Differential pressure measurement

Closed Vessels. Liquid level can be measured by the static pressure method also at non atmospheric pressures. However, ia such cases the pressure above the Hquid must be subtracted from the total head measurement. Differential pressure measuriag instmments that measure only the difference ia pressure between the pressure tap at the bottom of the tank and the pressure ia the vapor space are used for this purpose. At each tap, the pressure detected equals the Hquid head pressure plus the vapor pressure above the Hquid. Siace the pressure above the Hquid is identical ia both cases, it cancels out. Therefore, the change ia differential pressure measured by the instmment is due only to the change ia head of Hquid ia the vessel. It is iadependent of the pressure within the tank and is an accurate measure of the level. 

Pressure reducing valves should be of steel constmction, designed for minimum and maximum operation conditions. Pressure gauges should be of ak-kon constmction. Pressure rehef valves should be of the spring-loaded type. Rupture disks may be used only as auxkiary equipment. Differential pressure measurements using mercury manometers should be avoided in ammonia service. 

Head Devices A variety of devices utilize hydrostatic head as a measure of level. As in the case of displacer devices, accurate level measurement by hydrostatic head requires an accurate knowledge of the densities of both heavier-phase and lighter-phase fluids. The majority of this class of systems utilize standard-pressure and differential-pressure measuring devices. 

Liquid Column Density may be determined by measuring the gauge pressure at the base of a fixed-height hquid column open to the atmosphere. If the process system is closed, then a differential pressure measurement is made between the bottom of the fixed height liquid column and the vapor over the column. If vapor space is not always present, the differential-pressure measurement is made between the bottom and top of a fixed-height column with the top measurement being made at a point below the liquid surface. 

The catalyst bed that was charged to the reactor is now a restriction, calibrated for flow vs. pressure drop. The pressure drop equals the pressure generated by the blower, which in turn depends on the RPM. In essence, the differential pressure measurement was eliminated by calibrating the flow directly with RPM. 

Differential Pressure Measurement Differential manometers are instruments that measure the difference in pressure between two adjacent rooms. Cleaner environments must have a higher pressure than adjacent, less clean environments to prevent flow of relatively dirty air into the cleaner environment. This differential pressure must be monitored and controlled. 

The pitot tube is a device for measuring v(r), the local velocity at a given position in the conduit, as illustrated in Fig. 10-1. The measured velocity is then used in Eq. (10-2) to determine the flow rate. It consists of a differential pressure measuring device (e.g., a manometer, transducer, or DP cell) that measures the pressure difference between two tubes. One tube is attached to a hollow probe that can be positioned at any radial location in the conduit, and the other is attached to the wall of the conduit in the same axial plane as the end of the probe. The local velocity of the streamline that impinges on the end of the probe is v(r). The fluid element that impacts the open end of the probe must come to rest at that point, because there is no flow through the probe or the DP cell this is known as the stagnation point. The Bernoulli equation can be applied to the fluid streamline that impacts the probe tip ... 

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 ... 

Differential pressure measurements were made between several vertical elevations within the bed. The probability density function of the cold model and combustor gave very close agreement (Fig. 35). The solid fraction profiles were obtained from the vertical pressure profile with a hydrostatic assumption. The cold model solid fraction profile showed very close agreement with data taken from pressure taps in two different locations within the combustor (Fig. 36). The solid fraction shows a... 

C, static-differential pressure, measured range 99.926-314.982°C, Morgan Kobayashi... 

Suppose the density can change with time. What is the transfer function relating a perturbation in density to the differential-pressure measurement Assume the fluid moves up the vertical column in plug flow at constant velocity. 

K. Ito, K. Ashikaga, H. Masuda, et al. Estimation of flooding in PEMFC gas diffusion layer by differential pressure measurement. Journal of Power Sources 175 (2008) 732-738. 

Differential Pressure Measuring for Examination of Propellants. This subject is discussed in a paper by H. Rumpff, Explosivsc 5, 43-45 (1957)... 

Differential pressure measuring for examination of propints 5 D1255... 

There are two elements in a head flow meter the primary element is the restriction in the line, and the secondary element is the differential pressure measuring device. Figure 1 shows the basic operating characteristics of a head flow meter. 

The time constants of pressure and differential pressure measurements are on the order of 0.1 seconds. Temperature measurement time constants are usually between 1 and 10 seconds. Composition measurements (analyzers) are even slower, varying from 5 seconds to 10 minutes. 

Use of piezoelectric gages in closed bomb tests of solid proplnts) 55)H.Rumpff, Explosivst 1957, 43-5 [Differential pressure measurements on burning of proplnts, using an oval-shaped bomb developed in 1953-55 by Rumpff previously used bombs for this purpose were of Krupp-Schmitz (1913), Liebes-sart (1921), Thring (1927), Rumpff (1930) ... 

Fig. 12. Differential pressure measurement of osmotic effect on a soft gel column...

Vacuum Extraction. Vacuum extraction requires the sample to be heated in an evacuated system with determination of the evolved gas, either volumetricaUy or by differential pressure measurement. This method is recommended whenever possible because of the speed and precision, particularly where the amount of hydrogen is below 0.01%. 

The mobility-control surfactant increased the apparent viscosity of CO2 sufficiently to prevent gravity override and viscous fingering. The bulk CO2 phase passed through the core in a piston-like manner. Oil and most of the brine were displaced from the core ahead of the bulk-phase CO2. Differential pressure measurement across the length of the core indicated an average gradient, 1.3 psi/ft, similar to that observed during the brine flood. 

---