Evaluación de la ejecución del test y de los resultados obtenidos

Pump definitions and terminology, as given in Hydraulics Institute (HI) 1.1–1.5–1994 (Hydraulics Institute, 1994), are as follows:

Definition of a centrifugal pump. A centrifugal pump is a kinetic machine converting mechanical energy into hydraulic energy through centrifugal activity.

Allowable operating range. This is the flow range at the specified speeds with the impeller supplied as limited by cavitation, heating, vibration, noise, shaft deflection, fatigue, and other similar criteria. This range to be defined by the manufacturer.

Atmospheric head (h_atm). Local atmospheric pressure expressed in ft (m) of liquid.

Capacity. The capacity of a pump is the total volume throughout per unit of time at suction conditions. It assumes no entrained gases at the stated operating conditions.

Condition points

• Best efficiency point (BEP). The BEP is the capacity and head at which the pump, efficiency is a maximum.

• Normal condition point. The normal condition point applies to the point on the rating curve at which the pump will normally operate. It may be the same as the rated condition point.

• Rated condition point. The rated condition point applies to the capacity, head, net positive suction head, and speed of the pump, as specified by the order.

• Specified condition point. The specified condition point is synonymous with the rated condition point.

Datum. The pump’s datum is a horizontal plane that serves as the reference for head measurements taken during a test. Vertical pumps are usually tested in an open pit with the suction flooded. The datum is then the eye of the first-stage impeller (Fig. 5.1).

Optional tests can be performed with the pump mounted in a suction can. Regardless of the pump’s mounting, its datum is maintained at the eye of the first-stage impeller,

Elevation head (Z). The potential energy of the liquid caused by its elevation relative to a datum level measuring to the center of the pressure gauge or liquid surface.

Friction head. Friction head is the hydraulic energy required to overcome frictional resistance of a piping system to liquid flow expressed in ft (m) of liquid.

Gauge head (h_g). The energy of the liquid due to its pressure, as determined by a pressure gauge or another pressure-measuring device.

Head, Head is the expression of the energy content of the liquid referred to any arbitrary datum. It is expressed in units of energy per unit weight of liquid. The measuring unit for head is ft (m) of liquid.

High-energy pump. High-energy pump refers to a pump with a head greater than 650 ft (200 m) per stage and requiring more than 300 hp (225 KW) per stage.

Impeller balancing

• Single-plane balancing (also called static balancing). Single-plane balancing refers to correction of residual imbalance to a specified maximum limit by removing or adding weight in one correction plane only. This can be accomplished statically using balance rails or by spinning.

• Two-plane balancing (also called dynamic balancing). Two plane-balancing refers to correction of residual imbalance to a specified limit by removing or adding weight in two correction planes. This is accomplished by spinning on appropriate balancing machines.

Overall efficiency (␩OA). This is the ratio of the energy imparted to the liquid (P_w) by the pump to the energy supplied to the (P_mot): that is, the ratio of the water horsepower to the power input to the primary driver expressed as a percentage.

Power

• Electric motor input power (P_mot). This is the electrical input power to the motor.

• Pump input power (P_p). This is the power delivered to the pump shaft at the driver to pump coupling. It is also called brake horsepower.

FIGURE 5.1 Terminology for a pump with a positive suction head.

• Pump output power (P_w). This is the power imparted to the liquid by the pump. It is also called water horsepower.

(5.1)

(5.2)

where Q=flow in gal/min (U.S.) or m³/hr (SI), H=head in feet (U.S.) or meters (SI), S=specific gravity and P_w=power in a horsepower (U.S.) or kilowatt (SI)

• Pump efficiency (ηp). This is the ratio of the energy imported to the liquid (P_w) to the energy delivered to the pump shaft (P_p) expressed in percent.

Pump pressures

• Field test pressure. The maximum static test pressure to be used for leak testing a closed pumping system in the field if the pumps are not isolated. Generally this is taken as 125 percent if the maximum allowable casing working pressure. In cases where mechanical seals are used, this pressure may be limited by the pressure-containing capabilities of the seal.

Note: See “Maximum allowable casing working pressure” below, consideration of which may limit the field test pressure of the pump to 125 percent of the maximum allowable casing working pressure on the suction split-case pumps and certain other pump types.

• Maximum allowable casing working pressure. This is the highest pressure at the specified pumping temperature for which the pump casing is designed. This pressure shall be equal to or greater than the maximum discharge pressure. In the case of some pumps (double suction, vertical turbine, axial split case can, or multistage, for example), the maximum allowable casing working pressure on the suction side may be different from that on the discharge side.

• Maximum suction pressure. This is the highest suction pressure to which the pump will be subjected during operation.

• Working pressure. (p_d). This is the maximum discharge pressure that could occur in the pump, when it is operated at rated speed and suction pressure for the given application.

Shut off. This is the condition of zero flow where no liquid is flowing through the pump, but the pump is primed and running.

Speed. This is the number of revolutions of the shaft in a given unit of time. Speed is expressed as revolutions per minute.

Suction conditions

• Maximum suction pressure. This is the highest suction pressure to which the pump will be subjected during operation.

• Net positive suction head available (NPSH_A). Net positive suction head available is the total suction head of liquid absolute, determined at the first-stage impeller datum, less the absolute vapor pressure of the liquid at a specific capacity:

(5.3)

where (5.4)

or (5.5)

• Net positive suction head required (NPSH_R). This is the amount of suction head, over vapor pressure, required to prevent more than a 3 percent loss in total head from the first stage of the pump at a specific capacity.

• Static suction lift (I_s). Static suction lift is a hydraulic pressure below atmospheric pressure at the intake port of the pump.

• Submerged suction. A submerged suction exists when the center line of the pump inlet is below the level of the liquid in the supply tank.

• Total discharge head (h_d). The total discharge head (h_d is the sum of the discharge gauge head (h_gd) plus the velocity head (hv_d) at the point of gauge attachment plus the elevation head (Z_d) from the discharge gauge center line to the pump datum:

(5.6)

• Total head (H). This is the measure of energy increase per unit weight of the liquid imparted to the liquid by the pump and is the difference between the total discharge head and the total suction head. This is the head normally specified for pumping applications since the complete characteristics of a system determine the total head required.

• Total suction head (h_s), closed suction test. For closed suction installations, the pump suction nozzle may be located either above or below grade level.

• Total suction head (h_s), open suction. For open suction (wet pit) installations, the first stage impeller of the bowl assembly is submerged in a pit. The total suction head (h_s) at datum is the submergence (Z_w). If the average velocity head of the flow in the pit is small enough to be neglected, then:

(5.7) where Z_w=vertical distance in feet from free water surface to datum.

The total suction head (h_s) referred to the eye of the first-stage impeller is the algebraic sum of the suction gauge head (h_vs) plus the velocity head (h_vs) at the point of gauge attachment plus the elevation head (Z_s) from the suction gauge center line (or manometer zero) to the pump datum:

(5.8) The suction head (h_s) is positive when the suction gauge reading is above atmospheric pressure and negative when the reading is below atmospheric pressure by an amount exceeding the sum of the elevation head and the velocity head.

Velocity head (h_v). This is the kinetic energy of the liquid at a given cross section.

Velocity head is expressed by the following equation:

(5.9)

where v is obtained by dividing the flow by the cross-sectional area at the point of gauge connection.