include:
Reduce operating pressure of vessel upstream of the storage tanks
Typical oil and gas production tanks operate near atmospheric pressure. By reducing the
operating pressure of the separator or heater treater just upstream of the storage tanks, the facility
can reduce its flashing losses of methane (CH4) and VOC emissions. Typical LP separators and
heater treaters operate in the range of 100 to 25 psig. Lowering the vessel’s operating pressure by
10 to 50 psig can dramatically lower the flashing losses from the storage tanks.
This would require a reduction in the suction pressure of the booster compressor that receives the
LP separator or heater treater flash gas. Likewise spare compression capacity would be needed to
handle the additional flash gas recovered. The result would be less flash gas liberated at the
storage tank and more collected by the gas boosting system.
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Use vapor recovery towers (VRT) to reduce flash losses
In synch with the above method, operators are using vapor recovery towers (VRTs). A vapor
recovery tower is a tall 2-phase, pressure vessel which is installed between the production
separator(s) and the liquid storage tanks (See Figure 2). The VRT sends it oil to the storage tank
and sends its gas to a vapor recovery system or combustion device.
As stated earlier, typical LP separators and heater treaters can operate in the range of 100 to 25
psig. If this gas from the LP separator or heater treater is sent directly to the storage tank, then
there is a 25 to 100 psig pressure drop for the oil sent to the atmospheric storage tank. This can
produce a substantial amount to flash liberated at the storage tank.
The VRT will be located between the separartor/heater treater and the storage tank. The VRT will
operate at a range of 1 to 10 psig creating a lower pressure drop to the storage tanks. This causes
the bulk of flash gas to be generated in the VRT. The VRT flash gas can then be sent to a vapor
recovery unit (preferred) or to a combustion device and lower pressure oil flashing in the storage
tanks.
Properly operating, the VRT design prevents introduction of air (oxygen) into the storage tank.
Since the VRT reduces the storage tank flash gas, a VRT may lower VOC emissions such that the
storage tank is not required by regulation to have emission controls.
Reference: EPA GasSTAR Presentation - VRT/VRU
Figure 2. Vapor Recovery Tower Used With VRU
VRT
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