Friday, November 14, 2008

maintanance and inspection of screw compressor

The installation should be inspected regularly at intervals that will be determined by the severity of operating conditions. For convenience, suggested maintenance routine may be segregated into two separate but interdependent classifications as follows:

I] OPERATION INSPECTIONS
An accurate operational inspection system is the best means of detecting the need for maintenance work.

a) Continuous log or graph of all pressure and temperature readings should be kept. For the first few weeks of operational break-in, 4 readings per 8 hour shift are recommended. After that, the readings could be reduced to a suggested minimum of two per shift. Any rapid changes in consecutive readings would indicate possible malfunction and should be investigated immediately. Any gradual but consistent change not related to normal ambient or process variations should also be investigated.
b) Any change in the characteristic should or increase in vibration of the unit should be investigated.
c) Oil and water leaks should be repaired when first observed.
d) The water flow from jacket, after cooler and oil cooler should be observed when each reading is taken.
e) Check lube oil level in reservoir.
f) Make all checks recommended for the main driver.

II] PERIODIC INSPECTION
a) Every 500 – 700 hours: Check operation of alarms and controls.
b) Every 1000 – 1500 hours: Drain oil sample from reservoir. Check oil for oxidation, contamination and water. Frequency of oil change depends upon this analysis.
c) Every 7500 – 8500 hours (Annual):
  • Inspect rotors through inlet and discharge nozzles. Some polish areas on the rotor lobes are to be expected, due to float and relative shift of the rotors during start up and shutdown. Excessive rubbing between rotors or wear on radial and end sealing strips will require readjustment or possible replacement of the bearings.
  • Remove discharge end cover and inspect timing gears. There should be even wear pattern on gear teeth.
  • Measure thrust bearing end clearance with a dial indicator. Measure dial bearing clearance with a dial indicator mounted through thrust bearing housing drain opening.
  • Loosen inlet end cover and visually inspect radial bearings.
  • Remove compressor jacket cover plates and inspect for sediment or calcium deposits. Clean if necessary.
  • Remove coupling guards and inspect couplings for broken shims.
  • Recheck alignment at all couplings.
  • Check and reset, if necessary, all temperature and pressure gauges and pickups.
  • Inspect inside of silencers for deterioration of foreign materials.
  • Inspect water side of after cooler and oil cooler for foreign material or calcium deposits and clean if necessary.

d) Every 15000 hours (2 years):

  • Inspect per paragraph 2c.
  • Remove coupling hub at inlet end cover. Measure inlet radial bearing clearances with a dial indicator mounted adjacent to the bearing.

e) Every 30000 hours (4 years):

  • Inspect per paragraph 2d.
  • Remove and inspect timing gears, thrust bearings, radial bearings and seals.
  • With radial bearings in place, indicate shaft runout at all diameters of each shaft with a dial indicator.
  • If condition dictate, remove the rotors from the housing, clean and inspect the rotors and housing bores.


shut down procedure of screw compressor

When unit is to be taken off the stream and shutdown, the following procedure should be followed:
1. Gradually open bypass to reduce discharge pressure to ½ normal pressure. Operate unit at this condition for approx. 20 minutes or until the gas discharge temperature levels off.
2. Stop unit. If unit starts turning backwards, immediately close the block valve.
3. Allow lube oil to circulate for 10 minutes after the unit is stopped to properly cool the rotors.
4. If unit is to be shutdown for an extended period in a freezing environment, drain all water from oil coolers, compressor jackets and piping.
5. If unit is shutdown for an extended period of time, following maintenance program should be followed to keep unit ready for service:

Open compressor inlet casing drains to allow condensate to drain out Circulate lube oil for a minimum of one hour once a week and rotate unit several revolutions.For extended down periods, remove silencers and piping from compressor inlet and discharge and spray a light film of lubricating oil on rotor surfaces and machines internal housing surfaces once every two weeks to prevent rusting If the unit shuts down instantly under full load (power failure, emergency shutdown), following procedure is to be followed if unit is to be started:

1. Let the cooling water flow through the oil cooler and compressor
2. Circulate the lube oil for one hour and rotate the compressor drive shaft ¾ turn by hand every 10 minutes
3. Turn until over by hand three revolutions. The unit is ready to restart when it turns normal with no tight areas.
4. Restart unit in normal manner. Operate at approx. ½ normal pressure for 15 min before applying rated pressure

screw compressor-normal operation

Some conditions are subject to slight variations as listed below:

1. Process gas pressures and temperatures - changes in process requirements may cause a variation in operating temperatures and pressures. System temperatures will vary directly with incoming gas temperatures. Temperature differential between inlet and discharge nozzles should remain fairly constant. Pressures will be dependent upon and affected by incoming gas and barometric conditions.

2. Bearing temperatures - bearing temperatures will vary slightly with ambient temperature and with lube oil temperature. Higher lube oil temperatures will produce proportionately higher bearing temperatures. A variation of 6 C to 11 C in bearing o o temperatures is of no consequence.
A running plot of all temperatures and pressures is advisable. Log all gauge pressure and temperature readings at initial start after unit has reached normal operating conditions. Use these initial readings as a guide and check frequently as recommended in “operational inspection”

Any rapid or gradual change, not due to process conditions, may indicate a possible malfunction.

screw compressor-normal start procedure

After all the precautionary measures have been taken, the unit can be started as follows:

1. Turn on the seal buffering gas (if applicable)
2. Start the main lube oil pump for prefabrication. Prelube compressor approx. One hour at initial start and for 10 minutes after each shutdown and at each subsequent start. Start pressure gauges and sight glasses for oil flow.
3. Turn on the cooling water so that a steady continuous supply flows into the unit
4. Check all valves external to the machine for proper adjustment
5. Bar unit over using strap wrench.
6. Make sure process shut off valve and bypass valve is open. Start the driver in accordance with the manufacturer’s instructions and bring unit up to speed. Gradually choke down on bypass valve and bring unit up to pressure over a period of approx. 15 min.
7. Check all lube oil pressure gauges and sight flow indicators to see that the machine is being properly lubricated. Also observe temp. Gauges.
8. Observe the action of the machine to make sure there is no undue vibration or noise.
9. Check auxiliary equipment. Be sure the unit is not operating beyond the rating stamped on the nameplate
10. In the event of an emergency shutdown while unit is operating, do not attempt to restart compressor until it bars over freely.

screw compressor-initial start procedure

1. Run driver up to speed without load as follows:

  • disconnect coupling between driver and gear
  • pump driver and check rotation
  • run driver up to speed, checking for undue noise and vibration

2. Run driver and gear.

  • disconnect coupling between gear and compressor
  • check alignment between driver and gear
  • connect coupling between driver and gear
  • start lube oil pump and check pressures and flows to gear
  • pump driver and check gear rotation
  • run driver and gear up to speed, checking for undue noise and vibration

3. Run in compressor on gas as follows:
  • Install temporary wire screens at the joint between the compressor and inlet silencer to catch any weld berries, slag or dirt remaining in the pipe. Screen information is found at the back of this section. Remove screen before putting unit into service.
  • Bar compressor over by hand to check for freeness. If any metal- rubbing-on metal sounds are detected in the compressor, determine the cause and correct the same before starting. Use strap wrench to bar compressor.
  • Check alignment between the gear and the compressor. Couple compressor to gear.
  • Turn on buffering gas and/or other sealing fluid to compressor.
  • Start main oil pump and check flows and pressures to bearings. Oil temperature should be 16 C to 27 C before starting unit.
  • Make sure process shutoff valve and bypass valve are open.
  • Run driver for a period just long enough to bring the unit up to approx. ¼ speed, trip off driver and observe the unit while it coasts to a stop to make sure there is no undue vibration or noise. Bar unit over again to check for freeness.
  • Start driver and bring unit up to speed. Do not operate for more than two minutes with zero discharge pressure.
  • Gradually close down on the bypass valve and bring unit up to 50% of discharge temperature over a period of 15 minutes. (if applicable) Run for 30 minutes at above condition and observe operation of unit and auxiliary equipment. Check for any unusual vibration or noise.
  • Stop driver and let unit coast to a stop.
  • Bar unit over to check for freeness.
  • Check inlet screen and clean if necessary
  • Continue to operate unit for 30 minute interval until inlet screen is no longer picking up dirt
  • Always turn injection water off immediately before shutting off main driver. (if applicable)

screw compressor-pre start procedure

Before starting the unit for the first time, and always after a major overhaul or repair,check the following:
1. Be sure the intake, all oil and air piping and internal parts of the unit are free from dirt and moisture.
2. Remove paint and preservative coating, if used, from all moving parts. (See instructions for removing preservatives)
3. Extreme care must be taken to prevent foreign material such as nuts, bolts, tools etc from remaining in or dropping into any of the piping or compressors.
4. See that all bolts and nuts are secure. Some may have loosened in shipment.
5. Check alignment
6. Lubrication- in preparation for start up, flush the lube oil system and adjust the lube oil pressure switches. Check all switch settings in accordance with the lube / seal oil console section.
7. See that all valve internal to the unit are properly adjusted.
8. Check all other auxiliary equipment including safety alarm system to see that it is in proper order
9. Check compressor side inlet drain for condensation at initial start and after a shutdown period.

Tuesday, November 4, 2008

flexible element coupling

1. Flexible elements may be straight sided, contoured, or convoluted. Acceptable arrangements include a single or element, multiple series elements, or multiple parallel elements (elements packs) at each end of the spacer.

2. The flexible elements shall be positively secured to adjacent parts of the coupling by splines, bolts, or welds. Alternative methods shall be used only when approved by the purchaser.

3. When the flexible elements of a coupling in intermediate-or-high-speed service are combined in a factory-assembled disk pack, the coupling spacer shall be removable without disturbance of the factory assembly of the elements (disks).

4. Unless otherwise specified, the vendor shall treat the coupled equipment as an infinite mass when calculating the ANF. The ANFs of the coupling shall not fall within 10 percent of any of the following speeds or ranges:

a. Any speed within the range extending from the minimum allowable speed to the maximum continuos speed.
b. Tow times any speed within the range specified in item a.
c. Any other speed or exciting frequency specified by the purchaser. (For some applications these restrictions may preclude a practical coupling design. In this case the vendor and the purchaser should investigate and agree upon means of relaxing these criteria).

5 If the coupling must operate within a close-fitting enclosed coupling guard, the purchaser will furnish details of the guard for the vendor to inspect. The vendor shall determine and so advise if cooling is required and, if necessary, shall recommend a cooling system for the coupling.

6 When a tapered hub is specified for one or both ends of the coupling, the vendor shall furnish spacer shims to adjust the spacer gap. The shims shall provide a variance of +1 1/6 inch (1/8 inch total) [+ _ 3.2 millimeters (6.4 millimeters total)] in couplings for shafts with a nominal diameter of less than 4 inches (102 millimeters). In coupling

trim-balance holes

When specified, tapped holes shall be provided in the coupling for trim balancing. The number, size, depth, and location of such holes shall be agreed upon by the purchaser and the vendor. The optimum location for keyed hubs is generally on the outboard faces of the hubs, midway between the inside and outside diameters of the hub barrel. The optimum location for keyless (hydraulically fitted) hubs is generally on the coupling flanges, between the bolt holes of the flange.

Note: Because of eccentricity of the shaft end or incompletely filled keyways trim balancing the rotor after the coupling hub has been mounted may be advisable. The practice normally precludes moving the hub to another rotor, unless balance is achieved by using balance holes. When balance holes are used the hub can always be returned to its original state of balance by removing the weights inserted into the holes.

Materials

1. Except as required by the data sheets or this standard, materials of construction shall be the vendor’s standard for the specified operating conditions.

2. Material shall be identified in the proposal with their applicable ASTM, AISI, ASME or SAE numbers, including the material grade. When no such designation is available the vendor’s material specification, giving physical properties, chemical composition, and test requirements shall be included in the proposal.

3. Neither copper nor copper alloys (excluding Monel and precipitation-hardened stainless steels) shall be used for coupling parts.

4. All metallic components of the coupling shall be made from high-quality material manufactured by hot rolling, cold finishing, or foreign and shall be appropriately heat treated. Hubs and sleeves shall be made of alloy steel. Flexing elements in flexible-element couplings shall be of corrosion-resistant material or shall be suitable coated. The vendor shall state the nature of the coating and how it must be applied. The purchaser will specify whether all other parts shall be made from corrosion resistant material or suitably coated.

5. The purchaser will specify any corrosive agents present in the environment, including constituents that may cause stress corrosion cracking.

6. All fasteners shall be of heat-treated alloy steel, SAE J 429, Grade 5, or better. The threads shall be American Standard unified fine thread series. Materials shall be corrosion resistant to the specified environment. If plated bolts are required, they shall be treated properly to avoid cracking caused by hydrogen embrittlement. The quality of the nuts shall be at least equal to that of the bolts.


balancing options in coupling

Unless otherwise specified, couplings shall be component balanced. Each component, such as the hubs, sleeves, flexible elements, shims, spacer, and adapter plates, shall be balanced individually. All machining of components, except for the keyways of single-key hubs, shall be completed before balancing. Two-plane balancing is preferred, but single-plane balancing may be used for components with a short axial length. Each component shall be balanced so that the level of residual unbalance for each plane does not exceed the greatest value determined by the following expressions:

U = 4W/N

U = 0.0008W

U = 0.01

In SI Units,

U = 6350W/N

U = 50.8W

U = 7.2

Where:

U = Residual unbalance, in ounce-inches
(gram Millimeters)

W = Weight of the component, in pounds (kilograms),
apportioned to the balance planes so that the sum
of the weight apportionment for both planes equals
the total weight of the component.

N = Maximum continuous operating speed,
In revolutions per minute.

1 When specifiedm residual unbalance checks shall be performed on components of couplings with maximum continuous speeds of 5000 revolutions per minute or greater. These residual unbalance checks shall be performed after balancing is complete and before the component is removed from the balancing machine.
2 Unless otherwise specified, couplings balanced in accordance with the above shall be assembled and the balance verified. The residual unbalance for the randomly assembled coupling shall not exceed the greatest value determined by the followin expressions:

U = 40W/N

U = 0.008W

U = 0.1


In SI Units,

U = 63,500W/N

U = 0.8W

U = 72.0

Where:

U = Residual unbalance, in ounce-inches
(gram Millimeters)

W = Weight of the coupling, in pounds (kilograms),
apportioned to the balance planes at the two coupling
hubs so that the sum of the weight apportionment
equals the total weight of the coupling.

N = Maximum continuous operating speed,
In revolutions per minute.


Couplings that fail to meet these criteria shall be balance corrected by repeating the component balance, not by trim balancing the assembly.

Sample plot and calculation of Resiudal Unbalance

3 When specified, an assembly balance shall be performed, and the components shall be match marked. The coupling shall then be match marked and two-place balanced, with corrections being made only to the component or subassembly that was not periously balanced. The final residual unbalance of the assembled coupling in each of the two correction planes shall not exceed the greatest value determined by the following expressions:

U = 4W/N

U = 0.0008W

U = 0.01

In SI Units,

U = 6350W/N

U = 50.8W

U = 7.2

Where:
U = Residual unbalance, in ounce-inches
(gram Millimeters)
W = Weight of the component, in pounds (kilograms),
apportioned to the balance planes so that the sum
of the weight apportionment for both planes equals
the total weight of the component.
N = Maximum continuous operating speed,
In revolutions per minute.

4 When specified, the coupling shall be checked after the assembly balance to ensure that the assembly balance can be repeated. The coupling shall be disaaembled to the same extent required for normal field disaaembly and remounted on the balance fixture or fixtures. The unbalance of thr reasseambled coupling shall then be measured on the balancing machine, and the residual unbalance shall not exceed the greatest value determined by the following expressions.

U = 40W/N

U = 0.008W

U = 0.1

In SI Units,

U = 63,500W/N

U = 0.8W

U = 72.0

Note: Assembly balancing corrects for overall coupling unbalance caused by eccentricities of the pilot fits that are used to center components during assembly. However, assembly balancing may prohibited subsequent interchange of duplicate coupling components and may require that the entire coupling be maintained as a unit, except for the bolts and nuts.

basic design of coupling

1. TORQUE

The vendor shall furnish couplings with the maximum practical service factor, considering torque transmitted by the coupling, overhung movement, speed, and lubrication. Unless otherwise specified, the coupling, coupling-to-shaft junctures, and machinery shafting shall be capable of continuos operation at a torque determined by equation given in Chapter 3 using an experience factor of 1.75. the purchaser will specify the expected magnitude, nature and number of occurrences of transients to which the coupling will be subjected in service. The coupling, coupling-to-shaft juncture and shafting shall be capable of transmitting 115 percent of the purchaser-specified maximum transient torque without damage. The vendor shall state the coupling design without damage. The vendor shall state the coupling design rating (in horsepower per 100 revolutions per minute). The coupling size selection shall be submitted to the purchase of approval.

Note: Should reasonable attempts to achieve the specified experience factor fail to result in a coupling weight and subsequent overhung moment commensurate with the requirement for rotor dynamics of the connected machines, a lower factor may be selected by mutual agreement of the purchaser and the vendor. The selected value shall not be less than 1.25.

2. CRITERIA FOR CONTINOUS OPEATION

The coupling design shall permit continuos operation at the maximum continuos torque level with at least 125 percent of the purchaser-specified maximum steady-state for transient axial displacement (whichever is larger) occurring simultaneously with 125 percent of the purchaser specified maximum angular misalignment and 125 percent of the purchase-specified maximum parallel offset. (the maximum changes in misalignment and parallel offset are normally experienced during start-p of a machinery train).

3. SPACER

All couplings shall be if the spacer type. The spacer shall be of sufficient length to allow removal of coupling hubs and to allow for maintenance of adjacent bearings and seals without removal of the shaft or disturbance of the equipment alignment. The minimum spacer length shall therefore correspond to a between-shaft-ends dimension of 18 inches (457 millimeters), unless otherwise specified by the purchaser.

4. HUBTYPE

The purchaser will specify whether integral or removable hubs are to be used.

5. DRILL JIG

Unless otherwise specified, when the coupling is to be used with integrally flanged shaft ends, a drill jig (or template) shall be used to locate the flange holes. The purchaser will specify whether he or the coupling vendor is to furnish the jig (or template).

6. CORROSIVE ENVIRONMENT

When the purchaser specifies that the coupling is to operate in a corrosive environment, either oil mist or an inert-gas purge may be necessary. The vendor shall advise the purchaser when material limitations demand such protection for the coupling.

7. MOMENT SIMULATOR/ADAPTER PLATE

7.1 When specified, a moment simulator shall be supplied. The purchaser will supply the vendor with the measurement of the distance from the end of the shaft to the first bearing. The simulator shall also serve as an adapter plate.


7.2 The vendor shall supply an adapter plate for the price end of the coupling to allow uncoupled operation of the driver. The adapter plate shall be designed so that the late can rigidly bolted to the sleeve and centered on the hub by a pilot fit.

8. Removable Hubs

8.1. Removable coupling hubs shall be secured to the shaft by means of interference fit. The degree of interference will be specified by the purchaser and is subject to approval by the vendor. The choice of non-keyed (tapered-bore, hydraulically fitted) or keyed (tapered-or-straight-bore) hubs will be specified by the purchaser.

8.2. The surface finish of the hub bore shall be 125 micro-inches (3.2 micrometers)
arithmetic roughness (R ) or better.

8.3. The eccentricity of the hub bore, whether straight or tapered, shall not exceed 0.0002 inch (5.1 micrometers) TIR for bores less than or equal to 4 inches (102 millimeters) in diameter and shall not exceed 0.0005 inch (12.7 micrometers) TIR for hub bores greater than 4 inches in diameter, Eccentricity measurements shall be made before any keyways are cut.

8.4. The following guidelines are recommended for hub-to-shaft fits:

a) The interference fit for straight-bore keyed hubs shall be from 0.0005 or 0.00075 inch per inch of bore diameter. Shaft sizes and coupling bores shall conform to AGMA 9002.
b) The interference fit for tapered-bore keyed hubs shall be at least 0.001 inch per inch of bore diameter. The inspection procedure shall be in accordance with AGMA 9002.
c) The interference fit for tapered-bore hydraulically fitted hubs shall be at least 0.0015 inch per inch of bore diameter. The inspection procedure shall be in accordance with AGMA 9002.

For tapered-bore hubs and keyed hubs, please refer the API standard 671.

9. COMPONENET FIT TOLERANCES

9.1 Components of intermediate and high-speed couplings shall be centered by means of piloted fits. The eccentricity of these fits shall not exceed 0.001 inch TIR per foot of diameter or 0.0005 inch TIR, whichever is greater. Fits that tighten under centrifugal loading are preferred. The fit clearance shall range from a loose fit of 0.001 inch to an interference fit, with actual fit determined by balancing equipment.

9.2 The face run-out of mating faces (except for flexible elements) shall not exceed 0.001 inch TIR, whichever is greater.

9.3 Radial indicator surfaces used to align equipment shall be concentric to the hub bore within 0.001 inch TIR per foot of diameter or 0.001 inch TIR whichever is greater.

9.4 For hubs, pilot fits at fear coupling teeth shall be concentric to the bore within 0.001 inch TIR per foot of diameter or 0.0005 inch TIR, whichever is greater.

9.5 For sleeves, pilot fits at gear coupling teeth shall be concentric within 0.001 inch TIR per foot of diameter or 0.001 inch TIR, whichever is greater. Pilot fits shall be round within 0.002 inch TIR per foot of diameter or 0.0015 inch TIR, whichever is greater.

10. BOLTING CONSIDERATIONS

10.1 Bolting between coupling components shall be designed to transmit the required torque without dependence on flange-face friction.

10.2 Bolts for piloted flanges shall have a diametrical clearance of from 0 to 0.0005 inch (127 micrometers) in the bolt holes.

10.3 Bolts for low-speed couplings with nonpiloted flanges shall be of the body-bound style and shall be through-fitted into line-reamed holes, with the assembly’s diametrical clearance ranging from 0 to 0.0015 inch (38 micrometers).

10.4 Self-locking nuts shall be used. Lock washers shall not be used.

Note: The self-locking feature of nust may lose effectiveness with each removal of the nut. The coupling vendor shall recommended the interval at which nuts should be replaced.

10.5 the coupling vendor shall specify the required bolts torque, and shall state whether this value is for dry or lubrictaed applications.

10.6 the bolt shall be held within dimensional tolerances sufficient to permit both interchange within the same sset of bolts and substitution of spare bolts withut affecting coupling integrity and balance by more than 1 percent.

11. ELECTRICAL INSULATION

When specified, the coupling shall be electrically insulated.

12. MACHINGING

All coupling parts, except for flexible disks, shall be machined all over to minimise inherent unbalance.

coupling selection criteria

Usually couplings are supplied as part of any new equipment. Instead of having to select a new coupling, one is faced only with the need to replace an old one, or part of an old one. Assuming that the equipment manufacturer selected the right coupling type and size, couplings generate few problems. There are cases, however, when wither the coupling does not live up to expectations or when a new piece of equipment is purchased without a driver and a coupling must be selected. The process is not simple because there is no application for which only one type of coupling would work. The best approach is to let an application engineer from a coupling manufacturer make the selection. Today most manufacturers make more than one type of coupling and can objectively recommend the best one for the application.

Choosing a coupling of the correct size is very important. To do this one must know not only the required power and speed, but also the severity of service the coupling must accommodate. A correction factor, or service factor, must be applied.

Coupling manufacturers rate their couplings in horsepower per 100 r/min. for instance, if a pump required 50 hp (37.3kW) at 1750 r/min, it needs a coupling that can handle 2.86 hp (2.13k W) at 100 r/min. This is correct only if the pump is centrifugal and is driven by an electric motor. In this case the service factor is 1. If we have a double-acting reciprocating pump driven by an internal combustion engine, we have to use a service factor of 2.0 + 1.0 = 3.0 for a gear coupling and 2.0 + 0.5= 2.5 for an elastomer coupling, according to one manufacturer. As a result, we must choose a gear coupling that can handle 8.58 hp (6.4 kW) at 100 r/min or an elastomer coupling that can handle 7.15 hp (5.3 kW) at 100 r/min. it seems that we can choose a smaller coupling if we choose the elastomer type.

However, the elastomer coupling will be about 8 ¾ in (222 mm) in diameter, while the gear coupling will be only half that size! If size is not important, price can be the next selection criterion. But the price of the coupling alone is not a good guide; one should consider the total cost including maintenance, replacement parts, lost production, etc.

Although couplings represent a small percentage of the total cost of a piece of machinery, they can cause as much, if not more, trouble than the rest of the equipment if they are not properly selected. Buying an inadequate size or type of coupling will never be economical in the long run. Maintenance personals are frequently faced with the problem of replacing a worn-out or broken coupling. After the cause of the failure has been determined, careful consideration should be given to the type size, and style of the coupling that will used as a replacement. Whenever possible, it should satisfy all the needs of the drive.

Proper selection as to type of coupling is the first step of good maintenance. A well chosen coupling will operate with low cross loading of the connected shafts, have lower power absorption, induce no harmful vibrations or resonance into the system, and have negligible maintenance costs. The primary consideration in selecting the correct type of coupling, as well as its size and style are:

1. Type of driving and driven equipment
2. Torsion characteristics
3. Minimum and maximum torque
4. Normal and maximum rotating speeds
5. Shaft sizes
6. Span or distance between shaft ends
7. Changes in span due to thermal growth, racking of the bases, or axial movements of
the connected shafts during operation.
8. Equipment position (horizontal, inclined or vertical)
9. Ambient conditions (dry, wet, corrosive, dust, or grit)
10. Bearing locations
11. Cost (initial coupling price, installation, maintenance, and replacement)

The coupling should be conservatively selected for torque involved. Consideration must be given to all peak loads and shock loads encountered in normal service. If the coupling is to operate at high speeds, it should be dynamically balanced. Special coupling modifications dictated by the connected equipment should be made. If any doubt exists as to proper type or size of coupling to use, it is recommended that the manufacturer be consulted. Most manufacturers are usually qualified to make recommendations and assist in the coupling procurement.


GENERAL SELECTION CRITERIA:

TORQUE

The vendor shall furnish couplings with the maximum practical service factor, considering torque transmitted by the coupling, overhung movement, speed, and lubrication. Unless otherwise specified, the coupling, coupling-to-shaft junctures, and machinery shafting shall be capable of continuos operation at a torque determined by following equation (I) or (II), using an experience factor of 1.75. The purchaser will specify the expected magnitude, nature, and number of occurrences of transients to which the coupling will be subjected in service. The coupling, coupling-to-shaft juncture and shafting shall be capable of transmitting 115 percent of the purchaser-specified maximum transient torque without damage. The vendor shall state the coupling design rating (in horsepower per 100 revolutions per minute). The coupling size selection shall be submitted to the purchaser of approval.

Ts=
63,025 X Pnormal X SF
-------------------------- - Eq. (I)
Nnormal





In SI Units,


Ts=
9549 X Pnormal X SF
------------------------- - Eq. (II)
Nnormal


Where:

Ts = torque used to make the coupling selection, in inch-pounds (joules)


Pnormal = input power required by the driven machine at the specified normal operating point, in horsepower (kilowatts)

Nnormal = speed corresponding to the normal power, in revolution per minute

SF = experience factor derived from various modes of off-design operation that may result from such factors as a change in the density of the pumped fluid (molecular weight, temperature or pressure variation), unequal load sharing, fouling, and driver output at maximum conditions.


Note: Should reasonable attempts to achieve the specified experience factor fail to result in a coupling weight and subsequent overhung moment commensurate with the requirement for rotor dynamics of the connected machines, a lower factor may be selected by mutual agreement of the purchaser and the vendor. The selected value shall not be less than 1.25.