Liquid

The liquid end consists of the cylinder, the plunger or piston, the valves, the stuffing box, the manifolds, and the access covers. Figure 1 shows the liquid end of a horizontal pump, and Figure 2 shows the same for a vertical pump.

Cylinder (Working Barrel) The cylinder is the body where the pump pressure to overcome discharge pressure is developed. It is continuously under fatigue. Cylinders on many horizontal pumps have suction and discharge manifolds integral with the cylinder. Vertical pumps have separate manifolds.

A cylinder containing the passages for more than one plunger is referred to as a single cylinder. When the cylinder is used for one plunger, it is called an individual cylinder. Individual cylinders are used where developed stresses or replacement costs are high.

Cast cylinders are usually limited to the developed pressures listed in Table 1. Forged cylinders are made from 1020 and 4140 carbon steel; 304L, 316L, 17-4 PH, and 15-5 PH stainless steels; and nickel aluminum bronze. In recent years, duplex stainless steels have seen an increase in usage when higher strength and corrosion resistance is required.

FIGURE 1 A liquid-end horizontal pump (Flowserve Corporation)
FIGURE 2 A liquid-end vertical pump (Flowserve Corporation)

TABLE 1 Maximum pressure in cast cylinders

TABLE 1 Maximum pressure in cast cylinders

Forged cylinders have a 4:1 to 6:1 forging reduction on all sides of the cylinder to obtain a homogeneous internal structure. This type of cylinder requires heat treatment after forging to eliminate the stresses that occur during the forming operation. The pump designer must minimize the section thickness of the cylinder or residual stresses may remain after machining the internal bores.

The highest stresses occur at the intersection of the horizontal and vertical internal bores. Unlike a simple cylinder where the internal stresses are analyzed as a single hoop stress, the stress at the intersecting bores of a power pump cylinder are based on a double hoop stress. In Figures 3 and 4, the distances b and d are the internal diameters, a and c are the diameters to the nearest obstruction of the solid material of the cylinder, P is the developed pressure, and S is the resulting stress. Stress concentration factors at the intersecting bores can be omitted if the radius is not less than 0.25 in (1.5 mm). Above 3000 lb/in2 (207 bar), the internal bores should have a surface finish of 63 rms minimum.

During each plunger cycle, the developed pressure goes from suction pressure to discharge pressure and back to suction. For a pump operating at 360 strokes per minute, over six million fatigue cycles will occur in less than 12 days. For this reason, very conservative allowable stress limits are used. Normally, these limits range from 10,000 to 25,000 lb/in2 (69 to 172 Mpa), depending on the material of the cylinder, duty cycle, and liquid being

FIGURE 3 Stress dimensions for a horizontal FIGURE 4 Stress dimensions for a vertical liquid liquid end (Flowserve Corporation) end (Flowserve Corporation)

FIGURE 3 Stress dimensions for a horizontal FIGURE 4 Stress dimensions for a vertical liquid liquid end (Flowserve Corporation) end (Flowserve Corporation)

pumped. The allowable stress is a function of the fatigue stress of the material for the liquid being pumped and the life cycles required.

Due to improper suction system design, misapplications, or process upsets, instantaneous pressure in the cylinder may be much higher than design pressure. When the liquid contains entrained gas that can be released because of inadequate suction pressure, the resulting cavitation can cause instantaneous pressure four to five times the design pressure. This results in reducing the life of the cylinder and other liquid-end components, as well as damaging pressure pulsations in the suction and discharge piping systems.

For continuous duty pumps operating above 10,000 lb/in2 (690 bar) of discharge pressure, special liquid-end designs have been developed. In Figure 5, the intersecting bores have been eliminated by arranging the suction and discharge valves on the same axis as the plunger. This means the stresses in the cylinder will be only half of those in a comparable T-block design.

Plungers The plunger transmits the force that develops the pressure. It is normally a solid construction of up to 5 in (127 mm) in diameter. Above that dimension, it may be made hollow to reduce its weight. Small-diameter plungers used for 6000 psi (414 bar) and above should be reviewed for possible buckling. Plunger speeds range from 150 to 350 fpm (46 to 107 m/s). The surface finish normally is between 14 and 20 rms. A finish below 8 rms should be avoided because excessive packing leakage may occur due to the inability of the packing to seal properly on the smooth surface.

Although some plungers are made of heat-treated or case-hardened steel, the most common are the hard-coated or solid ceramic. The hard coatings are normally flame sprayed powders of Colmonoy or tungsten carbide, or ceramic oxides. These can be applied over base materials of 1020 carbon steel or 316L stainless steel. Ceramic oxide is normally limited to 200°F (93°C) and is used for soft water, crude oil, mild acids, and mild alkalis.

The porosity and bond strength of coatings must be carefully evaluated for use with higher operating pressures. Under those conditions, the liquid may penetrate the pores of the coating and lift the coating off the base material. Ceramic plungers also have special requirements and limitations. This type of plunger is often constructed as a solid bar, closed-end tube of ceramic, bonded to a metal end cap or plug. A vent must be provided to allow the pressure inside the plunger to equal the atmospheric pressure or the ceramic-to-metal bond may fail or the plunger may explode. In addition to the normally fragile nature of any ceramic plunger, the solid ceramic plunger is susceptible to failure due to thermal shock.

Pistons Pistons are used for water pressures up to 2000 lb/in2 (138 bar). For higher pressures, a plunger is usually used. Pistons are cast iron, bronze, or steel with reinforced elastomer sealing rings (see Figure 6). They are most frequently used in duplex, double-acting pumps. The latest trend is to use pistons in single-acting triplex pumps.

FIGURE 5 Special high-pressure liquid-end designs arrange the suction and discharge valves on the same axis as the plunger, thus eliminating intersecting bores (Flowserve Corporation).

REINFORCED ELASTOMER' LINER

FIGURE 6 Elastomer face piston (FWI)

REINFORCED ELASTOMER' LINER

FIGURE 6 Elastomer face piston (FWI)

Stuffing Box The stuffing box assembly consists of a stuffing box, upper and lower bushings, packing, and a gland. For ease of maintenance, the stuffing box assembly is usually removable (see Figure 7).

The stuffing box bore is machined to a 63 rms finish to ensure packing sealing and life. A single hoop stress equation is used to determine the wall thickness with an allowable stress limit of 10,000 to 20,000 lb/in2 (69 to 138 MPa).

The bushings that guide the plunger have a 63 rms finish with approximately 0.001 to 0.002 in (0.02 to 0.05 mm) of diametrical clearance per inch of the plunger diameter. The

FIGURE 7 A stuffing box (Flowserve Corporation)

FIGURE 7 A stuffing box (Flowserve Corporation)

FIGURE 8 Chevron packing (Flowserve Corporation)

FIGURE 8 Chevron packing (Flowserve Corporation)

FIGURE 9 Single-acting piston stuffing box (Flowserve Corporation)

lower bushing is sometimes secured in an axial position to prevent movement of the packing. Bushings are made of bearing bronze, Ni-resist, or cast iron. Stainless steel can be used if its antigalling properties with the plunger coating can be verified.

Packing is either square cross-section, woven construction or molded V or chevron-shaped (see Figure 8). Some types use metal backup adapters. A packing set consists of top and bottom adapters and one or more packing sealing rings. A stuffing box can use two to five rings of packing, depending on the pressure and the fluid being pumped. Packing rings are usually made up of a number of composite materials selected for their strength, wear resistance, and lubricity, including neoprene, Teflon, cotton duct, and Kevlar.

Packing can be made self-adjusting by installing a spring between the bottom of the packing set and the lower bushing or bottom of the stuffing box. This arrangement eliminates overtightening the packing and enables a more uniform break-in. Packing can be lubricated through a grease fitting or by an auxiliary lubricator driven through a take-off on the crankshaft.

For chemical or slurry service, a lower injection ring is used for flushing. This prevents concentrated pumped fluid from impinging directly on the packing. This injection can be continuous or synchronized to inject only on the suction stroke. Flush glands at the outboard end of the stuffing box are employed when a toxic vapor is present or when the leakage may flash.

The stuffing box for the piston rod of a double-acting pump is similar in construction to that of a stuffing box for a plunger. The primary difference is that it must seal against pressure being developed while the rod moves back through the packing. Single-acting pistons do not employ a stuffing box. Leakage past the piston-sealing rings goes into the frame extension to mix with the continuously circulating lubricant.

Cylinder Liner The cylinder wear liner (refer to Figure 6 and see Figure 9) is usually of Ni-resist material. Its length is slightly longer than the stroke of the pump to enable an assembly entrance taper of the piston into the liner. In double-acting pumps, the liner has packing to prevent leakage from the high-pressure side to the low pressure side of the cylinder. Because of the brittleness of the liner, the construction should be such that the liner is not compressed. The finish of the bore of the liner is approximately 16 rms.

Valves Many types of valve designs exist. Which type is used depends on the application. The main parts of a valve assembly are the seat and sealing member, usually a disc, ball, or plate. The plate movement is controlled by a spring or retainer. The seat usually uses a taper where it fits into the cylinder or manifold. The taper not only gives a positive fit but permits easy replacement of the seat.

Some pumps use the same size suction and discharge valve for interchangeability (refer to Figure 2). Some use larger suction valves than discharge valves for improved NPSH reasons. Others use larger discharge valves than suction valves because the cylinder configuration requires the suction valve to be removed through the location of the discharge valve (refer to Figure 1). Because of space considerations, valves are sometimes used in clusters on each side of the plunger to obtain the required total valve area that reduces valve velocity.

Table 2 shows seat and plate hardness for some valve materials. Seats and plates made of 316 stainless steel material are usually chrome-plated or flame-sprayed to give them the desired surface hardness.

The flow area of the valve must be large enough to prevent significant pressure drop or restriction to flow. Normally, suction valves are sized for 6 to 8 ft/s velocity, and the discharge valves and sized for 8 to 12 ft/s.

The valve springs must be made of corrosion-resistant material and designed to withstand high-cycle fatigue stresses. Under no situation should the valve operation enable the spring to go to a solid stack height or wire-to-wire condition. The ends of the spring must be ground flat and square with a maximum perpendicularity of 3 degrees.

Manifolds These are the chambers where the liquid is disbursed or collected for distribution before or after passing through the cylinder. In horizontal pumps and some vertical pumps, the manifolds are cast or machined integral with the fluid cylinder. Most vertical pumps have the suction and discharge manifolds separate from the cylinder (refer to Figure 2).

Suction manifolds are designed to eliminate air pockets from the flange to the valve entrance (see Figure 10). Separate suction manifolds are cast iron, cast bronze, or fabricated steel. Discharge manifolds are steel forgings or fabricated steel. The manifolds have a minimum deflection to prevent gasket shifts when subjected to maximum operating pressures.

The velocity through the manifolds of a clean liquid is 3 to 5 ft/s (0.9 to 1.5 m/s) at the suction and 6 to 16 ft/s(1.8 to 4.9 m/s) at the discharge. Suction and discharge manifold velocities in a slurry service are 6 to 10 ft/s (1.8 to 3 m/s). Slurry services have a minimum velocity of 6 ft/s (1.8 m/s) to prevent the heavier slurry particles from falling out of solution.

In USCS units, V (ft/s) = pump gpm X 0.321/cross-sectional area of the manifold, in2 In SI units, V (m/s) = pump m3/h X 277.8/cross-sectional area of the manifold, mm2

TABLE 2 Recommended material hardness for valve plate and seat

Material

Plate

Seat

Rockwell C hardness

Renewable Energy Eco Friendly

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.

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