Critically Important Piping Systems
‘When do you replace a pipe?’ an operator is asked. ‘When it bursts’, he replies strikingly. So goes a tale of caution in shipping. Tales aside, such bursting is a painfully grave business.
Some years ago, a cargo ship foundered because the engine was stopped to deal with a burst seawater cooling-pipe in the engine room. Damages of $1million were later paid as compensation to the four dead crew members. Preventive inspection and repairs would not have cost more than some hundred dollars.
Pipes form the invisible ‘steel frame’ for a ship’s operating systems conveying fluids and air to and from the equipment, machinery, and spaces. They go through most enclosed spaces, the weather deck, and the shell. Impaired pipes therefore possess unparalleled destructive potential i.e. total ship loss, fire, floods, and pollution.
Piping Systems @ Ships
Piping includes pipes, flanges with gaskets and bolts, flexible hoses, valves, pump housings, and supports. Piping systems include piping and secondary elements connected to piping viz. tanks, heat exchangers, pumps, and evaporators. Main components viz. boilers, engines, and turbines are not a part of the piping system.
Most ship pipes are made from mild steel and other ferrous material that can corrode. Supervision and maintenance are momentous considering the number of pipes and the inaccessible areas they go through.
Classification societies determine the design and installation norms for piping systems viz. materials, strength, pipe routes, testing, and inspections. Inspection criteria are different for tankers and cargo ships.
Onboard piping systems:
• Hull Piping Systems for:
- Fire Fighting
• Machinery Piping Systems for:
- Engine Cooling
Pipe Failures: Causes & Prevention
Pipes fail mainly due to corrosion-induced vulnerability. Erosion, fatigue, and corrosion – abrasive, uniform, and galvanic, attack pipes internally while atmospheric corrosion acts externally. Proactive inspection, maintenance, and repair are the only viable alternative. Causes:
• Corrosion is the gradual deterioration of materials caused by their environment:
- Uniform Corrosion is uniformly distributed over pipes and is the most common form of metal corrosion. It intensifies with temperature, humidity, and salt and oxygen content of water. Steam and seawater pipes are particularly prone.
- Pitting Corrosion occurs in stagnant ambiences and causes random, localized pits/cavities when the protective layer over pipes breaks down. Stainless and mild steel are particularly vulnerable. It is more hazardous than uniform corrosion as it is tougher to detect, forecast, and design against
- Galvanic Corrosion results when the pipe made of more-active metal corrodes because of its contact with another, less-active metal through an electrolyte
Graphitic Corrosion is inconspicuous corrosion that affects cast iron. Iron is removed leaving graphite intact particularly at bends where water gets left behind. It is lethal because pipes lose strength without dimensional change
• Fatigue Damage results from repetitive mechanical stresses such as vibrations caused by machinery and/or other pipes
• Water Hammer is the generated pressure-wave when steam enters pipes with condensed, cold water. Rapid expansion can cause pipe failure. Any sudden change in velocity or direction of flowing fluids also causes water hammer
• Abrasion and Erosion result from fluid flow. Minute debris in the flowing fluid cause abrasion giving the pipe a pitted look. Turbulent fluid flows erode internal pipe surfaces particularly where fluids change direction
• Faulty Pipe Alignment cause failure at valves and flange connections
• Extreme Temperature Ranges cause pipe-weakening contraction-expansion
•Use seawater-tolerant duplex stainless steel
• Employ painted, plastic-coated, or galvanized pipes or those made from non-corrosive metals
• Replace excessively-pitted pipes and those with uniform corrosion at levels above or equal to design allowance
• Sacrificial pipe-sections made of more-active metals control galvanic corrosion
• Inspect cast-iron pipes regularly and prevent stagnant water conditions to restrict graphitic corrosion
• Regularly replaced pipe filters prevent abrasion
• Lower pumping rates and use large diameter pipes to nullify erosion-causing turbulence
• Bellows/Compensators slash fatigue damage
• Draining fluid from steam pipes before operations eliminates water hammer
• Don’t use doublers to repair shell plating
• During maintenance, complement visual testing with pressure tests to check:
- corrosion-accelerating leakages especially below steam-pipe insulations
- pipes exposed to wet, damp areas
- corrosion-indicating green color on copper pipes
- fretting-indicators viz. rust and black-dust around clamps
- damaged pipe-coatings
- wall thicknesses of pipes
- threaded joints between pipes of dissimilar metals for galvanic corrosion
- bellows for deformation
- pipe insides where they connect with equipment for erosion
- bends and supports for fatigue damage
Known perils are bad, hidden pitfalls are worst. Voluminous reports and lengthy discussions on hull damage make us aware of the destructive potential of a damaged hull. Damaged piping systems can be more catastrophic.