## 34 31 30 275

Slip Slip (S) is the loss of capacity due to internal and external pump leakage. External leakage occurs primarily through the stuffing box via the packing. Internal leakage is primarily the backflow past the suction and discharge valves. Backflow occurs when a valve remains open for a fraction of a second as the plunger or piston reverses direction. A small amount of leakage may occur across the piston in a double acting pump from the high pressure side to the low pressure side. Slip is expressed as a percentage loss of the suction capacity and is typically 1% to 4%:

B = Leakage through the stuffing box V = Backflow across the valves L = Internal leakages

Fluid viscosity, pump speed, and discharge pressure can all have an effect on slip, which is shown in Tables 1 and 2.

Mechanical Efficiency The mechanical efficiency of a power pump is

In USCS units ME = power out/power in = Pout/Pin = Q X Pdt/1714 X Pin

In SI units ME = power out/power in = Pout/Pin = Q X Pdt/36 X Pin where P^ is the input power from the driver, bhp (kW).

The mechanical efficiency of a power pump is the sum of all the frictional losses in the fluid and power ends. These include the plungers and packing, the crossheads, the rod seals, and the bearings. The efficiency of a single acting pump often exceeds 90%, while a double-acting piston pump will be 88% due to the additional piston and rod seals. If the pump is equipped with internal gearing, an additional 2% loss is common.

Most power pumps are designed to accept a range of plunger or piston sizes. When the larger plungers are used, the increased diameter of the packing/seals and plunger/liners will result in higher frictional losses than with smaller components. As a rule, doubling the plunger/piston diameter will decrease the mechanical efficiency by 8%. Mechanical efficiency is also affected by speed and, to a lesser extent, by developed pressure, as indicated in Tables 3 and 4.

Speed Pump speed, or, more correctly, stroke rate, is one of the most critical selection criteria for power pumps. The rotating and reciprocating parts of the power end, as designated, are often capable of speeds twice that of the actual pump rating. The maximum pump speed is determined by the design of the fluid end, the hydraulic capability of the anticipated suction system, and the required life of the plungers, packing, and valves. Most power pump standards limit the plunger speed from 140 to 280 ft/min (0.71 to 1.42 m/s). The plunger speed is

s = stroke of the pump, in (mm) (half of the linear distance the plunger or piston moves in one revolution)

All pumps have a minimum speed limit, usually determined by a decrease in the adequate lubrication to the bearings in the power end.

Volumetric Efficiency Volumetric efficiency (VE) is the ratio of the discharge volume to the suction volume, expressed as a percentage, plus the slip. It is proportional to the ratio r and the developed pressure where r is the ratio of the internal volume of fluid between valves when the plunger or piston is at the top of the peak of its back stroke (C + D) to the plunger or piston displacement (D) (see Figure 2).

TABLE 3 The effect of speed on mechanical efficiency at a constant developed pressure

% of full speed 44 50 73 100 ME, % 93.3 92.5 92.5 92.5

TABLE 4 The effect of pressure on mechanical efficiency at constant speed

% of full-load developed pressure 20 40 60 80 100

FIGURE 2 The ratio r (Flowserve Corporation)

FIGURE 2 The ratio r (Flowserve Corporation)

FIGURE 3 Volumetric efficiency (Flowserve Corporation) (lb/in2 X 0.69 = bar)

Since the discharge volume cannot be readily measured at discharge pressure, it is taken at suction pressure. Taking the discharge volume at the suction pressure results in a higher volumetric efficiency than using the calculated discharge volume at discharge pressure because of fluid compressibility. Compressibility becomes important when pumping water or other liquids over 6000 lb/in2 (414 bar), and it should be taken into consideration when determining the actual delivered capacity into the discharge system.

Figure 3 shows the approximate volumetric efficiency for water (not including slip). Based on the expansion back to suction capacity,

VE = (1 - Ptd X b X r)/(1 - Ptd X b) - S Based on discharge capacity,

## Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook