The vapor a tank battery produces is rarely a steady stream. It comes in bursts when tanks fill or trucks load, and it can dwindle to almost nothing on a quiet night, which is a problem for a vapor recovery unit built around a compressor that does not like starting and stopping. Recycle and standby sequencing is how a VRU rides out those lean and intermittent periods without abusing the machine: instead of tripping off and restarting every few minutes, it recycles gas to keep running through the lulls and steps into an orderly idle or standby when there is genuinely nothing to do. This guide explains how recycle and standby logic protect the compressor, how auto start and stop work, and what SCADA sequencing coordinates when a tank battery has more than one unit.
VRU Recycle & Standby Sequencing in one line: VRU recycle and standby sequencing is the control logic a vapor recovery unit uses to handle low or intermittent vapor loads without short-cycling its compressor. Recycle, or spillback, routes compressed gas back to the suction so the machine keeps running smoothly when vapor is scarce, while standby or idle sequencing lets the unit stop in an orderly way and auto-start when vapor returns, rather than tripping and restarting repeatedly. On multi-unit tank batteries, SCADA sequences which unit runs, which recycles, and which stays in standby so the machines share load and starts evenly.
A compressor is happiest running steadily and unhappiest starting and stopping, because each start draws high current, stresses the driver, and disturbs lubrication. The trouble is that tank vapor comes and goes, so a VRU is constantly faced with periods where there is less vapor than the compressor's minimum capacity can move. Left to a crude on-off scheme, the machine would pull the header down, trip on low suction, wait for pressure to rebuild, restart, and trip again within minutes, a pattern called short-cycling that wears the unit out and leaves gaps in coverage during which the tanks can vent.
Recycle is the first answer to this. A recycle or spillback valve on the compressor discharge routes some of the compressed gas back around to the suction, so when vapor is scarce the machine draws partly from its own discharge instead of dragging the header toward vacuum. This lets the compressor keep turning at a steady, safe throughput through a lull without tripping, effectively giving it something to move when the tanks have little to offer. The recycle valve rides the suction pressure, opening as vapor falls and closing as it returns, so the unit floats through the lean periods rather than cycling through them.
Recycle keeps the machine running, but running a compressor purely to recycle gas is not free, since it consumes power and compresses gas just to spill it back. So recycle is the right tool for short and moderate lulls where it is worth staying online to catch the next surge, but for a genuinely long, dead period it makes more sense to shut down cleanly and wait. Deciding between recycling through a lull and stepping into standby is exactly the judgment the sequencing logic is built to make, balancing the cost of running against the cost and wear of starting.
When vapor stays scarce long enough that recycling is no longer worthwhile, the unit steps into standby, an idle state in which the compressor is stopped but the whole system is armed and ready to restart the moment vapor returns. Good standby logic does not just kill the machine; it shuts it down in an orderly sequence, unloading and closing up so the next start is clean, and it keeps watching the header pressure so it knows when to come back. The unit sits quietly, consuming almost nothing, until the tanks begin producing vapor again.
Auto start and auto stop are the transitions in and out of that standby state, and they are driven by the header pressure crossing thresholds with deliberate hysteresis. When the tanks begin breathing and the header pressure rises past a start threshold, the unit auto-starts and begins pulling vapor; when the vapor tails off and the pressure falls past a stop threshold, and recycle can no longer justify staying online, the unit auto-stops into standby. The gap between the start and stop thresholds is what prevents short-cycling: it must be wide enough that a small wiggle in pressure cannot start and stop the machine in quick succession, so the unit only cycles when there is a real, sustained change in vapor supply.
Together, recycle and standby give the unit a graceful response across the whole range of loads. A heavy surge is met by the compressor running flat out, a moderate lull by recycling to stay online, and a long dead period by an orderly stop into standby with an armed auto-start waiting. The logic protects the machine at every step, avoiding both the vacuum risk of overrunning the vapor and the wear of needless starts. What the operator sees is a unit that quietly follows the tanks' rhythm rather than fighting it, running when there is vapor to catch and resting when there is not, without ever hammering itself.
A large tank battery often has more than one VRU, sized so the fleet together can swallow the biggest surges while individual units can drop out when the load is light. Coordinating them is a sequencing problem: at any moment the load might justify one unit, two, or none, and the control has to decide which units run, which recycle, and which stay in standby. Done well, this is where SCADA sequencing adds the most value, because it can look across all the units and the shared header rather than letting each machine react in isolation, which is what would happen with independent local controls fighting over the same header.
The sequencing logic typically ranks the units into a lead, a lag, and standby roles, bringing the lag unit online only when the lead cannot hold the header alone and dropping it back to standby when the lead can cope again. To keep wear even, it rotates those roles over time so no single machine takes all the starts and running hours, and it staggers starts so two compressors do not slam on together and jolt the header. When the load is light it lets one unit carry the battery, perhaps recycling through the lulls, while the others rest, and when a surge hits it brings the reserves up in an orderly order rather than all at once.
A cloud SCADA and monitoring platform such as Merobix is a natural home for this coordination and for keeping it honest. Merobix can trend the header pressure, log each unit's run state, recycle position, and start count, and alarm on the tell-tale signs that the sequencing is not doing its job, such as a unit that is short-cycling, a lead machine pinned in recycle for hours, or run-hours that are drifting far apart between units. Seeing the whole battery in one view lets an operator confirm the fleet is sharing load and starts evenly, spot the unit that is quietly cycling itself to death, and rebalance the sequence with evidence rather than by walking the pad. For an unattended battery, that continuous, coordinated view is what turns a set of individual compressors into a fleet that behaves as one.
Recycle keeps the compressor running through a lull by spilling some compressed gas back to the suction, so the machine has something to move when tank vapor is scarce and does not trip off. Standby is an idle state in which the compressor is stopped but armed to auto-start when vapor returns, used when a lull is long enough that running to recycle is no longer worthwhile. Recycle handles short and moderate lulls; standby handles genuinely dead periods.
Each compressor start draws high current, mechanically stresses the driver, and disturbs lubrication, so frequent stops and starts wear the machine far faster than steady running. Short-cycling also leaves coverage gaps during which the tanks can vent vapor. Recycle and standby sequencing exist to avoid it, keeping the unit running smoothly through lulls and using wide start-stop thresholds so the machine only cycles when there is a real, sustained change in vapor supply.
SCADA typically assigns units into lead, lag, and standby roles, bringing a lag unit online only when the lead cannot hold the vapor header alone and dropping it back when the lead can cope. It rotates the roles to even out run-hours and starts, staggers starts so machines do not slam on together, and can alarm on short-cycling or unbalanced wear. Coordinating the units against the shared header is far better than letting each machine react on its own.
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