The SWC does have a drain on the collection cup, at least the 160 does although it is straight not an elbow like the BM. You can see the drain in my photo below on the left side of the photo bottom of cup. I think I can see in the photo you attached where the drain is in the back of the skimmer you can see it through the tube below the cup. I have attached a photo to illustrate. Also, it is not really a gate valve, it is a knob that turns a piece of pipe inside of the outer pipe that controls the water flow rate. The inner pipe has roughly a V at the bottom to go from almost total blockoff of water flow to wide open with no restriction. Seems to work very well.
I have 2 of the SWC 160's in my 125 and 150 sumps. My sumps are 40B tanks with 1/4" thick glass bubble traps I have
glued in with the skimmer sitting in 9.5" of constant level water.
The 160 calls to be run "best" in 7 to 10 inch deep water and I have found by experience that they produce best (Photos below) in deeper water. The problem is that on a power failure they will not restart when you run then even 9.5 deep which I did at first.(at least neither of mine will) Now I have them sitting on 4" high risers to where they are basically in about 6" of water and I am finding them a lot more tricky to fine tune but with effort, I am getting there again AND they restart no problem every time. I say all of this to explain that depending on your sump depth you may need to put the SWC on a stand in your sump. I understand from talking to others with the 120 they wish they had gotten the 160 and that the 120 operates very similar and water depth is also a factor with them restarting in deep water and that the 160 produces dark dry skimmate more rapidly.
Here is a photo of the first one I got when I was still running it in a ALS 1500 Wet Dry which is also now for sale. Notice the DARK and DRY Skimmate. These 160's produce some KILLER AWEFUL smelling skimmate but like any skimmer I have ever had, you have to learn them. I plan to most likely buy another one for my 93G Cube with custom 26x18x16 Sump I am building.
SWC Pros:
Very good production of DARK and THICK skimmate.
Easy to clean.
Fairly easy to adjust.
Affordable.
Good customer service from where I got mine, reef filtration, I had a leak in my collection cup and it was immediately replaced no questions asked. (I hope I can say this, if not I will remove the vendor name)
Pretty high quality construction and easy to take apart to clean/maintain even for a computer geek like me. Good fit and finish of all parts and easy reassembly after cleaning.
Low about 20" tall profile to fit under minimal clearance stands, like my 125 stand.
Cons:
Tricky to get to restart in deeper than about 7 or 8 inches of water after a power cut.
They need a 90 fitting coming out of the collection cup like the BM instead of one that comes out straight down.
Cord could be about a foot or so longer would be nice.
Overall, I am happy with the performance to price ratio.
