How to Select Gland Packing for Different Applications
The selection of gland packing is a very important part of the marine engineer’s job. An incorrectly selected gland packing would soon fail in service and lead to leakages. As a number of packings are available, the field of selection is very wide. There is no single gland packing that can handle all kinds of fluid or which is suitable for all types of valves and applications. A valve is only good if its sealing is good and a leaking sealing arrangement amounts to a leaking valve with all its associated faults. Not only is it displeasure to the eyes it is also a work, health, and fire hazard. Gland packings are made of the combination of the materials discussed earlier and are manufactured in plaited, braided, twisted, jacketed, corrugated foil, foil wrapped deformable core, and full foil form. It is prudent to consult the manufacturer’s manual about the type of gland packing to be used. The gland manufacturers also give a list of applications where their gland packing can be used; these should be consulted before use.
Gland Packing Details
How to Cut Gland Packing
The gland packing should be cut with a sharp knife to avoid fraying of the edges. Do not use scissors or hack saw blade etc. A heavy duty engineer’s knife or a sharpened ordinary knife would do.
There are two methods of cutting the gland packing, the first one is straight or at 90 degrees i.e. Butt joint and the second one is at 45 degrees i.e. skive joint. The taper method gives better sealing but is more difficult and skill oriented and the angle on both the ends should match properly or otherwise it would lead to more leakages. In places where there is an accessibility problem it is best to cut the gland packing straight.
How to Remove Gland Packings
Removing a gland packing can be very tough sometimes, especially if it is old and well set, you need to have specially contoured tools which the marine engineer learns well to make with experience. There are also gland packing extractors available in the market, which look like a cork screw used to open the wine bottles but are flexible. They just have to be twisted to get a grip and then the packings can be pulled out. The problem comes when the old packings have lost their properties and the packing comes outs in fibers. Sometimes in sea water pumps when the packings are not coming out, a little opening of the sea water suction valve would push out the packing with ease due to the water pressure. But it should be done very cautiously as you can flood the bilges and you must shut it promptly.
Packing Removal Tools
Standard sizes of Gland packings
In case the instruction manual is not available, then the correct size of the packing can be determined from a simple method. Simply measure the inner diameter of the bore of the stuffing box and subtract from it the outside diameter of the shaft. The figure obtained must be divided by 2 to get your gland packing size. But generally the following sizes of the gland packings are recommended for the different shaft sizes as follows:
- For shafts of size 16 to 28 mm diameter the gland packing of size 8 mm is used.
- For shafts of size 30 to 46 mm diameter the gland packing of size 10 mm is used
- For shafts of size 50 to 75 mm diameter the gland packing of size 12.5 mm is used
- For shafts of size 75 to 120 mm diameter the gland packing of size 16 mm is used
- For shafts of size 125 to 300 mm diameter the gland packing of size 19 mm is used
But the above is only a guide line and it varies from manufacturer to manufacturer.
How to Measure the length of the Packing Required
For measuring the gland packing size the following methods can be used.
- Take the reference of the old removed packing and cut accordingly.
- Put the new gland packing on the shaft circumferentially or on a template and then mark and then cut the required length.
- The most accurate is to measure the size of the shaft and calculate its perimeter or circumference. For example if the shaft is of 50 millimeters diameter then the circumference is given by the formula P = Pi x Diameter. That would make the perimeter 157.07 millimeters. This measured length can be cut from the gland packing reel considering a 2 mm clearance in packing which would swell in presence of sea water.
How Much Gap is to be Kept?
Some types of packing swell a little on contact with water. This must be considered and an appropriate clearance should be provided. Otherwise it would seize the shaft leading to overheating and burning of the packing, as well as increasing the load on the shaft and the motor. In some cases the shafts have even broken due to excessively tightened packing.
Also sometimes the packings do not go in the stuffing box spaces easily and they have to be hammered to make them a little thinner thus elongating them in return. This factor must also be considered in giving some gap. However the hammered and thinned packings would again gain their width after being compressed. As a rule hammering should be avoided as it damages the gland packings and should only be resorted to in an emergency, when the correct size packing is not available.
How to Insert the Gland Packing
The following are the cautions and the instructions while installing new gland packings.
- After the gland packings have been removed the spaces must be thoroughly cleaned with compressed air to ascertain than no residues of the old packing remain inside.
- The shaft must be carefully inspected for any abrasions and pitting marks and if any must be removed with 300 grit emery paper. In case it is not possible to smooth the surface, the sleeve must be changed at the next possible opportunity.
- Gland packing once used must not be reused and never try to reverse a gland packing as the old packing must have lost its properties in service.
- The packings must be selected as per the service required.
- The correct size of the packing should be referred from the instruction manual or measured.
- Cut the gland packing to the correct size and give a little clearance if using natural fibers like hemp and jute which would swell in contact with water.
- Fit each packing one by one with lots of lubricant and do not forget to stagger the same by 90 degrees each.
- Compress with the gland to ascertain that it goes inside evenly.
- Check that the shaft is free to rotate after the insertion of the gland packings.
- Slightly tighten the gland till a little resistance is felt in the turning of the shaft by hand.
- After the tightening slackens the nuts and finger tighten. The tightening must be done during the running in process.
- Remember that a gland packing is not supposed to completely prevent leakage of the fluid, a small leakage of about ten drops a minute is required for lubrication and cooling.
Running in of the Gland Packing
After the gland packing has been inserted in the pumps (both reciprocating and rotary) the pump must be allowed to run with a moderate leakage for ten minutes and after that start tightening the nuts one turn every five minutes. Keep monitoring the temperature of the gland and if it is getting too hot slacken the same amount. After another five minutes tighten again and monitor the temperature. Please remember that a little leakage of about ten drops a minute is required for the lubrication and the cooling of the gland packings. Do not attempt to stop the leakage entirely because it would look all right for ten or fifteen minutes, but the moment you turn your back and are busy with some other work, the gland packings would burn triggering all the smoke alarms in that area and the leakage would flood your bilges. It must also be noted that over tightening the gland also leads to premature wearing of the shaft sleeves.
Gland packings are used extensively in the marine field yet they are one of those things that engineers take for granted. Normally engineers just check the size and put the first packing they come across if it is of the same color. This negligent attitude results in leakage and scored shafts. It is also due to the fact that training material is not available to the junior engineers when they join this field and by the time they come up the ranks they are used to the idea of wrong working practice. In this article the subject has been discussed in brief and it is hoped that it would be useful to the new marine engineers as well as the practicing marine engineers.
- Gland Packing details: https://www.roymech.co.uk/images/gland_packing.gif
- Packing Extractor: https://lh6.ggpht.com/_rD7JWhpQBzk/Sf28xL_w-lI/AAAAAAAAAkU/mymqhM3aNCU/Gland+packings%E6%B8%85%E9%99%A4%E5%99%A83.JPG