Piston and Liner

Liners of high-pressure mud  pumps are always locked in place by metal-to-metal contact. A special liner lock nut is used in triplex pumps. Duplex pumps are provided with a dual cage metal-to-metal liner retention system (Figure 11).

The latter arrangement holds the liner metal-to-metal against a shoulder in the housing utilizing set screws), which are tightened against a liner cage. The liner packing is adjusted separately with set screws on the cage. Some types of pumps have special tell-tale holes (a small hole connecting the liner packing spacer ring area to the atmosphere) to check the proper functioning of the packing. If fluid drips out of these holes, the liner packing has to be tightened. These holes should never be plugged off to stop the “leaking”!

Liner wear is worst in the middle of the stroke because the piston velocity being highest at that point.

The maximum allowable liner wear depends on the pressure the pump has to overcome (see Table 1).

Figure 13 shows the pistons of a double-acting and a single-acting pump. Note that the piston bodies have been provided with a special tell-tale wear groove to judge the piston wear.

The piston rubbers are made of polyurethane. The clearance between the piston and liner determines the life of the rubbers (see Figure 14). If the play is too great, it is possible that the rubber will be extruded into the gap and become torn.