Flow Control - Jan 2012
the pump guy Example No 2 Lets say you need to complete another operation You want to transfer a liquid from a constant level holding tank and pump into a constant level elevated tank at 500 GPM over the next three hours This system design also contains 70 ft of energy In this example the system curve is composed of 60 ft of static head Hs 0 ft of pressure head Hp and 10 ft of friction Hf and velocity head Hv at 500 GPM The pump must overcome this energy Lets say the pump is designed to develop 70 ft of head at 500 GPM rotating at 1750 RPM on a VSM It could even be the exact same pump from the previous example At 90 percent speed on the VSM 1575 RPM the pump will only develop 56 ft of head The system static head is 60 ft This pump is deadheaded 0 flow Youre pumping nothing and the pump is under severe stress This pump is deadheaded at any speed less than 1633 RPM or 94 percent max velocity because of the design of the system If you want to complete this operation in three hours youll need maximum speed from the beginning Is this the same as a throttled control valve or soft start No In this case you cannot use the pump with VSM to perform the duties of a throttled control valve or a soft start And there is no reason to purchase a VSM if the effective operating range is 98 percent to 100 percent maximum speed Strangely the second example facetiously answers your original question Starting a pump slowly with a VSM will act precisely like a closed valve They will both deadhead the pump 0 flow depending on the design of the system This is why I say that it depends on the design of the system and the profile of the system curve Does this answer your question Is it the answer you expected Later Larry the Pump Guy P S A pipe wrench is a mechanical tool A screwdriver is also a mechanical tool The mechanic must know the difference and he must know when and where to apply each tool For a successful dinner party the host or hostess must understand when and where to use a steak knife and a serving spoon Likewise pump curves and system curves are important tools for the engineers and technicians to resolve pump problems These tools are as important to pump reliability as vibration analysis or laser alignment You will graduate from the Pump Guy Seminar with a complete understanding of pump performance curves and dynamic system curves As you incorporate the curves into your reliability program youll watch the unplanned pump maintenance disappear Your pumps will behave I guarantee it Yes I stole that line from the Mens Warehouse commercial Everything I know I learned from someone else Well be in Chicago in March and New Orleans in June Come if you can FlowControlNetwork com PumpGuy FC Larry Bachus founder of pump services firm Bachus Company Inc is a regular contributor to Flow Control magazine He is a pump consultant lecturer and inventor based in Nashville Tenn Mr Bachus is a retired member of ASME and lectures in both English and Spanish He can be reached at larry@ bachusinc com www bachusinc com Do You Know Understand Your Pumps Larry Bachus The Pump Guy is the co author of Everything You Need to Know About Pumps one of the best selling technical books on pump systems in the world This book is written exclusively for people who must maintain pumps Whereas other pump books are written from a design point of view this book is written with maintenance in mind While most technical books sit on a reference shelf gathering dust this book gathers dirt smudges Its pages get creased and folded when mashed by the lid of a photocopy machine It gets sneezed on and splashed with snot on cold mornings It gets soaked with leaking oil grease and coffee Basically it gets used because its tremendously useful The straightforward guidance it provides will help you ensure the efficiency and lifespan of your pumping systems To order your copy of Everything You Need to Know About Pumps call 615 361 7295 or order online at www bachusinc com books htm Circle 22 or Request Info Instantly at www FlowControlNetwork com 32 January 2012 Flow Control
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