Sunday, July 7, 2013

More notes on Marine, Electrical and Piping technical know-how for marine engineers

At any one time in a rig or vessel's machinery space there will be a considerable variety of liquids on the move. The lengths of pipework will cover many kilometres, the systems are often interconnecting and most pumps are in pairs. The engineer must be familiar with each system from one end to the other, knowing the location and use of every single valve. The various systems perform functions such as cooling, heating, cleaning and lubricating of the various items of machinery. Each system can be considered comprised of a set of pumps, piping, valves and fittings, which could be controlled or bypass if they are redundancy in place in the design. 

A rig or vessel'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. The influences of operational and safety requirements, as well as legislation, result in somewhat complicated arrangements of what are a few basic fittings. Valves, strainers, branch pipes, etc., are examples of fittings which are found in a pipe system.
Pipes Machinery space pipework is made up of assorted straight lengths and bends joined by flanges with an appropriate gasket or joint between, or very small-bore piping may use compression couplings or socket welded. The piping material will be chosen to suit the liquid carried and the system conditions. Where piping is to be galvanised, the completed pipe with all joints fully welded is to be hot dipped galvanised or in some cases the welded joints could be cold galvanised touch up if the quality and standard permit. 

Mud boxes are fitted into the machinery space bilge suction piping. The mud box is a coarse strainer with a straight tailpipe down to the bilge. To enable the internal perforated plate to be cleaned when necessary, the lid of the mud box is easily removed without disconnecting any pipework.
Suction pipes in tanks should be arranged with a bell mouth or foot. The bell end or foot should provide an inlet area of about one-and-a-half times the pipe area. It should also be a sufficient distance from the bottom plating and nearby structure to provide a free suction area, again about one-and-a-half times the pipe area. 

The bilge main is arranged to drain any watertight compartment other than ballast, oil or water tanks and to discharge the contents overboard. The number of pumps and their capacity depend upon the size, type
and service of the vessel. All bilge suctions must be fitted with suitable strainers, which in the machinery space would be mud boxes positioned at floorplate level for easy access. A vertical drop pipe would lead down to the bilge.  The emergency bilge suction or bilge injection valve is used to prevent flooding of the ship. It is a direct suction from the machinery space bilge which is connected to the largest capacity pump or pumps. An emergency bilge pump is required for passenger ships but may also be fitted as an extra on cargo ships. It must be a completely independent unit capable of operating even if submerged. 

The ballast system is arranged to ensure that water can be drawn from any rig ballast tank or the sea and discharged to any other tank or the sea as required to trim the vessel. Combined or separate mains for suction and discharge may be provided. Where a tank or cargo space can be used for ballast or dry cargo then either a ballast or bilge connection will be required. The system must therefore be arranged so that only the appropriate pipeline is in service; the other must be securely blanked off.

Domestic water systems usually comprise a fresh water system for washing and drinking and a salt water system for sanitary purposes. Both use a basically similar arrangement of an automatic pump supplying the liquid to a tank which is pressurised by compressed air. The compressed air provides the head or pressure to supply the water when required. The pump is started automatically by a pressure switch which operates as the water level falls to a predetermined level. The fresh water system has, in addition, a calorifier or heater which is heated, usually with steam. 

Some of the marine equipment in the machinery space is dedicated to servicing the ship in general and providing amenities for personnel or passengers. Thus the bilge system is available to clear oil/water leakage and residues from machinery and other spaces as well as to provide an emergency pumping capability. The domestic water and sewage systems provide amenities for personnel

Bilge systems and oily/water separators
The essential purpose of a bilge system, is to clear water from the ship's 'dry' compartments, in emergency. The major uses of the system, are for clearing water and oil which accumulates in machinery space bilges as the result of leakage or draining, and when washing down dry cargo holds. The bilge main in the engine room, has connections from dry cargo holds, tunnel and machinery spaces. Tanks for liquid cargo and ballast are served by cargo discharge systems and ballast systems respectively. They are not connected to
the bilge system unless they have a double function, as for example with deep tanks that are used for dry cargo or ballast. Spectacle blanks or change over chests are fitted to connect/isolate spaces of this kind where necessary.

Oil/water separators
Oil/water separators are necessary aboard vessels to prevent the discharge of oil overboard mainly when pumping out bilges. They also find service when deballasting or when cleaning oil tanks. The requirement to fit such devices is the result of international legislation. Legislation was needed because free oil
and oily emulsions discharged in a waterway can interfere with natural processes such as photosynthesis and re-aeration, and induce the destruction of the algae and plankton so essential to fish life. Inshore discharge of oil can cause damage to bird life and mass pollution of beaches. Ships found discharging water containing more than 100 mg/litre of oil or discharging more than 60 litres of oil per nautical mile can be heavily fined, as also can the ship's Master.

Oil content monitoring
In the past, an inspection glass, fitted in the overboard discharge pipe of the oil/water separator permitted sighting of the flow. The discharge was illuminated by a light bulb fitted on the outside of the glass port opposite the viewer. The separator was shut down if there was any evidence of oil carry over, but problems with observation occurred due to poor light and accumulation of oily deposits on the inside of the glasses.
Present-day monitors are based on the same principle. However, whilst the eye can register anything from an emulsion to globules of oil a light-sensitive photo-cell detector cannot. Makers may therefore use a sampling and mixing pump to draw a representative sample with a general opaqueness more easily registered by the simple photo-cell monitor. Flow through the sampling chamber is made rapid to reduce deposit on glass lenses. They are easily removed for cleaning.
Bilge or ballast water passing through a sample chamber can be monitored by a strong light shining through it and onto a photocell. Light reaching the cell decreases with increasing oil content of the water. The effect of this light on the photo-cell compared with that of direct light on the reference cell to the left of the bulb, can be registered on a meter calibrated to show oil content.

Waste heat losses arise both from equipment inefficiencies and from thermodynamic limitations on equipment and processes. For example, consider reverberatory furnaces frequently used in aluminum melting operations. Exhaust gases immediately leaving the furnace can have temperatures as high as 2,200-2,400°F [1,200-1,300°C]. Consequently, these gases have high-heat content, carrying away as much as 60% of furnace energy inputs. Efforts can be made to design more energy-efficient reverberatory furnaces with better heat transfer and lower exhaust temperatures; however, the laws of thermodynamics place a lower limit on the temperature of exhaust gases. Since heat exchange involves energy transfer from a high-temperature source to a lower-temperature sink, the combustion gas temperature must always exceed the molten aluminum temperature in order to facilitate aluminum melting. The gas temperature in the furnace will never decrease below the temperature of the molten aluminum, since this would violate the second law of thermodynamics. Therefore, the minimum possible temperature of combustion gases immediately exiting an aluminum reverberatory furnace corresponds to the aluminum pouring point temperature 1,200-1,380°F [650-750°C]. In this scenario, at least 40% of the energy input to the furnace is still lost as waste heat.

Some general considerations for waste heat recovery design are:

1. All waste heat recovery options, if installed, should be done with full consideration of:
– Simplicity of operations
– Ease of maintenance
– Prevention of corrosion
– Proven technology

2. Dewpoint and corrosion of waste heat recovery equipment.
The dewpoint of the flue gas in waste heat recovery is very critical to the efficient operation and maintenance of boilers, fired heaters, gas turbines, and heat recovery steam generators. At temperatures below the dewpoint, sulfuric acid condenses on surfaces and corrodes the metal. At temperatures above the dewpoint, corrosion is not a problem. Accordingly, each project must select the optimum exit temperature and the
appropriate alloys to maximize the efficiency and minimize corrosion. 

These systems have many benefits which could be direct or indirect.
  • Direct benefits: The recovery process will add to the efficiency of the process and thus decrease the costs of fuel and energy consumption needed for that process.
  • Indirect benefits:
  1. Reduction in Pollution: Thermal and air pollution will dramatically decrease since less flue gases of high temperature are emitted from the plant since most of the energy is recycled.
  2. Reduction in the equipment sizes: As Fuel consumption reduces so the control and security equipment for handling the fuel decreases. Also, filtering equipment for the gas is no longer needed in large sizes.
  3. Reduction in auxiliary energy consumption: Reduction in equipment sizes means another reduction in the energy fed to those systems like pumps, filters, fans,...etc.
  • Capital cost: The capital cost to implement a waste heat recovery system may outweigh the benefit gained in heat recovered. It is necessary to put a cost to the heat being offset.
  • Quality of heat: Often waste heat is of low quality (temperature). It can be difficult to efficiently utilize the quantity of low quality heat contained in a waste heat medium. Heat exchangers tend to be larger to recover significant quantities which increases capital cost.

Waste heat recovery systems are frequently implemented, but constrained by factors such as temperature limits and costs of recovery equipment.
There are a number of cases where heat recovery equipment is installed, but the quantity of heat recovered does not match the full recovery potential. Key barriers include heat exchanger material limits and costs for extending recovery to lower-temperature and higher-temperature regimes.

• Most unrecovered waste heat is at low temperatures.
The waste heat streams analyzed in this study showed that roughly 60% of unrecovered waste heat is low quality (i.e., at temperatures below 450°F [232°C]). While low-temperature waste heat has less thermal and economic value than high-temperature heat, it is ubiquitous and available in large quantities. Comparison of total work potential from different waste heat sources showed that the magnitude of low-temperature waste heat is sufficiently large that it should not be neglected in pursuing RD&D opportunities for waste heat recovery. New technologies are developing that may provide significant opportunities for low-temperature heat recovery. 

The complete electrical plant on board vessel or rig is made up of power generation equipment, a distribution system and the many power utilising devices. Electricity is used for the motor drive of many auxiliaries and also for deck machinery, lighting, ventilation and air conditioning equipment, etc. A constant supply of electricity is essential for safe rig and machinery operation, and therefore standby or additional capacity is necessary together with emergency supply equipment. Emergency equipment may take the form of an automatically starting emergency alternator or storage batteries may be used. The complete range of electrical equipment will include generators, switch gear for control and distribution, motors and their associated starting equipment and emergency supply arrangements. 

Alternating current AC has now all but replaced direct current as the standard supply for all marine  offshore installations. The use of alternating current has a number of important advantages: for example, reduced first cost, less weight, less space required and a reduction in maintenance requirements. Direct current does, however, offer advantages in motor control using, for example, the Ward-Leonard system which provides a wide range of speed. 

Example, Motors rated 200 HP and above are often wound for Medium Voltage (2300VAC or 4160 VAC) instead of Low Voltage (480 VAC to 600 VAC).  Above 1000 HP it is difficult to find motors that are not wound for Medium Voltage.  There are many reasons why you should consider the use of a Low Voltage AC Drive with an Input and Output Isolation Transformer instead of a Medium Voltage AC Drive.
First Cost:
Even if you own an existing motor wound for 2300 VAC or 4160 VAC your first cost to install a Low Voltage AC Drive with an Input and Output Isolation Transformer will typically be lower than your first cost to install a Medium Voltage AC Drive.  If your Input Voltage is 2300 VAC or 4160 VAC you are going to need an Isolation Transformer.  Medium Voltage AC Drive suppliers always require an Isolation Transformer to protect the AC Motor from high voltage transients.  The cost of a 4160 VAC to 480 VAC Step Down Input Isolation Transformer is no more expensive than the cost of a 4160 VAC to 4160 VAC Input Isolation Transformer.
Medium Voltage AC Drives are not inexpensive.  The cost of a typical Low Voltage AC Drive and an Output (Step-Up) Isolation Transformer is typically only 50% to 75% of the cost of a Medium Voltage AC Drive.  Why you might ask?  First of all, semiconductor technology still favors higher current at a lower voltage over lower current at a higher voltage. Second, there are relatively few Medium Voltage AC Drives manufactured.  The manufacturing process for a Medium Voltage AC Drive still involves a good deal of custom engineering and manufacturing.  Low Voltage AC Drives (even 1000 HP Low Voltage Drives) are more of a production line product.  Third, there are just more suppliers of Low Voltage AC Drives.  The competition is more intense and the prices are more competitive.
The physical size of a Medium Voltage AC Drive and Input Isolation Transformer is typically 125% to 150% larger than the size of a typical Low Voltage AC Drive with Input and Output Isolation Transformers.  Medium Voltage AC Drives are often constructed in their own walk in enclosures.
Service and Repair:
Plant Maintenance generally has a healthy fear of Medium Voltage applications.  They are usually quite comfortable with Medium Voltage Transformers and Medium Voltage Motors.  They are less comfortable with Medium Voltage Switchgear and Medium Voltage AC Motor Starters.  Medium Voltage AC Drives are very complex products, much more complex than a Medium Voltage AC Motor Starter.  Most Plant Maintenance Engineers are not comfortable with Medium Voltage AC Drives.  Medium Voltage AC Drives are almost always Installed, Started Up, Repaired, and Maintained by the manufacturer’s Field Service Engineer.  This can become very expensive and it leaves you at the mercy of the manufacturer.  A Low Voltage AC Drive with Input and Output Isolation Transformers is a product that your maintenance staff will feel comfortable with

Motors and generators, both d.c. and a.c., are rated as Continuous Maximum Rated (CMR) machines. This means they can accept a considerable momentary overload and perhaps even a moderate overload for a longer duration. Temperature affects the performance of all electrical equipment and also the useful life of the insulation and thus the equipment itself. The total temperature of an operating machine is a result of the ambient air temperature and the heating effect of current in the windings, Temperature rise is measured above this total temperature. Adequate ventilation of electrical equipment is therefore essential. Classification Societies have set requirements for the various classes of insulation. The usual classes for marine installations are E, B and F where particular insulation materials are specified and increasing temperature rises allowed in the order stated. 

An a.c. distribution system is provided from the main switchboard which is itself supplied by the alternators. The voltage at the switchboard is usually 440 volts, but on some large installations it may be as high as 3300 volts. Power is supplied through circuit breakers to larger auxiliaries at the high voltage. Smaller equipment may be supplied via fuses or miniature circuit breakers. Lower voltage supplies used, for instance, for lighting at 220 volts, are supplied by step down transformers in the distribution network.
The distribution system will be three-wire with insulated or earthed neutral. The insulated neutral has largely been favoured, but earthed neutral systems have occasionally been installed. The insulated neutral system can suffer from surges of high voltage as a result of switching or system faults which could damage machinery. Use of the earthed system could result in the loss of an essential service such as the steering gear as a result of an earth fault. An earth fault on the insulated system would not, however, break the supply and would be detected in the earth lamp display. Insulated systems have therefore been given preference since earth faults are a common occurrence on ships and a loss of supply in such situations cannot be accepted. 

The operation of paralleling two alternators requires the voltages to be equal and also in phase. The alternating current output of any machine is always changing, so for two machines to operate together their voltages must be changing at the same rate or frequency and be reaching their maximum (or any other value) together. They are then said to be 'in phase'. Use is nowadays made of a synchroscope when paralleling two a.c. machines. The synchroscope has two windings which are connected one to each side of the paralleling switch. A pointer is free to rotate and is moved by the magnetic effect of the two windings. When the two voltage supplies are in phase the pointer is stationary in the 12 o'clock position. If the pointer is rotating then a frequency difference exists and the dial is marked for clockwise rotation FAST and anti-clockwise rotation SLOW, the reference being to the incoming machine frequency.
To parallel an incoming machine to a running machine therefore it is necessary to ensure firstly that both voltages are equal Voltmeters are provided for this purpose. Secondly the frequencies must be brought into phase. In practice the synchroscope usually moves slowly in the FAST direction and the paralleling switch is closed as the pointer reaches the 11 o'clock position. This results in the incoming machine immediately accepting a small amount of load. 

In the event of a main generating system failure an emergency supply of electricity is required for essential services. This can be supplied by batteries, but most vessel or rigs have an emergency generator. The
unit is diesel driven and located outside of the machinery space. The emergency generator must be rated to provide power for the driving motors of the emergency bilge pump, fire pumps, steering gear, watertight doors and possibly fire fighting equipment. Essential services such as Emergency lighting for occupied areas, navigation lights, communications systems and alarm systems must also be supplied. Where electrical control devices are used in the operation of main machinery, these too may require a supply from the emergency generator.
A switchboard in the emergency generator room supplies these various loads. It is not usual for an emergency generator to require paralleling, so no equipment is provided for this purpose. Automatic start up of the emergency generator at a low voltage value is usual on modern installations. 

The automatic provision of electrical power to meet varying load demands can be achieved by performing the following functions automatically:
1. Prime mover start up.
2. Synchronising of incoming machine with bus-bars.
3. Load sharing between alternators,
4. Safety and operational checks on power supply and equipment in operation.
5. Unloading, stopping and returning to standby of surplus machines.
6. Preferential tripping of non-essential loads under emergency conditions and their reinstating when acceptable. 

A loading in excess of each power generator will result in the start up and synchronising of another generator machine. Should the load fall to a value where a running machine is unnecessary it will be unloaded, stopped and returned to the standby condition. If the system should overload through some fault, such as a machine not starting, an alarm will be given and preferential tripping will occur of non-essential loads. Should the system totally fail the emergency alternator will start up and supply essential services and lighting through its switchboard.

Over recent years, there has been a significant increase in the installation and use of power electronic equipment onboard ships and on offshore installations. The operation of this equipment has in many cases significantly degraded the ship or offshore installation electrical power quality to such an extent that measures have to be implemented in order to minimize the resultant adverse effects on the electrical plant and equipment.
The quality and security of voltage supplies are important to the safety of any vessel and its crew and to the protection of the marine environment. Any failure or malfunction of equipment such as propulsion or navigation systems can result in an accident at sea or close inshore with serious consequences. Many power quality issues are transient, for example, the starting of a large electric thruster motor resulting in a momentary dip before the generator regulators correct the situation and reinstate the correct level of voltage and frequency.
The harmonic distortion of voltage supplies caused by the operation of electronic devices which draw nonlinear (i.e., non-sinusoidal in nature) currents from the voltage supplies, the same items of “nonlinear” equipment can also be affected by harmonic currents and the subsequent voltage distortion they produce, as can the majority of “linear” equipment (particularly generators, AC motors and transformers). As harmonic distortion is “steady state” and continuous, the issue of electrical power quality associated with harmonics is an important concern to the marine safety aspects and in addition, to any adverse effects harmonic distortion has on the performance and reliability of the majority of marine and offshore systems and equipment.

“Linear” equipment, such as generators, AC motors and transformers, is also known to produce harmonics,
albeit in limited qualities compared to large “nonlinear loads”. However, in the context of the marine and offshore installations, it is the electric variable speed drive (a combination of an electric motor and electronic power converter) whether AC or DC based, which, due to its increased popularity in a host of applications, is the main source of harmonic currents and subsequent voltage distortion.

In the late 1970s and early 1980s, AC variable frequency drives (“VFDs” or “inverters”) were developed in various forms, “quasi square wave drives” and “current source drives”, being two common examples. However, it was only in the late 1980s that AC drive technology (most notably in the form of “pulse width modulation” (“PWM”) drives) started to appear on ships in any numbers, although they were installed on offshore oil production installations from the mid 1980s for a few duties, especially including down-hole pumps, where the initial benefit was as “soft-starters” for the pumps. Shortly thereafter, the speed control feature was utilized as oil reservoirs became depleted and flow rate had to be controlled.
Offshore, on applications where robust, variable speed control was needed, DC SCR drives were most
common. DC SCR drives went largely unchallenged until the early-to-mid 1990s. However, from that time on, as the physical size, performance, reduction in maintenance of AC motors compared to DC motors, cost, and reliability of AC drives continued to improve, their popularity increased making AC drives comparable to DC SCR drives.

The harmful effects of harmonic voltages and currents on transformer performance often go unnoticed until an actual failure occurs. In some instances, transformers that have operated satisfactorily for long periods have failed in a relatively short time when plant loads were changed or a facility's electrical system was reconfigured. Changes could include installation of variable frequency drives, electronic ballasts, power factor improvement capacitors, arc furnaces, and the addition or removal of large motors.
Many industrial and commercial electrical systems have capacitors installed to offset the effect of low power factor. Most capacitors are designed to operate at a maximum of 110% of rated voltage and at 135% of their kVAR ratings. In a power system characterized by large voltage or current harmonics, these limitations are frequently exceeded, resulting in capacitor bank failures. Since capacitive reactance is inversely proportional to frequency, unfiltered harmonic currents in the power system find their way into capacitor banks, These banks act like a sink, attracting harmonic currents, thereby becoming overloaded.
A more serious condition, with potential for substantial damage, occurs as a result of harmonic resonance. Resonant conditions are created when the inductive and capacitive reactances become equal in an electrical system. Resonance in a power system may be classified as series or parallel resonance, depending on the configuration of the resonance circuit. Series resonance produces voltage amplification and parallel resonance causes current multiplication within an electrical system. In a harmonic rich environment, both types of resonance are present. During resonant conditions, if the amplitude of the offending frequency is large, considerable damage to capacitor banks would result. And, there is a high probability that other electrical equipment on the system would also be damaged.

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