Section 3.2 Pumps and Pumping Systems Flashcards
What are the types of pumps commonly used in the maritime industry?
i. Roto-dynamic / Centrifugal Pump (Rotaty Pump)
ii. Positive Displacement Pump (Reciprocating Operation)
Some pumps are designated to be bi-directional and uni-directional.
Describe a roto dynamic pump.
The roto dynamic centrifugal pump is fitted with an impeller that can be either single-sided or double-sided. The free-standing pumps can be either horizontally or vertically mounted (ballast water pumps in the engine room); they can also be fitted with an air extractor assembly that will manage the variation in the suction in the case of removing water with a negative suction head.
Normally a vessel is fitted with roto dynamic centrifugal pumps have these pumps installed below the water line or with a positive head in the case of jacket cooling pumps used on the main engine in the case of a motor ship.
In the case of boiler feed pumps, the use of multistage pumps utilising more than one impeller to increase the pressure without altering the total water throughput.
The impellers are generally closed type impellers except where the water can be contaminated, as in the case of pumps used to handle dredge spoil. This type of pump can be used as operational pumps for transferring fluids from sewage and wastewater systems, including bilge water treatment.
Describe positive displacement pumps.
The positive displacement pumps are used to pump a large range of fluids that can be highpressure discharge and generally higher than centrifugal pumps and also lower than the volume of the discharge.
Positive displacement pumps are generally arranged with a relief valve arrangement to avoid overpressure situations that may damage the pump casing impeller or drive shaft.
The positive displacement pump is able to be designed in a rotary or reciprocating design.
The rotary positive displacement pump is suitable for use at very high pressures and, therefore, ideal for use as the prime mover for hydraulic pumping systems.
Reciprocating pumps are suitable for use as the bilge pumps being able to be primed easily.
The air-operated reciprocating pump, such as the Wilden pump, is able to pump almost any fluid regardless of temperature, chemical composition or viscosity. This type of pump is designed as a portable pump and this allows for the pump to be used for a wide range of operational requirements.
The rubber impeller (Jabsco) pump is normally not fitted with a relief valve in operation but the impeller can be damaged while the shaft and casing remain undamaged in operation.
The bore hole windmill pumps often used to service water on farms in the Australian countryside are a further example of a positive displacement pump.
Describe the materials used to manufacture valves.
Valves fitted to the shell of the vessel are all subject to ‘Class’ survey. The material that is used should preferably be corrosion-resistant such as cast steel, bronze or 316 stainless steel.
The valve trim may not always be the same as the body of the valve and you may have a cast steel body with bronze trim; this could also be a cast bronze body with stainless steel trim. The valve should always be a class-approved valve that is used for the purpose of the design.
Grey cast iron cannot be used for class-approved work. The grey cast iron is subject to corrosion and while the valve may appear to be sound, the iron particles may have corroded, and a sharp tap with a hammer will reduce the valve to powder in extreme circumstances.
If a valve is replaced in a suction position in a sea chest or a ship side valve, the valve should be specifically designed for the purpose and should be purchased with a Class approved certificate and the valve stamped accordingly.
Pumping System Details
These pumps include mono pumps which are manufactured as a positive displacement pump with a ‘worm’ rotor turning in a hard rubber stator. These pumps can pump soft material, including water oil and, in some cases, material such as sewage and galley waste material.
Two or three screw pumps, also positive displacement pumps, are used to pump oil at various pressures and are often used as the main oil pump in large slow-speed engines.
All pumps require maintenance from time to time. Most pumps are fitted with mechanical seals that, while very reliable, will require the replacement of some items, which may include the carbon seal in the sealing face is damaged.
O rings and gaskets are required from time to time. The impeller and drive shaft on the centrifugal pump is likely to be affected, particularly by erosion
Describe high pressure special pumps
High-pressure pumps specifically designed for boiler feed pumps which may be able to pump boiler water to 100 bar pressures. These are often turbo-feed pumps. They are described as having a horizontal ring section, radial flow centrifugal pump with vertical suction and discharge
nozzles.
These pumps are a multi-stage operation to deliver boiler feed water into the boiler. The water is in a clean condition but may have a temperature in the order of 120 degrees Celsius.
Construction of these pumps, often referred to as a multistage pump, consist of the flowing base components, a stator that consists of a suction casing and a discharge casing, and a diffuser.
These primary parts are secured using external tie rods. The rotor that consists of a shaft fitted with impellers and a balance disc over a shaft sleeve.
Bearing parts include the bearing housing and covers with one bearing (fixed) and one expandable bearing (roller). Mechanical seals are used to retain water and protect against leakage.
List and describe some common types of valves.
- Screw-down non-return valves - are generally fitted as the overboard discharge valve; they are subject to survey. Other SDNR valves are used on the bilge system to avoid back flooding in the system.
- Screw lift valves - these are often selected for ballast valve isolation but almost never used for operational valves with the requirement of remote operation requiring a different valve type.
- Gate valve - a cheaper alternative to the screw lift valve often made from grey cast iron but these cannot be fitted to the ship skin because of the risk of corrosion.
- Butterfly valve - often used in areas where the use of remote operation is required. The valve is very cheap to purchase and is robust, using a stainless steel disc and rubber seat. The operation of these valves go from fully closed to the maximum open position in 90 degrees. The main use, of course, is for remotely operated ballast systems.
- Half rotary cocks - are one of the cheaper alternatives; these are generally mass-produced. They can be made from cheaper brass and then plated using chromium electro-plating. The valves can be produced in stainless steel bodies and ball with a teflon seating; in either case, the valve being mass-produced provides a very cheap and robust valve. It is not class approved but can be used in many positions.
- Quick closing valves - the valve is used for the emergency closing systems on tanks that are holding hydrocarbons - mainly fuel and lube oil. The tanks are situated above the doublebottom tanks and the purpose is to isolate the fluids. They can also be fitted in some water
systems to avoid losing the water from the tanks. The valves are operated using a mechanical connection to operate which can be as simple as flexible wire drawn in smaller vessels to air or hydraulically operated valves. In all cases, the valves are to be class approved. - Foot valves - the valve is fitted in the suction connection to a pump to assist with the priming, particularly with a centrifugal (roto-dynamic) pump. The pump is particularly
useful in the suction of an emergency pump to provide a close-up connection to the suction of the pump. They are generally made from non-ferrous or stainless steel. - NR (non-return) valves can be configured for fitting either horizontally or vertically; the purpose is to avoid loss of suction or to prevent water, oil or steam short cycling of the fluid or gas.
What are the different pumping operations that happens onboard?
- Ballast systems use centrifugal pumps.
- Bilge systems can use centrifugal pumps or reciprocating pumps.
- Fuel and lube systems. Using low-volume high-pressure pumps, positive displacement
fitted with a relief or pressure limiting pump. - Cooling water, both fresh and sea water, have closed-loop systems.
- Wastewater can use a variety of pumps, but centrifugal pumps are often used.
Bilge systems
- Planned maintenance of pumps and pipelines is essential.
- Strainers and strum boxes need to be regularly serviced and kept free from debris.
- All valves should be screw down non-return valves to avoid back flooding in holds
machinery spaces and storage spaces. - The oily water separator (OWS) should always be in working condition with the oil content
monitor. - Bilge levels should be checked and levels adjusted on a daily basis, and the bilge water
holding tank should be maintained at a working level along with sludge and waste tanks. - The direct bilge suction valve should always be ready for use.
When the oil content within the bilge water that is discharged into the sea exceeds 15ppm an alarm should sound.
Vessel needs to be underway and traveling more than 6kts. The oil record book needs to have an entry with start and stop positions with amount of discharge. 12nm from the nearest shore.
What are the reasons to maintain watch over the bilge water levels?
- It is necessary to check and adjust the bilge levels not only in engine and machinery spaces but holds, forecastle and storage spaces.
- Engine room bilges, because they often contain bilge water soiled with fuel or oil residues, they provide a possible fuel source for fires.
- Engine room bilges should be maintained and kept free of any fluids located above the bilge wells in the machinery and engine spaces.
- If in sufficient quantities, bilges can influence the free surface effect.
- Bilge water in sufficient quantities in holds can damage cargo.
- In some specific instances, methane gas can be produced in the atmosphere posing a risk to the crew.
Why is there a ballast system onboard?
Improve stability.
2. Adjust the trim.
3. Correct a list.
4. Reduce the stress on the structure.
5. Ensure the propeller immersion.
Ballast Water Management Systems
The plant and equipment associated with the system to avoid carrying pathogens and vegetable matter includes the use of fine filter intakes that can be back flushed and maintained in continuous working condition and either ultraviolet sterilisers, or alternatively, the use of chemical sterilisers utilising chlorine systems in various forms.
It should be remembered that there is no single system that meets all requirements. The filter operation is capable of operating in a continuous manner and the back flushing and the filter element maintains the size of particle that passes through the screen. The flushing system should prevent the passage.
Ballast Filtering Systems
The primary treatment is provided by a filter system.
This can be installed with additional features, including the use of automatic back-flushing systems.
The filter system can be set at a 20-micron range and the automatic system for backflushing arrangements relies on the pressure drop across the filter to start the back-flushing operation.
