How To Analyse Tapes Under Flushing

Tutorials and 'How To' information.
Post Reply
User avatar
jovivier
Site Admin
Posts: 739
Joined: Thu Mar 12, 2009 2:57 pm
Company: LVL
City / Town: Blenheim
Location: Lincoln, New Zealand
Contact:

How To Analyse Tapes Under Flushing

Post by jovivier »

See "How To Simulate Tapes Using Spraylines" first.

Because IRRICAD cannot have two submains on a block of tapes, the following method must be used to analyse tapes under flushing conditions. The flushing valves used need to be entered as outlets in the database. The radius equation is unimportant so use a K of 1.0 and an n of 0.0. Values for the pressure / flow equation (Constant and Index) should be derived using the tabulated headloss vs. flow values for the valve in the Outlet Flow curve fitting utility. Tape needs to be mirrored as a sprayline as tapes cannot have a submain on both ends.

SUMMARY
  • Simulate the tape as a pipe and outlet
  • Use Change Type to change all tapes to spraylines using the pipe and outlet
  • Add the flushing main
  • Analyze
SETTING UP DATABASES
Entries to mirror the tape need to be made in the pipe and outlet databases.
In the pipe database add a pipe with the internal diameter as the tape entry (do not forget to give it a unique warehouse code). The roughness for the pipe can be determined from trials to achieve the same headloss through the pipe that the tape gives. To do this, in a clean design, run out a tape a set length (similar to the run length in the design), e.g. 300ft (100m). Then run out your new pipe the same length. Connect with a large submain pipe and a control valve. Turn off "Minor Losses" in Design|Zone Design Configuration, analyze the zone, and check the Zone Full report. Change the roughness factor for the new pipe until it achieves the same headloss through the pipe that the tape is giving.
In the outlet database make an entry for the tape outlet. The constant and index (for the Iteration calculation method) for this can be derived in a number of ways:
  • Directly from tape manufacturers data
  • From manufacturers data using Outlet Flow curve fitting utility
  • Assuming the index is 0.5 and calculating the constant by substitution in the equation (this method is normally accurate enough) Flow (lph) = Const * Pressure (m) ** (0.5) (for PC emitter the index is 0 and the constant is equal to the flow rate)
  • Contact AEI Software
CREATING FLUSHING DESIGN
Draw a block of tapes.
It is generally better to duplicate your tape design and make the changes required to carry out the flushing analysis on a separate copy. To do this, use the File|Save As… command and save it with a different file name.
In the copy of your design select the tape block and change the tapes to connected spraylines (use Tools|Select|Window then select Tools|Tapes to Spraylines). Select the tape pipe you entered above then the tape outlet in the dialog.
Sprinklers are always assumed to be at ground level. If in your design they are not at ground level, you must select an outlet connector with the appropriate height previously entered in the "Height" field (in the database) to ensure that IRRICAD uses that height in the pressure calculations at the sprinkler. Note the height entered into the database is the height above ground, not the length of the outlet connector from a buried submain to the height of the outlet. For subsurface outlets (e.g. subsurface dripline) the height above ground will be negative (-ve).
Add the flushing main to the tape block using Zone|Cut Pipe.

ANALYSIS
Set the valve pressure required for the zone in Design|Zone Design Configuration (you can get this from the Zone Summary report from the design of the real tape block) and turn off "Minor Losses" if it is not off already (this specifies what type of connection and therefore headloss to include).
Run Design|Zone Design|Detailed Analysis, this will analyze the tape block under normal conditions.
Now insert flushing valves where required (from the Outlet database) onto the flushing main.
You can vary the valve pressure as you wish and examine the results in reports.
Note: A word of warning - Do not use a flush valve that is too big, i.e. make sure you use one that has 1.4 - 2.1psi (2m - 3m) of headloss at the flushing flows you expect. If you use a flushing valve that has too low a headloss, you will have problems getting a solution.

If you do have problems with convergence, there is a detailed analysis factor in Design|Design Parameters|Analysis Parameters that you can increase (e.g. use 0.85 rather than 0.75). Remember to return the parameter to its original value after you have finished this design.
The required flushing velocity rule of thumb is 1ft/s (0.305m/s) down the tapes. Set the valve pressure to achieve the correct velocity.
Post Reply