To be successful in its duty and to attain the objective of successful sealing and trouble free operation the gland packing must have the following properties.
Anti-friction properties. The gland packing basically rubs along the shaft and stationary along the stuffing box side. If the gland packing has friction it would score the shaft or the shaft sleeve and also would heat up and fail and even burn due to consistent overheating.
Chemical resistance to the fluid being contained. If the gland packing reacts with the fluid it is supposed to seal it would later disintegrate and be flushed away leading to leakage of the fluid which is not desirable.
Temperature resistance. The gland packing should be able to resist the working temperature of the fluid being sealed without failing.
Compressibility and resilience. The gland packing should be able to compress and confirm to the shaft under the force of the gland flange and when the tension is released it should come back to its original shape. The latter is important as when the gland nuts are loosened the gland packing should spring back releasing the shaft.
Retention of lubricants. All the gland packings are coated with lubricants like graphite, grease, petroleum products etc., also external lubricant is applied before insertion. The gland packing should be able to retain this grease for anti-friction properties.
Should not score the shaft. Normally on pumps a shaft sleeve is inserted over the portion where the gland packings are inserted and which is replaceable. The gland packing should nonabrasive to avoid the scoring on the shaft sleeve. Of course on a long interval this is not possible, but it should at least sustain till the next overhaul Should not contaminate the fluid being sealed. In some applications like food and pharmaceutical industries the gland packing should not contaminate the fluid, while in others it does not matter.
It should be non-corrosive to avoid damage to the shaft and the housing.
It should be wear resistant to last a long time.
It should retain its property over a time period.
Different Types of Gland Packing (Materials used)
There are many different types of gland packing depending on the type of applications such as for valve stem sealing, centrifugal pump shaft sealing, reciprocating pump shaft sealing, static duties like hatches sealing and pressure vessels manholes sealing, etc. Gland packings are made from the following basic substances and are a combination of these basic materials and are sometimes reinforced with metal wires for extreme conditions.
Wrapped metal foils
Poly acryl nitrile fibers
All the gland packing are a combination of the above mentioned basic materials and great research is done by the manufacturers to develop new and more efficient types of packing. The packing made of a combination of graphite, carbon, glass, etc. are used for harsh applications whereas gland packing made of materials like Jute, flax, hemp, and cotton are used for light applications like sea water, potable water etc. The compound PTFE is versatile and can be used for a wide variety of applications.
Preconditions for a Successful Gland Packing Job
No matter how many times you change the gland packing and how well you have done the job, the success of the gland packing job done would also depend on the following factors.
The trueness of the shaft. A shaft that is bent would eat the gland packings prematurely and there nothing you can do except change the shaft. You must make it a point to check the trueness of the shaft during each overhaul using a dial gauge.
Concentricity of the shaft with the stuffing box bore. The shaft might be true but if not aligned properly would again destroy the gland packings in no time.
Surface roughness of the shaft. If the shaft or the sleeve is pitted or unevenly worn it would again damage the gland packings very fast.
Whipping of the shaft due to worn bearings.
Consistent cooling water supply in case of pumps with a lantern ring.