Performance and safety features of a 2 x cbm LPG-import terminal built in China

Transcription

1 Performance and safety features of a 2 x cbm LPG-import terminal built in China Dr. Manfred Kuver Christof Fey Li Qin Yi Executive Vice President Tractebel Gas Engineering GmbH Deputy Engineering Dept. Manager Tractebel Gas Engineering GmbH General Manager Shanghai Golden Conti Petrochemical Co. Ltd. Abstract: This paper shall describes the performance and safety features of the largest LPG above ground import terminal in the Shanghai area and was started up in mid The storage tanks of cbm each are of a modern double containment design with pre-stressed concrete outer tank and low temperature carbon steel inner tank. The two fully refrigerated tanks are connected by pipeline to a secondary site with four spherical tanks truck and bottle filling plant.

2 PERFORMANCE AND SAFETY FEATURES OF A 2 X CBM LPG TERMINAL 1. INTRODUCTION The LPG market in P.R. China has grown considerably in the last few years. As the local refineries could not meet the demand of the market, huge amounts of LPG have to be imported. In the mid 90 s the bottleneck for import was that no refrigerated terminals were available. Only small quantities could be traded through fully pressurised terminals with maximum capacities of about m 3. A number of FSO s with capacities of up to m 3 were serving as buffer import storage in order to try to satisfy the import demand. As a logical consequence a number of refrigerated terminals were planned and put into operation at the end of the 90 s. Starting in 1996 and completing in 1999 Tractebel Gas Engineering (TGE) built a 2 x m_ refrigerated LPG Terminal on a turn-key basis at Jinshanwei near Shanghai, China. The plant was built for Shanghai Golden Conti Petrochemical Corporation, a joint venture of Shanghai Petrochemical Corporation, Mitsubishi, and Continental Grain Company. The terminal is intended to be used as an import and distribution terminal. The following main operations are provided: LPG storage operation Ship unloading operation BOG reliquefaction operation Send out operation to a spherical tank farm Send out operation to coastal tankers The terminal was erected on a green field site i.e. all auxiliaries and infra structure had to be provided too. Dr. Kuver -1-

3 2. PERFORMANCE DATA 2.1 LPG STORAGE OPERATION The imported LPG is stored in two tanks, each with a size of 50,000 m_. One tank is dedicated for propane, the other for butane (see sketch 1). The design for both tanks is identical, i.e. design pressure is 5 to 200 mbar g, design temperature 45/+38 C. The boil-off rate for the tanks is 0.05 wt% at a product temperature of 45 C and 38 C ambient temperature. 2.2 SHIP UNLOADING sketch 1 For ship unloading operations two hydraulically operated marine loading arms are provided, one dedicated for propane, the other for butane. Each loading arm is connected with the respective tank via a 16 ship unloading line of approx. 1 km length. The ship unloading operation is designed for an unloading rate of m_/h propane and butane at a pressure in the ships cargo tanks of 100 mbar g. The ship unloading operation is performed without vapour return. Dr. Kuver -2-

4 2.3 BOG RELIQUEFACTION Each tank is equipped with its own Boil Off Gas (BOG) reliquefaction system. The systems are designed for the reliquefaction of the boil-off gas from the tanks as well as for the displaced volume and flash gas which is generated during ship unloading operation. The reliquefaction capacity required for each tank is about 800 kw, taking also into account that one send out pump is running in bypass operation. Each BOG reliquefaction system consists of two oil injected, single stage screw type compressors with oil system, compressing the vapour phase from the tank to condensation pressure. After compression the injected oil is separated from the LPG down to 5 ppm by means of a two stage oil separation system. Thereafter the LPG is condensed at approx. 40 C and released back to the tank. 2.4 SEND OUT TO SPHERICAL TANK FARM Both tanks are connected via two approx. 35 km long pipelines with a spherical tank farm consisting of four spheres of 1,500 m_ volume each. Making use of one intank pump and a booster pump the pressure of the product can be increased and the LPG is injected into the pipelines. Prior to feeding the propane into the pipeline it is heated up to approx. 3 C. Butane is injected into the pipeline without heating. The send out takes place at a pressure of approx. 30 bar g. Each send out system is designed for a send out rate of 50 t/h. At the spherical tank farm it is possible either to store the products in dedicated propane or butane spheres or both products can be mixed inline and stored in a dedicated sphere for the mixture. The product from the spheres is exported to trucks, bottles, or can be used as feed for a cracker. Dr. Kuver -3-

5 2.5 SEND OUT TO COASTAL TANKERS Propane as well as butane can be sent out to coastal tankers. For this operation the product pumped with the intank pumps is led to one ship loading arm via one ship loading line of 1.5 km length. Prior to propane ship loading the product is heated from 42 C to approx. 3 C. Vapour from the ship is returned to the terminal where it is condensed against the ship loading stream and sent back to the ship. To perform the max. ship loading rate of 300 t/h both intank pumps of one tank have to sent out in parallel. If mixed product is to be loaded, the ship will be fed with butane first and thereafter with propane. Product composition is controlled by a flow integrator closing a valve in the ship loading line when the pre-selected quantity has been sent to the ship. 2.6 SIMULTANEOUS OPERATION The terminal provides a high degree of flexibility allowing the owner immediately to react to the market demand. Ship unloading operation with both products, send out operation to the spherical tank farm with both products, and ship loading operation can be performed simultaneously or as a single operation. 3. SAFETY FEATURES The terminal was built taking into account the latest recognised technical standards and fulfilling the regulations of the P.R. China. 3.1 SAFETY FEATURES OF THE STORAGE TANKS Tank Concept The British Standard BS 7777 distinguishes between the following three major design concepts for storage tanks: Dr. Kuver -4-

6 Single Containment Tank Either a single tank or a tank comprising an inner tank and an outer container designed and constructed so that only the inner tank is required to meet the low temperature ductility requirements for storage of the product. The outer container (if any) of a single containment storage tank is primarily for the retention and protection of insulation and to contain the vapour purge gas pressure, but is not designed to contain refrigerated liquid in the event of leakage from the inner tank. A single containment tank is normally surrounded by a low bund wall... to contain any leakage. /1/ (see sketch 2). Double Containment Tank sketch 2 A double tank designed and constructed so that both the inner tank and the outer tank are capable of independently containing the stored refrigerated liquid. To minimize the pool of escaping liquid, the outer tank or wall is located at a distance not exceeding 6 m from the inner tank. The inner tank contains the refrigerated liquid under normal operation conditions. The outer tank or wall is intended to contain the refrigerated liquid product leakage from the inner tank, but it is not intended to contain any vapour resulting from the product leakage from the inner tank. /1/ (see sketch 3). sketch 3 Dr. Kuver -5-

7 Full Containment Tank GAS-TECH 2000 Houston A double tank designed and constructed so that both the inner tank and the outer tank are capable of independently containing the refrigerated liquid stored. The outer tank or wall should be 1 m to 2 m distant from the inner tank. The inner tank contains the refrigerated liquid under normal operation conditions. The outer roof is supported by the outer tank. The outer tank is intended to be capable both of containing the refrigerated liquid and of controlled venting of the vapour resulting from any leakage after a credible event. /1/ (see sketch 4). sketch 4 The selection of design concept is done by the plant owner comparing safety and environmental aspects with economic issues. The LPG terminal is located next to the Shenshan crude oil terminal of Shanghai Petrochemical Corporation, i.e. a failure of the tank may jeopardise safe operation of the crude oil terminal. The original tender asked for a double wall, single containment tank (inner tank made of low temperature carbon steel, outer tank made from normal carbon steel) with breather tank built according to API 620 Appendix R. Both tanks were surrounded by an impounding wall. Liquid inlet and outlet nozzles had been located at the tank bottom. During the bidding procedure, TGE introduced an alternative proposal with a double containment design in accordance with European safety standards. The tanks have the following main features (sketch 5): Dr. Kuver -6-

8 sketch 5 The inner tank, which is located on a wooden load distribution ring, is made of low temperature carbon steel. The tank bottom insulation is made from foam glass, which is installed inside the load distribution ring. The tank wall insulation, made from Polystyrene panels, is attached to the outer side of the steel tank. The roof insulation is made from mineral wool which is installed on a suspended deck. The inner tank with insulation is located in a reinforced pre-stressed concrete container. In case of inner tank failure the liquid LPG is kept inside the concrete container, which is liquid tight. For safe operation of the terminal there are no tank nozzles in the liquid phase. All tank nozzles are located on the tank roof. This tank concept offers the following advantages compared to the original tank concept: The safety distance between the two tanks could be reduced from one times the tank diameter to half the tank diameter. Due to the smaller safety distance the land consumption for the two tanks is reduced and there is now space for the erection of a third tank with a volume of 30,000 m_. There is no need for an impounding wall around the two tanks. The reinforced pre-stressed concrete container protects the tank against impact of flying objects. Because there is no bottom nozzle in the liquid phase, LPG spillage to the atmosphere is prevented. For a bottom outlet there is always the potential risk of major liquid leakage to the atmosphere because of a failure at the first flange connection or a failure of the stuffing box of the first valve. Such a leak is impossible to tighten and a considerable quantity of LPG is released to the atmosphere creating a hazardous scenario if the LPG is ignited. The higher safety standard combined with the more economic utilisation of the tank area finally convinced the client to implement the TGE proposal. Dr. Kuver -7-

9 3.1.2 Overpressure protection The tanks are designed for an overpressure of 200 mbar g. In order to avoid an overpressure in the tank there are several measurement and control protection measures implemented. During normal storage operations without ship unloading the tank pressure is kept between 110 mbar g and 70 mbar g by a BOG compressor re-liquefying the BOG from the tank. During ship unloading however the tank pressure may increase up to 140 mbar g. At this pressure a pressure control valve installed in the ship unloading line starts to control the flow to the tank in order to avoid a further increase of tank pressure. If, nevertheless, the tank pressure increases further the ship unloading will be interrupted by closing the shut-off valve in the ship unloading line at a pressure of 160 mbar g. At a pressure of 175 mbar g a control valve opens the line from the storage tank to the flare and at 180 mbar g a safety shut down with two out of three selection closes all import and export lines to isolate the tank and opens the control valve to the flare 100 %. As a final overpressure protection a set of pressure relief valves (one in operation, one as installed spare) opens to the atmosphere Vacuum protection The tanks are designed for a vacuum of 5 mbar g. During normal operation the BOG re-liquefaction plant is switched off at a tank pressure of 70 mbar g. If, nevertheless, the tank pressure drops further evaporated LPG is injected into the tank at a pressure of 60 mbar g and there will be a safety shut down with two out of three selection closing all import and export lines to isolate the tank. At a pressure of 40 mbar g nitrogen is injected into the tank in order to build up the tank pressure again. As a final vacuum protection two vacuum relief valves (one in operation, one as installed spare) open at a vacuum of 2.5 mbar g Overfilling Protection To avoid overfilling of the tanks each tank is equipped with two independent level measurements and one level switch. In case of danger of overfilling a two out of three level switch will close all filling lines Roll-over Protection Due to import of a LPG quality which differs from the quality already stored in the storage tank there is the possibility of stratification in the tank. If there are two adjacent layers where the upper one is colder than the lower one there is risk of roll-over, resulting in a sudden evaporation of a huge amount of product. Such phenomena can also occur if product is stored over a longer period of time. In this case stratification can take place due to heat ingress from the tank bottom. To detect such stratification in an early stage there are nine temperature elements installed at different elevations in the liquid. A temperature alarm is issued if the lower temperature measurement of two adjacent temperature elements indicates a higher temperature than the upper one. In this case the operators have to stop the product send out immediately and run both discharge pumps in parallel in bypass operation. For this purpose the pump bypasses are designed for the max. flow rate of the intank pumps. Thus the tank volume can be circulated with a flow rate of about 600 m_/h. This means, that the volume of a full tank can be circulated in approx. 80 hours. Because the stratification will built up over a longer period of time this circulation is an adequate measure for the protection against roll-over. Dr. Kuver -8-

10 3.1.6 Butane Storage in Winter Time As described above both tanks are equipped with a suspended deck on which the mineral wool as tank roof insulation is installed. Due to low ambient temperature (min C) the tank roof itself can cool down to a temperature below saturation temperature of butane (approx. 3 C). This would result in condensation of butane at the tank roof. The condensed butane would drop down on the suspended deck. The insulation becomes wet and the insulation quality deteriorate. Furthermore there is the risk of mechanical damage of the tank because the suspended deck is designed for a load of dry mineral wool and does not accept liquid product on it. As a second effect the pressure in the storage tank would decrease dramatically resulting in a vacuum. To avoid both, i. e. overloading of the suspended deck and vacuum in the tank, the TGE patented winterising concept is implemented. This systems decreases the dew point of the vapour phase above the suspended deck by injecting vapour of low molecular weight hydrocarbons. In this case propane from the second tank is taken. With in tank pumps it is pumped from the propane tank to an electrical evaporator. The evaporated product is then injected into the tank dome of the butane tank. To make sure that the propane is equally distributed a sophisticated piping system is installed above the suspended deck of the tank. The dew point is determined by analysing samples taken from different locations in the tank dome. The winterising is initiated prior to start of the cold season. Propane is led to the tank dome of the butane tank until the dew point of the mixture is below the min. ambient temperature. Due to a concentration gradient between the product above and below the suspended deck there is a certain convection of mixture from above towards below the suspended deck and of butane from below towards above the suspended deck. This convection takes place at the gaps of the nozzle penetrations through the suspended deck. Furthermore some mixture is sucked into the compressor suction line by compressor operation. Therefore frequent samples are analysed to verify that the mixture is still sufficient to avoid condensation. If the mixture is diluted by butane such, that condensation at the tank roof is not excluded anymore, again propane vapour has to be fed to the tank dome in order to re-establish a non-condensation condition. 3.2 PROCESS SAFETY FEATURES Ship Loading / Unloading Arms For safe ship loading / unloading the ship loading arm as well as the two ship unloading arms are equipped with a double ball valve / emergency release coupling (DBV/ERC) installation. In case the gas carrier drifts out of the operational range of the marine loading arm, at first an alarm is generated and the ship loading / unloading is interrupted by closing the double ball valve combination at the marine loading arm and by closing the shut off valve in the ship loading / unloading line. If the ship drifts further ship and marine loading arm are disconnected by the emergency release coupling. This ensures that the marine loading arms can be operated safely and there is no risk of major damage of the loading arm due to the drift of the gas carriers. The closing of the double ball valve combination reduces the liquid spillage to the trapped volume between the two ball valves in case of disconnection. Furthermore the DBV/ERC enables the gas carrier to escape quickly in case of danger (e.g. fire at the jetty). In this case normal disconnection of the connection flanges, which takes a considerable time, is not required. Dr. Kuver -9-

11 3.2.2 Intank Pumps As described above all tank nozzles are located on the top of the tank. Because there is no nozzle in the liquid phase a leakage at a flange connection only results in vapour leakage. Due to this nozzle arrangement submerged motor pumps are necessary for the product send out. The pumps deliver the product to a discharge nozzle at the top of the tank where the send out piping is connected. The power for the pumps is also supplied via the top of the tank. The pump is connected with the junction box via the electrical cables and two gas tight feeders with a distance piece in between. The distance piece is purged with nitrogen to avoid the formation of an explosive mixture in case of feed thru leakage Booster Pumps For the send out to the spherical tank farm, booster pumps are required to achieve the necessary pressure for pipeline injection. For this purpose a horizontal canned motor pump had been chosen. This type of pump offers a high degree of reliability combined with a high safety standard. Due to the construction of the pump - the motor is an integral part of the pump - there is no mechanical seal which can leak or fail LPG Heating Prior to the product send out the LPG is heated to +1 C. The heating is performed by steam. The condensate is routed to the drain system (client did not want to feed it back to the boiler). For a safe and economical heating process an indirect heating system was chosen. Therefore two heat exchangers, which are saddled on each other, are connected by several pipes on the shell side. In one case a heat exchanger with one big shell and two U-bundles is installed. On the shell side of the lower heat exchanger ethanol is evaporated against condensing steam. The evaporated ethanol is condensed again in the upper heat exchanger, while the LPG is heated on its tube side. This installation offers the following advantages in respect to the plant safety compared with a direct heating with steam: If the indirect heat exchanger is in operation and if there is a failure (crack) of a tube of either bundles there will be no leakage of flammable gases or liquids to the atmosphere. A crack results in a pressure increase of the shell side. If the indirect heat exchanger is not in operation and if there is a failure (crack) of a tube of the upper bundle there will be no leakage of flammable gases to the atmosphere. A crack results in a pressure increase of the shell side. If the indirect heat exchanger is not in operation and if there is a failure (crack) of a tube of the lower bundle there will be only a leakage of flammable liquid to the disposal system. In the disposal system the flammable liquid is diluted and the danger of ignition is minimized. Furthermore the quantity of ethanol in the shell side, which can spill into the drain, is restricted. A failure (crack) of a tube of a direct heat exchanger always results in a leakage of LPG to the sewing system. The quantity that spills into the sewer is dependent of the time until countermeasures are taken. An ignition of the spilled LPG can take place at a location far away from the source of leakage. The freezing of the steam / steam condensate is excluded, because it is separated by the inter-medium ethanol from the cryogenic LPG. If there were a direct heating the steam / steam condensate system Dr. Kuver -10-

12 can freeze. If there is steam, condensate is trapped in the heat exchanger and cold LPG flows on the other side Blow down / flare system For a safe and environmentally friendly disposal of waste gases a blow down and flare system is installed. To avoid inadmissible pressure increase in equipment and piping, pressure safety valves are installed, releasing the LPG to the blow down system. All lines with a "locked-in" liquid volume of more than 40 litres are equipped with a thermal expansion valve also connected to the blow down system. The blow down gases are led to a blow down vessel where liquid and vapour phase are separated from each other. The liquid phase is evaporated by a steam tracing of the blow down vessel and thereafter led to the flare together with the vapour phase from the safety valves. At the flare the gases are combusted smokelessly. 3.3 SAFETY INSTALLATIONS Gas Detection In a LPG terminal there is always the potential risk of accumulation of explosive gases, e.g. due to failure of a gasket resulting in LPG leakage into the atmosphere. To detect any accumulation of those explosive gas mixtures in the plant in an early stage gas detectors are installed at strategical locations. These gas detectors generate an alarm at 15 % LEL alerting the operations personnel to look for the source of the gas. If there is a heavy leakage the accumulation may increase beyond 15 % LEL. To avoid that an explosion takes place the plant is shut down and brought to its safe position at 40 % LEL. The following areas are protected by gas detection installation: Jetties Tank roofs Compressor shelter Apparatus area Pump platform Control room building Fire Detection In case of fire it is most important that the operations personnel are informed immediately to commence fire fighting measures. Therefore a fire detection system is installed in areas with an accumulation of flange connections and at machines with mechanical seal. The fire detection system consists of a PE hose which is pressurised. In case of fire the hose melts and the pressure drop effects a pressure alarm in the control room. Dr. Kuver -11-

13 A fire detection system is installed in the following areas: GAS-TECH 2000 Houston Tanks Compressor shelter Fire Fighting For the supply of fire fighting water three fire fighting pumps are installed with a capacity of 680 m_/h each pump. Sea water is used as fire fighting water. Different kinds of fire fighting equipment are installed all over the plant. On the tank roof and in the compressor shelter a fixed sprinkler system is installed. The primary task of the sprinkler system is to cool the equipment which is adjacent to the fire. The system is designed for 4.2 l/m_min for the sprinkler system on the tank roof and 10 l/m_min for the sprinkler system in the compressor shelter. Six manual operated monitors with a capacity of 3,000 l/min each are installed around the two tanks to have a well concentrated fire water jet to extinguish a limited fire and to cool the concrete container of the tanks. Additionally each jetty is equipped with two remote controlled fire fighting monitors installed on stand post. These monitors are mainly for fire fighting purposes on the gas carriers. For the supply of additional fire fighting water hoses can be connected to hydrants which are installed at various places in the plant. 4. Conclusion The Shanghai refrigerated LPG terminal is a modern installation with a high safety standard according to European level. The design is based on the latest recognised technical standard taking into account the safety requirements which are required because of the location next to the Shenshan crude oil terminal. Especially the design of the two tanks is very economical, because it combines the most advanced safety features with low investment cost. Literature /1/ British Standard BS 7777: Flat-bottomed, vertical, cylindrical storage tanks for low temperature service, Part 1: Guide to the general provisions applying for design, construction, installation and operation, 1993 Dr. Kuver -12-