pump theory & characterstics ch.7

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Transcript of pump theory & characterstics ch.7

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7 CARGO AND BALLAST PUMPS Learning objectives -that the suction action of a pump is really atmospheric pressurepushing the liquid into the inlet side of the pump-how low vapor pressure of the liquid being pumped will improve suction-that the discharge pressure will fluctuate when the liquid boils-the meaning of head-the meaning of NPSH-the meaning of typical Q-H curve and the shore installation curve-that the actual discharge rate also depends on static and dynamicbackpressure of the shore installation -the meaning of pressure surge -that the system is liable to serious pressure surges if valve-closure time is equal to or less than the pipeline period 7.1PUMP THEORY AND CHARACTERISTICS 7.1.1 Classification and selection of pumps There are a number of different pump types. Each type has its own special quality and therefore certain advantages and disadvantages. The selection of pumps is determined by a thorough study of the capacity needs and under which operational conditions the pump will operate. The following factors are important when you evaluate these conditions: Estimated back pressureCapacity requirementCapacity rangeRequirement for installation and arrangementExpenses for purchase, installation and maintenanceAvailability of parts and serviceSuction termsCharacteristics for the liquid to be pumpedSelection of the right pump for a determined purpose qualifies a close co-operation between the customer and the producer of the pump. The customer has a special responsibility to clarify all conditions concerning the pump installation, so the producer can choose the best pump from his product range with the best match. When you choose a pump you must find out how much the pump needs to deliver under a specific condition. Definition of capacity range is important. Demand for capacity or capacity range and expected discharge pressure must be specified. The capacity requirement is determined by the intended use of the pump. The discharge pressure is determined by various conditions where the pumps delivery pipeline design, the capacity of the pump and the liquids characteristics, is the essential. Alternative installation locations of the pump are limited due to special demands from Class and Shipping Authorities and also from lack of space.

Purchase and installation cost is important. Future maintenance expenses, availability of parts and service now and over the next years, are also important and must be included in the evaluation of alternative pump supplies. The liquids properties and which other arrangements you have to consider, often limits the options. Density, viscosity and boiling point are important properties to consider. The liquid temperature and corrosive properties are important factors when pump material is selected. The pumps suction condition is determined from where the pump is located in relation to the liquid to be pumped. A given suction pipe creates a certain resistance that will have influence on the pump capacity. The main principle is to minimise resistance on the suction side by decreasing the suction pipe length, have the largest diameter possible and few as possible restrictions in form of bends, valves and so on. The different types of pumps are divided into two main groups, displacement and kinetic pumps. The displacement pumps displace the liquid by reducing the volume inside the pump. An example is a piston pump where the piston is moving up and down inside a cylinder or when the screws revolve inside a screw pump. Kinetic pumps (kinetic energy is equal to movement energy) increase the liquids velocity through the pump. The diagram below gives a brief view of the different available groups and types of pumps. The diagram would be more comprehensive if the pumps were divided in all details according to number of rotors, design of pump inlet/outlet and flow directions. Single action Double action Piston pumps Resiprocal pumps Single rotor Gear pumps Screw pumps Multi rotor Rotating pumps Displacement pumps The Ejektor pump Special pumps Single stage pumps Multi stage pumps Single suction Axial flow Diagonal flow Radial flow Centrifugal pumps Kinetic pumps Types of pumps A kinetic pump like the centrifugal pump increases the liquids velocity in the pump by means of a rotating impeller. A displacement pump, like the piston pump, mechanically displaces the liquid in the pump, either by help of a piston or screws. Resistance on delivery side gives a liquid pressure rise (pump delivery pressure). One should be aware of this difference for these two pump types. The pressure rise on a kinetic pump is restricted by the increase in velocity over the pump, which is controlled by the pump design. All kinetic pumps therefor have a designed or built- in limitation for maximum discharge pressure. The displacement pumps limitation depends only on available power and the constructional strength. In contrast to a kinetic pump, such a pump will operate against resistance with all its available power. A closed-delivery valve after a displacement pump is damaging. The same closed valve for a kinetic pump will not bring any immediate danger. Piston pumps and screw pumps have good suction capacity and are used where these characteristics are required. The weakness of these pumps is the complex construction and the relatively low capacity.

Centrifugal pumps are simply constructed with few parts and no valves. There are no immediate problems if the outlet of the pump is closed. These qualities result in relative low purchase and servicing costs. Operation at high speed makes the pump small in proportion compared to the capacity and flexibility in relation to the pumps location. The most negative side of using a centrifugal pump is the lack of self-priming capacity. This weakness is improved by constructional efforts and positioning, which consolidate the free flow of liquid. Location of a pump, for instance below the liquid level, can reduce the flow resistance. High viscosity liquids are therefore particularly difficult to pump due to this condition. A centrifugal pumps efficiency is high only within a small range. This is the reason it is especially important to have a clear understanding of what capacity range the pump will operate under, in connection with the selection of a centrifugal pump. The differential pressure over each impeller is relatively low. Using so-called multistage pumps where several impellers are mounted in serial, increase the pumps capacity to deliver against higher backpressure. A centrifugal pump will, without a non-return valve on delivery side, give complete back flow at the time the pump stops. For all operators of centrifugal pumps, this relationship is important to know. Centrifugal pump 7.1.2 The ejector The ejector design is simple and is used for stripping. This ejector has no revolving or reciprocating parts and is thereby especially easy to maintain.

The propellant (driving water), a liquid or gas, is forced through a nozzle into a mixer tube. The velocity of the propellant will naturally increase as it passes through the nozzle. Due to the propellants velocity and direction, plus the friction force between the propellant and the liquid, the surrounding liquid will be sucked into the ejectors mixer tube. The mixer tube is connected to an expanding tube, the diffusor. Here some of the kinetic energy supplied to the liquid in the mixer tube is transformed into potential energy. The capacity depends on the friction force between the two mediums, suction head, delivery head and the propellants velocity. The ejector has the advantage that it does not lose the suction capacity even if it sucks air or vapour. The ejectors efficiency is between 30% and 40%. Even if the propellants efficiency is up to approximately 70%, the total efficiency for the whole ejector system is far less than compared to a pump system, such as a centrifugal pump. Another drawback with ejectors is that the propellant is mixed with the pumping liquid. This implies that if the ejector is to be used in cargo transfer operation, the cargo itself must be used as propellant liquid.The ejector is frequently used as a bilge pump in hold spaces. A common arrangement for a hold space is as follows: The ejector is usually submerged in a bilge sump and the propellant is normally supplied from a seawater pump. Onboard gas carriers where the hull is the secondary barrier, the ejector may also be used to pump cargo from hold space. In that case, the liquefied cargo itself must be used as a propellant . 7.1.3 Tips Be aware that the ejector has a limitation on the propellants pressure. Higher pressure than recommended by the supplier may result in reduced suction capacity. Start the ejector by opening all valves on delivery side first, and then adjust the correct propellant pressure. The ejectors suction valves should be opened last, which will prevent the propellants flow back into the tank that is to be stripped. Stop the ejector by using the opposite procedure.

As the drawing shows the ejector is positioned 3 meters above the liquid level. The liquid level in the slop tank is 15 meters above the ejector and the propellant's pressure is 8 bars. The ejectors capacity can be found by use of the performance curve for the specific ejector. In the performance curve the ejector capacity is set as a function of the propellant pressure. Observe that this curve has curves for different suction lifts. The different performance curves are marked with different suction lifts. The ejectors suction lift in this example is 3 meters; this specific curve shall be used. You can find the capacity of the ejector by drawing a vertical line from 8 bars on the scale for a delivery head of 15 meters and up to the performance curve with a suction lift of 3 meters. From this point of intersection, draw a horizontal line to the left and over to the ejectors