Flow Control - Jan 2012
the pump guy By Larry Bachus Chaos on Steroids Revisited When When Not to Use a Variable Speed Motor for a Soft Start Larry I read your article Chaos on Steroids in the September 2011 edition of Flow Control page 40 I have a question You did not address the use of a variable speed motor in lieu of a nearly closed valve If I use a variable speed motor and start the pump slowly will this do the same thing that a closed valve will do Or do I still need to install a control valve Thank you Ernie C Engineer Houston TX Hello Ernie That article was about imprecise language in the engineering profession specifically confusion regarding the word throttle used with industrial pumps and valves The word throttle has a variety of interpretations from control obstruct reduce choke strangle or suffocate Control and reduce evoke certain meanings and images Choke obstruct strangle and suffocate evoke totally different images An engineer or supervisor might say Throttle that control valve and then start the pump The operator might understand Reduce the flow through that control valve and then start the pump However another operator might understand Close that control valve and then start the pump The misunderstanding can be significant maybe disastrous Having said that it is desirable to restrict a control valve about 80 percent meaning about 20 percent open as the pump is started in a system with no backpressure The restricted valve provides artificial resistance on the pump to prevent damage Using a pump to fill a vessel from the bottom is a classic example While the discharge vessel is mostly empty there is no backpressure on the pump Another occasion to provide artificial resistance backpressure is when the pipes are empty and the pumps first duty is to fill the discharge pipes with liquid There is no backpressure on the pump while the pipes are filling with liquid And now your question If I start a pump slowly with a variable speed motor is it the same as starting the pump against a restricted valve The answer is It really depends on the design of the system and the profile of the system curve Ill explain with the following two examples Example No 1 Lets say you need a pump to recirculate a liquid through a filter and heat exchanger at 500 GPM for the next three hours The system design horizontal vertical pipe runs elbows flanges the filter valves instrumentation and other process devices contains 70 ft of energy The system curve is composed of 0 ft of static head Hs 0 ft of pressure head Hp and 70 ft of friction Hf and velocity head Hv at 500 GPM The pump must overcome this energy The pump is designed to develop 70 ft of head at 500 GPM rotating at 1750 RPM on a variable speed motor VSM In this case the pump with VSM will perform the duties of a control valve or a soft start And you can operate the pump with VSM at any speed as long as the RPM is above the minimum performance velocity of the pump and the VSM This system features negative static head and very little resistance on the pump The control valve throttled about 80 percent can provide the artificial resistance to prevent stressing the pump 30 January 2012 Flow Control
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