Orifice flowmeters

Fig 1. Schematic diagram of the ICFB gasifier. 1. steam generator, 2. flowmeter, 3. orifice flowmeter, 4. air preheater, 5, gas plenum, 6. draft tube, 7. orifice, 8. gas separator, 9. freeboard, 10. screw feeder, 11. mono-pump, 12. sludge feed nozzle, 13. cyclone, 14. condenser, 15. collector, 16. filter, 17. ID fan. 

Differential pressure flowmeters are suited to high- and moderate-velocity flow of gas and clean, low-viscosity liquids. Venturi flowmeters (Fig. 18.9(a)) are the most accurate, but they are large and expensive. Orifice flowmeters (Fig. 18.9(b)) aresmaUer, less expensive, and much less accurate than venturi flowmeters. Nozzle flowmeters (Fig. 18.9(c)) are a compromise between venturi and orifice flowmeters. Pipe-bend flowmeters (Fig. 18.9(d)), which can essentially be installed in any bend in an existing piping system, are used primarily for gross flow rate measurements. Pitot-static flowmeters (Fig. 18.9(e)) are used in flows which have a large cross-sectional area, such as in wind tunnels. Pitot-static flowmeters are also used in freestream applications such as airspeed indicators for aircraft. 

FIGURE 18.9 Differential pressure flowmeters (a) Venturi flowmeter, (b) orifice flowmeter, (c) nozzle flowmeter, (d) pipebend (elbow) flowmeter, (e) pitot static flowmeter. 

Flow is an important measurement whose calibration presents some challenges. When a flow measurement device is used in applications such as custody transfer, provision is made to pass a known flow through the meter. However, such a provision is costly and is not available for most in-process flowmeters. Without such a provision, a true cahbration of the flow element itself is not possible. For orifice meters, calibration of the flowmeter normally involves cahbration of the differential pressure transmitter, and the orifice plate is usually only inspected for deformation, abrasion, and so on. Similarly, cahbration of a magnetic flowmeter normally involves cahbration of the voltage measurement circuitry, which is analogous to calibration of the differential pressure transmitter for an orifice meter. 

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... 

Flow measurements using nonintrusive or low mechanical ac tion principles are desired, such as magnetic, vortex-shedding, or Coriolis-type flowmeters. Orifice plates are easy to use and reliable but have a limited range and may not be suitable for streams which are not totally clean. Rotameters with glass tubes should not be used. 

Blockages or faulty pumps. Pumps should be checked periodically as instructed by the manufacturer. They can be calibrated using rotameters or bubble flowmeters. Unless pumps possess a limiting orifice they should be calibrated with the air indicator tube in position. 

Dali Flow Tube - The advantage is this type of flowmeter is that it has a permanent head loss of only 5 % of the measured pressure differential. This is the lowest pressure drop of all orifice meter designs. Flow ratios as high as 1 10 (e.g., 1.0 to 10 kg/s) can be measured within + 2% of actual flow. Dali flow mbes are available in different materials and diameters up to 1500 mm. 

To repair a flowmeter, a man had to walk six times from the orifice plate to the transmitter and back. To get from one to the other, he had to walk 45 m. cross a 30-in.-diameter pipe by a footbridge, and walk 45 m back—a total of 540 m for the whole job. Instead, he climbed over the pipe while doing so he hurt his back. 

Flow Rate. The values for volumetric or mass flow rate measurement are often determined by measuring pressure difference across an orifice, nozzle, or venturi tube. Other flow measurement techniques include positive displacement meters, turbine flowmeters, and airflow-measuring hoods. 

The flow of fluids is most commonly measured using head flowmeters. The operation of these flowmeters is based on the Bernoulli equation. A constriction in the flow path is used to increase the flow velocity. This is accompanied by a decrease in pressure head and since the resultant pressure drop is a function of the flow rate of fluid, the latter can be evaluated. The flowmeters for closed conduits can be used for both gases and liquids. The flowmeters for open conduits can only be used for liquids. Head flowmeters include orifice and venturi meters, flow nozzles, Pitot tubes and weirs. They consist of a primary element which causes the pressure or head loss and a secondary element which measures it. The primary element does not contain any moving parts. The most common secondary elements for closed conduit flowmeters are U-tube manometers and differential pressure transducers. 

Class 11 flowmeters have no wetted moving parts to break and are thus not subject to catastrophic failure. However, the flow surfaces such as orifice plates may wear, eventually biasing flow measurements. Other disadvantages of some flowmeters in this class include high pressure drop and susceptibility to plugging. Very dirty and abrasive fluids should be avoided. 

The indicated flow of acetic acid is 9000 liters per day. The instrument technician checks the flowmeter to see if it has drifted, by opening valve B, with A and C closed (see Fig. 6.7). It should go back to zero— but a reading of 2000 liters per day is noted. The full range on the flowmeter is 10,000 liters per day. What is the real flow rate of the acetic acid The answer is not 2000 liters. Why Because flow varies with the square root of the orifice plate pressure drop. To calculate the correct acetic acid flow ... 

The lesson is that near the top end of its range, the indicated flow is likely to be accurate, even if the meter is not well zeroed, or the measured AP is not too accurate. On the other hand, flowmeters using orifice plates cannot be very accurate at the low end of their range, regardless of how carefully we have zeroed them. Digitally displayed flows also follow this rule. 

Let us say we have an ordinary orifice-type flowmeter, as shown in Fig. 21.5 (see Chap. 6, How Instruments Work ). What happens if the low pressure (i.e., the downstream) orifice tap plugs Does the indicated flow go up or down ... 

If a pressure tap plugs, the measured pressure will decrease. The measured pressure difference across the orifice plate will increase. The indicated flow will go up. If the flowmeter is controlling a flow-control... 

Flow generally is considered to be fully laminar at Reynolds numbers below 1500 in transition between 1500 and 4000 and turbulent above 4000. Thus, Ihe Reynolds number is a most useful tool in constructing piping system designs and in sizing flowmeters. Orifice or throat Reynolds number, R,i = Ro/d/D, where d = diameter of orifice bore (inches) D = inside diameter of conduit or pipe (inches). 

Two or more of these conditions can occur at the same time, resulting in asymmetric axial, radial and tangential velocity vectors. Some flowmeters are more sensitive than others to particular types of flow distortion, e.g. orifice meters are affected by pure swirl more than venturi meters are magnetic flowmeters are unaffected by changes in the radial velocity component whereas ultrasonic time-of-flight meters are highly susceptible thereto swirl and asymmetry have the least effect on positive displacement meters and the greatest effect on variable area meters. 

Head-type flowmeters include orifice plates, venturi tubes, weirs, flumes, and many others. They change the velocity or direction of the flow, creating a measurable differential pressure, or "pressure head," in the fluid. Head metering is one of the most ancient of flow detection techniques. There is evidence that the Egyptians used weirs for measurement of irrigation water flows in the days of the Pharaohs and that the Romans used orifices to meter water to households in Caesar s time. In the 18th century, Bernoulli established the basic relationship between the pressure head and velocity head, and Venturi published on the flow tube bearing his name. 

In a target flowmeter, a target or impact plate is inserted into the flowing stream, and the resulting impact force is detected. The target meters are more expensive than orifices, but because they have no pressure taps to plug or freeze, they are better suited for applications where the process fluid is "sticky" or contains suspended solids. 

Impeller and shunt flowmeters are widely used in steam and gas flow metering. They comprise an orifice plate in the main flow line and a self-operating rotor assembly in the bypass. As gas flows through the meter body, a portion of the flow is diverted to drive the fan shaft assembly. The rotational speed of the shaft is proportional to the rate of flow at all rates within the normal range of the meter. 

The variable-area flowmeter is a head-type flow sensor, but it does not measure the pressure drop across a fixed orifice instead, the pressure drop is held relatively constant, and the orifice area is varied to match the flow (Figure 3.98). In gravity-type variable-area flowmeters, increase in flow lifts the float, piston, or vane, and it is the weight of these flow elements that has... 

The V-cone flowmeter requires less upstream straight pipe and maintains the square root relationship between flow and pressure drop at lower Reynolds numbers than does an orifice plate. (Courtesy of McCrometer Div. of Danaher Corporation.)... 

The basic rangeability of this meter is the same as that of an orifice plate (3 1), but if two (a high span and a low span) transmitters are used, and the flow element is accurately calibrated over the complete flow range, it can be increased to 10 1. This performance can be obtained from all properly calibrated d/p flow elements, not just from the V-cone design. The V-cone flowmeter should be installed horizontally so that the two pressure taps are at the same elevation. This guarantees that the d/p cell will detect a zero pressure differential when there is no flow. 

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