A glycol dehydrator is supposed to send dry gas out the top of its contactor and keep the glycol circulating in a closed loop below. Glycol carryover is what happens when that separation fails and droplets of lean glycol leave with the gas, riding out the top of the tower into the sales line. It is a double loss: the operator pays to replace the glycol that walks out the top, and the glycol that leaves goes somewhere it does not belong, fouling equipment downstream. This guide explains what carryover is, why it costs money and causes downstream trouble, what makes a contactor start losing glycol, and how the differential-pressure and make-up trends in SCADA reveal it before the losses mount.
Glycol Carryover in one line: Glycol carryover is the entrainment of lean triethylene glycol droplets out the top of a dehydrator's contactor along with the treated gas, instead of the glycol staying in its loop. It is both a direct cost, because the lost glycol must be continually replaced, and a downstream contamination problem, because the escaped glycol collects in pipelines, compressors, and other equipment. It is caused by conditions that overwhelm the tower's separation, such as excessive gas velocity, foaming, or a failed mist eliminator, and it shows up in SCADA as rising differential pressure and climbing glycol make-up rate.
Inside a contactor, gas flows up while glycol flows down over trays or packing, and the gas is supposed to leave the top stripped of water and free of liquid. Glycol carryover breaks the second half of that promise: fine droplets of lean glycol get swept up with the gas and carried out the top instead of draining back into the loop. The glycol inventory is a closed circulating charge, so every droplet that leaves is glycol that must be bought and added back, and a contactor losing glycol steadily runs through make-up far faster than a healthy one. That make-up cost is the first and most visible penalty of carryover.
The second penalty is what the escaped glycol does after it leaves. Glycol that rides out with the gas does not vanish; it travels down the sales line and settles wherever the gas slows or cools. It collects in low spots and separators, coats the internals of downstream compressors, and can foul or upset equipment that was never meant to see liquid glycol. What began as a loss at the dehydrator becomes a contamination problem for everything downstream, and tracing a fouled compressor or a glycol-laden pipeline back to a leaking contactor is not always quick.
Because it is both a cost and a downstream hazard, carryover matters more than its modest-looking glycol bill suggests. A little carryover looks like nothing more than slightly high glycol consumption, which is easy to write off as normal, but the same droplets that raise the make-up rate are also the ones fouling equipment further along. Treating rising glycol consumption as a symptom worth investigating, rather than a cost to top up and ignore, is what separates operators who catch carryover early from those who discover it only when something downstream fails.
The most common driver of carryover is simply too much gas velocity through the tower. A contactor is sized for a gas throughput, and above that the upward gas moves fast enough to tear droplets off the glycol and carry them out rather than letting them fall back. This is why carryover often appears when a unit is pushed above its design rate, or when gas that should be split across two contactors is crammed through one. The tower can dry the gas at the higher rate, but it can no longer keep the glycol out of it.
Foaming is the second major cause and often the more insidious one. When contaminants such as hydrocarbons, salts, corrosion products, or degraded glycol build up in the circulating charge, the glycol can foam as the gas bubbles through it, and foam is far easier to entrain than clean liquid. A foaming contactor may carry over glycol at gas rates it handled cleanly before, because the problem is the fluid, not the throughput. Foaming tends to follow poor glycol housekeeping, so carryover that appears without a rate change often points back to a contaminated or degraded glycol charge.
The last line of defense is the mist eliminator, a demister pad or vane pack near the top of the contactor whose job is to coalesce entrained droplets and drain them back before the gas leaves. When it is doing its job, it catches the ordinary fine mist and keeps carryover low even under some stress. When it becomes plugged, damaged, or dislodged, that protection is gone, and glycol that the pad would have caught passes straight out with the gas. A failed mist eliminator can turn a tower that was operating cleanly into one that loses glycol steadily, sometimes with no other change in operation.
The two signals that most reliably expose carryover are the contactor's differential pressure and the glycol make-up rate, and both are natural things for SCADA to trend. Differential pressure across the contactor tends to rise when the tower is running above its comfortable capacity or when foaming is building, because both conditions make it harder for gas to pass, so a climbing differential is an early warning that the conditions which cause carryover are present. Watching that differential against its normal operating band turns an invisible internal condition into a visible trend.
The make-up rate is the more direct evidence, because glycol leaving as carryover is glycol that has to be replaced. When a contactor is losing glycol out the top, the rate at which fresh glycol must be added to keep the system topped up climbs, and a steadily rising or stepped-up make-up rate is one of the clearest fingerprints of carryover. On its own a single top-up tells you little, but the trend over days and weeks distinguishes a healthy unit with slow, minor losses from one that has begun bleeding glycol into the gas. Read together, a rising differential and a rising make-up rate point strongly at carryover as the cause.
A cloud SCADA and monitoring platform such as Merobix is well suited to catching this because the individual signs are slow and easy to miss on a periodic round, but obvious in a continuous trend. Merobix trends contactor differential pressure and logs glycol make-up over time, and it can alarm when the differential drifts above its normal band or when make-up consumption climbs beyond expectation, so a foaming charge, an over-pushed tower, or a failed mist pad shows up as a signal rather than as a fouled compressor discovered weeks later. Across a fleet of dehydrators the same view highlights the outlier unit whose glycol consumption has quietly stepped up, letting an operator investigate the carryover before its cost and its downstream contamination have had time to grow.
The most common causes are excessive gas velocity through the contactor, which tears droplets off the glycol and carries them out, and foaming of the glycol caused by contaminants or degradation, which makes the liquid far easier to entrain. A plugged, damaged, or dislodged mist eliminator removes the tower's last defense against carryover. Often several of these act together, such as a foaming charge combined with a unit being pushed above its design rate.
The glycol that leaves the top of the contactor does not disappear; it travels downstream with the gas and settles in pipelines, separators, and compressors, where it fouls equipment and can cause upsets. So carryover is both a direct cost, because the lost glycol must be replaced, and a contamination problem for everything downstream of the dehydrator. That downstream damage is often more expensive and harder to trace than the glycol make-up bill itself.
The clearest signs are a rising glycol make-up rate, because glycol leaving as carryover must be continually replaced, and a rising differential pressure across the contactor, which points to foaming or an over-pushed tower. SCADA that trends both signals over time exposes carryover as a pattern rather than a one-off, and alarms on climbing make-up or high differential flag it early. Physical evidence such as glycol found in downstream separators confirms it.
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