A complete system of differential pressure measurement consists of several components (Figure below):
- Main element (a) - A narrow passage or a Pitot tube
- Shut-off (b)
- Capillaries (c) to transmit the measured pressures
- Manifold (d)
- Differential pressure transmitter (e), including the power supply
- Chamber of condensation (f) in steam
Fig 1. Typical systems for PD meters.
A = liquid, B = steam, gas C =, a = parent element (eg orifice plate or pitot tube), b = Shut-off, c = capillary, d = Manifold, e = transmitter differential pressures (including power supply), f = Chamber of condensate or condensate pot
Capillaries (c) connect the parent (a) with the differential pressure transmitter (e). The layout varies depending on equipment design, application and type of fluid. The capillaries also reaches the shut-off valves, (b) behind the pressure taps and Manifold (d). The transmitter maintains Manifold isolated and zero differential pressure process conditions. The differential pressure transmitter can be replaced without interrupting the process.
The capillary systems are essential in applications with high process temperatures (eg> 300 ° C / 570 ° F) so that excessive heat does not damage the electronics of the transmitter. For temperatures below 300 ° C (570 ° F), E + H offers the measuring system "Deltatop" with integrated hair, so it provides us with Invensys Foxboro measurement systems for pressure differential. (Figure below).
Photo: "Deltatop" E + H. It clearly shows the orifice plate (below), the manifold (center) and differential pressure transmitter (above).
Different types of Invensys Foxboro Differential Pressure
Figure: Model of an orifice plate.
Liquid installation configuration (Fig. 1 / A):
The differential pressure transmitter is located at the bottom of the parent or main. Thus, the capillaries are always in contact with the liquid and gas bubbles can escape into the process pipe. Pressure taps are usually located in the lower third of the pipe to minimize the risk of bubble formation, which could even enter the capillaries.
Steam installation configuration (Fig. 1 / B):
The differential pressure transmitter is located at the bottom of the parent or main. Thus, the capillaries are always in contact with the liquid and gas bubbles can escape into the process pipe. Condensation occurs constantly in the condensation chambers. Excessive condensation returns to the process pipe and re-evaporate. Condensation chambers guarantee a hydrostatic height of the liquid constant in both capillaries, which must be exactly the same length to eliminate bias effects due to the static pressure in the diaphragm of the transmitter.
Gas installation configuration (Fig. A / C):
The differential pressure transmitter is located at the top of the parent or main. This prevents any moisture or to precipitate in the capillaries and these are kept dry. Condensed moisture droplets slide down the process piping.
