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Pump repair and restoration guidelines

12/18/2005

By Heinz Bloch

Pump manufacturers usually supply pump maintenance manuals with detailed assembly and disassembly instructions that are either generic, or specific to a  particular pump style and model. Nevertheless, a number of important checks should be performed by users whose goal it is to systematically eradicate some of the often overlooked quality checks.

Unless a process pump manufacturer gives specific and different values or measurements for a particular make, size, or model, experience shows the following guidelines to be useful and valid. Perhaps your pump shop would benefit from making it a habit to use and apply this assembly dimension checklist. Some of the listed diametral clearance and/or interference tolerances will be more strict than what certain pump manufacturers allow for internal cost reasons, but then again, we see the need to improve on some products.

In any event, some plants have made copies, laminated them in plastic, and often even handed these to each shop technician, or posted these standards near the mechanics’ or repair technicians’ work stations.

01.   Radial ball bearing I.D. to shaft fits: 0.0001”-0.0007" interference

02.   Radial ball bearing O.D. to housing fits: 0.0001”-0.0015” clearance

03.   Back-to-back mounted thrust bearing I.D. to shaft fits: 0.0001”-0.0003” interference

04.   Back-to-back mounted thrust bearing O.D. to housing fits: 0.0001”-0.0015” clearance

05.  Shaft shoulders at bearing locations must be square with shaft centerlines within 0.0005”
   
06.   Shaft shoulder height must be 65-75% of the height of the adjacent  bearing inner ring

07.   Sleeve to shaft fits are to be kept within 0.0010”- 0.0015” clearance

08.  Impeller to shaft fits, on single-stage, overhung pumps, are preferably 0.0000”-0.0005” clearance fits. However, some pump manufacturers allow diametral clearances from 0.0015” to 0.0025”, depending on impeller diameter and pump speed

09.  Impeller to shaft fits, on multistage pumps, sometimes require interference fits and must be checked against the manufacturer’s or plant reliability group’s specifications

10.  Keys should be located in keyways with 0.0000”- 0.0001” interference. There are sound, experience-based reasons why we advocate “tightening” the manufacturer’s allowable 0.001” clearance
11.  In view of (10), above, keys should be hand-fitted to a “snug fit”

12. (a) Throat bushing to case fit is to be 0.002”-0.003” interference, depending on diameter

(b) Throat bushing to shaft fit is to be 0.015”-0.020” clearance

(c) Throat bushing to shaft fits of inline pumps (depending on    shaft size) where the throat bushings may act as intermediate bearings, will have clearances ranging from 0.003” to 0.012”
    
13.   Weld overlays can be substituted for metallic impeller wear rings. When separate wear rings are used, the impeller fit should be 0.002”-0.003” interference (Note: Applies to     metallic parts only).

14.  Impeller wear rings should be secured by either dowelling,     set screws that are threaded in the axial direction partly into the impeller and partly into the wear ring, or tack-welded in at least two places

15. Clearance between impeller wear ring and case wear ring should be 0.010” to 0.012” plus 0.001” per inch up to a ring diameter of 12 inches.

Add 0.0005” per inch of ring diameter over 12 inches. Use manufacturer’s guidelines when applying high-performance polymers such as Dupont Vespel®, good up to 500°F---note that tighter clearances are allowed!

16.   For pumping temperatures of 500°F and over, add 0.010” to the wear ring clearance.  Also, whenever galling-prone wear ring materials such as stainless steel are used, 0.005” are added to the clearance.
 
17.  Impeller wear rings should be replaced when the new clearance reaches twice the original value
 
18.  Case wear rings are not to be bored out larger than 3% of  the original diameter

19. Metal case ring-to-case interference should be 0.002”-    0.003”, depending on diameter
        
20.  Case rings should be doweled or spot welded in 2 or 3 places
              
21.  Metal oil deflectors should --if possible-- be fitted with an O-ring, and should be mounted with a shaft clearance of   0.002”-0.003”  

22.  On packed pumps, the packing gland typically has a shaft clearance of 1/32” (0.7 mm) and a stuffing box bore clearance of 1/64” (1.5 mm)
        
23.  On packed pumps, the lantern ring clearance to the shaft is typically 0.015”-0.020” (~0.4-0.5 mm)
 
24.  On packed pumps, the lantern ring clearance to the stuffing box bore is typically 0.005”-0.010”  

25.  Coupling-to-shaft fits are different for pumps below 400 hp (~300 kW) driver rating and pumps with drivers of 400 hp and higher: The below 400 hp pumps are often supplied with clearance fits of about 0.002”. To upgrade your pumps, employ fits metal-to-metal to about 0.0005” clearance. For 400 hp and larger models, interference fits ranging from 0.0005”-0.002” (0.012-0.05 mm) are recommended and routinely implemented by reliability-focused repair facilities

26. Taper bore coupling and hydraulic dilation fits should be as defined by either the manufacturer or your in-plant reliability group.

27.  On pumps equipped with mechanical seals, the seal gland alignment boss to stuffing box dimension should have 0.002”-0.004” (0.05-0.1 mm) clearance
 
28.  (a) Seal gland throttle bushing to shaft clearance should typically be 0.018”-0.020”, unless the pump is in “hot” service 
        (b) For “hot” service applications, a somewhat large clearance may be specified by the manufacturer or your plant reliability group.
        (c) Seal locking collar to shaft dimensions are typically 0.002”-0.004” clearance

29.  On cartridge seals, the manufacturer has set the correct seal spring compression. On non-cartridge seals---which are generally avoided by reliability-focused users---the recommended, and typical, spring compressions are:
 
(a)  for 7/8” (22 mm) long springs---3/16” (~5 mm);
(b) for ¾” (19 mm) medium-length springs---5/32” (4 mm); and
(c)  for ½” (short) springs--1/16” (1.5 mm)   

30.  On all mechanical seals, face flatness should be 3 light bands minimum
      
31.   Heads, suction covers, adapter pieces (if used), and bearing housing to case alignments should have 0.001”-0.004” (0.02-0.1 mm) clearance

                [Material contributed by Heinz P. Bloch and excerpted from Bloch/Budris “Pump User’s Handbook: Life Extension,” ISBN 0-88173-452-7]