Positive displacement pumps efficiency

In general, overall efficiencies of positive-displacement pumps are higher than those of centrifugal equipment because internal losses are minimized. On the other hand, the flexibihty of each piece of eqmp-ment in handling a wide range of capacities is somewhat limited. 

Figure 32.27 shows how a centrifugal pump is affected, particularly at low flow rates, and the behavior is typical of conventional centrifugal pumps. Figures 32.28 and 32.29 present well-known information on the effects of dissolved and entrained gas on the volumetric efficiency of a positive displacement pump. 

Reciprocating FW pumps may have either one or two steam cylinders. They are positive-displacement pumps and usually operate at only slow speed. They are efficient but can rapidly lose efficiency when either the steam or water valves are worn. 

Another efficiency which is important for positive displacement pumps is the volumetric efficiency. This is the delivered capacity per cycle as a percentage of the true displacement per cycle. If no slip occurs, the volumetric efficiency of the pump is 100 per cent. For zero pressure difference across the pump, there is no slip and the delivered capacity is the true displacement. The volumetric efficiency of a pump is reduced by the presence of entrained air or gas in the pumped liquid. It is important to know the volumetric efficiency of a positive displacement pump when it is to be used for metering. 

A simple positive-displacement sampler system is shown in Figure 3. The basic system contains a battery and motor connected to a positive-displacement pump mechanism and provides an efficient means for moving air through the sampler. In order to provide feedback flow control the system must be expanded to include the means to monitor air flow through the pump to compensate for flow variations. 

Centrifugal pumps require appropriately large circumferential speed of the impellers and a number of serially arranged stages, in order to obtain the high-pressure differences. For efficient and economic operation the specific speed, nq, of the individual pump stages (n<, = n V0,5 / H0 75 V m3/s H, m n, min 1) should stay above 10 to 20. Too small a capacity or hydraulic power transmission will not provide favourable operating conditions and it is then recommended to focus on positive-displacement pumps alternatively. 

High-shear pumps will reduce the molecular weight of starch and thus its efficiency. Progressive cavity positive displacement pumps are recommended. Wet-end starch solutions are generally stored at 4% concentration, being reduced to below 1% concentration at the point of addition for fine paper and below 2% concentration for board and industrial grades. 

Huid power is the product of flow rate by head. In an ideal positive displacement pump, flow rate is independent of head. Therefore, the maximum fluid power is approximately P = p Q = p ct V/3T. One actuator closes at each step, and two actuators must be held closed at each step. If energy act is required to close an actuator, and power / act is required to hold it closed, we see that each cycle requires total energy 3 act + PactT (we assume no energy is required to open an actuator). The total power consumed per cycle is (fiact/7) + 2/ aot> and the maximum efficiency is r] = pact W(3 act + 6PactT). 

Positive-displacement pumps have an almost vertical head—flowrate curve the decline in capacity at increased pressure results mainly from increased internal leakage, a relatively small quantity in an efficient pump. It is not, however, normal to present the performance of a positive-displacement pump on a head—flowrate basis it is essentially a constant flowrate device (for constant speed), where the discharge pressure is determined by the discharge system only. Such pumps usually require a relief valve as protection against overpressure in the event of flow restriction (typically, a closed valve), and capacity is controlled by a bypass flow rather than by a series control valve. 

As a result of the low moisture content of the slurry and the short mixing times, the slurry can have inorganic lumps, which would block the atomization nozzles. The typical way of resolving this is to pump the mix through a filter and provide a disintegrator to break up any such lumps before passing to the main slurry pump. To achieve the high pressures (up to 100 bar) required for atomization, one or more piston pumps are typically used. In most cases these require a booster pump, typically a positive displacement pump, to keep them efficiently filled with viscous aerated slurry. 

Power consmnption the operating point of centrifugal pump should be chosen at the highest efficiency possible. Generally, the vendors/manufacturers offer a range of models, which cover the requirements of capacity, head, etc. However, the final selection should be done while keeping in mind the most likely (normal) condition in which the pump will be ran. In case of positive displacement pumps the performance curves should be studied to select proper speed for the capacity and required head... 

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