Interpretation of Test Results
Pure glycol is water clear. With use, however, the glycol solution can become discolored. Generally this discoloration can be attributed to one or more of the following:
- Thermal decomposition components resulting from excessive reboiler temperatures.
- Corrosion components resulting from poor pH control.
- Oxidation components resulting from entry of air into the system.
- Solids or corrosion inhibitors in the incoming gas.
- The carbon filters are saturated with hydrocarbons and no longer active.
To prevent corrosion damage to the glycol system, the pH of the glycol/water mixture should be maintained between 6.5 and 8.0.
The iron content is a measure of how much corrosion has occurred in the glycol system. An iron content of 5 ppm is usually the maximum value for a non-corroding glycol system. An iron content of 10-15 ppm would indicate some corrosion components are present in the glycol and measures may need to be taken to control corrosion. The iron content should not be allowed to exceed 100 ppm.
Any chlorides in the glycol come from salt produced with the gas. The salt may enter the glycol system either through a malfunction of the bulk separator, inlet gas separator or as a result of these vessels being incapable of handling large slugs of liquids. Chlorides increase corrosion and reduce heat transfer in the reboiler and exchangers. The glycol inventory should be replaced or cleaned up when the chloride concentration based on water content reaches the saturation limit (approximately 360g/L).
- For TEG with 1% water this corresponds to 3.6 g/L NaCl (2.2 g/L chloride).
- For DEG with 25 % water this corresponds to 90 g/L NaCl (55 g/L chloride) in the glycol.
Suspended solids in glycol are usually the result of corrosion caused by a low pH, degradation of the glycol or contamination by the gas stream. The suspended solids content should normally be less than 10 mg/L. If higher than 100 mg/L, the inlet gas separator and the glycol filters should be checked.
Water Content, mass (%)
Generally, the glycol solution should be reconcentrated to a minimum of 99 % glycol to obtain the desired dewpoint depression in a TEG contactor. If the test report indicates a lower degree of reconcentration, a dewpoint measurement should be made to ascertain that contract quality gas is being delivered by the contactor. If the water specification limit is being exceeded, one or more of the following adjustments should be made:
- Raise the reboiler temperature to the maximum allowable.
- Increase the gas stripping rate (if the unit is equipped with gas stripping).
- Increase the UCR until the reboiler cannot maintain the maximum allowable temperature.
- If a glycol-balanced pump is being used, check for leakage from the discharge of the pump back to the suction.
Hydrocarbon Content, vol (%)
The hydrocarbon content should be less than 1% by volume. If higher values are reported, following should be checked:
- Carrying over from the bulk separator or inlet gas separator.
- A faulty air-cooled lean glycol cooler may lead to condensation of hydrocarbons in the top of the contactor.
- For best results, the temperature of the glycol at the inlet to the glycol flash vessel should be between 140°F and 160 °F. The glycol-glycol heat exchangers can become fouled which will prevent the glycol from being heated to this design temperature.
- The glycol solution may be foamy and not separating in the glycol flash vessel. A foaming condition can be detected by determining the length of time required for the foam to dissipate. If the foam seems to be stable it may be necessary to add small amounts of an anti foaming agent to the glycol system..
- The carbon filters are saturated with hydrocarbons.
- Feed gas could contain very heavy hydrocarbons which accumulate in the glycol.
- Glycol/condensate separation is poor due to a low separation temperature or a blocked condensate outlet.