How to distinguish between Grinding wheel and Cutting wheel (Cut-off wheel) ?

Some useful tips to distinguish Grinding wheel and Cutting (or Cut-off) wheel are given as below:


1.      Remember: All grinding is cutting operation but all cutting is not grinding operation.

2.      Thumb rule : “THICKNESS”  is the the major difference between cutting and grinding wheels. A newly procured disc or wheel  up to 3 mm(1/8 inch max) thickness  is used as  Cutting wheel. Its radial periphery is utilised for cutting or slotting  operation as it can withstand a great deal of radial stress but not any side stress . Cutting wheel is not designed to take side load. Hence its use as grinding wheel is not recommended.

3.      Grinding wheel or disc start with minimum 6 mm thickness onwards. The shape and thickness of a grinding wheel decide a wheel's type. Grinding wheel can be used for side  as well as radial grinding as per job requirement.

Bellows seals -Material of construction for Thermic fluid application

Selection of  AISI 316Ti  Vs AISI 321 as materials of construction for bellows seals for Thermic fluid application (such as Santhotherm-66, dow therm ,therminol etc) for high temperature application +200 to +300 Degree C)

Generally both the materials are recommended for selection of bellows seals so long maximum temperature is below the creep range.
AISI 321 is Titanium stabilized Austenetic Stainless steel. It is very good corrosion resistant material for high temperature environment where presence of chloride is negligible.Presence of Ti has tendency  to suppress the formation of Chromium carbide under elevated temperature specially at welded construction .Thus depletion of Cr is avoided due to presence of Ti.
But it has less resistance to Pitting-Corrosion as compared to AISI 316 Ti

On the other hand AISI 316 Ti is having 'Mo' in addition to 'Ti:'. Therefore it has very good resistance to Pitting corrosion and has all property of AISI 321.


Obviously AISI 316 Ti shall be comparatively costlier than AISI 321   

Why No-Load Testing is important in Reciprocating compressors after complete overhaul ?

Why No-Load Testing is important in Reciprocating compressors after complete overhaul ?

After complete overhaul (i.e. replacement of worn out bearings -main bearings,conn rod big end bearings,small end bearings,cross head shoe replacements,piston rings and rider rings ,stuffing box pair of rings,etc etc) following should be checked during idle run (no-load ) test for establishing soundness of assembly .
Prior to start up for idle run:
1.Check and ensure that CE (Crank end  or BDC) and Cylinder Head End (TDC) clearances are measured correctly by lead wire and micrometer and values maintained as per OEM recommendation.
2.Check piston rod run out (vertical and horizontal) to ensure correctness of piston and cylinder assembly with respect to slide body or distance pieces wherever applicable.
3.Start auxiliary oil pump and observe that oil is going to all the bearing locations as well as cross head pin and crank shaft main bearings through oil passages .
4. keep barring the crank shaft while oil is flowing to all required locations. Lube oil pressure should not be less than 2.5 kg/cm2 (g).
5.Close all the doors of cross-head guide .
6.Ensure valve and valve covers from cylinders are removed prior to No load Testing.

Observations during No-load Testing

1.Run the compressor for 15 minutes and stop to measure/observe  followings:
  - Temperatures at crank shaft main bearings and intermediate bearings if provided.
  - Temperatures at all conn rod big end bearings and small end bush bearings.
  - Lube oil pressure and Temperature before and after oil cooler.
  - Water temperature at in let and outlet of oil cooler.
  -Temperatures at all cross heads shoes.
  - Piston rod temperatures at stuffing box packing locations.
  - Gear box bearing temperatures
  -Any other abnormality if any.

2. If all the readings in step 1 above are within satisfactory range, run the compressor for next one hour and repeat all those readings mentioned in step 1 above.
3.If all the readings in step 2 above are satisfactory within acceptable range ,run the compressor for next two hours and repeat the readings mentioned in step 1 above..
4 If the readings in step 3 above are within acceptable range run the compressor for next one hour and repeat the readings mentioned in step 1 above.
5. If readings in step 3 and 4 are almost steady ,stop further running. This completes the satisfactory idle run test for minimum 4 hours.In case readings are not steady even after 4 hours of no load  trial , repeating the running further is not advisable rather we should check  for cause of increasing trend of temperature .It could be the misalignment or  poor workmanship which needs attention to be addressed / fixed the problem. After correction , repeat the fresh no load trial as per step 1 to 5 above till the parameters are steady within 4 hours of cumulative running.

Many end users are not following above procedures .They directly start the compressor for load trial .There are two main disadvantages in this case :
1. It may result in catastrophic failure of compressor or bearing seizure because of  possibility of  poor
    workmanship/practices which could have been noticed in first 15 minutes no load trial run.
2.Very fast wear rate of rider rings and piston rings resulting in their low MTBF. As most of the rider rings
   are made of PTFE with graphite filler,during no load a layer of PTFE is formed on sliding surfaces of
   cylinder liners which further reduces the coefficient of friction during load testing or load trial.

What is risk based inspection (RBI)?

 

The inspection of plant and machinery has traditionally been based on statutary requirement  backed up by local health & safety legislation. The type of equipment has determined inspection frequency, methods employed and locations examined - with little focus given to its age, specific duty or likely condition. Increased operational experience and a greater appreciation of the hazards are now leading some parts of industry to adopt a more informed approach to inspection planning, targeting the inspection required to reduce the risk as low as reasonably practicable.
Risk based inspection is the process of developing an inspection plan based on knowledge of the risk of failure of the equipment. The essential element is a risk analysis. This is the combination of an assessment of the likelihood (probability) of failure due to damage, deterioration or degradation mechanism with an assessment of the consequences of such failure.
The information gained from this process is used to identify
1.The type and rate of damage that may potentially be present and
2.The equipment or locations where failure would give rise to danger of different degrees.
 Higher risk equipment may have active damage mechanisms or high consequences of failure, or a combination of the two. A suitable inspection scheme is then planned to increase confidence about the equipment's current and future condition, taking account of the potential damage mechanisms and the reliability of the inspection techniques used.
Risk based inspection may be applied in any industry, but there has been most interest from the power and petrochemical sectors.  The American Petroleum Institute has published guidance for risk based inspection relevant to refineries. ^[1]
Industry sees RBI as a means of using inspection resources more effectively which can result in economic benefits from extended run lengths or from the use of advanced NDT or non-invasive schemes. Regulatory pressure will ensure that the process of RBI is carried out rigorously so that non-prescriptive inspection decisions are based on adequate information and expertise. .

Reference

1. API Recommended Practice 580 and Base Resource Document 581

How to keep copper and aluminum apart to avoid corrosion ?

Guidelines for how best to keep copper and aluminum apart to avoid corrosion?

In many industrial installation use of aluminium and copper combination can not be ruled out completely.
To minimize galvanic corrosion,  always attempt  to maximize the size of the anode and minimize the
size of the cathode. In an aluminum/copper couple, copper is the cathode and aluminum is the anode. If
you paint the copper and the paint gets scratched, then you have a small cathode and a very large anode
which is exactly what you want. We must know that in a cathode-anode combination, larger is the anodic area lesser will be the corrosion rate.
On the other hand If you  paint the aluminum and it gets scratched, then you have a small anode and a very large cathode, which is not what you want.This will accelerate the corrosion rate.

Tips on Belts drives for better performance and reliability

1.Maximum speed that a drive belt can safely handle:
Pulley rim speed is the limiting factor, rather than the belt. This limiting speed depends on the
pulley material and design.
For gray iron casting and statically balanced, pulley  rim speeds up to 6,500 fpm is normally satisfactory. A pulley running at more than 6,500 fpm may cause vibration, noise, poor bearing life, and high fatigue stresses. Therefore, pulleys that exceed 6,500 fpm should be dynamically balanced.

2.Causes of vibrations in belt-drive and corrective measures.
 Drive belts experience both vertical and lateral vibrations when their natural frequencies coincide with excitation frequency of connected equipment.Belt tension can affect the amplitude of this vibration. Therefore, to correct the problem, first check for proper tension.
If this does not work, consider changing other drive parameters to reduce the amplitude of vibration or alter its frequency. Such parameters include span length, belt type, misalignment, inertia of driving or driven machinery,pulley diameter and weight (inertia), speed, and the number of belts. In some cases (where original unit was oversized), it may be possible to downsize the drive by reducing the number of belts or belt width, and increasing the static tension to alter the belt’s natural frequency so it doesn’t coincide with the excitation frequency of the machinery. When it can be done safely, it is preferable to reduce the static tension to keep the operating belt tension below the belt’s natural frequency range.
To reduce lateral vibration, increase flexural rigidity in the lateral direction. This can be accomplished by using joined belts.

What causes a squealing belt?
 A V-belt squealing is usually caused by belt slip, often due to under tensioning. When a new belt replaces one belt in a multi belt drive, the new belt may be tensioned properly, but all of the old ones are undertensioned. To avoid this problem, replace all belts in a multibelt drive at the same time, and with belts of the same construction from the same manufacturer.
Replace worn sheaves, which can lead to noise and belt rollover, as well as worn or damaged belts.
Sudden, high startup torques or peak loads also cause belt slip. Usually, this condition lasts only a few seconds.But, it can lead to heat build-up which reduces belt life. If belt slip and heat build-up is suspected, turn off the drive and place a gloved hand on the belt to feel if the belt is too hot.
Grit, oil, or grease cause belts to slip. Therefore, keep the drive components clean. And don’t use belt dressing.
This only masks the real problem of inadequate tension.
Large pitch, wide synchronous drives may generate noise at high speeds. This can be caused by too-high or too low belt tension, or misalignment, which prevents the belt teeth from smoothly entering or leaving the sprocket grooves. Because of this, alignment requirements are tighter for synchronous belts than standard V-belts.
Note:: when inspecting a problem drive, review all components. Noise can be caused by nonbelt sources,
such as bearings, guard vibration, and loose mounts.

Enhanced Shelf Life of V-Belts- By Good Storage practices.

Under proper storage conditions, belts can be used for many years without appreciable deterioration of quality and service life.Shelf life can be enhanced between 4-6 years provided following guidelines are followed as a good preservation practices:

1. Store belts in a cool, dry, dust-free area, away from radiators and direct sunlight.
2. Temperatures ideally between 5 degree C and 30 degree C and relative humidity below 70
    percentage are recommended near storage location of material stores
3. Store belts away from ozone producing unguarded fluorescent lights, mercury vapor lamps, and high
   voltage electrical equipment.
4. Do not store belts near chemicals, oils, solvents, lubricants and acids.
5. Belts can be coiled on shelves or hung on pegs having diameter not less than minimum diameter sheave
    recommended for the belt cross section.
6. However, avoid sharp bends and stresses that can cause deformation, cracks or any other damage to the
   belts.
7. Stack belts no higher than 12" to prevent damage to the belts at the bottom of the pile.
8. When hanging, coil longer belts to prevent distortion due to belt weight
9.Do not store belts on the floor unless they are in a protective container. Floor locations are exposed to
   traffic that may damage the belts.
10.When the belts are stored, they must not be bent to diameters smaller than the minimum recommended
sheave or sprocket diameter for that cross section.