View source for TEG contactor(PV-1106)



The TEG contactor is an absorption tower where lean TEG dehydrates sweet gas on bubble cap-equipped trays. Dehydrating the gas removes water vapor from the gas so that the water content is below TCPL’s 4 lbs/mmscf water content sales specification.


==Purpose and Internals==

[[File:TEGcontactor.png|center|frame|TEG Contactor]]

Sweet gas enters the TEG contactor below the contactor’s trays to begin its upward path through the trays. At the same time, pumped and cooled lean TEG enters at the top of the contactor to begin its downward, water-absorbing path.

<u>'''Note:'''</u> The pumped and cooled lean TEG arrives at the TEG contactor as described in Module 6 – [[TEG Regeneration]]. The TEG is pumped by [[TEG pumps(PM-1108A1/A2)]] and cooled in the [[TEG cooler(HT-1108)]] before flowing to the TEG contactor.

When the lean TEG arrives at the TEG contactor, the TEG has been cooled so that it is only about 10 to 20°F warmer than the gas entering the contactor. This specific temperature difference ensures that an abnormal TEG temperature does not upset contactor operation:
*overcooled lean TEG could result in the condensation of hydrocarbons in the TEG contactor. Condensed hydrocarbons in the TEG trigger foaming-based carryover from the TEG contactor and reduces dehydration performance.
*excessively warm lean TEG returned to the TEG contactor also decreases dehydration performance because the absorption of water works best at cooler temperatures.


[[File:TEGContTrays.png|center|frame|Weir and Tray Arrangement]]


The TEG contactor has 8 bubble cap trays (see Figure 5). At each bubble cap:
*there is a short riser at the hole in the tray through which the gas flows upwards
*a cap with slots (the bubble cap) is fixed above the riser and deflects the rising gas so that it is bubbled through the TEG liquid flowing crosswise over the tray

During the TEG/gas contact at each tray, the TEG absorbs water from the gas. The TEG becomes richer as it flows downward while the gas becomes drier as it rises through the bubble cap valve trays. Although the TEG primarily absorbs water, the TEG also absorbs some of the methane from gas flowing through the contactor. Dehydrated gas leaves the top of the TEG contactor through a demister pad that coalesces entrained TEG droplets. The gas then flows to the Dry Sweet Gas Scrubber (PV-1108).

Rich TEG descends from the bottom tray into the bottom storage section of the TEG contactor. Rich TEG leaves the contactor through the 2” outlet pipe through the TEG regenerator condensing coil. It than passes through the TEG Contactor (LC-1425) and the ESD block valve (XV-1446).

A weir (figure 5) at each tray maintains the level of TEG on the tray while a downcomer directs the TEG flowing over the weir to the tray below. The downflowing liquid in the downcomer provides a seal to prevent the rising gas from bypassing a tray through the downcomer.

==Level Controls and Instrumentation==

Rich TEG leaves the contactor under level control as follows:
*The TEG passes through the condensing coil in the stripping column of the Amine regenerator (HT-1116).
*the TEG then flows through a level control valve (LC-1425) that is modulated by the TEG contactor’s level controller to maintain a setpoint of rich TEG level at the bottom of the contactor
*the TEG passes through ESD valve (XV-1448) and into the TEG Flash Drum

===Rich TEG level controller (LC-1425)===

The rich TEG contactor is level controlled by level Transmitter (LIT-1425) which monitors the TEG level in the contactor. The signal is used to control the position of the contactor Level control valve (LC-1425) based on the current level measurement, the level control valve (LC-1425) modulates to maintain the desired DCS setpoint.

The control valve directs TEG flow from the TEG Contactor (PV-1106) via the ESD valve (XV-1446) to the TEG regeneration process.

<u>'''Note:'''</u> ''When flowing through the level control valve (LC-1425) , the rich TEG pressure drops so that some of the absorbed methane gas begins to flash from the TEG. The two-phase rich TEG/released gas mixture then continues to tie in with the recovered TEG drained from the Dry Sweet Gas Scrubber.

The pressure reduction across the level control valve (LV-1425) is a prerequisite condition for achieving the low pressure required for downstream TEG regeneration (as described in Module 6- TEG Regeneration.''

The TEG contactor also has a gauge glass (LG-1425) to locally show the rich TEG level in the bottom of the TEG contactor.

===Rich TEG outlet ESD valve (XV-1446)===

The rich TEG outlet ESD valve (XV-1446) is triggered to close automatically in response to a train or plant ESD. Because the upstream level control (LC-1425) valve may pass when closed, closing the ESD valve ensures a positive rich TEG shutoff; the shutoff prevents rich TEG draining from the contactor.

<u>'''Note'''</u> ''Preventing total drainage from the contactor is important because any high pressure gas that escapes the contactor through the contactor TEG outlet piping could overpressure downstream TEG handling equipment and piping.''

The rich TEG outlet ESD valve (XV-1446) is operated by a piston-type pneumatic actuator. The pneumatic actuator has a cylinder (supplied with instrument air) to open the valve and a return spring to close the valve.

The operation of the ESD valve is triggered by emergency conditions which trigger the automatic closure of the ESD valve The FNNPF Control Room Operator can:
*remotely trigger the ESD valve’s closure via onscreen controls
*remotely reset and reopen the ESD valve via onscreen selections
*There is also a mounted ESD panel in the Control Room.

==Pressure Instrumentation==

The TEG contactor is equipped with a differential pressure transmitter (PDI-1421) which relays the gas pressure drop across the contactor (i.e., between the contactor’s wet gas inlet and dehydrated gas outlet) to the DCS workstations for onscreen display. (Note that there is no local readout.) Increasing differential pressure is an indication that the contactor is foaming or flooding. There is also a local pressure gauge (PI-1426) for local read out.

==Alarms and Shutdowns==

At the TEG contactor, abnormal levels trigger the following alarms and shutdowns to appear as onscreen messages at the DCS workstations:
*low Level alarm (LAL-1425) at 30%
*low-low level shutdown (LALL-1425) is triggered if the level falls further to 5%. The shutdown triggers the closure of the rich TEG outlet ESD valve (XV-1446). Closing this valve (when the TEG level is excessively low) prevents high pressure gas from exiting the contactor via the rich TEG outlet piping and overpressuring downstream piping/equipment.
*high level alarm (LAH-1425) at 64%
*high-high level shutdown (LAH-1425) is triggered if the level rises further to 75%. The shutdown actions are as follows:
:-the online TEG pump shuts down
:-T1’s raw gas inlet ESD valve, the T1 sales gas flow control valve, and the T1 sales gas outlet ESD valve all close because without TEG circulation, the T1 train cannot dehydrate sweet gas to sales specifications

At the TEG contactor, a differential pressure switch (PDI-1421) activates an alarm if the differential pressure across the contactor exceeds 4 psi. The alarm appears as an onscreen message at the DCS workstations.

==Additional Specialty Lines and Valves==

The TEG contactor is equipped with the following:
*a 65# steam purge line (normally blinded and blocked in) is used for startup/shutdown steam purging
*manual drain valves on the TEG contactor and its associated level columns tie into the TEG drain which empties into the TEG drain tank (PV-3218) (see Module 6-TEG Regeneration)
*a 2’ top vent valve for use during startup/shutdown steam purging
*A 3’ drain to the TEG drain tank for shut downs and draining purposes.
*A pressure relief device ( PRV-1420) to HP flare SET AT 1300psi in the event of a high pressure process upset
*A local temperature gauge (TI-1423)

==Operations Monitoring==
===<span style="color: red; text-decoration: bold;"><u>⚠ Warning!</u></span>===
''The Area Operator may be exposed to TEG during operational activities such as drawing TEG samples and draining instrumentation columns. When direct exposure to TEG is possible, wear chemical handling PPE and take the chemical handling precautions.

Exposure to TEG involves the following hazards:''

'''Refer to the most up to date MSDS.'''
*''TEG is irritating/damaging to the eyes, respiratory system, and skin''
*''the BTEX carcinogens may be present in TEG. TEG has a high affinity for the BTEX aromatics (benzene, toluene, ethyl benzene, and xylene).''

During monitoring rounds at the TEG contactor, the Area Operator:
*ensures that the rich TEG is draining correctly and checks the level glass to confirm that the rich TEG level is being held at setpoint.
*blows down the gauge glass and level columns as required to ensure they remain clear of debris

Much of the TEG Contactor operating data that indicates effective TEG contactor operation is only relayed to the DCS workstation for onscreen display; therefore the Control Room Operator monitors contactor operation to ensure effective gas dehydration. The Control Room Operator and the Area Operator work together to investigate and correct internal TEG contactor operating problems noted in this sections 5 and 6 of this module.