V. SUPPLY SYSTEM CONSIDERATIONS
F. Acceleration head
Whenever a column of liquid is either accelerated or decelerated, pressure surges exist. This condition is found on the suction/inlet side, as well as discharge side, of a reciprocating pump. Not only can the surges cause vibration in the inlet line, but they can restrict and impede the flow of liquid and cause incomplete filling of the inlet valve chamber. The magnitude of the surges, and how they will react in the system, is impossible to predict without an extremely complex and costly analysis of the system. Since the behavior of the natural frequencies in the system is not easily predicted, as much of the surge as possible must be eliminated at the source. Proper sizing, installation and charging of a dampening device will absorb a large percentage of the surge before it travels into and through the system and cause trouble. The function of the device is to absorb the peak of the surge and feed it back at the low part of the cycle. The preferred position for the device is in the liquid supply line, as close as physically possible to the reciprocating pump, or alternately attached to the blind side of the pump inlet. In either location, the surges will be significantly dampened and the possibility of harmful vibrations considerably reduced.
The experimental formula for calculating acceleration head is:
ha = L V n C and V = GPM
K g (2.45) (ID)2
Where,
ha = acceleration head (ft.)
L = length of liquid supply line (ft.)
V = average velocity in liquid supply line (fps)
n = Pump speed (rpm)
C = Constant depending on the type of pump (see page A35)
K = liquid compressibility factor:
K = 2.5 for relatively compressible liquids (ethane, hot oil)
K = 2.0 for most other hydrocarbons
K = 1.5 for amine, glycol and water
K = 1.4 for liquids with almost no compressibility (hot water)
g = standard gravity = 32.2 ft/sec2
ID = inside diameter of pipe (in.)
Example 12:
Find the acceleration head (ha) for a
single acting 3-1/2" x 4" triplex plunger pump operating at 350 rpm and
175.35GPM capacity. Supply to the pump is through 50 feet of 4" Schedule 40 pipe fed
from an open tank.
Assume the liquid is water @ 80°F.
ha = (50) (175.35) (350) (0.066)
(1.4) (32.2) (4.026)2 (2.45)
ha = 113.13 feet
The acceleration head would be reduced to 49.85 feet if 6" Schedule 40 pipe were used, but this is still too high. An 8" schedule 40 pipe would reduce ha to 28.78 feet. Proper application of dampener device could reduce this value to approximately 10 feet of 4" schedule 40 pipe. This would appreciably help the net positive suction head available (NPSHA) - in addition to the benefits in avoiding harmful vibrations.
