Have you become so focused on the efficiency of the motors around your plant that you’re losing sight of the equipment those motors are driving? In many applications, pumps included, the answer to the efficiency question is best addressed when the complete system is studied.
Sooner or later, most maintenance professionals who work with pumps will encounter a pump curve and its key parameters, one of which is Best Efficiency Point (BEP). The BEP graphically represents the point on a pump curve that yields the most efficient operation. For electric motors, efficiency varies with load, with the best efficiency being at about 75% of load. With rotodynamic pumps (which includes centrifugal and axial flow types), efficiency depends on three important pump curve parameters—head, flow (i.e., capacity or volume) and power—as expressed in this simple equation:
As the equation shows, power is inversely proportional to efficiency, which basically means pumps use less power when operating more efficiently. But power is also directly proportional to flow X head (Q X H), both of which vary with demand in a rotodynamic pump system. If the system restricts the discharge of the pump, as when a discharge throttle valve is closed, the flow decreases and the head increases. Conversely, less restriction from the system means greater flow and less head. This relationship is illustrated 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 a function of flow through the pump, although it is not strictly linear (see Fig. 2).
Effect of flow rate
To visualize how flow affects pump efficiency, imagine the flow of traffic on a highway, with efficiency measured as cars per minute. Late at night with no cars on the road (and therefore no traffic), efficiency is zero. Early in the morning, traffic moves quickly, 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 when the highway handles the most cars per minute—i.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 the discharge valve opens (i.e., the discharge restriction is gradually reduced), flow and efficiency gradually increase, until the flow through the pump becomes more turbulent. At that point, efficiency will start dropping and then continue to drop as the pump approaches “run out” condition (zero). As with traffic flow on a busy highway, somewhere between “dead head” and “run out” condition, there is a flow rate at which the efficiency is maximum—i.e., the BEP.
Note that the BEP in Fig. 3 occurs at a flow rate of about 1600 units—which 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 that pump efficiency directly correlates with turbulence in the flow—i.e., the greater the turbulence, the lower the efficiency. Thus, it makes sense that the BEP is where turbulence is minimal.
Effect of impeller design
Impeller design is the most significant factor for determining the BEP of a pump because it dictates how efficiently power (brake horsepower or BHP) is transmitted to the liquid being pumped (“pumpage”). A properly designed impeller optimizes flow while minimizing turbulence.
Pumpage enters the impeller eye and accelerates as it travels radially outward toward the impeller discharge. As the liquid discharges from the impeller, it merges with liquid already in the impeller housing. If the impeller vanes are at just the right angle relative 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 Vt represents the speed of the vane tip (tangent and relative to the impeller), and Vr represents the radial velocity of the pumpage flowing out of the impeller. The discharge angle of the flow is Vm, the sum of vectors Vt and Vr, which should match the impeller vane angle at the discharge. The length of vector Vr changes with flow rate, so greater flow through the pump means the pumpage must move faster as it exits the impeller.
Note that the flow rate changes the discharge angle, but the impeller vane angle remains constant. The BEP is the flow rate where the discharge angle matches the vane angle. Similar design factors apply to the impeller intake. Although impeller housing characteristics also play a role, the impeller design is the primary factor that determines the flow rate at which the BEP occurs.
Points to remember
The most important thing to remember from this discussion is that any modification of the impeller will change the BEP of the pump. Trimming the outside diameter (OD) of an impeller, replacing an impeller with one of different diameter or number of vanes or 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 efficiency curve and the BEP. UM