We are carrying out installation of a new single diesel centrifugal fire pump house package (complete with test return line, pressure relief, etc). Pump has operating duty of 165psi, 17,500 L/min. Typical discharge conditions would be 17,600 l/min @ 12 barg. Basic layout attached. Please not the engine cooling line exchanges heat with the engine but exhange is not shown.
We are planning to return the pressure relief line to the suction line, which is permitted by NFPA 20 - 22.214.171.124 - 2016 as long cooling water for diesel engine is taken from the pump discharge and the controller has a high engine temperature alarm/signal.
I've been told returning to suction causes operational issues and with the valve opening and closing and also the pressure is returned straight back into the pump and not dissipated? If so then why would NFPA 20 permit the relief line to be returned to suction?
Someone advised that the relief line should always be returned to an open end (ie top of a tank) however this is much more costly in terms of materials and labour, as opposed to routing back to the suction line.
Can anyone advise on returning to the suction line?