Pressure measurement question

Discussion in 'Irrigation' started by 1idejim, Dec 3, 2013.

  1. Kiril

    Kiril LawnSite Fanatic
    Posts: 18,308

    Can't do it with just a pressure gauge. You would need to monitor pressure at a given flow rate in order to have any faith in your data.
     
  2. 1idejim

    1idejim LawnSite Fanatic
    Posts: 9,322

    My dad says the same thing more or less.
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  3. stebs

    stebs LawnSite Senior Member
    Posts: 373

    Not sure I can agree with that... The pressure at the return fitting will almost always be less than pre-filter pressure... In a sand filter, the filter itself is the biggest restriction. Cartridge filters, especially oversized ones, the wall fittings are most likely the biggest restriction until the cartridges start to load up...

    One of my pools used to have a large sand filter on a 1hp pacfab challenger pump and would run about 17 psi clean with 1.5" plumbing throughout. Owner replaced the sand filter with a very large pentair cartridge filter that ran about 5 psi clean. The pump was extremely noisy after this change because it could not suck enough water to fully load up. I ended up manifolding the suction side with 2" which helped it draw more water, quieted it down quite a bit and bumped my clean pressure to around 10 psi.
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  4. 1idejim

    1idejim LawnSite Fanatic
    Posts: 9,322

    My suction is always .5" larger than my returns. Typically 2.5 and 2.0 respectively. A certain amount of back pressure is required for proper operation. As I said 8-12 psi is optimal at the filter.

    I may be over thinking this and may be wrong, I may be trying to confuse flow and pressure in my theory. Centrifugal pumps are easy to work on but the actual diagnosis can be confusing at times. That's why so many pumps are R&R if it's not the obvious impeller, bearing or seal.

    We'll see next year, I'll install a gauge or two close by and monitor them.

    Thanks for replying stebs, you don't post often enough and have a lot to bring to the forum.
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  5. stebs

    stebs LawnSite Senior Member
    Posts: 373

    I'll have to build a gauge for the return fittings next spring... you've sparked my curiosity now....
     
  6. 1idejim

    1idejim LawnSite Fanatic
    Posts: 9,322

    Ok......
    When I got started on a remodel I pressured the system up with water. You've prolly seen my induction rig. It's a three way brass valve with vent and adapters for water and air. I placed a liquid filled gauge on the rig. I build individule manifolds from a tee, gauge and no kink hb.

    Cut to the chase, I noticed that when inducing water, the induction gauge read a few pounds higher than the gauge on the manifold. At a point during the pressure build the pressures on the gauges swapped. After twenty and before thirty psi the induction gauge went from being a couple psi higher to a couple of psi lower than the manifold gauge.

    Well, FMS, this piqued my curiosity a bit. When the weather forecast was for a hard freeze I blew out the lines and brought the pressure back up using air. Same thing happened only the psi swap was at a different spot on the gauge and the spread was only 2 psi. I even swapped out my gauge and the results were the same. Now I'm hooked and want to find out if there could be a differnce on the return than there is on the filter.

    VGB drains were created to eliminate Accidental drownings and eservications due to entrapment or inlet obstruction. If the suction is so great it can turn your body inside out, the return pressure has to be higher than 8-12 psi doesn't it?

    When you talk restrictions, the inlet on the pump should be the first bottleneck. The filter is a cleansing chamber and not a real pressure building vessel in my line of thought. At least not like a pressure tank. I may be wrong but without installing a few gauges on a few pools, I'll never know.
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