Questions and Answers

Questions and Answers

DEPARTMENTS

wherein are answered questions relating to current problems in the fire protection

Piston Pumps Defended

The Questions and Answers column of May 1960 contained an answer to a query on the use of piston pumps (volume) in the fire service. Following its publication, Mr. R. S. Howe, president of the Howe Fire Apparatus Co., disputed our answer and asked that he be permitted to voice the other side of the question. The following is his statement:

Although the piston pump is heavy and bulky, present-day trucks are entirely adequate to carry them. The usable space which they occupy is scarcely more than a typical two-stage centrifugal pump. In regard to weight distribution problems, these can easily be handled by merely extending an L from the booster tank on the side opposite the piston pump to correctly distribute the weight.

Although piston pumps have many movable parts, the modern pump operates at only 180 rpm maximum, as compared to centrifugal pump speeds of 3,000 to 5,000 rpm.

The sealing between pistons and cylinders is done with a flexible leather seal, which gives a tight seal with practically no slip. The pump will handle sand and gravel under normal pumping conditions, without ill effect because the leather seals in the pistons, and rubber seals in the valves will deform temporarily as required, rather than scratch cylinder walls, etc. Due to the clearance through the center of the pistons and the check valves, foreign objects of considerable size can be forced through the pump without ill effects.

The operating speed of the pump has not changed with engine speed changes. It has always required a speed reduction to relate engine speed to pump speed, just as most centrifugal pumps require a set of gears to provide a proper impeller shaft speed. The use of high-speed engines has no effect on the efficiency of the piston pump. In regard to the performance of the piston pump as compared with centrifugal pumps, there are occasions when the piston pump will out-perform the centrifugal pump. The volume piston pump can draft water at extreme lifts up to approximately 29 feet, whereas the centrifugal pump cannot draft water over 22 to 25 feet. The piston pump is a very easy pump to operate, requiring no priming, as it is a self-pruning pump, and only requires two positions, one for road operations and one for pumping operation. Water is immediately available to piston pumps, whereas the centrifugal pump has to first be primed before operation.

We might add that with the piston pump pumping with a lift of 29 feet, capacity operation can be performed, whereas with a centrifugal pump drafting at 22 feet to 25 feet, only a small volume can be produced at this lift.

Correction

The answer to the question concerning “Finding Nozzle Diameter” on page 429, May 1960, FIRE ENGINEERING, inadvertently omitted the square sign in the last line of the problem. The correct statement should read: D2=1.256; D=1.12, or 1 1/8 inch nozzle.

Watch That “Cycle” Rating

To The Editor:

We recently acquired a portable electric generator. The information plate stated it is rated at 1,500 watts, 115 V.A.C., 800 cycle. Can we employ this unit to run small motors, etc.?

D. H.

Answer: The “800 cycle” is a warning sign not to employ this unit for motors other than those designed for the same cycle rating. Power plants of this type, were originally manufactured for use with military aircraft radio, special tools, etc., where compact high power, and light weight equipment is a necessity. To employ them for use with motors not of the same rating may result in damage to the generator. This warning includes the use of AC-DC or “Universal” motors.

Aerial Ladder Measurement

To the Editor:

Is the height of an aerial ladder measured from the turntable or the ground?

S. W.

Answer: It is the practice in this country to denote the height of an aerial ladder by measuring it fully extended at maximum elevation from the ground to the top rung of the uppermost fly ladder (see Pamphlet No. 19, NBFU Specifications for Motor Fire Apparatus, Chapter 8, Sec. 8012).

It may be noticed that some European-style ladders for use in this country are designated by the actual ladder length. This is done because they are intended for mounting on a U. S. chassis and depending on the final vehicle choice, the over-all height of the completed apparatus may vary.

Questions and Answers

70

Questions and Answers

DEPARTMENTS

Horizontal Range

To the Editor:

How is the reach of a fire stream calculated?

N. B.

Answer: Maximum range of a stream when the nozzle pressure is greater than 30 psi may be calculated with satisfactory results by the formula S = 1/2 N.P. +26. The above is for use with a 3/4-inch nozzle where S equals the horizontal distance in feet and N.P., the nozzle pressure in pounds per square inch. Add 5 feet to the 26 for each 1/8-inch increase in nozzle diameter.

Example: What is the maximum range of a fire stream when a 1-inch nozzle is employed at 50 psi?

S = 25 + 26 + (2 X 5)

= 25 + 26 + 10

= 61 feet

(See “Eire Service Hydraulics,” page 171.)

Sprinkler Calculations

To the Editor:

What is the method used for calculating discharge from a sprinkler head and for supplying sprinkler systems during a fire?

H. P.

Answer: Discharge from a sprinkler head may be computed with comparative accuracy by the formula Q = 1/2 P + 15, where Q is the discharge in gallons per minute and P the pressure in pounds per square inch at the head. For example, if the pressure is 20 psi, the discharge will be Q = 1/2 X 20 + 15, or Q = 25 gpm.

The use of the formula is excellent for determining required quantities to supply a building system during pre-fire planning. However, a simple procedure for use during an actual fire is recommended, A pumper is connected to the fire department Siamese and a pressure of 150 psi is maintained. The pump operator then notes the tachometer during the fire, and if excessive engine speed is required to maintain the pressure, it signifies a second pump is necessary to supply the number of heads which have opened. No further calculations are necessary.

(See “Fire Service Hydraulics,” page 188; FIRE ENGINEERING, page 552, July 1958; NBFU Bulletin 270.)

Ladder Mounting

To the Editor:

Should the heel plates of a 24-foot ladder carried on the side of a pumper extend toward the rear of the apparatus or to the front?

K. S.

Answer: Carrying a ladder on the side of a pumper is usually a matter of original mounting design, custom and regulations of the department concerned. Safety to the men riding on the vehicle is, however, an important consideration.

If the ladder projects beyond the side grab rail, it may be a better practice to place the heel plates in a forward position. This may possibly prevent injury to the men riding the tailboard in case of sudden stops and hard bumps while the apparatus is in motion. If there is no projection, then custom or regulations will dictate the method.