Negative slip occurs in reciprocating pumps, when delivery pipe is

Concept:

Slip: It is the difference between the theoretical discharge and actual discharge.

Slip = Q_the - Q_act

Positive slip: When the theoretical discharge is more than actual discharge is known as positive slip. i.e.  Qthe > Qact

Negative slip: When the theoretical discharge is less than actual discharge is known as negative slip. i.e.  Qthe < Qact

Conditions of "Negative slip":

• Due to High speed
• Due to short delivery pipe
• Due to long suction pipe

This happen because of inertia pressure in suction pipe will be large as compared to pressure of the delivery valve. This causes delivery valve open before suction stroke is completed.  

Coefficient of discharge

C_d = \(\frac{Q_{act}}{Q_{the}} \) and % slip = \(\frac{{Q_{the}}-~Q_{act}}{Q_{the}}\)