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

continue to increase.) Static head is found

simply by measuring the appropriate vertical

distances in feet or meters.

In a system that circulates water, like a

closed cooling system, there is no static head.

The water begins and ends its travel at the

same level—at the pump.

Velocity Head

Velocity head is the head that results from the

kinetic energy in the water flow. It is the head

(height) the water would have to fall from to

reach the velocity it’s moving at in the sys-

tem. Velocity head is important for high-

volume systems, but at under 200 gpm or

12,000 gph (760 lpm or 45,600 lph), velocity

head is small, usually under 0.4 foot (0.12 m).

For our purposes here, we can ignore it.

Pressure Head

In Figures 19-6 and 19-7, we are pumping to

and from open containers. The height of the

water in the container we’re pumping into

(above the pipe’s inlet) is the pressure head,

but it is already taken into account in the

height of the discharge head. If you want a

given pressure at a nozzle outlet for, say, a

fire hose or a deck washdown, you can con-

vert the required pressure in psi or kPa to

head in feet or meters and add it to the total

required head.

head (fresh water), ft. ÷ 2. 31 =psi

psi × 2. 31 =head (fresh water), ft.

or

head (fresh water), m × 9. 8 =kPa

kPa ÷ 9. 8 =head (fresh water), m

Seawater is 1.028 times denser, but for most

ordinary pump calculations this can be neg-

lected for static head and pressure head.

A bilge pump simply pumps into the at-

mosphere. You don’t care about the exit pres-

sure, only the flow rate, so the pressure head

is zero. However, if you are selecting a pump

to fill a tank and the pipe inlet is near the bot-

tom of the tank, you will have to add the pres-

sure head, either by accounting for the total

height of water in the tank as static head or

by adding it separately as pressure head.

(Note that such a pipe to the bottom of a tank

would require a check valve.)

Friction Head

As water flows through pipes and fittings, it

loses energy due to the friction of rubbing

against the pipe walls. All pumps must over-

come this significant component of the

head. It has to be taken into account in any

piping system. The longer the pipe run and

the more fittings or bends, the greater the

friction head. Similarly, for a given flow of

water, the larger the pipe diameter and the

fewer the fittings or bends, the less the fric-

tion head.

Calculating Friction Head

Friction head can be calculated as follows.

Formula 19-1. Friction Head

or

Where

hf =friction head, ft. or m

L =total length of pipe, ft. or m

gpm =flow, gallons per minute

lpm =flow, liters per minute

ID =inside diameter of pipe, in. or cm

(NOTE: ID in cm, notmm, in this

formula)

C =coefficient of roughness for the type

of pipe:

Iron and Steel Pipe = 100

Plastic, Lead, Copper, and Brass Pipe =

130

Smooth Hose = 140

Corrugated Hose = 150

Friction head from fittings can be deter-

mined from Table 19-2.

To determine the total friction head in the

system, you list all the fittings and then multiply

them by their equivalent lengths. For instance, a

90-degree elbow is 30 D; for a 1^1 / 4 - inch pipe, its

equivalent length is 37. 5 inches or 3.125 feet; for

hf, m

82 .54 L, m lpm

C (ID, c m)

1. 85

= 1. 85 4. 86

××

×

hf, ft.

10 .44 L, ft. gpm

C(ID,in.)

1. 85

= 1. 85

××

×^44.^86

PART SIX:PLUMBING SYSTEMS WITH NOTES ON FIRE SUPPRESSION

Formula 19-1.