What is the difference between suction lift and suction head?

Suction lift is only negative. Suction head is positive or negative.

Suction head is the distance between the surface of the liquid being pumped and the centerline of the pump. It can be positive or negative, and it can be affected by pressure or vacuum on the surface of the fluid. A negative suction head is also called suction lift.

Example 1: A D/A under 15 psig is set 20 feet above the feed pump the effective suction head is 54 feet. (15 psig is equivalent to 34 feet of water).

Example 2: A Condenser hotwell under 28 inches of vacuum (Hg) is set 16 feet above the suction of a propeller pump. The effective suction head (lift) is -15 feet.* (28"Hg = - 31 feet of water)

Example 3: A sump pump is set 4 feet below the surface of the well. The effective suction head is 4 feet.

*Note: Suction lift is only effective to a maximum of 21 feet, and 15 feet for hot water depending on temperature and pump slip.