How to set Diameter units in MSX to meters

G'day Team,

Can someone please tell me how to change the MSX units for the pre-defined term 'D' from feet to meters?

The MSX Manual states that Diameter can either be in Feet or Meters, however no-where does it say how to change it or what the unit selection is based on.

I have the Area units set to M2, which I thought would have set the MSX to use meters for D, however it's still using feet.

Context:

I'm back after 4 years trying to solve the problem of modelling wall decay in MSX. It had fallen off my priority list before I was able to get it to work last time but here I am again.

I intend to model two wall decay theories, EXPBIO (referenced in this previous query) ( RE: How to include Mass Transfer Coefficient into MSX )  and first order. The first order method still needs to be done in MSX to incorporate bulk decay modelling.

Yashodhan provided a great answer to the EXPBIO query outlining how the STEP function can be used to select between different equations here: Adding custom parameters and equations with Multi Species Extension (MSX) . However please note that he states that the equations used in that example are indicative only, they don't combine wall decay with bulk decay correctly.

P.S.

If anyone can also explain how the 'Wall Species' in WaterCAD MSX works it would be greatly appreciated. I ran a test model where I simply set the wall species RATE to 1 (e.g. RATE W 1) and the results make no sense to me whatsoever. 

Thank you,

Ryan Catling

Parents
  • If anyone else comes across this problem a workaround you can use to substitute for D is to replace it with 1/(250*AV). This will work when you have Area units set to M2 and your equation requires diameter in meters.
    i.e. D (m) = 1/(250*Av)    when Av is in m^2/L 

    Answer Verified By: Ryan Catling 

  • Hello Ryan,

    Based on what the developers are seeing in SewerGEMS, the diameter units will need to be in feet. In MSX, an INP file is used and the units (feet or meters) is based on the the type of units being used in the INP file itself. However, the MSX module within WaterGEMS does not use the INP file. All calculations within WaterGEMS is done in US units. If a model uses SI units, internally they are converted to US units when running the model. For this reason, pipe diameter units will be expressed in Feet. If you workaround works, that should work as well.

    For the wall species results, this appears to be working as expected. The developers tested with a wall specifies of 1, which would mean that the there is a constant linear growth. Note that wall species only applies to pipes, and not tanks. Though the wall species is defined in the [TANKS] section, the wall species is not used. This is mentioned in the MSX manual.

    In the model, you are using the following global initial conditions (i.e., 0 for all pipes).

    [QUALITY]
    GLOBAL W 0

    With the other input, the pipe values start at 0, then grow linearly (rate = 1). Tank values start at 0 and stay at 0 because wall species don’t apply to tanks. I will be updating the MSX wiki article with this information.

    Regards,

    Scott

  • Hi Scott, 

    Thanks for getting back to me, I'll stick with the workaround for 'D'.

    For the Wall Species, what version of WaterCAD was this tested with?
    My version must be bugged (10.04.00.108 64bit) because for the majority of walls in the model the Wall Species does not grow linearly. I have screenshotted just a few below for example.

  • Hello Ryan,

    We have looked into this. It appears the that the W results are sensitive to the value used for FTOL in the MSX controls. WaterGEMS includes a feature where the the engine will skip calculations for pipes with small lengths and/or small flows. The FTOL (flow tolerance) setting in your model is currently set to 0.5. If you adjust this value to 0.001, the W results will have a linear increase with time. We looked at pipe RS317732 to see this. This pipe has a relatively small flow, which may be why this is occurring. I also looked at the other pipes from your screenshot and the W results for those are all linearly increasing as well.

    Please see if this adjustment helps with your results. The developers also noted that setting the tanks setting to:

    [TANKS]
    ;Bulk Species
    RATE WA 1

    This is instead of using RATE W 1. This is to avoid any confusion on the tank results, since wall species does not apply to tanks.

    Regards,
    Scott

  • Thanks Scott,

    That fixed the wall species results into something that makes sense!
    (i.e. linear in this example).

    I'm just wondering in what circumstances someone may want to set a higher FTOL, I assume the primary benefit would be to reduce  run times?

  • Hello Ryan,

    Smaller FTOL values should provide more accurate results, which is the primary reason to use a smaller value. If set too low, numerical error can be introduced from low flow pipes. The exact value to use may be different for each system and the user may need to experiment to get good results.

    I have updated the MSX wiki with this informaition.

    Regards,

    Scott

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