Pressure differential transmitter

For very low flow rates the orifice plate is often incorporated into a manifold, an integral part of the differential-pressure transmitter. This provides a convenient compact installation. 

Fig. 15. Differential pressure transmitter with remote seals.

Differential Pressure. Differential pressure transmitters designed for Hquid level measurements use soHd-state electronics and have a two-wire 4—20 m A d-c output. 

The Series 1151 differential pressure transmitter manufactured by Rosemount (MinneapoHs, Minnesota) uses a capacitance sensor in which capacitor plates are located on both sides of a stretched metal-sensing diaphragm. This diaphragm is displaced by an amount proportional to the differential process pressure, and the differential capacitance between the sensing diaphragm and the capacitor plates is converted electronically to a 4—20 m A d-c output. 

The proper installation of both orifice plates and Venturi-type flow tubes requires a length of straight pipe upstream and downstream of the sensor, ie, a meter mn. The pressure taps and connections for the differential pressure transmitter should be located so as to prevent the accumulation of vapor when measuring a Hquid and the accumulation of Hquid when measuring a vapor. For example, for a Hquid flow measurement in a horizontal pipe, the taps are located in the horizontal plane so that the differential pressure transmitter is either close-coupled or connected through downward sloping connections to allow any trapped vapor to escape. For a vapor measurement in a horizontal pipe, the taps should be located on the top of the pipe and have upward sloping connections to allow trapped Hquid to drain. 

Cahbration of some measurement devices involves comparing the measured value with the value from the working standard. Pressure and differential pressure transmitters are calibrated in this manner. Calibration of analyzers normally involves using the measurement device to analyze a specially prepared sample whose composition is known. These and similar approaches can he applied to most measurement devices. 

Flow is an important measurement whose calibration presents some challenges. When a flow measurement device is used in applications such as custody transfer, provision is made to pass a known flow through the meter. However, such a provision is costly and is not available for most in-process flowmeters. Without such a provision, a true cahbration of the flow element itself is not possible. For orifice meters, calibration of the flowmeter normally involves cahbration of the differential pressure transmitter, and the orifice plate is usually only inspected for deformation, abrasion, and so on. Similarly, cahbration of a magnetic flowmeter normally involves cahbration of the voltage measurement circuitry, which is analogous to calibration of the differential pressure transmitter for an orifice meter. 

If orifice plates are used as flow sensors, the signals from the differential-pressure transmitters are reaUy the squares of the flow rates. Some instrument engineers prefer to put in square-root extractors and convert everything to linear flow signals. 

Pneumatic differential pressure transmitter. Typical installation with orifice plate to sense flow rate. (Courtesy of Fischer and Porter Company.)... 

Electronic differential-pressure transmitter. (Courtesy of Honeywell.)... 

Concentric-orifice devices can be easily installed in high-pressure lenses. The high-end- and the low-end connection of such devices could be coupled with differential pressure transmitters, for example, with the before-mentioned devices of [54]. The devices are mounted in different ways. Mostly, the open pipe technique is used. Furthermore, both connected sides of the transmitter are fed with an inert gas, for example, nitrogen. For cases where the systems must separated, membrane devices can be arranged between them. All the mentioned orifice devices are technically proved and are applied in the high-pressure area. 

The differential-pressure transmitters are only available for moderate pressures, up to 400 bar. Membrane systems give the possibility of choosing corrosion-resistent materials for the parts of a device (wet system), or to protect the inside of the device by using an additional membrane which divides the instrument side from corrosive media (dry system). 

Differential pressure transmitters (or DP cells) are widely used in conjunction with any sensor that produces a measurement in the form of a pressure differential (e.g. orifice plate, venturi meter, flow nozzle, etc.). This pressure differential is converted by the DP cell into a signal suitable for transmission to a local controller and/or to the control room. DP cells are often required to sense small differences between large pressures and to interface with difficult process fluids. Devices are available that provide pneumatic, electrical or mechanical outputs. 

Fischer and Porter Ltd. Instruction Bulletin 10B1465, Revision 3 (1986). Differential pressure transmitters. 

Fig. 1. Schematic diagram of the experimental setup (a) Process diagram (all dimensions in centimeter) (b) packing element (c) liquid distributor. DP, differential pressure transmitter FI, flow indicator TI, temperature indicator.

When detecting the interface between two liquids, electrical conductivity, thermal conductivity, opacity, or sonic transmittance of the liquids can be used. Interface-level switches are usually of the sonic, optical, capacitance, displacer, conductivity, thermal, microwave, or radiation types. Differential pressure transmitters can continuously detect the interface, but, if their density differential is small relative to the span, the error will be high. On clean services, float- and displacer-type sensors can also be used as interface-level detectors. In specialized cases, such as the continuous detection of the interface between the ash and coal layers in fluidized bed combustion chambers, the best choice is to use the nuclear radiation sensors. 

Experiments to measure pressure drop and flooding limits were performed in a set-up accommodating monoliths with diameters of 43 mm (Fig. 8.16), while the length of the monoliths varied up to total length of 1 meter. The liquid was distributed by a nozzle the gas was introduced in countercurrent mode via mass flow controllers in the system. At the outlet of the monolith, a special device was mounted (Fig. 8.17), which improved draining of the monolith. The pressure drop along the column was measured using differential pressure transmitters. All experiments were performed at room temperature and atmospheric pressure. 

Not shown in the flowsheet is a 60-gal, heated, stirred tank used to prepare and store feedstock. The feed flows from this mixing tank to the reed tank, which is an 8-L vessel. The liquid inventory in the feed tank was monitored with a differential pressure transmitter from which the liquid flow rate was determined. The feed tank holds enough liquid for one balance period up to 8 hr. 

Now, we shall present an inexpensive means to meter small flows of gas. Accurately controlling and measuring such flowrates is often difficult. Control valves with small trims coupled to differential-pressure transmitters having small orifices (or long capillaries) are prone to plugging and calibration troubles. On the other hand, most mass flowmeters are expensive for small-scale uses. 

The principal process instrumentation included (1) thermocouples to monitor the temperature at various locations (2) a flow controller to measure the air flow rate to the calciner and (3) differential-pressure transmitters to monitor the pressure drop across the distributor plate, the fluidized-bed and the sintered-metal filters. 

Quite a number of technologies are available for measuring volumetric flow rates. These include differential pressure transmitters, vortex meters and magnetic flow meters. Each has its advantages and disadvantages. 

The differential pressure transmitter is the most popular and has been in use the longest. Its measurement principle is quite simple. Create a restriction in the line with an orifice plate and measure the pressure drop across the restriction. The measurement takes advantage of the physical relationship between pressure drop and flow. That is, the fluid velocity is proportional to the square root of the pressure drop, and in turbulent flow, the volumetric flow rate is essentially the velocity of the fluid multiplied by the cross-sectional area of the pipe (Fig. 11). 

---