Reciprocating pump operation: condition for negative slip Slip of a reciprocating pump becomes negative (i.e., actual discharge exceeds theoretical discharge) under which practical condition?

Introduction / Context:
Slip in reciprocating pumps is defined as theoretical discharge minus actual discharge. It is usually positive because of leakage and valve losses. However, under certain dynamic conditions the actual discharge momentarily exceeds the theoretical value, producing negative slip. Recognizing when this occurs is important for diagnosing performance anomalies.

Given Data / Assumptions:

• Single- or double-acting reciprocating pump with delivery and suction valves.
• Significant inertia and frictional effects in the delivery line.
• High-speed operation relative to system time constants.

Concept / Approach:
At high speeds with a long delivery line, the moving water column possesses substantial momentum. During the suction stroke, the inertia of this column may continue pushing water through the delivery valve, so flow does not reverse as quickly as the piston motion would suggest. This makes the measured actual discharge exceed the kinematic theoretical discharge based on piston motion alone, yielding negative slip.

Step-by-Step Solution:
Define slip S = Q_theoretical − Q_actual.At high speed and with long delivery pipe, inertia sustains delivery flow even during part of the suction stroke.Therefore Q_actual > Q_theoretical ⇒ S < 0 (negative slip).Hence the condition is long delivery pipe combined with high pump speed.

Verification / Alternative check:
Indicator diagrams and instantaneous flow measurements show continued positive delivery flow beyond the theoretical cut-off in such cases, confirming negative slip behavior.

Why Other Options Are Wrong:
Short lines and low speeds minimize inertia effects, so slip stays positive. Long suction with high speed mainly aggravates suction problems (cavitation), not negative slip in delivery.

Common Pitfalls:
Assuming leakage always makes S positive; ignoring water hammer and fluid inertia in long pipelines.

Final Answer:
delivery pipe is long and the pump is running at high speeds