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Thread: Dry Bulk Truck Unloading Procedure

  1. #1
    prosolid Guest

    Dry Bulk Truck Unloading Procedure

    Dry Bulk Truck operators keep blowing the filter bags out of the collector on top of the silo a causing quite a mess. Most of this is caused by inadequate high level controls which we are addressing. Another problem I believe the truck operators are operating at too high an air volume, and also they let the compartments go completely empty which causes a great rush of air to flow into the silo which the collector can not handle. I am looking for a published Dry Bulk Truck unloading procedure so our personnel can keep an I on the truck drivers and instruct them on how we wolud like the trucks unloaded. We are a waste water treatment facility and the product is lime. The bulk truck can back right up to the silo and connect to a 4" line. Right now our operators are telling the truck drivers not to go over 11PSI but I know we need to find a way to limit the CFM. Problem is different trucks use different blowers, Roots, GD, Duroflow etc. and the RPM needed for a specific ICFM is different. I am sure this problem is common and was wondering how it has been handled in the past

  2. We would be happy to assist in developng a procedure for your plant personnel.

    Please contact me directly at the e-mail address below if you would like our assistance.

    Regards

  3. storage tank problem

    Greetings from the "Frozen Eastern Wilderness",

    To solve your problem quickly and easily you should install a pressure relief valve/vacuum regulator valve in the delivery circuit before the dust control system and this will solve your problem and stop all the delivery problems, it works very well for a vacuum circuit and will work equally well on a low pressure air circuit. it is a fix all for all the different types of systems you deal with.

    quick fix and fool proof.

  4. There are a number of issues, first of all filters blowing off could be due to silo over filling or due to undersize filters. Over filling could be avoided by having high level alarm connected to a valve on the inlet pipe and as soon high level alarm goes off the inlet valve shuts off. But there is a catch to it: one is left with tanker at 1 or 2 bar depending on the blower. Some tankers have their on filters which could be used to depressurise the tanker, but some don’t this causes problem.

    Pressure relief valve will help and should always be used on silos, but still it will send the dust to atmosphere which in most cases is not acceptable. Correct size filter is required which should be generously sized to handle the pressure surge at the end, when the tanker depressurises. If different size tractor blowers are to be used size the filter for the highest air flow / operating pressure.
    Mantoo

  5. The common design criteria for silo vents is TWICE ther free air delivery from the tanker blower up the line with no feed.

    This will ensure that yr filter is adeq. sized.

    Also....silo over and under pressure relief valves are common as well...

    Cheers


    James

  6. #6

    Pressure tanker unloading

    The problem relates to the "clean blow" airflow rate at the end of the unloading cycle. The tanker is first pressurised by the operator then the discharge valve is opened and the material transfer takes place.

    When all the material is transfered, the pressure in the tanker is equalised with atmospheric pressure, via the transfer pipework and the silo vent filter. This can result in a very high flow rate for a short period. There is often damage to vent filters, filter bags and the transfer pipework.

    The tanker operator/driver must be educated to control the discharge rate during clean blow by regulating the discharge valve. Provide a pressure gauge for the operator which shows the pressure differential across the vent filter, with instructions not to allow it to exceed a set figure.

    This is not total protection, so the vent filter should be oversized. Sizing for two or three times the normal transfer flow rate will provide protection.

    Michael Reid.

  7. #7
    Dennis Hauch - Freeport, TX, USA Guest

    Bulk Truck Unloading

    Michael,

    The drivers of bulk trucks always want to unload in the shortest possible time. So they push the flow and pressure limits as much as possible.

    Bulk trucks are piped to split the blower flow between the conveying line and a line to the top of the tanker (it's this flow that controls the solids conveying rate). The driver manually adusts this split until the conveying pressure inside the tanker is maximized, 11 psig in your case.

    When the last high-velocity slug of product clears the conveying line you have, at that instant, a large reservoir of air (tanker and pipeline) at 11 psig with clear path to atomosphere. This results in a tremendous rush of air (it can be easily calculated) which overwhelms the filters.

    The solution has two elements. First, determine the maximum allowable flow for the baghouse that you now have. As a general rule bag filters should be capable of an instantaneous peak flow 10 cfm/sq ft of cloth area.

    Then by iterative calculations determine the pressure at which this peak flow will occur. This is the maximum conveying pressure which can not be exceeded when unloading the bulk truck. If it's 5 psig, it's 5 psig. The driver may not like it (unloading time will increase) but it will eliminate damage to the filters and EH&S issues associated with escaping dust.

    Dennis Hauch, PE

  8. #8
    There should be stickers around the tanker which instruct the driver & were presumably approved in the early days. Otherwise a publication should exist in the guise of the driver training manual presented at the beginning of his/her employment. I was given a manual when I started driving ADR tanker loads. Aside from these legal considerations...won't a relief valve clog occasionally?....have you considered an intermediate cyclone?.....can you fit a sonic nozzle to limit the air mass flow rate?...is there room atop the silo to halve the air to cloth ratio?

  9. If it is confirmed, that the problem is caused by the final rush of air, a usual solution is the following:
    Install a ball valve between tank nozzle and conveying pipe and a bypass (around this valve) with an orifice and a valve. During steady conveying the valve is open and bypass is closed.
    As soon as the vessel is empty and air expansion starts (detected by decreasing conveying pressure or increasing filter pressure), close the valve to prevent the rush and open the bypass for slow expansion via orifice.

    Reinhard Ernst

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