How To Read A Pump Curve – Part 1
If a picture is worth a thousand words then a pump curve must be worth several thousand. Make no mistake, there’s a lot of information on a centrifugal pump curve like the one shown here. It’s no wonder that many newly practicing engineers are a little l ittle intimated by them. But a solid understanding of pump curves is absolutely necessary for anyone involved in the specification of hydronic equipment. In this blog, we’re simply going to cover the elements of a pump curve -- more accurately referred to as a “pump performance curve”-- and explain why these elements are relevant to the selection of a pump. First, it is important to understand that manufacturer’s publish curves for every pump they make. These curves, which are the result of many hours hours of factory or laboratory laboratory testing, tell us how a pump curve will perform under a given set of conditions. Thus, they help us decide which pump is the best selection for a given application. Here are the main elements of a pump performance curve: Total Head and Pump Flow Capacity. These are the values that run on the vertical and horizontal axis of the graph.
The total head, shown on the y-axis, is measured in feet and tells us what sort of pressure a given pump is capable overcoming. Hopefully, the engineer will have calculated the total system head (or pressure) the pump will need to overcome in order to get the water where it needs to go. This number is the sum of all resistance values, such as friction head, static head, etc. – virtually any pressure that the pump must overcome to achieve a specific flow. The pump flow capacity, shown on the x-axis of the pump performance curve, represents the gallons per minute (gpm) that the pump will pump at any given point during its operation. The corresponding values of the flow capacity and head are plotted on the main curve on the graph at various impeller sizes. This curve tells us exactly how much gpm a pump will deliver under the various total head conditions.
Pump efficiency curves. We consult pump curves not only to see if they can do the work we need them to do – but if they will do it efficiently . Fortunately, manufacturers include this information on the curves as well. These lines intersect with the head-capacity curves and are typically labeled with percentages. Thus, the Bell & Gossett Series 1510 pump curve shown here tells us that this pump with an 8.5” impeller will deliver 600 gpm and a total head of 57 feet at 84% efficiency. It’s clear by looking at the curve that a pump’s efficiency varies with flow and head. Ideally, you will choose a pump that operates slightly to the left of the Best Efficiency Point (BEP) because typically the pressure in the system is less than what has been calculated. ASHRAE recommends a preferred selection range of 85% to 105% of BEP. Impeller size. Impeller size or “trim” greatly impacts the pump’s performance. The impeller, of course, is the moving element inside the pump volute, which drives the liquid. The pump performance curve shows the performance of a given pump with multiple size impellers, so the engineer can specify not only the best pump but the best impeller selection as well. Brake Horsepower. Brake horsepower (BHP) signifies the amount of horsepower required to operate the pump at any given point along the performance curve. These are the straight and sometimes broken lines that typically slope downward from left to right. Brake horsepower varies along with the impeller trim. Required Net Positive Suction Head (NPSHR). Every pump requires a certain amount of pressure at the suction to operate while delivering the head and flow values shown on its performance curve. If the pump does not have sufficient NPSHR, not only will it not
perform as designed but cavitation could also result. Cavitation can cause severe damage to the impeller and the volute. Fortunately, manufacturer’s pump curves also include the NPSH requirement for each operational point along the curve. This is shown in feet of head above the performance curve.
How To Read A Pump Curve - Part 2
One of the most important lines on a pump performance curve is the Net Positive Suction Head (NPSH) curve. Discreetly applied either below or above the pump performance and efficiency curves, this single plotted line is the key to avoiding cavitation. Required NPSH or NPSHR for a given pump increases with flow. So, using the pump curve shown in Figure 1, we can see that the NPSHR for this Model 1510 B & G pump with a 8” impeller and 800 GPM and 33 Feet of Total Head is 12 feet of head.
As we discussed in an earlier blog on NPSH, cavitation occurs when the pressure at the eye of the impeller drops below the vapor pressure of the fluid inside the pump. When this
occurs, vapor pockets will form on the veins of the impeller and then “implode” once they reach the higher pressure inside the pump. Cavitation is not only loud, it wreaks havoc with pump performance and can seriously damage the impeller and the pump shaft. The NPSH curve on the pump curve is critical because it tells you exactly how much pressure is required at the pump suction to keep the pump from cavitating. Figure 1 This is especially important information to have when selecting pumps for an open system like cooling towers where the water is only under atmospheric pressure. In many cases the available pressure at the suction of the pump is minimal after total head losses are subtracted. Since water boils faster under lower pressures, this increases the chance of cavitation in an open system. Note the vapor pressure of water at various temperatures in Table 1. Under a negative pressure of -14 Psig water will boil or flash to steam at only 85°F!
Table 1
What Happens Between Suction and the Impeller You may wonder why this supplemental pressure (NPSHR) at the pump suction is even necessary. The answer lies in the small space between the suction and the eye of the impeller. Every centrifugal pump will exert a negative pressure in this small space. This pressure drop occurs because of the sudden change in velocities between the suction and
discharge of the pump, the directional change of the fluid, and the increased turbulence. The negative pressure created within this space is the NPSHR value. It is the minimum amount of pressure required at the pump suction in order for the pump to operate correctly.
Figure 2 represents the pressure drop that occurs between the suction and the impeller of two pumps. Note that the lower curve dips below vapor pressure. This pump will surely cavitate, while the other is a suitable range above the NPSHR. Figure 2 This is one reason why it is so important that the design engineer accurately calculate the total head loss in the system and subtract it from whatever static head pressures that exist. This value (the NPSHA) must be equal to or greater than the NPSHR. Finally, keep in mind that atmospheric pressures also vary geographically, so total head calculations must include the proper atmospheric values – whether it be Denver, CO or Myrtle Beach, SC.
How to Read a Pump Curve - Part 3
Factors that Impact Centrifugal Pump Efficiency
Pump selection can have dramatic impact on the overall operating cost of a hydronic system. Consulting several pumps curves prior to the selection of a pump is the key to minimizing these operating costs. In this blog we will discuss the factors that impact pump efficiency and how pump curves can be used to take the guesswork out of efficienct pump selection. First, it is important to understand what pump efficiency is . Pump efficiency is the ratio of energy delivered by the pump in liquid horsepower to the energy supplied to the pump shaft in brake horsepower. So, a pump that delivers 75% efficiency at a given point on the pump curve is converting 75% of the brake horsepower it uses into hydronic energy or liquid horsepower. Table 1 shows how brake horsepower and water horsepower are calculated and how both are used to determine pump efficiency.
Table 1 A pump’s efficiency is impacted by several factors, all of which can be known by consulting a pump curve. These factors include: Flow and Head. It is fairly clear by examination of virtually any pump curve that pump efficiency will varies depending on the total head (vertical axis) and the flow (horizontal axis). By knowing the dominant flow and head range that the pump will typically be pumping at, you can select a pump so that its primary operating range falls within or near its best efficiency range. Note that every pump curve has a Best Efficiency Point (BEP) at
a given impeller trim. Any point to the left or right of the BEP represents a drop in efficiency. ASHRAE recommends pump selection between 66% to 115% of flow at the BEP. Within this range the combined effects of circulatory flow, turbulence, and friction losses are minimized. However, 85% to 105% of flow is the preferred range of for pump selection. (See Figure 1)
Figure 1 ©ASHRAE, www.ashrae.org. 2012ASHRAE Handbook-HVAC Systems and Equipment .
Impeller Size. Change the impeller in a given pump and you will change that pump’s efficiency. Pump efficiency is greatest when the largest possible impeller is installed in the pump casing. This is because of the fluid that escapes through the space between the tips of the impeller blades and the pump casing. Vibration. Pump impellers are subject to axial and radial forces, which increase as the pump operates further away from the BEP shown on the curve. The deflection of the pump shaft increases the amount of vibration, which in turn can diminish the efficiency of the pump. Pump Size. Pump efficiency tends to increase with larger pump size. This is because the losses associated with bearing, mechanical, and internal hydraulic friction decrease in
proportion to the required brake horsepower to drive the shaft as the pump gets bigger. That said, it is best to avoid over-sizing pumps in a given system since this will greatly decrease system pumping efficiencies.
