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Nash rotary liquid ring pumps & Condensate extraction pump for motor ships

Nash rotary liquid ring pumps, in association with atmospheric air ejectors, may be used instead of diffuser-type steam ejectors and are arranged as shown in Figure 1. The pump, discharging to a separator, draws from the condenser through the atmospheric air ejector, creating a partial vacuum of about 600 mm Hg.


Figure 1: Nash liquid pump ring

At this stage the ejector, taking its operating air from the discharge separator, which is vented to atmosphere, comes into action, sonic velocity is attained and vacua in the order of 725 mm Hg maintained in the condenser. The liquid ring pump is sealed with fresh water recycled through a sea-water cooled heat exchanger.

The Nash pump, using recycled fresh water for sealing, may be also used to provide oil-free instrument air at a pressure of 7 bar


Figure 2: Recycled fresh water

Condensate extraction pump for motor ships

Removal of condensate from a condenser imposes very difficult suction conditions on the pump. The available net positive suction head (NPSH) is minimal because the condenser is situated low in the ship permitting a static suction head of only 450—700 mm and the condensate is at, or near its vapour pressure. It is necessary therefore to ensure that the pump's required NPSH is correspondingly low and to this end suction passages and inlets are given ample area. The purnps used for the duty, are two stage units the first stage impeller being arranged as low as possible in the pump with an upward facing eye. This impeller feeds a second impeller via suitable passages in the pump casing.

Where extraction pumps are fed from de-aerators or drain coolers, the pump suction level is maintained constant by a specially designed float control. In instances where the pump is drawing from the main condenser however, it is common practice to operate the pump on a free suction head. This means that the pump must operate with a variable NPSH at varying flow rates.

The natural H/Q characteristic is modified as the discharge pressure is reduced to that required to overcome the resistance of the system at the reduced flow rate. When the suction level increases the flow rate increases. Therefore, inherent control of the flow rate is achieved without the use of a float controlled regulator. Such a system is usually referred to as cavitation control. The extraction pump is a cavitating pump.


Summarized below various circulating systems for motorships, some of the basic procedure of heat exchangers & control of temperatures:

1. Sea water circulation-systems

The usual arrangement for motorships has been to have sea-water circulation of coolers for lubricating oil, piston cooling, jacket water, charge air, turbo-charger oil (if there are sleeve type bearings) and fuel valve cooling, plus direct sea-water cooling for air compressors and evaporators....




2. Shell and tube heat exchangers for engine cooling water and lubricating oil cooling

Shell and tube heat exchangers for engine cooling water and lubricating oil cooling have traditionally been circulated with sea water. The sea water is in contact with the inside of the tubes, tube plates and water boxes....


3. Plate type heat exchanger

The obvious feature of plate type heat exchangers, is that they are easily opened for cleaning. The major advantage over tube type coolers, is that their higher efficiency is reflected in a smaller size for the same cooling capacity....


4. Details of charged air cooler

The charge air coolers fitted to reduce the temperature of air after the turbo-charger and before entry to the diesel engine cylinder, are provided with fins on the heat transfer surfaces to compensate for the relatively poor heat transfer properties of air....


5. Maintenance of heat exchangers

The only attention that marine heat exchangers should require is to ensure that the heat transfer surfaces should remain substantially clean and flow passage generally clear of obstructions. Indcation that fouling has occured is given by a progressive increase in the temperature difference between the two fluids, and change of pressure....


6. Central cooling system & Scoop arrangement for motorships

The corrosion and other problems associated with salt water circulation systems can be minimized by using it for cooling central coolers through which fresh water from a closed general cooling circuit is passed. The salt water passes through only one set of pumps, valves and filters and a short length of piping.....


7. Circulating systems for steamships

The main sea-water circulating system for a ship with main propulsion by steam turbine is similar to that of a motorship with a central cooling system. The difference is that the sea water passes through a ....


8. Closed feed system and feed heating for motor ships

To ensure trouble-free operation of water-tube boilers the feed water must be of high quality with a minimal solid content and an absence of dissolved gases. Solids are deposited on the inside surfaces of steam generating tubes,....


9. Marine condenser assembly

A condenser is a vessel in which a vapour is deprived of its latent heat of vaporization and so is changed to its liquid state, usually by cooling at constant pressure. In surface condensers, steam enters at an upper level, passes over tubes in which cold sea water circulates, falls as water to the bottom and is removed by a pump (or flows to a feed tank)....


10. Three stage air ejector with internal diffusers

A steam-jet ejector may be used to withdraw air and dissolved gases from the condenser. In each stage of the steam-jet ejector, high pressure steam is expanded in a convergent/divergent nozzle. ...


11. Pressure governor for motor ships

The main feature of the governor is that if the pump loses suction the steam ports are opened wide, allowing the pump to accelerate rapidly to the speed at which the emergency trip acts....


12. Liquid ring pump- Nash rotary liquid ring pumps

Nash rotary liquid ring pumps, in association with atmospheric air ejectors, may be used instead of diffuser-type steam ejectors and are arranged as shown...


13. The Weir electro-feeder - a multi-stage centrifugal pump

A multi-stage centrifugal pump mounted on a common baseplate with its electric motor. The number of stages may vary from two to fourteen depending upon the capacity of the pump and the required discharge pressure....


14. Feed water heaters for motor ships

Surface or direct contact feed heaters, play an important part in the recovery of latent heat from exhaust steam. Direct contact feed heaters are also known as de-aerators....


15. Devaporizer & turbo-feed pump

If the de-aerator cannot be vented to atmosphere or to a gland condenser satisfactorily, a devaporizer is connected to the vapour outlet condensing the vapour vented with the non-condensable gases and cooling these gases before they are discharged. ...


16. Typical de-aerator & Cascade trays

Normally, the de-aerator is mounted directly on a storage tank, into which the de-aerated water falls, to be withdrawn through a bottom connection by a pump or by gravity. The tank usually has a capacity....

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