System Integration & Control Systems Design


Optimising the cost of steam generation

January 2012 System Integration & 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



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