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    Topic: Gravel?
    Andy Macqueen
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    Gravel?
    on: May 18, 2015, 10:12

    My three-cell ACC was constructed with a permeable bed of 50mm gravel underneath. I am considering replacing this with a concrete hardstanding to use as a laydown area during our outages. Obviously, any temporary storage would be cleared before returning the plant to service. Are there any technical reasons why the gravel would have been used? Or just cost? Thanks for any ideas.


    ray.ohern
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    Re: Gravel?
    on: May 18, 2015, 10:22

    Andy, the gravel may be part of noise control, far field noise levels could increase with a solid surface under the ACC.


    Field Service Rep. GEA Heat Exchangers, Inc. Dry Cooling Solutions Division.

    Andy Howell
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    Re: Gravel?
    on: May 18, 2015, 11:00

    A couple of thoughts to consider:


    (1) Some gear box oil winds up underneath the ACC due to leaks. It sort of 'disappears' as it soaks into pea gravel, but might require clean-up if it accumulates on a concrete pad. This is not necessarily trivial, in some units the majority of gearboxes leak significantly. On the other hand, if you would rather collect and dispose of this oil due to environmental concerns, a concrete pad would help in this effort.


    (2) When the tube fins are washed (typically once or twice a year), the rinsewater accumulates under the ACC. Again, with pea gravel it will soak into the ground and disappear, but with a concrete pad it would run off, and presumably soak up elsewhere off the pad. I don't know that either option matters, but if you build a pad you should channel the rinsewater so it doesn't runoff to a random undesirable location. A side benefit might be that the debris rinsed off the ACC could be directed with rinsewater away from the structure, so that the debris is not picked up again with airflow and re-deposit on the fins.


    chemistron
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    Re: Gravel?
    on: May 18, 2015, 13:00

    I hate to be a nagging conscience, but from an environmental standpoint, oil does not just disappear into the gravel. It contaminates the gravel and underlying soil with oil and whatever additives it may have. Some of this will naturally attenuate from biological activity but not quickly. If you are using a synthetic oil, it may be even more toxic and biodegrade even more slowly. Frequent, uncontrolled oil loss and soil contamination will eventually turn into a cleanup activity, either voluntary or as a consequence of environmental enforcement actions. There are also ethical and social responsibility issues with this should your organization subscribe to such.


    Super Dave
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    Re: Gravel?
    on: May 18, 2015, 13:17

    Even if one overlooks the oil hitting the dirt/gravel, it shows poor maintenance practices. Input seal leakage or other gear reducer leakage should be caught in a weekly walk down type inspection, work order written to repair and scheduling/parts and so on to be done as soon as practically possible.


    Leaking oil will also build up dirt on the fan blades, shrouds, and on the tubes themselves, which I believe will reduce the efficiency of the tube sheet significantly over time. The oil attracts dirt which will begin to stick to the structure as well.


    Oil leakage should be targeted with a high priority since over time that leak can reduce output / efficiency of a cell.


    Gravel is a sound deadener. If your ACC is in a noise reduction zone you may be stuck with gravel. Unless you engineer a special form of concrete to be noise reducing.


    Maintenance Specialist for NV Energy at Walter M Higgins III Generating Station South of Las Vegas NV.

    Experience includes machinist, millwright, mechanice, welder and general industrial maintenance for over thirty years.

    rvillafuerte
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    Re: Gravel?
    on: May 18, 2015, 14:14

    Esteemed Colleagues:


    All of the above are sound advice. Here’s a new variable for discussion; the last couple of years we have entertained the idea that the gravel and dirt below the ACC contribute significantly to fouling. In addition to the gravel being on top of dirt that eventually flies into the ACC, whatever you water wash out of the ACC falls onto the gravel, water evaporates and leaves behind the fouling elements, and in the dry seasons eventually this “stuff” gets picked up again by the air flow and sticks to blades, cells and panels once more.


    For a while we thought of the concrete option with all the fixings like run-off tank and so forth, but it is expensive for a 15 cell large ACC.


    What do you think?…


    Andy Howell
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    Re: Gravel?
    on: May 18, 2015, 14:18

    Sorry the suggestion that pools of oil might form due to gearbox leaks is not realistic and would merit environmental evaluation as suggested. Of course Dave's advice to have an aggressive leak monitoring & repair program is most appropriate, but even with un-addressed leaks very little oil makes it to the ground, probably less over the ACC life than one major storm runoff at a Walmart parking lot. The upward air flow in the cell will move any oil droplets upward to the walls and tubes/fins, and this is what is seen in cells. I've never seen any evidence of gearbox oil underneath our ACC. So given the few droplets that might occasionally escape the gap on the fan cage, while 'disappear' is not technically correct, it's probably accurate in terms of detectability.


    Since the main issue seems to be sound dampening, it would be a matter of whether the sound increase from 3 fans in the particular facility location would be a problem. Including any impact on workers throughout the facility, including hearing protection etc.


    Francis Stanton
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    Re: Gravel?
    on: May 18, 2015, 15:01

    The original question was why Gravel? The answer is Cost $$$. The pros and cons of gravel versus concrete discussed here are valid. However, when design engineers are faced with capital cost restrictions to meet a capital budget, gravel is the choice, and an acceptable covering for power and process plant yards. In my opinion, the answer is that simple to keep the project costs down.


    If operations can justify the concrete or bituminous surface out of the capital budget, then it may be changed.


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