Taps, pressure-sensing

The pressure-sensing tap must go all the way through the wall— metal skin and refractory. Flare the refractory opening into a cone so that crumbs of refractory and... 

The elevation of the pressure-sensing tap does not necessarily have to be at the elevation desired for the neutral pressure plane. The most desirable height for the zero pressure plane may be at a point that turns out to be bad for good measurement, for example, below the hearth, at a level where scale might plug the pressure tap, or in a place where liquid metal may splash into the tap. In such cases, a very workable solution is to locate the sensor tap at a convenient higher position and then adjust the controller s setpoint in accordance with the correction for the rise in pressure for the chosen higher elevation from table 6.2. (See example 6.2.)... 

In case of a major problem in meeting the 3% line loss criterion, a pilot-operated relief valve can be installed with a pressure sensing tapping taken from the protected equipment. With such an installahon, it is still required to reduce the inlet pressure drop as... 

Local Static Pressure In a moving fluid, the local static pressure is equal to the pressure on a surface which moves with the fluid or to the normal pressure (for newtonian fluids) on a stationary surface which parallels the flow. The pressure on such a surface is measured by making a small hole perpendicular to the surface and connecting the opening to a pressure-sensing element (Fig. 10-8a). The hole is known as a piezometer opening or pressure tap. 

A more economic solution is to replace the valves with POSRVs with remote pressure sensing. Instead of having a pressure pick-up to the pilot at the inlet of the main valve, pressure can be measured on the system, bypassing the inlet piping and its pressure losses. This solution is also easily field convertible if the snap-action pilot has integral pressure sensing. A typical remote sense point could be a tee into a gauge tap. 

The inductance-type transducer consists of three parts a coil, a movable magnetic core, and a pressure sensing element. The element is attached to the core, and, as pressure varies, the element causes the core to move inside the coil. An AC voltage is applied to the coil, and, as the core moves, the inductance of the coil changes. The current through the coil will increase as the inductance decreases. For increased sensitivity, the coil can be separated into two coils by utilizing a center tap, as shown in Figure 7. As the core moves within the coils, the inductance of one coil will increase, while the other will decrease. 

Pitot Tubes. The fundamental design of a pitot tube is shown in Eigure 9a. The opening into the flow stream measures the total or stagnation pressure of the stream whereas a wall tap senses static pressure. The velocity at the tip opening, lA can be obtained by the Bernoulli equation ... 

Pressure measurement for piping and vessels is achieved by the installation of dial indicators or by distributed control systems. The piping and vessels are tapped at the required location and furnished with a 3/4-in threaded or flanged conneaion and a block valve. Tbe dial indicator is screwed into the block ralve if a remote readout system is required, the valve becomes the sensing connection. Like the thermocouple, the dial can be either fixed or swivel-headed to fecilitate readout. A dual local indicator and transmitter s) em needs only one tapping point. Exhibit 14-9 depicts a typical pressure gauge and a dual-pressure system hookup. 

The pressures were sensed through taps located in the walls of the tank. Two taps at the same station were located near the bottom of the tank and three more were located in a vertical row approximately 36 in. apart near the top. U-tube water manometers were used to measure the pressure differences. Since it was not known beforehand how far out from the tank wall the liquid—gas interface would occur, the %-m. connecting tubes were held horizontal for the first 5 ft from the tank wall. Leveling was accomplished by having the tubes attached to adjustable channel irons. The liquid—gas interface was then at a constant vertical station. 

Other instrumentation in the tank included a tree of hot-wire liquid—gas detector probes to determine the approximate liquid level in relation to the upper tap, a mercury manometer sensing the tank ullage pressure, and three temperature probes in the water manometers to determine the water temperature. Additional equipment was installed near the end of the test program. This included a motion picture camera to observe the appearance of the surface of the liquid oxygen and a float device for the accurate measurement of the level drop when boiling was halted by pressurization of the tank. 

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