Orifice taps

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 ... 

International Organization for Standards Report DIS 5167, Geneva, 1976). Similar equations are given for other kinds of orifice taps and for nozzles and Venturi meters. 

Figure 5-1 shows the locations and names of the orifice taps. The true mass flow rate for both liquids and gases is obtained by multiplying the theoretical mass flow equation by the discharge coefficient, the gas expansion factor, and the thermal-expansion factor. This is expressed as ... 

Example 2.1. A manometer of the type shown in Fig. 2.4 is used to measure the pressure drop across an orifice (see Fig. 8.19). Liquid A is mercury (density 13,590 kg/m ) and fluid B, flowing through the orifice and filling the manometer leads, is brine (density 1260 kg/m ). When the pressures at the taps are equal, the level of the mercury in the manometer is 0.9 m below the orifice taps. Under operating conditions, the gauge pressuret at the upstream tap is 0.14 bar the pressure at the downstream tap is 250 mm Hg below atmospheric. What is the reading of the manometer in millimeters ... 

Blow clear both the high- and low-pressure legs running from the orifice taps to the flow meter (see Fig. 17-5). 

Instrumentation malfunctions (distillation tower), 372-373 plugged orifice taps, 372-373 Insulated float chamber, 270 Insulation (energy savings), 218-219 Integral trusses (support lips), 84 Interface level, 303 Internal reflux, 13 Ionic hydrogen, 105 Iron oxide scale, 79 Iron sulfide, 372... 

Plugged instrument taps. Flow rates in dirty amine systems tend to be erratic. Orifice taps on flow meters and level taps on float chambers often plug. Level control in the bottom of the scrubbers becomes unreliable and massive carry-overs of amine are frequent. 

Open the orifice taps bypass valve. The indicated flow will then drop to zero, and the trip should be activated. 

Suddenly, a long-time shift operator hit on the solution The orifice taps on the reboiler steam flow meter were plugged. The taps were drilled out. The tower s control immediately reverted to its former smooth operation. An investigation revealed what had happened. 

The feed-forward control logic was designed to reduce the reboiler heat input when the debutanizer feed rate dropped. The measured steam flow was relayed back to the feed-forward controller. If the measured steam flow did not decline in step with the reduced feed rate, the controller signaled the steam flow control valve to continue closing. Then the plugged orifice taps caused a misleading steam flow rate to be transmitted back to the feed-forward controller. This controller continued to throttle the reboiler steam, trying vainly to reduce the measured steam flow rate. Of course, the actual heat input to the reboiler dropped precipitously. With reduced boil-up, the reflux flow dried up. As the tower cooled off, its pressure also fell. 

Flow indicators should be zeroed and calibrated. The orifice taps for flow meters should be blown out, and the process engineer must make sure that every flow meter needed for the test actually exists in the field as an operable entity. A preliminary unit material balance should be made. This will delineate if any of the feed or product meters are badly out of calibration. 

Trace the small lines from the orifice taps back to the flow transmitter. This is done to ensure that one will be working with the correct transmitter. 

Instmment calibration should also be checked. Often in a difficult service it is common for errors to occur in the flow measurements - for example orifice tappings or the orifice itself may become partially blocked. An apparent maldistribution of flows may not be real -particularly if control valve positions (and/or pass outlet temperatures) are approximately equal. 

A better way of measuring flows than the ordinary orifice plate method is by inducing vortex shedding across a tube in the flowing liquid and then measuring the velocity of the vortices. This is a nice method, as there are no orifice taps to plug. Then there are Doppler meters, which measure the velocity of a fluid based on how the speed of sound is affected by the flow in a pipe. More commonly, we have rotometers, which measure how far a ball or float is lifted in a vertical tube by the velocity of the liquid. But regardless of their relative merits, perhaps simply for historical reasons, the vast majority of flows in most process plants are measured with the orifice plate flowmeter, shown in Fig. 10.7. 

Naturally, just like level indicators, the flow orifice taps can plug. 

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