Being after the FW control valves does not make sense. You have to have enough pressure to overcome the pressure of the steam being desuperheated or attemperated and the BFW is the only water with that much pressure, but once it takes a pressure drop across the control valve, then it is only minimally above drum (steam) pressure.
Plus, if the controls sense no need for BFW and close the valves, then there is no excess pressure for overcoming the steam pressure for attemperation.
The spray water ones I have seen are always taken BEFORE main feedwater isolation valves and control valves, except that you might have s surface (heat-exchanging) type of attemperator.
For the surface type attemperator designs, yes, the take-off points are always located after the control valves. I figure that is because, after going trough the attemperator, the water needs to come back and return to the main feedwater before going to the economizer but since this water has already lost some pressure head by going through the coils inside the attemperator, a throttle valve has to be installed on the main line, which is parallel to the attemerator line, so as to create a pressure drop that is equal to or a bit higher than what the attemperating water has lost thus the two streams can possibly join together. Now imagine, if attemperating water is taken before the feedwater control valve then this control valve will then be in serial with the throttle valve and become another throttle valve. When it tries to change feedwater flow its pressure drop changes this can mess up the pressure balance with the attemperator then messes up the attemperator flow control.
If the feed pump is a constant speed pump and the feedwater flow is regulated by a control valve, then the pressure upstream of the feedwater control valve is very high at low loads, and is at a minimum pressure value at high boiler loads. review the pump performance curves of gpm vs developed head to confimr this characteristic. This pressure characteristic is opposite of teh required spray water pressure characterisitic needed for a normal turndown for the spray water control valve.
If the spray water is sourced downstream of the feedwater control valve, then the pressure available for spray water attemporation will increase as load increases, which improves the turndown performance of a simple spray water station.This is because the pressure drop from economizer inlet to superheater interstage increases as boiler load increases. However, a spray water nozzle typicall has low turndown for attemporation purposes, unless one uses spring-loaded nozzles with wide turndown ability.
