When you take delivery of new product, are you sure your filters are working properly? The delivery truck uses a high pressure air pump to force the product up the pipe and into your silo. This high volume of air will escape from the silo but, depending on the material, the filters can clog and then the pressure builds up. A very small air pressure of 1 to 2 psi, acting on a large area such as the top of your silo, the weakest link, will buckle it or tear it apart but you can avoid damaging your silo by adding a pressure switch and relief valve. Global Cement magazine has a good article on this in their July-August edition, page 32.
To understand how damage can occur, just take a moment to consider the area of your silo top. Assuming a diameter of 8 ft, the area is a little more than 48 square feet which is around 7000 square inches. A pressure of 1 psi becomes 7000 lb and 2 psi gives 14,000 lb; 7 tons! It’s easy to see how a small overpressure rips welded seams apart and structurally damages your silo.
When your delivery truck blows cement, fly-ash or other material into your silo, it uses a powerful air compressor to force the material up the pipe and into the silo. This creates a lot of exhaust air, which has to be filtered before it can be released from the silo. The fine dust that doesn’t settle lands in the silo filters. This must be released from them by mechanical shakers or blasts of compressed air from the outside. Eventually the dust starts to clog the filter. When the partially clogged silo filter impedes the exhaust air flow, typically the filter rips. Cascades of dust descend over the neighborhood. This, in turn, alerts the EPA and an unwanted inspection can occur. Alternatively, the pressure difference exerts huge forces on the top of the silo. It rips apart and, of course, triggers the EPA inspection as well!
John Hasbrouck of WAM USA advises that you should pulse the filter cleaning jets periodically during the fill and for 10 minutes following the fill. Also, be sure to fit an adequately sized filter. You can avoid damaging your silos by fitting them with pressure relief valves such as the VCP at left. This has a spring-loaded diaphragm inside a rain-proof housing which opens when the pressure exceeds a low value consistent with a partially clogged filter. This, in turn, prevents the filter from rupturing or the silo from bursting but will release unfiltered dust into the atmosphere. Note that most silos have these fitted before you receive them.
Before the pressure relief valve opens, it makes sense to warn someone that the pressure is rising. If this is done in time, you can change the filters before disaster strikes. If the pressure relief valve opens at 0.7 psi, a pressure switch such as the IPM, set to close at 0.3 or 0.4 psi will give adequate warning to allow workers to order new filters and install them in plenty of time.
SiloWeigh II Pro
Scale-Tron’s SiloWeigh II Pro Weighs up to 6 silos, which can be standard or divided, using sensors bolted onto the silo’s legs or skirt. As one of its many features, it displays a warning message when it detects overpressure. The warning message appears when the pressure switch closes and remains until cancelled. The warning messages reappear periodically, which gives personnel time to order new filters and replace them before disaster occurs. It can also activate an output for a horn or warning light which pulses or remains steady. The same holds true for the SiloWeigh.Net version with Internet viewing of up to 32 silos.