Key characteristics of bulk cement, its limited shelf life, tendency to harden, dustiness during filling and significantly decreased density when freshly aerated, can make cement difficult to handle during storage and transfer.
Associated challenges for cement suppliers and concrete manufacturers include the need to monitor bin levels regularly, de-clog connections and weighing mechanisms, manage custody transfer, maintain filter efficiency and avoid over-filling.
Endress Hauser offers a comprehensive range of solutions and strategies for meeting these challenges, and thus optimising the efficiency of cement silo storage, and minimising costs for both vendors and customers.
Cement silos should ideally be sized to hold at least two days supply of cement. Bin levels must be frequently monitored and topped up to ensure that product will always be available on demand.
Endress+Hauser's Fieldgate-based Silo Guard Monitoring System (SGMS) enables a silo to be monitored remotely, so refill level alarms can be automatically triggered and supply chain logistics can be optimised.
Each Fieldgate module can link up to 30 plant control instruments (continuous level, overfill level switch, temperature, pressure, etc) via standard process industry communication protocols (4...20mA HART, Profibus or Ethernet) and transmits measurement data over a WAN (via standard telecom lines or GSM).
The collected data can then be accessed via the Internet using standard web browsers (e.g. MS Internet Explorer), exported and saved in standard formats (e.g. OPC, CSV).
Concrete hardening is an inevitable problem within cement silos, which must be regularly inspected and cleaned to keep connections unclogged. Mechanical systems for weighing bulk cement are also be affected by concrete hardening, compromising the accuracy of custody transfer and making frequent recalibration essential.
A far simpler measuring solution for bulk cement, and the only accurate and trouble-free way to determine the content of a cement silo, is to use a continuous level sensor such as Endress+Hauser's Micropilot FMR 250.
The Micropilot is ideal for this application because its accuracy is not affected by plant vibration, moisture, high process temperatures or excessive dust clouds, and no recalibration is required to maintain measuring integrity, after the initial set-up.
Numerous issues must be addressed to ensure the accuracy of custody transfer. Every silo, supply line and bin sector within a split silo must clearly display the type of cement it contains.
Since vehicles typically measure up to 15 metres long and weigh up to 40 tonnes, adequate ground strength and turning space are essential. The readouts of measuring instruments must be clearly visible and well lit, if deliveries are made outside daylight hours.
Endress+Hauser offer a range of bright, easy-to-read silo level or pressure displays that can be mounted on a wall or panel, either on the silo or near the tank loading nozzle in the truck loading area.
Installing an Endress+Hauser Ecograph T recorder (mounted on a panel) will clearly indicate the actual silo content (e.g. cement, sand, and aggregate) and corresponding bin level and pressure, and also record the mix ratios. The data gathered by these instruments enable optimum times for topping up the silos to be planned in advance.
Before a silo is filled, its level should always be re-checked. To avoid over-filling, it is important to minimise the gap between shutting off the cement supply and filling the silo.
Given that the bulk density of fresh cement varies between 1000 g/m2 and 1350 kg/m2, freshly aerated cement can reduce the capacity of a silo by over 25%. It is therefore advisable to be conservative, when estimating the weight of cement required to fill a silo.
A pressure relief valve and sensor are used to ease the build-up of air pressure within the silo, during the filling operation. A 100mm-diameter valve that responds to a pressure of 5 kM/m2 is sufficient.
Air filters are used to extract cement dust from the excess air. These have often been located on top of the silo, so that extracted dust can drop into the silo below, but this makes accessing the filter difficult. A better solution is to place the filter closer to ground level and connect it to the top of the silo with a 200mm-diameter feed pipe. Extracted cement dust can then be conveyed to a weighing hopper at ground level.
Many modern filters are self-cleaning pressure filters, which require minimal attention. Depending on frequency of use, the filter bags should be visually inspected every two to three months. It is also recommended that the efficiency of the filters be checked on a weekly basis, using the pressure sensor electronic control unit located at ground level.
Endress+Hauser's comprehensive range of pressure gauges includes the cost-effective Cerabar S gauge and the Deltabar S differential pressure sensor. Deltabar S can be flange-mounted to the silo and monitors for blocked or burst filters. Deltabar S comes with an abrasion-proof ceramic or stainless steel cell, and complies with SIL2 safety requirements.
Older, static type filters (i.e. not fan-assisted) require a much higher degree of maintenance to perform properly. A high-resolution differential pressure sensor should be installed to ensure prompt detection of burst filters. To avoid blockages that would cause excessive dust and dangerous internal pressures during filling, the filter bag should be inspected and cleaned at least once a week.
An air filter needs to be large enough to handle the maximum flow of air it is required to process. An air flow of up to 20m3/minute is expected for normal road tankers operating at pressures of up to 200kN/m2, and an extra allowance of 10m3/minute should be added for filtration of air from the silo discharge point. A total filter capacity of at least 30m3/minute is therefore recommended.
As well as causing severe dust emissions, over-filling can put the structural soundness of the silo at risk and pose a serious threat to the safety of personnel. The cement, concrete and quarrying industries as a good practice, should use a high level device to prevent over-filling, and to operate the silo within the manufacturer's specifications.
Endress+Hauser has a range of cost-effective high level switches for bulk solids, including the Soliphant vibrating fork, the Solicap capacitance probe and the Soliswitch rotating paddle stitch. Any of these can easily be mounted as a rod from the tank side, or suspended from the silo roof on a rope or cable.
As well as optimising cement logistics and the reorder process, Endress+Hauser's Fieldgate-based Silo Guard Monitoring System functions as a permanent silo guard that will automatically distribute data triggered by silo overfills, excessive internal pressure or burst filters. Alarms are sent immediately by phone, fax or email to a service centre, and/or directly to the maintenance engineer's mobile phone.
Endress+Hauser's user-friendly Applicator Selection software tool at endress website can help customers explore their cement silo management options and determine exactly which devices and models they need.