Water conservation on rotating equipment

Scott Boysen, Ian Forsyth and Marco Hanzon
A W Chesterton Co, Stoneham, MA 02180, USA

ABSTRACT
Water use and its reduction has become a major focal point for pulp and paper mills around the world. The adoption of environmental laws and ISO14000 have driven many pulp and paper mills around the world to reduce plant water usage.

Water use on rotating equipment is often high and with today's advanced sealing systems can be dramatically reduced. Packing leakage and the resulting effluent treatment can also be addressed and reduced at the same time.

Current packing and flush water control arrangements on rotating equipment typically use large quantities of water. These flow rates are often uncontrolled. Studies done illustrate flush water use on packing arrangements.

Reducing water use on rotating equipment can negatively impact equipment reliability. High flow rates will mask any inadequacies in the packing currently used. Packing material and construction becomes much more critical when flush flow rates are reduced or eliminated. Quick break in and long term leakage control are two critical attributes packing requires to function well at reduced or zero flush rates. Tests were developed to easily illustrate these two attributes.

New braided packing fibres and construction allow for reduced water consumption. Injectable packing with low heat generation allows for zero water consumption. Testing methods are used to illustrate their performance and field testing is described. Use of a centrifugal force device as an ancillary device is also discussed.

Single mechanical seals have been developed to run flush-free in stock service up to 3%. The use of single or split mechanical seals with an advanced throat bushing / expulsion device offers reduced or zero water consumption in a variety of mill applications. Dual mechanical seals with proper environmental controls offer water conservation in more severe services.

INTRODUCTION - DRIVING FORCES ON WATER CONSERVATION
Water use and its reduction has become a major focal point for pulp and paper mills around the world. Water use on rotating equipment is often high and with increased focus can be dramatically reduced. Packing leakage and the resulting effluent treatment can also be addressed and reduced at the same time.

The adoption of the Environmental Protection Agency's (EPA's) Cluster Rules in the United Stated has increased focus on sources of effluent and its treatment. European mills need to comply with Integrated Polution Prevention and Control Directive (IPPC-Directive). Eco-Management Schemes such as ISO-14000 and EMAS have already been largely adopted within the Pulp & Paper Industry. The Confederation of European Paper Industries (CEPI) reported that as of October 1999, almost 50% of the european paper production was certified with an Eco-Management Scheme or ISO-14000. By reducing sealing water usage and leakage from rotating equipment, mills can realize reductions in the amount of effluent treatment. This can be a significant factor in achieving compliance with legislation and environmental standards. In some cases, this can assist in averting capital improvements in current wastewater treatment facilities.

Various economic concerns with water are also driving water conservation. The cost of water ranges widely depending on access, region and accounting practices. Biological effluent treatment, advanced effluent treatment, limited water supply or municipal water use results in charges from as low as $0.03 to $1.05/m3. In pulp mills using black liquor as fuel, 1 l/min of water dilution can cost over $1,000 per year to evaporate. An area often neglected is the reheating cost incurred when diluting an elevated temperature process such as stock. Flush water reheating costs can easily reach over $100 per year for each liter per minute of flush water injected into an elevated temperature processes.

By combining newer packing materials and constructions with a focus on water reduction, large flush water savings can be realized. Proper packing selection is critical to ensure both minimized water use and no decrease in equipment reliability. Several mechanical sealing systems are available today which require little or no flushwater.

FLUSH WATER USE ON PACKING
Mechanical packings are perceived by industry as low cost commodity items with minimal performance ability. It is for this reason that in the Pulp and paper industry, packing materials cost considerably less than the flush water used to protect the packing. Little attention has been paid to the amount of flush water used on packed rotating equipment such as hydropulpers, refiners, centrifugal/positive displacement pumps and agitators, etc. Water has been used at relatively high flow rates. The concern has been that any flush water reduction would be detrimental to equipment reliability. High flush flow rates can hide a multitude of deficiencies in the packing, its installation, break in, leakage control, etc. Conversely, lowering flush flow rates can lead to increased shaft sleeve wear, short packing life due to thermal degradation, and increased effluent flow from packing leakage. Several important considerations must be taken into account before selecting the correct packing for reduced flush or flush free service. Parameters such as a quick break in period and excellent leakage control become much more critical to the success of a water conservation program on packed rotating equipment.

In the process of shutting the water off to packing, quite often there is interest in how much water is being used and the associated costs. Industry predominately uses two types of piping arrangements, a "flow through method" and a "conventional" method for flushing packing. While the flow through method is increasing in popularity, the conventional method with one line connected to the lantern ring is the industry standard. Flush flow rates and leakage rates on both these arrangements were analyzed.

Flush Through Packing Arrangement
This flush arrangement uses a flush line that connects to a lantern ring port on the stuffing box and allows water to exit out of a similar port on the opposite side. This arrangement is also called an in-line flush. The primary advantage of this arrangement is that it typically has no product dilution. The flush pressure in the lantern ring connection when using this arrangement is very close to atmospheric pressure. Therefore, flush water does not typically enter the process.

The disadvantage of this arrangement is that product leakage rates are often indistinguishable from the lantern ring outlet flow. High volumes of process fluid can easily be sent to drain as the flush dilutes and masks the process leakage. Ultimately, the bottom rings, those below the lantern ring, become sacrificial. Flush water does not lubricate them and leakage through them is not easily detectable.

Also, the packing gland load can be too low for adequate sealing on the bottom rings. The packing gland is typically adjusted by looking at the flush water leakage from the top ring (packing ring closest to the gland). Pressure in the lantern ring is usually low as it is open to drain in this arrangement resulting in a low pressure drop across the top packing rings (rings between the lantern ring and gland). The applied gland load is now very light. This low gland load does not provide adequate pressure on the bottom rings of the packing set and can result in high process leakage to drain and poor packing life.

Surveys were done to determine flush flow rates on equipment using flow through flush arrangements. One study looked at flush flow rates on 36 pieces of rotating equipment. Flow meters were installed in-line on the flush line inlet. Inlet and outlet flows were assumed to be the same. Outlet flows would typically be higher from process leakage through the bottom rings of packing into the lantern ring area. A time period of 350 days was used for the yearly calculations. Calculations were rounded off for clarity. The results of this study are indicated in Table 1.

Table 1 Water Use on Flow Through Packing Arrangement

Table 1

Equipment was categorized by type. While the concern at this mill was reducing water to lower the load on the treatment area, costs associated with using a flush through packing arrangement were not ignored. Filtered water and treatment costs for the 36 pumps is estimated to be over $5,000 dollars per year.

Another study on ten centrifugal pumps yielded similar results.

Table 2 Water Use on Flow Through Packing Arrangement

Table 2

The average flush flow on these pumps was 16 l/min or approximately 8,000 m3 per pump each year. The costs for the filtered water and its treatment is thousands of dollars each year. The concern for maintenance for both mills was minimize effluent. This became a priority for a number of reasons not least of which was the adoption of the EPA Cluster Rules.

Conventional Packing Flush Arrangement
The most common flush arrangement on packing is to connect a flush water line directly to the lantern ring connection on a stuffing box. The flush water pressure is higher than stuffing box pressure. This ensures flow of a relatively clean, cool fluid to both the packing above and below the lantern ring. The idea of the conventional packing flush arrangement is to eliminate process fluid and abrasives from entering the packing set. It should also eliminate leakage of process fluid and the cost associated with this. In reality this is not always the case as it is extremely common to see paperstock leaking from flushed stuffing boxes because of the inefficiencies of the lantern ring arrangement. Leaking paperstock is a direct loss of expensive fibre and due to lantern ring inefficiencies fibre loss of 1-3 tonnes/ stuffing box is commonplace throughout the industry. The primary disadvantage of conventional flushing is that water is injected into the process. This dilution can affect quality and increase costs. It also has a cooling effect on an elevated temperature process. This cooling cost can be significant.

Table 3 shows the results of a study on a papermachine in Europe. The total flow entering into the stuffing box was measured on 44 pumps based on 340 days of production. The pumps listed in Table 3 represent 7.5% of the papermachine's process water requirement.

Table 3 Water Use on Conventional Flushed Packing Arrangement

Table 3

When using a conventional flush arrangement, the flush flow separates into two directions. One flow is towards the packing gland and becomes leakage. The other flow component enters the process fluid at the bottom of the stuffing box. Flush water theoretically is only required to be 1bar higher than stuffing box pressure. In practice however there may only be one or two supply pressures available resulting in flush pressures being 3-4 times higher than necessary. In some incidences flush pressures 10 times greater than necessary have been found. The amount of flush water entering the process is not easily determined. Preliminary testing on packed stuffing boxes shows that it is a function of a number of different factors such as packing construction and condition, gland adjustment, sleeve wear, and stuffing box and flush pressures. Various studies have shown that the highest sealing forces occur at the gland with reduced sealing forces at the bottom of the stuffing box. It is for this reason that the greatest sleeve wear occurs at the contact area between the top ring of packing and the sleeve. With a conventional flush arrangement approximately 80% of the flush water enters the process. However as the packing wears out leakage can often become equal to, or greater than process dilution.

Studies were carried out to determine typical flow rates, amounts of dilution and its cost when using a conventional packing flush arrangement. In both cases flow meters were installed on the flush supply line to determine the total flush flow. Leakage from the packing set was measured and recorded. Dilution flow rate and dilution ratio was then calculated using :

Q DILUTION = QSUPPLY - QLEAKAGE

Dilution Ratio = QDILUTION / QSUPPLY

where :

QSUPPLY = Total flush flow into the lantern ring (l/min)

Q DILUTION = Flush flow into the process (l/min)

QLEAKAGE = Leakage flow from packing to the drain (l/min)

The total flush water use on ten pumps was over 80,000 m3 per year. Leakage and treatment was over 22,000 m3 per year. Process dilution, averaging 72% of flush flow, was over 58,000 m3 per year. Fresh water and water treatment costs per pump was over US$2,000 per year.

Process dilution can also have a cooling effect on the process. This is more pronounced in Chemical Pulping where process temperatures are often 30-100oC higher than flush water. As the flush water cools down the process, additional heating is required to maintain desired process temperatures. The amount of additional heat required to heat the flush water to the process temperature can be calculated by the formula :

Q = mcdt.

where :

Q = the heating requirement

m = the mass flow of the flush dilution

c = the specific heat of the flush

dt = the temperature difference between the flush water and the process.

The costs associated with reheating the flush water diluting the process can then be calculated.

Another smaller study yielded similar results. This study was done using an non-invasive ultrasonic flow meter to monitor supply flow.

Table 4 Water Use on Conventional Packing Arrangement

Table 4

On these five pieces of equipment, total flush water use is over 68,000 m3 per year. Effluent flow from leakage is over 38,000 m3 per year. Process dilution, averaging 45% of flush flow, is over 30,000 m3 per year on the five pieces of equipment analyzed.

The studies show that both the flush through arrangement and the conventional flush arrangement use similar flow rates. In fact, the conventional arrangement used slightly higher flows. Dilution ratios varied substantially as expected. Many factors contribute to this ratio, as in-house testing illustrates. Factors which effect dilution ratios are numerous, such as equipment speeds, shaft deflection, packing materials and construction, flush pressure, shaft/sleeve condition and maintenance practices. With a conventional flush, a high dilution ratio is expected although in practice due to excessive leakage from poor quality inferior packing materials dilution ratios may drop considerably resulting in excessive gland leakage.

A further problem associated with flush water entering into the process is that it dilutes the strength of chemicals in the process, such as white liquor or bleaching chemicals. This subsequently requires additional chemicals to be added to the process so that the same required pH or chemical reaction is achieved.

In the pulp and paper industry carbon yarn and graphite packings are often not permitted due to risks of black particles appearing on the paper or in the pulp. Flushing is the main reason why this occurs, as excessive amounts of water are forced under pressure through the packing rings below the lantern ring. On a typical 50mm shaft approximately 2,500 m3/year of flush water or forced past the packing rings below the lantern ring, and wash all particles and lubricants into the pump. Where flush water can be eliminated (85% of Pulp and Paper applications ) this problem can be avoided. Through the use of advanced high performance packings considerable water savings can be achieved resulting improved operation of the water treatment plant, improved mean time between failure and environmental compliance.

PACKING PERFORMANCE TESTING
Two parameters were considered vital to the success of a zero or low flush packing. These parameters were break in and long term leakage control. These parameters become critical when water is reduced or eliminated. Performance tests were developed to evaluate both of these parameters. Various types of packings were tested to determine their performance characteristics. To eliminate variables in the test, shaft diameter and speed, stuffing box pressure and installation procedures were held constant. Break in adjustments were performed to keep break in leakage between 5 -15 ml/hr. The adjustment amount was kept to a turn of one flat or 60 degrees of turn on the packing gland bolt. A data acquisition system was used to record packing gland temperature, packing leakage, and amperage draw. The testing data clearly illustrates desirable and undesirable break in and long term leakage control behavior in packing.

Critical Break In Period
Breaking in packing becomes much more critical when flush water is no longer used. If too much leakage is allowed on abrasive fluids, such as stock and green liquor, solids will imbed into the packing. When the packing is tightened, the abrasive particles create excessive wear on the shaft sleeve resulting in premature failure. If too little leakage is allowed during the break in period, packing can overheat damaging the packing. Not only will some yarns such as polytetrafluoroethylene (PTFE) breakdown or glaze, break in lubricants, blocking agents, and fillers may be lost causing unrecoverable damage.

Flush water on packing supplies a clean cool lubricant to the packing set. A flush will hide most problems associated with break in on packing. A poor break will result in only an increase in flush use and clean water leakage. Re-tightening the packing set will reduce excessive leakage with no permanent packing damage caused by abrasives. When the flush is eliminated a fast break in period is desirable. The following test data illustrates the break in behavior of two types of packing.

Slow packing break in is characterized by power consumption, temperature fluctuations and high leakage rates during the initial hours of start up. Figure 1 illustrates undesirable break in behavior. Seven gland adjustments were made to control leakage. A half a flat (30 degree) gland bolt turn was used each time. Sharp spikes in power consumption, followed by a gland temperature increase can be clearly seen as a result. Packing leakage was reduced initially but soon increased. Packing leakage rate is excessive at over 50 ml/hr and adjustments are difficult to make due to the sensitivity of the packing. Very small adjustments created large power and temperature increases making this packing very susceptible to thermal damage during break in.

Figure 1

Clearly a packing that exhibits stable power and temperature characteristics while allowing little leakage with few packing adjustments, allows the user to break the packing in quickly. Figure 2 illustrates this type of performance.

Figure 2

Leakage was maintained at below 5ml/hr during the break in. No adjustments were made to control leakage. Power consumption and temperature reached a steady-state condition within minutes. This type of performance will allow the user to establish desirable sealing quickly. Thermal and abrasive damage are minimized during the critical break in period. Once break in has been established, long term leakage control becomes the next critical packing performance parameter.

Long Term Leakage Control
To operate flush free it is also critical that the packing exhibit excellent leakage control over time. Lower leakage rates lead to higher packing temperatures. At these higher temperatures, packing thermal damage can occur. To prevent this it is critical that the packing have a combination of high thermal conductivity, low coefficient of friction, and utilize high temperature yarn and lubricants. Without this combination, packing volume loss, consolidation and thermal damage will result in high leakage rates as low leakage is attempted.

If leakage increases quickly with time, it is much more likely that process fluid particles will move between the packing and shaft causing premature failure. One of the most common failure modes of packing is that the gland adjustment is not made soon enough when the packing begins to leak. Even when using a flush, long term leakage control can be critical if the flush supply has solids and abrasives in it. The tests below clearly illustrate different long term leakage control characteristics.

Figure 3

Tests run on this PTFE packing with graphite dispersion demonstrate unstable leakage (Figure 3). After a three hour break in period and a number of adjustments leakage dropped below 5ml/hr. Power consumption and temperature, while at their maximum during this period, are still low. Packing leakage increased over the next eighteen hours, however, to over 30 ml/hr. The packing was adjusted, leakage dropped with corresponding power and temperature increases. The packing leakage began to increase again. This cycle was repeated for a third time. Clearly, this packing cannot operate at low leakage rates for extended periods of time. Certainly not a desirable attribute for a flushless or reduced flush packing.

Figure 4

No gland adjustments were necessary during this test. Power consumption and temperature remain stable and achieve steady-state conditions after break in. The long term leakage control of this packing was excellent. As a result of this testing on break in and leakage control, three types of packings were evaluated in the field as integral parts of water reduction programs.

ZERO AND REDUCED FLUSH PACKING SOLUTIONS FOR ROTATING EQUIPMENT
Three new packing alternatives have recently been developed to reduce or eliminate water. Each one of these has demonstrated excellent test results with quick break in and long term leakage control. These characteristics are essential when flush water is reduced or eliminated. The three types performance tested and evaluated in pulp and paper mill services are :

  • heat resistant, thermoset fibre packing
  • twisted, pure graphite tape with carbon reinforcement yarn
  • injectable packing compound

The thermoset fibre packing is a braided packing with extremely high temperature capability. The graphite packing is a braided, twisted tape packing strengthened by carbon yarn fibres. The injectable packing offers a unique approach to flush free sealing and repacking. Each of these types offers unique attributes that may be desirable to the user.

Heat Resistant, Thermoset Fibre Packing

Recently, new heat resistant thermoset fibres have become viable for use in braided packing. These fibres offer outstanding resistance to temperature. This is extremely critical when flush water is reduced and low leakage is required. These materials are extremely resistant to thermal breakdown of the base yarn or glazing as a result of the material's high temperature capability. This material is available in white which eliminates concerns about carbon/graphite packings on stock pumps. This can be a concern when flushing a pump. Performance testing is illustrated in Figure 5.

Figure 5

During the first few hours, the packing ran with zero leakage. As it broke in, a small amount of leakage developed (<10 ml/hr), dropping both temperature and power consumption. No gland adjustments were made during the length of the test. Break in adjustments were not required as high initial leakage, necessary with many PTFE based packings, is not necessary due to the high temperature resistance of the yarn. Long term leakage control is very low and stable over the length of the test.

This packing worked extremely well in reduced water applications. Water use has been reduced by 90% in some cases. This type of reduction was not possible with PTFE based packings. At slower speeds, 1200 feet per minute and below, eliminating flush water entirely is possible. This packing offers industry a new alternative in a tough general purpose white packing for reduced water consumption as a result of its quick break in, leakage control, and high temperature resistance.

Braided Graphite Packing With Carbon Reinforcement

Graphite is often seen as an ideal material for flush free service. It has low heat generation, high temperature capability, and excellent thermal conductivity. Each one of these characteristics becomes much more critical when flush free service is required. Braided graphite yarn, however, is costly and does not yield the strong Fibre required for pulp and paper applications. Braided graphite tape packing has recently been introduced to the pulp and paper industry and has shown good success. The graphite forms a homogeneous mass in the stuffing box. In effect, the entire packing set becomes die formed in the stuffing box. This is much different from other braided type packings. Typical braided packings contain voids which must be filled with blocking agents, such as PTFE, to prevent wicking and provide good leakage control. When used in large amounts these lubricants and blocking agents can contribute to packing volume loss, leading to relaxation and increased leakage.

The conformability of graphite is excellent resulting in a packing that seals well on worn sleeves. This is a significant advantage over carbon fibre packing which requires sleeves in very good condition. The disadvantage of conventional graphite tape packing, however, is that the graphite may have a tendency to extrude out through the bottom and top of a worn stuffing box and packing gland. For this reason, graphite tape packing has typically been used with braided carbon end rings to prevent extrusion. This results in two types of packing being used on one stuffing box. Industry feedback has been that one type of packing per stuffing box is preferred.

Reinforced graphite packing minimizes the extrusion problems associated with older graphite tape packings. The reinforcement can be provided by braiding corners of carbon yarn into the foil (Figure 6). Sealing performance remains exactly the same. An additional benefit of this strengthening is that packing removal is simplified. Now one type of packing can be used for many different applications within the mill - flush free.

Figure 6

Break in is extremely short with little adjustment required. Thermal degradation of the packing is not a concern compared to packing constructed of PTFE and other low temperature fibres. Short term volume loss is negligible as no break-in lubricants are required. High temperatures will not damage the packing so it is very user friendly. Long term leakage control is excellent as the packing tends to form a homogeneous solid mass under it with little chance for wicking. The test data below demonstrate these characteristics (Figure 7).

Figure 7

Figure 7. Graphite Packing

Field performance has been excellent. The reinforced graphite packing has been used in numerous applications flush free. Nine refiners were converted from carbon yarn packing to the graphite packing. Total previous flush water leakage was 96 litres/min or over 48,000 m3/year for the nine refiners. Total flush flow and dilution rates were not measured. Repacked with the graphite tape packing, these refiners not only use no flush and have no dilution, total leakage rate for all 9 refiners is well below 4 litres / minute. Four agitators using well over 36,000 m3/year and twelve stock pumps using over 76,000 m3/year were converted with similar results. Large leakage rates were so common, to prevent mechanics and operators from loosening the packing gland to achieve "normal" leakage, "Waterless Packing" tags were applied to the gland studs.

In another mill, over 30 centrifugal stock pumps have been sealed in both the pulp and paper machine areas for over a year - flush free. Previously each one was flushed. Other types of equipment have been sealed successfully, vacuum pumps, white, green, and black liquor pumps, hydropulpers, agitators, soot blowers, steaming vessels, have all been sealed successfully - flush free.

Injectable Packing
Injectable packing material also using white, heat resistant thermoset materials can also be used with no flush. The packing material is injected into the lantern ring connection of the equipment. Two braided end rings are used to contain the injectable packing compound. The compound is injected by means of a hydraulic piston pump that pressurizes the compound into the stuffing box of the equipment (Figure 8). As an injectable material, part of the compound rotates with the shaft. Sealing is done within the material itself. As an extremely formable material it works well on worn sleeves. Repacking is not necessary. Sealing can be reestablished by injecting more material.

Figure 8

Leakage control is excellent. No thermal degradation due to sleeve friction exists. No flush is needed as heat generation is extremely low. Break in is immediate. Installation requires both the stuffing box and gland to be in good condition. Also, end rings must be properly installed for the compound to establish pressure.

The test data below demonstrates the negligible break in period (Figure 9). No injections were necessary during the test. Steady-state temperature conditions were quickly obtained. Long term leakage control was easily established and no gland adjustments were necessary during the test.

Figure 9

Numerous successful zero flush installations have been performed on white water, stock, weak filtrate and waste water. Hydropulpers, agitators and centrifugal pumps have all been sealed with zero flush.

Centrifugal Force Devices
A method using white water rather than clean water flush can be used to flush a packing set. The concern in the past is that white water would also reduce the life of the packing set. Centrifugal force devices can be installed in the bottom of the stuffing box. This device centrifuges flush particles towards the impeller and away from the packing set (Figure 10). The cleaner flush fluid is now sealed by the packing. This device replaces the bottom two rings of packing and the lantern ring. Flush fluid containing particles, such as white water, can be used to flush the packing to assist in closing the water loop.

Figure 10

When using a water flush with the centrifugal force device, low flush flow rates can be established and maintained. Packing wears over time, allowing greater flush flows to enter the process. The centrifugal force device does not wear so flush rates stay consistently low. This device also is currently being field evaluated as a device that will effectively centrifuge particulate away from the packing in flushless services.

MECHANICAL SEAL SOLUTIONS ON ROTATING EQUIPMENT
Mechanical seals have seen mixed acceptance within the pulp & paper industry. Whereas one mill may have converted the majority of its packed stuffing boxes to mechanical seals with documented savings, another mill may had to convert back to conventional packings due to lack of success with mechanical seals.

Standard mechanical seals do not perform well in mills, particularly in stock slurry applications. The very nature of the pumpage as well as the operational conditions in mills require advanced sealing systems and consideration of the environment is the stuffing box.

With respect to water usage reduction single mechanical seals typically have not utilized flush water reduction methods. As a result, flow rates of the flush water and process dilution can be quite high. Yet, these mechanical seals depend on this constant flush flow for reliable operation. Interruption in the flush flow may lead to premature failure. Dual mechanical seal arrangements typically have not addressed water consumption, as they are 'open' or flow-through systems. At the same time these flushing arrangements typically have not been provided with a means of detecting process dilution in the event of inboard seal leakage.

Mechanical seals have been developed for use with zero flush water in stock service. Special, advanced throat bushings can significantly reduce or eliminate flush water use and eliminate particulate build-up around the seal. The combination of such a seal and throat bushing creates an flush-free, intrinsically reliable sealing method. On advanced thermally efficient dual seal designs, flow through flush water use can be eliminated and replaced by 'closed' systems.

Flush-Free Single Mechanical Seals
The seal design and type plays an important role in determining the amount of flush water required to operate the seal reliably. This is especially true with single seal designs. To select the best mechanical seal design to work reliably at low or zero flush flows, it is important to analyze a number of important attributes.

Non-Clogging
The mechanical seal should always have the spring mechanism out of the process fluid for optimum reliability. This will prevent the solids and particles in the process from clogging the spring which would lead to premature failure.

Cool Operation
A seal that runs hot will have excessive amounts of dewatered and packed solids around its faces. Solids packing will eventually cause the seal face to hang-up leading to premature seal failure. To minimize this problem, water flush is used to remove the frictional heat generated by these seals. The two components that are involved in creating seal operating temperatures are heat generation and heat dissipation. Cooler operating seals have features that minimize heat generation and maximize heat removal. The need for a water flush for cooling the seal is minimized and often eliminated.

A cool operating seal requires low heat generation. A hydraulically balanced seal reduces the hydraulic closing forces on the seal faces to reduce heat generation. Seals that are unbalanced and have wide seal faces can require three times the flush rate compared to cooler designs to achieve similar operating temperatures.

The other component of a seal that operates cool is the amount of heat that is pulled away from the seal faces. The hardface, typically tungsten carbide or silicon carbide, is approximately ten times more thermally conductive than the carbon. The majority of heat developed at the seal faces moves through the hardface and out into the process fluid. To optimize heat dissipation in a seal design, the hardface should be located in the fluid, rotating and exposed to the process fluid. If the hardface is the stationary face, air surrounds it, acting as an insulator

Seal Face Wobble
Once the temperature at the seal face is reduced, it is important to look at how the seal faces are kept closed during pump operation. The first step is to minimize seal ring movement. The seal design should have the ability to keep the unsprung or fixed seal face completely square (90) to the shaft. Any out of squareness will require the spring loaded face to wobble twice each revolution. At the high speeds that rotating equipment operates, the spring loaded seal face will not respond perfectly to the wobble of the fixed face. This time lag or hysteresis will allow solids to move across the faces causing seal face scoring and premature seal failure. High flush rates are typically used to minimize the solids in the stuffing box and extend seal life as a result of this wobble and resultant scoring. An ideal seal design will ensure the unsprung or fixed seal face is square to the shaft centerline after tightening.

Shaft vibration should also be minimized to prevent momentary seal face opening. Shaft vibration is caused by a variety of reasons such as impeller imbalance, poor coupling alignment, pump cavitation, etc. Since this movement cannot be completely eliminated, it is critical to ensure the seal face can move freely. Many mechanical seals use o -rings as secondary seals. It is important to minimize the drag underneath these dynamic o-rings. Drag on o-rings can cause the seal faces to open from hysteresis or lack of responsiveness resulting in face scoring and premature seal failure. This is an even greater concern when these o-rings cause fretting corrosion of the stainless steel surfaces.

To minimize the drag caused by the o-ring, many seal designs have completely eliminated fretting corrosion from the seal. The dynamic o-ring rides on the seal face material itself. In the case of carbon, this material contains graphite thereby minimizing o-ring drag and the resulting hysteresis and seal face opening. Another step that can be taken is to polish this surface to a fine finish to further reduce drag.

Mechanical seals have been specifically designed with these features to work reliably, flush free, on 3% stock. Clearances around moving parts are maximized to prevent clogging and enhance heat dissipation. The narrow balanced faces, coupled with the increased clearances, will not allow clogging or binding. This enables successful start-stop operation in flushless service.

Figure 11

Advanced Throat Bushing / Exclusion Devices
On stock applications at 3% and above, advanced centrifugal force expulsion devices have been used very successfully in conjunction with mechanical seals. These devices simply slide over the shaft and sit in the bottom of the seal cavity against the backcover. During operation, this type of device converts some of the rotating flow in the seal cavity into a strong axial flow component. This axial flow is driven along the seal cavity bore in the direction from the gland towards the throat (Figure 12).

Figure 12

Since contaminants are centrifuged to the bore during pump operation, the flow directs them into the expulsion device. The expulsion device increases velocity and centrifugal force on the particulate. A small groove, machined at the end of the lead in ramp, is then able to collect the particulate.

The collection groove leads directly into the main spiral, which conveys the contaminants radially inward and out through the exit groove at the shaft. The main spiral continually decreases in diameter and the steadily increasing angular acceleration forces abrasives deeper and deeper into the groove. This enables the groove design to decrease in depth and width as it approaches the shaft, spilling most of the excess fluid to drive the axial flow pattern in the seal cavity. Only the apex of the spiral containing the abrasives needs to continue out to the exit controlling fluid exchange. Abrasives are removed from the cavity with or without flush.

Dual Mechanical Seals With Environmental Controls
In more severe pulp & paper services such as black liquor, green liquor, chlorine dioxide etc., dual mechanical seals are often recommended. With the proper environmental controls in place water use can be reduced.

Flow Through Arrangement ('Open' System)
Dual seals require external fluid lubrication. This is typically supplied by a "flow through arrangement". Water is supplied to the seal gland's barrier fluid ports and exits to drain. Conventionally, there is no flow monitoring equipment used in this arrangement. This can result in excessive water consumption sometimes higher than 15 liters per minute. Also, inboard seal leakage on the dual seal is undetected and can be very costly.

Figure 13

Barrier Fluid Tanks
The use of a barrier fluid tank in conjunction with a dual mechanical seal offers the greatest water conservation available with dual seals. The barrier fluid tank is a closed loop system with no need for continuous feed from an external water source (Figure 14). The tank is typically pressurized 1 to 2 barg higher than the stuffing box pressure in the equipment using external gas pressure. An integrated pumping ring circulates the barrier fluid and provides maximum cooling. Water savings vary, but can be as high as 15 l/min per minute or 7,500 m3 per year, or US$400 per pump per year.

Figure 14

SUMMARY
There are many opportunities to conserve water throughout a pulp and paper mill. Rotating equipment is often overlooked as a source of water consumption. Careful selection of the proper sealing device can greatly reduce water consumption and its associated costs.

High flush flow rates are commonplace on many types of mill rotating equipment. Packing break in and long term leakage control become critical factors when flush flows are reduced. Packing performance tests illustrate these attributes. Packing constructed of new heat resistant fibres, reinforced graphite, and an injectable compound demonstrate excellent results in both performance testing and water reduction field testing.

Mechanical seals have no break-in period and run leak-free. Single seals have been developed to run flush-free in 3% stock applications. Employing advanced throat bushings / expulsion devices with single mechanical seals will perform equally well up to 6% service. Thermally efficient dual mechanical seals with the proper environmental controls can dramatically reduce water consumption in more severe services such as liquors, bleach, coatings, etc. Water reduction methods are available and proven. Correct selection, however, becomes critical for high reliability.