Monday 17 June 2013

Insulation resistance test.... courtesy: openelectrical.org

Insulation Resistance Test

The insulation resistance (IR) test (also commonly known as a Megger) is a spot insulation test which uses an applied DC voltage (typically either 250Vdc, 500Vdc or 1,000Vdc for low voltage equipment <600V and 2,500Vdc and 5,000Vdc for high voltage equipment) to measure insulation resistance in either k $Ω$ , M $Ω$ or G $Ω$ . The measured resistance is intended to indicate the condition of the insulation or dieletric between two conductive parts, where the higher the resistance, the better the condition of the insulation. Ideally, the insulation resistance would be infinite, but as no insulators are perfect, leakage currents through the dielectric will ensure that a finite (though high) resistance value is measured.
Because IR testers are portable, the IR test is often used in the field as the final check of equipment insulation and also to confirm the reliability of the circuit and that there are no leakage currents from unintended faults in the wiring (e.g. a shorted connection would be obvious from the test results).
One of the advantages of the IR test is its non-destructive nature. DC voltages do not cause harmful and/or cumulative effects on insulation materials and provided the voltage is below the breakdown voltage of the insulation, does not deteriorate the insulation. IR test voltages are all well within the safe test voltage for most (if not all) insulation materials.

Test Equipment

IR test set (courtesy of Megger)

The Megger company were the original manufacturers of IR test equipment over 100 years ago and have become synonymous with insulation resistance testing. Most modern IR testers are digital, portable / handheld units and some have multi-functional capabilities (e.g. built-in continuity testing).

Test Procedure

Firstly ensure that the equipment to be tested and the work area is safe, e.g. equipment is de-energised and disconnected, all the relevant work permits have been approved and all locks / tags in place.
Next, discharge capacitances on the equipment (especially for HV equipment) with static discharge sticks or an IR tester with automatic discharging capabilities.
The leads on the IR tester can then be connected to the conductive parts of the equipment. For example, for a three-core and earth cable, the IR test would be applied between cores (Core 1 to Core 2, Core 1 to Core 3 and Core 2 to Core 3) and between each core and earth. Similarly for three-phase motors, circuit breakrs, switch-disconnectors, etc the IR test can be applied at the equipment terminals (and earth connection).
Note that when applying an IR test to earth, it is good practice to connect the positive pole of the IR tester to earth in order to avoid any polarisation effects on the earth.
Once connected, the IR tester is energised for a typical test duration of 1 minute. The IR test measurements are recorded after 1 minute.
When the IR test is finished, discharge capacitances again for a period of 4-5 times the test duration.

Interpretation of Test Results

The minimum values for IR tests vary depending on the type of equipment and the nominal voltage. They also vary according to international standards. Some standards will define the minimum IR test values for the general electrical installations.
For example, for low voltage installations in the IEC world, IEC 60364-6 [1] Table 6A gives the minimum IR values and also suggests test voltage, i.e.

Nominal Circuit Voltage (Vac)	Test Voltage (Vdc)	Insulation Resistance (M $Ω$ )
Extra low voltage	250	$\geq$ 0.5
Up to 500V	500	$\geq$ 1.0
Above 500V	1,000	$\geq$ 1.0

In the ANSI/NEC world, the standard ANSI/NETA ATS-2009 [2] provides test procedures and acceptance levels for most types of electrical equipment. Table 100.1 provides representative acceptance values for IR test measurements, which should be used in the absence of any other guidance (from the manufacturer or other standards):

Nominal Equipment Voltage (Vac)	Min Test Voltage (Vdc)	Min Insulation Resistance (M $Ω$ )
250	500	25
600	1,000	100
1,000	1,000	100
2,500	1,000	500
5,000	2,500	1,000
8,000	2,500	2,000
15,000	2,500	5,000
25,000	5,000	20,000
34,500 and above	15,000	100,000

NFPA 70B [3] also provides some guidance on insulation resistance testing for different types of equipment.

Factors Affecting Test Results

There are two main factors that will affect IR test results:

Temperature

Electrical resistance has an inverse exponential relationship with temperature, i.e. as temperature increases, resistance will decrease and vice versa. Since the minimum acceptable IR test values are based on a fixed reference temperature (usually 20^oC), the measured IR test values must be corrected to the reference temperature in order to make sense of them.
As a rule of thumb, the resistance halves for every 10^oC increase in temperature (and vice versa). So if the measured IR test value was 2M $Ω$ at 20^oC, then it would be 1M $Ω$ at 30^oC or 4M $Ω$ at 10^oC.
ANSI/NETA ATS-2009 Table 100.14 provides correction factors for IR test measurements taken at temperatures other than 20^oC or 40^oC, which were in turn based on the correction factors in the freely available Megger book "A stitch in time..." [4].

Humidity

The presence (or lack) of moisture can also affect the IR test measurements, the higher the moisture content in the air, the lower the IR test reading. If possible, IR tests should not be carried out in very humid atmospheres (below the dew point). While there are no standard correction factors or guidance for humid conditions, it is good practice to record the relative humidity of each IR test so that they can be used for baseline comparisons in future tests. For example, having past data on the IR test values for dry and humid days will give you a foundation for evaluating future test values.

Sunday 9 June 2013

Free surface effect

Free surface effect:

Liquid that only partially fills a compartment is said to have a free surface that tends to remain horizontal (parallel to the waterline). When the ship is inclined, the liquid flows to the lower side (in the direction of inclination), increasing the inclining moment.

Background:

If the tank contains a solid weight, and the ship is inclined, the center of buoyancy shifts in the direction of the inclination and righting arms (GZ) are formed.

Replacing the solid with a liquid of the same weight, when the ship is inclined, the surface of the liquid remains horizontal. This results in a transfer of “a wedge of water,” which is equivalent to a horizontal shift of weight, causing gravity to shift from G₀ to G₂.

The wedge of water transferred increases as the angle of inclination increases, therefore, the center of gravity shifts a different amount for each inclination.

Due to the horizontal shift of the center of gravity, the righting arm is now G₂Z₂. To determine the effect on stability, a vertical line is projected upward through G₂ (see below). Where this line crosses the ship’s centerline is labeled G₃. The righting arm G₃Z₃ is the same length as the righting arm G₂Z₂. Therefore, moving the ship’s center of gravity to position G₂ or G₃ yields the same effect on stability. Movement from G₀ to G₃ is referred to as a Virtual Rise of the center gravity.

To calculate the virtual rise in the center of gravity due to the Free Surface Effect, use the following equation:

B = The breadth (width) of the compartment

L = The length of the compartment

W_F = The ship’s final displacement (after flooding water added)

Factors Effecting Free Surface Effect

Pocketing

Free Surface Effect can be reduced, to some extent, by creating pocketing. Pocketing occurs when the surface of the liquid contacts the top or bottom of the tank, reducing the breadth (B) of the free surface area.

Since the effects of pocketing can not be calculated, it is an indeterminate safety factor. The Free Surface correction will therefore indicate less overall stability than actually exists.

Surface Permeability

Impermeable objects (engines, pumps, piping systems, etc) inside a flooded space project through and above the liquid surface. These objects inhibit the moving water and the “shifting of the wedge” may or may not be complete, thus reducing Free Surface Effect. The impermeable objects also occupy volume, reducing the amount of flooding water (movable weight) that can fill the space.

Swash Bulkheads (Baffle Plates)

In addition to some structural support, these bulkheads are designed to reduce Free Surface Effect. They are longitudinal bulkheads that hinder, but do not prevent, the flow of liquid from side to side as the ship rolls or heels. They are found in tanks, voids, double bottoms, bilges, etc.

Sluice Valves

Free flow (Sluice) valves on tankers allow opposing tanks to be cross-connected. When large, partially filled tanks are connected, Free Surface Effect increases, and the vessel becomes less stable.

Conditions of Free Surface Effect

1. FSE increases with increased length and width of compartment

2. FSE increases when displacement decreases (de-ballasting)

3. FSE is independent of the depth of the liquid

Boiler Burners.....courtesy:marineengineering.org.uk

Burners

Combustion of fuel in furnace and burner design

Process

The heat producing constituents of the fuel are hydrogen, carbon and sulphur.

Carbon to carbon dioxide - 34
Hydrogen to water - 120.5 ( assuming the water vapor is not allowed to condense)
Sulphur to sulphur dioxide - 9.3

The main cause of heat loss with the process is that taken away by nitrogen. Therefore, to achieve maximum efficiency the excess air should be kept to a minimum. However there is a limit to the reduction in the excess air in that the combustion process must be fully completed within the furnace and within a finite time.
The main type of combustion process is called the suspended flame. The flame front remains in the same position relative to the burner and quarl.. The fuel particles pass through the flame completeing their combustion process and exiting at the same rate as the fuel entering.

Primary Flame-To burn oil the temperature must be raised to vaporisation temperature, this can not be done in heaters due to gassing but is done by radiant heat in the flame. The lighter hydrocarbons in the atomised spray are rapidly heated and burnt in the primary flame. The heavier fractions pass through this achieving their vaporisation temperature. The primary flame is essential to good combustion. By design the primary flame exists where it receives maximum reflected heat from the shape of the quarl. The size of the primary flame ( shown smaller than actual in drawing) just fills the quarl space. Too large and impingement leads to carbon deposits building up. Too small unheated secondary air reduces combustion efficiency. The tip plate creates vortices reducing the mixing time for the air/fuel and reduces the forward speed of the flame

Secondary Flame-Here the heavier fractions are burnt. The velocity of the air and fuel must be matched to the required flame propogation rate.

Combustion in furnace space

For proper combustion of fuel in the furnace and adequate supply of air must be supplied and intimately mixed with a supply of combustible material which has been presented in the correct condition.

Air- it is the purpose of the register, swirler vanes and (vortice) plates, and quarl to supply the correct quantity of air for efficient combustion suitably agitated to allow proper mixing.

prevent thermal shocking
improve the combustion process
improve plant efficiency (bled steam and regenerative)

Fuel It is the purpose of the burner to present the fuel in suitable condition for proper combustion. Generally this means atomising the fuel and giving it some axial (for penetration) and angular (for mixing) velocity. For effective atomisation the viscosity of the fuel is critical, for fuels heavier than gas or diesel oils some degree of heating is required. It should be noted that the temperature of the fuel should not be allowed to raise too high as this can not only cause problem with fuel booster pumps but also can cause flame instability due to premature excessive gassification (is that a real word-answers to the normal address)
The smaller the droplet size the greater the surface areas/volume ratio is, this increases evaporation, heating and combustion rate.

Combustion zones

Register- supplies the correct quantity of excess air. Too little allows incomplete combustion, smoking, soot deposits and flame instability. Too much excess air reduces combustion efficiency by removing heat from the furnace space, may cause 'white' smoking and promote sulphurous deposits. In addition too much excess air increases the proportion of sulphur trioxide to dioxide promoting increase acid corrosion attack in the upper regions.
The register and to some extent the quarl determine the shape of the flame, short and fat for side fired boilers, long and thin for roof fired.

Flame burning off the tip- may occur after initial ignition or after a period of high excess air. The effect of this is to move the primary flame away from the quarl thereby effecting the combustion process leading to black smoke and flame instability. Two methods of bringing the flame back are to reduce excess air and introduce a hand ignitor to ignite the fuel correctly, or to rapidly close then open the register damper

Types

Pressure jet
Spill type pressure jet
Variable orifice pressure jet
Spinning cup
Steam assisted
Ultrasonic

Turndown ratio ratio of minimum to maximum flow ( roughly the square root of the ratio of maximum to minimum pressure)

Pressure jet

This is the simplest and oldest design of burner. Atomisation of the fuel is achieved by forcing the fuel under pressure through an orifice at the end of the burner, the pressure energy in the fuel is converted to velocity. Spin is given to the fuel prior to the orifice imparting centrigual force on the spray of fuel causing it to atomise. The disadvantage of this burner is its low 'Turn-Down' ratio (in the region of 3.5). The advantage is that it does not require any assistance other than supplying the fuel at the correct pressure. Due to this it is still seen even on larger plant were it is used as a first start or emergency burner.
Anouther disadvantage over assisted atomisation burners is the lack of cooling from stam or air means the burner must be removed when not in use from lit boilers to prevent carbonising in the tube

Ultrasonic

Wider turn down ratio with lower excess air (15 :1)
Low O₂ levels
Simplified operation
Reduced acid corrosion problems

Atomisation is achieved primarily by the energy of ultrasonic waves imparted onto the fuel by the resonator tip which vibrates at a frequency of 5 MHz to 20 MHz under the influence of high speed steam or air impinging on it. Extremely small droplet sizes result which allow for a very stable flame.

Spinning Cup

Fuel is introduced onto the inner running surface of a highly polished fast spinning cup (3 to 7000 rpm). Under centrifugal force this fuel forms a thin film.
Due to the conical shape of the cup the fuel flows to the outer edge spilling into the primary atomising air stream. The fuel is broken into small droplets and mixed with the primary air supplied by the shaft mounted fan. Secondary air is supplied by an external fan for larger units.
Packaged units of this design have the air flow valve controlled by the fuel supply pressure to the distribution manifold.

Wider turn down ratio with lower excess air
Low O₂ levels
No requirement for atomising air or steam
Low fuel pressure requirements to an extent that gravity flow is sufficient
stable flames achievable with very low fuel flows although maximum flow limited by size of cup. This, allied to being limited to side firing making the design more suitable for smaller installations.

Blue Flame

This highly efficient and claen burning method is very close to stoichiometric combustion. Under normal conditions a portion of the hot gasses from the combustion process is recirculated. Fuel is introduced into the gas were it is vaporised. The resultant flame is blue with little or no smoke

Safety Valve in A Boiler..... Courtesy:marinenegineering.org.uk

SAFETY VALVES

At least two safety valves have to be fitted to the boiler. They may be both mounted on a common manifold with a single connection to the boiler. The safety valve size must not be less than 38mm in diameter and the area of the valve can be calculated from the following formula C x A x P = 9.81 x H x E
where
H= Total heating surface in m³
E = Evaporative rate in Kg steam per m² of heating surface per hour
P = Working pressure of safety valves in MN/m² absolute
A = Aggregate area through the seating of the valves in mm²
C = the discharge coefficient whose value depends upon the type of valve.
C=4.8 for ordinary spring loaded valves
C=7.2 for high lift spring loaded valves
C= 9.6 for improved high lift spring loaded valves
C= 19.2 for full lift safety valves
C= 30 for full bore relay operated safety valves

LIFT PRESSURE

The safety v/v must be set at a pressure not exceeding 3% of the approved boiler working pressure. It is normal to set the suphtr safety below that of the drum to ensure an adequate flow of stm for cooling purposes under fault conditions. Similarly the superheater should be set to close last.

10% ACCUMULATION OF PRESSURE RULE.

With all the flames in full firing the stm stop is closed, the boiler pressure must not increase by more than 10% in 7 minutes for water tube of 15 mins for tank boilers with the safety lifted. this is normally waivered for superheater boilers. Instead calculations and previous experience used.

BLOWDOWN

The pressure drop below the lifting pressure for a safety v/v is set at 5% by regulation although it is more normal to set v/v's at 3% to prevent excessive loss of stm. For boilers with a superheater it is important that the superheater v/v not only lifts first but closes last. Adjustement of the blowdown may be necessary following adjustment of the popping setpoint (Increaseing set point lengthens blowdown). Adjustment is achieved by altering the height of the 'adjusting guide ring' on the full lift safety valve design shown below. Over raise adjustment of this ring can lead to mal-operation with the valve not fully opening

SETTING

Must be set with the surveyor present except when on the waste heat unit. A chief engineer with three years experience may then set the safety valve but must submit information to surveyor for issue of certificate.
Superheated steam safety valves should be set as close to operating temperature as possible as expansion can alter the relationships between valve trim and guide/nozzle rings which can effect the correct operation of the valve.

Two safety valves- each set independently
Each safety valve must release entire steam flow in pressure accumulation test
Surveyor uses specially checked gauge
One valve gagged
valve initially set to approximately the correct position then steam pressure increased to set pressure
adjust valve to lift
raise and lower pressure to check
fit locks to both valves on completion

Easing gear to be checked free before setting valves. Steam should not be released as this can damage seat.

Improved high lift safety valve

Differences in the ordinary and high lift designs

Ordinary	High Lift	Improved high lift
Winged valve	Winged valve	Wingless valve
No waste piston	Waste piston	Waste piston
	No floating ring	Floating ring

For superheated steam the aggregate area through the seating of the valves is increased, the formula is
As = A(1 + Ts/555)
where
As = Aggregate area through the seating of the valves in mm² for superheated steam
A = Aggregate area through the seating of the valves in mm² for sat steam
Ts = degrees of superheat in ^oC
As is greater than A due to the higher specific volume of superheated steam requiring more escape area.
The manifold pipe must have an area equal to at least Н of A, the exhaust must have a diameter dependent on the type of valve but up to 3 x A for a full bore relay operated valve.
A drain pipe must be fitted to the lowest part of the valve, it should have no valve or cock and should be checked clear on regular occasions.

Materials

Materials for all parts must be non corrodible. Common materials are Bronze, Stainless steel or Monel metal, depending on the conditions of service. The valve chest is normally made of cast steel.

Full lift safety valve

This is a modern version of the high lift safety valve incorporating the piston and reaction force effects to improve valve lift. In addition the inlet pipe is tapered to give a nozzle effect increasing the reaction on the lid.
The initial lift is produced when the steam pressure under the disc exceeds the spring pressure. As the valve begins to open a thin jet of steam escapes and is deflected by a small angle on the nozzle ring. As the lift increase the steam begins to react against upper guide ring increasing to 'full bore'lift. Full Bore lift is defined as that point where the area of the nozzle, rather than the lift, limits the discharge capacity of the valve. The form of the valve offers an increased area to the steam jet stream and the design allows for a piston effect of the valve trim assembly as it enters in the guide ring cylinder, both these effects increase lift and improve action of the valve
The guide sleeve is adjustable allowing alteration of the blowdown.
With boiler pressure dropping the valve begins to close. When the lid just exits the guide sleeve there is a loss of the reaction and piston effect and the valve tends to snap shut cleanly.
Blowdown adjustment is achieved by altering the height of the adjusting Guide Ring. On some designs a second adjustable ring is mounted on the nozzle, this allows adjustment of the 'warn' or 'simmering'period and increases the popping power. Adjustment of this ring is critical to operation, after factory setting it is generally unnecessary and no attempt should be made to remove slight 'warn'

Full lift safety valve

Seen fitted to large high pressure boilers.

Complicated design to achieve high lift is obviated
Pilot valve may be mounted on the drum and the main valve maounted on the superheater thus making the system more sensitive to load changes (over pressurisation will first be seen in the steam drum before the superheater. In addition the pilot valve and main valve piston arrangements are subject to lower steam temperatures
Boiler pressure will assist to close the main valve rapidly leading to very small blowdown

Easing gear

This is fitted to safety valves to allow manual operation of the valve in an emergency.

Marine Engineering

Monday 17 June 2013