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Preparation for receiving nitrogen from shore : Chemical tanker procedure

It is a frequent practice at chemical loading ports to control the atmosphere in cargo tanks with nitrogen supplied from shore, for the purpose of drying a tank and its associated piping system, purging a tank before loading the cargo or padding cargo in a tank. The nitrogen may be supplied at high pressure (up to 10 bar) and at a high flow rate. Agreement on the procedure for handling the nitrogen is paramount, and should be part of the pre-loading checklist between ship and shore, with emphasis on a clear understanding of the transfer rate and pressure

Although the operation is an important stage in cargo handling, it is also potentially hazardous because high pressure gas is being introduced into a tank which is not designed to withstand internal pressure, and whose structure may fail at less than 0.5 bar overpressure. The associated risks of the operation should therefore be thoroughly understood. Procedures should be in place to ensure safety during the operation, and all personnel involved should be made conversant with those procedures.

It is possible to overpressurise and even rupture a cargo tank if the nitrogen supply from shore is at too high a flow rate or too great a pressure. There have been incidents where structural damage has occurred.

When a liquid is being loaded through the cargo manifold and pipeline system on a chemical carrier, the existing atmosphere in the tank can escape through a vent system that is notably smaller than the liquid filling line, because friction and turbulence are far greater impediments to liquid flow than to gas flow. Ships are designed with this in mind.

However, when a gas is being introduced through the liquid filling line, especially a gas under pressure that will expand within the tank, the same condition does not apply, and the disparate sizes between inlet and outlet can allow an overpressure to develop. To avoid such an eventuality, the outlet for the existing atmosphere in the tank should be as big as or bigger than the pipeline supplying the gas. That is usually achieved by having the cargo tank lid or a tank washing hatch open.

But when vapour control and emission regulations require a closed operation (with the existing tank atmosphere forced to exhaust to shore), the incoming flow of nitrogen must be restricted to a rate equal to or less than the maximum flow of vapour possible through the venting system. If the capacity of the vapour return system is exceeded by the flow of nitrogen into a closed cargo tank, then the only other outlet is through the relief valve, which will prevent overpressurisation (though contravening the vapour control regulations). However, if the capacity of both outlets is exceeded, then overpressure will occur and damage to the tank structure may follow.

The pressure and the flow rate of the incoming nitrogen must therefore be controlled. Use of a small hose or a reducer prior to the manifold will restrict the flow rate, but pressure must be controlled by the shore. A gauge will allow the ship to monitor the pressure. It is not appropriate to attempt throttling a gas flow by using the ship's manifold valve that is designed to control liquid flow. However, the manifold valve can and should be used as a rapid safety stop in an emergency: pressure surge in a gas is not as violent as in a liquid.

When shore supplied nitrogen is to be used for drying or purging an empty tank that has been cleaned and gas freed, the volume of nitrogen required should be calculated and agreed (tank volume multiplied by number of atmosphere changes needed to reach the desired level of dryness or oxygen exclusion), together with the flow rate, during the pre-transfer planning conference.

When a cargo is required to be carried under a pad of nitrogen, and it is necessary to use nitrogen supplied from shore, it is better to purge the entire tank before loading. After such purging is completed, loading the cargo in a closed condition will create the needed pad within the tank. The risk of overpressurisation can be substantially reduced by avoiding padding with shore supplied nitrogen as a separate procedure on completion of loading.

However, if padding with shore nitrogen has to be performed after loading, planning and good communication are essential. The supply should be through a small diameter connection to restrict the flow, and the rate must not exceed the vent capacity of the cargo tank.

The operation should be stopped when a slight overpressure exists in the ullage space, but which is less than the tank pressure relief valve setting. The vapour space in a loaded tank is usually small, so overpressurisation can occur very suddenly, especially if cargo is forced into the vent lines which then become restricted or blocked and add to the rapid increase in tank pressure.

Receiving nitrogen from shore - Ship safety checks

When preparing to receive nitrogen from shore special emphasis should be placed on the following points:

Ship and shore should agree in writing on the gas supply, specifying the volume required, the flow rate in standard cubic metres per minute, and the maxima in each case.

Care should be taken to ensure that the valves on the loading line between the shore manifold and the ship's tank are operated in the correct sequence, so that the ship is in control of the nitrogen flow. The ship should station a crew member at the loading manifold valve during the operation, even where remotely operated valves can be closed more quickly by a person in the cargo control room who is monitoring tank pressures. The crew member at the manifold is in the best position to react promptly to any other external indication of trouble.

Care should be taken to ensure that a tank to be dried or inerted has open vents with a greater flow rate capacity than the inlet, such that the tank cannot be overpressurised. This is usually achieved by having the cargo tank lid or a tank washing hatch open.

If local requirements for vapour control demand a closed venting of the tank through a vapour return line to shore, the nitrogen flow rate and pressure should not exceed the capacity of the venting system. Positive measures to ensure this should be agreed.

The tank pressure should be closely monitored during the operation.

As with all inert gases, there is a potential health hazard, and it is necessary to ensure that crew members are not unnecessarily exposed to vapour being vented from a tank while it is inerted or purged with nitrogen.

Special care is necessary when nitrogen as a gas is supplied to a ship directly from evaporating liquid nitrogen, sometimes delivered by a road tanker fitted with a vaporiser, because the volume and flow rate can be difficult to control and the agreed delivery figures may be unexpectedly and suddenly exceeded. The vaporisation ratio of nitrogen from its liquid form to its gaseous form is approximately 1:640. When any of this expansion is happening in the delivery pipeline the flow rate becomes uncontrolled, and it is the rapid expansion in volume that causes high pressures to be reached extremely quickly.

In general, nitrogen should not be delivered to the ship this way, and the ship should request that it is provided from gas held in a buffer tank. If a ship suspects that traces of liquid nitrogen are arriving at the manifold valve (possibly indicated by ice forming on the ship's lines and valves), or that other agreed procedures are not being followed, the operation should be suspended until the apparent problems have been satisfactorily resolved.

Related Info:

Purging requirement of chemical tank(s) with Nitrogen - safe method

Nitrogen blanketing - safety method

Supply of Nitrogen from shore - safety guideline

Failure prevention and maintenance guideline for inert gas systems onboard

Gas freeing safety precautions for chemical tankers

Supply of Nitrogen from shore - safety guideline

Isolation of cargo tanks and piping systems

Ship shore cargo connection safe method

Venting of cargo tanks safety procedure

Ship to ship transfer operation

Ship to ship transfer guideline using VEC systems

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