GAS DEHYDRATION UNIT

CONTENTS                                                                                                                 PAGE

1.0       INTRODUCTION                                                                      -           01 -10

General                                             

Design Capacity

Sweet Gas Feed Specifications                                                     

Product Specifications                                                                    

Utility Specifications                                                                        

Effluents

Process Flow Diagram

2.0       PROCESS DESIGN                                                                 -           11 - 17            

Process Description

Chemicals

3.0       PLANT SHUTDOWN PROCEDURES                                       -           18 - 27

                General

Plant Shutdown (Short Duration)

Plant Shutdown (Long Duration)

            Emergency Shutdown

4.0       ALARMS AND TRIP SETTINGS                                               -           28 - 29

5.0       OPERATIONAL TIPS                                                               -           30 - 33

6.0       SPECIAL FEATURES, MODIFICATIONS AND SCHEMATICS          34 - 39

1.0     INTRODUCTION

1.1           GENERAL

1.2           DESIGN BASIS

1.2.1   DESIGN CAPACITY

1.2.2   SWEET GAS FEED TO GAS DEHYDRATION PLANT

1.2.3   PRODUCT SPECIFICATIONS

1.2.4   UTILITY SPECIFICATIONS

1.2.5   EFFLUENTS

1.2.6   PROCESS FLOW DIAGRAM

1.0  Introduction

1.1    General                           

This design and operating manual covers the design and operating instructions for a Gas Dehydration Plant. Each Gas Dehydration trains have the capacity to treat sweetened gas as mentioned in the following table.

Total capacity of GDU trains is 41.0 MMSCMD with all 07 trains operating.

The basic philosophy and operating methodology of operation of all the 8 GSU trains is similar. However the operating parameters of individual trains may vary depending upon the process requirement and healthiness. The operating procedures and methodology have been extracted from Phase III-A design package.

1.2             DESIGN BASIS

1.2.1            DESIGN CAPACITY:

The design capacity for each Gas Dehydration Train of 46 & 47 is 6.3 MMSCMD of sweet gas. Other GDU trains from 41 to 45 are having capacity of 5.697 MMSCMD each.

The plant has been designed such that the minimum capacity for each Gas Dehydration Train is 40% of the design capacity.

1.2.2   SWEET GAS FEED TO GAS DEHYDRATION PLANT:

Pressure at battery limits            :  51.9 - 74.9 kg/cm² Abs.

Temperature at battery limits       :  38 ⁰C

1.2.3  PRODUCT SPECIFICATION

          The product from Gas Dehydration Train will meet the following specification:

          Sweet and Dry Gas :

          High Pressure Case :

H₂S content                                        4 ppm vol. max.

H₂O content                               85 kg/mmNm3 max, (-7 ⁰C dew point)

Pressure at battery limits                 74 kg/cm²

Temperature at battery limits           40 ⁰C

Low Pressure Case

H₂S content                                        4 ppm vol. max.

H₂O content                               50 kg/mm Nm3 max, (-11 ⁰C dew point)

Pressure at battery limits                 51 kg/cm²  a

Temperature at battery limits           40 ⁰C

1.2.4        UTILITY SPECIFICATION

1.2.4.1  Steam:

1.2.4.2                       Instrument Air:

1.2.4.3                       Plant Air:

1.2.4.4                       Cooling Water:

Supply pressure at grade kg/cm²a                           5

Return pressure at grade Kg/cm²a                           2.5

Supply temperature ^oC for exchanger design  33

Max. return temp. at any exchanger outlet                 44

Turbidity in ppm                                 

Alkalinity, ppm                                                        15-20

Total hardness, ppm                                               300

Silica as SiO₂, ppm                                       125

Chlorides as Cl, ppm                                    

TDS, ppm                                                              850

PH                                                                        6.2-6.6

Conductivity at 20 ^oC, µmho/cm

1.2.4.5                       Service Water:

Supply pressure at grade kg/cm²a                      7.0

Return pressure at grade kg/cm²a                       -

Supply temperature ^oC for exchanger design       - Ambient

Max. return temp. at any exchanger outlet  -

Turbidity in ppm                                       5

Alkalinity, ppm                                                   15-20

Sodium as CaCO₃ ppm                                      127

Bicarbonate as CaCO₃, ppm                     174

Calcium                                                            67

Magnesium                                                       47

Silica as SiO₄, ppm                                            30

Chlorides as Cl, ppm                                          33

Sulphate as SO₄, ppm                              33

TDS, ppm                                                         66

PH                                                          7-8

Fe, ppm                                                            0.3

Organic matter normally                                     Nil

Conductivity at 20 ^oC, uohm/cm      

Mech. Design pressure, Kg/cm²                         11

Mech. Design Temp., ^o C                                   65

1.2.4.6  Inert Gas:

1.2.4.7  Fuel Gas:

1.2.4.8  Boiler feed Water:

Supply pressure at grade kg/cm2a                         2.0

Temp. ^oC                                                                 Ambient

Note: Quality required for steam generation is acceptable.

1.2.4.9  Methanol:

1.2.4.10          Electrical Power:

Introduction :

1.2.5      EFFLUENTS

•     Released liquid hydrocarbons will be sent to closed blow down system.

•     Released gaseous hydrocarbons will be sent to the central flare system.

Back Pressure of the Flare at battery limits end will be;

          Maximum   = 1.8 Kg/cm²

          Minimum    = 0.6 Kg/cm²

•     Liquid waste effluents will be sent to a central wastewater treatment plant.

1.2.6  PROCESS FLOW DIAGRAM:  (See Annexure­-I)

OPERATING CONDITIONS-G.D.U.

2.0            PROCESS DESIGN

2.1             PROCESS DESCRIPTION

2.1.1      GAS DEHYDRATION UNIT

2.1.2      DESCRIPTION OF A DEHYDRATION TRAIN

2.1.3      ABSORPTION   SECTION

2.1.4      TEG FILTRATION PACKAGE

2.1.5      REGENERATION SECTION

2.1.6      GLYCOL STORAGE TANK

2.1.7      GLYCOL SUMP DRUM

2.2             CHEMICALS

2.2.1      CHEMICAL CONSUMPTION

2.2.2      FIRST LOADS

2.2.3      CHEMICAL PROPERTIES

2.0     Process Design

2.1     PROCESS DESCRIPTION

Sweetened gas is dried by washing with TEG.

The MDEA Gas Sweetening Trains and the TEG Gas Dehydration Trains are connected with one another by a pipe rack- supplying products (raw gas, de-sulfurized and dried gas) and required utilities.

2.1.1  GAS DEHYDRATION UNIT

The removal of water from sweet gas is made by contacting the gas with a  tri-ethylene glycol solution.

Due to their hygroscopicity, glycols are widely used for this purpose.

The hygroscopicity is directly related to the solution concentration. So water vapor will be absorbed by a glycol solution as long as the partial pressure of the water in the vapor phase exceeds the water vapor pressure of the solution.

Furthermore, the molecular compatibility of the couple solvent-solute plays an important role. So the greater the molecular attraction between solvent and solute, the lower the vapor pressure of the water. In liquid state, water is highly associated through hydrogen bonds.

Among all the glycols, tri-ethylene glycol represents the optimum choice between hygroscopicity, price, losses and regeneration ability; in this case, the required specification is 85kg of water/million Nm³ of gas.

2.1.2  DESCRIPTION OF DEHYDRATION TRAIN

Treated gas leaving the gas-sweetening units enters the dehydration unit. It pressure ranges from 74.9 kg/cm2 a to 51.9 kg/cm2 a, and its temperature is 38⁰C. The dehydration is supposed to be run with same liquid flow while the pressure is varying.

So the gas is dried to 85 kg/million Nm³ (-7 ⁰C dew point) in the higher-pressure case and to 50 kg/million Nm³ (-11 ⁰C dew point) in the lower pressure case.

2.1.3  ABSORPTION SECTION

Sweet gas at the TEG unit battery limit enters to the Feed Gas KO. Drum V-404. In V-404 entrained or condensed liquids are removed. To avoid or minimize condensation of liquids due to ambient cooling, lines going from the gas sweetening trains to the gas dehydration trains are insulated.

Liquid collecting in the base of V-404 are sent to the rich amine flash drum (V-303) under level control by LV-1101.

The gas leaving the top of V-404 then flows to the absorption column C-401 where it is contacted with the lean tri-ethylene glycol solution (TEG) (99.7% wt). The column C-401 is fitted with 9 bubble cap trays, 8 of which are absorption trays and a top dry tray (Tray No. I).The purpose of this dry tray is to retain the major part of carry-over to reduce the glycol losses.1

The feed gas enters the lower part of the column below the bottom tray (Tray No.9) and is scrubbed by the lean glycol as it passed up the column counter-current to the glycol, which enters above Tray No.2.

The scrubbed gas leaving the top of C-401 passed to the dried gas scrubber (V-401) where entrained glycol carry-over is removed. The gas leaving the top of 401 passed to the glycol unit B.L. and thence to the hydrocarbon dew point depression units.

Rich glycol collected in the bottom of the Absorber (C-401) is sent under level control (LV-1106) to the Rich Glycol Degassing Drum (V-402). Before entering V-402 this stream is combined with the glycol from the scrubber V­401. The level in V-401 is controlled by LV-1110.

Due to the lower operating pressure (IOkg/cm2a) of the Degassing Drum (V-402) absorbed hydrocarbons are released from the glycol. The released light hydrocarbons are sent to the fuel gas header. If insufficient gas is available from V -402 for use as stripping gas then this will be made up directly from the sweet, dry gas stream from V-401.

The Degassing Drum V-402 is fitted with 2 skimming lines for the removal of any condensed hydrocarbons, which accumulate on the surface of the glycol. These condensed hydrocarbons must be drained manually to the flare header.

The rich degassed glycol from the base of V-402 goes to the filter package.

Process Design

2.1.4  TEG FILTRATION PACKAGE

Tri-ethylene glycol will not exhibit a high degree of foaming if it is kept free of surfactant-­type materials. These materials may be introduced through compressor oil, plug-cock lubricant, and corrosion inhibitors used in either the formation or in the gas gathering system. So such products must be chosen carefully.

Special attention has been given in the design to foaming and fouling by use of:

•       TEG degassing and hydrocarbon condensate removal in V-402.

•       Cartridge filter X-40INB on the full rich glycol stream with a standby unit.      

•       Charcoal filter X-402 on 30% of the rich glycol stream.

Each of the cartridge filters is designed to take 100% of the glycol flow, with one filter in service and the second on standby. The cartridge filter removes any solid particles from the glycol stream. An activated carbon filter is located downstream of the cartridge filter and is designed to take up to 33 percent of the glycol flow with the major stream by­passing the charcoal filter under control of FV -1208.

2.1.5   REGENERATION SECTION

Before entering the regenerator column (C-402) to be regenerated the glycol is preheated in a heating coil at the top of the regenerator. The flow of glycol to the heating coil is controlled by a 3-way valve TV-1215 which controls the top temperature of C-402 from 97.8.C to 98.4"C. Temperature controller TV-1215 opens to allow cold rich glycol to flow to the heating coil. As the glycol flows through the coil it cools and partially condenses the hot vapors rising up the column C-402 there by reducing the overheads temperature and providing and internal reflux for the column. The glycol, which is not required to maintain C-402 top temperature, flows through the by-pass port of TV-1215 and rejoins the preheated glycol stream from the heating coil.

The rich glycol stream then flows to the rich/lean glycol plate type exchanger (E­401 A/B), where it is heated from 52 ^oC to 175 ^oC by exchange with the regenerated lean glycol" before entering the glycol regenerator column C-402.

The regenerator column C-402 is an atmospheric column, which contains 4 bubble cap type trays and the previously mentioned heating coil.

The temperature in the regenerator reboiler in (E-402) is controlled at 204 ^oC by TV-1212, which controls the flow of H.P. steam. Glycol from the reboiler E-­402 overflows to the stripper (C-403), which is end-mounted on to the reboiler. Then it is stripped by hot dry fuel gas to achieve a concentration of 99.7% wt. The fuel gas from V-401/402 is preheated in a second coil of the reboiler before it enters the stripper C­403.

The hot, stripped glycol from the base of C-403 flows by gravity through the rich/lean glycol plate type exchanger E-401 A/B, where it is cooled from 204^oC to 80 ^oC by heat exchange with the cold rich glycol feed to C-402, before going to the surge drum V-403. The gases from the top of the stripper C-403 are piped to the reboiler E-402 and the surge drum V-403 to maintain a slight positive pressure in these vessels.

The lean glycol collected in the surge drum V-403 at 80 ⁰C is pumped by the lean glycol injection pumps P-401 A/B to the trim cooler E-403, where it is cooled to 45 ⁰C by exchange with cooling water, it then returns to the absorber C-­401.

2.1.6   GLYCOL STORAGE TANK

A lean glycol storage tank is provided, which will hold the entire glycol inventory of the units in the event the unit has to be drained during a shutdown. Fresh TEG from the battery limit can also be sent to the tank.

The transfer pump P-402 is used to transfer glycol make-up to the circulation system either to the surge drum V-403 or to the rich glycol stream upstream of the filters. The pump will also transfer spent glycol from the tank to off-sites.

2.1.7   GLYCOL SUMP DRUM

The TEG section is provided with a sump drum to collect the drips and drains from the unit. All low point drains from columns, vessels, pumps etc. are piped into the sump drum (V -405) where the glycol is collected. If the TEG unit or a part of it is shut down then the glycol is first drained to the sump drum and then pumped by Sump Pump P-403 either to the storage tank or to the suction of P-402 for transfer to the regeneration section.

The sump drum is fitted with a pump P-403 that starts automatically when a high level is reached and stops automatically on low level.

2.2     CHEMICALS

2.2.1  Chemical Consumption : Expected chemical consumption is given for the gas dehydration units for a maximum design capacity of 6.3 million Nm³/day of sweet gas from the Gas  Sweetening Trains and for 365 days/year.

          Pure solvents

          TEG (Tri-ethylene glycol)  : 27.5 tons/ year x2

          Charcoal

Recommended quality                : Type AC 40 ( manufactured by CECA,

                                                  France)

          Typical consumption                  : 1.5 ton/year x2

2.2.2  FIRST LOADS

          TEG                                : 80 tons

          Charcoal                         : 1.0 tons

2.2.3  CHEMICAL PROPERTIES

          These physical properties correspond to typical commercial products:

          a) Tri-ethylene Glycol

              Chemical formula         : HO (C2H4O)₃H

          Commercial product characteristics

          -- Molecular weight                    150.17

          -- Color (Pt. Co Hazen )                       25 max

          -- Specific gravity at 20 ⁰C                        1.124 to 1.126

            -- Distillation 760 mm Hg                             IBP 278 ⁰C min.

                                                                                    DP 300 ⁰C max.

            -- Water content                                             0.10%weight max.

            -- Ash content                                                0.01% weight max.

            -- Flash point                                      165.6 ⁰C

            -- Freezing point                                            -5.5 ⁰C

            -- Refractive index                                         1.4561

            Packing and delivery

            Bulk or barrels        

b)       Charcoal AC 40

          ACTI CARBON 40 is manufactured by CECA (France)

          Chemical nature                        : Charcoal

Characteristics

          --  Pellet diameter                      :         1.8 mm

          --  Bulk density                          :         450 kg/m3

          --  Specific heat                        :         0.25 kcal/kg ⁰C

            --  Pore volume                                  :           0.85 cm3/g

            --  Specific area                                :           1150 m2/g

            --  Ash content                                   :           10% max.

            Packing and delivery

            25 kg plastic bags

3.0            PLANT SHUTDOWN PROCEDURES

3.1             GENERAL

3.2             PLANNED SHUTDOWN (SHORT DURATION)

3.3             PLANNED SHUTDOWN (LONG DURATION)

3.4             EMERGENCY SHUTDOWN

3.4.1      GENERAL

3.4.2      TYPE OF EMERGENCY SHUTDOWN

3.4.3      ACTION DURING EMERGENCY SHUTDOWN

3.1     GENERAL

A normal shutdown is a planned non-emergency shutdown such as, annual turnaround.

The gas sweetening and dehydration units are in connection with other upstream or downstream units; the process supervisors of these units should be advised before any scheduled shutdown.

During a shutdown, all equipment automatic isolation valves should be closed to minimize the release of hydrocarbons in the event of a leak. All precautions to be taken to prevent vacuum formation in vessel/piping during draining operation.

Two types of normal shutdown (short duration and long duration) and purging procedures are presented in this chapter. The major difference between the two types of normal shutdown is that, for the long duration shutdown, it is recommended that all the equipments be completely drained and purged.

All safety precautions must be taken during the shutdown period. Protective clothing should be worn by all personnel entering vessels, and water hoses should be provided to damp out any possible areas of combustion.

Note: Prior to any vessel being entered for any reason, please refer to Section 7.0 - "Safety Practices and Procedures".

3.2     Plant Shutdown (Short Duration)

          On a normal shutdown, the feed to the unit should be slowly reduced in a stepwise manner to minimize disturbances to the utility system and downstream process units. For shutdowns of a very short nature, i.e. less than a shift, consideration should be given to locking in the unit and maintaining the glycol circulation.

1.0             Slowly reduce the flow rate of feed gas to the unit using FV -1101 (in maximum steps of 5% design flow) down to 40% design flow.

2.0             Simultaneously, slowly reduce the glycol circulation flow on FR-1106 to               50% design flow.

3.0             Change the overhead gas from V-401 to flare via PV-1107. Close the battery limit valve MOV-1102.

4.0             Slowly close FV-1101. When FV-1101 is closed then close the feed gas battery limit valve MOV-1101.

5.0             Continue to circulate glycol for a minimum of 2 hours until it is regenerated.       During this period the plant pressure will be slowly reduced by the use of stripping gas in C-403 via PCV-1108 and PCV-1111. If required, the unit pressure can be further reduced using PV-1107.

6.0             When the solution is regenerated, the steam flow to the reboiler E-402 should be slowly reduced, and the steam flow blocked in.

7.0             The shutdown of the glycol circulation in the unit will normally be carried out only at a total plant shutdown.

8.0             Continue circulation of glycol until the solution is cool, helped by injection of inert gas or fuel gas to the rich glycol-degassing drum V-402.

9.0             The glycol circulation rate should be reduced to 30% by adjustment of the pump. After this the injection pump can be stopped.

10.0         The cooling water flow to the trim cooler E-403 can also be stopped.

3.3               PLANNED SHUTDOWN (LONG DURATION)

It is assumed in this procedure that the outlines as described in the short duration shut down have been completed.

All equipment in the dehydration unit should be drained of glycol, which may be stored in the lean glycol tank T-401 or transferred as necessary in the bulk storage tank in the off-sites.

1.0 Pressurize as much glycol as possible from the Absorber C-401 and the Scrubber V-401 to the Degassing Drum V-402 (i.e. until LSLL-1105 and LSLL-1133 close SDV-1102 and SDV-1105). Then depressurize the absorber and scrubber before drain these vessels to the sump V-405.

2.0 Similarly drain V-404 until LSL-1132 closes SDV-1104. Then drain the vessel to the sump tank.

3.0 Using LV-1116 on manual drain as much glycol as possible via the filters to the surge drum V-403. Thereafter, depressurize V-403 and drain the degassing drum to the sump drum. Drain any hydrocarbons accumulated in the rich glycol degassing drum to the flare, via the skimming nozzles.

4.0 Carefully monitor the level in the glycol drum V403. It may prove necessary to pump the glycol to the lean glycol storage tank T-401 using the pump P-401.

5.0 Drain all vessels, filters, pumps, reboiler exchangers, coolers and low            points in lines to the closed glycol drain and then to the glycol sump drum V-405.

6.0 Transfer glycol from the sump drum V-405 using the pump P-403 to the lean glycol tank T-401.

7.0 Depending upon the purpose and nature of the shutdown, glycol can also be   held in the reboiler E-402 and lean glycol surge drum V-403.

8.0      Any liquid, which cannot be routed to the hydrocarbon blow down, or glycol sewer may then be emptied into the oily water sewer.

9.0 After de-pressurization to the flare pressure, using blinds and block valves isolate any line or vessel that is to be entered. Open the vent on the vessels and using the steam out connections and steam from the utility stations, steam out the equipment until it is free of poisonous and combustible materials

Note : The equipment may be purged with steam using the same procedure described in  Section 3.5.2 ( Purging with Steam) or using the following procedure.

Before draining TEG to sump, depressurize the system to prevent splashing.

    PURGING

1)       After all liquid has been removed from the system and the system has been depressurized, the unit must be purged with steam to reduce the hydrocarbon content to less than 0.5 vol. %.

2)       Connect steam hoses from utility stations to the purge connections on all equipment.

3)       Open the steam connections and pressurize the unit to 3.5 kg/cm²a.

4)       Depressurize the unit by opening the vents to the flare system and draining any steam condensate to the process sewer.

5)       Close the vent connections when the pressure in the unit is reduced to             approximately 1.1 kg/cm²a.

6)       Repeat previous steps three times more for a total of four pressurize/depressurize cycles to achieve a hydrocarbon content of less than 0.5 vol. %.

7)       On the fourth depressurizing cycle, leave a pressure of 0.4 - 0.7 kg/cm²a in the unit using inert gas.

8)       Check hydrocarbon content throughout the unit with a portable analyzer. If the hydrocarbon content is above 0.5 vol. %, depressurize to 0.1 kg/cm²a and repeat the steps until hydrocarbon content is less than 0.5 vol. %.

Note:  Especially check the hydrocarbon content at the dead ends of pipes and ensure that the hydrocarbon in the liquid lines is blown through. Any hydrocarbon condensate accumulated in the vessels should be drained to the blow down system.

9)       When satisfied that all lines and equipment have been inertized, close all connections to flare and leave lines and equipment that will not be opened for maintenance under a slight overpressure using inert gas or nitrogen. Steam should not be left in the system because it will condense and may pull a vacuum on the vessels. These vessels should not be opened to atmosphere because of the presence of pyrophoric iron and it is recommended that inert gas or nitrogen are maintained in the system for safety.

10)     The spectacle blinds at the unit battery limits shall be swung to their closed position. -The dehydration unit, except the Flare and Blow down Systems is now completely isolated and is ready for maintenance and inspection.

Note: Maintenance personnel shall not enter any equipment in the unit without proper oxygen apparatus unless the unit has been purged entirely with air to displace all inert gas.

3.4    EMERGENCY SHUTDOWN

3.4.1 GENERAL

Shutdown system design depends on the selected control system type, which is a central distributed digital system.

All shutdown valves (SDV) actions are controlled by the central system for each train. Operating procedures are described in the following section of this procedure.

Generally speaking, main equipment such as pumps, compressors, reboilers, coolers etc. can be stopped at once without any mechanical damage.

The emergency system involves a complete automatic shutdown of the train and with complete isolation and possible depressurization. In case of emergency situations, shutdown can be initiated by operators from the control room.

In case of utilities failure, the general rule is to stop and isolate the train while maintaining the gas pressure as long as possible in order to allow a fast restart of the train.

3.4.2 TYPE OF EMERGENCY SHUTDOWN

Emergencies will generally require an immediate complete stoppage of operation with at least part of the plant shutdown and depressurized. In most instances, hydrocarbon must be eliminated to the maximum extent possible in the shortest time as determined by the urgency of the emergency. In some cases, the type of shutdown is complicated by the emergency situation itself, requiring in many cases, a split-second decision by the operator.

Conduct all emergency shutdowns in the most economical manner possible with primary considerations for the safety of personnel, with secondary concern to safeguarding the equipment and still less priority reserved for product's quality.

Determine the cause of the emergency including the exact situation; and if possible, revert to a normal shutdown at the first opportunity.

A unit shutdown system is provided to allow shutting down all incoming gas and treated gas.

Emergency shutdowns may be caused by:

-- Automatic shutdown following a programmed sequence and resulting, for example, from a product/utility failure.

-- Annual shutdown entailed by an emergency or induced to avoid an accident.

It is difficult to predict all the possible causes of emergency shutdowns and to define, for each case, the dispositions to be taken.

Recommendations hereafter, are only partial guidelines; the actual shutdown procedure will be defined in function of the actual situation and operator’s troubleshooting capability.

1.       Automatic shutdown

          This may be caused by a low flow of glycol to the absorber.

2.       Manual shutdown

Depending on the kind of trouble, the operators should be able to react at different levels. Total shutdown may be achieved by shutting down the lean glycol pump. Automatic trips in series will then ensure the unit isolation.

Shut down may be partial by blocking the feed and exit gas, the glycol flow will still be in circulation.

3.       Short Duration Stops

          They may be caused by utility failure, for example; shutdown in this case may be automatic or manual. (Next page)

3.4.3  ACTION DURING EMERGENCY SHUTDOWN

As mentioned in the previous Section 6.4.2 “Type of Emergency Shutdown”, there are a number of failure modes that will cause the Gas Dehydration Unit to go into a full or partial shutdown.

The following procedures deal with the various shutdowns mentioned in Section 6.4.2.

3.4.3.1 LOW LEVEL IN THE ABSORBER C-401 (INTERLOCK SIGNAL NO.1)

          The following automatic action will take place upon LSLL-1105 being       activated :

          -- Absorber bottoms shutdown valve SDV-1102 will close.

          -- Feed shutdown valve SDV-1101 will close.

          -- Treated gas shutdown valve SDV-1103 will close.

          -- Glycol injection pump P-401 will stop.

The following manual action will be taken by the control console operator:

          -- HP steam inlet valve TV-1212 (SDV-1212) to be closed, if required.

Depending upon the amount of time taken to get the plant back on stream, the following actions may be induced:

- Gradual closing of the V-402 rich glycol degassing drum level control valve LV-1116.

- Gradual closing of the C-402 glycol regenerator temperature control valve TV1215.

- Gradual closing of the HP steam inlet valve TV-1212 when not manually closed by the operator.

3.4.3.2 LOW LEVEL IN FEED GAS KO. DRUM V-404 (INTERLOCK SIGNAL NO.2):

             The following automatic action will take place upon LSL-I132 being activated :

•       Feed Gas K.O. Drum bottoms shutdown valve SDV-1104 will close.

             The following automatic action will take place upon LSL-1133 being activated:

•       Dried Gas Scrubber bottoms shutdown valve SDV-1105 will close.

3.4.3.3 LOW GLYCOL FLOW TO ABSORBER C-401  (INTERLOCK SIGNAL NO.4)

          The following automatic action will take place upon FSL-1106 being

          activated :

          -- Feed gas shutdown valve SDV-1101 will close.

          -- Treated gas shutdown valve SDV-1103 will close.

          -- Glycol injection pump P-401 A or B will stop.

          The following manual action will be taken by the control console operator:

          -- Absorber bottoms shutdown valve SDV-1102 will be closed.

Depending upon the amount of time taken to bring the plant back on stream the following actions will be induced :

•       Gradual closing of the V-402 rich glycol degassing drum level control valve               L V-1116.

•       Gradual closing of the C-402 glycol regenerator temperature control valve TV –1215.

•       Gradual closing of the HP steam inlet valve TV-1212.

3.4.3.4 HIGH TEMPERATURE IN GLYCOL REBOILER E-402

            (INTERLOCK SIGNAL NO. 5)

            The following automatic action will take place upon TSHH-1210 being

            activated :

          -HP steam inlet valve TV-1212 will close.

3.4.3.5 GENERAL UNIT SHUTDOWN (INTERLOCK SIGNAL NO.6)

The following automatic action will take place upon activation of the emergency shutdown push button.

- Feed shutdown valve SDV-1101 will close.

- Treated gas shutdown valve SDV-1103 will close.

- Absorber bottoms shutdown valve SDV -1102 will close.

- HP steam inlet valve TV-1212 will close.

- Feed Gas KO. Drum bottoms valve SDV-1104 will close.

- Dried Gas Scrubber bottoms valve SDV-1105 will close.

- Glycol injection pump P-401 will stop.

The following manual action will be taken by the control operator:

          - When required, the unit may be depressurized via PV-1107.

          - Shut off all electric motor drivers.

Restart of the unit will depend upon the reason that the Emergency Push button was activated. Depending upon the duration of the shutdown, refer to Section 4.0 Plant Start-up.

          Electric power Failure:      Refer to general unit shutdown.

          Instrument Air Failure:      Refer to general unit shutdown.

Note: The unit will depressurize when the instrument air reserve of PV-1107 is empty.

            HP Steam Failure:    Refer to general unit shutdown.

4.0  ALARM AND TRIP SETTINGS

ALARM AND TRIP SETTINGS

NOTE: 1. High pressure case                      : -8 ⁰C dew point

           2. Low pressure case                       : -11 ⁰C  dew point

1.     Process objectives:

1.1             To reduce the water-content of sweet-gas coming from GSU for pipeline transportation through HBJ pipeline.

1.2             Product specification (sweet and dry gas)

1.2.1      (85 kg/MMNm³ MAX. i.e. (-) 7 °C at BL pressure of 74 kg/cm²a) (Case-I)

1.2.2      (50 kg/MMNm³ MAX. i.e. (-) 11 °C at BL pressure of 51 kg/cm²a) (Case-II)

2        Principle of Operation:

2.1             Counter current scrubbing using hygroscope TEG (99.7 %wt). (Tri-ethylene glycol)

2.2             Regeneration of rich glycol solution to lean glycol solution in two stages

2.2.1      Stage-I: De-gasification at intermediate pressure at 10 kg/cm²a in to liberate fuel gas.

2.2.2      Stage-II: Re-boiling of Rich TEG in regenerator to remove absorbed water and concentrate the glycol to required level of 99.7 % Absorber water is vented to atmosphere in the form of water vapors.

3        Parameters & operating variables:

            Operating parameters and variables affecting the absorption & regeneration processes are listed below :

1.     Rerouting in fuel gas system of GSU/GDU of Phase-I and II trains through suction KOD of vapour compressor:

Commissioned in June’2001, fuel gas generated in GSU trains was routed through KODs installed in GSU trains of Phase-I and II, which were of very small capacity (0.65 m³). The fuel gas generated in GDU trains was directly joining the fuel gas header from degasser itself. Because of smaller capacity of KODs /direct routing of fuel gas from GDU trains, heavy carry over of liquid was observed whenever there was any operational upset in either GSU of GDU trains. The total fuel gas header used to get filled up with liquid affecting downstream consumers of fuel gas like KRU, boilers and incinerator. Due to carry over problem these units sometimes had to be shutdown.

In order to avoid recurrence of the above problem, it was thought of installing higher capacity KODs in the fuel gas streams of Ph-I and II. The vapour compressor system installed along with GSU trains of Phase-I and II trains are not in operation since the last 10 years. Each train is having two numbers of KODs installed at the 1^st and 2^nd stage suction of vapour compressor. They are designed for 7.5 kg/cm² and 143⁰C, which are suitable for the fuel gas system. Also there are 26.5 m³ capacity, which is much higher than existing KOD.

Suitable modifications in the fuel gas system have been done by re-routing the fuel gas from Ph-I  GSU trains through V-306 of train 33. Similarly the fuel gas from GDU trains of Ph-I is routed through V-305 of train-33. The outlet of these two vessels are combined before it joins the common fuel gas header. In addition the liquid outlet of V-305 is connected to pure TEG header to recover TEG. Provision exists in V-306 for recovery of MDEA. The vessels contain PT, LT, LG and level switches for high and low to monitor the operating parameters. The vessels are having two numbers each of safety valves set at 7 kg/cm².

On the same lines, the fuel gas system of Phase-II was also modified through the vessels V-305 and V-306 of train 34.

The modified drawings are enclosed at Serial No. 1 and 2 .

2.     Relocation of LV 116 of GDU-IIIA.

LV1116 of GDU trains 46 and 47 of Phase-IIIA were relocated, away from feed nozzle to regenerator. This is for avoiding flashing near feed-nozzle, which may cause liquid carry over.

he relevant drawing is placed at Serial No. 3

3.     Modification in H.P. Glycol Absorber of   GDU-IIIA.

          Gas feed nozzle was modified by reorienting the nozzle towards downcomer of 1^st tray of the absorber. The top bubble cap tray has been made dry. This modification has been done to minimize glycol loss.

     The schematic diagram is placed at Serial No. 4.

4.      PV 1107, (flare valves) of GDU train No 43,44 of phase-II were made fail close, in case of instrument power failure. However, it will remain fail open in case of instrument air failure. This modification was done to avoid sudden depressurization of GDU train causing heavy vibration in flare header, during 1990-91. In all other trains of GDU, this valve is fail-open in both the cases.

5.      The flame arrester of steam vent boom, of regenerators in train 46 and 47 were modified. This avoids the back flow of steam condensate into the regenerator, if any.

6.      Drain of condensed vapour in the regenerator vent was initially lined up to PWS and sump. The spent glycol was collected from sump to tank. When sufficient spent TEG was collected in the tank, gas was cut-off from the train, spent TEG was then regenerated in batches. By modifying the vent drain line, it has been routed back to the reboiler E-402. This has helped to avoid shutdown of GDU trains for regenerating spent TEG.

-End of the document-