JessCo solutions is dedicated to educating our customers on all aspects of vapor recovery and the regulations that govern it. We want to ensure that all of our potential clients are making well informed decisions when seeking a vapor recovery solution. We have curated a list of common industry terms as well as information relating to the operation and benefits of vapor recovery and the problems that it solves. Click on a link below to quickly navigate to that section.
Most common VRU packages are comprised of a control system, motor, compressor, cooling pack, skid, inlet scrubber and inter-connecting piping. The actual compressor may be a reciprocating, rotary vane, rotary screw or other type of compressor. The reciprocating Quincy type compressor is the most widely used, but as rotary screw manufacturing technology has gotten better, small oil-flooded rotary screw compressors are becoming more and more prevalent.
The control system is either on-off or variable speed using a VFD (variable frequency drive) based on high and low pressure settings. In an On-Off style unit, when pressure reaches a high set point, the unit kicks on and operates at a set RPM until the pressure reaches the low set point at which time the unit shuts off. In a variable speed style unit, when pressure reaches a high set point, the unit kicks on and operates at a variable speed based on the pressure until the pressure reaches a low set point at which time the unit kicks off. Either style of unit can have a recirculation valve that will provide make-up gas when the pressure reaches a lower point before the low set point shut-off setting. This helps keep the unit running and limits the amount of start-up shut-down cycles. The valve may be a KimRay style low pressure regulator, or an electronic valve operated by the control system settings.
Most typical VRU control systems have an operational and fault timer to log the amount of time the unit is in service. Quad O requires no less than 95% operational time (unit is available to operate or operating). An additional run timer is also common to log the amount of actual run hours to allow PMs and maintenance to be performed on a set schedule.
Common Faults or Kills are as follows:
Other kills may be included, but it's important to limit the amount of down time due to unnecessary or overly cautious kills due to the 95% operational time requirement. Fines and penalties for not maintaining minimum operational time can easily outweigh the cost of repair and replacement of the major components on these small compressors.
The most common problem with VRU packages is liquid issues. These issues are caused by the gases that are being compressed and the narrow window in which these gases will remain in vapor form and not change phase back to liquid. To counter this problem, it is important to understand the "boiling points" of each constituent in the flash gas stream and operate the system at a pressure below and a temperature above this boiling point to ensure the constituents remain in gas form instead of falling out back to liquid. A simple gas analysis will make this information available and allow the operator to set up operating parameters. Your VRU service company should be able to help you in the recovery and analysis of the sample and work with you to adjust the control system to the proper settings.
Vapor Recovery Towers basically have one purpose...to keep oxygen (O2) out of the flash gas stream. Storage tanks have thief hatches and vent stack valves with vacuum breakers which allow the tanks to "breathe" as pressures cycle. This also allows ambient air in occasionally when the conditions are right and pressure falls below ambient pressure. This introduces O2 into the flash gas stream. VRTs prevent this from occurring. They accomplish this by providing a secluded area in the top of the tower to allow gases to flash before they flow into the tanks. The liquid level in the tower produces a barrier to keep contaminated storage tank gas away from the cap gas in the top of the tower.
Typical downstream gas purchasers require O2 levels to remain below 10 ppm (parts per million) due primarily to corrosion concerns. When levels exceed this, the producers can be "closed-in" by the downstream purchaser causing major loss of revenue, and subjecting the producer to fines and penalties due to the emissions.
As discussed above, although rotary screw compressor packages may be more practical for the higher flow requirements of the horizontal well production facilities, the following three operating concerns must be addressed to ensure trouble-free operation and a long life span.
A high grade full synthetic oil is required to ensure dilution does not occur. Unlike reciprocating compressors that have oil lubricated parts typically outside of the gas stream in the cylinders, rotary screw compressors actually use the oil for sealing and cooling purposes by directly injecting the oil into the rotor case. The oil is mixed with incoming gas and all of the diluting constituents. Typical carbon based oils cannot separate once this occurs and the result is a much lower viscosity and lubricity, which rapidly causes excessive wear and compressor failure. High grade synthetic oil when operated at the right temperature will separate fast and efficiently.
As discussed before, maintaining a high enough temperature to stay above the boiling points of the constituents is critical to keep the gas in vapor form. Even if a high grade synthetic is used, if the constituents are liquid they will stay in the lubrication circuit and rapidly overfill the sump causing the good oil to be lost downstream and the remaining oil to be mixed with low lubricity hydrocarbons. Typical rotary screw packages have thermostatic valves to ensure temps stay high, but this alone may not be enough. A variable speed cooling package is the most effective way to maintain temperatures at pre-determined settings above constituent boiling points throughout the oil circuit. The added benefit is a lower power draw and quiet operation due to slower fan speeds on all but the hottest days of the year.
It's very important to keep the unit operating and limit cycles due to several factors. The most important issue related to this is shock load. When rotary screws turn off they still inject oil into the rotor case until the case discharge pressure is bled down. This causes the rotor flutes to be flooded with oil and if a restart occurs before the oil can weep back into the sump, the compressor will be in what is known as a hydro-lock condition. The oil will be forced through the flutes and squeezed past the rotor clearances causing high pressure and excessive force against the main and thrust bearings. This shock will also put a strain on the entire system from the power in and VFD in Amp loads, to the motor windings and drive coupling. Once the oil is cleared the load is reduced, but the damage has already been done.
The next concern related to excessive start-up/shut-down cycles, is oil carry-over issues. When the pressure drops in the sump during a shut down cycle, gas expansion occurs which increases flow velocity. This will cause oil droplets that have coalesced to be carried over into the discharge piping and sent downstream upon restart. The same situation occurs when a restart takes place and the pressure is rising. Over time, with excessive restarts, many gallons of expensive high grade synthetic oil can be lost.
Finally, the last concern related to excessive start/stop cycles is maintaining heat. As previously discussed, the system must be kept above the constituent boiling points at all times to ensure liquids do not drop out into the oil circuit. Multiple start/stop cycles allow the unit to cool and condensation will occur. These liquids are then entrained in the system until the boiling point is reached and there is adequate time for them to disperse back into the gas stream.
All of the above issues can easily be addressed by your VRU service provider. These units simply will not operate long if these conditions are not met. The larger VRUs are much more expensive to repair and replace, but most importantly, the fines and penalties are much larger due to the excessive emissions when these units are down.
RVP (Reid Vapor Pressure) is a common measure of the volatility of a liquid. It is defined as the absolute vapor pressure exerted by a liquid at 37.8 °C (100 °F) RVP levels directly correlate with the flash gas levels in oil. If the flash gas has been emitted and "flashed-off", the RVP will be lower. If the flash gases have not had a chance to flash/boil off, then the RVP will be higher. As discussed before, heat helps the release of flash gases in oil by lowering the boiling point of these lighter constituents and causing them to boil/flash. This is why RVP issues tend to be more prevalent in cooler months since cooler temperatures lower flash rates and keep lighter constituents entrained within the oil/liquids.
Downstream producers typically require RVP levels below 9.0 to allow custody transfer. If high RVP oil is injected into pipeline and processing systems that are normally at a higher temperature, flash will occur and the downstream pump and metering systems could be affected causing gas lock issues.
RVP levels can be managed by several methods, and it usually requires a combination of a few. All of these involve heat as this is the most efficient way to lower levels. Below are some suggestions from the simplest to most costly and difficult.
If a VRT is not employed and the producer is flowing directly to the storage tanks, the flash gas can easily become contaminated with O2 if conditions are not closely monitored. Most O2 introduction occurs during the drawdown/pump-off cycle of water and oil during normal production facility operations. This happens when product is removed from the tanks faster than it can be replaced by flash gas volume. Once the tanks fall below ambient pressure, the vacuum breakers open and allow ambient air laden with O2 into the gas cap. This gas is then later evacuated downstream by the VRU.
Maintaining the hatches and vent stack valves is important, but the best way to prevent O2 outside of installing VRTs or a blanket gas system is to control pump-off rates of product to prevent tank gas cap pressures from falling below ambient. This can be done with a simple control system installed with a VFD (variable frequency drive) on the water transfer pumps, truck loading LACT unit, and standard LACT units. This system is currently in development by JessCo and is patent pending at this time.