Friday 18th Sep, 2020

Life beyond the head pulley

Grant Wellwood, General Manager for Jenike & Johanson, explains how to master mechanical energy usage to reduce electricity costs and engineer better performing systems.

Q. My organisation is moving towards zero emissions and I was recently tasked with conducting a site energy survey. I was quite surprised by the magnitude of plug energy required by our conveyors. I never really thought about it before, but apart from the amount of energy we consume just moving material, what bugs me is that the ore often ends up static on a stockpile or as a feed for another conveyor, so where does it go? Although it’s been a while, I still recall that the first law of thermodynamics energy is that energy is conserved. As an example, we have one conveyor that travels at three metres per second and delivers 8000 tonnes per hour of bulk material through a transfer chute to another belt six metres below.

While I understand how energy dissipation plays out with furnaces and hydrodynamic systems, what happens to the plug energy in the case of bulk solids? Is there any way to recover some of this energy in a useable form to help contribute to our zero emissions objective and therefore make my management happy?

Yours Sincerely,

Flummoxed

A. Thanks “Flummoxed”, glad to read you are a conscientious member of an organisation focused on understanding and reducing its environmental impact. In most mining operations, materials handing is the major energy draw. For example, Terry Norgate and Nawshad Haque found that in relation to their model iron ore operation, 92 megajoules per tonne or 60 per cent of the total energy demand come from “loading and hauling” activities, therefore making it the prime target for improvement.

Within such operations, conveyors like the one you described are an efficient means of transporting and in many cases elevating, bulk materials, especially at high volumetric rates. As such, they are a familiar and critical element within many bulk solid handling systems so let’s take a closer look.

Your question can be rephrased as “what happens to the total mechanical energy of the flowing material between one point in the value chain (discharge from the head-pulley) and the next (dropping into a bin, or stockpile or wagon, bag or perhaps as in your case directly onto another conveyor)?” This critical yet unassuming function in the value chain is colloquially referred to as a “transfer point”.

While the plug power of the conveyor system will always be higher (to overcome the friction of running the conveyor-empty), there is always mechanical energy associated with the flowing solid that needs to be managed in the transfer. Using the elevation of the underflow belt in your example as the datum and applying the equations for power, there is around 140 kilowatts of total mechanical energy (130 kilowatts of potential energy and 10 kilowatts of kinetic energy) that needs to be dispersed within this transfer point, between the end of the head pulley and the underflow pick-up point below.

KE (W) = 1/2 m.v2-Equation 1

PE (W) = m.g.h-Equation 2

where:

m = mass flowrate (kg/s)

v = aggregate velocity (a vector quantity involving both speed and direction) of the bulk solid (m/s)

h = vertical drop under gravity (m)

g = gravitational constant (9.81 m/s2)

W = power in watts (J/s or kg m2/s3)

This is actually quite a lot of energy that needs to be continually dissipated beyond the head (or drive) pulley. Given the first law of thermodynamics holds, where does this energy go and what does it mean for the performance of your organisations value chain?

Within each transfer point there is a wide range of outlets (sinks) for the incoming mechanical energy including:

  • Heating (particles, sliding surfaces) and work done by heat-evaporation, heating air
  • Mechanical displacement of air
  • Noise
  • Attrition and abrasion of the bulk solid particles
  • Abrasive wear of the sliding surfaces
  • Driving segregation and stockpile movements
  • Vibration and elastic deformation of the infrastructure
  • Hard loading damage of the receiving belt conveyor (where applicable)
  • Driving compaction
  • Driving adhesion
  • Driving abrasive blasting of static material
  • Changing flow direction
  • Recovery via useful mechanical work

In most situations, the exact profile of mechanical work sinks at a transfer point will be some mix of these options determined by design and application-specific considerations. It is important to always consider life beyond the head pulley. Regardless of the operating circumstances, the issue of mechanical energy dissipation/management should not be left to chance, as it can hold your operating freedom to ransom. It requires careful and considered engineering focus, informed by a system view of all the components involved in order to get the best profile for the application. For example, if the bulk solid is dusty and fugitive emissions are an issue, the transfer needs to be designed with abrasion minimisation and displaced air movement in mind.

In the era of abundant and cheap energy, the primary sink for total mechanical energy within a transfer point has been wear. Depending principally on the geometry of the transfer, the wear-based energy sink can be associated with that of sacrificial (wear) surfaces. Or, if material properties permit and degradation of the bulk solid is not an issue, autogenous protection is afforded when moving particles impact on a layer of the same material (for example a “rock-box”, funnel flow).

In the case of engineered wear surfaces, a key consideration is the rate of wear in service.

Rotables

While wear plates designed to be regularly replaced represent a viable management option, premature failure between scheduled maintenance shutdown can be very costly. In this regard, there are some energy related issues to be aware of:

Elevated operating temperature

Due to friction, wear plates operate at an elevated temperature sometimes 100 to 150˚C above ambient. The higher the rate of wear, the higher the plate temperature (Figure 1).

Figure 1: Infrared thermograph showing dissipation of mechanical energy into wear plate as temperature increase.

Although operating temperature is seldom a consideration when selecting wear plates (outside extreme applications like kiln or smelter liners), abrasive resistance is generally a function of temperature. Therefore, it is something that should be taken into account when designing the geometry of your transfer system to accurately determine the risk-free service interval.

As it was only ever intended to qualitatively rank wear plates, accelerated wear tests that use a recycling sand applied at ambient temperature are not able to predict wear rate under service conditions (Wellwood, 2019). However, quantitative testers capable of emulating elevated operating conditions and generating a realistic wear rate are available. Reliance on qualitative, ambient temperature-based wear results can be a trap for the unwary in terms of service life estimation and competitive procurement testing.

Impact vs sliding abrasion

Particles within the flowing stream are essentially projectiles, so depending on the geometry and operation of the system, there can also be an impact dimension to wear plate life. Impact effects can be different depending on the type of surface or liner being used with the bulk material. 

For example, a ductile metal liner often experiences rapid wear from low impact angle and high velocity situations. A ceramic liner on the other hand experiences rapid wear from near perpendicular impact. This material is often not tough and experiences brittle fracture leading to failure.

Hidden consequences of over-rating throughput

A common misconception when it comes to the operation of bulk materials handling systems is that the elements in the value chain, like transfer chutes and the wear surfaces within, are invariant to throughput. However, when we look at things from mechanical energy perspective, we can see why this is not the case.

Say your asset’s economic optimisation model flags an attractive increase in overall profitability in exchange for a 10 per cent increase in throughput. In the case of the elements in the study system you provided, this would probably be achieved by increasing belt speed. But how does it play out in terms of total mechanical energy beyond the head pulley?

At the new condition of 8800 tonnes per hour with belt speed of 3.3 metres per second, the total mechanical work of the flowing solids increases by 12 per cent to 157 kilowatts. If the primary energy sink is wear plates in the transfer chute, the increased rate of abrasive wear could mean failure between scheduled maintenance shutdowns. The impact of such an event would almost certainly negate the throughput value gains (and probably then some) making the change in operating point a false economy that should never have been executed.

However, it is possible to quantify wear rates for a given material wear surface combination. These findings can then be incorporated into Discrete Element Method (DEM) models to predict flow patterns, service life and facilitate ‘what if?’ analysis in relation to the use of wear as a sink for mechanical energy.

In practice, mechanical energy dissipation is just one of the impacts of throughput change. When it comes to bulk material handling, everything is connected, and decisions need to be taken holistically. The emergence of digital twins featuring DEM elements offers the potential to change scenarios in the virtual world before being enacted. This approach has many benefits but critical to its success is giving transfer points a process identity.

Transfers as critical control points?

Bulk material transfers do not involve value increasing transformation (chemical or physical), which are the exciting aspects that most engineers are trained to deal with. This fact of life, combined with a lack of awareness in terms of issues like mechanical energy transformation, means that no-one takes accountability for the performance of transfer points beyond the basic requirements.

With no individual or discipline accountable, transfer point engineering tends to slip through the cracks during the detailed design phase only to emerge in the form of intractable performance issues during operation.

To remedy this situation, a simple technique known as Flow Analysis at Critical Control Points (FA@CCP) has been devised (Wellwood, 2020). Critical control points are defined as physical places in the value chain where there is an “aggregate velocity” (velocity) change, typically a transfer point (e.g. Figure 2).

Figure 2

A strategic tool, FA@CCP is based on establishing formal flowsheet identities for elements like transfer chutes and provide accountability for performance. Based on the extremely successful Hazard Analysis Critical Control Point (HACCP) framework, FA@CCP helps avoid baking-in problems by ensuring elements like transfer points are seen for what they are – a network of interdependent flow nodes whose individual and collective performance is critical for overall performance. Giving transfer points a flowsheet identity helps ensure they are considered as part of the detailed design process and ahead of executing every change.

Recovery

While most of the total mechanical energy sinks listed above represent loss from the process, hence waste, there is an opportunity to recover at least some of the total mechanical energy in transfer points.

For example, the inclusion of a mechanical device in the path of the moving material to harness mechanical energy, much like in the manner of a waterwheel (See Figure 3a), could be an option. This approach has been pursued with the support of a European grant and is currently being piloted (See Figure 3b).

Figure 3a Modelling of geometry and layout options for a turbine energy recovery device (used with permission).

In terms of technical feasibility, the results to date, such as those found in Michael Prenner’s Solid State Material Driven Turbine in 2019, appear encouraging and for many high-volume operations, seem worth pursuing in terms of value proposition.

Figure 3b Prototype mechanical energy recovery device (used with permission).

Assuming the wholesale electrical power price is $100 megawatt hours and a recovery efficiency of 55 per cent, the avoided electrical energy cost-based return from an investment from this one transfer point would be around $70,000 per annum. Of course, meaningful utilisation of this recovered electrical energy requires careful consideration. For example, could it be used to help drive conveyors in the vicinity or perhaps power dust collection at the transfer point?

While electrical energy is still relatively cheap, it is generally agreed that the current price does not reflect the cost to the environment and that this needs to change in the future. Getting ahead of the curve could be well worth considering and a good social and financial investment. If your operation is constrained by the power available from the grid, the recovered power can be used to generate more product/revenue, increasing its value to you.

In terms of the total mechanical energy dissipation profile, the inclusion of a recovery device reduces the load on the wasteful sinks, the impacts of which in many cases would be more valuable than the avoided power savings.

Additional, less tangible process benefits here could include:

  • Reduced particle segregation and degradation
  • Less dust
  • Less noise
  • Soft loading effect at a receiving belt conveyor (where applicable)
  • Reduced wear rates
  • More latitude in terms of managing particle trajectory
  • Tolerance of changes in cohesiveness
  • Rotable change out (compartment liners) during operation to reduce/avoid a complete flow shutdown
  • Reduced breakage and segregation
  • Reduced energy/environmental footprint

For many operations there is a need to demonstrate that our scarce resources like energy, are being used efficiently, with both the social licence to operate and environmental footprints becoming a key consideration.

This is an evolving area and there has been some interesting work done, however solutions like this need to be considered holistically from a life cycle perspective. The approach is perhaps better suited to very high throughputs (15,000 to 20,000 tonnes per hour) as the capital cost of the recovery device will scale sub-linearly while its installation infrastructure will be insensitive to installed capacity, giving good economies of scale.

A well-considered and engineered transfer point can help minimise undesirable outcomes like dusting, particle attrition and wear. By using tools like Discrete Element Method analysis, it is now possible to explore transfer point designs in the virtual world and reduce risk and maximise utility ahead of detailed design and fabrication. In cases where the amount of total mechanical energy to be dissipated is large, in-line devices that recovery energy in the form of electrical energy may be feasible.