Control Valve

What is valve packing?

What is valve packing and the importance of valve packing?

We can describe the packing as a sealing system that normally consists of deformable materials such as graphite, asbestos, etc. Mostly the packing material will be in the form of solid or split rings which would be contained in a packing box. The packing material would be compressed to provide an effective pressure seal between the fluid in the valve body and the outside atmosphere.

Valve packing would allow the valve stem to pass into the valve body without any loss of fluid and pressure. The valve packing would act as a dynamic seal between the valve stem and the bonnet. Valve packing is done for most of the valves to prevent leakage from the space between the stem and the bonnet. Mostly valve packing is done by using fibrous material such as flax or Teflon and this would form a seal between the internal parts of a valve and the outside of the stem.

How to do the valve packing?

The valve packing must be done properly which means it should be compressed properly. So while doing this we could prevent fluid loss and also damages to the fluid stem. In case the valve packing is too loose then the valve would leak and it would affect safety. In case the valve packing is too tight then it would damage the stem.

What is the purpose of the packing box assembly?

Control valve packing

The major purpose of the packing box assembly is to prevent the leakage of the process fluid by elastic means. Proper adjustments should be done in order to vary the compression of the packing material against the stem surface.

The packing box assembly consists of

  • Packing flange
  • Packing follower
  • Lantern ring
  • A certain number of equally spaced packing rings
  • The lantern ring would provide space for the insertion of the lubricating grease through an isolating valve

What are the major requirements for the valve packing material?

  • The ideal packing material must be elastic and it must be deformable
  • It should be chemically inert
  • It must be able to withstand high pressure and temperature
  • Durability

What are the types of packing material which is used to do the valve packing?

Valve packing

Inert gas

This can be considered the most expensive valve packing material. Inert gas would be used to pressurize the area between the two packings, this gas would be added at a specified pressure level which would be more than the fluid pressure. So due to this, the inert gas could leak into the valve but the fluid is not allowed to leak by the stem. This type of valve packing is very reliable when compared to other types of valve packing.

Teflon

This is mostly used valve packing material and this is due to the fact that it has good chemical inertness and better lubricating properties. This material can be used as a solid molded or turned form. We can use this material below 250 degree Celsius. If it is deformed into solid rings then it must be spring-loaded to provide a minimum initial pressure against the stem.

Braided asbestos

This is another type of valve packing material it can be used to make split rings. This would be wrapped around the valve stem. Mostly this type of valve packing material is used with mica or graphite and it can be used for high-temperature applications.

Grafoil

This type of material has been recently added as a valve packing material. This material is all graphite product this packing material is flexible and has direction-dependent properties. This material is mostly chemically inert except when strong oxidizers are used.

Valve packing

How to examine and clean the valve packing?

At first, lose the gland follower nuts and lift the follower carefully in order to remove any trapped pressure under the packing set. Old packing should be removed and then clean the shaft/stem and the stuffing box area. Check the shaft/stem for corrosion or wear. We should check the packing box for clearness. If any component is damaged then replace it, check the previous packing in order to get an idea about the damages that could happen. We should also determine the shaft or stem diameter, packing box bore, and depth and while using lantern rings we should check the distance of the port to the packing box bottom.

How to install and adjust the valve packing?

Properly install one ring of packing at a time and then twist each ring. Make sure that each ring is seated in the stuffing box properly before installing the next one. After that stagger the joints of the subsequent rings at a minimum of 90 degrees. If the last ring has been installed then draw the gland up evenly until the nuts are finger tight Then check the lantern ring if it is positioned properly relative to the port and then check if the shaft/stem turns freely.

Adjusting the packing

We should consult the packing manufacturer to determine the torque specifications or the compression percentage.

Tightening of the gland nuts

  • Tighten the gland bolts to approximately 30 percent of full torque or appropriate compression percentage
  • The valve must be cycled a certain number of times and then apply full torque while the valve is in closed stroke position

What are the different types of valve packing material used for different process fluids?

Process fluidValve packing material
Clean water, hot or cold sewage, slurries, calcium brine, neutral liquids (not acid or alkali) Maximum temperature 212FBraided white asbestos or cotton with general service lubricant
Clear hot or cold water, neutral liquids Maximum temperature 400FBraided special white asbestos with a high-temperature lubricant. Graphited
Acids Maximum temperature 250FBraided blue asbestos with acid-resisting lubricant. Graphited
Alkalis Maximum temperature 250FBraided with asbestos with non-soapy lubricant. Graphited
Hot or cold water, mild acids, mild alkali, brine, boiler feed water Maximum temperature 450F where suction pressure is more than 50 PSICrimped lead foil sheets with asbestos core
Alkali and other liquids with a PH above 7 Temperature from 90 – 400 FTeflon – filled braided blue asbestos
Acids with a PH of 4 or less Temperature from 90 – 450FTeflon – filled braided asbestos
Valve packing materials for different kinds of process fluid

What are the different types of valve packing?

List of different types of packing

  • Hydraulic packing
  • Graphite packing
  • Compression packing
  • Braided packing

Hydraulic packing

This type of packing is used where no fluid leakage is acceptable the most common type of hydraulic packing would be V-ring packing. Mostly this would be made up of rubber or other polymer materials.

PTFE (polytetrafluoroethylene)V-ring

V-ring packing

This type of valve packing is done by using plastic material and it would reduce the friction. Molded V-shaped rings would be used to do this packing. In order to seal properly, it would require a smooth stem finish. This packing can’t be used for nuclear service due to the fact that the radiation could destroy the PTFE.

  • Less friction
  • No need for packing lubricant
  • Resistant to most known chemicals
  • Good temperature range

Graphite packing

Valve packing

This type of packing can be used for nuclear service or where low chloride content is required. This would provide leak-free operation and it also has high thermal conductivity and long service life. It can be used with hard fluids and also high radiation.

Compression packing

Compression packing won’t stop the leak but it will reduce the leak.  Compression packing can come in rolls and thus it can be cut into required length, preformed rings, or tape according to our need. If the gland follower is tightened then it would compress the packing and thus it would make it shorter and fatter. This type of packing would require a small flow of liquid to lubricate and cool it. Mostly fiber materials are used for the packing and it would also use a thin metal sheet to do this packing.

Braided packing

This is another type of valve packing, it could be loosely or tightly braided. Tighter braiding would last longer there are certain types of braiding methods such as square braid, interlock braid, and braid over braid. Braided packing could be done with fiber or non-fiber materials. Mostly this packing is created by natural fibers such as cotton, and asbestos.

How to add and replace the gland packing in a valve?

Gland packing

If the control valve is used frequently then there could be slight leakage through the gland packing. In order to stop the leakage, we should tighten the eye bolt nuts equally on both sides. During the time we would need to add one or more gland packing to the packing box. To add the gland packing rotate the handwheel in an anticlockwise direction in a fully open position. Remove the eye bolt nuts then lift the gland bush and the gland flange in the upward direction and add the required number of gland packing.

If the valve is kept in service for a long time then the gland packing would be in the compressed position for a long time and thus it would lose its compressibility. So, in this case, the only remedy for this problem would be the replacement of the packing. We can remove the gland packing by using the angular scriber and then it must be replaced with the new packing of correct size having opening ends and placed 180 degrees to each other. If the valve is installed in a running pipeline then we must take proper care while doing the replacement. There should not be any leakage through the valve when the valve is in the backseat position. We should not change the gland packing if there is leakage through the packing. In case there is any leak the valve should be dismantled and the back seat must be repaired.

What are the major requirements of valve packing?

  • It should be compatible with the process fluid at the operating temperatures
  • Lubricant must be insoluble and immiscible with the sealed fluids
  • It should be non-abrasive to minimize the shaft erosion
  • Noncorrosive to avoid damage to the shaft housing
  • Wear-resistant
  • We should also consider the shaft speed
  • Plasticity to conform to the shaft under the influence of gland force

What are the advantages of valve packing?

  • The major advantage of the valve packing is to prevent the fluid leakage
  • It would prevent the dirt or any other things from entering the fluid
  • Packed glands can be used for stem sealing of various types of valves including gate valve, globe valve, and ball valves

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