Power End Theory

The power end or drive end of a power pump consists of a crankshaft, connecting rods, crossheads, and bearings, all housed in a rigid structure referred to as the frame. Details of the design and construction of these components are covered in Section 3.2, "Power Pump Design and Construction." The slider-crank mechanism that converts rotational driving energy to the reciprocating motion that actuates the pistons or plungers can be found in reciprocating gas compressors, automotive engines, and stationary and marine engines. However, the stress loading pattern of power pump components is unique to this type of mechanism.

FIGURE 12 Power end, vertical pump (Flowserve Corporation)

Applying Rod Load to the Power End As stated earlier, the loading on the power end is called the rod load and is the product of the area of the plunger multiplied by the maximum discharge pressure. In a gas compressor or engine, this loading increases over 90 to 180 degrees of crank rotation before maximum loading is reached. In a power pump, the maximum loading is reached in less than 30 degrees of crank rotation due to the relative incompressibility of the pumpage. Since this loading cycle is repeated with each plunger stroke, the actual loading resembles a shock load more than a simple cyclic fatigue load. The design criteria for the stressed components of the power end must therefore include the material's capability to absorb shock loads and overall safety factors greater than 3:1.

Another critical factor in the capability of the power-end components to handle the rod load is how the load is applied. Figures 11 and 12 show the direction of the load on the frame and connecting rod for two different power end configurations. On vertical pumps with outboard packed stuffing boxes, the frame is in compression and the crosshead and connection rod are in tension. With horizontal, single-acting pumps, the frame is in tension, and the crosshead and connecting rod are in compression. The material selection and factors of safety must be altered accordingly.

Liquid Separation from the Plunger It has already been explained that fluid flows through the suction valve (and suction manifold and related piping) to fill the increasing cylinder volume caused by the withdrawal of the plunger. If the plunger accelerates faster than the flow of incoming liquid, the liquid will lose contact or separate from the plunger. The void that forms will be at a pressure lower than anywhere else in the cylinder. If the pumpage contains entrained gas, the gas may come out of a solution in this low-pressure area. The gas bubbles, when recompressed later in the plunger stroke, can cause cavita-tion damage to the plunger and surrounding cylinder.

The geometry of the slider-crank mechanism affects the point at which liquid separation occurs. As the ratio of the connecting rod length to the crank radius increases, the pump speed at which liquid separation occurs will decrease. Since liquid separation can be the determining factor in a pump's NPSHR, the pump designer must carefully evaluate the slider-crank geometry in order to optimize the pump's hydraulic performance. The pump speed at which water will separate from the end of the plunger can be calculated from the following:

hf = piping frictional loss, ft (m) As = area of suction pipe, in2 (mm2)

L = length of connecting rod, centerline to centerline, ft (m) R = crank radius, ft (m)

l = length of pipe where resistance to flow is to be measured, ft (m) Ap = area of plunger, in2 (mm2)

Unbalanced Forces Due to the relatively slow speed of a power pump, the inertia loads of the rotating/reciprocating parts are low enough to avoid the vibration problems associated with centrifugal pumps. For that reason, power pump crankshafts are not normally balanced. However, when the power pump is coupled to a drive containing a high-speed motor and gear reducer, a torsional analysis of the pump/drive unit may be required. For this analysis, the unbalanced forces of the rotating and reciprocating pump components can be calculated as follows:

Unbalanced Reciprocating Parts Force (Frec) Parts are typically about one-third of the connecting rod weight (the crosshead, the crosshead bearing, the wrist pin, the pony rod, and the plunger). Additional parts on vertical pumps include the pull rods, yoke, and plunger nut.

In USCS units:

In SI units:

where Frec = reciprocating parts force, lb (N)

W = weight of all reciprocating or rotating parts, lb (N)

L = length of connecting rod, centerline to centerline, ft (m) d = crank angle; usually maximum force is at d = 0°, cos d = 1

Unbalanced Rotating Parts Force (Flot) Parts are typically about two-thirds of the connecting rod weight, crank end bearing, and crankpin, where the variables are as above.

rot g

Mechanical Efficiency The mechanical efficiency of a single-acting power pump without internal gears is typically 90 to 92%. Over half of the mechanical losses are due to the frictional drag of the plungers through the packing. The remaining losses are from the bearings, the crosshead-to-crosshead guide friction, and the extension rod-to-gland seal friction. If these components are properly lubricated, very few power-end design options are available that will produce a measurable increase in mechanical efficiency. Decreasing the diameter of the plungers and minimizing the number of packing rings will result in a small efficiency increase.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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