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Piping system failure and countermeasures- Ships machinery space guideline

A ship's machinery space contains hundreds of metres of piping and fittings. The various systems are arranged to carry many different liquids at various temperatures and pressures.



Causes of pipe failure

Pipes have a hard life: they carry abrasive and corrosive fluids; they are exposed to atmospheric corrosion and to general wear and tear; they sometimes operate at extremely high temperatures. The most common cause of pipe failure is corrosion-induced weakness.

Pipes corrode internally and externally. Internally, they may be affected by erosion, uniform and abrasive corrosion, fatigue and galvanic action. Externally, corrosion is caused mainly by atmospheric conditions, but pipes can corrode locally where liquids drip onto them.

pipes-corrosion
Fig: Pipes corrosion

Uniform corrosion

Uniform corrosion is the most common form of attack on metal. Its aggressiveness depends on relative humidity, temperature, oxygen content and salt content. It is widespread in pipes carrying saltwater. Pipes on deck, in locations prone to wetting, in bilges and in ballast tanks, as well as pipe supports are at risk of uniform corrosion.

It is a good policy to replace a pipe when the corrosion measured is equal to or greater than the design allowance. Accelerated corrosion can occur in steam piping fitted to the decks of tankers. If pipes are insulated, and the insulation gets and stays wet, any corrosion is accelerated.

Pitting corrosion

Pitting corrosion is defined as the localised breakdown of the inert surface layer that protects metal against the formation of cavities or small diameter holes in the material. Such corrosion can occur in mild steel and stainless steel. It has a random pattern, as the formation of a pit is dependent on the breakdown of a pipe’s protective film. Pitting happens more readily in a stagnant environment.

The Oil Companies International Marine Forum’s Guidance Manual for the Inspection and Condition Assessment of Tanker Structures contains pitting intensity diagrams for plates, and these can be used to categorise the extent of pitting. As a general rule, any badly pitted pipe needs replacing.

Abrasion and erosion

Abrasion and erosion are the wearing away of material by a fluid flow. Material that has been abrasively corroded or eroded looks pitted. To determine whether material has been lost by either abrasion or erosion, it is necessary to examine the processes involved in both.

Abrasion happens when solid particles, such as sand, suspended in a fluid flow scour a pipe. It is therefore a mechanical process. If the oxidised surface protecting a pipe’s base metal is abraded by such flows, uniform corrosion or pitting can result. The main characteristic of abrasion is the appearance of cracking in the direction of flow. Filters are fitted in ballast and bilge lines to prevent debris from being sucked into a pipe.

A slower than expected pumping rate may indicate that filters are clogged and that they need cleaning. Worn or damaged filters must be replaced. Erosion is caused when turbulent fluid flows hit a pipe’s inner surface; it is most common at points where a pipe bends and at elbows where fluid flow changes direction, or where an orifice, valve, welded joint or any other blockage impinges on fluid flow to cause turbulence.

Prevention of turbulence is the key to prevention of erosion. The use of larger diameter pipes, together with a reduced pumping rate, can eliminate flow turbulence and erosion.

Fatigue damage

Fatigue damage is the rapid deterioration of metal, the results of which are cracking and collapse. It is caused by cyclical mechanical stress, or when pipes are connected to machinery or other pipes that vibrate.

Galvanic corrosion

Galvanic corrosion is the electro-chemical process between different metals. It is most common where pipes connect to equipment made from a different metal and where there is an electrically conductive path between the metals through an electrolyte.

Graphitic corrosion

Cast iron pipes and fittings are affected by graphitic corrosion that is most commonly found at bends and elbows, locations where boundary layers cause water to flow at different velocities, or where water accumulates.

Graphitic corrosion attacks the inside of a pipe by oxidation and leaching of iron. It results in the formation of rust supported by graphitic flakes. The process occurs over a period of time and, if the pipe is not replaced, will continue until the pipe weakens and eventually fails, usually catastrophically.

The risk of failure through graphitic corrosion can be reduced by:

Water hammer

Water hammer can affect any pipe but is most common in steam pipes. It is a problem in pipes where internal condensation occurs. Water hammers are impulse pressures that happen when steam enters a cold pipe containing a small amount of water. The resulting stresses, along with possible rapid expansion, can cause pipe joints to fail. Prevent water hammers by draining fluid from pipes before injecting steam gradually.

Steam systems are most prone to damage by water hammer because they operate at high temperature and pressure, and because condensed steam will remain in them, unless regularly drained.

Steam heating coils on tankers are particularly susceptible to damage by a water hammer.

Pipe alignment

Irregular stress affects pipes that are forced into alignment. If they have been weakened by corrosion, stresses caused by thermal expansion or impulse loading, the pipes will fail. Forcing pipes into alignment is bad engineering practice. Failures are most likely at flange connections or valves.

Low temperature

Very low temperatures cause water to freeze and to expand in uninsulated pipes. In cold conditions, high-viscosity or solidifying substances will become difficult to pump because of their tendency to constrict the flow in pipes. Care must be taken to avoid over-pressurising the pipe in an attempt to increase flow. It may be necessary to add anti-freeze to a pipe system, or to arrange external heating, if conditions get really cold.

Expansion

Metallic pipes expand and contract as the temperature changes. A ship’s movement will cause them to stretch and bend, and unless these stresses are absorbed by an expansion joint, pipes can fail. Bulkheads pierced by pipes present special problems. The bulkhead’s strength has to be maintained and the stresses resulting from a pipe’s movement have to be absorbed. If the bulkhead forms part of a fire zone, insulation has to be repaired or replaced to ensure that fire integrity is not compromised.


Dealing with pipe failure

It is not always practicable to examine every pipe on a ship, which means that pipe failure is always a possibility. If a pipe fails, the following action should be taken:
  1. Switch off relevant pumps; isolate the affected section of pipe by closing valves or by fitting blank flanges.


  2. Investigate the source of the leakage and make a temporary repair by binding or clamping. At the first opportunity, have the pipe repaired or renewed by a specialist repair shop.


  3. Avoid getting electrical equipment wet. If electrical equipment is wet, take care to avoid electric-shock hazards. Switch off electrical equipment.


  4. If there is leakage from a fuel, lubricating or hydraulic pipe, use absorbent material to soak up the loose oil. Oil is both a safety and a pollution hazard.


  5. Fuel spraying from a fractured pipe into an enclosed space, or on to a hot surface, is an extremely dangerous fire hazard.


  6. If there is leakage from a fractured steam pipe, evacuate the boiler area to avoid the risk of personal injury. This type of leakage can be extremely hazardous, especially if the steam is superheated because superheated steam is invisible and therefore difficult to detect. Test for a steam leak with a piece of cloth on the end of a pole, the cloth will flutter in the jet stream.


  7. If ballast piping on an oil tanker fails, exercise caution before pumping ballast into the sea because the ballast could be contaminated with oil. Check the surface of segregated ballast for oil before beginning discharge.


  8. If cargo or fuel pipes on a tanker fail, be very wary of pumping ballast into the sea because the fuel or cargo pipe might pass through a ballast tank. If fuel becomes contaminated with water, then pump it into a settling tank and purify it before using.


  9. If cargo piping failure happens on a gas carrier or on a chemical tanker, take measures to avoid vapour inhalation or skin contact. A significant escape of flammable gas from cargo piping presents an extreme explosion hazard, especially if the gas gets into the accommodation.


  10. Failure of a pipe that connects directly to the ship’s shell can result in serious flooding, especially if the connection is located below the waterline because most ships are not able to survive engine room flooding. It is therefore essential to make sure that both suction and discharge valves can be closed, and that they are leak-free.





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