If the relief valve is on the turret it is liable to lift water when it goes off. That can lead to a slug of water going down the main steam line to the engine. With larger plant this is the last thing you want to happen. It is not so likely to cause problems with the sot of engine you would run off a five inch boiler. If you keep the original valve and set the one on the turret to lift at a slightly higher pressure, it will only open if the original valve is not quite coping.

Ideally the size of the valve should be such that the pressure does not rise more than 10% over the set pressure when the boiler is being fired and no steam is being taken for other purposes. That is called an accumulation test, eg how much pressure accumulates when the plant is running full bore, the load stops taking steam, and the valve has to control the pressure.

John