Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

SPECIFYING THECOMMONRAILThe common

rail—the single distribution line to the

branches for each component (or from each

return)—is usually made of pipe. It can be a

single section with welded-on branch pipes, or

several sections connected in line with

T-connectors forming the branch pipes. Some-

times a solid block of metal is machined to

form the common rail. In this case a long

block—say, solid stainless—is bored along its

center to form an internal common-rail chan-

nel. The end is drilled and tapped to accept the

main inlet (or outlet) and the branch exits (or

inlets) are drilled and tapped at right angles

(into the side of the common-rail block). For

most ordinary fuel-piping manifolds, however,

plain pipe is the most cost-effective approach.

A mistake in setting up a common-rail man-

ifold is making the common-rail pipe the same

diameter as the branch pipes. If you stop to

think about it, obviously the common rail has

to handle the combined flow of allthe branch

pipes. There can also be some internal turbu-

lence and flow restriction due to the sharp

bends in flow required by the fluid inside the

common-rail manifold. For this reason, the in-

ternal cross-section area of the common-rail

pipe needs to be equal to the total cross-section

area of all the branch pipes plus 10 percent, or

Common-rail pipe section area = total

combined branch-pipe section area × 1. 1

Say you have a common rail that needs to

feed two large main engines and a gen set and

the common rail is fitted with four standard-

weight^3 / 4 in. (or DIN 26. 9 × 2 .9) stainless

branch pipes, including the spare. See

Figure 4-9, which is also a detail view of the

common-rail manifold in Figure 4-8.

Note in Figure 4-9 that the larger

common-rail diameter must extend the full

length of the pipe from the source. This

means a large valve at the tank, which takes

up space and requires a bigger takeoff pipe.

This must be allowed for in design. The tank

manufacturer has to install the large takeoff

pipe leading from close to the bottom of the

tank (see “Taking Off” on page 78) as well as a

suitable valve or valve attachment. Note the

brackets or supports required on the manifold

piping to prevent flexing and cracking. Also

note the screw-cap closure fitting on the

spare branch pipe for added protection

against accidental spills. The quarter-turn ball

valves allow you to immediately see if a valve

is open or closed.

The common-rail manifold could be used

exactly as shown in Figure 4-9; however,

space and installation requirements may

make it necessary to locate the common rail,

with its branch pipes and valves, much far-

ther from the tank. This is fine as long as the

piping is properly supported against bending

PART TWO:FUEL SYSTEMS

Figure 4-9.
Common-rail fuel-
feed manifold