I/EC
Instrumentation
Pulsating Flow Measurement—Today The Isobe meter m a y be the answer to those vexing problems of measuring flows that cannot be handled by conventional flow measuring equipment by P. H. Stirling and Henry Ho, Canadian Industries, Ltd.
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ULSATING
FLOW
measurement,
which often plagues t h e instrument engineer, h a s been receiving more attention lately. Economic m o n i toring or control considerations frequently dictate t h e metering of a process stream which is subject to pulsations. Rhythmic pulsations arising from reciprocating machines are c o m m o n a n d these, together with spasmodic pulses from sudden process upsets, c a n m a k e flow measurem e n t with s t a n d a r d flow elements difficult—if n o t impossible. I t is not feasible to a d d capacity tanks a n d snubbers to cut d o w n pulsations solely for flow m e a s u r e m e n t . Choice of a suitable flow m e t e r is complicated by t h e great variety of available flow elements only a few of which a r e capable of metering pulsating flows a n d even fewer a r e universally applicable. T h e underlying principles of each instrument must be examined carefully a n d a few thoughts on some of t h e basic concepts are collected here. Some Common Pitfalls Most of t h e flow elements in use today a r e of the constant area variable head class, such as orifice plates, venturi, flow nozzles, a n d Dall tubes, etc. Flows are usually t h o u g h t of a n d stated as average flow rates F, such that F X time = total q u a n t i t y of material. T h e instantaneous flow for these flow elements is proportional to t h e square root of the pressure difference (F œ \ Δ Ρ ) . This implies t h a t t h e average flow rate is proportional t o the average of the square roots of the pressure dif ferences (F
