A steam flowmeter comprises two parts:
1. The 'primary' device or pipeline unit, such as an orifice plate, located in the steam flow.
2. The 'secondary' device, such as a differential pressure cell, that translates any signals into a usable form.

In addition, some form of electronic processor will exist which can receive, process and display the information. This processor may also receive additional signals for pressure and/or temperature to enable density compensation calculations to be made.

Figure 4.4.1 shows a typical system.

 

 

Fig. 4.4.1 A typical orifice plate steam flowmetering station Fig. 4.4.1 A typical orifice plate steam flowmetering station

Differential pressure cells (DP cells)

If the pipeline unit is a differential pressure measuring device, for example an orifice plate flowmeter or Pitot tube, and an electronic signal is required, the secondary device will be a Differential Pressure (DP or ?P) cell. This will change the pressure signal to an electrical signal. This signal can then be relayed on to an electronic processor capable of accepting, storing and processing these signals, as the user requires.
Fig. 4.4.2 Simple DP cell Fig. 4.4.2 Simple DP cell

 

A typical DP cell is an electrical capacitance device, which works by applying a differential pressure to either side of a metal diaphragm submerged in dielectric oil. The diaphragm forms one plate of a capacitor, and either side of the cell body form the stationary plates. The movement of the diaphragm produced by the differential pressure alters the separation between the plates, and alters the electrical capacitance of the cell, which in turn results in a change in the electrical output signal.

The degree of diaphragm movement is directly proportional to the pressure difference.

The output signal from the measuring cell is fed to an electronic circuit where it is amplified and rectified to a load-dependent 4-20 mA dc analogue signal. This signal can then be sent to a variety of devices to:

* Provide flowrate indication.
* Be used with other data to form part of a control signal.

The sophistication of this apparatus depends upon the type of data the user wishes to collect.

Advanced DP cells
The advancement of microelectronics, and the pursuit of increasingly sophisticated control systems has led to the development of more advanced differential pressure cells. In addition to the basic function of measuring differential pressure, cells can now be obtained which:

* Can indicate actual (as distinct from differential) pressure.
* Have communication capability, for example HART® or Fieldbus.
* Have self-monitoring or diagnostic facilities.
* Have 'on-board' intelligence allowing calculations to be carried out and displayed locally.
* Can accept additional inputs, such as temperature and pressure.

Data collection
Many different methods are available for gathering and processing of this data, these include:
* Dedicated computers.
* Stand alone PLCs (Programmable Logic Controller systems).
* Centralised DCSs (Distributed Control Systems).
* SCADAs (Supervisory Control And Data Acquisition systems).

One of the easier methods for data collection, storage, and display is a dedicated computer. With the advent of the microprocessor, extremely versatile flow monitoring computers are now available.

The display and monitoring facilities provided by these can include:
* Current flowrate.
* Total steam usage.
* Steam temperature/pressure.
* Steam usage over specified time periods.
* Abnormal flowrate, pressure or temperature, and trigger remote alarms.
* Compensate for density variations.
* Interface with chart recorders.
* Interface with energy management systems.

Some can more accurately be termed energy flowmeters since, in addition to the above variables, they can use time, steam tables, and other variables to compute and display both the power (kW or Btu/h) and heat energy usage (kJ or Btu).

In addition to the computer unit, it is sometimes beneficial to have a local readout of flowrate.

Data analysis

Data collection, whether it is manual, semi-automatic or fully automatic, will eventually be used as a management tool to monitor and control energy costs. Data may need to be gathered over a period of time to give an accurate picture of the process costs and trends. Some production processes will require data on a daily basis, although the period often preferred by industrial users is the production week.

Microcomputers with software capable of handling statistical calculations and graphics are commonly used to analyse data. Once the measuring system is in place, the first objective is to determine a relationship between the process (for example tonnes of product/hour) and energy consumption (for example kg of steam/hour). The usual means of achieving this is to plot consumption (or specific consumption) against production, and to establish a correlation. However, some caution is required in interpreting the precise nature of this relationship. There are two main reasons for this:

* Secondary factors may affect energy consumption levels.
* Control of primary energy use may be poor, obscuring any clear relationship.

Statistical techniques can be used to help identify the effect of multiple factors. It should be noted that care should be taken when using such methods, as it is quite easy to make a statistical relationship between two or more variables that are totally independent.

Once these factors have been identified and taken into account, the standard energy consumption can then be determined. This is the minimum energy consumption that is achievable for the current plant and operating practices.

The diagram in Figure 4.4.3 plots a typical relationship between production and consumption.

 

 

Fig. 4.4.3 Typical relationship between production and steam consumption

Once the relationship between steam consumption and factory production has been established, it becomes the basis/standard to which all future production can be measured.

Using the standard, the managers of individual sections can then receive regular reports of their energy consumption and how this compares to the standard. The individual manager can then analyse his/her plant performance by asking:

* How does consumption compare with the standard?
* Is the consumption above or below the standard, and by how much does it vary?
* Are there any trends in the consumption?

If there is a variation in consumption it may be for a number of reasons, including:P

• Poor control of energy consumption.
• Defective equipment, or equipment requiring maintenance.
• Seasonal variations.

To isolate the cause, it is necessary to first check past records, to determine whether the change is a trend towards increased consumption or an isolated case. In the latter case, checks should then be carried out around the plant for leaks or faulty pieces of equipment. These can then be repaired as required.

Standard consumption has to be an achievable target for plant managers, and a common approach is to use the line of best fit based on the average rather than the best performance that can be achieved (see Figure 4.4.4).

 

Fig. 4.4.4 Relationship between production and specific steam consumption

Once the standard has been determined, this will be the new energy consumption datum line.

This increase in energy consciousness will inevitably result in a decrease in energy costs and overall plant running costs, consequently, a more energy efficient system.

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