Pressure differential across

Variable-Area Flow Meters. In variable-head flow meters, the pressure differential varies with flow rate across a constant restriction. In variable-area meters, the differential is maintained constant and the restriction area allowed to change in proportion to the flow rate. A variable-area meter is thus essentially a form of variable orifice. In its most common form, a variable-area meter consists of a tapered tube mounted vertically and containing a float that is free to move in the tube. When flow is introduced into the small diameter bottom end, the float rises to a point of dynamic equiHbrium at which the pressure differential across the float balances the weight of the float less its buoyancy. The shape and weight of the float, the relative diameters of tube and float, and the variation of the tube diameter with elevation all determine the performance characteristics of the meter for a specific set of fluid conditions. A ball float in a conical constant-taper glass tube is the most common design it is widely used in the measurement of low flow rates at essentially constant viscosity. The flow rate is normally deterrnined visually by float position relative to an etched scale on the side of the tube. Such a meter is simple and inexpensive but, with care in manufacture and caHbration, can provide rea dings accurate to within several percent of full-scale flow for either Hquid or gas. 

Many of the variations developed to make pressure sensors and accelerometers for a wide variety of appHcations have been reviewed (5). These sensors can be made in very large batches using photoHthographic techniques that keep unit manufacturing costs low and ensure part-to-part uniformity. A pressure differential across these thin diaphragms causes mechanical deformation that can be monitored in several ways piezoresistors implanted on the diaphragm are one way changes in electrical capacitance are another. 

Figure 1. Condensate controller maintains a positive pressure differential across all the tubes.

One means of overcoming this problem is to install a condensate controller, which maintains a positive pressure differential across all the tubes (Fig. I). A condensate controller provides a specialized purge line, which assures a positive flow through the coil at all times. 

Droplet generation by air pressure differential across the film, e.g. bursting of liquid film in bubbles, froth across bottle mouths and pipette tips... 

When the cake structure is composed of particles that are readily deformed or become rearranged under pressure, the resulting cake is characterized as being compressible. Those that are not readily deformed are referred to as sem-compressible, and those that deform only slightly are considered incompressible. Porosity (defined as the ratio of pore volume to the volume of cake) does not decrease with increasing pressure drop. The porosity of a compressible cake decreases under pressure, and its hydraulic resistance to the flow of the liquid phase increases with an increase in the pressure differential across the filter media. 

The net pressure differential across a membrane, taking into consideration the osmotic pressure is given by (AP - AH), and hence, the expression for the permeate flux is ... 

Venturi meter A measuring instrument used to determine the fluid velocity, achieved by the comparison of pressure differentials across its throat. 

Rotary compressors are positive-displacement machines. Figure iO-7 shows a typical vane compressor. The operation is similar to that of a vane pump shown schematically in Figure 10-10 of Volume 1, 2nd Edition (Figure 10-9 in 1st Edition). A number of vanes, typically from 8 to 20, fit into slots in a rotating shaft. The vanes slide into and out of the slots as the shaft rotates and the volume contained between two adjacent vanes and the wall of the compressor cylinder decreases. Vanes can be cloth impregnated with a phenolic resin, bronze, or aluminum. The more vanes the compressor has, the smaller the pressure differential across the vanes. Thus, high-ratio vane compressors tend to have more vanes than low-ratio compressors. 

For compressible fluids flowng through nozzles and orifices use Figures 2-17 and 2-18, using hL or AP as differential static head or pressure differential across taps located one diameter upstream at 0.5 diameters dow nstream from the inlet face of orifice plate or nozzle, when values of C are taken from Figures 2-17 and 2-18 [3]. For any fluid ... 

The slip for this type of compressor is similar to that of the lobe units however, the passages are basically different, and this changes the approach to slip correction. The manufacturer should be consulted for data specific to a particular unit. The slip is dependent on the pressure differential across the unit and the gas density. It does not vary with speed or length of the rotor. 

Bore Rod Effective Pressure differential across cylinder ports ... 

Bore diameter (in) Rod diameter (in) Effective area (in ) Pressure differential across cylinder ports ... 

The spent catalyst slide valve is located at the base of the standpipe. It controls the stripper bed level and regulates the flow of spent catalyst into the regenerator. As with the regenerated catalyst slide valve, the catalyst level in the stripper generates pressure as long as it is fluidized. The pressure differential across the slide valve will be at the expense of consuming a pressure differential in the range of 3 psi to 6 psi (20 kp to 40 kp). 

A lower pressure differential across the slide valve and/or a higher than normal slide valve opening are common evidence of a unit s reaching its circulation limitation. The causes of low differential are... 

Similarly, a negative pressure differential across the spent catalyst slide valve can allow hot flue gas to backflow to the reactor and the main fractionator, severely damaging the mechanical integrity of these vessels. 

Some of the main causes of loss of pressure differential across the slide valves are as follows ... 

Maintain a mitrimum of 1 PSi pressure differential across the Slide Valves Install "Radial designed Feed Nozzles Install an automated shutdown System... 

AP = pressure differential across safety relief valve, inlet pressure minus back pressure or down stream pressure, psi. Also = set pressure + overpressure, psig-back pressure, psig. At 10% overpressure delta P equals 1.1 — P2. Below 30 psig set della P equals Pi + 3 - P2. 

Membranes act as a semipermeable barrier between two phases to create a separation by controlling the rate of movement of species across the membrane. The separation can involve two gas (vapor) phases, two liquid phases or a vapor and a liquid phase. The feed mixture is separated into a retentate, which is the part of the feed that does not pass through the membrane, and a permeate, which is that part of the feed that passes through the membrane. The driving force for separation using a membrane is partial pressure in the case of a gas or vapor and concentration in the case of a liquid. Differences in partial pressure and concentration across the membrane are usually created by the imposition of a pressure differential across the membrane. However, driving force for liquid separations can be also created by the use of a solvent on the permeate side of the membrane to create a concentration difference, or an electrical field when the solute is ionic. 

For osmotic drug delivery systems, Eq. (2) is of critical importance. This equation demonstrates that the quantity of water that can pass a semipermeable film is directly proportional to the pressure differential across the film as measured by the difference between the hydrostatic and osmotic pressures. Osmotic delivery systems are generally composed of a solid core formulation coated with a semipermeable film. Included in the core formulation is a quantity of material capable of generating an osmotic pressure differential across the film. When placed in an aqueous environment, water is transported across the film. This transported water in turn builds up a hydrostatic pressure within the device which leads to expulsion of the core material through a suitably placed exit port. 

Packer fluids are used to provide hydrostatic balance to partially offset the reservoir pressure and eliminate a large pressure differential across the packer element. TKPP solutions more than meet the requirements of being noncorrosive, nondestructive to elastomers, variable in density, and stable for many years. It is desirable to perforate with a well bore filled with packer fluid so that the packer can be set and the well produced as soon as the perforation operation is complete. Since TKPP solutions make superior perforation fluids, their dual use as a packer fluid is further enhanced. 

You must size an orifice meter to measure the flow rate of gasoline (SG = 0.72) in a 10 in. ID pipeline at 60°F. The maximum flow rate expected is 1000 gpm, and the maximum pressure differential across the orifice is to be 10 in. of water. What size orifice should you use ... 

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