Closed loading guideline for various noxious liquid chemicals in bulk- Chemical Tanker Solutions

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Closed loading guideline for various noxious liquid chemicals in bulk- Chemical Tanker Solutions

Chemical tankers operate under a closed loading program. The aim of closed loading is to keep gases clear of cargo deck. The cargo is not visible to the crew at stage of loading. The use of vapour return line prevents any gas release to the atmosphere. Special precautions are necessary onboard a chemical tanker during closed loading of various grade liquid chemicals. Tank over / under pressurization can lead to serious accidents that can lead to ship out of commision for several months.

As the cargo enters the tank vapours are expelled via the primary means of venting such as PV valves. Failure of this primary means of venting may lead to serious accidents. SOLAS requires all tankers to be equipped with a secondary means of protection.

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Closed loading/discharge enables loading or discharging with securely closed ullage, sounding and sighting ports. Additionally the venting must be controlled. Vessels equipped with a system such as Skarpenord (pressure gauges in the tanks) or radar ullage systems shall at all times carry out closed loading/unloading procedures for all cargoes. Closed loading should be used at all times unless not possible due to the design of the vessel or trade practices (e.g. vegetable oil trade loading over the top is normal.)

For gauging e.g. ullaging and sounding closed devices must be used. The level alarm systems must be operated during the entire closed cargo operation. Closed cargo operations must be stopped as soon as any essential system for safe loading or discharging becomes inoperative. Sampling to be carried out with closed sampler whenever possible. When more than one grade of cargoes is loaded, use of same sampler for different grades will contaminate the cargo sample unless the sampler has been thoroughly cleaned.

Cargo tanks over pressurization fatality

Fig: Cargo tanks over or under pressurization fatality

Gauging, sounding and sampling

A closed gauging device penetrates the cargo tank, but is part of a closed system and prevents the cargo or its vapour being released. Examples are the float-type systems, radar systems, electronic probe, magnetic probe and protected sight-glass.

For sampling and sounding, the Dovianus or Hermetic portable gauging and sampling systems may be used. It is important that sufficient of these devices are carried onboard and maintained in a fully operational and certified calibrated condition. The vessel must fully comply with ISGOTT “Measuring and Sampling Non-Inerted Tanks” and ISGOTT “Measuring and Sampling Inerted Tanks” as applicable.

Vapour locks, where fitted, are to be calibrated and certified by a recognised cargo inspection company which will also approve the datum level corrections including list and trim corrections for tank volumes. The approval certificate is to be readily available during cargo surveys.

Cargo tank venting

Controlled venting must be established if closed cargo operations are required. A controlled tank venting system is a system with pressure and vacuum-relief valves (P/V-valve) fitted on each tank in order to limit the pressure or vacuum in the tank. The P/V valve should operate in such a manner that neither pressure nor vacuum is created in the cargo tank during cargo operations that exceed the tank design parameters.

Secondary venting system must also be operational Information on maximum loading rates and venting capacities is to be readily available and displayed in the cargo control room. Deficiency with the ships venting system can lead to serious accidents and tank damage. Therefore it is important to make them free of defect at all stage. As per SOLAS II-2 part C cargo tanks must be protected against pressure or vacuum.

Vapour return

Vapour return should be used in accordance with IBC code, local, terminal or customer’s requirements. Very toxic and volatile cargoes should, if the shore is equipped with a vapour return, be loaded with vapour return in order to avoid vapour release into the atmosphere. VECS manual must be complied with.

Precaution against liquid raising in the venting system

High level alarms can be fitted to guard against liquid raising in the venting system
Overflow control device
Independent gauging device

Pressure sensor are fitted in each cargo tank which provide alarm facility to be activated in case of over /under pressurization of cargo tanks. Customers’ requirements

Specific closed cargo operations may be required by the customers under the charter party or Contract of Affreightment (COA).

Reference publications

IBC Code / BCH code
International Safety Guide for Oil Tankers and Terminals (ISGOTT)
ICS Chemical Tanker Safety Guide
Ship’s “Procedure and Arrangements Manual” (Approved by Class)
Certificate of Fitness for the Carriage of Dangerous Chemicals in Bulk
Ship’s “VEC System Operational Manual” (Approved by Class)
Ship to Ship Transfer Guide (Petroleum)
Tank Cleaning Manual

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