flame safeguard control phần 7 pps

WarcI make R to B when the conlrollEld variable falls 10 the

sat point minus the differential.

A controller w!lh an additive differentIal operates simi￾larty e.~CBpt that the set point is al the/ow end of the differ￾enliallnslBad at at the hiQh erd.

A controller with manual reset (also callEld lockout) can

beUMd as a limltta (1) shut down the System it it braaks a

Circuit or (2) ~rate an atarm It makes a circuit On most

CCJ1lrol1e~ wilh manual reset, lockout occurs at the sat

point, but on a few rT'l(XjeIS it occurs at the sel point plusor

minus-the differenlia.!'. The circuli will stay lockEld out until

the ternperaturereturns. 10 normal (lockout point plus or

minus the diNerenllaQ and the ~rator depresses a man￾ual rese! rever or button (Fig. 2 or 48). It the lever or bullon

has a trip-free feature, operation will not be returned 10

..,.

'--'

-

-

- ~~T S~OT

-

- - -------SRE~K M~KE----.RE~~"'·6.

.... ~E "'.W

- I.. -

­

- '0'""" 0. , - ) 0"""'"'' ! -

- ~~"T+_F-I.!!!L-----....KE .IlE~K--.RE ..KR.... fol~KE R-II

-

-

-

-

~==-=~.... FIG.8- OIFFERENTIAL FOR ON-OFF

CONTROLLERS.

normal until the manual reset lever or button Is released.

This prevents the COfltrolier from resetting automatically,

even if the manual reset lever or button is IiEld down or held

in by tape, wire, or some other device. Manual reset

makes sure someone is going to notice 6Ol1l8thing is

wrong. A good servica lechnician will correcllhe prOOIAf11

before reselling the switch.

PROPORTIONING RANGE

Tha proportiOning range (also caliEld Ihrottling range

usually edends above the set poim (Fig. 9). alihOugh Qrl

some controllers it extends on lxlth sides 01 the set point.

Pressure or temperature variations cause the bellows or

diaphragm to move, caUSing the potentiometer Wiper (R

lerminall to move across its windings. This varies lhe re￾sistanCe between R and B, and Rand W. The resu1ting cir￾cuit urtJalance drives a modulating motor or mod:.JlalinQ

valve aCtuator and regulates the firing rale. (For a detailed

explanation of this operation, refer to the Flame Safeguard

Reference on Firing Rale Controls, 10rm 70-8117.) As the

controliEld variable rises, the Wiper will move toward the W

end of lhe potentiometer, driving the motor or actuator to￾ward its Closed position ard decraasing the firing rate. As

lhe controliEld variable talis, lhe wiper will move 'award the

B end, driving the motor or actuator toward its open posi￾tion and increasing the firing rate. Thus, a CharlQ8 in the

controllEld variable wilt cause modulation of the firing rate

10 compensate for fhe Change and keep the pressure or

terrperalure nearly CQrlstanl.

t ,

B,,----r-> -co....,,,. IClO§W)

I T I

~"'OPOR­ I ( POTENT'O"UER T'ON'NG "''''lOG' (",~~;j] Ii::,

: :

-NIGNFlIlE

10'ENI 5h PO'''' +1'-Ol.L-~----L - +

•

..J & ON A'£>O CtlfITROlU"" (:weH "" THE TN' n"'ER",TURE [Or<TAOl·

lEftSl. THE k~ PO,,""" 'IT TNt. Cl..TU OF?tt:. .......0""0..'..'

""'....E_

ill SQ"f C","'ROlLEIl' (SUC" "S 1HE T1"0 TEM.E"..W"E COHfIlOl· LUI ....VE z POTEHTlO"Uf'lll OI"l:"''''T'''' IN UN'IoO".

FIG. 9- PROPORTIONING RANGE FOR

PROPORTIONING CONTROLLERS.

220

On some controllers. the proportioning range is 1ixed,

while on others it is adjustable. On adjustable models, the

proportioning range scale is usually graduated tram A to F

.'!yith. a MIN (minimum) value below A. The value of each

division depends on the cperaling raf1Q8 and set J,)Oint at

the controller. Proportionlng range charts like the one

shown in Fig. 10) are available for different cperaling

ranges to determine the pr~ seuing. (The propor·

tioningrange scales of some t~ralure controllers, like

the T991A ~d B, are marked directly in degrees For q.

M'N'" • , .. 0 • •

\ \ , "'OlO"? & rll.""RUU"l '

• 1\ , .. \ I'~ .. 1\ 1\ 1\ \.

" \ ,

1\ 1'­ ,

". .. .. • .. ,.

.

. ,

. ,

,

'''01'O''T10''''«;' .......Cl--alCRUS F

& FOR LOW n"PlfLl,TURl VAPOR_.RlUURl SENS''''' ELlMl..n (T'"

COH?f'OLLl"'l

FIG. 10- TYPICAL PROPORTIONING RANGE

CHART FOR A CONTROLLER'WITH AN

AOJUSTABLE PROPORTIONING

RANGE.

CHECKOUT AND MAINTENANCE ----------­

CHECKOUT

After the controller has been installed, wired, and set, it

should be tested with the system in operation. First allow

the system to stabilize. Then observe the operation of the

controller while raising and lowering its set point. Pressure

or temperature should increase when the set point is

raised and decrease when the SElt point is lowered. Use

accurate pressure Of tef'11)erature testing equipment

when checking out the controller. Do nOI rely on inexpen·

sive gauges.

For an on·off controller. make sure the swilch makes

and breaks at the proper points. Be sure to consider the

differential. If the controller is ~£1ad ol operatinglffi￾properly, it may be further checked as lollows:

,. Disconrlect the wires from the controller.

2. Connect an ohmmeter between the switch terminals.

3. Raise the set J,)Oint of the controller more than the dif￾ferential, The switch should either make or break, depenc:J.

ing on the model of the controller. (11 it makes, the

If a proportioning controller is suspected of operating ohmmeler will read zero; if it breaks, the ohmmeter will

imprope~ly, it may be further cheCked as follows:

read infinity.) 1. Disconnectlhe wires trom the cOf1troller. 4. Lower the set point of the controller more than the dif￾2. Connect an Ohmmelel between cOf1trolier terminals ferential. The switch shoold break or make, depending on 8 and W to measure the resistance at the potentiometer in its action in step 3. the controller. The Ohmmeter should read about 135 Ohms 5. An approximalion of the differential can be made by or 270 ohms, depending on the model 01 the controller.

observing the change in set point r9QJired 10r a resistance 3. Conneclthe ohmmeter between cQnlroUer terminals

change from zero to infinity. Wand R and raise the set point of the cOf1trolier above fhe

For a proportioning con/roller. make sure the modulat­ actual pressure Of' temperature being measured. The

ing motor or modulating valve acluator reaches the low ohmmeter should read the full value of the J,)Otantiomeler

and hiQh fire J,)Osilions at the proper points. If the motor or measured in step 2 (135 or 270 ohms).

actuator runs in the proper direction when (he set point is 4. Slowly lower the set J,)Oint of the controller while 00­

adjusted, il can be assumed that the controller is operating serving the ohmmeter reading. The resistance should

properly. It it runs in the wrong direction, reverse the 8 a.nd drq) to zero at some sel point below the actual pressure or

W wires. Observe the action of the motor or aCluator tosee temperature.

if it stabilizes. If the motor or valve is moving cOf1stantly. 5. An approximation at the proportioning raf1lijJEl can bEl

widen the proportioning raf1Q8 (if it is adjustable). a lillie at made by observing Ihe chaf1Q8 In set J,)Oint rElQUired tor a

a time, unlil the system is stable. resistance change from zero to full value.

221 71·97558-1

CALIBRATION

AU controllers are carefully tested and calibl"aled at the

factory under controlled conditions. If the actual ~rating

preslil,lr.es_ or terrperatures 00 not malch the set points,

most controllers can be recallbrated in the field. FirSI

slsl of occasional inspection and blowing or brushing

away any accumulaled dirt and dust. To ensure prq:er

functioning of lhe controller at all times, an ~raUona!

check of the Mtire system Should be performecl wring

routine maintenance checks.

check lhat the controller is level (i1 it hasa mercuryswilCh).

Ralevel il if necessary, and recheck the ~raling points. It

there is still a discrepancy:

1. On 6Om8 controllers, the scaleplate can be moved

slightly L;lP oroowQ u(\111 the sat point agrees with lhe actual

pressUre 0( tef'rl)erature.

2. On other controllers, the set point Indicating dial can

be turnecl with a special calibration wrench until it agrees

wilh the actual pressure or terrperature.

MAI~TENANCE

The ,cover of the controller should be in place al all

times to protect the internal components tram din, dust,

and ~ysical damage. ROUline maint9l'lBnce should con­

-,'

Occasionally, mishandling of the controller may cause

a malfunction. A gradual change Of control point may oc￾cur because of a very small leak In the lhermal system. On

temperalure controllers, this Is mosllikely to be causadby

bending the capillary tubing too sharply, or too close to a

joint. Aging althe factory would have revealed the leak if it

had been presen1 at the lime of assembly. If/he tLbing is

squeezed so hard or bent so Sharply that ils bore is com·

pletely closed l,4), the controller will, of course, be ren￾dered entirely in~ralive. Deforming the bulb (or Olher

sensing element) will change the VOlume, resulting in a

Shift in calibration. Tnese of malfunctions should be

lool<ed for ciJring maintenance. Controllers should be han￾dlecl carefully althe time of installation, et.Jring actual use,

and during mainlenance.

222