Primary Flow Elements

What are primary flow elements?

Primary flow elements are built into piping systems and generate a defined differential pressure through a partial restriction in cross-section within the flow meter. The square root of this differential pressure is proportional to the flow rate.

How can the differential pressure be measured with a primary flow element?

In order to measure the differential pressure, a combination of primary flow element and differential pressure transmitter is needed.

What is the flow coefficient Cv?

The flow coefficient Cv determines the ratio between the actual flow rate and the theoretically possible flow rate. Different primary flow elements that have the same geometrical form, under the same flow conditions and with equivalent Reynolds number, have the same flow coefficient.

Where can the discharge coefficients Cv be obtained?

The discharge coefficients Cv can be obtained from the standard (ISO 5167) for nozzles, Venturi tubes and orifice plates that are manufactured to the specified tolerances of the standard.

What is the beta ratio (β)?

The beta ratio (β) defines how strongly the pipe cross-section is restricted. The beta ratio should thus be considered as a factor. A beta ratio of 0.75 corresponds to a restriction of 75 % of the pipe cross-section.

Is a calibration required with a primary flow element?

With primary flow elements that have been manufactured to the defined tolerances no calibration is required. A calibration is generally required for applications where a higher measurement accuracy is required, e.g. for custody transfer or performance test applications.

What is the advantage of the compact orifice plate over a measuring flange?

The compact orifice plate integrates the primary flow element and the pressure tappings into a single assembly. In contrast to a measuring flange; fittings, tubing, valves, adapters and brackets can be dispensed with. It can be mounted between standard line flanges.

What is cavitation?

Cavitation is the generation of vapour bubbles. Vapour bubbles implode after a short time and can generate high pressures with this.

How does cavitation occur?

Cavitation occurs with high flow velocities or when an object moves through a flow with high velocity. It can also occur if the local pressure declines to some point below the vapour pressure of the liquid and subsequently rises above it.

Can multi-bore restriction orifices overcome cavitation when high pressure drops are required?

Multi-bore restriction orifices do not solve cavitation problems. However, multi-step type restriction orifices can provide a remedy here.

Which are the recommended upstream/downstream straight lengths for orifice plates and Venturi tubes?

The upstream and downstream straight lengths depend on many factors, such as the piping system and the beta ratio, for which reason no blanket statement can be made on this. If you have any questions on your specific application, you are welcome to contact us.

What are restriction orifices for and when are they used?

Restriction orifices are used to achieve a controlled or restricted flow. They prevent a too high loading of the primary flow element, and also prevent the possibility of cavitation. Restriction orifices can also be used for controlling the pressurisation in the commissioning of a process plant.

What are the advantages of the FlowPak and ProPak systems over other primary flow elements?

The advantages of the FlowPak and ProPak systems are that no upstream or downstream pipes are required, whatever the flow profile. Fitting is flexible and suitable for applications with limited mounting space. The pressure loss is reduced to a minimum, therefore achieving the highest energy efficiency of all flow meters.

What is the sonic condition?

The sonic condition is when a gas flows through an orifice and its pressure drops, a critical pressure value exists for which flow rate reaches the speed of sound in that gas. This happens when Pout/ Pin ratio is approx. 0.5. At sonic condition, flow rate and pout remain constant.

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