Already discussed the pros and cons that have the different basic elements. But it is also important to take into account other general characteristics:

General advantages of PD meters considered:

- Are universally suitable for applications involving liquids, gases and steam.

- They are an excellent method even in extreme process conditions (up to 400 bar / 5800 psi and up to 1,000 ° C / 1,800 ° F).

- Allow replace differential pressure transmitters in the process without stopping the machine.

- The basic elements are robust because they are entirely mechanical and no moving parts.

General drawbacks of PD meters considered:

- Are suitable for low viscosity to 50 mPas, except if used special orifice models.

- In measuring gases, the process conditions (pressure, temperature) should be as stable as possible. If you can not achieve this, a computer can use measurements of temperature and local pressure to compensate for flow measurement signals to process variations.

- Not possible batch cycles short (less than 1 minute).

- The measurement system in the remote versions may require valves and capillaries. The compact configurations (Fig. below), by contrast, reduced installation expense.

Specific advantages of orifice plates, nozzles and Venturi tubes:

- The method has been used for many years and is well accepted.

- Are global standard methods.

- Supports the widest range of nominal diameters.

- The ratio ß can be optimized to obtain maximum signal with minimum permanent pressure loss.

- The calculation methods are standardized and do not require calibration for each continuity.

- With orifice plates, the price increases with size (DN> 300/12 ") is marginal.

- They have no moving parts.

Specific disadvantages of orifice plates, nozzles and Venturi tubes:

- The standard does not cover diameters less than DN 50 (2 ").

- They are sensitive to variations in the velocity profile and turbulence.

- Not suitable for hygienic applications.

Specific advantages of the Pitot tube:

- Economical method of flow measurement, even in large pipe diameters.

- The method has been used for many years.

- Nominal diameters DN 25 to 2000 (1-80 ") are as standard. Special lengths up to 12 meters (40 feet).

- Also suitable for rectangular ducts.

- Easy to install (and can be fitted a posteriori).

- The losses are low compared to orifice plates.

- Cross-types can be used to check distortions in the velocity profile.

Specific disadvantages Pitot tubes:

- Accuracy is not as good as, the orifice plate.

- Not suitable for hygienic applications.

- A proper alignment is essential.

General Applications

Differential pressure gauges can be used to measure a wide range of flows in both liquids and gases or steam. The large amount of empirical data has been incorporated into numerous standards. Consequently, this measurement method is accepted in most countries and is widespread. The measurement of steam and condensate gases at high temperatures in secondary systems remains their main area of ​​application.

Pitot tubes are a viable alternative to orifice plates in situations where pressure losses must be kept at the lowest level possible, or in applications with large nominal diameters. The examples below represent devices in use for measuring the flow of a gas condensate, hot water and cooling systems.

Figure 33: Measurement of gas condensate with a Pitot tube Photo: "Deltatop" E + H.

Circulation condensate - Pitot tube measurement (Fig. above): This example shows a device for measuring the flow of gas condensate return pipe. Pitot tube pressure drop causes only minor and has been reassembled in the original pipe.

Hot water systems - measuring orifice plate (Figure below): For security reasons, this system has two shut-off valves installed in each of the capillaries. Heat insulation have been installed that prevents the capillaries are in contact with hot parts. The capillaries connect downhill with differential pressure transmitter (PMD75 Deltaset E + H). The five-valve manifold differential pressure transmitter has several functions, among which include preparing the team and keep it at zero differential pressure and isolated.

Cooling systems - measuring orifice plates (Figure below): The two-piece orifice plate (DP051 Deltaset E + H) design with angular step making annular groove is installed in a vertical pipe of DN 150 (6 "). This application also has a five-valve manifold.

Fig orifice plate in use