(1) Selection of Reference Plane
Rotary pumps, typically horizontal ones, are commonly used. The reference plane for these pumps is the pump shaft centerline. For reciprocating pumps, including metering pumps, the reference plane is defined as the plunger (or piston) centerline. This distinction is important for accurate performance analysis and system design.
(2) Calculation of Cavitation Parameters
For positive displacement rotary pumps, the Net Positive Suction Head available (NPSHa) and required (NPSHr) must be considered. Similarly, for vane pumps, the same calculation method applies, as outlined in Table 2-24. For volumetric reciprocating pumps, the NPSH must also account for the acceleration head loss (Hacc). This loss occurs due to the non-uniform flow in the suction line. If a suction buffer tank is installed, the initial acceleration loss can be minimized or eliminated.
The formula for calculating Hacc is as follows:
$$ H_{acc} = \frac{L \cdot Q}{D^2 \cdot N} $$
Where:
- $ L $ is the suction pipe length in meters,
- $ Q $ is the pump flow rate in liters per hour,
- $ N $ is the pump reciprocation frequency in revolutions per minute,
- $ D $ is the suction pipe diameter in millimeters.
This formula is suitable for single-cylinder pumps. For double-cylinder pumps, the flow $ Q $ should be taken as half of the total flow (i.e., the flow per cylinder). For three-cylinder pumps, divide by 2.5. If an inlet buffer is used, multiply the result by 0.2 to adjust for the damping effect. Proper consideration of Hacc ensures more accurate NPSH calculations and helps prevent cavitation.
(3) Harmfulness of Cavitation
Cavitation in positive displacement pumps can lead to similar noise and vibration as in other types of pumps. However, unlike centrifugal pumps, cavitation in positive displacement pumps usually does not cause severe corrosion damage to components. Despite this, it is still crucial to maintain a sufficient margin between NPSHa and NPSHr, ideally with a safety factor of at least 0.5 m or more. This precaution helps avoid potential operational issues and extends the life of the pump.
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