Orifice flow meter

Figure 13-6C. Effects of pulsation filter on orifice flow meter charts at 300 rpm compressor speed. (A) Before peak-to-peak differential pulsations were 160 psi, and (B) after installation of filter pulsation, levels dropped to 1.5 psi. (Used by permission von Nimitz, W. W., and O. Flanigan, Oil and Gas Journal, p. 60, Sept. 8, 1980. PennWell Publishing Company. All rights reserved.)...

A more subtle but frequently encountered problem is attributed to plugged orifice-flow meter taps. 

In this mode, the controller shown in Figure 28-1 compares the desired set-point flow to the flow measured by the orifice flow meter. The controller then moves the control valve to return the measured flow to the set-point flow. This is called dosed loop control. 

The most frequent application of signal conditioning is linearisation. Many of the common functions may not be obvious to the control engineer since they are often built into the DCS or transmitter as standard features. For example, where cj is the discharge coefficient, d the orifice diameter, dp the pressure drop across the orifice and p the fluid density, the flow (F) through an orifice flow meter is given by... 

The heating load here is usually a reboiler or heater. The practical problem is to determine valve downstream pressure. This requires cal ating shell-side pressure of the heat exchanger as a fimetion of heat load, peu/hr. Line pressure drop is usually negligible. Upstream pressures and temperatures are seldom constant so orifice flow meters should be temperature and pressure compensated. For reboilers we have calculator programs that calculate downstream pressure. 

A simple but very important instrument convention that should be fijllowed routinely is that of using linear flow measurements. If orifice flow meters are used, they should be followed by square root extraaors. There are three reasons for this convention ... 

Note that we assume the flow control loop to be very fast compared with other dynamics. Also, since we have a cascade system, the steam flow transmitter should have a linear relationship between flow and transmitter output. If an orifice flow meter is used, the AP transmitter should be followed by a square root extractor. 

Level control is cascaded to flow control. With floating pressure columns and with the trend toward small control-valve pressure drops for energy conservation, this is virtually mandatory to counteract the effect of control valve up- and downstream pressure variations. The flow measurement must be linear if an orifice flow meter is used, it must be followed by a sqiaare-root extractor. 

The liquid hydrogen was transferred from the pressurized supply dewar through the vacuum-insulated lines, the orifice flow meter, the test section, and into the receiving dewar. Data were taken after steady state, as indicated by a steady orifice reading, was attained the data consisted of pressures, temperatures, flow rates, time of day, and ambient conditions (i.e., atmospheric temperature and pressure, humidity, and wind velocity). Two wind velocities were used in these tests a steady wind velocity of 15mph supplied by two 24-in. fans, and zero wind velocity. 

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). 

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. 

Capacity. Pumps deHver a certain capacity, Q, sometimes referred to as flow, which can be measured directly by venturi, orifice plate (11), or magnetic meters (12) (see Flow measurement). The indirect way to determine capacity is often used. Whereas this method is less accurate than applying a flow meter, it often is the only method available in the field. The total head is measured and the capacity found from the pump head—capacity (H— curve (Fig. 2). More recently, sonic flow meters (13) have been used, which can be installed on the piping without the need for pipe disassembly. These meters are simple to use, but require relatively clean single-phase Hquid for reHable measurements. 

Once these traverse points have been determined, velocity measurements are made to determine gas flow. The stack-gas velocity is usually determined by means of a pitot tube and differential-pressure gauge. When velocities are very low (less than 3 m/s [10 ft/s]) and when great accuracy is not required, an anemometer may be used. For gases moving in small pipes at relatively high velocities or pressures, orifice-disk meters or venturi meters may be used. These are valuable as continuous or permanent measuring devices. 

Instrumentation Calibration may be required for the instruments installed in the field. This is typically the job of an instrument mechanic. Orifice plates should be inspected for physical condition and suitabihty. Where necessary, they should be replaced. Pressure and flow instruments should be zeroed. A prehminary material balance developed as part of the prehminary test will assist in identifying flow meters that provide erroneous measurements and indicating missing flow-measurement points. 

The most difficult part of a field test is the flow meter, if it wasn t planned in the construction phase. There is no way to simulate a meter run if you don t have the proper pipe length. Figure 10-8 is an example of the requirements. An ASME long radius flow nozzle is preferred by the author, though a short throat venturi will do. The probability is that an orifice is all that will be available. It should be examined before and after the test to verify not only the bore diameter, but the finish. The bore should... 

Flow Meter Orifice Plate - A flow meter orifice plate is permissible in normal process flow pressure reheving path, provided that it can pass the required emergency flow without exceeding pressure limits of the upstream equipment. However, it is not acceptable in PR valve inlets and flare headers. 

The orifice plate is simple to manufacture and has a relatively low cost. It does, however, create a quite large permanent pressure loss when installed in the ductwork. The venturi is smoothly shaped with a low permanent pressure loss but requires more space and is more expensive. The nozzle is a compromise betw een the orifice and the venturi. All three devices are standardized flow meters with very detailed descriptions of their geometry, material, manufacturing, installation, and use. -... 

The orifice plate meter factor should be adjusted for actual operating parameters. For liquid streams, the flow meters should be adjusted for °API gravity, temperature, and viscosity. For gas streams, the flow rate should be adjusted for the operating temperature, pressure, and molecular weight. 

ORFM —6, Orifice flanges and plate for Recording Flow Meter No. 6... 

In this chapter we will illustrate and analyze some of the more common methods for measuring flow rate in conduits, including the pitot tube, venturi, nozzle, and orifice meters. This is by no means intended to be a comprehensive or exhaustive treatment, however, as there are a great many other devices in use for measuring flow rate, such as turbine, vane, Coriolis, ultrasonic, and magnetic flow meters, just to name a few. The examples considered here demonstrate the application of the fundamental conservation principles to the analysis of several of the most common devices. We also consider control valves in this chapter, because they are frequently employed in conjunction with the measurement of flow rate to provide a means of controlling flow. 

If the perturbations are in the form of spikes of an irregular nature, the problem is likely to be detector contamination. Such spikes are especially observed when dust particles have settled into the FID flame orifice. Of course, the problem may also be due to interference from electrical pulses from some other source nearby. Regular spikes can be due to condensation in the flow lines causing the carrier, or hydrogen (FID), to pulse, or they can be due to a bubble flow meter attached to the outlet of the TCD, as well as the electrical pulses referred to above. Baseline perturbations can also be caused by pulses in the carrier flow due to a faulty flow valve or pressure regulator. 

Accessories to apparatus. — Orifice was placed in die circuit to indicate on flow meter i how the pump is operating in circulating the gases thru the apparatus. High rates... 

Different methods used for calibrating samplers include rotometers, wet-test meters, pressure gauges across fixed orifices, mass flow meters, hot wire flow meters and bubble tubes. Each of these calibration devices requires an appropriate correction factor. Some of the devices measure mass flow rather than volumetric flow. Sampling requires volumetric flow calibration. 

Feed tank with control valve and gravimetric flow the desired flow rate is ensured by the appropriate opening of the valve, that is, the actuator of a control loop using the weight of the reactor or of the feed tank, the level in the feed tank, or a flow meter as input. The maximal feed rate can be limited by the clearance of the valve or by a calibrated orifice. 

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