Answer MEP Question 20

Q. You have opened up the main sea-water circulating pump (centrifugal type) onboard your vessel for examination and overhaul, describe the checks you would make, and state your conclusion regarding any faults found. 

The pump needs to be checked for damage, wear and corrosion/erosion at the routine over-haul. The clearance may need to be checked, alignment should be checked while the pump is warm, as the expansion due to heat while running would be different from the cold condition. Make markings and remove the coupling bolt. Rotate the halves together, checking for angular alignment and rim out-of-true. 

    Clean surfaces of pins and bushes, check for roundness — replace damaged bushes. The key should be a snug fit, else it will lead to damage of key-way and further problems. The Impeller should be visually inspected for wear erosion and cavitation damage. The impeller should not be excessively worn at any point, either due to contact or due to corrosion. 

    The vanes at the eye and the discharge are the trouble spots, Diffuser blades which have worn at the tips need to be dressed up, to avoid stress raisers. The shaft needs to be checked, especially the sleeve in way of the stuffing box, which is excessively tightened. Slip the packing the correct way onto the shaft. 

    When opening the ring, do it sideways, to prevent distortion; especially with lead-filled or metallic types. Swab the insides of metallic packing with the lubricant supplied along with the packing or light oil, to keep the packing from scratching the shaft and damaging it. Stagger the packing to ensure that all the gaps do not come in a line. Install the packing so that the lantern ring lines up with the cooling liquid opening.

   Remember that this ring moves in as the packing is compressed by the gland. First, tighten the gland by wrench, then back off till finger-tight, and allow packing to leak slightly till it seals itself, before re-tightening the gland. A slight leak is beneficial for lubrication and cooling since there is high friction. Use a split Wooden bushing to install the first turn of packing. Then force inside by tightening the gland. Remove all of the old packings, taking care not to damage the shaft with scratches. After removing all the packing, check the straightness of the shaft. A bent shaft will cause leakage problems, besides damaging the bearings. If the neck bushing clearance is great, use a stiffer bottom ring or replace the bush. Revolve the shaft and check with the dial gauge, that the shaft is true. To find the right size for packing, measure the bore of the stuffing box and the diameter of the shaft. Subtract shaft diameter from the inside bore, and divide by 2. 

    This takes the guesswork out and ensures a perfect fit. When cutting packing, place on the shaft to cut rings: If you cut the ring by stretching out the packing, The ends will be at the wrong angle and sealing will be bad. If used for high-temperature service, leave a slight gap between the ring to allow for expansion, else the shaft may have excessive drag. Turn a worn sleeve down to a smooth surface, if wear is negligible or replace it with a new sleeve. Renew shaft packing. Ensure that the lantern ring is aligned with the sealing hole. Slight gland leakage is beneficial as it provides lubrication for the packing, and also prevents air ingress. The water flinger ring protects the bearing from water ingress and damage due to corrosion. The ring should be a snug fit on the shaft, else use sealing paste. For shell type bearings, clearances need to be checked, either with appropriate feeler gauges or lead wire gauges. Adjust for proper clearance by means of shims. 

Causes of bearing overheating are: 

a. Grease / Oil level too low or improper grade. 

b. Dirt in bearing or moisture. 

c. Bearing too tight. 

d. Oil seals fitted too closely on the shaft. 

e. Misalignment. 

Dismantle a Centrifugal pump for the survey:- 

Considering the fire pump is a centrifugal pump. Ensure the pump is stopped, shut the suction and discharge valve. Isolate the motor electrically by control room circuit breaker. Put Man at work tag. Electric isolation permit obtained. Local panel breaker put off. Check via purging cock that the line has no pressure. Place the dismantled part on a clean surface. When separating fit and flange faces, use jack bolts and wooden hammers, and never apply force with chisels or drivers. When removing the rotating element, take care to avoid flaws on sliding faces and machined surfaces. Particularly, never damage the mechanical seal mating faces. When removing rotating parts from the shaft, draw off each one carefully after removing the locking device. Handle the long sized-parts such as the shaft carefully so that it may not bend. At overhauling, put suitable match marks as many as possible to avoid mistakes when reassembling. 

Dismantling:-

1. Remove the distant piece, fitted between the pump and motor coupling after removing both motors and pump side coupling bolts and discs. 

2. Remove cooling connections to the mechanical seal. 

3. Remove casing top cover bolts. Once casing cover bolts are removed, the pump assembly is free to remove from the place along with the shaft, bearing housing, bearing, mechanical seal, impeller and impeller shaft with sleeve. 

4. On removing the pump assembly, slacken the impeller lock nut and remove the impeller from the shaft Remove shaft key. Care should be taken for not losing the shaft key. 

5. Remove distance ring. 

6. Slacken the holding screw and remove the mechanical seal's rotating part 

7. Slacken bearing housing bolts fitted on casing cover. 

8. Remove casing cover from the shaft. 

9. Remove shaft sleeve from the shaft. 

10. Remove the bearing housing cover 

11. Remove bearing retaining circlip. 

12. Remove bearing housing along with bearing. 

 Factors which will decide to replace the parts. The parts are to be renewed as per the manufacturers recommended period of services such as bearings and mechanical seals. In addition to that, every time upon dismantling a pump few consumable items are required to be replaced such as washers, seal rings, o-rings etc. Further, during its inspections as per the PMS and surveys, parts are checked inspected and tested. If they are not found in satisfactory condition, must be replaced. 

Cleaning and Inspection of parts: 

1. Remove bearing from housing for inspection. 

2. Clean impeller, bearing, shaft sleeve, casing and shaft. Inspect and evaluate each part for any damages, deformation, wear and tears. 

3. Measure the clearance between impeller and wear rings for the top as well as bottom near rings. If clearance is more than the maker's recommendation, renew it. 

4. Inspect shaft sleeve for any wear down. If sleeve found worn out, renew the same. 

5. Check the condition of the bearing. If found not good, renew the same. 

6. Check the conditions of the mechanical seal's rotary and stationary parts. If found damaged, renew it. 

7. Check bearing bush at the casing bottom and confirm any wear down. If worn out beyond acceptable limits, renew It. 

Assembling the pump: Assembly Carry out assembly, by reversing the order of disassembly and paying attention as follows. 

a. Remove dust and stain from each part by washing it thoroughly with kerosene. Repair it if a flaw is found. 

b. Fit the locking device perfectly in each rotating part if necessary. 

c. When fitting the parts with match marks, be sure no-follow them. 

d. Install the mechanical seal carefully and confirm its movement by hand after installation. 

e. Insert each packing ring in good order softly one by one from the bottom, staggering each joint by 90° or 180° 

f. Check up alignment. 

g. Turn the shaft by the band to see whether it tums smoothly.