Suction pipe friction

Hf = Friction head or friction losses expressed in feet in the suction piping and connections. 

This open system pumping water is at sea level (Figure 2-T). Therefore the Ha is 33.9 feet. The level in the tank is 15 feet above the pump centerline, so the Hs is 15 feet. The friction lo,s,ses in the suction piping give us 2 feet. The water is 70° F so the Hvp is 0.839. The Hi is a safety factor of 2 feet. 

Total suction pipe side friction loss ... 

For conditions of (1) high suction side (or inlet) friction loss, from suction piping calculations or (2) low available Net Positive Suction Head (10 feet or less), a large open eye on the impeller inlet is necessary to keep the inlet velocity low. NPSH is discussed in a later section. The manufacturer should be given the conditions in order to properly appraise this situation. 

When a pump is taking its suction from a tank, it should be located as close to the tank as possible in order to reduce the effect of friction losses on the NPSH available. Yet the pump must be far enough away from the tank to ensure that correct piping practice can be followed. Using a larger diameter line to limit the linear velocity to a level appropriate to the particular liquid being pumped can usually reduce pipe friction. Many industries work with a maximum velocity of about 5 feet per second, but this is not always acceptable. 

Pressure drops on the high-pressure side will be small enough to have little effect on the performance of the complete system. Pressure losses in the suction pipe and its fittings, especially if this is long, should be checked, and a correction made for the actual compressor suction pressure. For low-temperature applications, pipe sizes may have to be increased to avoid excessive frictional losses at these low pressures. 

The net head or pressure measured in ft. or m that causes a liquid to flow through the suction side of a pump, enter the pump chamber, and reach the impeller. When the source of liquid is above the pump, NPSH equals the barometric pressure plus the static head, less the entrance head, frictional losses in the suction piping and vapor pressure of the liquid. When the source of liquid is below the pump, NPSH equals the barometric pressure less the static head, entrance head, frictional losses in the suction piping and vapor pressure of the liquid. NPSH is specific for each pump design and application and must be supplied by the manufacturer. 

A well point system consists of several individual well points spaced at 0.6 m to 1.8 m intervals along a specified alignment. A well point is a well screen (length 0.5 to 1.0 m) with a conical steel drive point at bottom. Individual well points are attached to a riser pipe (diameter 2.5 to 7.5 cm) and connected to a header pipe (diameter 15 to 20 cm). At the midpoint, the header pipe is connected to a centrifugal pump. As yield at different well points may vary, a valve at the top of each riser pipe is used to control the drawdown so that the screen bottom is exposed. The pump provides 6 to 7.5 m of suction, but friction losses reduce the effective suction to 4.5 to 5.4 m. 

They lack sufficient available NPSH to overcome the frictional losses in the suction piping and the drain or draw nozzle. 

If I now open the discharge flow-control valve sufficient to increase the flow from 100 to 110 GPM, or by 10 percent, this would increase the frictional loss in the suction piping by about 21 percent, or about 0.5 psi, because AP varies with (flow)2. 

Lack of available NPSH may also be caused by high frictional loss in the suction piping. If this is the case, a small reduction in flow will not noticeably increase the pressure at the suction of the pump. A properly designed suction line to a centrifugal pump should have a frictional head loss of only a few feet of liquid. However, having a large-diameter suction line, and a relatively small draw-off nozzle, usually will lead to excessive loss of available NPSH. 

What is the maximum capacity of a double-suction condensate pump operating at 1750 r/min if it handles 100°F (311 K) water from a hot well in a condenser having an absolute pressure of 2.0 in Hg (6.8 kPa) if the pump centerline is 10 ft (3.05 m) below the hot-well liquid level and the friction-head loss in the suction piping and fitting is 5 ft (1.5 m) of water ... 

Compute the net positive suction head on the pump. The net positive suction head hn on a pump when the liquid supply is above the pump inlet equals pressure on liquid surface + static suction head-friction-head loss in suction piping and pump inlet-vapor pressure of the liquid, all expressed in feet absolute of liquid handled. When the liquid supply is below the pump centerline—i.e., there is a static suction lift—the vertical distance of the lift is subtracted from the pressure on the liquid surface instead of added as in the preceding relation. 

It adds to the friction and reduces the possible suction lift. Figure 29 shows a clack valve and strainer. Figure 30 shows one fitted with a number of valves. If the water carries impurities thau. damage to the pump a strainer should be provide, suction pipe. 

The ideal suction lift of a pump equals the water column sustained by the atmospheric pressure minus the vapor pressure the former is affected by the barometer and altitude, the latter by the temperature of the water. The hydraulic losses, which comprise the friction through the suction pipe, valves and... 

SUCTION LIFT - The combination of static suction lift and friction head in the suction piping when the source of liquid is below the pump centerline. 

To avoid cavitation, a pump needs net positive suction head (NPSH). This is the height of liquid required to ensure that the fluid is above its bubble-point pressure at the impeller eye. Assuming a pump needs 10 ft of NPSH to prevent cavitation, the minimum liquid level in the vessel the pump is taking suction from should be 12 ft above the impeller. The extra 2 ft is needed for piping friction head loss. 

The economics would depend upon the smoother flow of fluid without exce.ssive friction loss. A smaller section of pipe may not only require a higher h.p. for the same suction and lifting head due to greater frictional losses, but may also cause the pipe to deteriorate quickly as a result of the additional load on its surface. Losses due to bends ami valves should also be added in the total friction loss. 

It is important to recognize that a cenlrijugal pump will operate only along its performance curve [10, II]. External conditions will adjust themselves, or must be adjusted in order to obtain stable operation. Each pump operates within a system, and the conditions can be anticipated if each component part is properly examined. The system consists of the friction losses of the suction and the discharge piping plus the total static head from suction to final discharge point. Figure 3-51 represents a typical system head curve superimposed on the characteristic curve for a 10 by 8-inch pump with a 12-inch diameter impeller. 

S] = Friction losses for pipe valves and other system losses, suction side of pump R = Required by pump (NPSH) s = Suction side of pump... 

---