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.
