Pumps curve

Other fitctors will affect the shape of pump curve are width of impeller, pitch of vane on impeller, and number of vanes oti impeller. Pump curve will be flatter, if the impeller is wider, or its vane ia more swept-back, or it has more vanes on it. 

Sometimes, pumps are installed in series to boost suction pressure or to increase pump discharge pressure. For this case, the composite pump curve can be constructed by adding the pump head at a fixed pumping rate. In other cases, pumps are installed in parallel to increase pumping rate. For this case, the cnomposite pump curve can be constructed by adding the pumping rate at a fixed pump head. 

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Range No. Pump Curve Range No. Pump Curve... 

Mechanical seal in suction system under vacuum is leaking air into system, causing pump curve to drop. 

Certain pump designs use an internal bypass orifice port to alter head-flow curve. High liquid velocities often erode the orifice, causing the pump to go farther out on the pump curve. The system head curve increase corrects the flow back up the curve. 

Equipment Constraints These are the physical constraints for individual pieces of eqiiipment within a unit. Examples of these are flooding and weeping limits in distillation towers, specific pump curves, neat exchanger areas and configurations, and reactor volume limits. Equipment constraints may be imposed when the operation of two pieces of equipment within the unit work together to maintain safety, efficiency, or quahty. An example of this is the temperature constraint imposed on reactors beyond which heat removal is less than heat generation, leading to the potential of a runaway. While this temperature could be interpreted as a process constraint, it is due to the equipment limitations that the temperature is set. 

This matrix will contain information regarding loading characteristics such as flooding hmits, exchanger areas, pump curves, reactor volumes, and the like. While this matrix may be adjusted during the course of model development, it is a boundary on any possible interpretation of the measurements. For example, distillation-column performance markedly deteriorates as flood is approached. Flooding represents a boundary. These boundaries and nonlinearities in equipment performance must be accounted for. 

Measurement versus Equipment Performance Pumps that are in reasonable condition typically operate within 5 percent of their pump curve. Consequently, pressures and flows that are inconsistent with the pump curve imply that the indicated flow and/or pressure are incorrecl . Figure 30-16 shows a single impeller curve plotted as head versus flow. The point shown is inconsistent with the pump operation. Therefore, that pair of flow and pressure measurements is not validated and should not be used in the subsequent steps. 

FIG. 30-16 Typical pump curve showing inconsistency between measurement and curve,... 

The pump manufacturers publish the NPSHr values on their pump curves. We re saying that the NPSH reading is one of the components of your pump curves. Wc ll. see this in Chapter 7 on Pump Curves. If you want to know the NPSHr of your pump, the easiest method is to read it on your pump curve. It s a number that changes normally with a change in flow. When the NPSHr is mentioned in pump literature, it is normally the value at the best efficiency point. Then, you ll be interested in knowing exactly where your pump is operating on its curve. 

If you don t have your pump curve, you can determine the NPSH of your pump with the following foritiula ... 

The easiest thing to do is to get the pump curve from the manufacturer because it has the NPSHr listed at different flows. Nowadays, you can get the pump curve on the Internet with an e-mail to the manufacturer, you can send a fax, or request the curve in the mail or with a local call to the pump representative or distributor. If you wanted to verify the NPSHr on your pump, you ll need a complete set of instrumentation a barometer gauge, compound pressure gauges corrected to the centerline of the pump, a flow meter, a velocity meter, and a thermometer. Definitely, it s easier to get the curve from your supplier. 

To avoid stress from inadequate NPSHa during the draining process, we should consult the pump curve and be sure that the NPSHr is less than 35 ft at the duty point. 

It is a one stage, end suction, back-pullout design pump. There is more information on this pump in Chapter 7 about pump curves. 

The operating window is the effective zone around the BH.P on the pump curve that must be respected by the process engineer and/or the operators of the pump. How far away from the BHP a pump can operate on its performance curve without damage is determined bv its impellers suction specific speed. 

Pump performance curves are the least used, least consulted, least appreciated, and least understood aspect of the world of industrial pumps. The plant personnel who most need their pump curves, meehanics and operators, generally don t have the curves and accompanying information at their disposal. The people who control the performance curves store them in a file, in a drawer, in a cabinet that s almost never opened. They don t share the information contained in the curves with the people who need it. Maybe it s because they themselves don t understand the information to share it. In the next few paragraphs and pages, we re going to explain the pump performance curves. This might be the most important chapter of the book. 

Think of the pump curve like the dashboard or control panel of a car. No one would operate a car without the dash instrumentation panel. 

The matrix of the pump curve graph is the same as the mathematical x-y graph. On the horizontal line, the flow is shown normally in gallons per minute or cubic meters per second. The vertical line shows the head in feet or meters. See Figure 7-1. 

Ixt .s talk about the pump efficiency. Imagine a small pump connected to a garden hose squirting a stream of water across the lawn. You could direct the How from the ho.se up into the air at about a 45-degree angle, and the stream would arc upward and attain its best distance of reach from the nozzle or launch point. The stream of water would attain a specific height into the air and a specific distance. The efficiency curve of a pump is seen as the trajectory or arc of a stream of water. When squirted from a hose, the elevation that attains the be.st distance, when plotted onto the pump curve, is called the best efficiency point (RHP). On the pump curve, it is seen as in Figure 7. ... 

In the final analysis, pumps should be operated at or near their BEP. These pumps will run for years without giving problems. The pump curve is the pump s control panel, and it should be in the hands of the personnel who operate the pumps and understood by them. 

The majority of centrifugal pumps have performance curves with the aforementioned profiles. Of course, special design pumps have curves with variations. Eor example, positive displacement pumps, multi-stage pumps, regenerative turbine type pumps, and pumps with a high specific speed (Ns) fall outside the norm. But you ll find that the standard pump curve profiles are applicable to about 95% of all pumps in the majority of industrial plants. The important thing is to become familiar with pump curves and know how to interpret the information. 

On seeing and examining these different pump curves, notice that all curves contrast head and flow. And, in every ease the head is decreasing as the flow increases. 

Except for the curve of the PE) pump, the other pump curves show various diameter impellers that can be u.sed within the pump volute. And, on all the.se family curves, the effieieneies are. seen as eoneentrie ellipses. There is very little variation in the presentation of the BHp and... 

Up to this point, you probably didn t understand the crucial importance of the pump curve. With the information provided in this chapter, and this book, we suggest that you immediately locate and begin using your pump curves with suction and discharge gauges on your pumps. 

Let s continue with system curves. Up to this point, all elevations, temperatures, pressures and resistances in the drawings and graphs of systems and tanks have been static. This is not reality. Let s now consider the dynamic system curve and how it coordinates with the pump curve. 

The same previously mentioned critical tips apply, plus one more. Upon observing the system curve, with the pump curves, it appears that the operator can operate any one pump, or any two, or any three or four pumps. Actually there is no option to run three pumps in this... 

Consider the following graph. Figure 9-14. Radial loading on the shaft rises if the pump is operated too far to the left or right of the be.st efficiency zone. Another interpretation of the. same concept is to. say that the maintenance and problems rise when the pump is operated away from its BEP. Many pumps have a rather narrow operational window. These pumps can be very efficient if they are correctly specified and operated. This is discussed completely in Chapters 7 and 8 Pump Curves and System Curves. 

On starting the pump and motor, the operators control the service of the mechanical seal. The operators and the process engineers have a tremendous influence on the optimal life of the mechanical seal, just as the operator of a car has the most influence over the optimal life of his automobile. The pump must be operated at, or close to it s best efficiency point (BEP) on the pump curve. 

Actually, everything we said about bearings, mechanical seals, piping, TDH, system curves and mating the pump curve to the system curve, the affinity laws, cavitation, horsepower and efficiency arc as applicable to PD pumps as centrifugal pumps. 

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