Originally Posted by original kuhli
In the data sheets there's a kv or cv value, which I'm presuming is some coeffecient of flow, can somebody shed some light on the value?
Correct. "cv" = Coefficent of Volume. And it is indeed a measure of the freedom of flow through the valve. The higher the number the better.
Specifically, the cv number is the amount of liquid, measured in gallons per minute, that will flow through the valve when it has a pressure differential of 1 psi across it.
Along the lines of the example you gave, with an inlet pressure of 6.5 psi, and the outlet of the valve going to free air (so no pressure is needed to push fluid through after
the valve), the pressure differential across the valve is, of course, 6.5 psi. Use a valve with cv=1 and you'll get a flow of 6.5 gpm. Use a valve with cv=2 and you get 13 gpm, etc.
I guess I'll go ahead and post a little bit of info on solenoid valves, that I was going to post in a separate message here:
Solenoid valves come in different types, with different characteristics. Major types include:
direct-acting solenoid valves
The most intuitively obvious type. The solenoid rod acts as the valve stopper; i.e., when the magnetic coil pulls the rod out of the way, the fluid is able to flow through the resulting opening inside the value body.
Advantages: They require no differential pressure in operate; e.g., they'll work in even graivity feed situations where little/no head pressure is available. They work well in low-flow/low-pressure application. Operating time is very fast (measured in milliseconds); e.g., the low rider guys use these in their hydraulic systems that make their cars bounch up and down. Many can take a bit of back pressure in the closed state, although most do have a preferred direction, and may leak a bit when exposed to significant back pressure.
Disadvantages: Lower cv values than other valves. Prices climb fast with higher values; e.g., the cheapest McMaster-Carr has is $28, with a cv=0.2, and >$200 fir cv>2. In other words, these type of valves have the lowest flow per dollar of any type of valve.
The typical sprinkler valve falls in this group. The solenoid rod is used to control the flow of fluid into a chambe that acts on a (much larger) diaphram or piston, which is what actually stops or allows the flow of the fluid. Operation of the diaphram or piston is determined by the differential in pressure between the fluid being controlled, and the pressure in the chamber colntrolled by the solenoid. In effect, the diaphram/piston acts as an "amplifier" of the solenoid for controlling the flow.
Advantages: Good flow rates at low cost. For example, $10 commodity lawn sprinler valves can have cv values in the range of 3-4. Reasonably quick operating times (~1 second).
Disadvantages: Most do not operate at low pressures; e.g., lawn sprinkler valves are typically spec'd to require at least 15 psi to operate. This is due to the fact that the fluid pressure itself is used to move the element (diaphram or piston) that controls the flow. Also, inherent in their design is that they cannot take any backpressure
in the closed state. Any backpressure merely pushes the diaphram out of the way, amd the fluid flows past.
Hydronic ball zone valves
I just stumbled across this type of valve. I haven't seen any posts about this being put to aquarium use, but I think it may be ideal for many aquarium applications.
Unlike the previous two types, these are not solenoid valve at all. Ever thought that a simple way to make an electrically-controlled valve would be to simply take a ball valve, and mount a small electric motor on it? That's exactly what these are. They're commonly used for zone control in HVAC heating systems that use water, such a underfloor radiant heating system.
Advantages: High flow at relatively low cost. Due to their use in HVAC systems, they a high-volume mass-market devices (as usual, eBay is the best source). CV=10 for 3/4" valves are not uncommon. Operates at low pressure, and some will take backpressure.
Disadvantages: Slow operating time; ten seconds is not uncommon. Not quite as "fail-safe" as other types (may fail in the open position).