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More information can be found at www. This guide is intended for engineers, production managers, or anyone concerned with proper pump selection for pharmaceutical, biotechnology, and other ultra-clean applications. We use cookies to personalize content, to provide better marketing, and to analyze our traffic. You consent to our use of Cookies if you continue to use this website. Search Close Search Bar.
A pump performance curve helps you select the right pump for the specific needs of your application. Centrifugal Pump Curve. Reading pump curves accurately helps you choose the right pump based on application variables such as: Head water pressure Flow the volume of liquid you have to move in a given time period As you will see, you can also use pumps in parallel to increase flow. What is Head? Fig 1.
Every 2. A basic pump performance curve for centrifugal pumps show it's performance range. In this curve, head is measured in PSI; flow is measured in gallons per hour. What is Total Dynamic Head? Total Dynamic Head TDH is the amount of head or pressure on the suction side of the pump also called static lift , plus the total of 1 height that a fluid is to be pumped plus 2 friction loss caused by internal pipe roughness or corrosion. Static Height is the maximum height reached by the pipe on the discharge side of the pump.
Friction Loss or Head Loss are the losses due to friction in the pipe at a given flow rate. In this pump performance curve, he pump can generate 80 PSI of discharge pressure at a flow rate of gallons per hour. Pump curve charts indicate flow rates on the horizontal axis and pressure on the vertical axis. Reading Centrifugal Pump Curves that Contain Additional Information Because some centrifugal pumps operate across a range of horsepower, their curves will include additional information.
Impeller Trim Size Impeller size is another variable for meeting performance requirements. Without enough net positive suction, the pump will cavitate, which affects performance and pump life. Efficiency and Performance Variables Good pump efficiency means that a pump is not wasting energy in order to maintain its performance point. Assuming two identical pumps, flow rate doubles when used in parallel. The system curve shows the rate of pressure loss.
As flow rate increases, pressure loss increases. Figure 6. A Positive Displacement Pump Curve answers several important questions during the pump selection process: What flow rate is the pump capable of? How much HP is required for the anticipated pressure? Curves answer those questions by displaying intersections of several important variables, including capacity, work horsepower, viscous horsepower, and Net Positive Suction Head required NPSHr.
Capacity Capacity, as illustrated in Fig. A PD pump curve indicates pump capacity, on the horizontal lines, in units per minute. In this example, the curve indicates gallons per minute GPM and liters per minute LPM in the left margin and the vertical lines indicates pump speed in revolutions per minute RPM.
Figure 7. The importance of viscosity in pump selection Positive displacement pumps deliver a constant flow of fluid at a given pump speed. As discharge pressure increases, keeping viscosity constant, more fluid slips from the discharge side to the suction side of the pump, so the pump must spin at a higher RPM to maintain output. Slip correction accounts for variations in pump performance while factoring fluid viscosity resistance to flow and discharge pressure.
Figure 8. By comparison, Newtonian liquids , such as water, do not change their viscosity, regardless of shear. You put a pressure gauge on the outlet of a pump and you read the amount of pressure. Figure 1 Measuring pressure with a pressure gauge. You read 60 psi on the gauge of the pump in your house and you know everything is fine, everything should work properly. So what is it about head? Why do people even talk about it, and what does it have to do with pressure?
So we are gonna get over this head problem right now, and you'll never get the googly eyes when you here that term ever again. Assume that you have a pump that you can disconnect the discharge pipe or tube and are able to extend it vertically. Head is the height at which a pump can raise water up, that's it, it's that simple. Figure 2 The meaning of head. Connect a tube to the discharge of a pump and measure the water height, that the head of the pump. OK, so head is somehow linked to pressure, in what way?
We will get to that later. For now let's agree to state that the more pressure the pump delivers the higher the head will be in the Figure 2.
Let's say the head we measure in the above situation is 60 ft 18 m , what happens to the head measured if the level in the suction tank is higher.
Will the head measured be higher or lower? If h2 is the head measured in Figure 2, will h 3 be higher than h 2 in Figure 3? Figure 3 How discharge head varies with suction tank level. The answer is yes. If you lower the suction level the head measured will be less and the opposite is true if you raise it. That's all normal since the pump is just doing a mindless job and if you provide more energy to it in the form of more pressure at it's suction then that pressure increase will add to the pump's ability to produce pressure and create a higher water level at the discharge.
The pump manufacturer's want to tell you how much head their pump's will produce but they don't know what type of water supply will be available, so how can they get around this. Ingeniously simple, they subtract the head available at the suction from the head produced at the discharge, they call this Total Head.
Then it doesn't matter what the suction tank level is, they are telling you only what the pump can do regardless of the water supply pressure at the suction. Figure 4 Total head vs. OK smart guy, what if I don't have a tank and I'm pulling water from a lake and the lake is lower than my pump. Figure 5 The effect of low level on the pump suction.
The pump will still produce the same total head but the discharge head will go down. This means you may not have enough pressure to run your devices and you may need to consider getting a pump with a higher total head.
When you buy a pump, you will try to find a pump that has the total head you require at the flow you require. Imagine a pump where you have a pipe that extends straight up vertically from the discharge see Figure 1. Figure 1: Imaginary vertical discharge pipe used to measure head. Intuitively, if a pump can produce more pressure, it can pump water higher and produce a higher head. Also note that the higher the liquid in the tank, the higher the pump will be able to pump the water into the vertical discharge pipe, due to the head exerted by the liquid in the suction tank see Figure 2.
Figure 2: Head depends on the pump itself and the height of the liquid in the suction tank. A much more useful measure of head is the difference between the liquid level in the suction tank and the head in the vertical discharge pipe. Increasing the level of the liquid in the suction tank will give rise to increased head, and decreasing the level will give rise to a lower head.
Total head is independent of the level of liquid in the suction tank. Figure 3: Total head is independent of the suction head. Where H t is the total head, H d is the discharge head and H s is the suction head. Also be aware that this equation holds true whether the suction head is positive level of liquid in the suction tank is above the pump or negative level of the liquid in the suction tank is below the pump. See Figure 4 for an example of the latter situation.
In this case, the pump will still produce the same total head, but because the suction head is negative, the discharge head will be reduced by this amount, according to our equation. Figure 4: Example of how a negative suction head affects the discharge head. In this situation, the flow of the pump is zero. To choose your required pump, you need to know two things: the total head and what flow rate you require. As you might expect, these two quantities are related.
The maximum head shut-off head is achieved at a flow rate of zero.
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