What is the difference between a Throttle Control Valve (TCV) and a Flow Control Valve (FCV)? In what situation should I use each?
Flow Control Valves
These valves limit the flow rate through the valve to a specified value in a user chosen direction. A flow rate is used to control the operation of a flow control valve. These valves are commonly found in areas where a water district has contracted with another district or a private developer to limit the maximum demand to a valve that will not adversely affect the providers system.
Throttle Control Valves (TCVs)
Throttle Control Valves simulate minor loss elements whose head loss characteristics change over time. With a throttle control valve, the minor loss 'K' is adjusted based on some other system flow or head.
The FCV will only throttle if the flow through it is greater than the setting. It will attempt to induce a headloss that causes the network to balance in such a way that the flow through it is reduced down to that setting. If the flow through it is already less than the setting without any headloss, then the FCV will be "inactive" (fully open) and you'll get a notification stating that the valve "cannot deliver flow".
In contrast, when you use a TCV and enter a minor loss or discharge coefficient, then set it to "Active", it will apply that headloss right away, regardless of the flow.
With that being said, if you place a TCV and set it up in such a way that you get a desired flow through it, then you should expect to see the same results if you place a FCV there with the same flow setting. The FCV should induce the same amount of headloss, to bring the flow down to the same amount. However, this won't work very well if there's only demands downstream, with only one path (through the FCV) to reach them. In this case, the flow must be the sum of the downstream demands.
About Rationing Control Valve (RCV) or rationing valve
Sometimes a "rationing control valve" might be used to control flow. This type of valve is usually associated with "hand pump" operation. The valve operates by hand allowing a fixed quantity of fluid to be "rationed" and extracted out. If this needs to be modeled when modeling standposts or hand pump operation on field an FCV can be used to "control" the flow downstream if a fixed outflow is expected. Alternatively, if the "rationing" is based on a fixed headloss applied at the valve, you can try using a TCV.
See related forum post about it here: Rationing Control Valve
Forum Post - TCVs vs. FCVs