The liquid is pulled downward by gravity and creates a low pressure under your fingertip. The bubbles subsequently collapse downstream as they enter a region of higher pressure. The pump characteristic curve has a similar appearance to the previous curve shown that I also called a characteristic curve that showed the relationship between discharge pressure vs. flow (see Figure 21). Almost all pump problems occur on the suction side. Classic because it is due to insufficient NPSHr. 173 0 obj <>stream Various requirements may be calculated using these relationships: According to Table 1, the copper tubing size should be somewhere between 1/2" and 3/4", I choose 3/4". 151 0 obj <>/Filter/FlateDecode/ID[<0CE76E54B5E1234C8484F18FDFA9A82B><98FB71DA2B976A438D49F50A60C50F84>]/Index[116 58]/Info 115 0 R/Length 152/Prev 194488/Root 117 0 R/Size 174/Type/XRef/W[1 3 1]>>stream The mechanical seal will need to be set after these other steps are completed. friction head loss in this part of the pipe. I will design my system so that from the pump there is a 3/4" copper tube main distributor, there will be a 3/4" take-off from this distributor on the ground floor to the second floor level where the bath is located. The force is the weight of water. Also the difference in velocity head of the pump discharge vs. the suction should be accounted for but this is typically small. Refer to above as a reference. This collapse occurs on a nanoscale (1.0 x 10-9 or billionth). Normally these are gravity and/or atmospheric pressure. The rpm determines the Total head and flow are the main criteria that are used to compare one pump with another or to select a centrifugal pump for an application. Cold water is typically the worst fluid for the consequential damage from cavitation. Cavitation is the formation of vapor bubbles in the fluid stream due to a drop below the vapor pressure of the fluid. Understand the system curve. %%EOF A pump running backwards does not reverse the flow direction. Checking for factors like oil level and color can make a big difference. Total head is the height that the liquid is raised to at the discharge side of the pump less the height that it is raised to at the suction side (see Figure 25). It is often expressed in pounds per square inch or psi. In this case, the distances are 10 feet of run on the main distributor and another 20 feet off of the main distributor up to the bath, for a total length of 30 feet. I recently heard a phrase that the pump becomes grumpy and grouchy when there is an insufficient NPSH margin. More attention should be paid to optimum selection of pumps, manufacturers experiences, application details and the specific requirements for each service. endstream endobj 117 0 obj <. There is such a criteria and it is called TOTAL HEAD, and it is defined as the difference in head between the inlet and outlet of the pump. In the same way that head was calculated to lift a certain weight, the friction head is calculated with the force required to overcome friction times the displacement (pipe length) divided by the weight of fluid displaced. Insufficient NPSHa will result in cavitation in the pump impeller. These calculations have been done for us and you can find the values for friction head loss in Table 1 for different pipe sizes and flow rates. BLDG. Typically the hardest part of the job is topic selection so it will be fresh, educational and interesting. A centrifugal pump is simply a machine, where for a given set of fluid properties, impeller geometry and operating speed it will react to the system in which it is installed. Valve timing is a major factor in achieving good volumetric efficiency, and is affected by plunger speed, plunger diameter, poppet weight, spring rate, and poppet travel distance. For lower specific speed pumps the flow will be approximately 50 percent of rated and the head will be 60 percent of rated. You can find the friction loss for a 1 pipe at 10 gpm in the Cameron Hydraulic data book of which the next figure is an extract: Friction loss on the discharge side of the pump. If you are a home owner, find out which of your uses for water is the biggest consumer. vertical distance. There is a common and pervasive misunderstanding about how pumps work. Most centrifugal pumps become too inefficient or exceed their horsepower (hp) limits in a viscosity range between 400 and 700 centipoise that depends on pump size. Join the General Pump Newsletter for new pump announcements, company updates, and more! This is not terribly useful to know Jim Elsey helps you avoid common centrifugal pump mistakes, To read other articles in the 'Common Pumping Mistakes' column, go. This calculation is useful if you want to troubleshoot your pump or verify if it is producing the amount of pressure energy that the manufacturer says it will at your operating flow rate. As I mentioned this is not a practical way of describing the performance because you would have to know the suction pressure used to generate the curve. dumbbell that the weight lifter needs to provide is 6 feet. shown in these next figure. If you hear the cavitation noise (sounds like pumping gravel), it is likely cavitating. But the static head of the fluid surface with respect to the bottom is the same. Like in the straw, the pressure close to the pump suction connection must be low for the liquid to be supported. Calculate the pump discharge pressure from the pump total head That hydraulic condition of one point of head and flow is the best efficiency point (BEP), also known as the best operating point. What is the pump rating? typical velocity of 10 ft /s. This field is for validation purposes and should be left unchanged. We can now go to the store and purchase a pump with at least 47 feet of total head at 10 gpm. Hopefully it will satisfy your appetite. Compare two tanks with the same cylindrical shape, the same volume and liquid level, the tank with the denser fluid will have a higher pressure at the bottom. in facts. OR we need to know the weight. If the fluid is dense, such as a salt solution for example, more pressure will be produced at the pump discharge than if the fluid were pure water. Crankshaft Bearing Life is determined by lubrication, speed, and load. The general relationship for pressure vs. tank height is: SG or specific gravity is another way of expressing density, it is the ratio of a fluid's density to that of water, so that water will have an SG =1. Load is calculated as follows: Local pressure forces involved can be higher than 10,000 pounds per square inch gauge (psig) (689 bar) or more, plus there is heat generated. Positive Displacement pumps are used for liquids or to compress gases. Lastly, fluids do not have tensile strength. Selecting the right flow rate may be as simple as determining that it takes 100 gpm (6.3 L/s) to fill a tank in a reasonable amount of time or the flow rate may depend on some interaction between processes that needs to be carefully analyzed. If you take a straw, fill it with water, cover one end with your fingertip and turn it upside down you will notice that the liquid does not come out of the straw, try it!. PARSIPPANY, NJ 07054. A web app for pipe friction loss is available here https://www.pumpfundamentals.com/web-apps.htm, The performance or characteristic curve of the pump. The speed of sound in water is 4.4 times faster at about 3,350 mph (5,391 k/h or 1,490 meters per second [m/s]). This is why buying a centrifugal pump is more complicated than buying a positive displacement pump which will provide its rated flow no matter what system you install it in. You may be interested to know that 324 foot-pounds of energy is equivalent to 1 calorie. The friction loss in feet is then 30 x 0.068 = 2.4 feet. to determine the discharge pressure. A pump is in many ways like a centrifuge, and so the heavier water is expelled to the outside diameter and the lighter air remains in the middle or center. Hydrocarbons have minimal effect from a damage aspect. What is the relationship between head and total head? The volume of the tank is the cross-sectional area A times the height H. The cross-sectional area is pi times the diameter squared divided by 4. The friction head depends on the flow rate, the pipe size and the pipe length. leave the pump turned on, you don't start and stop the pump for every pound of fluid displaced. The horsepower stated may or may not be corrected for specific gravity or viscosity. I have chosen 10 ft/s as a target velocity because it is not too large This a matter of taking measurements of the height between the suction tank fluid surface and the discharge pipe end height or the discharge tank fluid surface elevation. An insiders guide to maintaining electrical enclosure temperatures and protecting heat-sensitive equipment. To preclude the ingestion of air, do not operate the pump when the fluid level is below the critical submergence. So, in a perfect world, if there was no fluid friction or vapor pressure working against the system you might be able to lift cold water 33 feet. Cavitation damage can occur at different locations on the impeller. Pressure is equal to a force divided by a surface. Terms like simplex (for one) and triplex (for three) indicate the number of cylinders a particular pump features. The total head is the sum of the static head (remember that the static head can be positive or negative) and the friction head. Because I started my career in the submarine world, I have to point out that the speed of sound is even faster in salt water. Give it a look for another way how this topic can be approached. This is the type of curve that all pump manufacturers publish for each model pump for a given operating speed. When that pressure exceeds 150 psi, that pump is deemed high pressure.. Pumps are really designed to operate at only one point. Elsey may be reached at jim@summitpump.com. If you have been reading my column, many of these tidbits will be a review. You can find the friction loss for a 0.75 pipe at 10 gpm in the Cameron Hydraulic data book of which the next figure is an extract: The total friction loss for piping in the system is then 9 + 3.1 = 12.1 feet. We use cookies to ensure that we give you the best experience on our website. endstream endobj startxref The main factors that affect the flow rate of a centrifugal pump are: - friction, which depends on the length of pipe and the diameter, - static head, which depends on the difference of the pipe end discharge height vs. the suction tank fluid surface height. One shows the friction head on the discharge side and the other the friction head on the suction side. What is the unit of head? (( Plunger Diameter in inches)2 x p) x Pressure (psi) = Plunger Load (in pounds), Volumetric Efficiency compares the pumps actual output to its theoretical output. The driver will need to be aligned to the pump. HS is equal to 15 feet because it is a lift therefore: Now you can check your pump to see if the measured discharge pressure matches the prediction. Therefore the total head will have to be at least 25 feet plus the friction head loss of the fluid moving through the pipes. In reality, fluid friction and the negative consequences of vapor pressure will work against you and preclude fluid lifts of much more than 26 feet. Pay attention to the published pump curves. How to Make Viscosity Corrections for Centrifugal Pumps, Practical Guidelines for Centrifugal Pumps at Oil, Gas, Petroleum & Petrochemical Plants, Visual Inspections Increase Centrifugal Pump Life, Impeller Alterations for Better Performance, Part 2, Variable Frequency Drives: A Powerful Tool for System Optimization, How IIoT Early Anomaly Detection Increases Reliability & Productivity, Software Helps Water Utility Avoid Unplanned Downtime, Basics of AODD Pumps & the Technical Advantages, Vacuum Technology Fundamentals & Innovations, Increasing the Lifespan & Reliability of Electronic Components, In-depth articles on pump industry issues, Expert insights into important topics in the field. If we use head, we only need to know the vertical distance of movement. The density of water is 62.3 pounds per cubic foot. The installation is as shown in Figure 37, a domestic water system that takes its water from a shallow well 15 feet lower than the pump suction. for a weight lifter but we will see how very useful it is for displacing fluids. Most pumps start to lose performance around 2 to 3 percent air entrainment. First let's deal with the unit of energy. Velocity head is also present, but typically too small of a component to be concerned about. Why is this pressure less than atmospheric pressure or low? Critical submergence is necessary to prevent vortexing. This is very useful for fluids because pumping is a continuous process, usually when you pump you Exceptions can be disc pumps, self-primers and some vortex or recessed impeller type pumps. In many cases, this will be the bathtub which requires approximately 10 gpm (0.6 L/s). Gas engine horsepower required = Pressure x Flow / 1100. The formation of the bubbles typically occurs just in front of the impeller eye since this is typically the lowest pressure in the system. The manufacturer will rate the pump at its optimum total head and flow, this point is also known as the best efficiency point or B.E.P.. At that flow rate, the pump is at its most efficient and there will be minimal amount of vibration and noise. The total friction loss for the suction side is then 2.4 + 0.7 = 3.1 feet. Note the speed of sound in air is approximately 768 miles per hour (mph) (1,236 kilometers per hour [k/h]) and varies somewhat with humidity levels. However, the curve does exist and if you insist you can probably get it. Head is defined as energy divided by the weight of the object displaced. Example 1 - Sizing a pump for a home owner application. He is the general manager for Summit Pump Inc. and the principal of MaDDog Pump Consultants LLC. Think again if you are pumping thick fluids using water pump performance curves. This page outlines a few fundamental concepts you should find useful when working with our pumps. This webinar covers vacuum basics including gas laws, vacuum measurements, vacuum pump sizing and much more. The pump manufacturer has no means of knowing what these constraints will be. Think of any pump system as three separate systems when trouble shooting issues in the field. pumps output volume for any given combination of cylinder bore and piston stroke. ), it has the same meaning. It should in fact be slightly more than 15 feet because a higher suction lift will be required due to friction. Viscosity is the kryptonite of centrifugal pumps. hb``Pd``: $n-g@@,XX30d>yl.UwL@y?&aRf[%2GXtwpXAh` V Horsepower Requirements The total head is then 100 feet. My pump bearing feels hot. This is the pressure in pounds per square feet, one more step is required to get the pressure in pounds per square inch or psi. Not all manufacturer's will provide you with the pump characteristic curve. The horsepower required to achieve a given output depends on the energy source, because different types of prime movers vary in their pumping performance capabilities. A low specific speed impeller will have the flow enter parallel to the shaft centerline and leave the impeller at 90 degrees to the centerline. If the bubbles collapse in the middle of the fluid stream there is almost no damage. This next figure shows how much head is required to do the same job. But when the bubbles collapse near or at the metal surface, they collapse asymmetrically and cause a small microjet. This means that the pressure will be negative (relative to atmosphere) at the pump suction. Specific speed (Ns) is a tool used by designers to look at the performance and geometry of a hypothetical impeller. We are mainly interested in establishing a continuous flow rate. A high specific speed impeller will operate with the flow entering parallel to the shaft centerline and leave parallel to the centerline. The total head is equal to the difference between the pressure head at the discharge HD and the pressure head at the suction HS. The vertical distance from the surface of the fluid to the pump inlet is the submergence level. But lets assume that the pipe is generously sized and that the friction loss is small. I have been writing Common Pumping Mistakes for Pumps & Systems for more than three years. Almost all pump designs will cease to perform at around 14 percent entrainment. Learn the ins and outs of how AODD pumps work in this webinar. To calculate the discharge head, we determine the total head from the characteristic curve and subtract that value from the pressure head at the suction, this gives the pressure head at the discharge which we then convert to pressure. It is difficult for the typical person to hold their hand on a bearing housing that is over 120 F. It is perfectly normal for a bearing to be operating at 160 to 180 F. Use a thermometer or infrared device to measure the temperature and deal Elsey is an active member of the American Society of Mechanical Engineers, the National Association of Corrosion Engineers and the American Society for Metals. The suction system, the pump itself and the system downstream of the pump. It depends how complicated your system is, if the discharge pipe has a constant diameter then Therefore the total head is 35 + 12.1 = 47 feet. which would create allot of friction and not too small which would slow things down. the velocity though out will be the same. All rights reserved. There is some friction loss in the fittings, let's assume that a conservative estimate is 30% of the pipe friction head loss, the fittings friction head loss is = 0.3 x 6.9 = 2.1 feet. Industrial pumps do not come from the factory ready to plug and play. There are exceptions to this comment, but never assume. In an industrial setting, the flow rate will often depend on the production level of the plant. This is also why it is common to start up these types of pumps with the discharge valve open so as to not overload the driver. NPSHr is NPSH3. The pump produces pressure and the difference in pressure across the pump is the amount of pressure energy available to the system. A jet pump does not require a check valve therefore I will assume there is no check valve on the suction of this system. There is some friction loss in the fittings, let's assume that a conservative estimate is 30% of the pipe friction head loss, the fittings friction head loss is = 0.3 x 2.4 = 0.7 feet. 2022 General Pump Company. Pumps cannot efficiently move fluids mixed with air if the percentage is greater than 4 or 5 percent.

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