Moving towards Industry 4.0 with a factory full of ageing assets

Over 50% of the salt used in the UK is produced at the British Salt factory in Middlewich, taken from the Cheshire Plain salt reserves – which are expected to last for the next 300 years. 

Since the factory was opened in 1969 ago it has produced over 30 million tonnes of high purity dried vacuum salt along with millions of tons of industrial and technical salt grades. Over 1,000 tonnes every day for the last 50 years. 

Salt is extracted a few miles away from the processing plant. The brine field operation employs a technique called ‘solution mining’ which sees bore hole drilled to a depth of around 200m. Water is then injected into the salt strata dissolving the salt to form brine which is then pumped up to the surface and piped to the processing facility. There are currently a number of active brine field boreholes which produce enough brine on a daily basis to keep the British Salt factory operating on a 365, 24/7 basis.

The first step of the production process is purification, where unwanted impurities are removed from the brine solution. The purified brine then moves on to the evaporator hall where it goes through a series of evaporators that drive off the water to create a brine slurry. The final stage of dewatering process uses centrifuges to further reduce the water content. At this point the pure undried salt has a moisture level of around 2%.

The high purity salt designated for use as a food ingredient is then further dried, moving through a series of fluid bed driers to drive out the final 2% of moisture. The next step is grading, which decides whether the salt becomes a traditional table salt product or whether it is designated for use as an ingredient in processed foods. Once the salt has been graded it either leaves the site in a bulk load on a road tanker or is sent on to the bagging plant, where around eight million bags, sized from a minimum of 10kg upwards, are processed each year. 

Because salt is such a corrosive and erosive product many of the assets in the British Salt facility need to be specialised. The environment will, for example, result in mild steel corroding seven times faster than in a normal environment. Materials of construction is one of the biggest considerations when specifying new equipment. “We employ a number of exotic materials across the plant,” explained Stephen Crabb, head of operations at British Salt. “Everything from conventional 304L and 316L stainless steel through to some real super high corrosion resistant alloys in the evaporator tray and we still use wood and GRP for walkways around the site as these materials do not corrode. We undertake constant maintenance around the plant, continually checking for points of corrosion,” continued Crabb.  

“Some of the plant is 50 years old – a time when control was mostly achieved manually. Technology moves quickly and we have, of course, added varying degrees of automaton to our processes over the years and we continue to do so.”

When it comes to moving the plant towards Industry 4.0 there are many challenges as Crabb explained further: “One of the biggest challenges for us is that the factory runs 24/7 for 365 days of the year so we cannot stop the plant for any length of time to add new systems. We have made the decision to move carefully towards Industry 4.0 via a series of mini projects where we are able to add cost-effective off-the-shelf automation solutions. 

“Utopia for us would be to have real-time information from across the plant displayed at a single point but this is a huge challenge for 1960’s designed factory such as ours,” said Crabb. “We would first need to role the DCS out further into the factory and this would allow us to employ artificial intelligence (AI) technology. If we can put in place more control loops that can monitor and control themselves then the plant would become more reliable. This is why we want to take the approach of having a single control solution across the plant. Today we still rely on a human to react to the information provided by the data.

“Employing AI technology could also help improve the plant’s overall energy efficiency. Today we rely on operators to make informed decisions. In the future we hope to see this process being undertaken using AI technology, using real-time data to ensure the plant is always run at optimum efficiency.”

Recent automation technology investment at the plant has been focussed on the end-of-line processes. A new packing line was installed in October 2018 and another new line is due to be installed imminently, with yet another on order. The packing area is one of the newer parts of the facility and so is currently the most automated. “In this area we are already able to collect real-time information about packing rates – mostly using off the shelf technology solutions. Other equipment is so specific to us that a bespoke solution is needed,’ said Crabb. 

“Our current strategy is to automate locally,” said Crabb. “We have a top level control layer, provided by Metso, which monitors and controls the entire factory and this works very well. To enable us to benefit from the use of predictive maintenance technologies we also need to focus on specific assets. In a factory such as ours the pumps and valves are all of differing ages, with some even being obsolete, so we either need to add automation as and when we replace assets or we need to employ retrofittable solutions.  Of course, with a strategy such as this is connectivity is a key enabler. We need to get data from a wide variety of disparate sensors and systems to a central point and this is not an easy task!”

Three senior members of British Salt’s control electrical team have been tasked with working on these siloed automation projects. For example, they are looking at how to integrate the new pack lines into the existing control system. At the micro scale they are also working on projects to find out how to best monitor and capture data from factory assets using retrofittable sensor technology. 

One of the automation projects is focused on one of the plants main circulating pumps – if this pump isn’t working efficiently it could increase energy costs by up to 10%. “We are in the process of fitting monitoring sensors to this pump. The control system to monitor the pump is already in place and this gives us some very basic data. However, we will need to replace the pump to fit the new sensors to ensure that the pump has all the correct fittings, explained Crabb. “We cannot take this step until the next planned plant shutdown. When it is all up and running this new pump control and monitoring system will link directly to our DCS system.” 

British Salt is also currently trialling an ABB Smart Sensor on a flue gas compressor drive. “We capture C02 from flue gas emissions for use in the purification process. This system has no standby drive so it is important that we know in good time about any motor deterioration,” said Crabb.  While the Smart Sensor is wireless the team does still need to download information locally, using a Bluetooth tablet. The sensor holds data for up to 40 days so the engineering team is currently collecting data on a monthly basis. “If this data proves useful our next step will be to install more of these sensors across the site on other motors and to install a Bluetooth infrastructure which flows automatically onto the ABB cloud to send us a warning if an asset exceeds its defined pre-set specifications.” A number of other manually gathered condition monitoring operations are being undertaken by the engineering team – including vibration analysis.  

In conclusion, Crabb admitted that funding for new technology to help steer the factory towards Industry 4.0 is always going to be a challenge. “In an 24/7 50-year old plant there is always a need to replace existing assets so adding new technology has to compete for financing with these vital asset replacements and repairs.

“There will also always be a level of uncertainly when it comes to making Industry 4.0 technology decisions and it does require a leap of faith. No one knows how quickly technology will change or whether a better solution might become available in the near future,” he said. Despite this the engineering team at British Salt is already starting to make these difficult decisions in a bid to ensure that the plant continues to run at optimum levels of productivity and efficiency. “As digitisation technology reduces in price then some of these futureproofing decisions will become much easier to make,” concluded Crabb.