In today’s episode of the Dust Safety Science podcast, we’re talking about alignment sensors for bucket elevators as per NFPA 61. Joining us is Brian Knapp, Vice President of the Electronics Division with 4B Components in Morton, Illinois. He’s helping us answer this HelpDesk question: “Is there a requirement from NFPA or other sources where rub block slash misalignment sensors should be placed on bucket elevators? I’m seeing two different standards in industry. One is two per head and tail and one is four per head and tail.”
Our team tried to answer this question by looking at NFPA 61. We found that in the appendices, it recommends using belt alignment monitoring devices for all elevator legs. It also suggests bearing monitoring systems for the head, tail, and bend pulley bearings of elevator legs, as well as checking the alignment of the forehead, tail, and the pulley. However, there was no mention of using redundant or multiple sensors on each head or tail.
Wasked Brian for his opinion, and his experience in the industry provided us with some very helpful insights. This episode answers the following questions:
- What is the purpose of alignment sensors?
- What are the best practices and benefits or challenges associated with using multiple sensors in this way?
What Is The Purpose Of Alignment Sensors?
Brian explained that in bucket elevators, we aim to prevent several potential ignition sources through mechanical means. The main sources of ignition include:
- Heat from Belt Slip or Misalignment: If the belts are misaligned and rub against the elevator’s casing, this friction can generate significant heat. This heat is a potential fire risk, especially in environments with combustible dust.
- Wear and Tear on the Equipment: Misaligned belts can also cause damage to the elevator itself, wearing down the belt and eventually damaging the casing. This not only poses a fire risk but also affects the overall lifespan of the elevator, even in non-combustible dust environments.
- Overheating Bearings: Bearings can overheat due to poor maintenance or simply because they are nearing the end of their lifespan. Overheating bearings can generate enough heat to potentially ignite materials like grain dust.
The goal is to detect and address these issues before they lead to more serious problems like fires or explosions. This is done by monitoring belt alignment and the alignment of pulleys at both the head and the tail of the elevator. By detecting any misalignment early, one can either alert the operator or automatically shut off the equipment to prevent dangerous situations.
What Are The Different Types Of Alignment Sensors?
There are several types of sensors available for detecting belt misalignment in machinery. A popular option is the ‘rub block’. This sensor works by making contact with the misaligned belt or pulley. Typically, a hole is cut in the machine casing to mount this sensor, which has a brass block as the sensing part. This brass block is thick enough to extend through the wall of the bucket elevator. When the belt rubs against this brass due to misalignment, the friction creates heat. This heat is detected by a temperature probe in the brass block, which then alerts the operator or automatically shuts off the equipment if it gets too hot.
Another type of sensor is force-activated, providing instant detection of belt misalignment. Different customers prefer different sensors based on their comfort, historical usage, and operational needs of their facility.
Contact-type misalignment sensors tend to trigger more alarms. Some facilities might not want the equipment to shut down every time the belt slightly misaligns, such as during the initial loading phase when the belt shifts temporarily. Others might prefer an immediate shutdown to prevent any heat buildup in the brass. The choice of sensor and response depends on the safety protocols, operators, and managers of the facility.
Regarding guidance on the type of sensor or the number of sensors to use, standards like those from the NFPA and OSHA are generally open-ended. They don’t mandate specific technologies for these applications, leaving the decision to the discretion of the users based on their unique requirements.
What Does NFPA Require?
Brian explained that NFPA 61, may require monitoring inside bucket elevators, including for pulleys located at the head, tail, and knee (sometimes near the boot section). This standard applies to both bucket elevators and belt conveyors, specifying the need to monitor the alignment of the head and tail belts.
He specified that a bucket elevator is a vertical conveyor. It has a pulley at the bottom, known as the boot or tail pulley, and another at the top, called the head pulley. The belt moves around these two pulleys. Misalignment in the belt usually occurs at these extreme points – either at the head or the boot.
Why Are There Different Placements?
Typically, equipment manufacturers provide one pair of sensors at the head and another pair at the boot. These sensors are placed where the belt’s edge would make contact if it misaligns, and they can detect misalignment in either direction.
When looking at a bucket elevator, it has a right and a left side as the buckets move up and discharge material. Therefore, a pair of sensors is placed at the head – one on the right and one on the left – to detect any misalignment on either side. The decision to use two or four sensors depends on the specific requirements and realities of the operation. While in an ideal scenario, the belt would misalign evenly on both sides, the reality can be different, hence the variation in sensor placement and number.
“The explanations that I’ve heard are- ‘let’s start with the head section because there’s some differences between the head and the boot that will play into this as well,’” Brian said. “So at the head of the bucket elevator they’re going to get everything installed mechanically with those bearings and the shaft and the pulley and everything to try to get it all level, to get it so that everything is going to be even there. And once that’s adjusted, that pulley is not going to be moved around to tension the belt. It’s just going to stay in that same orientation. So it’s generally going to be fairly easy to mount a sensor at the head pulley that can detect both the belt and the pulley, because the belt’s in contact with that pulley for the top half of the pulley. So, like a 180 degree arc there.
“Generally you can detect either one with one sensor and then this or one pair of sensors. And then this question about do we need a second pair of sensors? Some of the things that I’ve heard people say maybe this is why would be if there’s something mechanically with that machine casing, let’s say one side of the the casing is bowing in and the other side maybe is bowing out, we’ll say the up and the down sides of that pulley. So if you’re looking at the edge of the pulley we’ll say like 3:00 o’clock and 9:00 o’clock on there. Maybe the 3:00 o’clock is bowing out a quarter of an inch, and then the 9:00 o’clock is bowing in a quarter of an inch.”
“If you position a sensor at 3:00 o’clock on a pulley, it will be half an inch further from the pulley than a sensor placed at 9:00 o’clock. This is one way to set up the sensors. Another issue could arise if a bearing fails completely, causing the balls to scatter and the race to break apart. This failure could make the shaft within the housing loose or even cause the housing to crack, leading the shaft to tilt. In such cases, if the pulley or belt is misaligned, one edge may make contact before the other. However, these scenarios are rare and not commonly encountered.”
It’s assumed that in large commercial grain elevators, which operate numerous locations globally, there might have been instances where having just one pair of sensors wasn’t enough to detect misalignment. Something could have happened where the belt and pulley didn’t touch the sensors. For these companies, adding an extra pair of sensors is a way to manage risk. They believe that more sensors reduce the chance of missing a significant issue like this, helping to avoid catastrophic outcomes.
In the boot section of a bucket elevator, the belt tension is adjusted using the boot pulley. Over time, belts tend to stretch. For example, a new belt might stretch by about 1% in its first year. While 1% might not seem like much, on a 200 ft long belt, this equates to two feet of stretching, which needs to be tightened. Sometimes, if the belt stretches too much and runs out of room for adjustment in the boot pulley, it may need to be spliced (reconnected) again.
Adjusting the boot pulley and splicing the belt may require additional alignment sensors at the boot. This is because the take-up (the part that adjusts the belt tension) can be quite long, say two feet, and it’s important to detect any misalignment of both the belt and the pulley regardless of their positions. With just one pair of sensors, positioning them to consistently detect misalignment can be challenging. Therefore, extra belt alignment sensors are often installed at the boot section of the bucket elevator. These ensure that at least one pair of sensors is always aligned with the belt and the pulley, no matter where they are in the take-up range.
Have You Ever Seen More Than Two In The Boot?
“It’s possible that some facilities might have installed extra alignment sensors for their belts, although I haven’t personally seen this,” Brian said. “A company might require monitoring both sides of a belt – the upward and downward paths. If they find that the alignment sensors can’t always detect the pulley, they might choose to install as many as eight sensors at the boot.
“When discussing the pros and cons of adding more belt alignment sensors, especially for a company that manufactures hazard monitoring equipment, there are several factors to consider. Firstly, we don’t usually advise companies to keep adding sensors. The main downside of installing more sensors is the cost. This isn’t just about buying the sensors themselves; you also have to consider the expense of installing them, wiring them up, and integrating them into an alarm system. Then there’s the ongoing maintenance cost. More sensors mean more potential for damage and more parts that might need replacing, which adds to the overall maintenance workload.”
Are There Issues With Rub Blocks?
Rub blocks, made of brass, are used to monitor temperature when a belt misaligns. However, they don’t provide instant detection of misalignment. With contact-type sensors that are force-activated, you tend to get more misalignment alarms. This is often due to how the belts are made and maintained. A belt isn’t just a single piece of rubber; it has ends that are spliced together. Over time, the belt might get damaged and need to be spliced again, leading to multiple splices in one belt. These splices are hard to align perfectly, so when they pass over the pulley, they can cause the belt to shift slightly.
Most of the time, the belt might not touch the sensor, but when a splice passes by the pulley, it can cause slight contact with the brass block. This contact might not be enough to heat the block and trigger an alarm, but if it happens regularly, like every 30 to 60 seconds during an eight-hour operation, it will gradually wear down the brass block.
One issue that can arise is that the brass might wear down without ever triggering an alarm. That’s why regular inspection of these blocks is crucial as part of maintenance, to check for wear and ensure they can still generate heat when needed. Additionally, if the belt misaligns and triggers an alarm, but then the alarm stops, it might mean the brass has been completely worn through by constant rubbing. In such cases, someone should inspect the rub block immediately to see if it needs replacing.
Brian said that the industry has evolved significantly over the years, particularly regarding education about the dangers of combustible dust incidents, including fires and explosions. Often, conversations with clients start with complaints about equipment causing shutdowns. However, these shutdowns are a good thing; they’re a safety feature designed to prevent fires and explosions. Part of his job is to educate clients about this and help them understand the root causes of their problems.
For instance, if a rub block is triggering alarms, it’s usually because the belt is misaligned. The solution seems simple – realign the belt – but for maintenance workers, it can be quite complex. They might face multiple issues causing the misalignment. For example, the belt might have several splices, requiring a complete belt and bucket replacement. Or, the lagging on the pulley – which helps maintain friction and keeps the belt in place – might be worn out, leading to belt slippage and misalignment. Another issue could be that the pulley isn’t level anymore, causing the belt to lean to one side.
Maintenance plays a crucial role in resolving these issues. The initial reaction from customers or operators when equipment causes a shutdown should be to investigate what mechanical issues are causing it, rather than assuming there’s a problem with the shutdown system itself.
Conclusion
Asked if he had any closing thoughts, Brian said, “Just keep learning. Keep educating yourself on what your facility is doing and audit. Make sure that you have the monitoring equipment and protection that you need both for. We’ve talked about the NFPA standards, but also what makes sense for your facility. It doesn’t have to just be an insurance policy for preventing these dust explosions. Obviously that’s a very important reason to have it. But it helps keep your facility running smoother and providing a longer life for your equipment and all those sorts of things as well. So it is extremely valuable as a preventative maintenance tool as much as safety.”
If you would like to discuss further, leave your thoughts in the comments section below. You can also reach Brian Knapp directly:
Email: [email protected]
Website: https://www.go4b.com/usa/
If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode.
Resources mentioned
The resources mentioned in this episode are listed below.
Dust Safety Science
Combustible Dust Incident Database
Dust Safety Science Podcast
Questions from the Community
Dust Safety Academy
Dust Safety Professionals
Companies
Thanks for Listening!
To share your thoughts:
- Leave a note in the comment section below
- Ask a question to be answered on the show
- Share this episode on LinkedIn, Twitter or Facebook
To help out the show:
- Subscribe to the podcast on iTunes
- Leave a review and rate our show in iTunes to help the podcast reach more people
Download the Episode
DSS244: Helpdesk Question: Alignment Sensors for Bucket Elevators in NFPA 61 with Brian Knapp
