<a href=showthread.php?s=&postid=8803161#post8803161 target=_blank>Originally posted</a> by alizarin
Could you give an example of how you would represent those stats because I'm not following you.
Sure, let me try to explain.
First the SCWD in the line behave as any other restriction to dynamic flow be it an elbow, a reduction in the pipe diameter, a union fitting or the friction of the water upon the pipe walls, etc.
The higher the flow the higher the pressure drop across the fitting will be.
As an example, when passing 600 gal per hour in a 1 " pipe, a "T" fitting will create a pressure drop of only about 0.04 psi as the flow increases the pressure drop also increases. For the same elbow but now passing 850 gal per hour the pressure drop near quadruples to about 0.15 psi. This increase pressure drop (resistance to flow) creates a higher flow loss the higher the flow is.
Similarly with a SCWD the higher the flow, the higher the flow loss would be.
Now lets take a look at that table again to see if that holds water:
According to that table if you increase the input of flow to the SCWD by 90 gph from 180 to 270 gph the output increase by the same 90 gph (from 120 to 210 gph). Because the SCWD is a restriction this behaviour of not loosing flow with an increase in flow is an engineering impossibility.
Even more, according to the table if you increase the input flow by 70 gph from 330 to 400 gph the output flow will increase by 120 gph (from 240 to 360 gph). All in a sudden according to the table the SCWD is acting as a pump increasing the flow!
If I were to know instead, what is the pressure drop at different flows, that will allow me to really identify for my particular operating conditions if a SCWD will be the right choice.
Of course marketing wise it is not smart to talk about "pressure drop" but instead they talk about efficiency of the device. This is equivalent to say that a T fitting is X or Y% efficient. You can mention how much efficiency it will take away from the system as a cost penalty for creating the switching of the flow but again that is not good marketing.
Why do I want to know the pressure drop at different flows? Well adding a given pressure drop to a system will have a lot different effects depending on the operating parameters of that system.
Take a look at the performance curve for the Mag Drive 9.5 below.
Now lets assume we add 1 psi of pressure drop in that system (approx. 2 ft) at 800 gph then I would know that if my system is operating at an existing 4 ft (800 gph) and add 2 more ft of head pressure to 6 ft my flow will drop by 100 gph from 800 gph to 700 gph so I will be reduced by 12% an may decide to use the SCWD.
If they tell me that the pressure drop will be 2 ft at 400 gph and my system is operating at the 10 ft point (400 gph), if I add two more feet my flow will drop by 200 gph to half of what I had so I may not decide to use it or switch to a different pump to compensate for the difference.
So if they tell me or give me a table or a chart that display the pressure drop across the device for different flow I will be able to decide how the device fits in my system and be able to make useful decisions, otherwise it is just marketing fuzz.
In summary, it is futile for the manufacturer to try to tell me how much flow I will loose as it depends on the performance characteristics of my system and my pump, something they will never be able to even guess but measuring the pressure drop across the device at different flows will be very simple and straight forward. Why don't they do it? Beats me.
