THE VOLUNTEERS CORNER
DRAFTING is probably one of the most commonly misunderstood fireground operations. It’s what you can’t sec—atmospheric pressure—that makes drafting of water possible with a centrifugal pump.
First of all, a centrifugal pump must be primed. This means that a partial vacuum must be created in the pump. Then the atmospheric pressure on the surface of the water provides the energy that sends the water up the suction hose to the pump. How great the vacuum must be depends on the height of the lift.
At sea level, atmospheric pressure is 14.7 psi. Couple this with the fact that a column of water 2.304 feet high exerts a pressure at the bottom of 1 psi. Then, 14.7 is multiplied by 2.304 to determine that the highest vertical lift theoretically possible for a centrifugal pump is 33.9 feet.
However, this is not a practical possibility. A fire pump in excellent condition might be able to draft between 26 and 28 foot vertically. A pump in very good condition should be capable of drafting from a height of 22 to 25 feet above the water.
Running the pump faster will have no effect on the height. The atmospheric pressure and the airtight condition of the suction hose, the truck piping and the pump itself determine the draft limitations. Any air leak will reduce the ability of the pump to draft.
Fortunately, a slight leak will not prevent a pump from drafting at small “lifts.” However, a “slight leak” sometimes means only another eighth of a turn on a suction hose coupling.
Speaking of suction hose couplings, if the gaskets are in good condition, it should not be necessary to use a hammer on the lugs or wings. With arms held stiff and throwing the body weight into the arms, two men should be able to couple suction hose successfully. However, if the gaskets have hardened with age, a hammer blow on a coupling wing will often eliminate an air leak at that point.
One of the bits of information that sometimes is not known is that when priming a centrifugal pump, it makes no difference whether the pump is wet or dry. The little extra time taken priming a wet pump is probably less than the time taken to drain a pump before priming. Just don’t be fooled by the initial discharge of water through the priming pump exhaust pipe and think that the pump has been primed.
The sound of the pump will often tell you when you are getting a prime. The suction gage is a more accurate indication. Roughly, the suction gage should show an inch of mercury on the vacuum side for each foot of vertical lift.
As a rule-of-thumb, vertical lift is measured from the surface of the water to the midpoint of the suction intake pipe. Actually, it should be measured to the midpoint of the pump—which cannot be seen behind the body steel of the truck.
As far as horizontal distance is concerned, this is no great problem in drafting. if you have to use four or five lengths of suction hose to reach the water, it just means that you have to pull a vacuum throughout more lengths of suction hose than in a two-length draft, and the suction friction loss is increased. The only important thing is that the vertical height is not greater than the pump is capable of handling. I can recall a fire at which a truck started drafting from the shore with three lengths of suction hose and added two more as the tide receded. This did not frustrate the pumping.
Use care not to create a whirlpool around the suction hose. The strainer should be 18 inches below the surface for safety so that air will not be drawn into the hose through the whirlpool. In practice, a strainer only about 6 inches below the surface may be satisfactory if the amount of water being drafted is small. Sometimes you can move your boot through the whirlpool and keep it broken up. Another trick is to float a couple of boards over the strainer to prevent the formation of a whirlpool. There are strainers available that float and accomplish the same thing. They are bulkier than an ordinary strainer, but they are a great advantage to a department that often has to draft from shallow water.
Clogging of the strainer is one of the annoying problems in drafting. The collection of leaves, twigs, water plants or whatever, will cut down the amount of water going to the pump. The pump operator can get an indication of this situation by watching his suction gage. When the strainer starts to get clogged, the gage will begin to show a larger and larger vacuum reading. If you are drafting at a height of about 10 feet, the vacuum reading should be about 10 inches of mercury. (Actually, 1 inch of mercury indicates 1.13 feet of water.) If the needle starts to head for 15 inches of mercury or so, then the strainer is probably getting obstructed. This same thing would happen if there was any obstruction in the suction hose itself. There have been cases where the inside lining of the hose has been torn so that a flap has entered the waterway.
A rule to remember is that using the same size suction hose, the greater the lift, the smaller the potential volume of water. At sea level, with a 10-foot draft, 500 gpm can be expected with a 4-inch suction hose, 750 gpm with a 4 1/2-inch hose, 1,000 gpm with a 5-inch hose, and 1,250 gpm with a 6-inch hose. With each size hose, greater or lesser volumes can be expected at lower or higher drafts.
Sometimes pumpers are positioned on a bridge with a railing. If possible, put the suction hose through the railing at a point below the level of the suction intake. If you have to put the hose over a railing, above the level of the suction intake, you may get in trouble. Sometime an air lock may develop at the top of the looped hose and the priming pump may be unable to eliminate the air lock.
If the priming pump fails to work, don’t give up; if the lift is slight, it is possible to drop water from the booster tank into the pump and down the suction hose. A little practice will enable you to speed up the pump at the right moment and pick up water.
Another problem is friction in suction hose. The rule is that, through the same size hose, a pump must exert as much energy lifting water from draft as it does forcing the same amount of water up the same height under pressure. This is why larger suction hoses must be used for pumps with greater capacities. If the suction hose is not large enough for the pump capacity, the pump may “run away from the water.” This condition, known as cavitation, is indicated when an increase in engine speed does not cause an increase in pump pressure.
High altitudes also have an effect on drafting because of the reduced atmospheric pressure. This can be reduced by increasing the suction hose size.