Pumps & Best Efficiency Point

There are four important locations on any head/flow (H-Q) pump curve.

The operating point (in blue). This is the point at which the system curve intersects the H-Q curve.

The shut-off point (in red). This is the point at which the flow shuts off. The H-Q curve intersects the Y-axis.

The run-out point (in green). This is the point at which the flow is at its maximum. The H-Q curve intersects the X-axis.

The Best efficiency point (in yellow). (BEP). This is the point at which “power in” is closest to the “power out”. (Normally, this is between 70% and 85% of the shut-off head.)

Today, we will concentrate our discussion on the latter of these four points, the BEP. A pump’s best efficiency point (B.E.P.) is defined by the point at which power input is closest to power output, so let’s first talk about power loss.

Energy Loss

The primary loss within the pump is due to the recirculation that occurs due to the “designed in clearance” between the impeller and the cutwater. All pump designers must leave some space to avoid mechanical contact and to minimize the effect of “vane passing syndrome” ( I will get to VPS in just a minute). The larger the space, the more recirculation within the pump and less fluid power available at the discharge flange ( the energy does not just disappear, it is lost in the form of heat in the fluid).

Note: Slurry pump designers have to leave additional room to minimize the chance of a solid becoming wedged in the opening. As a result slurry pumps may be as much as 40% less efficient than a similar sized water pump.

Vane Passing Syndrome

I know today’s subject is BEP, but vanishing passing syndrome or VPS, is closely related as it is also about cutwater clearance. Briefly put, VPS occurs when the OD of the impeller vanes pass too close to the discharge neck or the cutwater of the pump casing.

It should not be an issue for properly designed and unmodified pumps. However, if aftermarket oversized impellers are installed or coatings are applied to either the impeller or casing, it may become an issue.

The decrease in clearance between the impeller and the cutwater creates turbulence and cavitation. Damage is very similar to that caused by operating close to shut off, but can occur at rated BEP.

Back to Energy Loss

Secondary power losses within a pump occur mainly as a result of mechanical and or hydraulic friction. Manufacturers strive to design their pumps to have minimal losses at the rated flow. For example, if a manufacture wants to market a 6 inch pump with a nominal flow of 900 usgpm, he will adjust the internals so that both mechanical and hydraulic friction is at its minimum when the pump is flowing 900 usgpm.

The internal adjustments center around; impeller vane number and shape, impeller to casing clearance, bearing configuration and cutwater angles size and shape. A successful design is one in which hydraulic friction in both the casing and impeller is minimized. Radial load should also be minimized to reduce mechanical friction.

Pump Selection

Selecting a pump that will operate close to, or at BEP, obviously saves energy, but it also reduces operating expenses as pumps run at BEP wear more slowly and have increased reliability. It is unrealistic to expect every application to be matched with a pump running right on BEP, so how close is it enough?

Well, most engineering houses will immediately disregard submissions for pumps quoted to operate at below 50% of rated BEP or over 120% of BEP.

There are several reasons for this approach. In addition to the wasted power, increased wear, and higher radial loads, it can indicate that the selected pump is sized incorrectly for the desired flow. A pump selected to operate at below 50% of BEP can usually be replaced by a pump one size smaller, saving capital dollars. ( The smaller pump may also offer a more stable flow for flat system curves.)

Pumps selected at over 120% of BEP are simply too small and are being pushed to the limit resulting in premature wear at the cutwater and high radial loads.