Discharging LPG - Safety & operational matters
Preparations for LPG discharging
When the vessel arrives at the discharge terminal, cargo tank pressures and temperatures must be at values appropriate to the terminal requirements to allow maximum discharge rates to be achieved. Before the discharge operation begins, the pre-operational ship/shore procedures must be carried out, i.e. ship/shore information exchange, ship/shore safety check list.
The following information will be required to set the principle discharge plant parameters:
NOTE: THE COMPANY AND CHARTERER ARE ALWAYS TO BE ADVISED PRIOR TO USING A SHORE VAPOUR RETURN AS THIS MAY HAVE IMPORTANT IMPLICATIONS AFFECTING THE CHARTER PARTY OR OTHER COMMERCIAL CONSIDERATIONS. Three basic methods may be used to discharge cargo:
When discharging using the cargo pumps running in parallel, it should be noted that trying to increase the rate of discharge by running all the pumps may not result in the expected increase due to constraints in the overall system. Also, the increased energy imparted to the cargo is dissipated as heat and results in an increase in liquid temperature, which in turn increases the flash gas produced when the cargo discharges into the shore terminal. This may cause the terminal to ask for a reduction in the flow.
Observing the manifold pressure gauges will provide a good indication of whether or not the correct number of pumps is being used. The discharge rate must not be reduced by throttling the valves at the crossover if the shore cannot accept the discharge rate. This will only heat up the cargo. The principle method of flow control is by throttling the booster pump discharge or the main pump circulation or by a combination of the two. Control of flow solely by throttling the main pump discharge may cause loss of booster pump suction.
The required NPSH (Net Positive Suction Head) valve of the cargo pumps is relatively low due to the fitted inducer, and it is possible to unload down to a level of approximately 65cm. For further unloading, the capacity of the pumps must be throttled or the pressure of the gas pad above the liquid increased, otherwise the tank pressure can fall below atmospheric pressure causing the pump to stop. Replacement of the unloaded liquid in the tank by additional gas is normally not necessary, except for a cargo under atmospheric pressure, and this only at the end of the discharge.
Where cargo is being transferred into pressurised storage, it will almost certainly be necessary to warm the cargo on discharge.
Deep well and booster pumps are fitted with differential pressure switches which shut down the pumps if the pressure difference across the pumps and/or the outflow falls below set limits.
The maximum capacity when all deep well pumps are running is a rated figure which is unlikely to be achieved throughout a total discharge when pumping against shore pressure.
The upper pressure limit switch is set to approximately 22 barg and when this is reached, the quick closing valves on the cross over line will close, initiating the shutting down of both deep well and booster pumps. Therefore, it is necessary to set the maximum operating pressure to below 22 barg, using the pressure controllers between the pump suctions and discharged.
When running two deep well pumps in conjunction with a booster pump, the hand controlled discharge valve must not be throttled because this may have a detrimental effect on the suction pressure of the booster pump. Instead, the discharge valves on the deep well pumps are to be used to control the discharge from the two tank compartments.
To avoid cavitation or gassing-up of a pump when handling boiling liquids, the pressure at the pump suction must exceed the saturated vapour pressure (SVP) of the liquid by an amount termed the Net Positive Suction Head (NPSH). The required minimum NPSH, expressed as an equivalent head of liquid above the pump suction, may vary from 1 metre at maximum pump capacity to 200mm at practical reduced flow. If the vapour space pressure can be increased above the SVP by the supply of extra vapour from shore vapour return or shipboard vaporiser, the onset of cavitation as the liquid level approaches the bottom of the tank can be delayed. This procedure is most frequently used where maximum possible cargo outturn is required prior to gas freeing.
Before starting booster pumps, ensure that the combined pressure produced by the main plus booster pumps will not exceed the maximum system operating pressure, otherwise the pumps will be stopped automatically and the relief valves may lift.
VCM must not be unloaded with deep well and booster pumps running in series, because the high density of VCM may cause overpressure. The booster pumps on their own can be used to discharge VCM in emergency conditions.
Prior to commencing the discharge, all preparatory liaison with terminal staff is to have been completed, including communications systems, safety and emergency procedures, inspections and preparation of an agreed discharge plan. After the manifold valves are opened, the first pump should be started discharging back to the tank. When it is running satisfactorily the discharge valve can be opened slowly and the cargo discharged ashore. The other pumps can then be started in sequence.
If booster pumps are used to increase the discharge pressure they must be primed from a deep well pump. Booster pumps can be used to discharge at low rates of flow.
CARGO HEATING DURING THE DISCHARGE
When cargo is being transferred into pressurised storage, it may be necessary to warm the cargo on discharge; this means running the cargo booster pump and cargo heater in series with the main cargo pump. To operate the booster pump and heater, it is necessary to first establish the correct sea water flow through the heater.
Thereafter, to avoid thermal shocks, the pump and heater may be slowly cooled down prior to operation by carefully bleeding in liquid from the main cargo pump discharge. Once cooled down, the discharge valve can be opened until the desired outlet temperature is reached.
It is important to ensure that the main cargo pumps maintain adequate suction to the booster pump at all times
Cargo heating always carries with it the risk of freezing the heater circulating water. As well as checking the cargo outlet temperature and the booster pump suction during operation, attention is to be paid to the sea water inlet and outlet temperatures and pressure.
A minimum sea water temperature of +10 degree C is normally required to provide satisfactory cargo heating.
In the case of a lower temperature, it may be possible to achieve satisfactory heating by slowing the rate of discharge, but under these circumstances great care will be required if freezing of water in the heater tubes is to be prevented.
The heat exchanger is protected by a temperature switch which stops the flow of cargo if the sea water drops below +5 degree C.
Water for the heater is supplied by the condenser cooling water system. The main danger with cargo heating is water freezing in the heat exchanger tubes. This can be prevented by maintaining a high rate of sea water flow through the tubes.
The minimum discharge temperature of the cargo is set by the transmitter controller and limiting valve. Control of the cargo temperature above the minimum temperature is made by controlling the flow through the heater by pass line.
For cargoes with an inlet temperature at the heater warmer than -15 degree C, the sea water temperature must not be less than +5 degree C; for cargoes with a temperature colder than -15 degree C the sea water temperature is not to be less than +10 degree C.
On completion of cargo heating, the heater is to be drained back to a cargo tank. When it is confirmed that all liquid has totally evaporated the flow of seawater can be stopped.
CARGO TANK STRIPPING
The following procedures are used when the loading of a compatible cargo follows the discharge. With the deep well pumps running at full capacity, the tank can be emptied down to approximately 65cm from the bottom of the discharge well. At this level, the discharge is to be throttled at the discharge control valve, to maintain suction. Pressure above the liquid may be increased by shore supplied vapour or by using the compressors. When using compressors vapour can be drawn from an already discharged tank, or from the vaporiser.
When using this method to strip a tank containing VCM the pressure difference between the two tanks should be 2.5 bar, 1.5 bar when stripping propane and 1.7 bar for ammonia.
For a short time during the early stages of stripping, the compressed vapour will condense on the cold tank walls and tend to inhibit the stripping process, but as the tank walls begin to warm condensation stops and stripping proceeds.
A calculated minimum quantity of liquid is to remain in the tanks to keep the tanks cold during the ballast passage.
On completion of the cargo discharge and stripping, liquid must be drained from all deck lines, cargo hoses or hard arms. This can be done either from ship to shore using a cargo compressor, or from shore to ship, normally by blowing the liquid into the ship's tanks using nitrogen injected at the base of the hard area. Only after depressurising all deck lines is the ship/shore connection to be broken.
If, during this operation, liquid quantities in excess of the designed amount flow to the mast, a level switch will shut down the plant and close all hydraulic valves. The cause of this excessive flow must be investigated before attempting to re-start the plant.
STRIPPING FOR COMPLETE DISCHARGE
Stripping for complete discharge is required prior to changing between incompatible cargoes, when gas freeing and when specific instructions are given to avoid any mixing of previous and new cargoes. It is important not to under-estimate the quantity of vapour remaining in the tanks after total unloading of all liquid.
The tanks are to have pressure restored to above the SVP before the cargo pumps start to drain the tanks to delay the onset of pump cavitation. Pressure above the liquid is to be increased by introducing vapour from the vapour return line or by running the compressors. Vapour can also be drawn from an already emptied tank or from the vaporiser.
When using this method to strip a tank containing VCM the pressure difference between the two tanks should be 2.5 bar, 1.5 bar when stripping propane and 1.7 bar for ammonia. For a short time during the early stages of stripping, the compressed vapour will condense on the cold tank walls and tend to inhibit the stripping process, but as the tank walls begin to warm condensation stops and stripping proceeds. Final clearance of liquid will be accomplished by vaporising the remainders - this will also help to warm the tank walls.
Vapour is drawn from the tank, heated in the superheater section of the vaporiser and returned to the tank sump through the stripping line or the bottom distribution line. The warm vapour jet sprays onto the liquid collected in the sump which quickly vaporises. An increase in the temperature of the tank sump signals the completion of vaporisation.
THIS PROCEDURE MUST NOT BE HURRIED. During boiling off it is important to obtain a positive temperature reading on the sump thermometer. Any subsequent drop in temperature indicates that liquid may still be present hence it is important to monitor the temperature even after the flow of hot gas has stopped.
If there is insufficient cargo in the system to support this closed cycle operation, this will result in low pressure, and in this case a quantity of liquid cargo from ashore or the deck tank is to be vaporised and injected into the circuit. Inert gas or nitrogen can also be used to accelerate this procedure, but it must be remembered that the total contents of the tank, (nitrogen/inert gas and cargo vapour mix) will be lost.
When all liquid has finally been cleared from the tank the vapour sucked on by the compressors can be condensed and unloaded to shore or the deck tank.
On completion of the cargo discharge and stripping liquid must be drained from all deck lines, cargo hoses or hard arms. This is usually done from ship to shore using a cargo compressor. Only after depressurising all deck lines is the ship/shore connection to be broken.
Liquid quantities in excess of the designed amount flowing to the mast will cause a level switch to stop all machines and close all hydraulic valves. To restart the system, the drainings are to be drained to atmosphere via the bottom valve and a flexible hose, and allowed to evaporate or hosed with water.
One of the following procedures will follow the unloading:
Ice in suspension
With some cargoes ice can be held in suspension, and some ice can remain in the tank atmosphere after the discharge. This will cause little or no problem providing the tank temperature is maintained below 0 degree C on the ballast voyage.
Serious problems can occur if the tank temperature is allowed to rise above freezing point - ice held in suspension will melt and collect in the sump, this water will then refreeze in the sump when cooled below zero. This may cause problems with the gauging system.
Methanol can be injected into the pump discharge tube but this is not always completely successful. If the tanks are allowed to warm-up above 0 degree C the sump sampling line is to be used to drain off any accumulated water.
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