For pumps we recommend an angled butt cut with approx. 75 degrees, which results from the different circumferences of Stuffing box ID and OD with an circumferential over length adder of approx. 9% for small shaft dimensions and 4% for big shaft dimensions.
For valves and reciprocating pumps we recommend a 45° skive cut with a circumferential adder of approx. 2%.
The reason is, that some valve packing styles have no or little cross section impregnation and tend to fray. The 75° butt cut is the shortest way through the packing and minimizes the risk of fraying.
First we need to clarify the wording: Precompressed.
It means that a individual ring is densified in a die to its working density. Other expressions like preformed, die formed, precut or similar do not necessarily mean a densification has taken place.
The advantage is that all installed rings will have the same density and will react more promptly on gland pressure. As a result, less gland pressure is needed. The cut will close up better and additional over length to compensate shrinkage and to seal the OD better is easy accommodated in a die. In operation the settling of the ring stack is less and a longer way of adjustment is available at the bolts.
The limits should consider the weakest component in a packing. A carbon fiber may have a temperature capability of 400°C/750°F but the cross sectional impregnation maybe for example PTFE, therefore the limit is reduced to 280°C / 550°F. Not observing the limit will result in decomposition of PTFE and the resulting split products can attack the stuffing box components and harm the health of people working in the area.
Life loading is used on valve gland bolts at pressures above 16 bar/200 psi. The Belleville Springs have a linear spring load and compensate compression loss on the packing stack due to wear settling or thermal expansion. If life loading is not used a failure in not adjusting the gland bolt's can result in mechanical destruction and blow out of the packing when compressed products expand rapidly.
First select if it requires a pump, valve or special packing. Consideration must be done on the temperature of the application. The technical figures give guidance. An important criterion is the available shaft hardness expressed for most products in HRC. Further information is available on pressures, pH range and shaft speeds. Important is if the product contains solids the characteristics will describe that. If dry running can appear, packing with high heat conductivity and which runs on a softer surface is preferred.
This figure needs to be understood in combination with pressure to be sealed. If the pressure is low, even packing with a low heat conductivity, due to missing carbon content like aramid fiber packing, can run higher shaft speeds. In principle the higher the carbon content of a packing the better its capability to high shaft speeds. Pure PTFE packing runs lowest speeds, carbon and graphite packing run highest shaft speeds.
The given parameters are based on the ability and strength of the packing not to extrude. The value is determined on the recommended gap width between gland ID and shaft and SB bore. These are 5% of the used packing cross section for pumps and 2% for valves. Nigger gaps will reduce the pressure rating and vice versa. Higher temperatures can as well reduce the values with many packing styles mainly PTFE packings or PTFE impregnated packings.
Typically 3/16" is a 5mm size and vice versa. The braid of these sizes is 2-track and therefore very pliable. Imperial braiders sell 3/16" as 5mm in Europe and metric braiders sell 5mm as 3/16" in US markets. As well only a small difference is found with 16mm and 5/8”, 19mm and 3/4", 22mm and 7/8", 25mm and 1".
An individual size should be braided for 6mm and 1/4", 10mm and 3/8" and 12mm and 1/2".
In principle exist for all pump packing qualities we supply, recommended measurements for the shaft hardness in HRC hardness Rockwell. If slurries are the product to seal, automatically a harder surface should be chosen, as well considering the chemical aggressiveness. Some of the surface treatments will attack surface coatings. In principle the result of running a soft packing on a soft sleeve against abrasives maybe better, than ruining a hard packing requesting HRC 50 and more in the same application against a soft sleeve which only offers HRC 25.
In principle this is a marketing idea aiming for an understanding that eventually less maintenance is required. All good working practice would say, remove and inspect SB and sleeve condition.
For die moulded rings it makes a big difference if the same crossection packing is formed around a 30mm or 100mm ID and makes 30 x 50mm or 100 x 120mm. The smaller ring will bulge out and creates a bigger height and the bigger ring will tend to be smaller height.
Packing with a high percentage of run in lubricant will tend to bounce up in height after compression and a dry packing will actually be densified. Multiple bending rings may open as well the height.
In principal we differentiate between corner- and running track reinforced hybrid packing. Two different yarns are used, one may have reinforcing characteristic like aramid yarns, and the other may have high graphite content for improved heat transfer capability. Corner reinforcement makes sense in applications with axial movement for example in plunger pumps. An additional side effect of corner reinforcement is to prevent against extrusion of packing in bigger gaps.
In applications with rotating shafts running track reinforcement is preferred over corner reinforced braided packing. The reason for that is the reinforcement material is over the complete packing surface equal distributed, which leads to a reduced wear on shafts. A side effect is that solids in a sealed product are stopped in their dynamic activated by shaft rotation and this prevents the wear of the softer example heat conductive component of a hybrid packing.
In pumps so called bull rings of wear resistant packing material are placed as bottom ring in a stuffing box when sealing products with solids to secure softer packing. If a lantern ring is used to flush the packing chamber for example water often the wear resistant packing is used from stuffing box bottom up to the lantern ring. The softer packing is then used from lantern ring up to the gland and will run in clean fluid.
The packing is axially put under compression, to activate a radial acting sealing force. If the gaps between gland or stuffing box throat and valve stem is bigger than 2% of the used packing dimension, the risk exists, that packing material will be extruded through the applied compression in the gap and the sealing force is reduced. Especially thermoplastic material has a tendency to extrude. As well expanded graphite foil packing is exposed to the risk to shear off its first layer into the gap as the foil thickness is only between 0,4 and 0,5mm. Bullrings made from extrusion resistant material will prevent gap extrusion and support the sealing performance of the packing. Due to tolerances in valve parts especially after revision it can be found that the recommended 2% maximal gap width is not kept in general and bullrings are recommended to use.
Previous valves had often stuffing boxes housing 7 and more packing rings. Today’s experience with modern packing material limits the recommended amount of rings between 4 and 6 depending on the sealed pressure. The rest of the available space in a stuffing box is filled with distance rings made for example from 2 pressure resistant filled carbon. These rings are usually split in 2 halves for ease of installation.
As well other temperature and pressure stabile materials can be used. Physical sense behind this measure is to distribute the available compression force from the gland bolts more equally.
A reduction from 7 to 5 rings can result for an internally rough packing with a low K-Value = distribution factor of applied axial force into radial sealing force of a packing in less than half gland pressure. This reduces the risk of extrusion of packing material between gland and stem and still delivers the same compression at the bottom product next packing ring.
First it needs to be differentiated if the reinforcement is a processing aid or out of technical view an improvement of the application characteristics.
A processing aid can be viscose fiber content supporting the spun process of recycled aramid fiber yarns or on expanded graphite yarns of the first generation a glass or cotton carrier. These contents should be low as they result in chemical and/or thermal caused volume losses.
A technical reinforcement improves the packing in an application. Expanded graphite yarns of newer generation have an integrated carbon fiber or integrated or over knitting reinforcing ultrathin inconel threads, which raises the pressure capability and eases quite substantially the extraction of used packing material from the stuffing box.
PTFE yarns with an integrated reinforcement rise in the packing the compactness, wear resistance and safety against extrusion.
In principle this is possible and depends from the application values. Pump packing usually has a 15-25% content of run in lubricant, which will leave the packing in relation to temperature and compression.
Multiple readjusting of the gland follower is inevitable. Once all oil has left the pump packing it will operate stabile. An important side effect of run in lubricant is in a positive way that pump packing is easy to install and compresses easily, wherein in a negative way it will extrude more easily though gaps between gland and stem. Therefor bull rings are necessary to prevent extrusion.
This is as well possible. Valve Packing implements no run in lubricant, which will leave the typical oil impregnated pump packing under high compression, herewith reduces the load on the packing and secures it against burning.
Through the absence of run in lubricant a valve packing should have in a dynamic application a very high heat conductivity like expanded graphite packing.
Most packings consist of yarn and impregnation like PTFE and oil, which are applied in different dipping processes. The content may vary depending on tension in the yarn and as well during braiding of the packing. A typical band width can be +/-15%. The braider endeavor with control mechanism on the braiding machine to compensate this but a remaining deviation is inevitable.
Even when a packing ring shows no signs of wear and looks like new on the OD, a packing should never be turned and re-used. Reason for this simply that the cut will never close up again as the packing has shrunk and hardened in use.
The settling of pre-compressed packing rings is reduced as their delivered density is closer to their operational density when in use. This results in less adjustment work and safer run-in process.
The basic benefit lays in the possibility to repack in operation.
The setup time for pre-cut rings is much less than for pre-compressed rings. These setup costs need to be split by the amount of rings processed. The actual time to produce pre-compressed rings needs to be seen in relation with the weight of the rings respectively the material input.
You are better adviced to order a small amount of rings made of a cheaper quality rather as precut rings. For bigger rings of a higher quality, with a substantial weight and material input, the cost of pre-compressing is negligible.
We believe that professionally factory made rings are usually technically superior and as well cheaper than in situ cut rings from a bulk spool. Our ring department employs very experienced and trained personnel and is well equipped with special tools and machines to produce all kind of packing rings.
The idea behind this construction is, that stuffingbox packing with an integrated elastomer core will follow radial shaft movement without being destroyed. Actually the braid is usually softer than the elastomer with a typical shore hardness of 70. However, as the shaft moves the packing will be deformed over the harder core and at the end no improvement compared to a standard packing situation is achieved. An exception is elastomer tube core like styles A19 SKV, S6 SKV and P5 SKE. The tube core deforms much easier than a full core. This technology is useful in applications with a low revolution shaft or static applications like tank lids.
The amount of rings should not exceed 7 rings in a normal relation of shaft diameter to crossection of stuffingbox. The reason is that the transfer from gland load to the bottom ring gets less efficient the deeper the stuffingbox is. This is caused by friction along the stuffingbox surfaces and internal friction in the packing, expressed in a K-factor.
Suitable spacer to fill the remaining space can be made of any material suitable to the process fluid and temperature. If radial movement of the shaft should be controlled special bearing strength plastic or bronce can be used as well. Installation in front of the packing to productside. Tolerances of the gap between bushing and shaft should not exceed 5% of the packing size for pumps and mixers and 2% for valve application.
Technical information tend to give higher limits then a product can actually handle. Footnotes may say: "…only for a short periode of time" or "…not all values should appear at the same time at their maximum". In principal pumppacking will create friction heat which needs to be taken of the temperature limit.
This is in normal operation 30 to 50°C/65 to 90° F.
If the compression on a packing is changed by: shrinkage of packing, overcompression due to leckage appearing from missanlignment, change of settings by loss of flushwater etc., packing can create much higher friction in an area of more than 400°C/750° F. To be on the safe side only graphite packing can withstand these temperatures, but sleeve material will get soft and displacable.
We do not recommend this. As the packing will overheat when the springs are too strong or the packing may lift off if they are to weak.
Real run in of the packing is not done at all and the packing is permanently under stress.
The lifeloading system on valves will help to reduce the total leckage over a periode of time, for example over a month compared to a valve with no lifeloading system, as it maintains the gland load constantly, but it does not reduce the actual leckage measures expressed in ppm or cm³/min.
A typical value for new valves is max. 0,3 cm³/min gasleckage at 6 bar or 1,5 of nominalpressure of valve. This is fixed in EN 12266. This leckage needs to be obtained with or without lifeloading system.
288SKE and 730SKV are the styles we have in some dimensions on stock.
If the ordered amount regarding MOQ on braid and elastomer length is right, we can make a special of almost all packing styles with all elastomers. Packing with solid elastomer cores start at 10 mm - 12 mm packing size and tubecore packing may start at 15 mm.
In principal we favour tubecore packing as these are the only ones which act dynamically on a shaft deflection.
We don`t give out torque recommendations, we encourage mechanics to develop a good feeling for cutting and installation itself and suggest to use pre compressed rings to achieve a good result.
Practical tests on torque meters showed that the torque is very depending on the installation itself, the seating of the rings and the product pressure.
Calculations you find in the market have too many variables in and turned out to be up to 11x out of practical measured values. The key aim at installation is to avoid that the rings lift off from the bottom.
We are not designing TP Packing to an angle, it is a volumetric design. This means we place more material in an sector sealing the OD, then in the middle area of a packing and very little to the section which seals the ID.
The effect that a TP Packing balances the bulk on the ID, which appears when bending a square packing to a ring, is therefore highly efficient. The TP braid will be a compromise in its effect when sealing smaller and bigger shaft diameters.
To further optimized packing we recommend precompressed rings.
When we state pre-cut rings, this means we cut the packing at a mandrel example a sleeve to fit the required dimension stuffingbox.
When we state pre-compressed rings then we take that precut packing segment, insert it in a die form which has the dimension of the stuffing box, and densify it with a hydraulic press. The advantage is an easier installation and less consolidation.
There is a cost difference due to the set up costs and the proportional cost. If you have little amount, for example 5 or 10 of pre-compressed rings, this cost share is bigger than you have a larger amount, for example 50 or 100 pre-compressed rings.