Static Electricity

In document DCOM Petroleum & Chemical (Page 136-144)

Cargo Operations - -General

4. Static Electricity

4. Static Electricity

Static electricity precautions are necessary on vessels carrying static accumulator cargoes in non-inert tanks.

Static electricity precautions are not necessary under the following conditions:

• If cargo tanks are inerted;

• If it can be guaranteed that the atmosphere in the tank is non-flammable;

• If static non-accumulator cargoes are being carried;

• For ullaging if vapour locks are fitted with full depth sounding pipes.

 

4.1 Electrostatic Charge Generation

Static electricity presents fire and explosion hazards during the handling of petroleum and during other operations such as tank cleaning, dipping, ullaging and sampling. There is no risk of ignition unless a flammable mixture is present, or if the compartment is protected by inert gas.

Risk of the generation of an electrostatic charge and thus an incendive spark may occur in several ways, amongst them:

On the surface of the cargo:

• The flow of liquid petroleum through pipework;

• The flow of liquid through fine filters (less than 150 microns). This has the ability to charge fuels to a very high level, as a result of all the fuel being brought into intimate contact with the filter surface, where charge separation occurs;

• Contaminants, such as water droplets, rust, or other particles, moving relative to oil as a result of turbulence in the oil as it flows through pipes;

• The settling of a solid or an immiscible liquid through a liquid (e.g. water, rust, or other particles through petroleum). This process may continue for up to 30 minutes after the completion of loading into a tank;

• Gas bubbles rising up through a liquid (e.g. air, inert gas introduced into a tank by the blowing of cargo lines or vapour from the liquid itself released when pressure is dropped).

This process may also continue for up to 30 minutes after completion of loading;

• The splashing or agitation of a liquid against a solid surface (e.g. tank cleaning, or the initial stages of filling a tank with cargo).

Within the compartment:

• The ejection of particles or droplets from a nozzle (e.g. during tank cleaning, steaming, or the injection of inert gas);

• Turbulence and splashing in the early stages of loading oil into an empty tank. A mist can form above the liquid;

• The development of a charged mist during tank cleaning.

On equipment lowered into a tank:

• The vigorous rubbing together and subsequent separation of certain synthetic polymers (e.g. the sliding of a synthetic rope through gloved hands);

• The actual act of lowering equipment into a tank.

In summary there are three main ways in which an electrostatic charge can generate an incendive spark:

• The cargo surface can develop a charge. A spark can then pass from the surface to the equipment being lowered into the tank. In such cases, bonding of the equipment can facilitate the development of a spark;

• Unbonded equipment can generate a charge as it is lowered into the tank. A spark can then pass from that equipment to the surface of the cargo;

• A charge can develop within a mist in the tank, such as when tank cleaning.

 

4.2 Bonding

Bonding is an essential precaution for preventing electrostatic charge accumulation and its importance cannot be over-emphasised. However, while bonding facilitates relaxation, it does not prevent the accumulation and production of hazardous voltages. Bonding must not therefore be seen as a universal remedy for eliminating electrostatic hazards.

Examples of objects which might be electrically insulated in hazardous situations and which must therefore be bonded are:

• Ship/shore hose couplings and flanges, except when an insulating flange or a single length of non-conducting hose is used to provide electrical isolation between the ship and shore;

• Portable tank cleaning machines and hoses;

• Manual ullaging and sampling equipment;

• The float of a permanently fitted ullaging device if its design does not provide an earthing path through the metal tape.

The best method of ensuring bonding and earthing will usually be a metallic connection between the conductors. Any earthing or bonding links must be established whenever the equipment is set up and must not be disconnected until after the equipment is no longer in use. For example, portable tank cleaning machines and hoses must all be connected together and to the tank washing main before being introduced into the compartment; UTI tapes must be effectively bonded before the tape is lowered into the tank.

4.3 Portable Tank Washing Machines and Hoses

The outer casing of portable machines should be of a material that will not give rise to an incendive spark upon contact with the internal structure of a cargo tank.

The coupling arrangement for the hose should be such that effective bonding can be established between the tank washing machine, the hoses and the tank washing water supply line.

Washing machines should be electrically bonded to the hose by means of a suitable connection or external bonding wire.

When suspended within a cargo tank, machines should be supported by means of a natural fibre rope and not by means of the water supply hose.

Bonding wires should be incorporated within all portable tank washing hoses to ensure electrical continuity.

Tank cleaning hoses should be indelibly marked to allow identification.

Tank cleaning hoses should generally be tested for electrical continuity prior to each use if such use is infrequent. If hoses are in regular use then they should be tested at least once per week or whenever there is evidence of damage. In no case should the electrical continuity exceed 6 ohms per metre length.

The record ECF80 - Tank Cleaning Hose Condition and Continuity Test must be maintained of the testing of tank cleaning hoses, and the results.

It is important that hoses and portable tank cleaning machines are connected together before the machine is introduced into the compartment to be washed. Connections must not be broken whilst the machine is still in the tank.

Portable tank washing machines must not be introduced into a tank containing a flammable atmosphere until the tank has been ventilated to 10% LFL or less.

Particular care must be taken when using portable tank cleaning equipment if the vessel is experiencing rolling that the machines are not allowed to strike the tank structure whilst suspended within the tank.

4.4 Vapour Locks and UTI Tapes

Whenever UTI tapes are used with vapour locks the tape must be bonded to the vapour lock.

Although not strictly necessary on cargo tanks which are either inerted, or which contain non-static accumulator cargoes, or where full-depth sounding pipes are fitted, Columbia require UTI tapes to be bonded on every occasion before being introduced into cargo tanks.

Bonding may be achieved in two ways:

• The UTI tape may be of a design which incorporates internal bonding;

• An external bonding wire may be supplied which should be properly connected before the tape is introduced into the tank.

Where internal bonding is provided the continuity of such internal bonding must be checked at periods in accordance with the manufacturers’ instructions but in any case at intervals not exceeding 6 months. The record ECF63 - UTI Tape Condition and Bonding Check must be maintained.

Where full depth sounding pipes are not fitted below vapour locks or any other ullaging or sampling aperture, a relaxation period of at least 30 minutes must be allowed after loading the tank has ceased before introducing any portable equipment or UTI tape into a tank containing a static accumulator product.

This requirement applies when carrying static accumulator products, or any product which is flammable, or any non-volatile cargo being carried at a temperature within 10oC of its flashpoint.

4.5 Synthetic Ropes

The use of synthetic rope, for example for ullaging, sampling, dipping or portable tank cleaning equipment, is not permitted at any time.

4.6 Precautions with Static Accumulator Cargoes in Non-Inerted Tanks

These precautions apply to vessels handling static accumulator cargoes in tanks which are not inerted. No precautions are necessary if the tank is inerted or if static non-accumulator oils are being handled.

4.6.1 Static Accumulator Oils

The definition of a static accumulator product is one with an electrical conductivity less than 50 picoSiemens/metre (pS/m) so that it is capable of retaining a significant electrostatic charge.

A static accumulator product may carry sufficient charge to constitute an incendive ignition hazard during loading into the tank, and for up to 30 minutes after completion of loading.

All petroleum products should be considered to be static accumulators with the exception of the following:

• Fuel with an anti-static additive;

• Heavy black fuel oils;

• Conductive crude;

• Bitumen;

• Alcohols;

• Ketones.

Known static accumulator chemicals are as follows:

• Alcohol;

• Cumene;

• Cyclohexane;

• Diethyl ether;

• Heptane;

• MTBE;

• Nonene;

• Octane;

• Styrene;

• Toluene;

• Xylene.

Where there is any doubt as to whether a product is a static accumulator or not it must be assumed that it is and the necessary precautions observed.

4.6.2 Precautions against Electrostatic Discharge

There is a possibility of electrostatic discharges whenever equipment is lowered into cargo tanks.

The discharges may come from charges on the equipment itself or charges already present in the tank, such as on the surface of the liquid, or on water or oil mists.

If there is any possibility of the presence of a flammable hydrocarbon gas/air mixture, then precautions must be taken to avoid incendive discharges.

When handling static accumulator cargoes in non-inert tanks:

• Metallic equipment should always be bonded;

• Metallic equipment which has been bonded must not be introduced into a tank during loading and for 30 minutes thereafter, unless a sounding pipe is used;

• Metallic equipment which has been bonded may be introduced into a tank whilst tank washing. Metallic equipment which has not been bonded must not be introduced into a tank whilst tank washing and for 5 hours thereafter, unless a sounding pipe is used. If the tank is continuously ventilated then the period is reduced to 1 hour.

4.6.3 Initial Filling of a Cargo Tank

In order to control electrostatic generation during the initial stages of loading the velocity of oil entering into the tank must be restricted to 1 metre/second until the tank inlet is well covered and all splashing and surface turbulence in the tank has ceased.

The 1 metre/second limit applies in the branch line to each individual cargo tank and should be determined at the smallest cross sectional area including valves or any other piping restrictions.

The reasons for such a low linear velocity as 1 metre/second are threefold:

• At the beginning of filling a tank, there is the greatest likelihood of water being mixed with the oil entering the tank. Mixtures of oil and water constitute the most potent source of static electricity;

• A low product velocity at the tank inlet minimises turbulence and splashing as the oil enters the tank. This helps reduce the generation of static electricity and also reduces the

dispersal of any water present, so that it more quickly settles out to the bottom of the tank where it can lie relatively undisturbed when the loading rate is subsequently increased;

• A low product velocity at the tank inlet minimises the formation of mists that may accumulate a charge, even if the oil is considered not to be a static accumulator. This is because the mist droplets are separated by air, which is an insulator. A mist can result in a flammable atmosphere even if the liquid has a high flash point and is not normally capable of producing one.

4.6.4 Spread Loading Static Precautions

Spread loading presents a number of potentially significant static generation risks which must be assessed and properly managed. For example:

• Uneven flow in the ship’s cargo lines can create a back flow of vapour (gas or air) from other open tanks to the tank receiving product. This eductor effect will create a two phase mixture of product and vapour which will result in increased turbulence and mist formation within the tank;

• The possibility of exceeding 1 metre/second product velocity at a tank inlet due to poor distribution of product amongst the open tanks;

• The overall loading rate should be selected so as to ensure a maximum product velocity of 1 metre/second into any one tank, assuming even distribution of cargo between tanks;

• Possible different flow distributions into different tanks should be considered and best efforts should be made to ensure equal flow distribution between cargo tanks;

• Not more than four cargo tanks should be loaded at any one time;

• Tank inlet valves should not be used to control cargo flow in the initial loading phase. Their use will reduce the cross sectional area of the inlet, resulting in increased tank inlet velocity and greater turbulence and mist formation.

4.7 Sample Bottles, Cages and Lines, Sounding Rods and Lines

Non-conducting and intermediate conducting materials may be acceptable in some circumstances, for example plastic sample bottle holders can be lowered safely with natural fibre (intermediate conductivity) rope. Synthetic rope generates significant static charge when sliding rapidly through an operator’s gloved hand and must not be used for cargo operations at any time.

A material of intermediate conductivity such as wood or natural fibre, generally has sufficient conductivity as a result of water absorption to avoid the accumulation of electrostatic charge. At the same time these materials have low enough conductivity that instantaneous release of a charge is not possible.

4.8 Re-Circulated Wash Water and Water Mists

The spraying of water into tanks, for instance during water washing, gives rise to electro-statically charged mist. This mist is uniformly spread throughout the tank being washed. The electrostatic levels vary widely from tank to tank, both in magnitude and in sign – either positive or negative.

When washing is started in a dirty tank, the charge in the mist is initially negative, reaches a maximum negative value, then goes back through zero and finally rises towards a positive equilibrium value. Among the many variables affecting the level and polarity of charging, the characteristics of the wash water and the degree of cleanliness of the tank have the most significant influence.

Under no circumstances must recirculated washing water be used for washing cargo tanks which are not inerted and which have not been cleaned and gas freed, regardless of the type of cargo.

Recirculated water may be used either from a slop, residual or cargo tank:

• In non-inert cargo tanks provided the washing water is clean and the cargo tanks have been cleaned and gas freed;

• In inerted cargo tanks at any time provided that the conditions for washing in an inert atmosphere are complied with.

4.9 Steaming of Cargo Tanks

Steaming of cargo tanks can present significant risks with respect to the generation of electrostatic charges and must under no circumstances be carried out in any compartment which has carried an Annex I cargo without the Company first being consulted. If steaming should be required then the procedures in Cargo Operation Chemical, Chapter 8: Tank Cleaning – Chemical (8.13) should be complied with.

4.10 Free Fall of Liquid

Free fall is defined as the unrestricted fall of liquid into a tank. It is essential to avoid the free fall of liquids into a non-inerted tank which contains a flammable atmosphere.

Loading or ballasting from the top - overall - delivers charged liquid to a tank in such a manner that it can break up into small droplets and splash into the tank. This may produce a charged mist as well as an increase in the petroleum gas concentration in the tank.

4.11 Maximum Flow Rates

Flow rate is the linear velocity of flow of liquid in a pipeline, usually measured in metres per second (m/s). The determination of the flow rates at locations within cargo pipeline systems is essential when handling static accumulator cargoes in non-inert tanks.

4.11.1 Flow Rates in Loading Lines

A number of loading rates need to be determined for each cargo tank according to whether carrying static accumulator or non-static accumulator cargoes. These loading rates will be dependent on the maximum flow rates in the cargo lines for different products.

The following flow rates must be calculated for each section of the cargo system, and displayed in the Cargo Control Room:

• A loading rate based on a linear velocity of 1 metre/second at the tank inlet for the initial loading rate for static accumulator cargoes into non-inerted tanks;

• A loading rate based on a linear velocity of 7 metres/second for bulk loading static accumulator cargoes into non-inerted tanks;

• A loading rate based on a linear velocity of 12 metres/second for loading non-static accumulator cargoes and also for loading static accumulator cargoes into inerted tanks.

This velocity is provided for guidance only and is generally considered as a rate above which pipeline erosion may occur at pipe joints and bends. 

Where a number of tanks are loaded through a common manifold, the maximum loading rate may be determined by the flow rate through the manifold or drop lines. For this reason, it is important that a constant check is kept on the number of cargo tank valves that are open simultaneously and that a suitable loading rate is determined for the particular loading operation.

An offshore floating hose supplied in accordance with OCIMF Guidelines and having a nominal diameter of less than 400 mm is suitable for continuous operation at a flow velocity of 21 metres/second. Offshore floating hoses having a diameter greater than 400 mm are suitable for continuous operations at a flow velocity of 15 metres/second. However, the maximum loading rate may be controlled by the size of the ship’s loading line inboard of where the hose is connected.

5. Stability

5.1 Requirements for Stability, Stress and Bending Moments

All vessels are supplied with stability data. This data occasionally includes restrictions in the way the vessel may be loaded and Masters and Deck Officers must be aware of any such restrictions.

These might include:

• The maximum number of slack tanks at any one time;

• Maximum filling capacities;

• Filling restrictions. Occasionally, in order to reduce sloshing effects, there are restrictions on how much a tank can be loaded to;

• A lack of GM under certain conditions of loading.

If assumptions and estimates have to be made when calculating stability, they should be made with caution.

The company requires all vessels to comply with SOLAS requirements of a minimum GM of 0.15 metres, with all additional dynamical stability criteria set out in the Trim & Stability Booklet for each particular vessel, and to maintain stress limits and bending moments within the class approved parameters at all times. If for any reason operational requirements may result in not being able to maintain these limits then the company must be immediately informed.

Under no circumstances may a vessel sail from a port with the sheer forces and bending moments exceeding the seagoing limits, with a GM of less than 0.15 metres, or with any other minimum stability criteria not being met.

There are vessels which might conceivably have an issue with a GM of less than 0.15 metres. On

There are vessels which might conceivably have an issue with a GM of less than 0.15 metres. On

In document DCOM Petroleum & Chemical (Page 136-144)

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