Calculating Velocities

Chris,

You are going to need to be a bit more specific. Are you wanting to:

-Display somewhere what velocity was used to size your valve/mainline? 

-Change what velocity is used when sizing your valve/mainline?

-Something else?

Any additional light you would like to shed on this request might help us better understand what you are after.

Hey Jake,

We're trying audit an existing irrigation system - To figure out if there's a change in velocity through a fixed mainline size, say 3".  The site also doesnt have a flow sensor to take readings from either.  

The property is experiencing lots of mainline breakage, and in some cases imploding.  Pressure may or may not be an issue, so were recommending ARV's in certain areas to hopefully help with water hammering, possibly even considering adding purge valves and PRV's to relieve some pressure and vacuum.  There's about 60-80' drops in elevation with POC at the very top.  Static pressure mainline pressure at the top is about 55, and climbs as we head lower towards the 60-80 drops...we measured 90 psi at the bottom part of the elevation.  Our concern is if the flow is exceeding 5FPS through the drop in elevation while the system is operating.

Chris,

This is definitely an interesting concept... but I am not sure you will ever get to know the exact speeds going through portions of pipe simply from a calculation. I would assume you could start working this problem backwards by installing a flow sensor of sorts, to read what is coming from the pipe, both at the beginning and end of the pipe run.

I did stumble upon this little bit of info on Magnetic flow meters that was sort of interesting:

Magnetic Flow Meter

Magnetic flow meters work by applying a magnetic field to the flow passing through the pipe. This induces a small electronic potential difference (thanks to Faraday’s law and electromagnetic induction) that can be measured by electrode sensors. The magnitude of the electronic potential difference is proportional to the velocity of the water, and flow can then be computed using the continuity equation. 

One advantage of magnetic meters is that the metering section is the same diameter as the adjacent pipe, so a magnetic meter causes no additional head loss. Most often magnetic meters are used in full (pressure) pipe applications, but open-channel magnetic meters are now available as well. 

Could be something worth exploring, but as far as having the design program figure this out, I am at a loss of how it would do this.

At the end of the day, you might just need to draw a few versions of this system and play around with the velocity sliders to see what the pipe sizes start showing you regarding the pressure losses at different velocities.

I would love input from others to see if there is indeed something that could be done!

This how I would troubleshoot. Always start at the controller before you get a shovel. 

• Only run 1 valve at time. Check for controller running concurrent programs, multiple controllers, battery operated controllers, and ganged wired valves. 
• Ganged valves can be identified at the controller using an ohm meter. 
• If you have a water meter at the POC. Run each station and record the GPM. Install a meter if your on a water well. 
• Verify the size of the main.
• Use the flow charts at the back of a Hunter/RainBird/Toro catalog.   Locate pipe size/class and gpm to find your velocity. Record this for all valves and look for a pattern based on location. 
• If you notice a pattern, install a pressure regulator or a master valve with flow control in that area. 
• High flow rate at valve. Reduce the flow with flow control at the valve, renozzel heads, or use pressure regulated heads
• If a lateral lines draining out do to gravity use check valves.  
• Manual ball valves or quick coupling valves  can shatter pipe if not closed properly. 

This could be as simple as a repair tech wired 2 valves together because a station wire developed a fault. Check your ohms, amps, and gpm's. 

Jake,

yes...this system is peculiar.  The clients were recommended by a contractor to install a normally closed master valve with a battery timer, this only saves one thing...their water bill, which was their primary concern.  Now they're experiencing the breakages, and I believe that MV is the culprit.  The usual suspects - MV closes, mainline empties from it's leaks, charges back up - rattling their pipes, and causing more progressive breaks.  We've also narrowed some other things. We've found out that the installers havent installed  any thrust blocks on the fittings, and guess where their breakages occur mostly.  Also, the mainlines depths are quite deep...anywhere between 3' to 5' deep, with the bedding eroded away,  and backfill with rocks as big as a basketball resting on the mainline; their site office kept one of them as a souvenir to show us during our site visit.

They're treating this as a case by case item, just repair things as they go.  Ultimately we want to go back to a Normally Open valve, or none at all and keep the line charged, with new irrigation mainlines with Ductile Iron fittings, and possibly go with Class 200 pipe instead of Sched - whether do decide on that or not, that's up to them.  But we've made recommendations to hopefully reduce the rattling by suggesting installing ARVs and Purge valves, and to install Ductile Iron fittings where repairs are needed.

Sean,

That's a good approach.  I'll keep this in mind when we make our next visit.  I'll need to confirm with the site manager if their meter shows current flows, some of the meters here...especially older ones, I think it just show total flows.  If not, they'll need to install a flow sensor kit.  Then they'll need to watch the meter and zones on their end and report their findings.

THANKS!!!

I'd still like to find out if there's an equation or a variable to find out velocities.  keep chiming in everyone.