you need to refear Train engineering Hand book 1996 edition the concept and working of decoupler has been explained if you have defficulty send a mail to me

crm

decoupler means kinda bypass line

when you insatlled a decoupler header it serve as a by pass header when the pressure insaide the system increases due to any pump cut off the pressure goes high at the discharge line at that time this v can opn the decoupler line to balance or minimize the pressure it should be insatalled on the secondary pump delivery with the primary pump delivery

thanks