It has been quiet on he support desk lately so I have been working on an
automatic flush system for the watermaker. I have breadboarded a simple
little circuit made from a 555 timer, a few transistors and some relays.

When the system shuts down or the flush button is pressed it turns on
the pressure relief solenoid and the fresh water supply solenoid just
downstream of the pre filters. The high pressure pump turns on and the
boost pump turns off. It pumps through the membranes for a set period
of time then shuts down the HP pump and closes the pressure relief
solenoid. The fresh water then back flushes the pre filters for about
15 seconds and the second solenoid closes. (For the negative thinkers
out there, there ARE manual valves just in case and the fresh water goes
through a charcoal filter to remove any chlorine.)

The question is, how long should the flush stage be set for? The high
pressure piping and pressure vessels hold a little less than 2 gallons
and the HP pump is operating at about 3.7 GPM. To displace the sea
water in the system would take about 40 seconds but is that enough?
If it stays on to long I waste fresh water but if to short I don't get
complete flushing.

The time will be adjustable +/- 20% by a trimmer pot but I need to
figure out where the center is.

BTW, The watermaker companies want upwards of $1,000 for this option.
The high pressure stainless solenoid does cost $150+ (I found mine on
eBay for $25) but everything else is under $15.00 total.

BTBTW, the total cost for this engine driven 800 GPD fully automatic
watermaker with stainless Cat pump has ended up right at $2,675.00.
(Gloat Gloat) :-)

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

Glenn,

you are so far outside the mainstream design experience, that I don't
think you are going to get all that many informed responses.
So here's an uninformed response.

I am abysmally innocent of the cost/benefit equation in flushing a
water maker - so I fall back on those rules that don't even merit the
term, "rule of thumb", namely:

1) The punters won't object to a 10% price rise too much.
2) 10% of the effort fixes 90% of the problem
3) If you don't like it, reduce it by 90%
4) If people are dying, reduce it by 99%

Looking over this short list, I see you have a rule #3) application.
Sample the salinity during the backflush.
When it's down to 10% of its sea value, note the time.
Add 10% to that.
You're done!

Brian (Zen school of Design) Whatcott Altus OK

p.s.If you are math compulsive, try this approach:

a 3.7 g/min pump feeding a 2 gal container,
given PERFECT mixing, obeys the exponential mixing
rate equation following:

fresh proportion = 1 - exp( -t / 0.54 ) for t is time in minutes
84% in 1 min
97% in 2 min
99.6% in 3 min etc.

for 90% fresh, we use
..90 = 1 - exp( -t / 0.54 )
0.1 = exp( -t / 0.54 )
ln (0.1) = -t / 0.54
so t = - 0.54 ln( 0.1 ) = 1.24 min
add 10% for luck: result - 82 seconds.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
p.p.s.
Hope you are designing for
1) a spike on each axis from a piezoelectric gas lighter.
2) a complete immersion in sea water.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


On Sat, 23 Aug 2003 10:29:30 -0400, Glenn Ashmore
wrote:

It has been quiet on he support desk lately so I have been working on an
automatic flush system for the watermaker. I have breadboarded a simple
little circuit made from a 555 timer, a few transistors and some relays.

When the system shuts down or the flush button is pressed it turns on
the pressure relief solenoid and the fresh water supply solenoid just
downstream of the pre filters. The high pressure pump turns on and the
boost pump turns off. It pumps through the membranes for a set period
of time then shuts down the HP pump and closes the pressure relief
solenoid. The fresh water then back flushes the pre filters for about
15 seconds and the second solenoid closes. (For the negative thinkers
out there, there ARE manual valves just in case and the fresh water goes
through a charcoal filter to remove any chlorine.)

The question is, how long should the flush stage be set for? The high
pressure piping and pressure vessels hold a little less than 2 gallons
and the HP pump is operating at about 3.7 GPM. To displace the sea
water in the system would take about 40 seconds but is that enough?
If it stays on to long I waste fresh water but if to short I don't get
complete flushing.

The time will be adjustable +/- 20% by a trimmer pot but I need to
figure out where the center is.

BTW, The watermaker companies want upwards of $1,000 for this option.
The high pressure stainless solenoid does cost $150+ (I found mine on
eBay for $25) but everything else is under $15.00 total.

BTBTW, the total cost for this engine driven 800 GPD fully automatic
watermaker with stainless Cat pump has ended up right at $2,675.00.
(Gloat Gloat) :-)

Subject: Watermakers -How long to flush?
From: Brian Whatcott

Glenn,

you are so far outside the mainstream design experience, that I don't
think you are going to get all that many informed responses.
So here's an uninformed response.


Lots of look at me ain't I clever stuff snipped

How about this, just call/Email the manufacturer and ask them how long their
fresh water flush runs.

Capt. Bill