Document 85rB9851LpdV1YxY1gjaN5XVK

2Using o bsllows to position nozzls gives controller lineor ond proportionol oction (or every position of boffle Employing 2-tonk system end restricting volve ltlustretei/>] effect of introducing time tog between inflow ond Host'*# 3 4Opposing prsssure-Iooded bellows reploces loading spring of Fig. I ond 2 to give o reset oction to o controller Bellows kxxded by output olr pressure moves the nozzle proportionately to keep it o proper distonce from boffle i Working Model Simplifies thl Theory of Automatic Control With a demonstration panel it's easy to instruct plant men in operating and- caring for control instruments. Con clusion of a 2-part lecture by G F Akins, instrument engineer, Eastman Kodak Co .Recalunc the statement in Tort One (Power, Sept 1948, p 82)--that one major requirement in a controller is to allow a baffle travel of 0.25 in.-- let's find out how we can permit such movement and still hold nozzle-baffle clearance within 0.002 in. If the nozzle can be mode to move through a given distance, and its travel be proportional to the instrument's out* put pressure, the baffle can be moved a .lota) distance of 0.25 by using o suit* able mechanical arrangement. Yet the nozzle will always maintain a alight clearance with the baffle. Minute clearance changes noted in SI (694) <3 the low-preaaure nozzle become useful sure at the slightest change in low-prtw*^ in the new scheme, since by utilizing an sure impulse from the nozzle. Air eoMf-j air relay, the slightest change in baffle put line from the relay divides into I*?',' clearance causes an immediate change- branches--one line going to the in output from the relay. The problem control valve and the other to bellows'.fj now becomes one of converting output X. A spring opposite the bellows pressure change into proportional mo tion of the nozzle to cause it to seek out the baffle's position. vides o movement proportions] lo.tkty^ change in output pressure. Bellows ud epring ere designed for a total travel f.Sj Btllowi Pressure. A ready means of % In. when output from the instm*); ^ transforming pressure changes into mo* lion is a spring-opposed bellows. By increasing or decreasing pressure with* changes from 3 to 15 pel. `/j-J Lever T, pivoted at its upper nd.b.;.h| ao arranged that any change in be!lo**v;l in the bellows, motion ot its free end is position is transmitted to the nozzle./. proportionol to the pressure change. adjustable link 5. With llhk in Incorporating the bellows motion to treme lower position (point L), 1***,', position the nozzle. Fig. I, we have a * motion of % in. causes the controller that produces linear and pro* move 0.25 in. For every value of portion^] action for every position of put pressure, there is a definite ./j the baffle. corresponding nozzle position. In Fig. 4, pen motion through a con* Neule Follows Baffle* As ** netting link (upper left) and a pivoted the boffle in midposition of its 0-*aV*:. lever raises or lowers the baffle 0.25 travel and turn on the air, the **Y*g`' in. as the pen travels lull chart width. clearance between nozzle and Below the baffle, a nozzle rotates about causes the output pressure 10,^74' a pivot to hold continuous alignment rapidly to 9 psl, and thus force hriJsgf with the baffle. X to expand to its midposilion* ^ Backpressure created by the nozzle is motion, through link S, c,ufe* carried to the air relay (at right) which nozzle to move to its mldposit 00 gives an instant change of output pres* establish a clearance of 0.004 in* .*. POWER Nv,inb,r:||| the boffle. Slightest deviolion from the 0.004-In. clearance causes an Immediate change in output, thereby producing corrective action through the bellows - tad linkages to reset the nozzle. For new position ond output pressure regis ters 12 psi, causing valve to reach its 75%-flow position. Process is again in balance, bur not at desired control point of SO on chart. >ny other baffle position, a correspond This demonstrates that any propor- ing output pressure positions the nozzle tional-typc controller, whether the de* si prescribed distance from the baffle. sign it simple or complicated, follows Placing link S in. its lowest position the same pattern; that Is, to correct for to produce a throttling range of 100%, a process load change, measured level ht's investigate proportionol response must rise-ot fall to tend corrective im f the new device. pulse lo the control valve. Setting the pen ot O on the chart, Retulti Gained. At first glance this titput from the controller becomes 3 controller appears little better than a and the control valve is fully closed. simple float valve but remember we Moving the pen to 50 gives 9*pai output, wanted to operate Urge valves from ,Dd finally os the pen is moved to 100, small impulses generated by a bourdon output increoses to 15 psi ond control spring or similar device. Our control jn . nl is wide open. For any other pen ler not only fulfills these requirements Ration, Fig 7, output pressure.has a but allows far greater adjustability corresponding and proportional value, than does the mechanical leverage of T,klo I, p 92. ' the float valve. A new control point Setting the loading petcocks ot 50% can bo readily established by merely ""< the controller creates 9*psl oot- moving the set pointer, Fig- 4, to change Put pressure. Fig. 1, to open the control. baffle-link pivot position. The throt 50% flow rale. The process tling (proportioning) range of the in Si i"** td temalns to ss long strument can easily be altered by mov - 0ad does not change. If we lo- ing link 5 to o new position on arm T, ` sad *6 l4t*10 ^%- tank level drops Suppose we move 5>to its midposition. [' f*1 moves to 75; baffle assumes a Pen now traverses only 50% of chart to 5>OW Nevsmbsr 1948 produce full output change of 3 to 15 psi. To reduce amount of offset with .load change we may wish to-reduce the throttling range to the minimum, as, for Instance, placing link 5 in its upper* most position to produce 6% throttling. If the process has favorable control characteristics, this setting may over come the difficulties of offset because a very slight motion of the pen is enough lo reposition the valve to a new flow rale. If the process is of n more difficult nature such narrow throttling U unde sirable. As an example, let's investi gate the influence upon controller set tings as established by the process. Putting in operation a controller with 6% throttling range, we find it very unstable. Chart curve is similar to that of an on-off instrument As an ex periment, let's transfer this controller to a process having a greater tank capacity, Fig. 2. We can do this by in stalling a second tank in the line. Following a number of ovcrahois, the control finally becomes stable. This shows process characteristics, such as capacity, affect proper adjustment of o controller. The double-tank system represeat* (695) 09 - SB !? l\