SIZING CENTRIFUGAL PUMPS

Many factors affect performance of a centrifugal pump and must be considered during pump selection. This chapter describes conditions that affect the centrifugal pump and is followed by details that will assist in eliminating negative conditions that cause pump failure.

During rig design, the centrifugal pump is often considered a low-cost product that does not warrant a great deal of engineering consideration. Many times the centrifugal is sized and ordered based on existing packages utilized on other rigs. This can cause serious problems because each rig has unique operating conditions and piping designs. Centrifugal pumps are used for a variety of applications and feed other, much more expensive equipment. If centrifugal pumps are not properly sized, they and other equipment can be adversely affected. Proper sizing, design, and installation of centrifugal pumps can directly affect the efficiency and operating cost of the rig.

Centrifugal pumps are available in a variety of materials, configurations, sizes, and designs. Normally, a single size, configuration, and speed can be selected to best meet the intended application. Accurate centrifugal pump selection can occur only with knowledge of system details. It is imperative to obtain accurate information such as fluid temperature, specific gravity, pipe diameter, length of pipe, fittings, elevations, flow required, head required at end of transfer line, type of driver required, and type of power available. Without all of this information, assumptions have to be made that could cause pump failure, high maintenance costs, downtime, and/or improper performance.

Standard Definitions

Terms associated with centrifugal pumps have been defined in a variety of ways, but for our purpose the terms will be referenced as defined below:

• Flow rate: volume of liquid going through a pipe in a given time. If a hose stream will fill a 10-gal bucket in 2 minutes, then the flow rate is 5 gpm. The bottom axis of most pump curves is flow rate and is usually measured in gallons per minute or cubic meters per hour (m3/hr).

• Friction loss: resistance to movement of fluid within the pipe, or head loss caused by turbulence and dragging that results when fluid comes into contact with the ID of pipe, valves, fittings, etc. This value is
  normally measured in feet per 100 feet of pipe and is noted in Tables
   18.1 through 18.14 under the column heading ‘‘Friction Loss in Feet Head per 100 Ft of Pipe.’’ These tables are based on Schedule (SCH)-40 new steel pipe. Other piping material or older scaled or pitted pipe will have higher friction losses. Consult engineering handbooks for piping other than SCH 40 new steel pipe.

• Head: distance, in feet, that water will rise in an open-ended tube connected to the place where the measurement is to be taken. Throughout
  this chapter, head is measured in terms of distance (ft) and not pressure (psi, for example). Units of psi vary with the weight of the fluid, but head in feet or meters is constant regardless of fluid weight.

• Net positive suction head available (NPSHA): amount of head that will exist at the suction flange of the pump above absolute zero. Friction losses, atmospheric pressure, fluid temperature/vapor pressure, elevation, and specific gravity affect this value. This value must be calculated.

• Net positive suction head required (NPSHR): amount of inlet head above absolute zero required by the centrifugal pump to operate properly. This value varies with pump size and flow rate and is normally represented on the pump curve.

• Total differential head (TDH): amount of head produced by a centrifugal pump in excess of pump inlet head. This value is found on the left axis of most pump curves.

• Total discharge head: sum of the inlet head þ total differential head of a centrifugal pump, measured in feet or meters.

• Velocity (V ft/sec): refers to the average speed that liquid travels through a pipe. Velocity is measured in feet per second (ft/sec). Velocities measured in ft/sec can be found in here(Centrifugal Pump Velocities measure.pdf).  where one can look up the velocity for recommended flow rates in a pipe size or the flow rate in a pipe for any velocity desired. For example, 10 ft/sec in an SCH 40 4-inch-diameter pipe will flow 400 gpm.

Head Produces Flow

Most water used at home and in industry comes from tank towers or standpipes. Figure 1. shows water flowing through a straight pipe of constant diameter lying on level ground. If a clear sight tube is installed on the pipe near the open end, it can be used to measure head at that place. By closing the end of the pipe, flow will stop, and the water in the sight tube will rise to a level equal to that of the standpipe. When the end of the pipe is opened fully, the flow will be the most the standpipe head can deliver, and the water in the sight tube will drop to the bottom. Almost all of the head is consumed while pushing the water through the pipe and overcoming friction.

The velocity head is used first, to speed the water from a standstill in the standpipe up to the velocity in the pipe as it enters. The velocity head (amount in ft) depends on only the velocity of the flow (in ft/sec), not on diameter or gpm. It is the same amount at any point in the pipe (constant diameter) even to the open end, where it shows as the strength of the flow stream. It is usually small, 3–6% of the total head for pipes of 100 feet or more in length. It is shown in the friction loss tables (Tables 18.1–18.14) under the column headed ‘‘V²/2g.’’

A sight tube installed somewhere near the halfway point will show the (pressure) head remaining at that point that pushes the water on to the end. The difference of the height in the sight tube from the height at shutoff is the velocity head plus the friction loss from the standpipe to that point.

The use of a standpipe to supply fluid for pipe friction problems is the clearest way to demonstrate how pipe friction tables are made. While a standpipe illustration explains the system head and flow, it is not a practical method of producing head in most applications, and pumps are substituted for standpipes. Pumps can be sized to produce the proper amount of head to achieve the desired flow rate and overcome the friction losses and elevation in a system.