Document 93dwqpxRe6DymwkKJbLbrbvqL

To prevent purchased power becoming the toil that wags Purchased Power Held the dog, here is a working control. It keeps power de mand within limits, avoids generating excess energy, maintains process pressure By PRANK O WAlltNS ContvJffop fnp/oeer, Clrritond, Ohio An industrial plant ihai requires steam for process the year round often finds it advantageous to buy some power and generate (he remainder. Sav ings con be substantia) when such a plant uses a turbine or engine in place of a reducing valve to bring boiler pressure down to required process lev* els, and--in the expansion to lower pressures--to generate electrical en ergy for power-plant needs. It takes only a fairly simple engineering study to determine whether a turbine-gen erator installation is feasible. Principal shortcoming of such an ar rangement has been that plant de mands for process and power seldom if ever vary concurrently. As a result costly peak demands on purchased power occur that seriously affect the over-all savings. Successful Experience. Ohio Leather Co, of Cirard, Ohio, met this situation by installing a turbine that (s provided with an automatic governor control. Pig. 1 and 2, responsive to both pur chased or primary power and plant de mands for process siaom. And the saving effected indicates a complete re turn of investment in less than three years after deducting interest, deprecia tion and taxes, despite operating only ten hours per day, five doys o week. The unit, a 750-kw straight noncon densing type, designed for 250 and 5 psig exhaust, operates in parallel with the purchased source of power. The automatic governor control varies the output of the unit within the range from no-load to full-load setting of the regular governor, hut not in excess of this setting. Thus all safety features of the standard governor are in control. Pig. 5 and table on p M show how automatic governor control provide tor the following conditions: (1) Limits maximum demands os tbs primary purchased source to a prede termined top level regardless of plant- process steam demands. (2) Limits minimum demands at fixed level so as to prevent return flow of power to primary source wbea pleat . steam demands may be high and eke* iric-power demands low. (3) With the minimum demand set at a fixed poim, (he maximum demand* ' are automatically varied In the range from the top fixed position to the mini mum one, depending on steom demand*Tho control is provided with mesa* to alter easily and accurately tho totsj' mum top fixed-limit while the onit is in operation. Thus advantage on he taken of power rates that permit high" peaks without penalty during cortaio periods. Control Design. "J*he standard design of control uses one wattmeter el*a* relay with the pressure clement a coofr 82 (688) POWER No*mb*r INI Under Control Aids Process Plant terpin ol the unit. Fig. 3. In the Ohio Leather Co's installation the turbine wit equipped with a backpressure regu lator built into the governor. The con* trol. therefore, has two wattmeter ele ment relays, one set Jor maximum, the other for minimum, as in Fig. 4. The relays have contact points C, , sad C. H, between which float Indica tor arms 0 and F. These relays are identical except contacts C, E are on the lower range of the scale, while C, H are 00 the upper range. Deflection from C to E and C to // is hoot 5% of full uale, and is adjusts"le. Settings here ore (1) 400-Vw >timum purchased-power peak de *nd (2) SO-kw minimum demand on Parchsrtd power. To meet these con~*kms contacts C ond 0 closa when Coinl power is 50 kw or less--D & *hen 100 kw or more--F and C "ea 3S0 kw or less--F and H when s^kw or more. when incoming power load Is be* 100 and 350 kw arm J stops ot point with clevis 1C in posi- .JOWtg , Novmbr 1948 ------1-------------1------1------1--T--i--i--i-- s^J-Pten/ dtmemf f/Jm mi p$ok, pvtrogt dori 7 1. I 1 rL r/ , 4 / "h t s. Pun Aoi td p t/ /\\ / Ti \ s Comporison between purchosed-power peek demands, before end otter governor control went to work leveling off purchased power pecks, shows In these cherts lion to permit exhaust-pressure regula tor to function through its full range of control. Arm R is spring loaded so the servo- regardless of portion of the exhaustpressure regulator. This permits the all-important "peak demand" to super, aede other factors. If excess exhaust (689) 8)