Valve Magazine - Fall 2008

BEYONDVALVES

BY DAVID W. SPITZER

Understanding Differential Pressure Flow Transmitters

Adifferential pressure flow measurement system consists of a differential pressure primary flow element and a differential pressure flow transmitter.

When the flow of a fluid in a pipe passes a restriction in the piping system, the pressure in the piping system is reduced. Most differential pressure primary flow elements are designed, constructed and operated in a manner such that the flow rate is proportional to the square root of the pressure drop across the restriction. These differential pressure primary flow elements include orifice plates, Venturi tubes, elbows, flow nozzles, low loss flow tubes, single-port and multiple-port Pitot tubes, segmental wedge and V-Cone flowmeters.

Some differential pressure primary flow elements, such as critical flow elements and laminar flow elements, do not follow this (squared) relationship. Therefore, some sections of this article do not apply to these technologies.

flow turndown) would require a differential pressure flow transmitter range of 1- 100 differential pressure units (100: 1 differential pressure turndown). Therefore, the “reasonable” 10: 1 flow turndown requires a 100: 1 differential pressure flow transmitter turndown.

Because many differential pressure flow transmitters measured accurately with an approximate 10: 1 differential pressure turndown, differential pressure flowmeter technology was often consid-

ered accurate from approximately 30- 100 flow units. Improved performance of differential pressure flow transmitters has increased the differential pressure turndown, so somewhat larger flow turndowns may be possible.

The upstream and downstream pressures associated with a differential pressure primary flow element are available at the taps of the element. Both of these taps are piped to ports on the differential pressure flow transmitter that measures

Flow Rates

As mentioned above, the flow rate through a differential pressure primary flow element is proportional to the square root of the pressure drop across the restriction. Table 1 illustrates this relationship.

This relationship can limit the ability of differential pressure flowmeter technology to measure large flow ranges. In the table, a “reasonable” flow measurement range of 10-100 flow units (10: 1

Table 1

Flow Rate

(in flow units)

100

50

31. 6

25

10

Pressure Drop (in differential pressure units)

100

25

10

6. 25

1.0

Differential Pressure Flow Transmitter Designs

The following principles are used in the design of differential pressure flow transmitters:

Capacitance. The differential pressure at the ports causes the wetted diaphragm to move an internal diaphragm located between two fixed plates. The movement of the internal diaphragm causes a capacitance change that can be converted into a signal that is proportional to the applied differential pressure.

Differential Transformer. The differential pressure at the ports causes the wetted diaphragm (or bellows) to move the magnetic core in a transformer. The movement of the core causes an electrical imbalance that can be converted into a signal that is proportional to the applied differential pressure.

Force Balance. The differential pressure at the ports causes the wetted bellows to create a force that is counteracted by a force generated by an electromagnet (or perhaps a servomotor). A measurement of the generated counteractive force can be converted into a signal that is proportional to the applied differential pressure.

Piezoelectric. The differential pressure at the ports causes the wetted diaphragm to apply force on a crystal. This force causes an electric signal to be generated that can be converted into a signal that is proportional to the applied differential pressure.

Potentiometer. The differential pressure at the ports causes the wetted diaphragm (or bellows) to move the wiper of a variable resistor (potentiometer). The movement of the wiper causes a resistance change that can be converted into a signal that is proportional to the applied differential pressure.

Silicon Resonance. A silicon resonance sensor is a micro-machined semi-conductor structure fabricated on a silicon crystal. The structure is shaped so it can oscillate and resonate at high frequencies. When a differential pressure is applied, part of the structure is under compression while another part of the structure is in tension. The compression and tension forces change the resonant frequency of the structure in a manner proportional to the applied differential pressure.

Strain Gage. The differential pressure at the ports causes the wetted diaphragm to apply a force on a strain gage. This force stretches the strain gage and causes the resistance of the strain gage to change. The resistance change causes an electric signal to be generated that can be converted into a signal that is proportional to the applied differential pressure.