Document G6dLvEyM4z602Jxx1kbY2DkpN

PUMP MAINTENANCE T o m k* os, . Overhaul That Pump at Right Time ,\ Which is better? Pulling a pump apart once a year to see if it's in good condition er letting it run until trouble shows up? __ , ,, There s more than one answer to this important question. Here one authority Jells you what he thinks is the best for your plant By W fi W JONES Encinsbrs don't acres on how often you should completely overhaul a cen trifugal pump. Thai's because ft's al most impossible to set up general rules applying to all classes. Design of unit, service It's used for, kind of liquid han dled, construction materials, overhaul cost, and power saved by replacing worn parts--all enter the decision of when a pump should be opened for in spection end repair. Overhaul Schedule. Some pumps in severe services may need a monthly overhaul; others need be opened only every two or four years, or even less often. So a schedule for inspection and repair of pumps in one service may not suit those in another. But, in general, a centrifugal pump need not be opened for inspection un less evidence shows overhauling Is neces sary. This means a decrease in pump performance, too much noise, or driver overload shows you have trouble. This isn't the whole story; we'll look at ex ceptions later. Instruments. You need the right in struments ond meters to run mediumand large-site centrifugal pumps suc cessfully. They alio help increase pump life. You must be able to find capacity and discharge pressure if you expect to have a good maintenance program. Set up a schedule for testing each unit. Compare the test results With per formance figures for the pump when ft was new. These tests, and not a fixed period of service, should tell you when a pump needs overhaul. Testing a unit costs lest than opening it for inspec tion; and unit doesn't hove to be taken off-the line. Pump Performance. Let's see what effect Internal wear has on centrifugal pump's performance. We'll assume its impeller delivers the same gross capac ity at any given head. But pump has internal clearances at running joints, like those between impeller ond casing wearing rings. A, ond stage pieces, B, Tig. 1. Leakage occur* through these joints and all fluid diacharged by the impeller does not go out the pump discharge. Some of It leaks back to the impeller suction eye. Net capacity of the pump is the difference between gross discharge and Interns} leokage. In a new unit, leakage may be at low at 2% so the net capacity is 98% of impeller discharge. But after pump Is on the line owhile. running clearances wear and internal leakage or short-circuit losses Increase. This reduces the net capacity of the pump for a given total head and horse power. If we want the same net capacity from the pump, total head on It mu* be reduced. Horsepower input increasa Head Capacity. Horsepower sat head-capacity curves, Fig. 3, show Hsv internal leakage affects centrifugal pump performance. When this pump is in good condition it delivers 1270 gpa at 550-ft head, point A, and needs 211 hp, point B. - After the running clearances west, leakage through them increases to when pump can deliver only 1000 gpm at SSfr ft head, point C. Horsepower input r mains the same, 215 hp, point D. Ym would expect this because the irapella delivers the some gross capocity undo the same total head in both cases- If you wont the same capaeity, 127 gpm,`under both conditions, then dw total head against which the pump mat must be reduced. For the worn pumpt* deliver 1270 gpm, total head must b* redueed to 520 ft, point B; power Inpd increases to 235 hp, point F. If you te* pump regularly, you'll get o relisbU picture of iniernahelcarance conditio Oooraltng Exporlonco. Informed* from past use of a given pump or * similar one can serve as a guide f* when it needs opening. For insianA assume you have several boiler ft* pumps that ran 50,000 hr without need; Ing an overhaul. That's enough evident* .It? riANT OPERATION AND MAINTENANCE SECTION - rovtO Capoelty.gpm 3Hsod-copoeHy ond horsepower curves show how increosed . leskooe will affect the performance of o centrifugal pump that similar units need not be opened for inspection until they've run 50,000 hr. On the other hand, pumps on severe service may need an overhaul every three months. To get longer running periods you can replace the pump with one built of better materials or of sturdier construction. But until new equipment proves itself in service it's best to open it at the end of three months to find out how the better materials stand up. Big Plants. It Isn't wise to advise public utility operators to neglect hav ing an adequate inspection schedule. They generally have spare pumps and plant won't be shut down If one foils. But'during the last 10 years, at least 76% of utility-plant operators have had to judge pump condition by outside signs and instruments because of plant overloads. Keep enough spare parts on hand when running on overloaded plant. Then II a. pump foils, you can overhaul It ftdekiy. This avoids possible delays in Betting needed parts from'i the builder. Chotk the Pump. Whether you do or doo't open pump for Inspection, make these scheduled checks. At least once *y six months check pump nnd driver ligament and correct if necessary. In spect stuffing-box packing. Drain oil* Jwricated bearings; flush ond refill. amount and condition of grease j* brings. Moke as accurate a lest ol lhe unit as you can with the instruments 7u have on hand. "sides the 6-month Inspection, make more^eompleie one once a year. Take out beoringe and inspect them for wear and defects. Repack stuffing boxes un less experience shows you the packing can .safely go for o longer time. On horizontal pumps with sleeve bearings, remove the packing ond open coupling. Check vertical movement of shaft at each end. Restore bearing clearance if it la more than 150% of the original. That is, if the original clearance was 0.002 in., recondition or replace bear ing when wear increases iu clearance to 0.003 in. Opening the Pump. Dismantling a centrifugal pump is usually done to in spect Internal surfaces and running clearances. This should show if mate rials are suitable for the liquid* han dled. If you find small pits or erosion on the casing, you may be able to fill these by welding, bracing, silver solder ing or metal spraying. Examine all metal-to-metal fits, aueh os where casing rings fit into the casing, as at C, Fig. 1. If the tongue or cut-water T In volute casings, Fig. 2, Is worn uneven, cut It square across, and file it round and smooth. Horizontally-split casing pumps, Fig. 4, must have damaged gaskets between top and bottom halves replaced before reassembly. Re sure new gasket Is the same thickness and material aa the original one. Check clearances between all running joints, as between cosing and impeller wearing rings, A, stage pieces and Im peller bubs, 0, Fig. ). When these clearances inereaae 100 to 150%, re place worn parts or renew their surfaces. While you have a generol-servica pump open, clean and paint casing waterways. If you work on shaft sleeve, check its concentricity after tightening it in place. Pump size, sleeve material, and size of your repair shop must all be taken into account when deciding if it's cheaper to install new shaft sleeves or resurface old ones. Running Pumps. Where you hove operators on constant duty, make hourly and daily Inspections of the pumps, investigate'immediately any change in a running pump's sound. Check bear ing temperatures and stuffing boxes every (tour. See that rings in ring-oiled bearings work right. Havo just enough stuffing-box leakage to cool and lubri cate packing. Too much leakage wastes fluid pumped. Too little may cause burned packing and scored sleeves. If pump has pressure gages and a flowmeter, check every hour to see thst pressure and flow correspond to plant operating conditions. Moke a daily check of recording instruments lo in sure that output, pressure, or power consumption Indicate right performance. B* Careful. There ore exceptions to the rule thst a pump needn't be opened unless outside evidence shows it's neces sary. On some jobs you may feel safer with clearance measurements and inter nal inspection. Just bo sure to open pump at regular intervals. Otherwise, you won't have too much confidence in your maintenance. Rules are made to be broken. A feel ing of confidence In your pump Is far more important than opening It a few more limes than necessary. MNUAIV 1933 PLAN?- OPERATION AND MAINTENANCE SECTION