Pump types--part 2 (Rotodynamic, part 3)

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__3.15 Marine pumps

Marine pumps are included as important components in, not only ships and oil rigs, but also underwater equipment. Pumps for ships can be classified as follows: Systems for propulsion, e.g. pumping fuel, lubricating oil, cooling water, feed water and condensate

--Systems dependent on the cargo

Safety systems, e.g. for bilge pumping, ballast pumping, for fire fighting

Domestic housekeeping type systems for making and pumping drinking water, hot and cold fresh water, sewage etc The types of pumps used for fuel is dependant upon the type of fuel. Many ships use heavy fuel which is about 3500 Redwood No 1. Rotodynamic pumps would be inappropriate. Double acting piston pumps act as transfer and low pressure feed pumps.

Plunger pumps, usually built by the engine manufacturer, inject the fuel into the cylinder. Special lubricators, driven by eccentrics or geared to an engine shaft, inject lubricating oil into the cylinders. Screw or gear pumps provide pressurized lubricating oil. Fresh water for engine cooling, and seawater for cooling are provided by centrifugal pumps. Packages are available using double ended motors which supply fresh and seawater simultaneously from two pumps on a common baseplate. Vertical units have been built with fresh and seawater pumps driven by a normal motor. Steam turbine ships use multi-stage centrifugal pumps for boiler feed and condensate extraction duties.

Again, the type of cargo pump is dependant upon the cargo.

Crude is a common load but is extremely variable, with low to very high viscosity. Steam driven, direct acting, vertical piston pumps are available which can operate on petrol through to the most viscous crude oil. Electrically driven piston pumps are also available but are not so versatile as the steam versions.

Liquefied petroleum gas and anhydrous ammonia have become popular as a sea cargo. Special centrifugal pumps are available for loading and unloading.

Safety systems for marine applications are usually designed around seawater because it’s readily available. Bilge pumps largely cater for seawater. Some modern pumps have monitoring systems fitted which shut down the bilge pumps if the oil content is higher than 15 parts per million (ppm). The amount of liquid able to be pumped from the bilges is dependant upon the age of the ship. Modern ships tend to have mechanical seals on the stern tube so that very little leakage enters via that route.

Soft packed stern tubes require some leakage for lubrication. In some vessels the level in the bilges must be watched very closely as it has a great effect on stability. Depending on the age of the ship the bilges may be pumped every two hours, at the end of each watch or automatically by float controls. The pumps may be reciprocating or centrifugal or ejectors powered by seawater. Reciprocating and centrifugal pumps would have a priming system to evacuate the suction line. Ballast and fire fighting pumps operate on seawater and would be centrifugal pumps.

Turbine and engine driven vessels can make fresh water from seawater by evaporation or reverse osmosis. Evaporation is a low pressure process requiring single-stage centrifugal pumps.

Reverse osmosis for seawater operates at 70 barg and requires a reciprocal pump or a multi-stage segmental pump.

Other domestic water circuits would have single-stage centrifugal pumps similar to those used onshore.

Ships at sea should be able to function safely and entirely independently, therefore special safety and reliability requirements are necessary. These are determined by marine insurance companies (classification societies) and each country's respective maritime authority. In certain cases additional requirements are imposed by, for example, port and canal authorities.

Various types of pumps are used for the pumping systems mentioned earlier in the same manner as those land based systems.

Rotodynamic pumps on board are often of the vertical type in order to reduce space. The mechanical design is adapted, generally reinforced, to take account of the ship's movement at sea and automatic, separate or built-in air ejection systems are present to avoid operational problems in the case of long suction lines or if the suction intake is momentarily above the surface of the liquid. As a general rule, bronze or gunmetal is used when pumping seawater and the same material as in the case of land based systems is chosen for other liquids.

Fig. 20 shows a typical vertical pump for general use on board ship. This is a double suction pump with the pump casing divided axially (in a vertical plane) in order to facilitate servicing.

Single suction impellers with axially or radially split casings are used for smaller flows. In the case of a radially split casing, the pump's rotor should be able to be disassembled in a simple manner, like for example, Fig. 5. Section 16, Section 16.3 shows the installation of cargo oil pumps as a practical example.

For marine use, there are many interesting pump applications, such as the steering/propulsion jet operation of non-anchored oil rigs with automatic positioning. A pump system as shown in Fig. 21 and comprising axial pumps designed to operate with any flow direction, is used to limit heeling when loading For example; or for the intentional generation of heel angle in the case of icebreakers.

Small pleasure craft, cruisers and yachts, use flexible vane pumps for many applications. Hand-operated diaphragm pumps have been used for bilge pumps. EN 28849, ISO 8849, specify the requirements for DC electric bilge pumps.

Fig. 20 Vertical ship's pump with soluble suction impeller

Fig. 21 Heeling pump for ships

Self-priming pumps

The majority of pumps must be vented and primed prior to start-up. With pumps that have flooded suctions, this usually involves opening small valves on top of the discharge pipework, so that all the air trapped in the suction pipework and in the pump will be forced out and replaced by liquid. Some designs require vent valves on the casing to fill the pump completely.

When the pump suction is not flooded the vent valve in the discharge pipework is coupled to an air ejector. The air is withdrawn by a partial vacuum and the liquid fills the void. Submersible pumps don’t suffer these inconveniences as the pump is always full of liquid.

Self-priming pumps are designed to overcome these problems.

The casing of a self-priming pump is constructed to retain enough liquid so that the impeller is always flooded. The casing needs priming only once provided the liquid does not evaporate or leak away through poor seals. The pump casing is shaped so that any gas or air bubbles become completely surrounded by liquid and are pumped from the suction to the discharge. Since there is only a limited amount of liquid during evacuation the gas and liquid must be separated, whereupon the liquid is returned to the suction side to recirculate. The internal design and shape of the pump casing to achieve this effect varies considerably depending on the manufacturer. When selecting a pump it’s important to take into account the wearing effect which for example sand, may have upon the pump's evacuating abilities.

When electric power is not available, on building sites for example, other sources of power must be used, such as engines, compressed air from mobile compressors or hydraulically from a separate power pack or one which is built into a vehicle. This type of pump is also suitable for intermittent use in construction work such as road making, rock blasting and excavating or as a fire precaution when carrying out stubble burning or site clearing.

Priming of engine-driven units can be assisted by using the engine inlet manifold vacuum or a separate engine-driven exhauster.

__3.16 Submersible pumps with electric motor

Submersible pumps placed in ponds, ditches and sumps are used to drain, mostly water from locations which are deeper than the sewage system. Three types are available:

--Simple, small, self contained for clean water

--Heavy duty for contaminated water

--Special purpose for sludge

Small submersible pumps tend to be made of plastic where ever possible. This style of construction removes many of the insulation problems associated with electrical equipment. The pumps are close-coupled and completely self-contained. The motor is totally enclosed and cooled by the water surrounding the pump. Level switches, either internally or mounted externally on the casing, detect the water level and switch the pump on and off. Low voltage versions, 110V and 240V, single phase as well as 380V three phase are available. EN 60335-1 and EN60335-2 should be consulted for relevant electrical safety requirements. The smallest units will pump 10m^3/h against a head of 7m. Pipe connections are usually screwed and a non-return valve is required on the smallest units.

Heavy duty electrically driven submersible pumps are close coupled pumps and comprise, together with external level controls, a complete unit. Their weight is the lowest possible due to the use of light metal alloys although their resistance to corrosion is low. The parts which are exposed to wear from the liquid are, on the other hand, of high quality. Typical features are a hard metal impeller, rubberized wear parts around the impeller and hard-faced seals. A non-return valve is built into the discharge connection.

The electric motor is completely encapsulated and cooled by the pumped liquid through a double jacket. In order to protect the motor in the case of blockage or operation without liquid, the motor is provided with a specially built-in motor protection unit.

Normally the motor windings have thermistors which stop the motor when the temperature is too high by means of an inbuilt contactor. The shaft seal is nearly always of a double type with an intermediate oil chamber, see Fig. 22. Normally construction pumps have suction strainers which limit the particle size to 5-10 mm, but there are special models with a through flow of up to approximately 100 mm for pumping larger solids.

Fig. 23 shows a heavy duty pump located in a sump with external level switches. Heavy duty pumps can handle up to 7000 m^3/h at heads up to 100 m.

Heavy duty submersible pumps are used for draining all types of building sites from the smallest ditch or hole to the large projects such as tunneling in mountains or harbor building. This type of pump is practical in the event of flooding and for all other temporary pumping requirements. Of particular convenience is the fact that pump companies in this field offer both small and large pumps for hire for periods ranging from one day to several months.

A special adaptation of the heavy duty submersible pump has been developed for sludge and heavy muds. The impeller and casing are made from hard wear resistant iron alloys, not rubber coated, with an agitator mounted on the shaft end where an inducer would be fitted. The pump is intended for use on liquid solid mixtures with very high solids content where the pump can sit, initially, on the surface. The agitator beats the mixture and induces moisture to travel towards the impeller. The extra moisture locally helps the mixture to flow and the agitator action allows the pump to settle in the mixture. The novel design of the agitator allows these pumps to handle liquid solid mixtures which would otherwise be moved by shovels.

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