Industry: Utilities

Advanced compressed air management helps Eskom keep the lights on

Goals

  • Improve the control of compressors with special attention to their optimal and efficient use as well as their reliability
  • Reduce the risk of downtime and plant failure through compressed air supply breakdowns
  • Give operating personnel more time to focus on other systems

Solutions and Products

  • InTouch
  • Historian
  • TOP Server

Challenges

  • Testing on a live process not possible due to its criticality and nature
  • Establishing reliable, redundant communications

Results

  • Increased reliability of compressed air availability
  • Energy savings
  • Automatic fail-safe system
  • Personnel freed up for other tasks
  • Reduced downtime
  • Accurate historical records

“The project included a complete redesign of the operator interface, taking into consideration the best practices for alarm management and situational awareness principles. The end result is a very simple yet highly effective interface which requires minimal operator intervention.”
– Barend Esterhuizen, Pr Eng, Senior Engineer, Lethabo Power Station

Background

Viljoensdrift, northern Free State, South Africa – In industry, compressed air is so widely used that it is often regarded as the fourth utility after electricity, natural gas and water. However, compressed air is more expensive than the other three when evaluated on an “energy packet” basis which makes its efficient management all that more important.

Compressed air is often used in a multitude of mission-critical applications such as at Eskom’s Lethabo Power Station which, with its six 618MW generators, is responsible for contributing to the supply of electricity to one of South Africa’s most densely-populated regions where the reduction of load shedding is an ever-present Eskom goal.

Compressed air is critical to Lethabo’s operations as various areas of the plant rely on sufficient air capacity or pressure. For example, in the event of the automatic cut off of the Water Treatment Plant and Service Air, should air pressure drop too low, then the fire protection systems (deluge valves) can be activated since the valves operate on the pressure difference between air and water. This could result in serious secondary plant damage.

The compressed air arrangement at Lethabo comprises six centrifugal compressors, two diesel compressors, cooling towers and an overhead control system. These are all critical elements to operations but were managed by an outdated monitoring system which required the frequent attention of operators who are routinely involved with many other pressing mission-critical tasks. In addition, the system used an unreliable ring network where failure of one node meant losing the entire network. Old-fashioned HMI graphics served to confuse rather than inform the operator who also had no control of the cooling towers or any feedback on their status.

“One of the important issues with the old system was that, whenever electrical isolations were done, such as on dust compressors, Operating would lose part or all remote indication and controls,” says Barend Esterhuizen Pr Eng, Senior Engineer, Lethabo Power Station.

In view of this, Eskom decided that the modernisation of Lethabo’s air management system was a priority and outlined three objectives for the project:

  • Improve the control of compressors with special attention to their optimal and efficient use as well as their reliability
  • Reduce the risk of downtime and plant failure through compressed air supply breakdowns
  • Give operating personnel more time to focus on other systems

Project Scope

To realise these goals, the following would have to be addressed:

  • Replacing the old compressor controllers with the latest version from the OEM (including new local custom-built enclosures and instrumentation)
  • Installing a new fibre optic network (not based on a ring topology)
  • Designing, programming and commission a new HMI SCADA system for the monitoring and control of the compressors which would involve:
  • Providing a mechanism for the sequencing of the compressors (duty selection)
  • Making provision for the automatic control of the compressors to eliminate manual operation
  • Supplying cooling tower monitoring and control facilities
  • Implementing alarm management best practices (ISA18.2 and according to The Alarm Management Handbook by Bill Hollifield and Eddie Habibi)
  • Adopting mimic design best practices for increased operator situational awareness (according to The High Performance HMI Handbook by Bill Hollifield)
  • Providing fail-safe procedures for increased reliability of compressed air delivery to the various users

“Our aim was to provide Eskom with a high performance HMI/SCADA system to control and monitor their complete compressed air system at Lethabo. We believe that we’ve designed and implemented a successful solution that can be used as a standard platform on their other plants.”
– Francois De Jager, Managing Member, FrandCorp CC

Implementation

The power station already had Wonderware’s Historian to which was added a new InTouch HMI/SCADA system with new software and mimics. The system was developed over a two-month period and thoroughly tested off site before being commissioned. TOP Server from Software Toolbox was used to better handle all Modbus communications.

“We designed the new compressor mimic overview display to be as uncluttered and informative as possible,” says Francois De Jager, Managing Member, FrandCorp CC. “At a glance and even from the other side of the control room, the operator can see the average system pressure, what it’s been doing over the last fifteen minutes and the alarm conditions. The display also shows which compressors are running, which ones have tripped as well as those ready to start.”

The operator can drill down into more detail for each of the compressors where, here again, the displays have been designed to be clear and unambiguous as to highlighting the problem as well as its cause while graphically indicating the state of each of the 22 measured parameters for each compressor. “This is a lot of information to be displayed but by adopting situational awareness principles, the operator can evaluate it all at a glance and take the appropriate action,” adds De Jager.

Eskom compressed air site overview

Site overview and network layout of the air management system at Eskom’s Lethabo Power Station

In the event that a compressor trips or becomes unavailable, the system will automatically adjust and start the next machine in the sequence when and if needed. This sequence is defined with the “Duty Selection” display which lets the operator set the operational limits for the compressors and guides him as to the order in which compressors should be selected in order to optimise energy efficiency. “The difference between our approach and those of the OEM control systems is that we try to run all available machines under normal load,” says De Jager. “Running compressors under any other condition means blowing air into the atmosphere and wasting energy. So we’d rather set different set points for each machine and have a standby machine supplement any additional demand when necessary. This means that we have the least number of compressors running at any time.”

“This automated solution to our compressed air needs means that we can focus on other issues to ensure that Lethabo continues its track record of reliable energy contribution to the national grid. Another plus is that we optimised the use of existing operational and control assets,” says Esterhuizen.

Eskom compressed air InTouch screen

InTouch screen showing the old and new compressor mimics (overview)

Benefits

  • Increased reliability of compressed air availability
  • Energy savings
  • Automatic fail-safe system
  • Personnel freed up for other tasks
  • Reduced downtime through automatic and prescribed system responses
  • Accurate historical records of trends, conditions and operator actions
Eskom

About Lethabo Power Station

Lethabo is an Eskom coal-burning power station situated in Viljoensdrift in the northern Free State. It comprises six 618MW Production Units each comprising one boiler, a turbine and a generator. Power is produced at 20kV and at 13.5kA and sent to a step-up transformer where the voltage is increased to 275kV for efficient transmission.

A unique aspect of Lethabo is the unusually low grade and quality of coal used. The coal burnt has an average calorific value of about 16 MJ/kg. Very few power stations can burn this quality of coal, with most boilers requiring coal fuel of values in excess of 20 MJ/kg. At full load, the power station consumes about 50,000 tons of fuel per day.

The ash content of the coal ranges between 35% and 42%, which means that the power station produces approximately 20,000 tons of ash per day. After mining operations, the open pit is filled with spoils and overburden, which was removed during mining. Ash is deposited onto this area to a level of 50 metres above ground level. The ash dumps (mountains) are then covered with fertile soil and vegetation.

Lethabo Power Station is also termed a ZLED-station (Zero-Liquid-Effluent-Discharge). This means that the whole station is a closed system and no water from its processes are allowed to leave the power station premises. An extensive water recycling and cleaning desalination is in place.