PREPARING A CARGO TANK ATMOSPHERE

5.7.1 General

For some cargoes the IBC Code requires vapour spaces within cargo tanks to have specially controlled atmospheres, principally when the cargo is either air reactive resulting in a hazardous situation, or has a low auto-ignition temperature, or has a wide flammability range. See Chapter 6 for guidance on

establishing the correct atmosphere in a tank, either inerting to prevent the formation of flammable mixtures of cargo vapour and air, or padding to prevent chemical reaction (between oxygen and the cargo.

It may also be necessary to reduce the humidity (dewpoint) of the atmosphere within the cargo system.

The extent of atmosphere control to protect the quality of the cargo will normally be specified by the cargo shippers. Some cargoes are extremely sensitive to commercial contamination or discoloration, and for quality control reasons are carried under a blanket of nitrogen that is very pure and which must often be obtained from shore.

5.7.2 Receiving nitrogen from shore

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.

Bit 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.

Figure 5.2 Damage in a cargo tank due to overpressurisation during purging with nitrogen

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 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.

Table 5.1 shows the volume of nitrogen that can be received in one minute through a known size of pipe at a known pressure. (The second figure in brackets indicates the associated hourly rate, which should be mentally compared to a liquid loading rate. Note that these tables are intended to be indicative only and any

discrepancies are due to rounding of figures.)

200mm (8") 150mm (6") 100mm (4") 50mm (2") 25mm (I")

1,286 (77,000) 662 (39,700) 243 (14,600) 48 (2,900) 9 (530)

2.1 bar (30 psi)

886 (53,000) 457 (27,400) 171 (10,300) 33 (2,000) 6 (360)

0.7 bar (10 psi)

471 (28,300) 214 (12,900) 80 (4,800) 16 (1,000) 3 (170)

Table 5 1

hour) 1 Cubic metres of

through hoses of Gas at various

sizes. gauge pressures received in 1 minute (and

Table 5.2 illustrates the time taken to receive gas into a tank at different pressures and hose sizes. The example used assumes a cargo tank of 1,250 cubic metres requiring four atmosphere changes, i.e. 5,000 cubic metres of nitrogen, to flow through.

5,000 cubic Metres of gas with various gauge pressures and

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.

5.7.3 Preparations/or receiving nitrogen from shore

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.