Iteration error on flushing

[sditape]

We are trying to calculate flushing on a field with 990 DripNet PC 18mil .16emitter on 27" spacing. Run length is 1,949' and we are getting an iteration error DES 557, stating that our run length is too long and that we need to shorten and rerun calculations. The Flushing v13 report shows that it should flush at 14.4 psi and pressure and flows have been verified by hand to be correct in the flushing report. Why are we getting this error?

[jovivier]

Normally for a "iterations exceeded..." message increasing the "Max. iterations for Detailed Analysis" in Design|Design Parameters|Analysis Parameters is the first option to try. The default is 100, try increasing this to 200. This may provide enough iterations for IRRICAD to finish the calculations and complete the report. When the "iterations exceeded..." message appears any information in the V13 report is incomplete. The "Detailed analysis flow close fraction" (in the same dialog) can also be changed to a higher number (such as 0.005) but this is rarely required.

Without knowing the exact situation - number of laterals in the block, valve pressure setting, back pressure, minimum required flushing velocity - it is a little hard to know the exact cause of message DES_557 in this case. I have tested your tape at the required length and used the "Required Flushing Inlet Pressure" reported in the V12 report (approx. 14.4psi) to set the tape inlet pressure for the V13 report and this does work as expected.

[sditape]

The number of laterals in the block is 131. The pressure is 13 as is the D/S pressure. Back pressure is set to 3 and the minimum flushing velocity is 1. We tried making the changes, but it did not fix the problem. We are also getting an error of too low of flow on a control valve.

On the Flushing V13 report, the zone irrigation flow at the top of the report is now showing 34.34 gpm at 13 psi and a run type of detailed analysis, the bottom Manifold flushing total flow = 657.02 and the Manifold End flush flow = 353.3, these last two number appear to be correct by our hand calculations, but the top number is not correct.

Does the minimum PC pressure in the database need to be set to 6 to get accurate flows with a minimum back pressure of 3?

We did change the grid origin after the irrigation was designed, but before the flushing was processed because our numbers were very high? Could this be a problem? We did try to change it back, but it still doesn't seem to be working.

Would you like a copy of this design to see what we are working with?

[jovivier]

I have made a block of 131 tapes at the correct length with the valve on one end (no elevation in my test) and used your valve, flush velocity and back pressure parameters. In this case if I increase the number of iterations to 500 the DES_557 message no longer occurs. If IRRICAD cannot finish the iterative calculations the report is not complete. After successfully completing the flushing run without DES_557 appearing the total zone flow should be the same as the reported "Manifold Flushing Total Flow".

The minimum PC pressure is the pressure below which the emitter no longer compensates and IRRICAD will then use an index of 0.5 to calculate the actual flow. To achieve accurate flows under flushing conditions enter the correct value in the database.

The issues you have mentioned so far do not point to a large coordinates or User Origin problem.

[sditape]

We have changed the iteration number and still get the same message.

In regards to the Zone Irrigation Flow, we have done flushing on a couple of designs and that number has never matched the Manifold Flushing Total Flow, sometimes it appears to be off by rounding errors, but this time it is off by almost 400 gpm.

What should we try next? Thanks for your help.

[jovivier]

Please send your design to Support@IRRICAD.com.

My apologies, the total zone flow will only be the same as the reported "Manifold Flushing Total Flow" if there is only one manifold in the block.