## Hydraulic Conditions

Low-Head Pumps For low-head pumps, up to approximately 150 ft (46 m), the installation must be carefully checked to prevent cavitation during periods of low-head operation. This is particularly important during one-pump operation on a parallel system. For

FIGURE 13 A sealed room in a coal mine housing three 1000 HP (746 kW) bronze constructed pumps (Hazleton Pumps, Inc.)

example, if two 5000-gpm (1136-m3/h) pumps are discharging into a common discharge line against a static head of 80 ft (24 m) and a frictional head of 60 ft (18 m), the frictional head will be only 15 ft (4.6 m) when one pump operates alone at 5000 gpm (1136 m3/h). The total head will now be only 95 ft (69 m), and the single pump will carry out to a much higher capacity. Unless sufficient NPSH is available for the single-pump runout point, the pump will cavitate. Although the two pumps should be selected for 5000 gpm (1136 m3/h) at 140 ft (43 m) total head, the required NPSH should be determined not at 5000 gpm (1186 m3/h) but at the capacity corresponding to the intersection of the pump and system curves.

High-Head Pumps For high-head pumps, 1000 ft (305 m) head or more, the risk of cav-itation for single-pump operation on a parallel system is less than for low-head pumps. For example, if the static head is 1000 ft (305 m) and the frictional head is 60 ft (18 m) when two 5000-gpm (1136-m3/h) pumps are operating, the total head will decrease from 1060 ft (323 m) to only 1015 ft (309 m) when one pump operates at 5000 gpm (1136 m3/h). This means that, for either one- or two-pump operation, the capacity of each pump will be approximately the same and the risk of runout cavitation is minimal.

Waterhammer and Pressure Pulsations A waterhammer analysis should be made of both high- and low-pressure pumping systems. Although the transient pressure pulsations are related to the rate of change of velocity rather than the magnitude of the steady-state condition, mine experience indicates that waterhammer problems can be anticipated when pipe velocities exceed 10 ft/s (3 m/s). In high-pressure pumping systems, it is not unusual for transient pressure pulsations to be as high as 300 lb/in2 (2068 kPa) above or below the steady-state pressure.

FIGURE 14 Three 2000 HP (1490 kW) pumps in one room in a zinc mine (Hazleton Pumps, Inc.)

Transient pressure pulsations have been experienced in low-pressure pumping systems. The danger here is that the low-pressure portion of the cycle will fall below atmospheric pressure and the pipe will collapse.

Although any pumping system for mine service should be analyzed in detail for transient pressure pulsations, experience has shown that adequate air bottles have proved to be one of the most effective and least expensive means of surge suppression. Slow-closing valves and flywheels have been used, but they must be sized correctly. This is especially true with high-speed pumps because these units possess little rotational inertia and will decelerate very rapidly on shutdown, with accompanying high-pressure surge.

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