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The Importance Of Best Efficiency Point (BEP)

Understanding the factors involved in pump performance is key to optimizing the fluid-handlingsystems in your operations.

ByEugene Vogel, Electrical Apparatus Service Association (EASA)

Have you become so focused on the efficiency of the motors around your plant that youre losing sight of the equipment thosemotors are driving? In many applications, pumps included, the answer to the efficiency question is best addressed when thecomplete system is studied.

Sooner or later, most maintenance professionals who work with pumps will encounter a pump curve and its key parameters, oneof which is Best Efficiency Point (BEP). The BEP graphically represents the point on a pump curve that yields the most efficientoperation. For electric motors, efficiency varies with load, with the best efficiency being at about 75% of load. With rotodynamicpumps (which includes centrifugal and axial flow types), efficiency depends on three important pump curve parametershead,flow (i.e., capacity or volume) and poweras expressed in this simple equation:

3027 6:18 pm May 15, 2013

As the equation shows, power is inversely proportional to efficiency, which basically means pumps use less power when operatingmore efficiently. But power is also directly proportional to flow X head (Q X H), both of which vary with demand in a rotodynamicpump system. If the system restricts the discharge of the pump, as when a discharge throttle valve is closed, the flow decreasesand the head increases. Conversely, less restriction from the system means greater flow and less head. This relationship isillustrated by a pump curve that is specific to each pump (see Fig. 1).

To understand BEP, it is essential to know that the flow through a rotodynamic pump varies from zero flow at dead head(discharge valve closed) to maximum flow at run out condition (no discharge restriction). Pump efficiency, it turns out, is afunction of flow through the pump, although it is not strictly linear (see Fig. 2).

Effect of flow rateTo visualize how flow affects pump efficiency, imagine the flow of traffic on a highway, with efficiency measured as cars perminute. Late at night with no cars on the road (and therefore no traffic), efficiency is zero. Early in the morning, traffic movesquickly, but with few cars traveling, efficiency remains low. During rush hour traffic volume greatly increases, so bottlenecks form,traffic slows to a crawl and efficiency plummets. Usually, there is a time just before rush hour with lots of fast-moving traffic whenthe highway handles the most cars per minutei.e., its BEP.

The BEP for a pump is similar (see Fig. 3). With the discharge valve closed (dead head) and zero flow, efficiency is zero. As thedischarge valve opens (i.e., the discharge restriction is gradually reduced), flow and efficiency gradually increase, until the flowthrough the pump becomes more turbulent. At that point, efficiency will start dropping and then continue to drop as the pumpapproaches run out condition (zero). As with traffic flow on a busy highway, somewhere between dead head and run outcondition, there is a flow rate at which the efficiency is maximumi.e., the BEP.

Note that the BEP in Fig. 3 occurs at a flow rate of about 1600 unitswhich coincides with the maximum value on the efficiency curve.That flow rate also intersects the pump curve at a point equal to head of about 220 units.

If the efficiency of the pump changes with flow rate, a logical question might be Why? As mentioned earlier, one reason is thatpump efficiency directly correlates with turbulence in the flowi.e., the greater the turbulence, the lower the efficiency. Thus, it makessense that the BEP is where turbulence is minimal.

Effect of impeller designImpeller design is the most significant factor for determining the BEP of a pump because it dictates how efficiently power (brakehorsepower or BHP) is transmitted to the liquid being pumped (pumpage). A properly designed impeller optimizes flow whileminimizing turbulence.

Pumpage enters the impeller eye and accelerates as it travels radially outward toward the impeller discharge. As theliquiddischarges from the impeller, it merges with liquidalready in the impeller housing. If the impeller vanes are at just the right anglerelative to the flow rate, incoming pumpage will merge smoothly with the swirling pumpage in the housing, minimizing turbulence,maximizing efficiency and yielding the BEP for that impeller.

Designers use a series of vectors to calculate impeller vane angle for a certain flow rate. As shown in Fig. 4,vector V representsthe speed of the vane tip (tangent andrelative to the impeller), and V represents the radial velocity of the pumpage flowing out ofthe impeller. The discharge angle of the flow is V , the sum of vectors V and V , which should match the impeller vane angle atthe discharge. The length of vector V changes with flow rate, so greater flow through the pump means the pumpage must movefaster as it exits the impeller.

t

r

m t r

r

Note that the flow rate changes the discharge angle, but the impeller vane angle remains constant. The BEP is the flow rate wherethe discharge angle matches the vane angle. Similar design factors apply to the impeller intake. Although impeller housingcharacteristics also play a role, the impeller design is the primary factor that determines the flow rate at which the BEP occurs.

Points to rememberThe most important thing to remember from this discussion is that any modification of the impeller will change the BEP of thepump. Trimming the outside diameter (OD) of an impeller, replacing an impeller with one of different diameter or number of vanesor changing the rotating speed of the impeller will alter the BEP for the pump.

Before modifying an impeller in any way, make sure that you determine how the change will impact the pump curve, the efficiencycurve and the BEP. UM

Eugene Vogel is a Pump and Vibration Specialist with the Electrical Apparatus Service Association, Inc. (EASA), in St. Louis, MO. EASA isan international trade association of more than 1900 firms in 59 countries that sell and service electrical, electronic and mechanicalapparatus. Telephone: 314-993-2220; email: easainfo@easa.com; Web: www.easa.com.

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