Generally you are allowed only one opportunity to blow up your boiler.   SAFETY FIRST!!
All connections to the boiler are through valves mounted as close to the boiler as possible and connected with Schedule 80 pipe fittings.  The safety valve is similarly attached , but with NO valve between it and the boiler.  The overflow from the safety valve is always a size larger that the input.  This is piped to a position where the exhausting steam will do no damage.  Globe or Ball valves are used throughout, mounted so that the seal is made against the boiler pressure, not with the pressure behind.  Many valves have arrows on them.  These always point away from the boiler.
The main steam valve (1) provides steam to the engine via 1/2" black pipe.  An optional super-heater in the stack may be used (W).  The throttle valve (2) can be a fancy and proper throttle valve or merely a ball valve.  It is important to have a flexible link close to the engine to eliminate vibrations, so that work harden an eventually break the steam line.  Pipe hangers to securely position the steam line may be necessary.
The engine cylinder lubricator in the main steam line is generally placed near or on the engine.  The steam chest and/or cylinder drains (K) are joined and dumped some convenient place such as the condenser input.  One can stall the engine by having the drains wide open while the engine turns slowly.  However, the engine need not turn more than several revolutions once it is hot and internal condensation has ceased, so this is not a problem.  The engine exhaust is also isolated mechanically, a piece of good heater hose will do the job.  The exhaust steam is passed through the exhaust pre-heater and then led to the condenser.  The condenser should be isolated at the thru-hulls by seacocks (L) and (M).  Condensed water is retrieved by the vacuum pump and emptied into the hotwell.
The hotwell contains oil absorbents and the automatic by-pass (H) from the engine driven feed water pump.  There is an overriding manual by-pass (I).  The hotwell is topped off from the fresh water tank controlled by the fill (J). 
The feed water is passed through the exhaust pre-heater to the boiler.  It may be additionally preheated in a preheater (V) coil within the boiler.  Note that locations of the boiler preheater and super-heater are not particularly  fixed.  One could reverse their positions or place both in the hood of a VFT or at the top

of a HWT boiler.
Some much-needed auxiliary items are the vacuum (pressure) gage and the vacuum pump; the feed water (pressure) gage between the feed water pump and the boiler; and the check valve (X) just before the boiler valve (B).  Note that if this valve is omitted the pressure gage will read the boiler pressure, and provide incorrect information about the performance of the engine feed water pump.
The boiler blow-down (N) and the output of the steam ejector are led through the boat hull via seacocks (O) and (Q).  A pipe is led from the bottom of the ejector to the compartment to be drained.
The boiler fill (6) is a standard 3/4" garden hose fitting which is very useful for initial filling of the boiler.  Never use when boiler is pressured!  When filling the boiler without the convenience of a garden hose, a funnel is needed.  The boiler pressure gage is protected from steam by the siphon (W) and petcock (T) which, like all gage protracting petcocks , is just cracked open.
Auxiliary boiler valve (3) delivers steam to the whistle which has its own quick-acting whistle valve (4) and for the steam lance control valve.  The stack blower is controlled by valve (a).
Sizes have been provided for all valves on the assumption that our system is 3 to 10 hp on a boat 18 to 28 feet.  Most of the piping nay be carried out using standard 1/2" pipe and fittings with the exception of close nipples and places where mechanical failure by vibration, shock, or a simple hip blow may result in failure.  In such instances as well as the high pressure line should be done using schedule 80.  Most certainly schedule 80 should be used between the boiler and the first valve in every line.  Engine exhaust ought to be 1" pipe down to the 3/4" copper pipe condenser thus avoiding back pressure;  1/4" copper tubing is good from the condenser to the vacuum pump.
Most of the rest of the piping can be a  mixture of pipe and copper tubing, the latter connected using flare, NOT compression fittings.  The blow-down line should be as large as the boiler fitting, generally 3/4".  Running the whistle line up inside the stack prevents condensation in the whistle line.   Attach the whistle to the stack.
Piping should be installed longitudinally and thwartships, not point to point.  Copper tubing should be formed using tubing benders, not by hand, which produces bumps and flat spots.