Flow measurements static pressure

Where conditions are such that it is impractical to measure static pressure at the wall, a combined pitot-static tube may be used. This is actually two tubes, one inside the other. The outer tube is plugged off at the end facing the flowing fluid, but small holes are drilled through it to receive the fluid pressure. These holes open into the annular space between the tubes. The static pressure is measured through two or more of these holes, and it is assumed that the flow follows along the outside of the tube in such a way that the true static pressure is obtained. 

Static pressure is the potential pressure exerted in all directions by a fluid at rest. In a duct, static pressure tends to expand or collapse a pipe, depending on whether static pressure is positive or negative. One measures static pressure in a duct normal to the direction of air flow. 

A pressure sensor which is well suited to measure dynamic pressure changes is a quartz crystal (piezoelectric measurement). A charge across the crystal is proportional to the force on the crystal. This force results in a deformation which causes a subsequent short lasting change in flow of electric charge. These sensors ate therefore not suited to measure static pressures. 

Figure 67 shows the velocity and pressure distributions measured wifti Montz-pak for perfectly fitting packing, obtained with a uniform initial profile. It can be seen that the ouftet velocity distribution is almost flat. There is only a small disturbance n the wall. The resuhs show that the pressure measured in the compartments which feed a channel that ends at the wall is slightly hi er. This is due to fee additional prrasure drop caused Ity a 90° turn of fee gas flow. Tbe measured static pressure distribution profile at the top of... 

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 ... 

Static temperature is the temperature of the flowing fluid. Like static pressure, it arises because of the random motion of the fluid molecules. Static temperature is in most practical instaUations impossible to measure since it can be measured only by a thermometer or thermocouple at rest relative to the flowing fluid that is moving with the fluid. Static temperature will increase in a diffuser and decrease in a nozzle. 

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 maldng a small hole perpendicular to the surface and connecting the opening to a pressure-sensing element (Fig. The hole is... 

Chile [Prog. Aerosp. Sc7, 16, 147-223 (1975)] reviews the use of the pitot tube and allied pressure probes for impact pressure, static pressure, dynamic pressure, flow direction and local velocity, sldn friction, and flow measurements. 

Since this temperature requires the thermometer or thermocouple to be at rest relative to the flowing fluid, it is impractical to measure. It can be, however, calculated from the measurement of total temperature and total and static pressure. 

After catalyst charging and the flow vs. RPM measurement is done, the reactor should be closed and flushed out with nitrogen while the impeller runs, until O2 drops below a few tenths of a percent. Then a static pressure and leak test should be made by turning off the forward pressure controller and the flow controller. If an observable drop of pressure occurs within 15 minutes, all joints and connections should be checked for leaks and fixed before progressing any fijither. 

Gas flows are often determined by measuring the associated pressures. Figure 32-4 illustrates several different pressure measurements commonly made on systems carrying gases. Static pressure measurements are made to adjust the absolute pressure to standard conditions specified in the test procedure. 

For critical measurement, where the drop p is more than the barometric pressure, flow should be measured with static-pressure taps upstream from the nozzle as illustrated in Figure 20-4. For exhaust measurements, differential pressure is measured at two static taps located downstream from the nozzle at the inlet as shown in Figure 20-5. 

Flow Low mass flow indicated. Mass flow error. Transmitter zero shift. Measurement is high. Measurement error. Liquid droplets in gas. Static pressure change in gas. Free water in fluid. Pulsation in flow. Non-standard pipe runs. Install demister upstream heat gas upstream of sensor. Add pressure recording pen. Mount transmitter above taps. Add process pulsation damper. Estimate limits of error. 

In order to avoid the need to measure velocity head, the loop piping must be sized to have a velocity pressure less than 5% of the static pressure. Flow conditions at the required overload capacity should be checked for critical pressure drop to ensure that valves are adequately sized. For ease of control, the loop gas cooler is usually placed downstream of the discharge throttle valve. Care should be taken to check that choke flow will not occur in the cooler tubes. Another cause of concern is cooler heat capacity and/or cooling water approach temperature. A check of these items, especially with regard to expected ambient condi-... 

Since the static pressure loss for a hood is dependent on form and flow rate it can be used alone to monitor rhe flow rate into the hood. If the flow rate and the pressure loss were measured at the same time as the efficiency, the pressure loss can be used for monitoring hood performance. These methods are also described in the literature mentioned above. 

The absolute, barometric pressure is not normally required in ventilation measurements. The air density determination is based on barometric pressure, but other applications are sufficiently rare. On the other hand, the measurement of pressure difference is a frequent requirement, as so many other quantities are based on pressure difference. In mass flow or volume flow measurement using orifice, nozzle, and venturi, the measured quantity is the pressure difference. Also, velocity measurement with the Pitot-static tube is basically a pressure difference measurement. Other applications for pressure difference measurement are the determination of the performance of fans and air and gas supply and e. -haust devices, the measurement of ductwork tightness or building envelope leakage rate, as well as different types of ventilation control applications. 

This states that the sum of the velocity pressure 0.5pv plus the static pressure / the total pressure, is constant along a streamline. In the case of standard air density (1.2 kg m ), 0.5pv becomes 0.6v. When a Pitot-static tube is immersed into the flow, as in Fig. 12.19, the velocity at the stagnation point at the tube nose is f = 0 and the local static pressure equals the total pressure p,. The flow static pressure p, is measured a short distance downstream from the surface of the tube. The flow velocity is obtained by applying Eq. (12.27) ... 

Pitot-static traverse The set positions of a Prandtl tube m a duct run required to provide a statistically valid set of readings, A series of measurements of the torn 1 and static pressure taken across an area of a duct to determine the air veloc-it> at that point. The sampling distance should be at least 7.5 times the diameter of the duct away from any disturbances of air flow. 

Simmer the audible or visible escape of fluid between the seat/disk of a pressure-relieving valve at an inlet static pressure below the popping pressure, but at no measurable capacity of flow. For compressible fluid service. 

The static pressure within a duct is too small to he measured hy a hourdon tuhe pressure gauge, and the vertical or inclined manometer is usually employed (Figure 27.2). Also, there are electromechanical anemometers. The pressure tapping into the duct must he normal to the air flow. 

In a stationary fluid the pressure is exerted equally in all directions and is referred to as the static pressure. In a moving fluid, the static pressure is exerted on any plane parallel to the direction of motion. The pressure exerted on a plane at right angles to the direction of flow is greater than the static pressure because the surface has, in addition, to exert, sufficient force to bring the fluid to rest. This additional pressure is proportional to the kinetic energy of the fluid it cannot be measured independently of the static pressure. 

When the static pressure in a moving fluid is to be determined, the measuring surface must be parallel to the direction of flow so that no kinetic energy is converted into pressure energy at the surface. If the fluid is flowing in a circular pipe the measuring surface must be perpendicular to the radial direction at any point. The pressure connection, which is known as a piezometer tube, should terminate flush with the wall of the pipe so that the flow is not disturbed the pressure is then measured near the walls where the velocity is a minimum and the reading would be subject only to a small error if the surface were not quite parallel to the direction of flow. A piezometer tube of narrow diameter is used for accurate measurements. 

The pitot tube is used to measure the difference between the impact and static pressures in a fluid. It normally consists of two concentric tubes arranged parallel to the direction of flow the impact pressure is measured on the open end of the inner tube. The end of the outer concentric tube is sealed and a series of orifices on the curved surface give an accurate indication of the static pressure. The position of these orifices must be carefully chosen because there are two disturbances which may cause an incorrect reading of the static pressure. These are due to ... 

For the flow not to be appreciably disturbed, the diameter of the instrument must not exceed about one-fiftieth of the diameter of the pipe the standard instrument (diameter 7.94 mm) should therefore not be used in pipes of less than 0.4 m diameter. An accurate measurement of the impact pressure can be obtained using a tube of very small diameter with its open end at right angles to the direction of flow hypodermic tubing is convenient for this purpose. The static pressure is measured using a single piezometer tube or a piezometer ring upstream at a distance equal approximately to the diameter of the pipe measurement should be made at least 50 diameters from any bend or obstruction. 

Example 4-1 Manometer. The pressure difference between two points in a fluid (flowing or static) can be measured by using a manometer. The manometer contains an incompressible liquid (density pm) that is immiscible with the fluid flowing in the pipe (density pf). The legs of the manometer are connected to taps on the pipe where the pressure difference is desired (see Fig. 4-2). By applying Eq. (4-7) to any two points within either one of the fluids within the manometer, we see that... 

The measured pressure difference AP is the difference between the stagnation pressure in the velocity probe at the point where it connects to the DP cell and the static pressure at the corresponding point in the tube connected to the wall. Since there is no flow in the vertical direction, the difference in pressure between any two vertical elevations is strictly... 

To measure the internal flow velocity in the duct, dust sampling was taken at various points along the vertical diameter. A pitot static tube and magnehelic gauge, shown in Figure 1, was the equipment used for these measurements. The duct humidity, tempertaure, and static pressure were measured to calculate the gas density. In determining the humidity, the wet and dry bulb temperature of a continuous sample stream was used. To prevent dust buildup on the wet bulb thermometer, an inline metal filter was inserted into the line. 

Also, in the cold jet case, pressure profiles were measured to assess possible thrust penalty associated with the flow-induced resonance. Near-held pressure prohles, which are plotted in Fig. 29.11 for typical forced and natural cases, again show the faster growth associated with the excitation. In the far held, the static pressure became identical to the ambient pressure. To obtain the thrust force, far-held total pressure prohles were integrated over the jet cross-sectional area. The measurement at 18 exit diameters downstream for the excited case showed that there was a force deheit of about 8% compared to the natural case. This appears to be the maximum amount of thrust penalty caused by periodic impingement of shear how on the cavity trailing edge. 

The EPA Method 2 probe uses a standard S-type Pitot tube to determine the velocity pressure by measuring gas flow as a unidirectional vector. This method is typically 10-20% higher than the calculated flue gas rate from the FCC heat balance. The newly develop EPA Method 2F probe is a five-holed prism tip with a thermocouple. A centrally located tap measures the stagnation pressure, while two lateral taps measure the static pressure. The yaw angle is determined by rotating the probe until the difference between the two lateral holes is zero. This method closely matches the... 

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