Piping pressure measurement

Disturbances upstream of the probe can cause large errors, in part because of the turbulence generated and its effect on the static-pressure measurement. A calming section of at least 50 pipe diameters is desirable. If this is not possible, the use of straightening vanes or a honeycomb is advisable. 

Minimum lengths of straight pipe are required for flow-measuring devices and for certain pressure measurements. Flow straighteners and/or equalizers should be used in the vicinity of throttle valves and elbows, as shown in Figure 20-1. 

A eommon failure in pressure measurement is the uneertainty of the eonfiguration of statie-pressure taps penetration through the pipe wall. This failure is another early-planning eoneern, sinee proper taps are easy to provide prior to plaeing the maehine in serviee, but inspeetion of the taps after operation has eommeneed is a luxury rarely afforded the test team. 

Another pitfall in pressure measurement, partieularly important in flow measurement, is the potential for liquids in gauge lines. All too often gauge lines eoming from overhead pipes have no provision for maintaining a liquid-free status, even though the flowing fluid may be eondensible at gauge-line temperatures. 

These techniques have been described previously in the pressure measurement section. Usually, one of the flow-measuring devices and the required instrumentation is incorporated as a part of the plant piping. The choice of technique depends on the allowable pressure drop, flow type, accuracy required, and cost. 

The net head or pressure measured in ft. or m that causes a liquid to flow through the suction side of a pump, enter the pump chamber, and reach the impeller. When the source of liquid is above the pump, NPSH equals the barometric pressure plus the static head, less the entrance head, frictional losses in the suction piping and vapor pressure of the liquid. When the source of liquid is below the pump, NPSH equals the barometric pressure less the static head, entrance head, frictional losses in the suction piping and vapor pressure of the liquid. NPSH is specific for each pump design and application and must be supplied by the manufacturer. 

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 rate of flow of water in a 150 mm diameter pipe is measured with a venturi meter with a 50 mm diameter throat. When the pressure drop over the converging section is 121 mm of water, the flowrate is 2.91 kg/s. What is the coefficient for the converging cone of the meter at this flowrate ... 

The flow of water through a 50 mm pipe is measured by means of an orifice meter with a 40 mm aperture. The pressure drop recorded is 150 mm on a mercury-under-water manometer and the coefficient of discharge of the meter is 0.6. What is the Reynolds number in the pipe and what would you expect the pressure drop over a 30 m length of the pipe to be ... 

The flowrate of a fluid in a pipe is measured using a pilot tube which gives a pressure differential equivalent to 40 mm of water when situated at the centre line of the pipe and 22.5 mm of water when midway between the axis and the wall. Show that these readings are consistent with streamline flow in the pipe. 

You are to specify an orifice meter for measuring the flow rate of a 35° API distillate (SG = 0.85) flowing in a 2in. sch 160 pipe at 70°F. The maximum flow rate expected is 2000 gal/hr and the available instrumentation for the differential pressure measurement has a limit of 2 psi. What size orifice should be installed ... 

A convenient way to measure the density of a liquid is to pump it slowly through a vertieal pipe and measure the differentia] pressure between the top and the bottom of the pipe. This differential head is directly related to the density of the liquid in the pipe if frictional pressure losses are negligible. 

Basically, the drill stem test involves attaching a tubing assembly to the end of the drill pipe, isolating the lest zone with rubber packers, and perforating the zone. The tool is then opened so that the fluids or gas in die formation can How up the drill pipe for metering at the surface. During this process, extensive pressure measurements are taken, which can help to indicate the extent of the reservoir and the rate at which the hydrocarbons could be recovered. Prior to describing how a well is finally completed (if the hole is not dry ), it is in order to describe the forces utilized to transfer the oil from the reservoir to the surface. 

A viscous fluid flows through a pipe with slightly porous walls so that there is a leakage of kP, where P is the local pressure measured above the discharge pressure and A is a constant. After a length l, the liquid is discharged into a tank. 

The value of the coefficient of discharge Cd for orifice meters depends on the properties of the flow system, the ratio of the orifice diameter to the upstream diameter, and the location of the pressure-measuring taps. Values of Cd for sharp-edged orifice meters are presented in Fig. 14-55. These values apply strictly for pipe orifices with throat taps, in which the downstream pressure tap is located one-third of one pipe diameter from the downstream side of the orifice plate and the upstream tap is located one pipe diameter from the upstream side. However, within an error of about 5 percent, the values of Cd indicated in Fig. 14-55 may be used for manometer taps located anywhere between the orifice plate and the hypothetical throat taps. 

The pressurized air network at the Depanment s workshop 6 bar) supplies the air for the installation. A fitter removes the eventual dust and moisture from the air and thereafter the air pressure is reduced to 3 bar. The air is then stored in a manifold that distributes it to 7 independent channels. The amount of air through each channel is measured with a rotameter and controlled with a valve. In addition, an air preheater has been installed before the manifold. Each air channel supplies air to a circular distributor around the gasifier. From each circular distributor five pipes evenly distributed supply air to the reactor. Such system guarantees an even air distribution, for any combination of channels in use. The location of the air levels is shown in Fig. 3, together with the position of temperature and pressure measurements. 

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