Safer distilling
22 Nov 2004
An Edinburgh distillery is saving thousands of pounds after an upgrade of its boiler control systems to a fully-fledged programmable safety system.
The North British Distillery Company, Edinburgh's last remaining working distillery, operates 24 hours a day producing 64million litres a year of whisky and neutral spirit for blends such as Famous Grouse and J&B, as well as for gin and vodka.
Steam is used extensively in the production process - for cooking the grain, mashing, distillation and as the heat source for disc dryers. Any interruption to the steam flow almost inevitably results in downtime, which can easily cost £5000 an hour.
Steam for the entire site is provided by a pair of Wellman Robey boilers, each with two burners, at a rate of 50tonnes an hour. When the plant is running flat out there is little spare capacity from the boilers, so a fault with any of the four burners is likely to lead to part of the plant being shut down.
Unfortunately, the ageing cam timers and electromechanical relays used to control the burners were occasionally causing problems. Additionally, it was rare for the burners to light first time following any routine maintenance or planned shutdowns.
To improve matters and bring the boilers up to current site standards, system integrator Elite Control Systems was awarded a contract to upgrade the boiler control systems. At the heart of Elite's solution was the Pilz programmable safety system (PSS), with one of these replacing the cam timers and relays for each burner.
The Pilz PSS is suitable for use in applications up to SIL 3 (safety integrity level 3, as set out in IEC 61508) and BS EN 954-1 Category 4. For the North British distillery, the PSS is a 3100-series modular system comprising a power supply, CPU, one digital input module, one test pulse input/output module, and two relay output modules. The I/O modules, wired directly to the field terminals and burner combustion control units, replace some 31 electromechanical relays. There is also a link to a supervisory PLC and Scada package to provide process visibility, especially during start-ups and shutdowns.
When asked by Elite, North British said the burner management system should comply with the requirements of IEC 61508 SIL 1. Elite worked in partnership with Pilz who provided an independent safety consultant to chair a Hazop study that confirmed this view.
All installation and commissioning had to take place within the summer and Christmas scheduled shutdowns of last year, with minimal risk to production. Elite therefore proposed to implement the PSS upgrade during the summer, with the PLC upgrade and new Scada system following at Christmas. Despite the limited time available on site, Elite was confident that the PSS upgrade would function as intended. The software was therefore written as a direct copy of the cam timer and relay logic of the existing system, rather than using prewritten standard function blocks from Pilz.
Installation and commissioning for Phase 1 took just over one week, while Phase 2 was completed in just four days. 'Elite did an excellent job,' says Jim Davie, assistant engineering manager at North British Distillery. 'But proof of their expertise was borne out when, after commissioning, the burners all lit first time - a rare event indeed with the original system.'
