Elbow Meters

Differential Pressure Meters Differential pressure meters or head meters measure the change in pressure across a special flow element. The differential pressure increases with increasing flow rate. The pitot tubes described previously work on this principle. Other examples include orifices [see also Eqs. (6-111) and (8-102), and Fig. 10-14], nozzles (Fig. 10-19), targets, venturis (see also Sec. 8 and Fig. 10-17), and elbow meters. Averaging pitot tubes produce a pressure differential that is based on multiple measuring points across the flow path. 

In addition to the foregoing standard devices for measuring the flow of fluids, there exist a number of supplementary devices less amenable to exact theoretical analysis but worthy of brief mention. One of the simplest for measuring flow in a pipeline is the elbow meter, which consists of nothing more than piezometer taps at the inner and outer walls of a 90° elbow in the line. The pressure difference, due to the centrifugal effects at the bend, will vary approximately as the velocity head in the pipe. Like other meters, the elbow should have sections of straight pipe upstream and downstream and should be calibrated in place [32],... 

Other methods to measure bulk velocity in slurry pipelines include venturi meter (22) and elbow meter (85). 

Kittredge, C. P., Elbow Meters , Proc. Flow Measurement Sym. by ASME 274-289 (1966). 

Hauptmann, E. G., Take a Second Look at Elbow Meters for How Monitoring , Instruments Control Systems 51 47-50 (1978). 

Enough space must be available to properly service the flow meter and to install any straight lengths of upstream and downstream pipe recommended by the manufacturer for use with the meter. Close-coupled fittings such as elbows or reducers tend to distort the velocity profile and can cause errors in a manner similar to those introduced by laminar flow. The amount of straight pipe required depends on the flow meter type. For the typical case of an orifice plate, piping requirements are normally Hsted in terms of the P or orifice/pipe bore ratio as shown in Table 1 (1) (see Piping systems). 

Elbow flow meters. The principle of centrifugal force at the bend is used to obtain the difference in pressure at the inside and outside of the elbow, which is then related to the discharge pressure. 

Bulk alum can be stored in mild steel or concrete bins with dust collector vents located in, above, or adjacent to the equipment room. Recommended storage capacity is about 30 days. Dry alum in bulk form can be transferred or metered by means of screw conveyors, pneumatic conveyors, or bucket elevators made of mild steel. Pneumatic conveyor elbows should have a reinforced backing as the alum can contain abrasive impurities. 

The downstream pressure-sensing pipe of each valve is connected to a straight section of pipe 10 diameters or 1 meter downstream of the nearest tee, elbow or valve. This sensing line should be pitched down, to drain into the low-pressure line. If it cannot drain when connected to the top of this line it can often be connected instead to the side of the pipe. The pipe between the two control valves must be drained through a steam trap, just as would the foot of any riser downstream of the pressure-reducing station. 

A liquid with a viscosity of 25 cP and an SG of 0.87 is pumped from an open tank to another tank in which the pressure is 15 psig. The line is 2 in. sch 40 diameter, 200 ft long, and contains eight flanged elbows, two gate valves, a control valve, and an orifice meter. 

To avoid swirl, elbows should be well separated and have large radii of curvature. If this is not possible then the flowmeter should be sited at least 40 pipe diameters downstream of fittings causing asymmetric flow only and a minimum of 100 pipe diameters downstream when swirl is likely to occur0 S). There should also be at least 10 pipe diameters allowed downstream of the meter free of any obstruction or fitting. If the flow is laminar then these distances should be doubled. 

Fit the flow meter in the discharge line. Use the feed bend (elbow) of the meter, connecting it to a pipe extending straightly for at least 400 mm immediately before the feed bend. 

There are many other flow measurement devices including Onlicc/Venturi meters, turbine meters, and more sophisticated instruments using ultrasonic, magnetic, and Coriolis effect techniques. Orifice/Venturi type meters have a restriction causing a pressure drop related to the flow rate of liquid. Such meters are popular because of their low cost however, their accuracy can be compromised by upstream elbows and valves. Turbine meters are designed so that rotation speed varies linearly with the... 

In certain cases, the Altering of a noisy sensor is required. Noisy sensors present a challenge. A nuclear-based level sensor, a pressure sensor located too close to a 90° elbow in a line, or an orihce flow meter located immediately downstream of a control valve are examples of noisy sensors. For these cases, tuning the filter is a compromise between removing the noise from the sensor reading and adding lag to the closed-loop response when one is forced to use a noisy sensor reading. 

Meters that measure differential pressures over the flowmeter and such pressure changes that can be interpreted as flowrates. Such flowmeters with a large number of designs include orifices, venturi tubes, pitot tubes, elbow taps, etc. Fluids that result in changes of the cross-sectional area due to erosion, corrosion, or deposition of solids obviously change the calibrations. These meters tend to be relatively cheap but are often not very accurate. 

When fluid flows through elbows, tees, and valves a swirling flow with an uneven velocity profile develops. For accurate flow measurement fhe flow upstream of a differential pressure meter should be free of these disturbances. In order to eliminate these disturbances, a long length of straight pipe is required. For example, swirl flow inside a pipe can require 50 to 100 diameters of sfraight pipe to eliminate the spin [16]. 

So far we have considered only flows which were in one direction, as in a pipe or down a straight riverbed. In the few cases in which the fluid flow was not one-dimensional, as around a sphere or in a pipe elbow or venturi meter, we have introduced experimental data to allow us to treat the problem as if it were one-dimensional. Although this one-dimensional approach adequately covers many of the practical problems in fluid mechanics, it is not satisfactory for the complicated ones, particularly for the aerodynamics problems. To solve these more complicated problems, two additional ideas are needed potential flow and the boundary layer. 

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