Hydraulics and Fluid Mechanics in ME
Slip of a reciprocating pump is negative, when

Suction pipe is short and pump is running at low speeds
Delivery pipe is long and pump is running at high speeds
Suction pipe is long and delivery pipe is short and the pump is running at high speeds
Suction pipe is short and delivery pipe is long and the pump is running at low speeds

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Hydraulics and Fluid Mechanics in ME
For similarity, in addition to models being geometrically similar to prototype, the following in both cases should also be equal

ratio of inertial force to force due to surface tension
ratio of inertial force to force due to gravitation
ratio of inertial force to force due to viscosity
all the four ratios of inertial force to force due to viscosity, gravitation, surface tension, and elasticity

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