when you guys do pressure pipe model, do you predefine the pipe minor loss coefficient?

I always use 1.5, no matter what pipe size, length. Today, when I read a report prepared by another engineering firm, they put 0.

what is you guys' suggestion?

If you are applying a single number, then it sounds like what you using is what is usually called a "complexity factor".  It is simply a fudge factor (safety factor) that is used to multiply the length of straight pipe by to make allowance for elbows, valves(Balance Valves) etc.  It will vary very much from installation to installation and it is hard to give a universally acceptable number.

I prefer to use the approach suggested by RWF7437 and include the losses for the actual fittings unless it is a very preliminary calculation and the details of the fittings are unknown - in which case I would agree with your 1.5 factor.  And then when you have the details of the fittings you can revisit the calc and confirm the pressure drop.

The question is.....how close is close enough?  I was not satisfied with the tabulated values I have seen published so I conducted my own study with surprising results.  For instance, when I measured the minor loss coefficient through a 90 degree elbow discharging freely, the number agreed exactly with published data.  However, when I added some pipe to the discharge side of the elbow the minor loss coefficient jumped dramatically.  Furthermore, minor loss coefficients are a function of pipe diameter as well.  In a system dominated by minor losses, it makes sense to know rather than guess...or even worse to hope that the person providing the data knows what they are talking about.

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