1. Name parts of the centrifugal pump?
Rotary parts;
· Shaft
· Impeller
· Coupling
· Mechanical seal
· Oil rings
Stationary parts;
· Casing
· Stuffing box
· Bearing housing
2. Explain overhauling steps for a back pullout pump?
- Remove coupling guards after getting clearance of safety aspects like electrical isolation, process lines drainage etc.
- Remove coupling spacer and loose the casing bolts connects the stuffing box frame after removing and draining oil level glass and lube oil from bearing housing.
- Pullouts back the pump from casing with care of casing gasket.
- Dismantle all parts from the pump unit as impeller, mechanical seal assembly, shaft sleeve, coupling, and bearings respectively with high care.
- The following factors should be checked and recorded for future to estimate the performance condition of the pump, are wear ring clearance, seal compression, shaft deflection, thrust, trueness etc while or after dismantle.
- Proper tracing of parts need to any vision able damage like crack, score, break etc.
- Best is changing new one comparing reconditioning of parts.
- Assemble is the reversing procedure of dismantle as before seen.
- Before coupled alignment should be checked or corrected
3. What is impeller back clearance?
The clearance between the backside of the impeller (shroud) and the stuffing box or crown
(Wear) plate.
Actually this clearance standardize the stuffing box pressure and should be strictly followed incase of depressurizing vanes on impeller and semi open impellers.
4. How do you check impeller back clearance?
By the use of feeler gauge and with dial indicator as possible.
5. How do you adjust impeller back clearance?
· Add or remove shims behind the stuffing box.
· Changing the position of the rotor depends clearance if possible.
· Machining of impeller hub if there is no another way.
6. What is impeller front clearance?
The clearance between the front side of the impeller (crown) and the casing or wear plate.
This clearance should be strictly followed incase of semi open impellers.
7. How do you check impeller front clearance?
By the use of feeler gauge and with dial indicator as possible
8. How do you adjust impeller front clearance?
Add or remove casing gasket thickness
9. What is fixed bearing?
The bearing fixed with out any allowable axial clearance to permit thermal expansion.
This bearings bears the thrust load and only has thrust allowance as 0.001 – 0.003 inches
10. Where is fixed bearing located?
· In back pullout pumps, fans – inboard
· Double suction single discharge pump – outboard
· Mixers (agitators) – top bearing
11. What is floating bearing?
The bearing fixed with allowable clearance to permit thermal expansion. And takes radial load normally.
12. Where is floating bearing located?
· In back pullout pumps, fans – outboard
· Double suction single discharge pump – inboard
· Mixers (agitators) – bottom bearing
13. What is necessity of floating bearing?
· Elongation due to thermal expansion of shaft
· Axial fluctuation effects by load
14. Why bearing back-to-back arrangement is provided?
The back-to-back arrangement can support combined axial and radial loads and will keep the pump shaft end movement within acceptable limits. A bearing pair will support an axial load equally in either axial direction. It should preferably have a slight clearance (0 to 0.002 inches) with the bearing housing end cover.
16 .In which series bearing back-to-back arrangement is available?
· Single row angular contact ball bearing
· Taper roller bearing.
17. How do you check plain journal bearing clearance?
· By taking measurement difference between shaft outer and bearing inner diameters.
· By the use of plastic gauge or lead wire.
18. Is there any thumb rule for journal bearing clearance?
Bearing clearance necessary depends upon oil viscosity, speed and load.0.001per 1 inch of journal diameter plus 0.0015 inches up to 9000 feet per minute journal velocity. Above this speed 0.002 inches per 1 inch of journal diameter for shaft larger than 2.5 inches.
19. Have you seen tilting pad journal bearing?
Yes.
20. What are the various ways to check tilting pad journal bearing clearance?
- keeping dial indicator on shaft vertically besides bearing and lift the shaft to find out clearance of bearing through indicator reading.
- By the use of mandrel as same diameter of shaft and moving bearing.
- By manual measurement = diameter of pivot – (2 x thickness of individual pad) – diameter of the shaft.
21. What is the advantage of tilting pad journal bearing over plain journal bearing?
Suitable for higher speeds and loads and has greater stability. It avoids the possibility of oil whirl and allows self-aligning.
22. How do you adjust tilting pad journal bearing clearance?
By add or remove shims behind the shoes of tilting pads.
23. Give the ‘go’, ‘no go’ gauge dimensions for a 100 mm journal diameter with a minimum bearing clearance of 0.16 mm and maximum bearing clearance of 0.25 mm?
Go gauge dimension - 100.16 mm
No go gauge dimension- 100.26 mm.
24. Why plastic gauge are preferred rather than lead wire?
· Lead wire expands after removal from cap
· It can be become embedded in the Babbitt; especially in soft, high lead Babbitt bearing.
· Plastic gauge doesn’t need skill more and it is easy to identify just comparing the compressed width of plastic with the gauge paper.
25. How do you polish journal bearing?
With the use of special polish oil and 4/0 fine emery sheet or sponge pad. A special tool named as bearing scrapper used to clear deep able scratches.
26. What is spherical seated journal bearing?
The spherical seated journal bearing in which the shell of bearing in the form of spherical by which it allows self-accommodation on housing depends shaft aligns.
28. What is bearing crush?
Actually it defines the compression fit of the split type journal bearings diametrically and will not exceed more than maximum of 0.003 inches.
29. How do you check bearing crush?
Keeping 0.004 to 0.010 inches of shim according the plastic gauge series on the face of half split bearing housing and measuring the clearance between the bearing shell and cap (top cover) of bearing housing with the use of plastic gauge after tightening cover.
Now the bearing crush = shim thickness – plastic gauge reading.
30. How do you adjust bearing crush?
By providing shim piece in-between bearing shell and top cover of the bearing housing.
31. How do you check thrust-bearing clearance?
Keep the dial indicator horizontally on rotor in any accessible point. Move the rotor shaft longitudinally to find out the thrust clearance of the bearing after fixation of radial and thrust bearings.
32. How do you adjust thrust-bearing clearance?
Add or remove shims on the thrust-bearing collar if provision given otherwise Machining carried on in case of no other way.
33. How do you identify bearing pads based on their pivot?
Left hand and right hand pads.
34. How do you identify RH & LH pivoted pads?
If the loading point of pad on the right side called RH pads when seeing in front of the pad face. If the loading point of pad on the left side called LH pads when seeing in front of the pad face.
35. If the DOR of a rotor CW looking from C/E, where will you locate RH & LH pads?
Right hand pads on opposite side of coupling.
Left hand pads on the coupling side.
36. What is adaptor sleeve & withdrawal sleeve?
These sleeves used to fix and remove bearings easily on that and to be fixed any accessible place on the rotor. In adaptor sleeve, bearing is fastened while the adaptor sleeve nut tightened. In withdrawal sleeve nut is used to withdraw bearing from its position from the withdrawal sleeve.
37. How do you specify a bearing lock nut & lock washer?
N for bearing locknut
W for bearing lock washer
38. What is rotor free float?
It defines the total axial movement of rotor without thrust bearing.
39. Explain rotor centering?
It is the process of keeping rotor half of the free-float through the position of thrust bearing in the bearing housing fixation.
40. How do you adjust rotor centering without affecting axial float?
Increase or reduce the thickness of centering spacer fixed behind the thrust bearing on shaft. Add or remove gasket on the end covers of bearing housing. It allows only slight adjustment otherwise machining carried out on end cover in case of no other way.
41. What is radial centering?
Aligning the rotor axis in the same line with centerline of casing confined recess provided for the fixation of wear ring, neck bush, etc. radially. This centering keeps and helps to maintain constant gap between wear ring, neck bush, etc.
42. How do you adjust radial centering?
By the use of adjusting screws provided on the bearing housing carrier.
43. Explain the overhauling sequence for a multi stage centrifugal pump?
44. What do you mean by wear rings?
Wear rings are removable rings fixed on either impeller eye and casing or both of them.
45. What is the purpose of wear rings?
To avoid the change of impeller and casing due to pitting corrosion.
46. What are the different types of wear rings?
· Serrated wear rings
· Stepped wear rings,
· Reverse thread wear rings,
· Thrust balancing wears rings,
· Flat wear ring,
· L shaped wear ring,
· Impeller wear ring,
· Casing wear ring,
· Double ring with labyrinth leakage joint wear ring.
47. How do you check wear ring clearance?
Means of measuring the outer diameter of impeller wear ring or impeller eye and the inner diameter of the casing wear ring inner diameter by the use of micrometer or vernier caliper and with the use of feeler gauge if possible.
48. How do you adjust wear ring clearance?
By machining wear rings or change as new one if the clearance more.
49. How do you fix eyewear ring & casing wear ring?
Interference fit with Allen screw lock, pin with spot weld, direct bead weld, threading etc.
50. What the different types of shaft sleeve?
Threaded sleeve, sleeve with locknut, sleeve with Allen lock, straight sleeve, stepped sleeve, throttle sleeve, throat sleeve, etc.
51. Where are throttle sleeve & throat sleeve located in a rotor?
Throat sleeves located on throat bush area to control the flow of pumping liquid in to stuffing box area. Throttle sleeves are used in slurry service to avoid the flow of slurry liquid into seal area with the use of throttle packing which allows only the flushing liquid into pumping liquid area instead of reversing that’s why flushing fluid pressure is kept 15 psi more than suction pressure. ENSIVAL pump design has the provision of both throttle and throat bush arrangement.
53. Why balanced opposed rotor arrangement is preferred?
This arrangement minimizes axial thrust load of rotor by keeping the low pressures (suction pressure) on both side by the arrangement of impellers.
The balancing drum is located, behind the last stage of impeller.
55. What is the purpose of balance drum?
Used to take 90% of thrust load by liquid and the residual thrust is supported by the thrust bearing.
56. What is the applicable API standard for centrifugal pumps?
API 610.
57. What do you mean by oil wedge?
Under normal operating condition, the shaft of the machine will rise slightly up the side of the bearing. The amount of rise depends on shaft RPM, rotor weight and oil pressure. The shaft, thus operating in an eccentric position relative to the bearing center, draws oil to produce a pressurized load-carrying film called as ‘oil wedge’.
58. What do you mean by oil whirl and oil whip?
The eccentricity of oil wedge is momentarily increased from its equilibrium position, perhaps due to a sudden surge, an external shock load or other transient condition, additional oil is immediately pumped into the space vacated by the shaft. The result is an increase in the pressure of the load-carrying film. The additional force developed by the oil-film can drive the shaft into a whirling path around the bearing. If the damping within this system is sufficiently high, the shaft returns to its normal position in the bearing; otherwise the shaft continues in a whirling path called ‘oil whirl’
Lack of lubrication or improper lubrication causes excessive friction between the stationary bearing and rotating shaft, and the friction excites vibration in the bearing and other related parts similar wiping a moisture finger over a dry plane of glass called ‘oil whip’.
59. What is hydrostatic lubrication?
The pressurized lubrication oil given on oil wedge for lubricating and creating oil film in-between the moving parts. This type of lubrication called hydrostatic lubrication.
60. What is hydrodynamic lubrication?
The moving parts make the lubricating film in between contact area while running itself. This type of lubrication named as hydrodynamic lubrication.
61. What is forced feed lubrication?
Actually it is a pressurized lubrication, oil wedge formed by the pressure induced from separate lube oil pump, which driven by either separate drive or with drive from running rotor.
62. What is oil thrower ring?
The ring provided on the shaft in bearing housing to make efficient lubrication.
63. What is the purpose of oil thrower ring?
It is used to lubricate the bearing effectively by splashing out lube oil quenched and rotates the contact motion of shaft while running.
64. What is oil scoop?
It is a small bucket shaped part fixed on rotate able crankshaft
65. What is the purpose of oil scoop?
Used to splash lube oil intermediately for effective lubrication.
66. What is oil control ring?
A ring made of aluminium, brass or bronze fixed in the end covers of bearing housing.
67. What is the purpose of oil control ring?
The labyrinth grooved ring (oil control ring) used to avoid the leakage of lube oil to atmosphere and entry of moisture in to the bearing housing.
68. Why auxiliary lube oil pump is used?
Normally it’s a motor driven pumps used,
1. To maintain the lube oil pressure when starting and running time.
2. Auto start, incase of main lube oil pump failure.
3. To give lube oil circulation when the rotor is in ideal.
70. What is torque wrench?
It’s a mechanical tool used for tightening the bolts of equipments with proper torques by the help of the torque indicator attached with it.
71. What is rotor run out?
Rotor run out defines the accuracy of trueness of the rotor. This should not exceed more than 0.002 inches.
72. How do you check rotor run out?
After keeping the rotor on the bearings or lathe center, with the use of dial indicator.
73. What are the various ways of impeller fixing arrangements?
1. Threaded
2. Sliding fit with key and lock nut
3. Snug fit with sleeve lock nut.
4. With use of collets
74. What is the general value of rotor run out at various locations?
· Impeller- 0.0025”
· Sleeve – 0.0015”
· Bearing – 0.001”
· Coupling – 0.0025”
75. What is NPSH?
Net positive section head= static head + velocity head of liquid – vapor pressure at ambient temperature
76. What are NPSH required and NPSH available?
NPSH required establish the pump capability of working in minimum of positive suction head.
NPSH available establish the original working condition of pump in process with positive suction head.
NPSH available should be more than that of NPSH required.
77. What is cavitation?
Cavitation is the process of air or vapor lock in a pump. Reason for cavitation
1. Wrong NPSH
2. Pin hole or any air leak on suction line
3. Starvation of pump
4. Low velocity of liquid in the suction eye.
78. What is inducer?
A small low head, axial flow impeller that attaches to the conventional impeller is called as inducer.
79. Why inducer is used?
Inducer is used to increase the pump’s suction head (pressure), and prevent cavitation problems. The inducer will reduce the net positive suction head required of the pump or permit the pump to run at a higher speed.
Not all pump manufacturers have this feature available.
80. What is different between discharge head and discharge pressure?
Discharge head expressed in feet of the liquid being pumped from the discharge port.
The head varies depends on its discharge pressure.
Example: 1 Psi liquid raises 2.32 feet of height of liquid with the specific gravity of 1.
Discharge pressure expressed as Psi.
81. What is minimum flow check valve?
Minimum flow check valve is a non-return valve with a regulating by-pass port in the application of high temperature (near saturation temperature on the discharge) liquid services. Example- boiler feed water pump
82. Why minimum flow check valve is used?
To avoid the backing of vapor lock in the application of near saturation temperature liquid service by giving circulation of liquid while pump in ideal.
When the pump in ideal condition the by-pass port opens and allows the circulation of liquid and when the pump starts, the by-pass port closes by the action of main check valve opening.
83. What is vertical in line pump?
The pump fixed in the line of flow of liquid itself in vertical axis called vertical in line pump. Actually it is a booster pump for giving extra energy to liquid while flowing.
84. How do you center the rotor and position the coupling of a vertical in line pump?
Normally this pump have rigid muff split coupling attached with help of Allen screws, the shaft and coupling steps and grooves matched with together while fixing.
Add or remove the metal shims in-between the pump and motor frame for centering the rotor.
85. Explain the procedure for mechanical seal renewal of vertical in line pump?
· To get proper work and safety aspect from operations
· Remove coupling and before ensure the compression of seal unit by removing gland bolts.
· Remove coupling and take out gland plate outside
· Pull out compression unit (retainer) by special puller, care seal parts
· Check the packing area of sleeve for fretting corrosion and confirm any other defects in sleeve
· If the sleeve condition is good assembly of seal can start otherwise sleeve should be replaced by the pump overhauling
· Assemble is the same reverse procedure of the dismantle procedure before seen
· While assembling the following aspects should be checked, retainer seating on sleeve slot with lock pin perfectly, seal setting compression, bearing condition of the motor and alignment.
86. What is vertical sump pump?
The vertical pump designed and fixed on a sump called vertical sump pump
87. What is steady bearing?
Steady bearings are fixed in the vertical pump intermediately between the drop columns. It is actually bush bearing fixed in steady rest housing.
89. What do you mean by balancing line in multi stage centrifugal pump?
The line connects intermediate labyrinth of balancing drum (discharge side) with the suction port for thrust balancing of rotor named as balancing line in multistage centrifugal pump.
90. What is the purpose of balancing line?
Used for thrust balancing by connects suction port and balancing drum.
91. Why mechanical seals are used?
Used for hazardous and toxic liquids
· Zero leak proof
· Reliable
· Less friction loss comparing packing
· High efficiency
92. How do you specify a mechanical seal?
As per API –682
X1 X2 X3 X4 X5
X1 – Balanced (B) and unbalanced (U)
X2 – single (S), double (D), tandem (T)
X3 – Type of seal gland (T- throttle bush, A- auxiliary sealing device)
X4 – gasket material (R- Graf oil)
X5 – face material (M – means tungsten carbide Vs carbon)
93. Name few seal manufactures?
John crane, Pacific, Borg Warner, Dresser rand, Dura metallic
94. What is 15WRS seal?
It is a seal made of john crane as per API – 610
95. What is applicable standard for mechanical seal?
API – 682 (shaft sealing systems for centrifugal pump as well as rotary pumps)
96. What is the average life expected out of a seal as per API –682?
2 Years in normal working condition
97. Name few types of mechanical seal?
1. Single coil mechanical seal
2. Multi coil mechanical seal
3. Balanced mechanical seal
4. Unbalanced mechanical seal
5. Pusher/ non pusher mechanical seal
6. Single/ double mechanical seal
99. Explain the complete dismantling and assembly procedure for a mechanical seal?
1. Remove the gland from stuffing box to access mechanical seal with care after checking the seal setting compression.
2. Take out sleeve with retainer set and check any vision able effects like carbon crack, struck, wear on facing area both carbon and seat
3. Remove the seat from gland and justify the condition of secondary packing of seat.
4. Remove retainer set by loosing Allen screws before make conform the retainer houses on the shoulder of sleeve other wise take measurement of retainer position.
5. Full view of inspection to be done on each and every mechanical seal parts like springs, thrust plate, drive lugs, snap ring, seat seating area on gland, sleeve surface due to fretting corrosion, seal matting face of both seat and seal ring.
6. Changing new parts is better than reconditioning (lapping)
7. Assembling is the reverse procedure of dismantling on time the following aspects should be check:
· Square ness and rectangularity of seat after fixation on the gland plate.
· Trueness of the shaft and sleeve.
· Correct fit of wedge packing/ “O” ring with backup ring on the sleeve.
· The recess confined with gland plate and stuffing box.
· Seal setting compression as per drawings or as same before.
100. What is seal setting compression?
The seal setting compression given by the retainer in to matting faces as per designed by the suppliers
101. What is cartridge type of mechanical seal?
The seal is mounted on a sleeve that can be secured to the shaft from outside the seal chamber. It’s an advance-designed seal supplying by the seal manufacturers to make the installation easier. There is no need measurement while fixing and just changing as new set of cartridge seal instead of reconditioning.
102. Can you change the seal settings compression of a cartridge type mechanical seal?
No.
103. Explain the assembly procedure of a cartridge type mechanical seal?
Is a simple and easy assemble procedure comparing a normal mechanical seal
Just fix the cartridge seal set on the shaft and tight the gland plate with stuffing box. Tight the Allen screws on sleeve to lock with shaft then remove the seal setting lock plate provided on gland plate by loosening the sets screw.
104. How do you identify a balanced / unbalanced mechanical seal?
Identification of balanced seal:
1. Stepped sleeve on seal ring area.
2. Reduced contact area of seal ring by the provision of step.
105. What is pusher/ non-pusher type mechanical seal?
When the secondary packing of seal ring is affected due to the thermal expansion in ambient condition and the axial end movement of rotor called as pusher seal. Otherwise called as non-pusher seal.
Normally pusher seals are single & multi coil mechanical seals
And non- pusher seals are bellow seals.
106. What is all the pre check to be made on a pump before starting positioning a mechanical seal?
· Shaft true ness
· Stuffing box bore square ness
· Shaft deflection
· Thrust movement of rotor
· Dust free condition
107. What is stuffing box square ness?
It’s a process of checking the concentricity of stuffing box bore with shaft rotation by the use of dial indicator. The maximum allowable square ness is 0.0025”
109. What type of mechanical seal prefer for hot oil? Why?
Non-pusher seal (bellow seals) to avoid fretting corrosion
110. What type of mechanical seal prefers sulphur duty?
Single coil mechanical seal with seal ring made-up of carbon and matting ring made – up tungsten carbide to with stand high temperature distortion and slurry service.
111. What is seal flushing fluid? Name few seal plans?
Seal flushing fluids used in the mechanical seal system for cooling, cooling and lubricating the mating seal faces.
112. What is seal quenching fluid? What is its purpose? Name few seal plans?
Seal quenching fluid used behind the mechanical seal system in the gland plate design provision to avoid the hazardous or toxic liquid with atmosphere if the seal leaks in the form of liquid or gas state.
113. What is stuffing box cooling water jacket? Why it is used?
The integral or removable jacket provided in the pump for circulating the cooling water to maintain the stuffing box temperature into desirable limit for attaining the mechanical seal life reliable.
114. Why seal coolers are used?
To reduce the temperature of flushing fluid while entering into the mechanical seal system to maintain stuffing box temperature and better lubrication in the seal mating faces in running condition.
115. Name few MOC of seal faces?
Carbon, tungsten carbide, silicon carbide, stellite, peramic, ceramic, bronze, etc.
116. What is seat ring and mating ring?
The ring has the provision (springs or bellows) to give seal setting compression called as seat ring.
The ring mates with seat ring for sealing called as mating ring.
117. How many elastomers are used in a mechanical seal assembly?
1. Between seat ring and sleeve or shaft
2. Between mating ring and gland plate
3. Between shaft and sleeve
4. Between gland and stuffing box.
118. Name few MOC of elastomers?
Neoprene, viton, glass/ asbestos filled Teflon, Graf oil, Buna-N, buna-S, etc.
119. What is snap ring in a mechanical seals?
Snap ring is a cut off circular ring used to hold seal ring with advance compression. Snap ring rests on the groove made on retainer.
120. What are the usual problems faced with mechanical seals?
1. Sleeve fretting corrosion
2. Retainer springs struck
3. Seat or mating ring crack
4. Lack of reliable of secondary packing
5. Excessive wear of mating faces, etc.
121. Does the axial float have something to do with mechanical seal?
Yes. Seal setting compression will change.
122. What is double mechanical seal? Why it is used?
It is the two sets of simple mechanical seal. It is used in the application of;
1. High hazardous of pumping liquid
2. Usage of separate flushing fluid
3. A seal leak could cause a pollution problem
4. The product is very costly
123. Name two types of mechanical seal? Where is it used?
1. Back to back. The worst possible choice if used in the rotating seal version. Stationary versions are acceptable because the sealing fluid is located at the outside diameter of the seal faces where we can take advantage of centrifugal force
2. Tandem. One seal behind the other requiring a low pressure buffer fluid between the seals. This arrangement cannot be used if a higher-pressure barrier fluid is required or desirable.
124. What do you mean by barrier fluid? Name few barrier fluid?
Any time you use two seals in an application you will need a fluid between them. If the fluid between the seals is higher than stuffing box pressure we call it barrier fluid
Water is a good barrier and buffer fluid.
125. What do you mean by buffer fluid? Name few buffer fluids?
If the fluid between the seals is lower than stuffing box pressure we call it buffer fluid.
126. What is the difference between barrier fluid and buffer fluid?
Any time you use two seals in an application you will need a fluid between them. If the fluid between the seals is higher than stuffing box pressure we call it barrier fluid. If it is lower than stuffing box pressure we call it buffer fluid. The liquid can be circulated either by forced circulation, a pumping ring or convection. The method that you will use will be dictated by the pressure, pump speed and shaft size. Water is one of the best barrier or buffer fluids because of its high specific heat and good conductivity. Petroleum oil is probably one of the worse because of its low specific heat and poor conductivity
127. Where do we use cyclone separator in a mechanical seal?
In the application of the stuffing box pressure is more than the suction pressure of the pump to separate the foreign materials if anything in flushing fluid from the discharge of the pump by the action of centrifugal force.
128. What is pumping ring in a mechanical seal? What is its purpose?
The thermodynamic grooves feature made on the ring or the circumference of the retainer of the seal system is named as pumping ring. This has two types of radial and axial pumping ring. Used to give circulation and pressure to flushing fluid
129. What is the difference between inside mounted seal & outside mounted seal?
The inside mounted seal. All components are in the pumping fluid.
· Advantage. The elastomer can move to a clean surface as the seal face wears. Centrifugal force throws solids away from the seal components allowing the lapped seal faces to stay in co
· Disadvantage. All the metal components must be corrosion resistant to the pumping fluid.
· If the product solidifies or crystallizes when the pump is stopped, the seal can become inoperable.
The outside mounted seal. None of the metal components are in contact with the pumping fluid. Most designs clamp to the shaft rather than using setscrews that damage ceramic or glass coated shafts.
· Advantage. This is the most common solution to non-metallic pump sealing.
· Disadvantage. Centrifugal force throws solids into the lapped seal faces and can prevent the sliding components from moving freely. Higher pressure applications can cause the retaining clamp to slide on the shaft
130. What is seal balance ratio?
It is the ratio between the effective area of fluid pressure acting and the loading area of mating seal faces. It is actually defines the balancing of hydraulic forces.
131. What is the seal balance ratio for balanced/ unbalanced seal?
For balanced seal- 0.75
For unbalanced seal- 1.25 to 1.35
132. Differenciate centrifugal pump and reciprocating pump?
Centrifugal pump:
1. Working in the principle of centrifugal force
2. The main part is impeller
3. In the application of high capacity
4. Constant pressure with variable capacity
5. High efficiency
Reciprocating pump:
1. Working in the principle of reciprocating compression
2. The main part is piston/ plunger
3. In the application of high pressure
4. Constant capacity with variable pressure
5. Low efficiency
133. How do you measure Valve lift in a reciprocating pump?
In feather plate valve type the valve lift is equal to the thickness of the spacer used in-between seat plate and spring (cushion) plate; valve lift – 0.8 to1.5 mm. For the wing disc valve type it is the gap between disc top faces with the valve cover; valve lift- 3 to 6mm.
134. What is the necessity of pulsation dampener?
To minimize the fluctuation of flow of liquid in the reciprocating pump (+ve displacement).
135. Name few types of pulsation dampener?
1. Bladder
2. Diaphragm
3. Mechanical
4. Bellow
136. How do you fill up gas in an accumulator?
Gas is filled in an accumulator by the use of flow regulator valve into the desired pressure, which is normally 40 to 70% of the working pressure of the line to be damp.
138. What is web deflection? How do you measure web deflection?
Any movement of crank webs from their ideal position during the 360-degree rotation of the crankshaft is web deflection.
It is measured with the deflection meter or electronic web deflection system.
139. How to check big end bearing & small end bush clearance?
Big end bearing clearance:
Keeping dial indicator on connecting rod and lift it by a jack
Small end bearing:
By measuring outer diameter of gudgeon pin with inner diameter of bush in the small end or with the use of dial indicator or feeler gauge or plastic gauge which is adaptable.
140. What do you mean by stroke length?
The traveling length of piston/ plunger from BDC to TDC. It is the equal to the distance between crank pin center to crank shaft center.
141. What is gudgeon pin?
The pin, which connects the small end bush of connecting rod with the piston or cross head. The pin is well hardened to with stand tensile and compression load acting on the piston/ plunger.
142. What is crosshead slide/ shoe?
It is the two Babbitt lined adjustable shoes fixed to guide the reciprocal motion of the crosshead with an allowable clearance.
143. Name few ways of locking arrangement of piston/ plunger?
· Threaded with locknut
· Flanged bolted
· Locked by pin
· With split threaded coupling
144. How to ensure in-line concentricity of crankshaft to cylinder bore?
Keeping the two-dial indicators on the piston/ plunger rod with 90 degree of position and checking the deflection run out of the longitudinal movement of the rod on both sides when rotating crankshaft. The maximum deflection should be less than that of neck bush clearance otherwise the position of cylinder bore with crankcase is adjusted to attain the reading with in limit.
145. What are dosing pumps?
These are all the tiny low capacity reciprocating plunger pumps.
146. Name few types of dosing pumps?
1. Simplex
2. Duplex
3. Multiplex
4. Single acting
5. Double acting
6. Horizontal
7. Vertical
147. Why dosing pumps are used?
Dosing pumps are used to pump a low capacity liquid injected into a high-pressure process operation.
148. What are the probable reasons for cylinder knocking in a reciprocating pump?
1. Valve seat or parts damaged
2. High temperature of pumping fluid
3. Foreign materials in suction area
4. Valve holder looseness
5. Plunger looseness
6. Air / gas mixed on liquid
7. Flow restriction, etc.
149. What are the probable reasons for valve knocking in a reciprocating pump?
1. Worn out seating area
2. Springs broken
3. Valve holder looseness
4. Improper valve lift
5. Valve loose by improper tightening
6. Reusing worn parts
7. Foreign matters in valves, etc.
150. What are the probable reasons for vibration in a reciprocating pump?
1. Foundation looseness
2. Cylinder or valve knocking
3. Misalignment
4. Worn parts
5. Failure of vibration dampener
6. Wrong installation and assembly, etc.
151. Where do we use screw pumps & gear pumps?
In the application of moderate pressure and capacity & constant capacity with variable pressure requirement.
152. What do you mean by flute?
Flute is a helical wave formed on the screws of the screw pump. In between the flutes pumping fluid passes.
153. What is idler screw?
The screw on the screw pump driven by the engagement of the flutes of the main screw coupled with prime mover.
154. How drive is transmitted to idler screw?
Through the engagement of the flutes of main screw of the screw pump.
155. How do you measure radial clearance & axial clearance between flutes?
Radial clearance: With the use of feeler gauge
Axial clearance: Keeping the dial indicator on the main screw shaft end and moving it longitudinally.
156. How do you measure gear backlash?
With the use of feeler gauge or dial indicator measurement reference with pitch diameter of the gear.
157. What is the usual gear hardness value?
Gears made of alloy- steel; maximum hardness value – 38 HRC & pinion 2 HRC more than gear.
Gears made of hardened alloy steel; surface hardness up to 60 HRC.
158. Name few parts of a centrifugal compressor?
1. rotor
2. casing
3. diaphragm
4. diffuser
5. labyrinth
6. seal
7. Bearings, etc.
159. What is the applicable standard for centrifugal compressor?
API- 617
160. How do you specify a centrifugal/ horizontally split compressor?
MCL- horizontal split type centrifugal compressor, simply specify as ‘M’.As per the design of BHEL.
162. How do you center a compressor rotor?
Measure the total float, which is the axial displacement of the rotor with out thrust bearing and with journal bearings on both sides to avoid the rest of rotor on the labyrinth edges.
Now the centering of rotor is the half of the total float while the thrust plate loaded on the active side of the thrust bearing and is achieved through:
1. Increase/reduce the width of centering spacer fixed behind the thrust plate
2. Machining on the active thrust bearing collar and adding spacer behind the in active thrust bearing
3. Add/ remove the shims on the thrust bearing if the adjustable requirement is less.
163. How do you check labyrinth clearance for a barrel type centrifugal compressor?
Securing the half split labyrinth with together by a hose clip firmly and checking the measurement difference with use vernier or micrometers.
164. What is the compound used in the split half of the casing?
Copalite, key-paste, permatex.
165. How do you leak check a barrel type compressor?
The compressor pressurized to operating pressure by an inert gas like N2 . the shaft turn slowly with seal oil delta pressure 0.5 Kg/ CM2, and leakage seal across HP seals is measured with this, it is possible to judge the fitness of the seals.
167. Name the different types of labyrinths?
1. Conventional labyrinth seal
2. Straight type labyrinth
3. Stepped labyrinth
4. Knife edge labyrinth
5. J type labyrinth
6. Turbine type labyrinth
7. Honey comb labyrinth
168. How do you measure labyrinth clearance in barrel type compressor?
Securing the half split labyrinth with together by a hose clip firmly and checking the measurement difference with use vernier or micrometers.
169. How do you measure labyrinth clearance in horizontally split type compressor?
1. With the use of feeler gauge
2. Securing the half split labyrinth with together by a hose clip firmly and checking the measurement difference with use vernier or micrometers.
170. What is diaphragm?
It constitutes the dynamic profile of the stator parts of the compressor. Then are divided into three types viz. suction, intermediate and delivery. Suction diaphragm guides the gas to the first impeller. Intermediate diaphragms have the function of transforming the kinetic energy into pressure, return to channel serves to guide the gas to the next impeller with shock less entry. It has contoured passage leading from the diffuser of one stage to the impeller of the next stage. Some diaphragms are constructed with baffled passages for the flow of coolant. Delivery diaphragm forms the diffuser for the last impeller and delivery spiral.
172. What is balance piston?
The balance piston or drum fixed behind the last stage of impeller to take 90% of thrust load by gas and the thrust bearing supports the residual thrust.
In balancing piston the area in suction pressure side through balancing line is more comparing discharge side of last stage of impeller. Labyrinths on the balancing drum effects to reduce leak off between the both sides of pressure acting.
174. What is bridge over arrangement?
Bridge-overs are provided to reduce the number of compression stages and the omission of an impeller in the rotor assembly is
1. To provide an internal for a large side stream in connection
2. To make the critical speed of rotor out of normal operating range
3. To avoid interrupt of adjacent nozzle flanges each other.
176. What is lube oil run down tank?
It is an overhead tank for lube oil to supply for bearing lubrication in case of emergency purpose when the both lube oil pump failured, designed still the rotor stops and coming into atmosphere temperature.
177. What is the normal filtration size of LO & SO filter?
Lube oil filtration size- 20 to 40 microns
Seal oil filtration size- 10 to 20 microns
179. What is the purpose of PCV?
The pressure control valve is used to maintain the pressure of the seal oil in the header line.
180. What is the purpose of LCV in the seal oil circuit?
The temperature control valve is used to maintain the level of seal oil in the overhead tank.
181. What will happen if PCV opens more?
1. The pressure of the seal oil header increases
2. The level in the over head tank increases
3. LCV tends to open more to lower the level of over head tank
182. What is flow through valve?
The recycle gas compressors are applied 3 rings seal configuration because of high sealing pressure. However at start up operation handling gas pressure for regeneration is so low that insufficient seal oil quantity makes over heat of seal rings if seal system is used as is so, this is considered to solve such problem at start up operation by apply by-pass flow (flow-through) system with the use of a control valve called as flow- through valve.
183. Why buffer gas is used in the seal oil circuit?
The buffer gas is an inert or non- hazardous gas used in the seal oil circuit to avoid the contact of process gas with the seal parts and seal oil, if the process gas has hazardous or toxic or corrosive. The buffer gas pressure is normally 7 psi more than that of process gas.
184. Is there any other source for isolating the seal oil from process gas?
Yes. Dry gas seal.
185. What is dry gas seal?
It is a non- contacting gas lubricated seal compressor seal with controlled buffer or flushing gas. They have a rotating seat with a broad face and V grooves. When the seat rotates, the V grooves force gas between the sliding faces. The resulting rise in pressure causes the sliding faces to lift off and run with out making contact.
187. What is polytropic head & polytropic efficiency?
Polytropic head (H);
The work usually required to compress unit gas flow along the polytropic compression curve is called polytropic head. i.e. actual head developed by the compressor.
Polytropic efficiency;
It is the ratio between the power required to develop polytropic head in to the power actually supplied.
188. What is surging?
Surge can be defined as the capacity below 55 to 70%, which the centrifugal compressor performance becomes unstable. When surging occurs, the high-pressure gas at the discharge of the machine flows back through the compressor in a complete reversal of the normal direction of flow causing the collapse of pressure producing ability will repeat and cycle through the same sequence. Surging controlled by;
1. Increase the flow from process to compressor suction
2. By-pass some discharge through coolers, back to suction of the unit
3. Decrease the pressure ratio.
189. What is stone wall effect?
Chocking or stone wall effect is in the large flow region, if the flow velocity reaches sound velocity anywhere in the compressor. The compressor can’t generate the head and all input energy are spend to over come the friction.
190. What is stalling?
Is a performance of vortex gas flow generated in the impeller internal or diffuser area as a symptom of surging when gas flow is decreased. To accompany this phenomenon, resonance of impeller and vibration sound (humming sound) will be generated. For corrective measure increase compressor flow
Simply stalling means fluctuation in flow rate.
192. What is contact type & non- contact type seal?
The sliding faces of seal mating together when running called as contact type seal.
The sliding faces of seal with out mating contact with together when running called as non-contact type seal.
193. Is the seal rings stationary, rotating or floating?
Floating.
194. How do you measure seal ring clearance?
With the use of vernier caliper or micrometers.
195. Does the seal ring have clearance with housing? Why?
Yes. Because of floating on its position.
196. What is the general value of seal ring clearance?
Inner seal ring (gas side) – 0.002 to 0.004 inches
Outer seal ring (oil side) – 0.005 to 0.006 inches
198. What is honeycomb labyrinth?
The use of honeycomb labyrinths offers even better control of leakage rates (up to 60% reduction compared to straight pass type). It operates at approximately half of the radial clearance of conventional labyrinth seals. It consists of S.S. foil about 10 mills thick. Hexagonal shaped cell make are enforced structure that provides a larger number of effective throttling points. In addition S.S. honeycomb retains its strength at high temperature and pressure levels, which would cause weakening of an aluminum labyrinth.
199. What is head?
Head used to close either side of the barrel and house the bearing end gas seals and oil seals made of solid forging. Depending gas medium either carbon steel or stainless steel are selected.
200. What is head puller?
It is arrangement of mechanical tool used to pull up head from the barrel of compressor on its position.
201. What is bellyband?
It is a large steel clamp fabricated to hold the full assembly of rotor with diffuser and diaphragm housings with together for lifting and transporting the assembly safely.
202. Differentiate bundle ‘o’ring & head ‘o’ ring?
Bundle ‘o’ ring avoids the leakage between suction and discharge side of gas flow.
Head ‘o’ring avoids the leakage of discharged pressure gas into atmosphere. This ‘o’ring should be used with a back up Teflon ring to neglect extrusion due to high-pressure ratio.
203. What is hydraulic fit coupling?
The coupling can be fixed or removed in the shaft easily with hydraulic pressure by a separate hydraulic pump. This coupling has interference fit with shaft.
205. What is pusher pump & expander pump?
The pump connected in the jack to pull or push the coupling into shaft named as pusher pump.
The pump used to induce high hydraulic pressure between the contacting face of coupling and the shaft to make work easy named as expander pump.
206.Is hydraulic fit couplings with ‘o’ rings or with out ‘o’ rings?
With ‘o’ rings.
207. How do you ensure fit of taper coupling?
By the projection of the coupling face with the shaft end face. It is between 1/16 to 1/8 inches.
208. What is diaphragm coupling?
It is one of the flexible coupling used between compressor and steam turbine. All parts of this coupling are solidly bolted together, no moving or no wearing parts, therefore no lubrication is necessary.
209. What is active shim & store shim?
The shims connected on coupling with drive or driven hubs to transmit load with flexible named as active shims.
The extra shims bolted steady with coupling and these have not take the load of transmission named as store shims. It should be fixed for the consideration of dynamic balancing of the rotor and add or remove the shims in between the coupling depends span length.
210. What is shear plate coupling?
This coupling has the several of round plates with holes for bolting coupling spacer with hub intermediately and it has be sheared & failured in case of high torsion force of drive exceeds ultimate limit. It is simply known as metal flex coupling.
211. Name different types of coupling?
Rigid coupling
1. Flanged coupling
2. Muff coupling
3. Threaded sleeve coupling
4. Four piece coupling (in Gould’s sump pump)
Flexible coupling
1. Metal flex coupling
2. Diaphragm coupling
3. Gear coupling
4. Grid coupling
212. What is the allowable vibration level as per API standard?
As per API 670
Vibration amplitude = root of (12,000 / RPM) mills per second.
213. Name few parts of a reciprocating compressor?
1. Cylinder
2. Piston
3. Valves
4. Cross head
5. Connecting rod
6. Crank shaft
7. Crank case
214. What is the applicable standard for reciprocating compressor?
API 618
216. What is top dead center? How do you measure it?
The end of transverse movement of the piston towards cylinder head is the top dead center of the reciprocating compressor.
It is measured by keeping lead wire in the cylinder towards head end and rotating the compressor till the piston come back after compressing the lead wire. Now the top dead clearance is equal to the thickness of the lead wire.
217. Is there any means to increase TDC? How to do it?
TDC can be increased through;
1. Increase or decrease the thickness of the cylinder head gasket
2. Loose or tightening the piston rod in the cross head if threaded with piston rod locknut
218. What is bottom dead center? How do you measure it?
The end transverse movement of piston towards crankshaft end is the bottom dead center.
It is measured by keeping lead wire in the cylinder towards crank end and rotating the compressor till the piston come back after compressing the lead wire. Now the bottom dead clearance is equal to the thickness of the lead wire.
219. Why TDC is more than BDC?
For the consideration of thermal elongation of crank, connecting rod, piston rod towards the cylinder head side at working temperature.
220. What is rod deflection? How do you measure it? When will it increase?
It is the deflection of the piston rod from its trueness position while running.
Measuring: keeping two dial indicators on the piston rod with 90 degrees each of the recess in the distance piece of reciprocating compressor and measure the reading of indicator while rotating compressor. Deflection will increase due to
1. Accelerated wear and scuffing – piston to liner, piston rings, piston rod packings.
2. Broken rod
3. Damaged cross heads
4. Damaged pin bushings
5. Frame failure
6. Misaligned concentricity of cylinder bore with crank shaft.
221. What are the various clearances measured during overhauling of reciprocating compressor?
1. piston end clearance
2. piston ring side clearance
3. piston ring end clearance
4. diametrical clearance between piston with cylinder/ liner
5. rider ring diametric clearance
6. web deflection
7. piston rod run out
8. connecting rod big & small end bearings clearance
9. crank shaft main bearing clearance
10. crank shaft thrust allowance
11. cross head with cross slide clearance
12. gas & oil wiper packings clearance
222. What is cylinder to piston clearance? How do you measure it?
It is the diametrical clearance of piston outer with cylinder or liner bore.
It can be measured by feeler gauge or micrometers.
223. How do you check piston ring end clearance?
By inserting the piston ring in to the cylinder bore and checking the clearance of the piston ring ends with the use of feeler gauge.
224. What is difference between piston ring & bearer ring?
Piston ring, which used to avoid leakage from the piston end compressed gas to outwards through piston circumference.
Rider or bearer ring used to guide the piston movement and neglect the scuffing of piston with cylinder liner when the piston rings are damaged. It has less clearance with the cylinder bore.
225. What is the MOC of piston ring & bearer ring?
1. Cast iron
2. Carbon
3. Bronze
4. Teflon
5. Phenolic
226. How much the normal clearance between piston ring to piston groove?
It simply specified as piston ring side clearance
For MOC of cast iron, bronze, carbon – 0.001 inches
For MOC of Teflon – 0.012 inches
227. Name different types of gas packing rings?
1. pressure breaker ring
2. radial cut ring
3. tangential cut ring
4. anti extrusion ring
228. What is pressure breaker ring? Where it is located? Which one is pressure breaker ring?
This is the simplest form of packing ring acts to break or slow down gas passage only with out actually sealing it completely. This prevents damage to packing rings and garter springs due to the shock effect of the differential pressure involved.
It has a limited clearance at the cut only and should not exceed more than 0.010 inches per inch of bore diameter. Obviously this ring should not be allowed to operate with out end clearance because its life would be very short. This ring located in prior towards gas side.
229. What is sealing ring? Where is it located? Which one is sealing ring?
The true sealing ring made up of a pair of rings, the first of which is radially cut with considerable clearance at each end. The second ring is tangentially cut forming an overlapping seal joint that prevents gas passage from the outside periphery toward the rod bore. The inner radial cuts of this tangent ring are arranged so that they occur approx. between the radial cuts of the first ring in the pair. In this manner, gas passage along the rod is blocked so that there is no through escape.
This type of ring is single acting or directional in that it seals pressure from one side only.
232. Is there any clearance between cup to packing? Why that clearance is given?
Yes. This clearance given for
1. space for oil film lubrication
2. shock effect of the differential pressure involved
233. What is the MOC of gas packing rings?
1. Tetra fluro ethylene
2. Carbon
3. Bronze
4. Teflon
5. Babbitt material
6. Cast iron
234. What are various methods of capacity control?
1. speed regulation
2. suction throttling
3. holding inlet valve open
4. clearance volume variation
235. What is clearance pocket?
Clearance pocket made up of a cylinder with a piston to regulate the capacity in high-power compressor by varying the clearance volume. It can be operated manually or by automatic means and divided into two types of
1. constant clearance pocket
2. Variable clearance pocket.
When this pocket in operation the compressor neither draws in nor delivers any gas: both valves are closed and a constant amount of gas is expanded and compressed in the cylinder.
236. What is suction unloader?
It is a pneumatically operated piston-type device overcomes the tension of a spring and moves down the rod of the piston like element terminates in fork, the prongs of which will keep the inlet valve plate off the seat. As a result no gas will be compressed and delivered because the inlet valve will be open, and the will be pushed out of the cylinder into the intake line until the air pressure relived in the piston of unloader. Thus the compressor capacity is reduced by skipping the discharge strokes.
239. How do you recondition the valves of a reciprocating compressor?
The main work of reconditioning the valve is to make the seating area of plates in perfect appropriate level by lapping. Out of this well vision able inspection of valve plate, seat plate, lock screw and the condition of springs should be adapted.
240. How do you leak check a valves of a reciprocating compressor?
1. liquid filled observe for 1.5 minutes and slots of seat must still be half full
2. Leak check by connecting to metered air supply system.
241. Second stage discharge pressure is high? What is the problem? How will you identify?
Causes
1. inter cooler between second and third stage or after cooler has not effective cooling – feeling cooling water circulation
2. unloader not working – check the unloader spindle position any marked
3. second stage discharge valve not opened properly – feel temperature of valve
4. improper piston end clearance
5. third stage suction valve not opened – checking with stethoscope
242. There is a capacity reduction in the compressor? What is the problem?
1. suction valve damage/passing
2. discharge valve damage/passing
3. clearance pocket may be in open condition
4. unloader loaded
5. damage piston rings or packing rings
6. excessive piston end clearance
7. speed less
243. There is a reduction in the discharge pressure of the compressor? What is the problem?
1. Suction valve damage/passing
2. Discharge valve damage/passing
3. Clearance pocket may be in open condition
4. Unloader loaded
5. Damage piston rings or packing rings
6. Excessive piston end clearance
7. Speed less
244. What is the normal pressure ratio of discharge to suction?
14: 1
245. Why intercoolers are used?
a. Remove heat of gas taken by compression
b. Reduce the volume of gas consistence to other stage
c. Save in power
d. Increase efficiency
246. What is distance piece?
This piece with inspection doors fixed in between compressor cylinder and the cross slide house. It accomplishes space for gas packings gland and wiper rings through inspection door. Drain line attached bottoms of it for identification incase of gas or oil leakage if any.
This can be aligned with cylinder or crankcase with measurement of rod deflection.
247. What is oil wiper packing?
Wiper packings fixed on the piston rod in the side of cross slide to avoid the leakage of oil outside and avoid the entry of gas into crosshead area incase of gas packing rings leakage. These packings are same like gas packing seal rings.
249. What is cylinder jacket?
The jacket made with cylinder integrally or passage sealed with cylinder liner ‘o’ rings for removing heat from gas by compression and friction heat of piston rings.
250. What is stuffing box jacket?
Cooling water circulated jacket provided in gas packing stuffing box for removing heat generated through the friction of packing rings with piston rod.
251. How the cooling water is prevented from entering the gas side?
With the use of ‘o’ rings on cups of stuffing box.
252. What is volumetric efficiency?
It is the ratio of the actual capacity of the compressor to displacement (swept volume) and is expressed as a percentage.
253. What is need for cylinder liner? What is its MOC?
Cylinder liner is used to avoid the change of cylinder or major recondition incase of high scuffing due to friction with damage or broken piston rings.
Liners are normally made of cast iron.
255. What is cylinder liner lubrication?
A force feed lubrication given to cylinder liner to minimize the friction with piston rings is called cylinder liner lubrication.
More lubrication causes the discharge valve plates to stick.
256. How do you lubricate the cylinder liner?
Force-feed lubrication by lubricator pumps given into the center of stroke length in the cylinder port provided.
257. what is single acting, double acting and balanced opposed?
Single acting:
Compression of gas takes place in one side of the piston only.
Double acting:
Compression of gas takes place on both sides of the piston.
Balanced opposed:
Two set of piston and cylinder arrangement made in the single crank shaft horizontally 180 degree oppose for dynamic balancing and admit of rotative speeds higher than other types.
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