Why Is My TEG Unit Producing Off-Spec Gas? A Troubleshooting Guide

Off-Spec Gas Is a System Symptom

A triethylene glycol dehydration unit rarely goes off specification because of one isolated number. Dry-gas water content is the final result of an entire loop: inlet separation, contactor temperature and pressure, gas rate, glycol circulation, lean TEG purity, tray or packing performance, filtration, heat recovery, regeneration, stripping gas, and the accuracy of the measurement declaring the gas off spec.

That is why the common first responses—raise the reboiler temperature or increase glycol circulation—often waste fuel and sometimes make the problem worse. More circulation can overload the contactor, increase foaming and glycol carryover, and exceed the regenerator's capacity. More firing can degrade TEG without materially increasing its purity.

Troubleshooting should move from evidence to mechanism to corrective action. Confirm the failure, establish when it began, determine which part of the loop lost performance, and change one controlled variable at a time. This guide provides that sequence.

For the process-design basis, see TEG Gas Dehydration — Designing the Contactor and Regenerator. For regeneration thermodynamics and duty, see TEG Regenerator Design — Reboiler Duty, Stripping Gas, and Lean Glycol Purity.

First Confirm That the Gas Is Off Specification

Before changing the plant, confirm the measurement. Water content and dewpoint sampling are unusually vulnerable to false readings because a small amount of liquid water, glycol, contamination, or condensation in the sample system can dominate the result.

Check the following:

• Is the measurement water content or water dewpoint, and is it being compared with the correct contractual basis?
• Are pressure and temperature corrections applied consistently?
• Is the analyser calibrated, and when was it last checked against a reference method?
• Is the sample probe in a representative flowing gas stream rather than a stagnant pocket?
• Are the sample line and regulator heat-traced or otherwise kept above the expected dewpoint?
• Is free liquid entering the sample system?
• Has the result been confirmed with an independent sample or portable instrument?
• Was the unit stable long enough for the dry-gas result to reflect the changed operating condition?

Trend the result rather than relying on one reading. A sudden step in indicated water content with no corresponding process change points toward the analyser, sample system, or calibration. A gradual decline over weeks suggests fouling, contamination, exchanger degradation, packing damage, or changing feed conditions. A repeatable daily cycle may track ambient temperature, air-cooler performance, gas rate, or upstream separator behaviour.

Do not dismiss the result simply because the process “looks normal.” Confirm it independently, then treat it as real until the evidence says otherwise.

Build a Timeline Before Touching the Controls

The best diagnostic question is often: what changed immediately before performance deteriorated?

Review operating history for:

• Increased gas rate, water loading, pressure, or inlet temperature
• New wells or a changed gas composition
• Upstream separator high level or liquid carryover
• Start-up following shutdown or maintenance
• TEG make-up from a new batch
• Filter or carbon-bed change
• Pump maintenance or stroke adjustment
• Reboiler, burner, exchanger, or cooler maintenance
• Stripping-gas interruption or source change
• Foaming, high differential pressure, or unexpected level behaviour
• Increased TEG make-up or still-overhead liquids
• An analyser replacement, calibration, or sample-system modification

Plot at least the following on a common time base: dry-gas water result, wet-gas rate and temperature, contactor pressure, lean and rich TEG temperatures, circulation rate, reboiler temperature and firing, stripping-gas flow, contactor differential pressure, levels, and relevant valve positions.

Correlations are more useful than snapshots. If dry-gas water increases whenever lean TEG temperature rises, the contactor equilibrium may be controlling. If it tracks gas rate while lean purity remains stable, contactor capacity or stage efficiency is more likely. If it follows reboiler firing at constant circulation, regeneration or heat recovery deserves attention.

The Four Numbers That Divide the Problem

Four measurements quickly separate contactor problems from regeneration problems:

1. Wet-gas water loading
2. Dry-gas water content or dewpoint
3. Lean TEG water concentration
4. Rich TEG water concentration

Taken together, these establish whether the solvent is arriving dry enough and whether it is absorbing the expected water.

Glycol concentration measurements must be reliable. Hydrometers and refractometers respond to dissolved salts, hydrocarbons, degradation products, and suspended solids as well as water. Field readings should be corrected for temperature and periodically checked against laboratory water analysis.

The difference between lean and rich concentration also requires accurate circulation measurement. A small apparent concentration change can still represent substantial water removal at a high glycol rate. Close the water balance before concluding that the contactor is doing no work.

Check the Feed and Inlet Separation

The dehydration unit may be responding correctly to a feed that has moved outside its design basis.

Gas temperature and water loading

Warmer saturated gas carries more water. A rise in contactor inlet temperature therefore creates two penalties: more water enters with the gas, and absorption equilibrium becomes less favourable. A cooler failure, seasonal ambient change, compressor operating change, or hot bypass can move a previously stable unit off specification without any fault in the glycol loop.

Confirm inlet gas temperature at the contactor—not at a distant upstream location—and calculate the saturated water content at actual pressure and composition. Compare this with the design and recent operating history.

Free-liquid carryover

The contactor is designed to remove water vapour, not slugs of produced water or hydrocarbon liquid. Carryover from the inlet separator can:

• Dilute TEG directly
• Introduce salts and solids
• Cause foaming and emulsions
• Contaminate filters and packing
• Increase hydrocarbon loading on the flash tank and still
• Produce unstable levels and glycol losses

Check separator level trends, dump-valve operation, demister differential pressure, high-level events, drain samples, and evidence of liquid in the contactor inlet. A healthy glycol unit cannot compensate indefinitely for failed inlet separation.

Gas composition and new production

New wells can change gas molecular weight, water loading, condensate content, acid-gas composition, and foaming tendency. Confirm whether the fluid model and operating target still represent the current feed.

Verify Glycol Circulation—Do Not Assume It

Pump stroke, speed, or indicated flow is not proof of delivered glycol circulation. Positive-displacement pumps can lose capacity through worn check valves, gas locking, internal leakage, low suction level, blocked strainers, or incorrect stroke settings. Gas-assisted pumps add further dependence on motive-gas pressure and valve condition.

Verify circulation using the best available independent method:

• Calibrated flowmeter
• Timed level change in a vessel of known geometry
• Pump displacement corrected for measured speed and volumetric efficiency
• Rich/lean water balance under stable operation

Then compare actual circulation with water removal:

Specific circulation = glycol flow / water removed

If circulation is below target, identify the hydraulic or mechanical cause. If it is above target, do not assume that more is better. Excessive circulation can:

• Increase contactor liquid loading and entrainment
• Overload the regenerator and lean-rich exchanger
• Increase reboiler fuel use
• Absorb and release more hydrocarbons and BTEX
• Raise TEG losses
• Reduce achieved lean purity when regeneration duty is fixed

A useful controlled test is to adjust circulation in small steps while holding gas conditions stable, allowing the loop to reach equilibrium after each change. Stop if contactor differential pressure, carryover, reboiler capacity, or dry-gas performance deteriorates. All changes must remain within the approved operating envelope and Management of Change requirements.

Test Lean TEG Quality, Not Just Concentration

TEG can measure “concentrated” and still perform poorly because its physical condition has degraded.

A representative laboratory programme may include:

• Water content
• pH or reserve alkalinity, using the operator's accepted method
• Hydrocarbon contamination
• Suspended solids and iron
• Chlorides and other dissolved salts
• Thermal and oxidative degradation products
• Foaming tendency and foam stability
• Viscosity, density, and colour trend

Common contamination routes include inlet liquid carryover, oxygen ingress through tanks or maintenance, corrosion products, degraded glycol from hot spots, compressor oil, chemical incompatibility, and dirty make-up TEG.

Foaming

Foaming reduces effective contact between gas and glycol, increases column differential pressure, causes glycol carryover, and destabilises levels. Hydrocarbons, corrosion inhibitor, solids, salts, and degradation products are common contributors.

Antifoam can suppress symptoms temporarily, but overdosing may create new separation or filtration problems. Find and remove the contaminant, restore filtration and carbon treatment where designed, and confirm inlet separation.

Filtration

Particle filters remove solids; activated carbon removes many dissolved hydrocarbons and degradation products. Neither is effective when bypassed, exhausted, incorrectly installed, or operated outside its design flow. Review filter differential pressure, change history, bypass position, cartridge condition, and carbon-bed service time.

Diagnose the Contactor

If lean TEG purity and circulation are on target, attention moves to vapour–liquid contact.

Temperature approach

Lean TEG normally enters close to the contactor gas temperature. Glycol that is too hot weakens absorption; glycol that is too cold may condense hydrocarbons from the gas, promoting foaming and contamination. Check the glycol/gas exchanger or final cooler and confirm actual top-of-column temperatures.

Hydraulic capacity

Increasing gas or glycol rate raises column loading. Warning signs include:

• Rising or unstable differential pressure
• Glycol carryover in the dry-gas outlet
• Sudden deterioration above a repeatable gas rate
• Unstable top or bottom levels
• Performance that improves immediately when throughput is reduced

Compare current gas density and liquid load with the original tray or packing rating. Capacity should be checked at actual pressure, temperature, and composition.

Damaged or ineffective internals

Tray damage, blocked downcomers, poor liquid distribution, collapsed packing, fouled distributors, channeling, corrosion, and an incorrect liquid level can remove effective stages. External data may show only reduced performance and abnormal differential pressure; confirmation may require shutdown inspection.

Bypass and leakage paths

Internal bypassing can occur through damaged trays, poorly sealed packing supports, or mechanical defects. External process bypasses, leaking valves, and incorrect line-up can mix wet gas with the dry outlet. Verify the physical line-up rather than relying only on control-system indication.

Diagnose the Regenerator

When lean TEG is too wet, the regeneration package is the immediate focus—but the root cause may still be excessive water load or circulation upstream.

Reboiler temperature and heat input

Confirm temperature with an independent calibrated measurement and inspect the trend, controller output, burner cycling, fuel pressure, and heat-medium conditions. A normal indicated temperature with declining purity can result from sensor error, local hot spots, high pressure, or poor still performance.

Do not raise the set point above the approved limit to chase purity. TEG systems commonly operate with bulk reboiler temperatures around 198–204°C; overheating accelerates degradation and can create an apparently stronger glycol density reading while damaging the inventory.

Heat recovery

If rich TEG enters the reboiler colder than expected, the available burner may no longer have enough duty. Check lean-rich exchanger inlet and outlet temperatures, bypass valves, fouling, flow direction, and circulation. The detailed regenerator design guide shows why sensible heating of circulating glycol can rival the water-vaporisation load.

Still pressure and vent restriction

Regeneration performance depends on low pressure. A fouled condenser, liquid-filled vent, frozen line, closed valve, undersized vapour-control connection, or excessive downstream backpressure can reduce water removal and tempt operators to increase temperature.

Check still pressure locally, inspect the complete overhead path, and verify that any condenser, separator, vapour recovery, burner routing, or flare interface operates within the package pressure basis. Never open or bypass a suspected restricted vent without an approved safe-isolation procedure.

Still packing, reflux, and liquid distribution

Damaged or fouled packing reduces separation. Too little reflux increases glycol loss; too much reflux adds water and duty back to the column. Review the temperature profile, overhead condensate rate, TEG content in condensate, packing differential pressure where available, and maintenance history.

Stripping gas

Confirm actual flow, not valve position. Check gas dryness, source pressure, control-valve operation, meter range, distributor condition, and whether still backpressure has changed. Wet stripping gas or poor distribution can provide far less benefit than expected.

Stripping gas passes into the still overhead. Increasing it may raise methane and VOC emissions and overload the condenser or vapour-control system. Any adjustment must remain within the emissions and disposal design basis.

Use a Symptom-to-Cause Matrix

The matrix narrows the search, but it does not replace a water balance and operating history. Several causes can exist at once—for example, inlet hydrocarbon carryover can create glycol foaming, foul the exchanger, increase still emissions, and lower contactor efficiency.

Plan a Controlled Performance Test

When routine data cannot distinguish the causes, run a performance test with a written plan. Define:

• Stable starting conditions and minimum stabilisation time
• Instruments and laboratory samples required
• Test steps and the single variable changed at each step
• Expected response and success criterion
• Maximum gas rate, differential pressure, temperature, level, emissions, and product-water limits
• Stop criteria and authority to terminate the test
• Method for returning to the original safe condition

A practical sequence may be:

1. Establish a reconciled base case.
2. Confirm wet- and dry-gas water content independently.
3. Verify lean and rich TEG water content and actual circulation.
4. Correct obvious instrumentation or line-up defects.
5. Test circulation in controlled increments.
6. Test contactor temperature or gas rate within approved limits.
7. Confirm reboiler duty, stripping flow, and still pressure.
8. Repeat samples after the complete glycol inventory has circulated long enough to reach a new equilibrium.

TEG loops respond slowly. Taking a dry-gas sample five minutes after changing circulation may capture a transient rather than the new steady condition. Estimate total inventory and circulation time, then allow several turnovers where practical.

Worked Case: More Circulation Made the Gas Wetter

A facility treating 32 MMSCFD begins exceeding its dry-gas water specification during afternoon operation. Operators increase the TEG pump stroke from 4 to 5.5 US gal/lb water removed. The gas becomes slightly wetter, reboiler firing reaches maximum, and glycol make-up increases.

The first data review finds:

• Wet-gas temperature rises from 35°C to 44°C during the afternoon
• Lean TEG concentration falls from 99.5 to 99.0 wt%
• Reboiler temperature remains near its set point, but burner output is saturated
• Rich-glycol temperature after the lean-rich exchanger is 18°C below its historical value
• Contactor differential pressure rises after the pump adjustment
• Dry-gas outlet filters contain glycol

The constraint chain is clear. Higher inlet temperature increases the water load and worsens absorption equilibrium. Lost exchanger performance increases the sensible reboiler duty. Higher circulation then overloads both regeneration and contactor hydraulics: lean purity falls, glycol entrainment rises, and the original problem becomes worse.

The immediate controlled response is to return circulation to the verified optimum and operate within the seasonal gas-rate limit. Inspection finds a partially open exchanger bypass and fouled final glycol cooler. Restoring heat recovery and lean-glycol cooling returns lean TEG to 99.5 wt% and the dry gas to specification without increasing reboiler temperature.

The longer-term study checks summer cooling capacity and establishes an ambient-temperature operating envelope. No larger reboiler or contactor is required.

Corrective Actions by Time Horizon

Immediate, controlled actions may include correcting analyser or sample-system faults, restoring the approved line-up, repairing a failed pump or controller, reducing rate to the demonstrated envelope, and restoring specified circulation or stripping flow.

Short-term maintenance may include filter replacement, carbon-bed service, exchanger or cooler cleaning, burner maintenance, instrument calibration, inlet-separator repair, glycol reclamation or replacement, and inspection of accessible distributors or mist eliminators.

Engineering modifications may include additional inlet separation, contactor internals replacement, more heat-transfer area, revised filtration, a larger or alternative regenerator, improved stripping contact, overhead recovery, or automated performance monitoring.

Every action should identify the mechanism it corrects and the evidence that will confirm success. “Add antifoam” and “increase reboiler temperature” are not root-cause solutions.

Data Worth Monitoring Continuously

A small performance dashboard can identify degradation before product gas fails specification:

• Wet- and dry-gas water content or validated dewpoint
• Gas flow, pressure, and inlet temperature
• Actual TEG circulation and specific circulation per unit water removed
• Lean and rich TEG water content from routine samples
• Contactor differential pressure and outlet glycol carryover
• Reboiler temperature, heat input, and fuel consumption
• Rich and lean temperatures around the heat exchanger
• Stripping-gas flow and still pressure
• Filter differential pressure
• TEG make-up, flash gas, and overhead condensate

Normalise energy and glycol consumption against water removed. A rising fuel per kilogram of water removed or TEG make-up per MMSCF treated often reveals declining performance earlier than the final dewpoint result.

Common Troubleshooting Mistakes

• Changing the plant before confirming the sample. Bad data produces convincing but irrelevant troubleshooting.
• Increasing both temperature and circulation together. The response cannot identify which mechanism mattered.
• Using pump stroke as glycol flow. Mechanical condition and volumetric efficiency can make the indicated rate wrong.
• Measuring glycol strength without checking contamination. Density and refractive index are not selective water measurements.
• Ignoring inlet liquid carryover. A contactor cannot act as the inlet separator.
• Treating reboiler temperature as lean purity. Pressure, duty, circulation, stripping, and still condition also govern regeneration.
• Using antifoam as routine treatment. It can hide contamination while the underlying problem worsens.
• Removing operating margin. Moving alarms, trips, or temperature limits does not create process capacity.
• Sampling too soon after a change. The full glycol inventory needs time to reach its new condition.
• Fixing the first bottleneck without retesting the system. Restored regeneration may expose contactor, cooling, or inlet-separation limits.

Conclusion

An off-spec TEG unit should be diagnosed as a complete water-removal system. Confirm the dry-gas measurement, reconstruct the timeline, close the wet gas–dry gas–lean TEG–rich TEG water balance, and then separate regeneration, circulation, contactor, feed, and sampling causes.

The most effective troubleshooting is disciplined and conservative. It uses independent measurements, controlled tests, adequate stabilisation time, and explicit stop criteria. It does not compensate for uncertainty by overheating glycol, flooding the contactor with circulation, or moving protective set points.

Once the mechanism is proven, the corrective action is usually clearer—and often smaller than expected. Restoring a cooler, correcting a bypass, repairing inlet separation, recalibrating circulation, or removing contamination can recover specification without replacing the contactor or reboiler. The objective is not merely to make one dewpoint reading pass; it is to restore a stable operating envelope the facility can sustain.