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Optimising the cost of steam generation
January 2012, Control Systems Design


Steam has been described as the ‘lifeblood’ of industry. It is the medium by which heat from a boiler is converted into an easily transportable form that can provide diverse services from office heating to the mechanical energy that drives turbine generators. It is still one of the most popular methods of providing an energy source to a process and its associated operations. However, the true cost of generating steam varies from installation to installation depending on how effectively the generation set is optimised and the how the steam is transported through the operation.

It is now well accepted that measuring energy consumption is an important ingredient in the quest to improve energy efficiency. Efficient and accurate metering is vital to ensure that excessive consumption can be detected, along with an accurate picture of where the steam is being used to enable cross-departmental energy charging to be applied.

The rationale for regarding steam monitoring and control as a process is just as valid as monitoring and controlling other parts of the manufacturing process. The more accurate and reliable the measurements that are made, the more informed the decisions that affect costs and product quality. The following is intended to highlight a few areas where for a relatively modest investment; improved efficiencies can be gained quite quickly through the use of the most appropriate technologies.

Steam measurement

A common method of steam metering is the traditional orifice and differential pressure transmitter technique. The main areas of concern with this type of measurement are the orifice plates’ susceptibility to wear introducing immediate inaccuracies, the relatively high pressure losses introduced into the system by the orifice plate and the small measuring range.

Another significant area of potential inaccuracy is the dp transmitter itself, even most so called smart dp transmitters still utilise analogue sensing systems. The primary analogue sensor is very susceptible to drift caused by static pressures and high temperatures. These factors, coupled with the inaccuracies evident at low measuring ranges, can cause the overall performance of the metering installation to be highly suspect. The performance of the transmitter can be improved by considering the Yokogawa DPharp series of transmitters. This series of ‘digital’ sensor transmitters have been extensively performance tested on numerous fiscal installations worldwide and have proven to be accurate and reliable.

Yewflow Vortex

Another approach to steam metering is to consider the use of a vortex shedding flowmeter. The Yokogawa Yewflow vortex flowmeter has been installed successfully in thousands of steam metering installations worldwide for over 30 years. The volumetric accuracy of the Yewflow is better than 1% of reading as opposed to 3-5% of full-scale when compared with the orifice method. Low pressure losses, only two connections to the process and high reliability coupled with zero maintenance are just some of the reasons why this meter has been applied in such numbers worldwide. The meter is available in line sizes from DN15 to DN400 with operating temperatures up to 400°C.

Pressure and temperature measurement

The majority of steam metering is still volumetric. With the output of the boiler measured in tonnes/kg/lbs per hour, any change in line pressure through demand can cause significant errors in inferred mass steam calculations on a volumetric measurement. For example, a 0,5 bar change in line pressure at 10 bar operating pressure will cause a 5% error in mass flow accuracy. The installation of a ‘steam computer’ along with pressure and/or temperature measurements (depending on whether the steam is saturated or superheated) can remove the errors caused by line pressure fluctuations.

Applications can be as diverse as condensate differential temperature control through to simple line pressure monitoring. A comprehensive range of pressure and temperature instruments can be supplied to give an accurate and reliable picture of what is happening in the system.

Fuel oil metering

Fuel oil metering is traditionally the domain of mechanical type flow meters, which can be highly prone to problems of blockage or simple mechanical wear and tear due to contaminants in the system. The mechanical meters also introduce a high-pressure loss in the system, which in turn can increase the transportation costs. A modern alternative would be to consider the Rotamass coriolis mass flowmeter. Some of the benefits that can be accrued by applying this type of measurement on boiler fuel oil metering are:

* High accuracy, typically 0,1% of mass flow for liquid applications.

* Wide measuring range.

* Outputs in kg, tonne, lb etc.

* Density outputs available.

Water flow rate monitoring

The Admag AXF electromagnetic flowmeter can be applied on all conductive fluids over 1 μS/cm conductivity. The meter has virtually no pressure losses with its full bore design and has one of the widest dynamic flow ranges available. The high accuracy of reading coupled with low maintenance requirements makes it a sensible alternative to more traditional mechanical type meters.

Boiler combustion measurements

Precise oxygen measurement is critical for boiler combustion control. To achieve optimum combustion efficiency and reduce emissions, the ratio of fuel to air is critical. Too much oxygen causes thermal losses through the exhaust stack, whereas too little produces unacceptable stack emissions. Customers applying the Yokogawa EXA series of oxygen analysers report extended calibration lifetimes due to the unique robust sensor design. In addition, field sensor replacement times have been reduced to only 30 mins instead of the usual 2-3 hours, a great saving of maintenance time and effort.

Conclusion

The benefit of making improved quality measurements on the boiler process is the recording of meaningful information that can be used to improve the optimisation of the boiler control system. Recording can consist of simple paper-based recorders, paperless devices that store the results on computer media or comprehensive data acquisition systems with full monitoring and logging capability.

Many different types of approach to optimised control can be applied, ranging from the single loop approach to a distributed control system based on an embedded operating system. Typical examples include:

* Three element boiler level control.

* Boiler fuel/air ratio control.

* Drum level pressure compensation.

* Automatic combustion control.

* Master steam pressure control.

Remember, the key to executing a successful control strategy is to start with an accurate definition of the desired result – Yokogawa offer’s consultancy and advice in this area.

For more information contact Johan van der Westhuizen, Yokogawa SA, +27 (0)11 831 6300, johan.vanderwesthuizen@yokogawa.com, www.yokogawa.com


Credit(s)
Supplied By: Yokogawa South Africa
Tel: +27 11 831 6300
Fax: +27 11 831 6370
Email: info@za.yokogawa.com
www: www.yokogawa.com/za
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Further reading:

  • Case History 143: Report after optimisation exercise – Part 4.
    August 2015, Michael Brown Control Engineering, Control Systems Design
    This is the fourth article taken from a report which was prepared for various departments in a process company after some optimisation work was performed for them. (The other three articles were published ...
  • Yokogawa’s integrated control ­system for power stations
    August 2015, Yokogawa South Africa, Electrical Power & Protection
    Convergence of control system platforms is a big productivity enhancement tool for plant O&M management today.
  • Yokogawa acquires label-free cell image analysis technology
    August 2015, Yokogawa South Africa, News
    Yokogawa has announced that it has obtained a label-free image analysis technology from Chip-Man Technologies of Finland. Using this technology, detailed computer analyses can be conducted of live cell ...
  • Yokogawa training courses
    August 2015, Yokogawa South Africa, Training & Education
    Who will benefit from this training? Automation Engineers       Field Instrument Product Training    Roodepoort 7-11 September       Centum VP Essentials    KZN 9-11 September       PCI Advanced Product Training Roodepoort ...
  • Yokogawa releases Plant Resource Manager (PRM) R3.20
    August 2015, Yokogawa South Africa, Industrial Wireless
    Improved management functionality for HART and ISA100 wireless field devices.
  • Yokogawa’s Enterprise Pipeline Management Solution
    August 2015, Yokogawa South Africa, IT in Manufacturing
    Yokogawa has announced the release of the Enterprise Pipeline Management Solution (EPMS), a modular software suite that is the product of over 20 years of practical experience in implementing pipeline ...
  • Yokogawa launches next-generation field wireless
    Technews Industry Guide - Wireless 2015, Yokogawa South Africa, Industrial Wireless
    In 2010, Yokogawa launched its first wireless field products conforming to the ISA100.11a standard and today continues to spearhead the development of industrial wireless technologies.
  • Yokogawa training courses
    July 2015, Yokogawa South Africa, Training & Education
    Who will benefit from this training? Automation Engineers          Stardom    3-7 August Roodepoort       Centum VP Essentials    12-14 August Roodepoort       PCI Measuring Principles    17-21 August Roodepoort       Centum ...
  • Dehn provides safety for automation systems
    July 2015, Dehn Protection South Africa, Control Systems Design
    Failure of automation systems, and the resulting drop in production capacities, can result in enormous costs for manufacturing companies. Therefore, high availability is critical for both measuring and ...
  • Yokogawa releases web-based enterprise automation solution
    July 2015, Yokogawa South Africa, IT in Manufacturing
    Yokogawa Electric Corporation has announced the release of FAST/TOOLS R10.02, the latest version of a web-based real-time operations management and visualisation software package that brings revolutionary ...
  • Yokogawa upgrades Debmarine diamond mining vessels
    July 2015, Yokogawa South Africa, PLCs, DCSs, & Controllers
    Debmarine Namibia mines for diamonds off the southern coast of Namibia and currently owns, operates and maintains five marine diamond mining vessels. Debmarine Namibia contracts De Beers Marine South ...
  • Case History 142: Report after optimisation exercise – Part 3.
    June 2015, Michael Brown Control Engineering, Control Systems Design, This Week's Editor's Pick
    This is the third article taken from a report which was prepared for various departments in a process company after some optimisation work was performed for them. (The other two articles were published ...

 
 
         
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