The primary elements provide the input to the secondary element which provides for indications, alarms, and controls.

EO 1.5

Given a block diagram of a typical flow detection device, STATE the purpose of the following blocks:

a. Differential pressure (DP) transmitter

b. Extractor

c. Indicator

EO 1.6

STATE the three environmental concerns which can affect the accuracy and reliability of flow sensing instrumentation.

Circuitry

Figure 11 shows a block diagram of a typical differential pressure flow detection circuit. The DP transmitter operation is dependent on the pressure difference across an orifice, venturi, or flow tube. This differential pressure is used to position a mechanical device such as a bellows. The bellows acts against spring pressure to reposition the core of a differential transformer. The transformer's output voltage on each of two secondary windings varies with a change in flow.

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Figure 11 Differential Pressure Flow Detection Block Diagram

A loss of differential pressure integrity of the secondary element, the DP transmitter, will introduce an error into the indicated flow. This loss of integrity implies an impaired or degraded pressure boundary between the high-pressure and low-pressure sides of the transmitter. A loss of differential pressure boundary is caused by anything that results in the high- and low-pressure sides of the DP transmitter being allowed to equalize pressure.

As previously discussed, flow rate is proportional to the square root of the differential pressure. The extractor is used to electronically calculate the square root of the differential pressure and provide an output proportional to system flow. The constants are determined by selection of the appropriate electronic components.

The extractor output is amplified and sent to an indicator. The indicator provides either a local or a remote indication of system flow.

Use of Flow Indication

The flow of liquids and gases carries energy through the piping system. In many situations, it is very important to know whether there is flow and the rate at which the flow is occurring. An example of flow that is important to a facility operator is equipment cooling flow. The flow of coolant is essential in removing the heat generated by the system, thereby preventing damage to the equipment. Typically, flow indication is used in protection systems and control systems that help maintain system temperature.

Another method of determining system coolant flow is by using pump differential pressure. If all means of flow indication are lost, flow can be approximated using pump differential pressure. Pump differential pressure is proportional to the square of pump flow.