A Little Bit Over a Long Time Equals A Whole Lot

The picture below is a floor basin serving the overflow line on a 600 ton cooling tower on a project I was visiting recently.

Make up leakage 03

If you look closely, you’ll notice that there is a little trickle of water coming out the overflow.  That trickle of water caused me and the folks I was working with to stop and take a look at what was going on inside the tower.  Here is what we saw.

Make up leakage 02

Most of the splashes you are looking at are the flow falling through the tower into the basin.  But if you look closely, you will notice that water is also dribbling out of the make up
valve even though the level is near the point where it will over flow the basin (notice the small distance between the level of the water and the drift eliminators in the rear of the picture).

The float has not sunk, but is actually up as high as the valve mechanism will allow it to rise.  The valve is seated but the seat is leaking, which is causing the overflow condition and the submerged float.

To troubleshoot the problem, we did two things.  One was to push the float down to verify that it would in fact float (it did).  We also did this to see if the leakage was caused by something that was hung up on the valve seat and could be flushed out by a high flow rate (it couldn’t). 

The other thing we did was measure the leakage out the over-flow to get a feel for the magnitude of the problem.  We used a very sophisticated measuring device for this;  an empty 10 ounce coffee cup and the sweep second hand on my watch.

What we discovered was that the tower was leaking about 1 to 2 fluid ounces of water a second;  not very much at first glance.  In fact, it sort of took an annoyingly long time to
fill the cup (we humans can be very impatient).  So, not a big deal, right?

Actually it is,  as illustrated by the table below.

Make-up valve leak v1

1 to 2 ounces per second can really add up over the course of a year.  And these projections don’t even look at the cost of the extra water treatment chemical that is used to accommodate the extra make-up water.

You’re probably wondering how likely it is that something like this would go unnoticed for a year.  More likely than you might think actually. 

Since the system this tower served had not been in use for a while, nobody had even been out to the tower yard for a couple of months.  And,, if the tower had been in a blow-down cycle or it had been raining, the trickle of water out the overflow would have been masked by the bigger events. 

And, if at the time we had walked by, the tower fan had been operating we might not have heard the trickling sound and looked down to see what was making it.

So, there are obviously a bunch of lessons here. 

  • Making rounds past a remote equipment location that is
    not active can still provide some valuable preventive maintenance
    information.
  • High level alarms on tower basins can probably pay for
    themselves.
  • Two position valves, either via a snap acting mechanism
    or via electric actuation probably have some advantages in terms of
    minimizing valve seat wear and preventing a problem like this from
    occurring in the first place.

But the big lesson probably is that little things can add up over time and become a big number, be it water, energy, bits of free consultation, what-ever.  Being aware of these things can often lead to big, easily captured savings.


David Sellers
Senior Engineer – Facility Dynamics Engineering
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