Positive displacement

A mud motor (Fig. 3.17) is a positive displacement hydraulic motor, driven by the circulated drilling fluid. A continuous seal is formed between the body ( stator ) and the... 

Production and Shipment. Estimated adiponitrile production capacities in the U.S. in 1992 were about 625 thousand metric tons and worldwide capacity was in excess of lO metric tons. The DOT/IMO classification for adiponitrile is class 6.1 hazard, UN No. 2205. It requires a POISON label on all containers and is in packing group III. Approved materials of constmction for shipping, storage, and associated transportation equipment are carbon steel and type 316 stainless steel. Either centrifugal or positive displacement pumps may be used. Carbon dioxide or chemical-foam fire extinguishers should be used. There are no specifications for commercial adiponitrile. The typical composition is 99.5 wt % adiponitrile. Impurities that may be present depend on the method of manufacture, and thus, vary depending on the source. 

Flow meters have traditionally been classified as either electrical or mechanical depending on the nature of the output signal, power requirements, or both. However, improvement in electrical transducer technology has blurred the distinction between these categories. Many flow meters previously classified as mechanical are now used with electrical transducers. Some common examples are the electrical shaft encoders on positive displacement meters, the electrical (strain) sensing of differential pressure, and the ultrasonic sensing of weir or flume levels. 

The output signal from positive-displacement meters may be mechanical, where the motion is transmitted by an output shaft through a housing seal, or it may be magnetically or inductively coupled. 

Positive-displacement meters are normally rated for a limited temperature range. Meters can be constmcted for high or low temperature use by adjusting the design clearance to allow for differences in the coefficient of thermal expansion of the parts. Owing to small operating clearances, filters are commonly installed before these meters to minimize seal wear and resulting loss of accuracy. 

There are at least five types of positive-displacement meters commercially available. 

Reciproca.ting Piston Meters. In positive-displacement meters of the reciprocating piston type, one or more pistons similar to those in an internal-combustion engine are used to convey the fluid. Capacity per cycle can be adjusted by changing the piston stroke. 

Fig. 5. Operating sequence for a rotating-vane positive-displacement meter.

The ease with which the separated products leave the bowl determines the richness of the fat. Fluid whole milk enters the separator under pressure from a positive displacement pump or centrifugal pump with flow control (Fig. 1). The fat (cream) is separated and moves toward the center of the bowl, while the skimmed milk passes to the outer space. There are two spouts or oudets, one for cream and one for skimmed milk. Cream leaves the center of the bowl with the percentage of fat ( 30 40%) controlled by the adjustment of a valve, called a cream or skim milk screw, that controls the flow of the product leaving the field of centrifugal force and thus affects the separation. 

Positive Displacement Pumps. Positive displacement pumps foUow HI convention (see Fig. 1). As a rule, these pumps work against significantly higher pressures and lower flows than do kinetic, particularly centrifugal, pumps. Positive displacement pumps also operate at lower rotational speeds. There are many types of positive displacement pumps, for which designs are constantly being developed. Some of these are discussed herein. 

Selection of pump for a given appHcation is not a trivial task. Often more than one pump type can accomplish the required job. Thus a final choice on a pump type is often a result of personal experience and usage history. As a rule of thumb, the choice of a kinetic, such as centrifugal, or a positive displacement pump is made on the basis of the specific speed. Whereas specific speed is appHcable primarily for centrifugal but not positive displacement pumps, the US value can be used as a guide. Generally, for calculated values of specific speed, eg, nS > 10 [NS > 500), kinetic-type pumps are usually selected. For nS < 10 [NS < 500), positive displacement pumps are typically appHed. 

Compressors used in a typical refrigerator/freezer are of the positive displacement type, utilizing either reciprocating or rotary motion (Eig. 5) and... 

Liquid Eeeders. Liquid feeders employ positive-displacement metering pumps for adding aqueous solutions of sodium or calcium hypochlorite. The feed solutions are typically stored in polyethylene tanks of various capacities up to about 0.19 m (50 gal). 

In some plants the pitches are received and stored as Hquids. Addition to the mixers can be either through a weighing system or positive displacement pumps (3). Except for equipment differences, the results of utilizing Hquid pitch are similar to bulk. 

Flow, defined as volume per unit of time at specified temperature and pressure conditions, is generally measured By positive-displacement or rate meters. The term positive-displacement meter apphes to a device in which the flow is divided into isolated measured volumes when the number of fillings of these volumes is counted in some man-... 

The principal classes of flow-measuring instruments used in the process industries are variable-head, variaBle-area, positive-displacement, and turbine instruments, mass flowmeters, vortex-shedding and iiltrasonic flowmeters, magnetic flowmeters, and more recently, Coriohs mass flowmeters. Head meters are covered in more detail in Sec. 5. 

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