Process Description

During the LP phase, the gases from the top outlet of the Production Separator

(302-V-001) reach the Field Gas Compressor Suction KO Drum (304-V-001A). There are two streams of Compressor Systems; one on duty and the other on standby in parallel mode. The split streams for the two compressor systems are identical, with a Suction Knock-Out Drum, Compressor and Compressor After cooler arrangement. The gas stream enters the Compressor Suction KO Drum-A (304-V-001A) through an ESD Valve 304-XV-1200 provided with an ESD Bypass Valve 304-XV-1201. ESD Valve 304-XV-1200 will open only if the differential pressure (304-PDI-1206) is below 2.5 barg. The Compressor Suction KO Drum consists of Vane-type Inlet Distributor through which the gas enters. The entrained condensate is separated and further fine droplets of

condensate (more than 3 micron) are removed in the Demister Pad located at the gas outlet on the KO Drum top. The demister can remove 99% of the particles larger than 3 micron size.

The Compressor KOD 304-V-001A is provided with two PSVs 304-PSV-110A and 304-PSV-110B which is set at a pressure of 95.2 barg. The condensate level in the Compressor Suction KOD is controlled by 304-LIC-1210. The Controller acts on Control Valve 304-LV-1210 to control the condensate flow to Flare Header. The Level

Transmitter 304-LT-1209 has High-High and Low-Low level alarms 304-LAHH-1209 and 304-LALL-1209 respectively. For details, refer Cause & Effect Diagram 250-EPR-CNE-

0500. The gas from 304-V-001A enters the suction side of the Field Gas Compressor

304-K-001A. The suction pipe of 304-K-001A is provided with a temporary hose connection for nitrogen purging, during the start-up period.

The suction pressure of the compressor is controlled by a self-regulating Pressure Control Valve 304-PCV-1294 which vents gas to Flare during the pressurized shutdown. Set pressure of 304-PCV-1294 above the settle out pressure. This PCV will be used to maintain pressure in the system during pressurized shutdown of the Compressor and not during normal operation. The compressor suction pressure is controlled by the Variable Speed Drive. The suction-side gas flow, temperature and pressure are

measured by 304-FI-1220, 304-TI-1222 and 304-PI-1221 which is also used for the anti-surge control of the compressor. Similarly the discharge pressure and temperature of the Compressor is indicated by 304-PI-1232 and 304-TI-1231 respectively. The compressed gas from the compressor discharge at a pressure of 87.5 barg and temperature of 166°C flows to the Field Gas Compressor Aftercooler-A (304-E-001A) where the temperature is cooled to 50C by Fin Fan Coolers. The compressed gas temperature is controlled by using 304-TIC-1215 which will control the Variable Speed Drive of the Fin Fan Motors. From the outlet of the Fin Fan Coolers, the gas is recycled to the suction of KO Drum 304-V-001A by the Antisurge Controller to minimise the surge. The compressed gas flows to the Gas Dehydration column through an ESD valve 304-XV-1219.

4.2.1.1 Compressor Auxiliary System

The Field Gas Compressor has two main auxiliary units – the Lube Oil system and the Seal Gas system.

Auxiliary Lube Oil System

The Lube Oil system comprises the following: Lube Oil Reservoir (304-T-1500), Lube Oil Heater (304-H-001A), Auxiliary Lube Oil Pump (304-P-002A), Main Lube Oil Gear Pump, Lube Oil Cooler (304-E-002A) and Lube Oil Filters 304-F-001A1/A2.

During normal operation, the lube oil is supplied by the gear-driven main pump. The Auxiliary Lube Oil Pump is used during start-up, shutdown and also as a standby pump to main oil pump when the oil pressure is not adequate. The Lube Oil Reservoir level and temperature are indicated by Level Indicator 304-LI-1505 and Temperature Indicator 304-TI-1514 respectively. The Reservoir temperature is maintained by an

electric Heater which maintains the lube oil in the reservoir at a constant

temperature. The temperature indicator 304-TI-1514 and level indicator 304-LI-1505 are provided with High and Low level alarms. The lube oil from the discharge of the Auxiliary/Main Lube oil pump is cooled in Lube Oil Coolers (304-E-002A) which are Air Fin Coolers. The lube oil header pressure is maintained at 2.5 barg by a Pressure Control Valve 304-PCV-1510 by regulating the lube oil back to the Lube Oil Reservoir. The lube oil is then filtered by Lube Oil Filters (304-F-001A1/A2) and distributed to the Motor and the Compressor (DE & NDE).

The lube oil is supplied to the following items: o Motor Bearings

o Gearbox

o Compressor (DE & NDE) bearings

Seal Gas System

The Seal Gas provides the sealing media to the compressor seals. The dry gas seals consist of an inboard seal and an outboard seal. The inboard-seal seals between the process gas pressure inside the compressor and the pressure in the primary vent line. The outboard-seal seals between the primary and secondary vent line pressures. A schematic sketch of the Compressor Seal gas system is illustrated in Fig. 3 as given below:

Fig. 3 – Schematic of Compressor Seal System

The primary seal gas from the discharge of the Compressor is filtered at the Seal Gas Filters 304-F-002A1/304-F-002A2. During Normal operation of the Compressor, Seal Gas is supplied from the Compressor Discharge. However, during the Start-up condition and Black-Start condition, bottled nitrogen will be used for the dry gas seal. In case of pressurized shutdown, the Settle-out Pressure is 46.4 barg. During this period, the seal gas supply can be supplied from the HP Fuel Gas tapped from the downstream (D/S) of Fuel Gas Heater (321-H-001A/B) to ensure dry heated gas supply to Dry Gas Seals. The suction pressure of the compressor is controlled by a self-regulating Pressure Control Valve 304-PCV-1294 whose set pressure is above the settle out pressure. This PCV will be used to maintain pressure in the system during pressurized shutdown of the Compressor and not during normal operation.

Similarly, the Secondary Seal Gas is nitrogen gas, filtered in Buffer Gas Filters

304-F-003A1/A2. The Nitrogen (from Utilities) from the Buffer Gas Filters is split into two lines - one supplying as secondary seal gas through 304-PV-1465 and the other supplying seal gas through 304-PV-1449 for bearing separation seal. The leaking gas from the primary and secondary seal is vented to the Flare Header under Back- pressure Controller 304-PCV-1464. The seal vent between the secondary seal and bearing separation seal is vented to the atmosphere at a safe location.