Orifice flow rate

Sutherland (1975). Orifice flow rates are underpredicted by about the same factor with the energy balance method and with the NEM. Discharge predictions for short (0.2-m) pipes are overpredicted by the energy balance method. In this region, the assumption of homogeneous equilibrium is not justified. A model that takes slip velocity into account may improve predictions for short pipes. 

Chowhan, Z.T. Yang, I.C. Powder flow studies IV. Tensile strength and orifice flow rate relationships of binary mixtures. Int. J. Pharm. 1983,14, 231-242. 

There are three modules that perform the necessary calculations required for flow measmement FL WRAT, AGAFPV, and ORFLC. The first is the orifice flow rate calculation, C. (See APPENDIX A for detailed information.) For this calculation the operator can select tap location, UPSTREAM or DOWNSTREAM for flange taps. Note pipe taps were not considered since they are not used. The second is the supercompressibility factor, Fpv. Finally, the gas flow rate is calculated in MMCFD and MMBTUD. 

The spring ensures a soHd closing action and is usually wound from stainless steel wire. The dip tube conducts the product from the container to the valve. It is usually extmded from polyethylene or polypropylene and has an inside diameter of over 2.54 mm, although it can be provided in capillary sizes having diameters down to 0.25 mm. These small tubes are used to reduce flow rate and may function in place of the Hquid metering orifice in the valve housing. 

For very low flow rates the orifice plate is often incorporated into a manifold, an integral part of the differential-pressure transmitter. This provides a convenient compact installation. 

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. 

In the manufacture of meltblown fabrics, a special die is used in which heated, pressurized air attenuates the molten polymer filament as it exits the orifice of the dye or nozzle (Fig. 9). Air temperatures range from 260—480°C with sonic velocity flow rates (43). 

Flow. The principal types of flow rate sensors are differential pressure, electromagnetic, vortex, and turbine. Of these, the first is the most popular. Orifice plates and Venturi-type flow tubes are the most popular differential pressure flow rate sensors. In these, the pressure differential measured across the sensor is proportional to the square of the volumetric flow rate. 

Knowledge-based systems typically use quaHtative methods rather than quantitative ones. For example, consider a simple tank system. The equation describing the flow rate of Hquid out of the tank is given below, where C is the orifice coefficient, d is the diameter of the orifice, and h is the height of Hquid in the tank. Based solely on the form of the equation, a human reasoner can infer that the flow rate F increases monotonically with the height b of Hquid in the tank. 

To reflect this type of reasoning, a KBS captures quaHtative relationships between variables. By contrast, a conventional program that implements the flow equation calculates the value of the flow rate for numerical values of the input variables, ie, orifice diameter, orifice coefficient, and Hquid height. 

Section 10 of this Handbook describes the use of orifice meters for flow measurement. In addition, orifices are commonly found within pipelines as flow-restric ting devices, in perforated pipe distributing and return manifolds, and in perforated plates. Incompressible flow through an orifice in a pipehne as shown in Fig. 6-18, is commonly described by the following equation for flow rate Q in terms of pressure drop across the orifice Ap, the orifice area A, the pipe cross-sectional area A, and the density p. 

Often, the pressure drop required for design flow rate is unacceptably large for a distributor pipe designed for uniform velocity through uniformly sized and spaced orifices. Several measures may be taken in such situations. These include the following ... 

Orifice Meter The most widely used flowmeter involves placing a fixed-area flow restriction (an orifice) in the pipe carrying the fliiid. This flow restriction causes a pressure drop that can be related to flow rate. The sharp-edge orifice is popular because of its simplicity, low cost, and the large amount of research data on its behavior. For the orifice meter, the flow rate for a liquid is given by... 

Head meters with density compensation. Head meters such as orifices, venturis, or nozzles can be used with one of a variety of densitometers [e.g., based on (a) buoyant force on a float, (b) hydrauhc couphug, (c) voltage output from a piezoelectric ciystal, or (d) radiation absolution]. The signal from the head meter, which is proportional to pV" (where p = fluid density aud V = fluid velocity), is multiphed by p given by the densitometer. The square root of the produc t is proportional to the mass flow rate. 

The orifice-riser distributor is designed to lay the hquid carefully onto the bed, with a minimum of contact with gas during the process. It can be designed to provide a large number of liquid streams, with the limit of sufficient liquid head to provide uniform liquid flow through the orifices. The gas risers must oe designed to accommodate the expected variations in flow rate, often with a minimum of pressure drop. For veiy distribution-sensitive packings, it is necessaiy to include pour points in the vicinity of the column wall (to within 25 mm). 

The perforated pipe distributor comprises a central feed sump and pipes that branch out from the sump to provide the liquid discharge. The level in the sump varies with liquid total flow rate, and the size of the lateral pipes and their perforations must be determined carefully to ensure that the ends of the pipes are not starved for hquid. The orifices are typically 4 to 6 mm diameter, and can be subject to phigging if foreign matter is present. The pipes must be leveled carefully, especially for large diameter columns. 

At high-flow rates through perforated plates such as those that occur in distillation columns, Calderbank and Rennie [Trans. In.stn. Chem. Engrs., 40, T3 (1962)] Porter et al. [ibid., 45, T265 (1967)] Rennie and Evans [Br. Chem. Eng, 7, 498 (1962)] and Valentin (op. cit.. Chap. 3) have investigated and discussed the effect of the flow conditions through the iTuutiple orifices on the froths and foams that occur above perforated plates. 

Air Flow Typical gas flow rates for an orifice scrubber unit are 0.47 to 24 standard cubic meters per second (sm /sec) (1,000 to 50,000 standard cubic feet per minute (scfm)). 

Hot water basins are used to distribute water in crossflow towers. Here, water is pumped to an open pan over the wet deck fill. The bottom of the pan has holes through which water is distributed. Manufacturers will fit specially shaped plastic drip orifices into the holes to give the water an umbrella shape for more uniform distribution. Different size orifices are used for different flow rates. Ideally, the basin will be almost full at maximum flow. This way, sufficient depth is retained for good water distribution as turn down occurs. The turn down ratio can be extended by the addition of hot water basin weirs- a pattern of baffles perhaps 2... 

When a gas is blown steadily through an orifice into an essentially inviscid liquid, a regular stream of bubbles is formed. A theoretical expression that relates the bubble volume V to volumetric gas flow rate G and gravitational acceleration g is the following ... 

Use physical limits of pipe size, restrictive orifices, and pump sizing to limit excessive flow rates. 

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