Safety control systems: Essential considerations, costs

Safety control systems have four typical architectures, each with advantages and disadvantages, and differing cost (per unit). See tables, photos.

05/21/2012


For a laminating line project, work included replacing the single channel emergency stop push buttons and guards with a dual-circuit system. Courtesy: Optima Control Solutions

According to the latest HSE reports [Health and Safety Executive is an independent U.K. watchdog group], each year 1 in 100,000 workers dies in a workplace accident. Another serious figure is that 1 in 125 workers will suffer a major injury preventing work for a week or longer. With almost 3 million people employed in the UK manufacturing sector alone, the impact of unreliable safety systems is obvious.

Industrial automation and increased production rates mean that many machines are bigger and faster than ever. Subsequently, the health and safety risks associated with any automated manufacturing process have risen too.

Increasing integrity and reliability of safety systems has been promoted in European law with the migration from EN954-1 to the latest SIL and Pl standards: ENISO13849 and IEC/EN62061.

For machine builders and owners, the ideal safety system architecture must conform with the minimum industry standards while allowing safe machine operation and not preventing operators from efficient production. The systems should be subject to a lifecycle cost analysis and a benefit-to-cost ratio analysis.

Many component manufacturers have, over the recent past, developed products specifically for use in safety-integrated systems.

Automation manufacturers that provide safety control equipment include:

  • Pilz Safety
  • Siemens
  • GuardMaster (part of Rockwell Automation)
  • Sick
  • Schmersal

By their nature, machine safety systems are an understood element of most control applications and are tailored to meet safe machine operation requirements. Consequently, engineers need flexibility in the product suite they choose to meet the varying demands of individual machines.

Selecting safety components for inclusion in a safety system is subject to the same rigors as any design engineering exercise. They must, obviously, be fit for purpose, satisfy the technical (or safety integrity) demands of an application, and be cost competitive. (Safety is probably one of the areas where cost is the lower priority of the two). Technical and product support are, as always, in the mix.

This is the most basic safety system configuration with the cost per unit starting between £100 [~$158] and £300 [~$475].*

Option 1: Safety Relays

  • All emergency stop buttons are wired in series (with dual channels for redundancy).
  • Dual contactors are also used for redundancy.
  • Reset circuit monitors contactors before allowing reset to occur.

 

Safety relays: Advantages, disadvantages

 

Advantages of safety relays

 

Disadvantages of safety relays

 

Cheap compared to other alternatives

 

Wiring can be complex on big systems.

 

Simple

 

Difficult to commission and fault find when the system is down.

 

No software programming required

 

Complete rewiring is required if changes need to be made later. 

 

 

Option 2: Configurable safety relays

A safety control system employing configurable safety relays is slightly more expensive if installed on small systems. Typical price per unit is between £300 [~$475] and £1000 [~$1582].*

  • Components are wired individually, making testing easier.
  • Again, dual contactors are used for redundancy.
  • Reset point is configurable in the software, in this case.

 

Safety relays are less expensive than other alternatives, but wiring can be complex on big systems. Courtesy: Optima Control Solutions

Configurable safety relays: Advantages, disadvantages

Advantages of configurable safety relays

 

Disadvantages of configurable safety relays

 

Configurable, flexible

 

More expensive if used on small systems

 

Easy to fault find

 

Need for a PC and software to program

 

Cheaper on large system due to saving on test time.

 

-

 

Easier to change

 

-

 

 

Option 3: Redundant PLC arrangement

A typical arrangement for programmable logic (PLC) controllers used in critical applications is to configure a redundant pair, often with "hot-swap" functionality. The redundant controller is used to support a safe and orderly shutdown in the event the primary controller fails.

Arranging multiple controllers and meeting the demand of a safety/critical application using standard PLCs involves cross-coupling additional, dedicated I/O from each PLC to monitor their individual conditions and initiate the “hot swap” routine in the event of a unit failure. In a nutshell this often proves to be a complex and time-consuming arrangement to design and implement.

Hence, designing safety systems with standard controllers at the core requires extended engineering time, hardware, and build time to implement the safety section of an application.

Rockwell, Siemens, and other major PLC manufacturers offer equipment specifically designed for these critical applications which, while carrying higher hardware costs, reduce the costs of design, hardware, and build activities and provide a certified solution to the application engineer.

The price for two parallel PLC systems starts around £10 000 [~$15,822], without considering other significant costs, such as software engineering and commissioning.

Option 4: Safety PLCs

  • Specific code for safety applications is written in addition to the normal PLC code.
  • Safe I/O modules can be centralized or remote.
  • At present, safety PLC technology is more readily available and cheaper (from £2000 [~$3,164]* per unit)

 

In a laminating line a safety “motion detection system” was included using star wheel plates and rotation detection proximity switches fitted on two dc motor-driven rollers. Courtesy: Optima Control Solutions

Safety PLCs: Advantages, disadvantages

Advantages of safety PLCs

 

Disadvantages of safety PLCs

 

Very flexible

 

More difficult to program

 

Perfect solution for large machines

 

Expensive software required

 

Easy to expand later

 

Understanding of PLC code is required and highly trained engineers required

 

 

* Prices cover hardware only; no engineering time is taken into account.

 

Examples: 3 safety control system projects

Safety control system project details show how the options above were implemented with laminating line redundancy, a machine retrofit, and an upgrade of a safety control system.

In a safety critical application, Optima engineers refurbished a production machine to improve control and reliability and to meet the stricter safety standard of EN13849. Courtesy: Optima Control Solutions

-- Laminating line redundancy: Recently Optima designed and installed a safety control system for API’s Laminating line in Poynton, UK. Optima replaced the single channel emergency stop push buttons and guards with a dual-circuit system. Six new lockable guard switches were fitted on the machine.

A safety “motion detection system” was included using star wheel plates and rotation detection proximity switches fitted on two dc motor-driven rollers. The redundant safety system was designed to ensure that if one contactor failed to open and remove the mains supply, a backup contactor would ensure the supply is removed.

 

Designing machine safety systems needs take into consideration machine operability. Use of a programmable safety PLC (Pilz) provided machine operators with the flexibility to enter some sections of the machine, for thread up, etc., without compromising the integrity of the machine safety circuit.

API’s engineering manager Tony Robinson noted that Optima did a “high-quality job, on time, and within budget.” API used Optima previously and expects to use them again.

-- Machine retrofit: In another equally safety-critical application at Dunlop Oil & Marine in Grimsby, UK, Optima engineers were to refurbish one of its main production machines to improve its control and reliability, and to meet the stricter safety standard of EN13849.

The project required that Optima’s engineers work closely with their Dunlop counterparts to design a complex, yet flexible, safety system, which could also monitor the rate of motion under various operational conditions. To successfully meet these demands, Optima selected the Pilz PNOZ Multiprogrammable safety relay to enhance the security of the machine and extend its lifetime. It also used Siemens’ Simotion D drive to precisely gear-lock the machine’s carriage drive, which applies the material to the main drive. Optima’s managing director Michael Hill explained: “By using the latest Siemens and Pilz technology, we have successfully enhanced the safety, security, and lifetime of Dunlop’s machine, therefore eliminating a major risk factor.”

In a papermaking, coating, and converting safety project, PLC, HMI, drives, tension control, safety equipment, and optics were integrated to increase machine productivity. Courtesy: Optima Control Solutions-- Safety control system upgrade: Another business for which Optima upgraded a safety control system was the papermaking, coating, and converting company Tullis Russell in Scotland. For this safety project, Optima integrated a Siemens PLC and HMI, Parker Hannifan SSD drives, ABB tension control, Pilz safety equipment, and Sick optics, subsequently elevating the machine’s productivity. The emergency stop system was designed to suit a risk level SIL2. A Pilz PNOZ multiprogrammable safety relay system was also used for this application. Hill from Optima said: “When considering a control system upgrade, safety improvements are unavoidable and are a fundamental component that keeps the machine running.”

- Ian Brady is a senior engineer at Optima Control Solutions Ltd., Blackburn, and is a TÜV functional safety certified engineer (CFSE). Edited by Mark T. Hoske, content manager CFE Media, Control Engineering, Plant Engineering, and Consulting-Specifying Engineer. Hoske can be reached at mhoske(at)cfemedia.com.

ONLINE extra

Ian Brady, CFSE, is a senior engineer at Optima Control Solutions Ltd., Blackburn, UK. Courtesy: Optima Control SolutionsAbout the author: Ian Brady is a senior engineer at Optima Control Solutions Ltd., Blackburn, UK. As a TÜV functional safety certified engineer, he supervises many safety control system projects for Optima.

Optima Control Solutions designs, builds, and commissions drive control systems, including safety-related control systems. The company has grown steadily since its start in 1995, and serves clients in the UK and overseas, including customers in Europe, China, and South Africa. Industries served include paper, plastics, and metals. 

ONLINE

controleng.com/safety

Optima:  www.optimacs.com 

http://www.hse.gov.uk 



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Leaders Under 40 program features outstanding young people who are making a difference in manufacturing. View the 2013 Leaders here.
The new control room: It's got all the bells and whistles - and alarms, too; Remote maintenance; Specifying VFDs
2014 forecast issue: To serve and to manufacture - Veterans will bring skill and discipline to the plant floor if we can find a way to get them there.
2013 Top Plant: Lincoln Electric Company, Cleveland, Ohio
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Why manufacturers need to see energy in a different light: Current approaches to energy management yield quick savings, but leave plant managers searching for ways of improving on those early gains.
Electric motor power measurement and analysis: Understand the basics to drive greater efficiency; Selecting the right control chart; Linear position sensors gain acceptance
Protecting standby generators for mission critical facilities; Selecting energy-efficient transformers; Integrating power monitoring systems; Mitigating harmonics in electrical systems

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.