Sunday, May 31, 2009

Fountain test

Belle, non?  (1/80 sec exposure)

Non. (1/4000 sec exposure)

Here it is with Martha for a sense of scale:

First, what went right?
  • The geometry is right.  In this spreadsheet, I calculated the height of the fountain (69.4 inches, was actually 65.5 inches), and how far it would throw the water, and where the jets would come down into the hot tub.  Although some of the jets land about 5 inches off where I expected, and two jets collide in midair just before they hit the water, the geometry is about as good as can be expected, and fulfills my goals, which were:
    • It should be possible to walk between the rising jets without being hit by them.
    • It should be possible to sit in the hot tub without being hit by the jets.
    • It should be possible for a child to stand in the middle of the hot tub and have the jets come down all around, without actually hitting the child.
  • All the jets rise to the same altitude, within about half an inch or less, which means the balanced binary tree distribution system with the shorted end terminals worked.
  • The pump-side pressure stack does not overflow.
  • The pumps don't cavitate.  They are incredibly quiet.  You cannot hear them unless you walk over to the pump vault and stand on top of it.  Once the lid is installed on the pump vault, I doubt you will hear the pumps even when you are on top of it.
However, the jets are not laminar.  Gloppy blobs of water fall into the water and make a dull roar instead of the quiet sizzle that I had wanted.  There is enough splashing from the jets entering the water that you wouldn't want that a few inches from your face.  The kids love it, of course, because it's loud, fast, and wet, but it's not so great for the adults.

I'm pretty bummed.  What happened?

The individual jets were flow tested in January, and this is what they looked like then:

As you can see, the jets were smooth, and landed smoothly and quietly, back in January.  Now Martha jokes that if we move the back yard table to the farthest corner of the yard, we can still have a nice conversation.

I'm not entirely sure why there is a difference.  Here are possibilities, ranked by my guess of most to least likely.
  • In January, the heads had no lateral ports in them.  In this latest trial, there are two ports in each head, connecting each to the heads on either side.  These ports keep the pressure even across all the jets, which makes them shoot to the same altitude.  But these ports may also be causing the water to tumble slightly as it passes the edges, and that turbulence may be causing the breakup that I'm seeing.
  • In January, the heads were surrounded by nothing, and so small amounts of water on top of the heads ran down the sides, away from the jets.  Now, the heads sit inside recesses in the gunite.  Each head has a small pool of water in it that terminates at the jet.  The water in this pool greatly disturbs the jet during startup, but it gets cleared in two or three seconds and then I don't think there is any more water recirculating through that pool and into the jet.
  • In the January trial, I had an open-topped pressure stack between the pump and the jets.  In the production version, I have a stack after the pump, but it's not quite the same.  In this one, the water from the pumps goes to a Tee.  In one direction, the water heads for the jets, and in the other direction, the water heads for the stack.  It's possible this alternate arrangement works less well.
  • This test has a closed air volume right before the jet, which was intended to be an additional flow smoothing device.  The January arrangement used open-topped pressure stacks either right before each flow straightener, or right after the pump, and both worked well.  The closed air volume is known not to work as well (since the pressure changes more with a small surge in water).  Also, since the current arrangement has two capacitors with an inductor between, it's possible that there is oscillating pressure being stored between the two capacitors.
  • The nozzle holes are 0.590 inches, rather than the 0.500 inch holes that I tested in January.  This makes the jet diameters about 0.500 inches, which is necessary for all 200 gallons/minute to flow.  As a result of the larger jet and the larger jet velocity, the flow through the flow straightener is perhaps twice as fast as it was in January.  It would be great if this were the problem, since I can reduce the flow later when I have that plumbing finished.
  • In January, the pump had air in the lint basket bowl, and the pump could be heard continually injesting air.  Now the pumps have no air in their lint basket bowls.  I would expect this to make things better now, but I thought I'd list it because it is a difference.
I also have two unexpected observations which may be a clue to a solution if I can figure it out:

The ports in the sides of the fountain heads are connected via riser pipes to a plenum that is fed from a pipe that will ordinarily lead to a blocked valve.  This valve is used when the fountain is off to backflush the flow straighteners.  However, that plumbing is not yet finished, and so the pipe currently leads to many other pipes that are currently filled with air.  There is also a hose bib and a pressure gauge connected to those pipes (this is how we did the pressure test).  I have calculated that the static pressure at the top of those fountain heads is about 3 psi above ambient, and so I expected the plenum to be pressurized at 3 psi.

But that's not what the gauge says.  The gauge shows zero pressure (I don't have any gauges that show negative pressures).  If I open the hose bib while the fountain is running, then cover the opening with my finger, I feel a little pull.  It's very feeble, but it's there.  WTF?

The flow in the head is moving at 1.6 inches/second, and I calculate a dynamic pressure of 0.85 Pascals, or 0.00012 psi.  That isn't diddly compared to 3 psi pushing out.

[Update: Mystery retired: it turns out that the pipe connected to the top manifold is capped off right now, and those other pipes are just not connected to the fountain.  I can't explain why I was thinking that there was a small pull of air, but it certainly wasn't measureable.]

The second unexpected thing happened the first time I started up two of the three fountain pumps.  All three pumps are in parallel.  I had difficulty taking the lid off the third pump's lint basket bowl, so I had left that bowl filled with air, and just started the other two (which were properly filled with water).  I expected the first two pumps to push water backwards through the third pump, flushing the air into the intakes of the first two, where it would be blown into the fountain or otherwise ejected from the system.

Nope.  There was no noticeable flow through that third pump.  Later, I pulled that lid off and removed the air.  When I ran just two pumps again, the third pump did have flow going backwards, and in fact the impeller was turning backwards at perhaps half the RPM of the two powered pumps.

It's not clear to me how the air can block a >3 psi pressure drop.  The total drop from the top of the pump to the bottom of any associated piping is perhaps two feet, which would account for a 1 psi drop block, but not 3.

I suspect that the solution to these mysteries, especially the first, will tell me something about the fountain behavior.

Friday, May 29, 2009

Relative safety of stairs and swimming pools

[Post updated: A friend called BS on my previous estimate of the number of houses falling into the CPSC's "Stairs, Ramps, Landings, Floors" category, so I've fixed that. The change affects the magnitude but not the polarity of the bottom line.]

As we're finishing up our swimming pool, my wife and mother-in-law and I were naturally led in a recent dinner conversation to consider whether the pool is dangerous. This is, of course, an ill-posed question.

So I changed the discussion to which of the stairs or pool was more dangerous. The stairs typify a common threat which which everyone is familiar. The pool typifies a threat for which there is plenty of hype.

I was able to remember the gist but not the exact numbers in my previous blog post on this subject. Now see, there's the value of my blog (I knew it was going to pay off someday!) -- I have a nicely written set of notes available online to which to refer. Sadly, I had not completely anticipated my mother-in-law's argument, so here's an update:

There are 8.6 million swimming pools in the United States, and 116 million homes. If we make an approximation that all those swimming pools are residential, 5322 deaths/year for 8.6 million residential pools is 62 deaths/year per 100,000 houses with swimming pools.

I'll assume that essentially all houses have stairs, ramps, landings, or [more than one] floor. That means the 202,104 deaths/year for 116 million homes equates to 174 deaths/year per 100,000 houses.

Since we have both a pool and staircases, my best estimate is that our stairs are 2.8 times more likely to kill someone than our pool.

The difference may be substantially larger. Our pool will have modern safety features like an automatic safety cover, parallel separated drains, and a properly engineered diving envelope for the diving board, along with a raised-periphery design that makes snapping your neck on the bottom at least very awkward.

Our stairs, on the other hand, are very much like stairs everywhere, and thus should be about as risky. I suspect that a disproportionate number of "Stairs, Ramps, Landings, Floors" fatalities are concentrated in the portion of houses with actual staircases, and so my estimate above understates our staircase risk. The two most used of our three staircases have turns part-way down, which I think makes them marginally safer since you are less likely to fall all the way down the flight, but I doubt that affects the polarity of my argument.

Bottom line: no, I'm not worried about the safety of my kids around the pool, but I have gotten noticeably more nervous about the stairs since running these numbers.

Friday, May 22, 2009

Laminar Fun Group

If anyone reading this is looking for help building a laminar flow fountain, let me know.  I now have a business: Laminar Fun Group.  Reach me at

Thursday, May 21, 2009

Too Big Has Failed

I've been wrestling with writing a blog post on how I think we should fix the financial mess.  Happily, it turns out that Thomas Hoenig has written it for me.  Whew!

Mr. Hoenig points out a basic problem we have now:
If an institution's management has failed the test of the marketplace, these managers should be replaced.  They should not be given public funds and then micro-managed, as we are now doing under TARP, with a set of political strings attached.
He reviews past financial crises and the mechanisms used to successfully deal with them:
financial crises continue to occur for the same reasons as always -- over-optimism, excessive debt and leverage ratios, and misguided incentives and perspectives -- and our solutions must continue to address these basic problems.
Then he points out flaws in the existing TARP mechanisms, that can be fixed by using the procedures that were used before.  That is, establish a simple metric for declaring a financial institution insolvent, fire the management of insolvent institutions, bring in new management, allocate losses to shareholders first, and then to unsecured lienholders, and take out all or a portion of the bad assets for seperate disposal.  This isn't new thinking; Mr. Hoenig is just saying we should do it now even though the failing institutions include the largest US banks (you know who you are, Citibank!)

Finally, he looks forward to avoiding our current problems in the future:
One other point in resolving "too big to fail" institutions is that public authorities should take care not to worsten our exposure to such institutions going forward.  In fact, for failed institutions that have proved too big or too complex to manage well, steps must be taken to break up their operations and sell them off in more manageable pieces.  We must also look for other ways to limit the creation and growth of firms that might be considered "too big to fail".
The underlying problem is that when a single entity or network grows to become vital to taxpayer interests, that entity achieves a claim on taxpayer resources.  Firms should have to pay for such a claim.  Many will find it cheaper to break themselves up.  Identification of networks vital to taxpayer interests is an extension of existing antitrust laws.

Mr. Hoenig is the president of the Federal Reserve Bank of Kansas City.

Financial Times opinion piece:

Fed White Paper

Let me add one thing:

One problem any kind of government takeover and cleanup of a failing bank incurs is that the new entity is unnaturally "clean" compared to the non-failing banks with which it competes.  Folks like Warren Buffet complain that they are penalized for having played well.

First, the complaint isn't entirely true.  As long as the shareholders of the failed institutions get wiped out, the shareholders of non-failing institutions do better in comparison, and so to the extent that shareholders guide bank operations in the future, they will tend to guide away from failure.

The complaint is true, however, for the individuals at the failing banks.  Folks working at Citibank and AIG have made more money than folks working at less spectacular non-failing banks, and they've kept their undeserved gains.  I think we need a better system for aligning the interests of bank shareholders and those who work at banks.  It's not enough to give the folks working at the banks options or shares, because shareholders do not have enough power right now.

Saturday, May 02, 2009

Gunite is in

Kathleen is spraying water on the gunite, Anya is directing, while Ava looks on. We're supposed to keep the gunite wet for the next two weeks.

I stayed home for the day to watch the crew shoot the gunite. We used Aqua Gunite (here is their web site), on the recommendation of our consulting engineer Charlie Adams. I found a listing for them here. It's listed as a two-person company, which I suppose would be Jose Aguayo and Sergio Garcia. For a two-person company this place has a lot of assets: at one point I saw three and my neighbor reports five trucks lined up to deliver the sand/cement mix. Those trucks had Aqua Gunite logos and Charlie tells me they cost $260k each. They also had what my friend Wes Grass reports to be the largest air compressor he's even seen (it was all of a large truck). Maybe the company is owned by those two guys.

They got here at 7:30AM and had the gunite going by maybe 8:00AM. That gun was shooting almost continuously until something like 5:45PM, and it took them another 30 minutes after that to finish up. We used almost five truckloads of gunite (our pool is 46 feet by 18 feet, and has a big cover vault at one end). Sergio, the foreman, told me that was 78 to 80 cubic yards of gunite, but I can't see how that's possible:
  • The shell surface area is 2015 square feet that average around 8.5 inches thick (53 yards^3).
  • We have about 38 feet of internal dam walls that are about a foot thick and average 4 feet tall (5.6 yards^3).
  • There is maybe 3 cubic yards of gunite in the steps and two pedestals.
  • We have a gusset which holds up the diving board that is 2 feet by 2 feet by 6 feet, so that's another yard.
  • 10% rebound would be another 6 yards, which is consistent with what I observed getting dumped and hauled away.
  • Total: 68 cubic yards.
Those trucks were claimed to hold 15 cubic yards, but they just did not look big enough. Maybe that's the volume of the containers, which they perhaps don't usually fill completely. For comparison, a 10-wheeler holds 10 cubic yards.

The cement and sand is mixed in the truck right before delivery, and the water is only added in the nozzle at the end. As a result, they don't have the usual concrete problem of having to order exactly the right amount of mix. Instead, they have the problem of disposing of "rebound", which is the portion of the stream that does not stick when it hits the wall. Sergio says they usually have 7 to 10% rebound. Aqua Gunite carefully arranges not to have the capability to offhaul the rebound -- they want to dispose of it somewhere on site. We had a nice big hole in which to dump 2 or 3 cubic yards, but after that we piled it up on what used to be our lawn and had some other folks cart it off for recycling the next day. In retrospect it probably would have been a good idea to negotiate this ahead of time with Jose.

It doesn't much matter, we had a fixed-price contract: $13145, $733 of which was for using thicker masonite so that we wouldn't have to strip the forms to make the form edge straight. This last bit is an artifact of our having a bond beam raised 15 inches out of the ground -- the sides have to be straight so that the masons can lay the siding stone properly.

One of the first things Sergio decided when he got here is that we didn't have enough rebar in the cover vault dam wall. The wall is 12 inches thick, and had just a single curtain of #3 rebar on 12 inch centers on the water side, plus four #4 rebar at the top. Sergio added another curtain of #3 rebar on 12 inch centers on the vault side. One nice side effect is that this will make the vault floor even more resistant to cracking from the applied torque should the gravel under the pool settle and leave the pool hanging on the soil under the cover vault.

Here's the top of the gusset that holds up the diving board. You can see the four two-foot bolts that actually go up to the diving board base. In retrospect, I should have had the gusset rebar tied into the vault wall rebar better, as that would help transmit loads between the two.

So now we wait four weeks for the gunite to harden, and shrink, and maybe crack, while we race to get the plumbing, electrical, and solar installations finished, and get the trenches closed up and filled in preparation for the new landscaping. In the meantime, the maintenance crew is keeping the shell wet.