.stp .sewage treatment plant

Overview of Sewage Treatment Plant on Ships

Sewage Treatment Plants (STPs) are crucial systems on ships for processing and treating wastewater before discharge. Proper maintenance and regular checks are essential for ensuring the STP’s efficient operation, compliance with environmental regulations, and prevention of system failures.

Routine Checks and Maintenance

Daily Checks

1. System pressure: Verify it’s within specified limits
2. Air lift return: Ensure proper flow through clear plastic pipes
3. Sludge content in aeration tank: Should not exceed 200 mg/liter
4. Chlorination levels: Maintain between 1-5 ppm

Weekly Checks and Tasks

1. Add bio-pac (aerobic bacteria) to enhance efficiency
2. Measure Biological Oxygen Demand (BOD): Should not exceed 25 mg/liter
3. Check sludge content using manufacturer-specified methods

Other Periodic Checks

1. Inspect internal coating for damage (cracking, blistering)
2. UV lamp replacement (if applicable)
3. Verify high and low-level limit switches for discharge pumps
4. Ensure stand-by sewage discharge pump is on auto mode

Key Maintenance Points

1. Back flushing procedure for clearing blockages
2. Proper use of cleaning chemicals as per manufacturer recommendations
3. Regular inspection and cleaning of tanks (with safety precautions)
4. Monitoring and adjusting chlorination or UV disinfection systems

Safety Precautions

1. Gas testing before tank entry (use dragor tube)
2. Proper Personal Protective Equipment (PPE) when entering tanks
3. Disinfection procedures after maintenance work
4. Proper hygiene practices for personnel involved

Troubleshooting and Common Issues

1. Blockages in sewage lines
2. Excessive sludge buildup
3. Inefficient bacterial action
4. Equipment malfunctions (pumps, switches, etc.)

Record Keeping and Documentation

1. Maintain logs of all checks and maintenance activities
2. Document test results and any corrective actions taken
3. Keep records of chemical usage and bio-pac additions

Environmental Compliance

1. Understand and adhere to MARPOL Annex IV regulations
2. Ensure effluent meets or exceeds MEPC 159(55) standards
3. Be aware of local regulations in different operational areas

Net Positive Suction Head and Its Significance?

.npsh

What is an NPSH curve and what is its significance?

Answer:

The NPSH curve displays the minimum required inlet pressure (expressed in m) allowing the pump to pump ir

accordance with the performance curve and in order to prevent evaporation of the pumped fluid so as to avoid cavitation inside the pump.

At a given flow rate, the NPSH value available at the pump’s suction ports must always be at least 0.5 m

greater than the required NPSH value: NPSHA > NPSHR + 0.5 m safety margin.

Operating a centrifugal pump below the NPSH curve can cause cavitation, which can damage the pump and reduce its efficiency. When a pump is operating below the NPSH curve, it means that the Net Positive Suction Head Available (NPSHA) is less than the Net Positive Suction Head Required (NPSHR). This can cause the fluid to boil and form bubbles, which can collapse when they reach areas of higher pressure, causing damage to the impeller and other components of the pump.

The NPSHA is affected by factors such as fluid temperature, fluid density, and suction piping configuration. If the NPSHA is too low, it can be increased by increasing the pressure at the suction port of the pump or by reducing the temperature of the fluid.

It is important to operate a centrifugal pump within its NPSH limits to prevent cavitation and ensure optimal performance. If you are operating a centrifugal pump below its NPSH curve, you should take steps to increase the NPSHA or reduce the flow rate to avoid cavitation.

Portable foam applicators 

A portable foam applicator unit shall consist of a foam nozzle of an inductor type capable of being connected to the fire main by a fire hose, together with a portable tank containing at least 20 L of foam-forming liquid and one spare tank of foam making liquid.

The nozzle shall be capable of producing effective foam suitable for extinguishing an oil fire, at the rate of at least 1.5 m3/min.

Advantages of foam applicator:

• long throw.
• Self-inducing models.
• Suitable for all foams.
• Range 18-22 meters at 7 bar.

• Balanced seals
• Unbalanced seals
• Pusher seals
• Non-pusher seals
• Conventional seals
• Cartridge seals

Design

A basic mechanical seal contains three sealing points.

1.The stationary part of the seal is fitted to the pump housing with a static seal –this may be sealed with an o-ring or gasket clamped between the stationary part and the pump housing.

^{(Highlighted in red below, left the stationary part and right the rotary portion)}

2.The rotary portion of the seal is sealed onto the shaft usually with an O ring. This sealing point can also be regarded as static as this part of the seal rotates with the shaft.

3.The mechanical seal itself is the interface between the static and rotary portions of the seal.

.corrosion

Corrosion is the deterioration of a material because of its interaction with its surroundings

Hot Corrosion:

It occurs due to the presence of Vanadium (Va) and Sodium (Na) in the fuel oil and affects exhaust passage of the engine. Creates a molten paste at temperatures more than 450 degrees and stick to the exhaust valve.

Cold corrosion: 

It occurs due to the presence of sulphur in fuel oil and affects the cylinder liner and other parts of combustion chamber

Hot Corrosion process:

Vanadium is a naturally occurring element in marine fuel oils in soluble form, which means, it will not be separated even when the fuel is treated in the centrifuge. Vanadium, when combined with Sodium, can cause damage to the engine under elevated temperature. Sodium and Vanadium compounds are formed at a high temperature, which plays a crucial role in hot corrosion.

The availability of abundant oxygen in the combustion chamber during the burning of fuel results in the oxidation of vanadium to form VO and VO2. During the temperature drop in the further combustion process, VO2 undergoes further oxidation resulting V2O5.

V2O5 has a low melting point and becomes semi-liquid, sticky in nature and adhere to the surface they come into contact with.

Sodium in the fuel reacts with water vapour during combustion to generate NaOH. This, in turn, combines with SO2 forming sodium sulphate.

Sodium sulphate condenses at a temperature approx. below 890 deg. C and will adhere to surfaces with already present V2O5. This resultant deposits block gas passages and corrode metal surfaces. If the ratio of Va:Na is 3:1, the resulting complex melting point is at it’s lowest, which is about 350 – 450 deg C, and there is an increased likelihood of deposit formation.

Fuels with high vanadium and sodium will increase the tendency for deposit formation in the exhaust passages. At higher temperature (>600 deg C), ash deposits can accelerate corrosion of metals and fouling of gas passages.

Effect of Hot Corrosion

1. Erosion: It mainly takes place along the exhaust gas passages, as ash and carbon deposits from high-temperature exhaust gases wear metals. Because of this, the exhaust valve is profoundly affected.

2. Fused salt corrosion: At high temperature, Na and Va form corrosive fluxes, attacking and corroding exhaust valves, turbocharger nozzles and blades. The salts dissolve the protective oxide layers, facilitating further gas phase oxidation.

3. Gas phase oxidation: It is the effect of oxygen on metal engine surfaces in the hot exhaust.

How to control hot corrosion?

• Maintain exhaust temperature well below melting point of Na and Va complex (about 400c)
• Use of Sterlite coating or Nimonic steels on exhaust valve seat for protection from corrosion
• Use exhaust valve rotators to smoothen radial temperature distribution and to prevent repeated impact damage at a single point on the valve face
• Fuel additives like ash modifiers can be used which can modify and increase the melting point temperature of Na and Va complex formed when the ash is not in a molten form and not corrosive
• Controlling fouling of exhaust passages and machinery, i.e. regular cleaning and inspection of the exhaust manifold, frequent water washing of turbocharger, overhauling of the exhaust valve, etc.

COLD CORROSION procedure

Sulphur is another element which is a naturally found in crude oil. Its level is indicated by the content of sulphur found in the residual fuel stream obtained during the process of crude oil refining.

Sulphur in the fuel acts as a natural EP (Extreme Pressure) additive, providing inherent lubricity in the fuel passing through the injectors and pumps.

With plenty of oxygen available in the combustion chamber, the Sulphur is converted to SO2 and it further combines with oxygen to form SO3 Sulphur trioxide.

When SO3 comes in contact with water or water vapour present in the scavenge air, it will react and form H2SO4.

If the engine is running inefficiently at low RPM, the liner temperature is on the lower side and below the dew point of sulphuric acid and water (120-160 deg C). Corrosive mixtures will condense on the linear walls causing cold corrosion of cylinder liner.

In low sulphur fuels, late or slow combustion will increase the thermal load on cylinder components, leading to overheating, lubrication problems and cold corrosion.

Why cold corrosion becomes an issue for newer engines?

New energy efficient marine engines are imposing severe operating conditions with ultra-long strokes and higher pressure while burning a low sulphur fuel. Adoption of slow steaming operation has also lead to an extremely cold corrosive situation in the engine.

Another important reason is that new marine engines are designed to comply with tier III NOX regulations and EEDI guidelines. To meet these new regulations, engine cylinders must operate under increased pressure and reduced operating temperatures (reduce NOX emission), thus creating conditions below dew point to allow water to condense on the cylinder linear walls. This then combines with sulphur from the combustion process to form H2SO4 which leads to cold corrosion.

EGR also brings acidic components into the air mixture and impacts temperature in the combustion chamber.

Thermal stress and pressure constant which can lead to the risk of corrosion are more severe in long stroke engine.

Engines of Older ships are often modified to run at low load operation. They are additionally installed with systems like VTA, gas bypass valves, jacket cooling bypass etc. to perform slow steaming. The older engines are provided with modification to run at low load but no additional modification is done to tackle cold corrosion.

The ultra low slow steaming engines operate at up to 10% of its full load, which again results in low temperatures in the combustion chamber. Once the temperature falls below the dew point, it will lead to cold corrosion.

Effects of Cold Corrosion:

1. Excessive cylinder oil fouling
2. Sticking up of ring grooves
3. Sticking of piston rings
4. Degradation of the surface by removal of iron particles
5. Decreased operational life of cylinder liner

How to manage cold corrosion?

• By using appropriate Toral Base Number (TBN) cylinder oil depending on the sulphur content of the fuel.

Fuel sulphur content (%)

Below 0.25 (approx cyl oil TBN 10 mgKOH/g)
0.25 – 1.0 (approx cyl oil TBN 10-20 mgKOH/g)
1.0 – 3.0 (approx cyl oil TBN 70 mgKOH/g)
Over 3.5 (approx cyl oil TBN >70 mgKOH/g)

• Using modern cylinder lubrication methods such as alpha lubricator (MAN) or Pulse Lubricating System (Wartsila)

To perform sweep test (done in MAN engines with high sulphur content fuel by supplying cylinder oil at different feed rate for a period of 24 hrs to check the effect) to find out acceptable ACC factor for a particular cylinder oil which corresponds to minimum corrosive wear.

• Implementing a condition monitoring program for analyzing iron wear (Fe) and residual TBN in scrape down oil

Use of latest technology and equipment such as

• Variable Geometry Turbocharger (VGT)
• Exhaust gas by-pass valve
• Provision of TC cutout, etc.

Types of Mechanical Seals for Centrifugal Pumps

• Balanced seals
• Unbalanced seals
• Pusher seals
• Non-pusher seals
• Conventional seals
• Cartridge seals

Design

A basic mechanical seal contains three sealing points.

1.The stationary part of the seal is fitted to the pump housing with a static seal –this may be sealed with an o-ring or gasket clamped between the stationary part and the pump housing.

^{(Highlighted in red below, left the stationary part and right the rotary portion)}

2.The rotary portion of the seal is sealed onto the shaft usually with an O ring. This sealing point can also be regarded as static as this part of the seal rotates with the shaft.

3.The mechanical seal itself is the interface between the static and rotary portions of the seal.

.rudder drop

Article link: https://sailorstaan.com/life-at-sea/jumping-clearance-and-its-purpose/

Rudder drop is defined as the wear down of the rudder carrier bearing as a result of the mechanical forces acting on it, namely buoyancy force (with which the rudder stock would ascend and damage the steering gear components), friction etc. The rudder drop would nullify the purpose of using the rudder carrier bearing which are supposed to reduce the friction during the rotation of the rudder stock during navigation. The rudder drop is measured using the trammel gauge.

How Rudder Drop measured ?

As We know that the drop is measured by a trammel gauge. The Trammel gage is an L-shaped instrument. Generally, a point marked on the rudder stock and another point is marked on the hull within the steering gear room (Here it is on the Deck head girder). The distance between these points shall be measured and recorded at the time of construction. The difference between the original and the measured points shall be referred to as the rudder drop or the rudder wear down as shown in the fig.

Measures to reduce the rudder drop

• More frequent greasing of the bearing
• Proving jumping clearance
• Regular care & maintenance of bearing

Jumping Clearance

Jumping Clearance is defined as the clearance or the distance between the pads one is welded onto the tip of the rudder and another one is on the hull opposite to the rudder pad. The jumping bar or the stopping bar is nothing but a rigid slab of metal which is being welded to the ship’s hull. Since, the bar needs to withstand heavy buoyancy force, acting on the rudder, it has to be made of high strength materials which has to be corrosion resistant and should be having a ductile nature.

Jumping clearance is increased due to rudder carrier bearing wear or rudder drop.

Why jumping clearance is provided:

• The maximum jumping clearance should always be less than the clearance between steering gear (the sliding ram) and the tiller arm.
• If the jumping clearance was not provided and no stopper, then it is very probable that the rudder would hit the hull with unimaginable force resulting to the damage of hull.
• Further, if the clearance is more because of the ascend, the rudder stock would hit the lower tiller which in turn strikes the sliding ram causing the ram to bend. In a nutshell, the entire steering system would break down. To prevent such undesirable situations to take place, jumping clearance is provided.  

Rudder survey:

.rudder survey

Planning:

1. Discussed with master and all officers.
2. Discussed with office superintend
3. Consult with dry-dock manager, safety officer, Repair manager
4. Informed class surveyor regarding rudder survey.
5. Risk assessment to be carried out.
6. Work permit to be carried out.
7. Check last dry-dock rudder service report.
8. Tool box meeting carried out among engine staff.
9. Wear proper PPE
10. Arrange proper tools for taking all clearance.

 Checks on rudder in dry-dock

1. Rudder survey will be done only in dry-dock; it should be done only by shipyard workshop team with presence of surveyor.
2. When ship enters dry dock and pumping out water, check water is coming out from rudder or not. If yes, then rudder is breached. If water ingress inside rudder, only the buoyancy of the rudder lost, no major casualty will occur. Internal parts of rudder might corrode.
3. Carry out a visual inspection for crack on rudder plate.
4. Open the top air plug and bottom drain plug in front of class surveyor. when water drain out it indicates crack in the rudder.
5. So crack is detected by air pressure and applying soap solution.
6. If the rudder is badly rusted or ship is older, surveyor may insist on thickness gauging of the rudder plate.
7. Check the condition of the sacrificial anode on the rudder. And any masking tape or paint is over there.
8. Check the cement on the palm coupling bolts for rudder and rudder stock. Remove the cement and check the condition of the palm nut.
9. Check the rudder pintle clearance.
10. Check the rudder jumping clearance.
11. Check the rudder drop using trammel gauge.
12. Check the rudder stock for corrosion and erosion
13. Check condition of external rudder stop.
14. Check the actual position of the rudder, compared to rudder angle indicator and see whether any difference is there by bending or deformations
15. Now lower portion of the rudder is cut open and pintle nut is checked for proper securing and later the plates are welded and tested.
16. Check the pintle bearing steel disc condition and drain passage clear or not.
17. Hydraulic Pressures test the rudder at a water head of 2.45 meters.
18. After draining and oiling the internal, plug the drain and check the effectiveness by a vacuum check and cement plug.
19. Checked the rudder stock gland packing and renewed.
20. For a new built ship, the standard clearance between pintle and bush is 1.5 mm.
    For the ship in service, Maximum allowable clearances between pintle and bush is 6 mm. IF the actual clearance exceeds above 6mm, the bush should be renewed.

TESTS AFTER RUDDER REPAIRS

1. At the completion of the repairs, the rudder system is reassembled and function tests are carried out to ensure satisfactory operation before undocking.

2. If any work was done on the rudder body, it should be tested by compressed air to an equivalent pressure head of 2.45 m above the top of the rudder.

3. Other tests include swing the rudder from one extreme position to the other

.rudder

Rudder types:

→Balanced Rudder
→Semi-balanced Rudder
→Unbalanced Rudder

The choice of rudder type depends on
→the shape of the stem,
→the type of vessel,
→the size of the rudder required and
→the steering gear available.

Parts of rudder:

1. rudder stock
2. rudder coupling
3. rudder blade (rudder body)
4. stem post with gudgeon
5. pintles (upper and lower)
6. rudder carrier bearing and g

Balanced rudders or spade rudders:

This rudder has 20-40% of the area forward of the stock, similarly there is no torque on the rudder stock at certain angles, this type of rudder is called balanced rudder. The axis of the rudder is placed near to the center of gravity, so torque required to move the rudder will be very less.

Semi Balanced:

→ A rudder with 20% of its area forward of its stock is called semi balanced rudder.

→ It is often found in twin screw ships.

→ it has fixed structure in upper half.

→ upper half is unbalanced.

→ lower half below the fixed structure is balanced

Unbalanced Rudder:

→A rudder with whole of its area aft of its stock is called unbalanced.

→ number of pintles fitted depends on the strength consideration.

→ torque required to move the rudder is very high.

Possible damages on rudder:

1. Fractured and loose coupling bolts

2. Loose Nut

3. Wear (excessive bearing clearance)

4. Fractures in way of Pintle cutout

5. Fractures in way of removable access plate

6. Fractures

7. Erosion

8. corrosion

What to look for in dry dock:

.rudder dry dock

→ deformation

→ fracture

→corrosion, erosion

→ clearance

Types of Works and Repair:

→Measurement of pintle clearance (the closing plate may have to be removed). Other clearances which are also taken are the jumping and the emergency carrier clearances.

→ Removal and replacement of rudder’s gland to renewal of rudder packing in the transom space.

→Disconnection of steering gear tiller or rotary vane and jacking up of rudder to inspect the stock steady bush.

→ Unshipping of rudder into the dry dock for survey/examination and repairs.

→Air test to check for leaks in rudder body. Repairs on damaged rudder body if any.

→Welding of erosion pits and wasted seams, replacement of zinc anodes.

→Repairs on damaged rudder stock.

→Repair on rudder couplings holes and bolts. Repair of sealing cement. Repair on rudder pintles and pintle bushes.

→Repair of pintle gudgeons

→ Common defects are wear, corrosion, twisting, cracks bending and fracture.

→Corrosion damage is observed mainly where the cylindrical part widens into the flange.

→Wear and corrosion can be rectified by means of deposit arc-welding with subsequent machining.

→ If cracks are found, then they are ground off and repaired by arc-welding.

→ Twisting and bending of the stock impede the normal working of the whole steering system. Too much twisting or bending requires a replacement of the stock. If slight bending, then a straightening force is applied by means of a hydraulic press.

Repairs of rudder coupling:

 The rudder coupling is subjected to similar bending stresses and twisting stresses as the rudder stock.

 These stresses give rise to fatigue of the coupling, bolts.

 A coupling defect that may occur is the opening of the horizontal coupling at ‘the forward edge. This could result in corrosion of the flanges of the coupling as well as loosening of the bolts.

 The situation could easily be aggravated by the weight of the rudder hanging by the coupling bolts. Repair could be affected ty building up the flange by means of welding and machining smooth. The bolt holes are then bored by a portable boring machine and new fitted bolts are then replaced.

Repairs on gudgeon:

 The alignment of the upper and lower gudgeons are first checked by means of Laser or using piano wire which is tensioned to ensure that it is in a straight line.

 Any misalignment discovered is then accurately recorded.

 One method of repair is to build up the gudgeon by welding and then machining the diameter by a portable boring machine.

 This method may prove to be difficult if the portable boring machine is not designed to bore taper accurately which can result in tedious fitting of the gudgeon. An alternative way to carry out such repairs is to fit a separate step-bush into the gudgeon which has been bored with a step as shown.

 After fitting the step-bush into the gudgeon, it is further welded onto the forging.

 The stern frame gudgeon, where corrosion damages are common, is also repaired the same way.

Clearances in rudder:

Jumping clearance

Pintle clearance

Rudder carrier bearing clearance

Tests of after repair:

 At the completion of the repairs, the rudder system is reassembled and function tests are carried out to ensure satisfactory operation before undocking.

 If any work was done on the rudder body, it should be tested by compressed air to an equivalent pressure head of 2.45 m above the top of the rudder.

 Other tests include swing the rudder from one extreme position to the other

Why Rudder Angle Limited to 35 Degrees?

• Beyond 35-degree rudder efficiency is reduced due to formation of eddies on the back of rudder as the flow is no longer streamlined. This is called stalled condition.
• The maneuverability does not increase beyond 35 degree, but rudder torque increases and ship’s turning circle increases.

Why Steering Test Rudder angle 35 degree to 30 degree?

• So that the point at which it is reached can be exactly judged as it crosses 30 degree.
• As hunting gear puts pump stroke to zero, the rudder movement slows down progressively as it approaches 35 degrees.

Why Astern Turning Moment much less than Ahead ?

• The propeller thrust adds to the force on the rudder when going ahead, but in astern that thrust is lost.
• The pivoting point (point about which ship turns) shifts aft to 1/3 rd the length from aft. This reduces turning moment greatly.

## Role of a Chief Engineer to Keep Steering Gear Healthy

.steering gear healthy .healthy .healthy steering gear

### 1. Service & Maintenance Schedule

– Follow the comprehensive service & maintenance schedule for the ship’s steering gear system as per SMS & maker’s instruction.

– Schedule should include:

  – Regular **inspection**

  – **Lubrication**

  – **Testing & calibration** (ILTC)

– Adhere to manufacturer’s guidelines & recommendations for maintenance intervals & procedures.

– Guide 2/E to check rudder drop regularly using a trammel gauge.

### 2. Visual Inspection

– Inspect the steering gear system daily for:

  – **Corrosion**

  – **Leaks or damage** (CLD)

– Check the condition of:

  – Hydraulic lines

  – Cylinders

  – Control valves

  – Mechanical linkage

– Look for any loose or worn-out parts.

– Take immediate action if any leak is noticed.

### 3. Calibration

– Verify the calibration of:

  – Rudder angle indicator

  – Position sensor

  – Feedback system

  – Control panel

– Adjust parameters if necessary.

### 4. Safety Devices/Electrical

– Inspect & test safety devices including:

  – Limit switches

  – Pressure relief valves

  – Emergency stop button

  – Alarm system

– Ensure all devices function properly and are connected to the control system.

### 5. Regular Testing & Drills

– Regularly test emergency steering, including:

  – Power supply

  – Control system

  – Mechanical linkage

– Conduct emergency steering drills as per SMS, at least once every 3 months.

– Test steering gear in different load conditions & verify response time, rudder angle feedback accuracy.

– Document all test results for record keeping.

### 6. Lubrication

– Ensure proper lubrication to prevent excessive wear.

– Follow maker’s recommendations for type, grade & frequency of lubrication.

– Lubricate bearings, linkage, hydraulic cylinder rods.

– Ensure sufficient lubricant is available onboard.

### 7. Hydraulic System Maintenance

– Regularly inspect & maintain hydraulic:

  – Filters

  – Accumulators (FA VP)

  – Valves (V/V)

  – Pumps (P/P)

– Check for leaks, pressure drop, abnormal noise.

– Monitor hydraulic oil level & quality.

– Conduct lab tests of hydraulic oil onshore as per SMS & maker’s guide.

### 8. Control System Calibration

– Calibrate the control system periodically to ensure accurate response & control.

### 9. Critical Spares

– Ensure critical & necessary spares are available onboard, such as motors, couplings, O-rings, etc.

Citations:

[1] https://pplx-res.cloudinary.com/image/upload/v1725385777/user_uploads/guusewxay/healthy-steering-gear-1.jpg

[2] https://pplx-res.cloudinary.com/image/upload/v1725385777/user_uploads/ohhybwavr/healthy-steering-gear-3.jpg

[3] https://pplx-res.cloudinary.com/image/upload/v1725385776/user_uploads/hgghssdbg/healthy-steering-gear-2.jpg

[4] https://pplx-res.cloudinary.com/image/upload/v1725385776/user_uploads/byvrfyznt/healthy-steering-gear-4.jpg

Engine room fire free  .engine room fire free

.fire free .no fire event .no er fire

.er fire free  .erf

The master solenoid is installed after the receiver, which is controlled by the control unit. In case of sudden stoppage of the compressor, the master solenoid also closes, avoiding the flooding of the evaporator with refrigerant liquid.

Thermostatic expansion valve

.tev .expansion valve .thermostatic expansion valve

Thermostatic expansion valve maintains a constant superheat of the vapour refrigerant at the end of the evaporator coil, by controlling the flow of liquid refrigerant through the evaporator. Thus, its operation is based on the principle of constant degree of superheat at the evaporator outlet by controlling the flow of liquid refrigerant through the evaporator.

Thermostatic expansion valve consists of a needle valve and a seat, a metallic diaphragm, a spring and an adjusting screw .

In addition to this, it has a feeler or thermal bulb, which is mounted on the suction line near the outlet of the evaporator coil. The feeler bulb is partly filled with the same liquid refrigerant as used in the refrigeration system. The opening and closing of the valve depends upon the forces acting on the diaphragm.

When refrigeration load on the evaporator increases

If refrigeration load on the evaporator increases, it causes the liquid refrigerant to boil faster in the evaporator coil. The temperature of feeler bulb increases due to early vaporization of liquid refrigerant. Thus, the feeler bulb pressure increases and this pressure is transmitted through a small diameter tube (also known as capillary tube) to the diaphragm. The diaphragm moves downwards and opens the valve to admit more quantity of liquid refrigerant to the evaporator. This continues till the pressure equilibrium on the diaphragm is reached.

When refrigeration load on the evaporator decreases

On the other hand, when refrigeration load on the evaporator decreases, less amount of liquid refrigerant evaporates in the evaporator coil. The excess liquid refrigerant flows towards the evaporator outlet, which cools the feeler bulb. Due to this, the feeler bulb pressure decreases due to decrease in its temperature. The low feeler bulb pressure is transmitted through the capillary tube to the diaphragm and moves the diaphragm in upward direction. This reduces the opening of valve and thus, reduces the flow of liquid refrigerant to the evaporator. The evaporator pressure decreases due to reduced quantity of liquid refrigerant flowing to the evaporator. This continues till the evaporator pressure and the spring pressure maintains equilibrium with the feeler bulb pressure.

faults:

High suction temperature (superheat) caused by the lack of insulation on the suction, or by too small opening of expansion valve and hence the admission of too little liquid to the evaporating coils.

Article link:

1. https://www.marineengineersknowledge.com/2020/08/operational-procedure-of-air.html

Sketch:

Direct expansion system:

.refrigeration system

→ evaporator coil situated inside the space to be refrigerated.

→ no duct and piping required

• The evaporator coil extracts the heat from the air inside.
• The compressor draws low pressure refrigerant vapour and delivers the high pressure refrigerant vapour through an oil separator to the condenser.
• The condenser liquefies the refrigerant.
• The liquid refrigerant from the condenser passes through the filter dryer, solenoid valve and thermostatic expansion valve to the evaporator.

Checks

Before starting the refrigeration plant, 

• Check the oil level in the crank case of the compressor.
• Insufficient oil wears down the components.
• In extreme cases, it may lead to seizure of the compressor.
• Check whether the cooling water inlet and outlet valves to the condenser are open.
• Insufficient flow of cooling water results in improper condensation which in turn increases the compressor discharge side pressure.
• Open the compressor outlet valve, condenser inlet valve, receiver outlet valve, filter drier inlet valve and outlet valves.
• Check whether power supply is available to the compressor motor.
• Ensure the compressor is in manual mode.

During start 

• Open the compressor inlet valve by half a turn and start the compressor.

• Open the compressor inlet valve slowly to fully open position.
• Change the operation of the compressor to automatic mode.
• Check the amperage of the compressor.

During operation:

• Check the compressor suction, discharge, and oil pressure.
• Inspect the compressor unit and check for any abnormality and vibration.
• Check the compressor oil level and check whether the oil returns from the oil separator to the crankcase

During auto Stopping:

• As the temperature reaches the set value, the thermostatic switch will cut off the compressor.
• The blower will continue to run.
• When the air temperature increases, the compressor will cut in.

To shut down for longer duration:

• To shut down for prolonged period, close the condenser liquid receiver outlet valve.
• The liquid refrigerant is collected inside the condenser receiver and the compressor stops due to low pressure.
• Shut the refrigerant line valves.
• Shut the condenser cooling water inlet and outlet valves.
• Switch off the power supply.

.impingement

Seawater flowing into the heat exchanger tubes at higher velocities tends to remove the thin protective film adhering to the base metal of the tube wall. This protective film is peeled and allow further corrosion of the tube wall. Due to this continuous process the tube wall is gradually thinned, and the tube-to-tube plate joint is weakened and ultimately fails or the tube walls just beyond the tube plate is perforated. This type of erosion is termed “Impingement Attack” or inlet end attack or bubble attack. This attack is usually taken place in a length of 4 x diameter of tube from the inlet side.

Causes:

• Higher velocity of cooling water
•  turbulence flow give rise to Impingement Attack
• Entrained air bubbles tend to accelerate this action, as do suspended solids.
• Impingement attack is related to cavitation damage, and has been defined as ‘localized erosion-corrosion

Remedy:

1. the velocity of the cooling sea water should not exceed 4 m/sec or fall below 1m/sec.
2. If it is more than 4m/sec, then it will aid in impingement attack
3.  if it is below 1m/sec, then salt and other solid particles will tend to settle on the tube surfaces, which in turn will form into small electrolytic cells resulting in the erosion of the tube material.
4. turbulent flow is NOT preferred in the shell and tube heat exchangers.
5. To maintain a proper laminar flow, the design of the header and fitting of the zinc anodes are important, care must be exercised that the zinc does not interfere with or add to the turbulence to the fluid within the header
6. the tube inlet ends are fitted with nylon ferrules to protect tubes of large heat exchangers

Chemical handling onboard:

.chemical handling

.chem handling

• Make sure all crew members have easy access to Safety Data Sheets and that these are kept up to date.
• Ensure all chemicals are stored safely – and, if possible, restrict access to authorised people.
• Provide specific training courses for crew members using or handling hazardous substances.
• Fully assess tasks before procuring personal protection equipment (PPE) and introduce processes to prevent improper PPE being used.
• Make sure all crew members are aware of the risks associated with chemicals as well as your vessel’s first aid steps.
• Make sure suitable first aid measures in place.

Article link: https://oceantimemarine.com/what-to-do-with-chemicals-on-board-your-vessel/

Storage

• DO store chemicals in a suitably contained safe area that is well marked. Class 8 (corrosive) and class 5 (oxidisers) for example should be in well segregated areas.
• DO store chemicals in containers kept at single level so ingredients cannot mix
•  DO keep chemicals below eye level to avoid accidental spillage over your face
• DO keep the lids on chemical containers on tight and secure so the contents cannot mix or spill if you are moving the product
• DO make sure the chemical is well labelled and can be easily identified.
• DO NOT mix chemicals. This can result in serious harm including death.
• DO NOT mix drum pumps between chemicals as a bad reaction may occur.
• DO NOT put chemicals into unmarked containers.
• DO NOT use old food or drink bottles as accidental ingestion can occur and will cause serious internal injuries or death.

Directions for use

• DO wear the correct PPE (personal protection equipment) as recommended on the material safety data sheet (MSDS) and the label. This may include goggles, gloves, respirator, suits and boots.
• DO read the label on the container as this gives usage and mixing directions. The container may have POISON or a Dangerous Goods Diamond with Class designation as to what type of hazard it presents on it.
• DO make yourself familiar with the MSDS. MSDS is provided with every container and provides information about that product such as description of the chemical, specifications, and safety and emergency instructions.
• DO mix or dilute the chemical to the supplier’s specifications only. Mixing an acid based chemical with a chlorinated chemical will result in a deadly chlorine gas being emitted.
• DO make sure cleaning is completed before leaving the area to prevent accidental skin contact with the chemical.

First Aid

• know where your first aid station is located
• read and be familiar with first aid instructions about the specific chemical being used which can be found on the label and MSDS sheet
• be familiar with the location of eye wash facilities around the vessel. In the event of a splash in the eyes you need to be able to automatically find the closest water source.
• a spill to the eyes should be washed under cool running water for 15 minutes

Emergency

In case of an emergency: know where emergency equipment, such as fire extinguishers, hoses and eyebaths, are located know what the vessel’s emergency procedures are and how to fulfil that task.

.plate type cooler

Description:

Methods of expansion

There are three methods of expansion

1. Shell and header fixed, tube stack expands:

1. One end of the tube stack is fixed(bolted)with shell and header
2. Other end of the tube stack is free to accommodate expansion
3. O-rings are fitted in the grooves circumferentially around the tube plates to prevent any leakage of oil into sea-water or vice versa
4. In case of o ring leakage, oil/water will come out through the telltale hole

2. Tube stack and header fixed, shell expands:

1. Both ends of the tube stack are fixed with shell and header
2. A bellows ring is welded circumferentially around the shell which gives room for expansion.

3. Shell, tube stack, header fixed, tube expands:

1. In this arrangement shell, tube stack and header are fixed
2. At the inlet end, the tube is fixed rigidly to the tube plate and at the outlet end, the tubes are allowed expand.
3. The holes, in the inlet side tube plate, are made slightly larger in diameter than the tube diameter.
4. The holes in the outlet side tube plate are stepped and threaded

.ftf    .frequent tube failure   .ctf .hetf    .htf

Link: https://www.fluiddynamics.com.au/six-causes-of-heat-exchanger-tube-failure/

1. Tube Corrosion

The biggest threat to shell and tube heat exchangers that use carbon steel tubes is oxidation (corrosion) of the heat transfer surface of its tubes.

The reaction between oxygen (O2) and iron (Fe2, Fe3) is the most commonly observed form of corrosion. This reaction yields a building layer of iron oxide (Fe2O3) on carbon steel tubes which results in decreasing thermal permeation and eventually the deterioration of the tubes. This problem is difficult to combat and is often only detected when tubes become so corroded their thermal performance levels decrease, the fluid flow is significantly reduced or the tubes are perforated and leak.

2. Tube Erosion

Erosion of tubes is the physical wearing of the metal by fluids. Fluids with high levels of total dissolved solids – such as silica, silt or sea water containing salt, sand and marine life – catalyze the erosion of tubes both internally and at the leading edges of the inlet tubes.

Although all tubes are subject to erosion over time, the weakest points for tubes are generally the U bend (if any) and the leading edge of the inlet tubes.

Tube-side fluid velocity in excess of manufacturers’ recommendations can lead to erosion damage along the internal face of the returning outer bend of the U-bend. The change in direction of flow at this point introduces resistance to its flow causing the force of the fluid, and any particulates in it, to concentrate against the far wall of the tube, constantly eroding the tube at this point.

Inlet Tube-end Erosion

Significant erosion of the tubes can also be found at the leading ends of the inlet tubes, where the tubes are connected through the tube sheets and face the full force of the incoming fluid. At this point the division of fluid flow from a single stream into many smaller streams results in turbulence and extremely-high localised velocities.

3. Steam or Water Hammer

Steam or water hammer is a powerful force and can cause the rupture or collapse of either the shell or the tubes of a heat exchanger. Hammer generally occurs where there has been a surge in pressure commonly caused by a sudden interruption in cooling water flow, the rapid vaporization of stagnant water or pump malfunction. The phenomenon can be observed in feed-water heaters where high steam pressures increase the chances of hammer. Hammer can often be heard, but only rarely will it damage the shell. Tubes, being weaker than the shell, are the more likely victims of hammer, however damage to tubes will only be detected on internal inspection or when leaks become apparent.

4. Thermal Fatigue

Heat exchanger tubes are vulnerable to tears and cracks due to accumulated stresses related to constant thermal cycling or high temperature differentials. Thermal fatigue occurs when extreme temperature differences between the shell and tubes result in tube flexing.

Thermal fatigue may cause the tubes to warp, producing stress loads that exceed the material’s tensile strength and will eventually rupture the tube.

Another result of high temperature differentials is the physical thermal expansion and contraction of tubes along their length, which may eventually compromise the integrity of a tube’s connection to the tube sheet, causing leaks.

The threat of thermal fatigue is almost impossible to diagnose until a failure has occurred.

5. Vibration/Resonance

Vibration and resonance, from whatever source and whether induced externally and internally, can impose powerful forces on heat exchanger tubes and, once vibration or resonance is commenced it can increase in intensity to a point where tubes rupture and fail or lose their seal with the tube-sheet and leak.

Baffles provide a vital support for the tubes in a shell and tube heat exchanger and direct the flow of the shell-side fluid to assist in thermal energy exchange. Heat exchanger tubes are normally either welded or tightly roller-expanded into their tube sheets to ensure the join does not leak. Both the sites of a tube’s contact with baffles and tube sheets are points of weakness.

Excessive tube-side velocities of fluids may result in a tube vibrating or resonating at high frequency, causing abrasion between it and the baffle edge. This can cause either the tube to rupture or the tube’s bond with the tube-sheet to fail.

Equipment or machinery, to which a shell and tube heat exchanger is attached, may also transfer its external vibration to heat exchanger tubes and cause damage or failure.

6. Pitting of Tubes

Chemically-induced corrosion can result in the pitting of heat exchanger tubes to the point where pinholes form and the tube fails and leaks.

Pitting results from the electrochemical potential set up by differences inside and outside of, what is commonly referred to as, a concentration cell. The oxygen-rich environment in this cell acts as an anode and the metal surface as a cathode, resulting in the metal surface being slowly pitted by the chemical reaction.

A concentrated electrochemical gradient of oxygen (O2) and carbon dioxide (CO2) is frequently the cause of tube wall pitting, as is the presence of excess chemical compounds such as chlorides and sulphates often found in inadequately treated cooling water.

2.1 General

• Valves and sea chests are to be easily accessible and permanently marked. Valves not easily accessible are in addition to be fitted with remote control.
• Shell valves are to be manufactured from non-heat sensitive materials 
• Normally bronze or other approved material
• Grey cast iron is not acceptable.

Shell and tube heat exchanger

Tube type cooler .cooler .heat exchanger

.tube cooler

Cooler article: https://marineengineeringonline.com/tag/l-o-cooler-leakage/

How many tube can be plugged: 10% tube can be plugged

.anode types .anodes .zinc anode .aluminium anode

Commonly used anodes are Zinc, Aluminium, Soft iron, Magnesium

Advantages of aluminium anodes

Weight: Aluminium is significantly lighter than zinc, by a factor of 2.5. Al anodes are lighter to ship and to fit.

Capacity: The electrochemical capacity is more than 3 times higher than of the same mass of zinc (we can protect more with less).

Driving voltage: Aluminium anodes has a relatively high driving voltage. This means that it provides better distribution of the current, compared with zinc.

Environment: Aluminium anodes carry a better environmental footprint than zinc anodes. Aluminium anode alloys do not contain cadmium, which is harmful to the marine population.

Cost: Aluminium anodes are less expensive, considering the significantly reduced weight requirement compared with zinc.

Advantages of zinc anodes

Availability: Traditionally used by the maritime industry, hence zinc anodes are widely available.

Geometry: Zinc anodes can be produced in rather complex geometry, as opposed to aluminium. This is particularly important for slender designs, such as rope guard anode rings.

No restrictions for use in tanks: Zinc anodes are not subject to the same class restrictions as aluminium for use in tanks with possible explosive atmosphere.

The anode surface corrodes more evenly: Zinc anodes tend to dissolve more evenly and completely; while typical aluminium anodes erode unevenly with visible “craters”.

Soft iron anode:

Soft Iron Anode

It’s mostly used for the protection of copper alloys of heat exchanger, freshwater facility, desalination plant (evaporator, brine heater) and condenser (used for marine and platform).

Magnesium anode:

It is mostly used in fresh water systems.

.mgps

The Marine Growth Prevention System (MGPS) has been developed for ships with the sole purpose of tackling marine organism growth, preventing it from depositing on the ship’s interior piping systems, which are continuously supplied with sea water.

The anode in the MGPS system generates ions that spread in the seawater system, producing an antifouling and anti-corrosive layer over the internal sides of sea pipes, heat exchanger (i.e. coolers and condensers), valves in seawater system, refrigeration systems, AC units etc.

The three types of alloys used for anodes are:

1. Copper Alloyed Anodes: This is the most used type to prevent marine fouling in piping, strainers, heat exchangers, pumps etc.
2. Aluminium Alloyed Anodes: This type is used in conjunction with copper alloy anodes to prevent corrosion throughout the ferrous piping system.
3. Ferrous Alloyed Anodes: They are used in conjunction with copper alloy anodes to prevent corrosion throughout Cu/Ni pipework.

An MGPS system can be installed on the ship in following ways:

1. Anodes Mounted on Sea Chest: They are commonly installed in new buildings and have a working life such that they can run till the next drydocking.
2. Anodes Mounted in strainers in the seawater pipeline: They have an advantage of replacing the anodes without affecting the seawater supply to ship’s system.

• Treatment tank setup with a spray nozzle in sea chest: In this system, a separate electrolysis tank with anodes is installed which sprays the ion through the nozzle in the sea chest. This system is installed on ships where sea chest or strainer mounting is not possible.

.hull protection .ships hull protection .ship hull protection

Sea Water System Protection against Corrosion:

Sea water systems such as ships hull protection may implement several methods for prevention against corrosion. Such systems are:

Sacrificial Anodic Protection

1. Sacrificial anodes work on the principle of electrolysis, and form a galvanic cell
2. When 2 dissimilar metals are in contact with each other in the presence of a corrosive medium (electrolyte), the more active metal in the galvanic series acts as an anode and undergoes corrosion. This means, in a galvanic series of metals, the more active metal acts as anode and undergoes corrosion and the less active metal acts as a cathode and stays protected.
3. an anode and a metallic strip are dipped in electrolytic solution,
4. Anode electron will dissolve and deposit over the metallic strip and make it a cathode.
5. Seawater acts as an electrolyte and transfers the electrons from the anode to the steel plate and making a protecting layer.
6. If the metal is more active, it will be easily oxidized and will protect the metallic compound by making it act as cathode
7. The anode will corrode first sacrificing itself for the other compound and it is thus called sacrificial anode
8. Here Active means more electrochemical potential. (magnesium, aluminium and zinc)

Impressed Current Catholic Protection

.iccp

1. Parts to be protected are made cathodic,
2. The anode does not corrode as electron is not generated by it but impressed on it
3. The electric current is supplied from ship supply and converted to low dc voltage
4. Current is impressed on anodes to reduce the potential difference between hull and anode. Hence no chemical reaction
5. Reference cells control the amount of current,
6. If too low, corrosion takes place
7. If too much will damage paint and protective coatings

Hull painting

1. AFS approved anti-fouling paints can be used on ship hull.
2. Anti-fouling paints are used to coat bottom of ship to prevent sea life from attaching themselves to hull, which may slow down the ship increasing the fuel consumption
3. Anti-fouling system is coating, paint, surface treatment, device used on ship to prevent fouling
4. In the past anti-fouling paint contain TBT, caused harm to marine life,
5. Todays technology of anti-fouling paint, do not contain TBT(tributyltin), It provides slippery surface preventing fouling and making easier to clean.

.corrosion prevention .seawater protection .sea water protection .sea water system

.prevention of corrosion

Sea water systems can be protected using some prevention measures, such as-

1. Material selection should be good.

            -steel

            -copper alloy

            – aluminium alloy can be used.

2. Improvement (alloy) in material should be done

3. Alteration of environment can improve corrosion such as

            – low temperature

            – lower flow rate

            – change pH – increase alkalinity

            -add inhibitors

4. Cathodic protection such as

            – sacrificial anode

and

            – ICCP (Impressed Current Cathodic Protection) gives protection

These Can protect  sea water system.

5. Coating protection

            – Paint

            – Metal painting

            – organic coating (rubber lining)