A vapor recovery unit only earns its keep if it keeps the tanks it serves at the right pressure - low enough to draw their vapors in, but never so low that it pulls a damaging vacuum. The mechanism that manages this is the suction pressure control loop, which continuously adjusts the unit so the pressure at its inlet stays on target no matter how fast the tanks are producing vapor. This guide explains what that loop does, how it modulates recycle or compressor speed to hold pressure, and why tuning and monitoring this single loop is what actually determines whether vapors are captured or lost.
VRU Suction Pressure Control in one line: VRU suction pressure control holds the pressure at a vapor recovery unit's inlet - the tank or vapor header pressure - at a setpoint by modulating the unit's throughput, either through a recycle valve that spills compressed gas back to suction or by varying compressor speed. Keeping suction pressure on target ensures the unit draws off exactly as much vapor as the tanks produce, so it neither pulls a vacuum on the tanks nor short-cycles when vapor is scarce.
The controlled variable in a vapor recovery unit is the suction pressure - the pressure in the vapor header or the tank vapor space that feeds the unit's inlet. The setpoint is chosen to sit just above the tank's vacuum limit and just below its pressure vent limit, so that when the unit holds suction at setpoint, the tanks stay comfortably inside their safe band. In practice this often means holding suction at a slight positive pressure, only a little above atmospheric, because the tanks it protects cannot tolerate being pulled down toward vacuum.
The unit is essentially a compressor, and a compressor removes vapor at whatever rate its capacity allows, which is rarely the exact rate the tanks are producing it. If it removes vapor faster than the tanks generate it, suction pressure falls and the tanks head toward vacuum; if it removes vapor slower, suction pressure rises and the tanks head toward their vent. The control loop's job is to continuously match the unit's effective throughput to the incoming vapor rate so suction pressure stays pinned at setpoint. Everything the control does is in service of that balance.
Because tanks breathe irregularly - a pump-in event floods the header with vapor, a quiet night produces almost none - the incoming vapor rate swings widely and unpredictably. A fixed-capacity compressor cannot simply be turned on and left; it must be modulated. The suction pressure loop is the modulation: it senses the pressure, compares it to setpoint, and continuously commands the unit to take more or less, absorbing the swings so the tanks never feel them. A well-behaved loop makes a variable vapor source look, from the tanks' point of view, like a steady, gentle draw.
There are two common ways to modulate a vapor recovery unit's effective throughput, and many units use both. The first is a recycle, or spillback, valve. The compressor runs at fixed capacity, but a control valve on the discharge routes some of the compressed gas back to the suction. When incoming vapor is scarce and suction pressure starts to fall, the recycle valve opens, feeding compressed gas back around so the compressor is pulling partly from its own discharge instead of dragging the tanks into vacuum. When vapor is plentiful and pressure rises, the recycle valve closes so more of the compressor's capacity goes to genuinely removing tank vapor. The valve position rides the suction pressure.
The second method is variable speed control, where a variable frequency drive changes the compressor's rotational speed and therefore its actual capacity. As suction pressure rises, the controller speeds the compressor up to take more vapor; as pressure falls, it slows the compressor down so it takes less. Speed control is generally more energy efficient than recycle, because the compressor only does the work needed to move the actual vapor rather than continuously compressing gas just to spill it back. Some designs combine the two, using speed control for the broad range and a recycle valve to handle the low end where the compressor cannot slow enough.
Both approaches exist to solve the same underlying problem: a compressor that runs faster than the vapor supply will, without intervention, pull the tanks into vacuum or, if it has a low-suction cutout, trip off. Recycle and speed control are simply two mechanisms for bleeding off surplus capacity gracefully. The choice affects efficiency and equipment wear, but from the tanks' perspective the outcome is identical - suction pressure held steady at setpoint - because in both cases the controlled variable and its target are the same.
Short cycling is the classic failure of a poorly controlled vapor recovery unit and the clearest sign that the suction loop is not doing its job. When vapor is scarce, an untuned unit pulls suction pressure down to its low-pressure cutout, trips off, lets pressure build back up, restarts, and immediately trips again - cycling repeatedly. Every one of those stop periods is a window in which the tanks can drift up to their vent limit and release vapor to atmosphere, and every restart adds wear. A properly tuned loop, whether by recycle or speed, keeps the unit running smoothly through the lean periods so it is always there to catch vapor when it appears. Tuning the loop is therefore directly a matter of how much vapor actually gets recovered.
The consequences of mistuning run in both directions. A loop that is too aggressive can drive suction pressure below the tanks' vacuum limit during a lull, risking tank collapse and drawing air into the vapor system, which is a safety hazard. A loop that is too sluggish lets pressure overshoot during a pump-in surge, venting vapor before the unit catches up. Getting the setpoint, the gain, and the neutral band right - so the loop responds quickly enough without hunting - is what keeps the tanks inside their band and the vapor inside the pipe. This is genuine loop-tuning work, not a set-and-forget installation.
This is where continuous monitoring proves its value, because a vapor recovery unit that has quietly slipped into short cycling or is riding at the wrong suction pressure often goes unnoticed until an emissions survey or a vented tank exposes it. A cloud SCADA and monitoring platform such as Merobix trends the suction pressure against its setpoint, records the recycle valve position or compressor speed, and counts unit starts and stops, so a maintenance team can see at a glance whether the loop is holding steadily or cycling. Alarms on frequent restarts, on suction pressure straying from setpoint, or on the recycle valve pinning open turn a slow, invisible loss of vapor into an immediate signal, and the recorded trends let an engineer retune the loop with evidence rather than guesswork. The unit captures vapor only when its suction loop is healthy, and monitoring is how that health is kept visible.
The setpoint is chosen to sit above the served tanks' vacuum limit and below their pressure vent limit, so holding suction on target keeps the tanks in their safe band. In many installations that means a slight positive pressure only a little above atmospheric, because atmospheric tanks cannot be pulled toward vacuum. The exact value depends on the tank design and the vapor header layout, and it is set so the unit draws vapor without ever dragging the tanks down.
Short cycling happens when incoming vapor is scarce and the suction pressure control cannot bleed off enough compressor capacity, so the unit pulls pressure down to its low cutout, trips, restarts as pressure rebuilds, and trips again. It matters because every off period lets the tanks drift toward their vent limit and release vapor, and the repeated starts wear the equipment. A properly tuned recycle valve or variable speed control keeps the unit running smoothly instead of cycling.
A recycle valve runs the compressor at fixed capacity and spills some compressed gas back to the suction to shed surplus capacity when vapor is scarce. Variable speed control changes the compressor speed itself so it only moves the vapor actually present. Both hold suction pressure at setpoint, but speed control is usually more energy efficient because it avoids continuously compressing gas just to recycle it, and some units combine the two.
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