13.7 MAJOR PROCESS COMPONENTS
3.7.2 LNG Un-loading Operation
The LNG from the carrier will be unloaded by means of the carrier’s on-board pumps.
Cool-down of the unloading arms and the auxiliary equipment will be started from the carrier, after which the LNG pumping rate will gradually be ramped-up until the maximum unloading flowrate of 52,834 gpm (12,000 m3/h) is obtained.
The LNG storage tanks will be maintained at an operating pressure of up to 3.5 psig during the unloading process. The unloading arms will be manifolded to a 32”
unloading line and a 6” recirculation line. The LNG will be transferred into each of the storage tanks via 32” pipes. The tanks can either be top or bottom filled depending on the compositions of the tank contents and the fresh cargo from the carrier. The LNG unloading rate will be controlled from the carrier as agreed with the terminal.
The unloading operation will continue until the LNG tanker is almost empty at which point the pumping rate will be ramped down. The jetty facilities and unloading lines will be designed to unload the contents of a 42.3 MM gallons (160,000 m3) carrier with adequate rail elevation and pumping capacity at a rate of 52,834 gpm (12,000m3/h) in approximately 14 hours, excluding time for docking, cooling and undocking.
The pressure in the carrier during unloading will be maintained by means of a vapor return system, which will enable the required vapor to flow from the storage tanks to the carrier. With the line pressure into the carrier controlled, the volumetric flow will adjust itself naturally to match the carrier’s liquid displacement. A desuperheater will be installed on the jetty in order to control the temperature of the vapor returned to the carrier to about -220°F by injecting LNG into the vapor.
LNG for desuperheating will be supplied from the jetty transfer line. A vapor return KO drum will be provided to prevent liquid slugs downstream of the desuperheater, ensuring single-phase vapor flow to the vapor return arm. The KO drum will also act as a drain pot for the unloading arms.
The carrier’s tank level gauges will be used for the fiscal measurement of the total cargo transferred from the carrier to the storage tanks. The LNG unloaded at Bradwood Landing from a carrier will be sampled on-line and analyzed for composition. The density, calorific value, and Wobbe Index of the unloaded LNG will also be determined from the on-line samples. An LNG sampling package will be installed on the unloading line to accomplish this.
A 32” unloading line will connect the jetty and the storage area. The size of this line is based on an unloading flowrate of 52,384 gpm (12,000 m3/h). A recirculation cooldown line (6”) will also be provided. The recirculation line is sized for no greater than a 4°F delta temperature rise or 114 m3/h (500 gpm) minimum, whichever is controlling.
The transfer lines coming across the jetty will be equipped with emergency isolation valves for isolating the carrier supply in case of an emergency situation.
During normal operation (when no carrier is berthed), the unloading lines will be kept cold by circulating LNG liquid from the send-out system to the jetty head via the re-circulation and unloading lines.
3.7.2.1 Vapor Return Blowers Knockout Drum
Refer to P&ID W00031-000-PR-PI-007.
A vapor return KO drum will be located on the jetty. Once the unloading activities have been completed and before re-circulation is started, LNG will be drained from the unloading arms to the vapor return KO drum and back to the LNG carrier by pressurizing with gaseous nitrogen. After the carrier has disconnected, the vapor return KO drum will be drained into the unloading line, again by pressurizing with nitrogen.
The main technical characteristics of the vapor return KO drum are as follows:
• Service: Natural Gas / LNG
• Design Pressure: Full vacuum / 174 psig
• Design Temperature: –274 oF / +99 oF
• Dimensions: 9 ft 6 in dia x 28 ft 6 in ht
See Appendix B13 for Vapor Return Knockout Drum datasheet (W00031-666-PR-DS-013).
13.7.2.2 BOG and Vapor Handling System
The BOG and vapor handling system is detailed in P&ID’s W00031-000-PR-PI-008/009/010/011.
The vapor handling system will be essentially comprised of:-
• Vapor handling pipework
• BOG compressors
• BOG condenser
The function of the vapor handling pipework is to provide a safe conduit for the vapors generated within the LNG storage tanks. These vapors are generated as a result of heat leakage into the system and the resulting vaporization of the LNG.
During the unloading operation, these vapors (BOG) are displaced by the LNG entering the tanks and therefore need to be safely removed in order to maintain the correct tank pressure.
Both LNG storage tanks are connected to a BOG vapor header line (24”) which is equipped with a connection to the process vent. Normally, the BOG is routed to the carrier (during unloading operations, to offset the unloaded LNG volume) or to the BOG compressors (where the BOG is compressed and subsequently condensed back into liquid form by mixing with a volume of LNG).
The function of the BOG compressors is to raise BOG pressure to a level at which it can be condensed in the BOG condenser. The BOG compressors will also serve to control tank pressure during carrier off-loading and periods of low send-out.
The function of the BOG condenser is to condense the boil-off vapors. This is necessary to avoid the high compression costs that would result if the boil-off vapors were simply compressed to export line gas pressure. The condensed boil-off (as liquid) is then raised to export pressure by pumping rather than compression.
During carrier unloading, vapor displaced from the LNG storage tanks will be returned to the LNG carrier via the vapor return line. The pressure control valve installed on this line will maintain the required pressure at the vapor return arm.
The energy of pumping the LNG out of the carrier and the heat leak into the unloading arms, unloading and fill lines will increase the vapor pressure of the LNG.
Hence, during carrier off-loading the LNG storage tanks will be operated towards the upper end of their pressure range to suppress flash from this increased vapor pressure. Normal tank boil-off and any extra boil-off gas from the unloading operation (nominally equivalent to the carrier’s boil-off) will flow to the vapor recovery system.
When there is no carrier unloading, the volume of LNG sent out from the storage tanks will frequently exceed the quantity of boil-off gas generated and “padding gas”
will be used to maintain “low” tank pressure. At lower send-out rates, boil-off gas production will exceed the LNG displacement and boil-off gas will flow out of the tanks to the vapor recovery system.
The tank vapor balance lines will be manifolded to the BOG header so that both tanks are at the same pressure. “High” tank pressure will be controlled by the action of the BOG compressors.
In the BOG condenser, the boil-off gas will be contacted with LNG from the in-tank pumps and, at the higher pressure of the BOG condenser, be re-condensed.
If the pressure of the boil-off gas header rises beyond the ability of the BOG compressors to control, the “relief policeman controller” will act to route excess gas to the process vent stack for disposal. The relief policeman controller will operate before the storage tank pressure rises to the set point of the tank pressure safety valves.
In the event of an LNG tank being isolated from the BOG header, the individual tank pressure safety valves will maintain a safe operating pressure in the tank.
13.7.2.3 Boil-Off Gas Compressor
The terminal design includes 2x50% reciprocating BOG compressors. The sizing of the compressors is based on the minimum send-out case (maximum BOG case) during start of carrier unloading. The thermal mass of the jetty line, which warms during periods of no off-loading means that at start of off-loading, the capacity of the system is reduced. The compressors will operate in a duty / standby arrangement. The BOG Compressors are sized for a maximum capacity of 7.68 MM actual cf/d.
A desuperheater will be installed on the BOG compressor suction line. This is to ensure that the compressor suction temperature will always be below -250°F to avoid unacceptably high discharge temperature when the compressor is operating at its maximum discharge pressure. (There are times, due to prolonged periods of high sendout, when there is no flow of BOG to the compressors and the compressor suction pipework, with considerable thermal mass, could warm up to close to ambient temperature, resulting in warm suction gas to the compressors.)
A knock-out drum will be provided on the BOG compressor suction to separate any injected liquid that is not vaporized in the compressor suction flow.
The main technical characteristics of the BOG compressor are as follows:
• Manufacturer: Burckhardt Corporation/IHI or similar
• Service: Natural Gas
• Suction Pressure: 18.2 psia at –251 oF
• Rated Discharge Pressure: 116 psia
See Appendix B13 for BOG Compressor datasheet (W00031-562-PR-DS-006).
13.7.2.4 BOG Condenser
The BOG condenser will perform two functions: a) it will condense boil-off gas using the cold capacity in the LNG from the in-tank pumps; and b) it will provide NPSHA and buffer capacity to the send-out pumps.
The upper section of the vessel will contain a packed section, which will be wetted by the downward flowing LNG. The packing will provide a large surface area for contact with the boil-off gas flowing co-currently through the packing. The LNG supply to the BOG condenser will be under pressure control and will increase if the BOG condenser pressure rises and vice versa. Additional pressure control valves will allow excess pressure to be vented or ‘padding gas’ to return from the send-out line.
The BOG condenser normally operates with the two objectives in the first paragraph above. However, there will be instances when there will be no boil-off gas coming from the LNG tanks to the BOG Compressor and BOG Condenser. In particular when the send out is at a high flow rate and its suction effect in the LNG storage tank due to the withdrawal of liquid from the tank exceeds the boil-off gas flow rate due to heat leak into the tank and associated pipework.
This situation will occur when there is no unloading from a carrier. In effect the BOG Condenser will not have any gas to condense, but LNG will be maintained in the condenser in order to keep it cold and to provide the required suction head for the send out pumps. When there is no boil-off gas flowing to the condenser there is no need to maintain its normal operating pressure. Padding gas which is normally used to maintain the pressure will not be required during this mode of operation and it can be considered to be operating in “flooded” mode. Padding gas will be utilized to restore the vessel to pressure and level control after flooded operation, providing the motive force to “empty” the vessel and the pressure to inhibit flashing of hot liquid from the previously flooded condenser.
The lower part of the vessel will be of larger cross-section and will act as the liquid buffer volume for the send-out pumps. It will be sized for the future send-out rate (1,500 MMSCFD) and a liquid hold-up time of 30 seconds at that rate.
The elevation of the BOG condenser will be set to provide the send-out pumps with adequate NPSHA assuming the handling of boiling liquid.
Under normal operating conditions, the majority of the LNG flow will bypass the BOG condenser. The level of liquid in the BOG condenser will be used to control this bypass flow (unless the condenser is flooded). At minimum send-out conditions, almost all of the LNG flow will be routed through the top of the BOG condenser.
The main technical characteristics of the BOG condenser are as follows:
• Service: Natural Gas / LNG
• Design Pressure: Full vacuum / 200 psig
• Design Temperature: –270 oF
• Dimensions: 12 ft 6 in dia x 26 ft 3 in ht
See Appendix B13 for BOG Condenser datasheet (W00031-566-PR-DS-009).