TIP 0404-54 Headbox approach piping guidelines · 2020. 1. 9. · The headbox approach piping system is defined as the network of piping and equipment including pumps, pressure screens,

TIP 0404-54 OLD NUMBER – TIP 0410-06

ISSUED – 2000 REVISED – 2006 REVISED – 2012 REVISED – 2017 REVISED – 2018

©2018 TAPPI

The information and data contained in this document were prepared by a technical committee of the Association. The committee and the Association assume no liability or responsibility in connection with the use of such information or data, including but not limited to any liability under patent, copyright, or trade secret laws. The user is responsible for determining that this document is the most recent edition published.

TIP Category: Automatically Periodically Reviewed (Five-year review)

TAPPI

Headbox approach piping guidelines Scope

This Technical Information Paper provides guidelines for design of paper machine headbox approach piping between the machine chest discharge and the headbox. The approach system has a significant influence on uniformity of the final sheet of paper. The goal of this TIP was to compile information on current approach piping guidelines from machine builders, other equipment suppliers, design engineers, and paper company representatives. The purpose is to provide design guidelines that paper companies can use to minimize machine direction, cross direction, and diagonal and basis weight variation on paper machines. The guidelines were developed for paper machines operating conventional headboxes. Changes may be required for application on machines operating at low (<0.2%) or high (>2.0%) headbox consistency. Safety precautions

All applicable safety procedures for working around the wet end of a paper machine must be followed. These include standard safety provisions and specific safety regulations required by the mill including vessel entry, confined space, and lock out procedures. Approach piping guidelines

Higher paper machine speeds, use of hydraulic headboxes, paper uniformity requirements, and other factors make it essential that stock delivered to paper machine headboxes be as uniform as possible with minimum consistency and pressure variations.

The headbox approach piping system is defined as the network of piping and equipment including pumps, pressure screens, cleaners, valves, deaeration tanks, silos, etc., which support flow of stock to the forming section of the paper machine. The approach system is the connecting link between the stock preparation system and the paper machine. A properly designed piping system provides: 1. Stability at desired flow conditions 2. Good stock mixing and fiber dispersion 3. Good stock cleanliness 4. Minimal air content 5. Good chemical introduction and mixing 6. Minimize potential for deposits of fiber, fines, biological materials or chemicals.

The approach flow system has a direct influence on basis weight variation. Pressure fluctuations and changes in fiber consistency affect basis weight uniformity of the finished product. Variations in pressure directly influence machine direction (MD) variation and fiber consistency variation affects both MD variation and cross machine direction (CD) basis weight profiles. Very low frequency basis weight variations (0.001 Hz to 1 Hz) are

TIP 0404-54 Headbox approach piping guidelines / 2

often related to thick stock flow. Thick stock flow at 3 to 4% consistency must be fed at stable flow rate and consistency to the inlet of the fan pump. Headbox and system consistencies can only be controlled when freeness, temperature, pH, conductivity, chemical additives and filler content are maintained at stable levels. Mechanical pulsation must be minimized since each irregularity will have a detrimental impact on overall system performance. The more stable the approach flow system, the better the final product.

The use of good piping principles is essential to good operation of paper machines. The guidelines that follow have been successfully used to design headbox approach piping installed on many paper machines. Thick stock flow introduction

All flows to the blend/mixing chest should be consistency controlled and should have volumetric flow rate measured. Deviations in furnish components are minimized by having relatively large capacities and good agitation. Stock consistency must be controlled between the blend/mix chest and the machine chest. Causes of thick stock consistency variation include:

• Variation in dilution water supply pressure. • Dilution steps smaller than the control valve dead band preventing control valve movement • Poor sensitivity of the consistency sensor. • Transmitter is installed too far from dilution water addition point so time lag is too long. • Control loop gain too high so diluted stock consistency oscillates. • Dilution too large (>0.5%). • Excessive valve backlash and stiction. • Poor mixing of different pulp streams

Thick stock flow should be uniformly supplied to the fan pump at constant head and with specific

differential velocities between components for good stock mixing with whitewater from the silo or tray water system. Concentric mixing experiments have indicated that the ratio of thick stock velocity to whitewater velocity should be at least 6:1, even up to 10:1, to achieve uniform blended stock consistency (1, 2). Improved mixing can also be achieved by modifying the end of the thick stock pipe (3). Thick stock supplied to the fan pump suction should have maximum total head variation of 0.6 in. (15 mm) water and consistency within ± 0.05% of target consistency. This can be achieved with a stuff box (see Figs. 1 and 2) or with a pressure control system. Stuff boxes

Stuff boxes are typically fed at the bottom of one end, discharge to the basis weight valve through a vertical drop-leg from the center compartment with continuous overflow at the far end. A properly designed stuff box has the following characteristics:

• Deaeration of thick stock (only free air, not dissolved air). • Reduction of pressure pulsations that arrive from preceding systems. • Constant head reference for the basis weight valve.

Stuff boxes must be designed to avoid dead corners and uneven flow characteristics and provide a free

surface for air to be released. Stuff box outlets should be designed with transition sections to gradually increase flow velocity. Use of constant overflow provides a cushion for minor upsets in the thick stock system and reduces foam and dirt buildup on top of the stock. Stuff box fluid surface level has to be calculated to provide sufficient differential pressure for the basis weight control valve. Stuff box level should be about 4 to 6 m (13 to 20 ft) above the silo/wire pit liquid level. Pressure loss in the pipes and control valve and dynamic head of the stock nozzle has to be considered. The stuff box should be located as close as possible to the silo to minimize pipe length to the fan pump suction.

Typical stuff box design guidelines are shown in Fig. 1(4).

3 / Headbox approach piping guidelines TIP 0404-54

Fig. 1. Example of an in-line flow stuff box

An alternative U-shaped stuff box design is shown in Fig. 2(5). Application guidelines, common to both stuff box designs, are as follows:

1. All interior surfaces in contact with stock or additives must be adequately corrosion resistant. All interior welds and surfaces must pass a cotton ball test.

2. Eight seconds deaeration time is required from points “X” to “Y” (refers to Figure 2). 3. Liquid surface level should be 13 to 20 feet (4 to 6 m) above silo liquid surface level. 4. The minimum submerged depth in feet should equal the maximum velocity in fps at “E” or 48" (1200 mm),

whichever is greater.

Stock flow velocity ranges are as follows: A. 2 to 4 fps (0.6 to 1.2 mps) B. 1 fps (0.3 mps) C. 1 fps (0.3 mps) maximum (applies to Figure 2) D. 1.5 fps (0.45 mps) maximum E. 4 to 8 fps (1.2 to 2.4 mps)

The key differences between these two stuff box designs are, referring to Figure 2, a longer dwell allowing

more air to release and a better chance for stability in the free surface. Other design recommendations are available from paper machine builders and engineering consultants.


Fig. 2 Stuff Box Flow Velocities and Design Principles

Pressurized systems

Thick stock can be piped directly to the fan pump suction without a stuff box with a pressurized thick stock system. Pressurized systems can be controlled with a basis weight valve, variable speed pump, or a combination of the two. The basis weight valve control arrangement is designed to operate with a constant speed pump. In the combination arrangement, a variable speed pump can be used to control the basis weight valve inlet to a constant pressure. In the variable speed control arrangement, a variable speed pump replaces the control valve and is directly controlled by the machine gauging system and/or thick stock flow and consistency measurements.

A pressurized system is more susceptible to pulsations than a stuff box system because there is no mechanism for decoupling system mass-surge oscillations and there is no thick stock deaeration. Some general guidelines when considering installation of a pressurized thick stock system follow:

• All means to minimize stock air content should be undertaken. • High accuracy of machine chest and silo level control is required. Well-designed chest overflows provide

good level control. • For variable frequency drive (VFD) pumps, the thick stock pump curve should be as steep as possible, i.e.

such that maximum flow change is possible with minimal pressure change. • The thick stock pump should not be belt driven. • Thick stock piping should be designed to minimize mass-surge oscillations, e.g. by minimizing elbows,

expansions, etc. • Final consistency adjustments should be made prior to the stock entering the machine chest. Consistency

should be measured at the machine chest pump discharge but it should not be used for automated adjustments in dilution water flow.

• Some applications may require a V-port ball valve in the thick stock line to provide fine-tuning for system backpressure versus pump speed. However, if a BW valve is used with a variable frequency drive (VFD) system, the pumping efficiency may be sub-optimal resulting in a loss of some of the financial incentive when considering the combination of valve cost/maintenance & pumping efficiency.


Fan pump suction

The distance between the end of the thick stock line (or where stock is introduced to the transition piece) and the fan pump suction is a critical variable in achieving good stock mixing and minimizing machine direction basis weight variation. Guidelines included in the TIP represent a consensus of experiences of Task Force members. Problems related to thick stock introduction too close to fan pump include:

1. Creating a “soda straw” effect, i.e. a too high-pressure differential across the basis weight valve that can cause the basis weight valve to cavitate.

2. Creating a hydraulic effect, e.g. too low NPSH in the fan pump that causes pump impeller vanes to crack within a few months of operation.

3. High frequency consistency variations. 4. Non-symmetrical introduction of thick stock lines into the pump suction can cause pump axial instabilities

of flow and thrust.

Often the most cost-effective solution to these problems is to shorten the thick stock line in steps until problems are reduced significantly or eliminated.

The ratio of thick stock to whitewater velocity at the point of thick stock injection is a primary factor in achieving good mixing. High differential velocity creates mixing motions in the flow leading to dispersing the fiber from the thick stock stream. Generally, the higher the ratio the better, but ratio of 6:1 to 10:1 is preferred for cases where a mixing cone is not used. This ratio should be evaluated over the extremes of a given machine’s grade range.

An alternative to high thick stock velocity is to attach a mixing cone to the end of the thick stock line as shown in Fig. 3. Thick stock flow impinges on a flat plate and is injected perpendicular to the whitewater flow stream. A cone is added to the downstream side of the plate to prevent a low pressure region (dead zone). Some suggested dimensions for mixing cones are shown in Fig. 4. Some machine builders and engineering consultants offer proprietary mixing to achieve good stock/whitewater blending. This applies mainly to cases where the high mixing speed ratios between thick stock and white water recommended in Table 1 cannot be achieved.

Fig. 3 Schematic diagram of a mixing cone


Wire pits and silos

Wire pit and silo design can have a major effect on system stability. Variations that occur in this area directly affect final paper quality. The primary purpose of the whitewater silo or wire pit is to efficiently mix multiple flows entering the silo and to deliver a homogenous, pulsation-free stock flow to the fan pump. Silos also provide whitewater storage and remove excess air. A properly designed wire pit/silo arrangement should reduce air content in dilution flow to the fan pump to less than 1.0% (for an open system). Silos maintain constant suction head pressure to the fan pump and also maintain constant backpressure against the basis weight valve and various stock return flows. Some builders prefer an off-machine silo to an under-machine silo to minimize potential cascading of whitewater. Paper grade, available space, and machine speed influence whitewater system design.

Uncontrolled mixing by free discharge into the whitewater system should be avoided. In some installations two or more recirculation lines may enter the suction transition piece between the wire pit silo and the pump suction. Recirculation lines can be from headbox overflow and/or recirculation, primary and secondary cleaner accepts or rejects, secondary screen accepts, or thick stock addition for basis weight control. Generally, pressure pulsations can be transmitted along these lines to the pump suction. The fan pump may dampen these variations but will not completely eliminate them. It is also possible for pressure pulsations caused by throttling valves in these lines to pass directly back through the recirculation line and appear at the headbox since, as fluid-borne noise, pressure pulsations travel in all directions.

Ideally, there should be no recirculation lines entering the pump suction transition piece to minimize pressure pulsations within a system. Where multiple lines return to the suction of the fan pump, it is advantageous to mix the lines in a pipe ahead of the pump suction and take a single line to the suction, preferably in a concentric design. Any return line should re-enter the system into or in close proximity to the pump suction transition to avoid random consistency variations at the headbox. Lines with consistency higher than silo water should be introduced preferentially toward the suction area transition piece. Recirculation line velocities should be a maximum of 1.1 to 1.5 times the velocity in the transition piece at the point of entry. Table 1 summarizes recommended flow velocities. The branch line nearest the pump should be no closer than a distance equal to twice the pump suction diameter.

Fig. 4 Mixing cone dimensions


Table 1. Suggested flow velocity ranges (applies to Figures 6, 7 and 8)

Location Velocity, ft/s

Velocity, m/s

Comments

i. Thick stock ahead of the basis weight valve (Heavy Stock in Figure 6, A in Figure 7)

1–4 0.3–1.2

ii. Stock return flow lines (Figure 6) 7.5–15.0 2.5–5 iii. White water silo downward velocity

(C in Figure 7) 0.3–0.5 0.1-0.15

iv. Standpipe downward velocity (Figure 8)

2.6-3.9 0.8-1.3

v. Headbox overflow (Figure 8) <3.2 <1.0 vi. Headbox recirculation (Figure 8) <3.2 <1.0 vii. Second stage cleaner accepts <3.2 <1.0 viii. White water flow from the silo

entering the fan pump transition piece (D in lower part of Figure 7)

0.5–3.0 0.15-0.9

ix. Differential velocity between thick stock and water recirculation outer pipes

6.0–10.0 1.8–3.0 Delta velocity greater than surrounding fluid. Discharge velocity from the thick stock line should be at least six (and up to 10) times the velocity of the surrounding fluid.

x. Velocity established by fan pump (F in Figure 7)

7.5–15.0 2.5–5 Normal values, but established by fan pump selection

xi. Thick stock after basis weight valve (G in Figure 7)

6.0–10.0 2.0–3.0

xii. Outer white water circulation line 3.0–6.0 0.9–1.8

Installation of current stock mixing concepts used for new paper machines can require major modifications

on an existing machine. An alternative top entry arrangement for double suction fan pump applications is shown in Fig 5. Thick stock enters the top centerline of the transition piece at an angle of about 45 degrees in the direction of flow. Other thin stock lines should also be introduced at a 45-degree angle well back from the pump to allow ample time for mixing if possible. This design does not provide good stock mixing but may be necessary when space is limited.

Fig. 5. Fan pump piping detail (alternate)

4d (min) or 2 times D suction, whichever is greatest


One efficient way to achieve good mixing of recirculation flows ahead of the fan pump suction is using a

collection pipe, or standpipe. The standpipe must be open to eliminate pressure pulsations. A typical modern stock/white water mixing arrangement ahead of a fan pump is shown in Fig. 6. It should be noted that the dimensions A-E in this figure are suggested relative distances to the critical points in this mixing arrangement. Thick stock is supplied through a basis weight valve or directly from a VFD pump and supplies the inner pipe in a “pipe-within-a-pipe” design (concentric mixing). Thick stock flows through an inner pipe and the white water silo to the transition piece ahead of the primary cleaner or fan pump. The outer pipe is supplied by cleaner accepts, secondary and tertiary screen accepts, deaeration overflow, etc. This line surrounds the thick stock line and flows through the silo and discharges into the transition piece ahead of the fan pump. Headbox recirculation stock is discharged into the bottom of the silo preferentially oriented toward the fan pump transition piece. This configuration is used when the headbox does not have an internal attenuating system, and helps prevent pump pulsations from traveling up the recirculation line to the headbox. A concentric piping arrangement, shown in Fig. 7, is used to provide good mixing and flow at the suction of the fan pump. Suggested flow velocity ranges are shown in Table 1.

Fig. 6. multiple mixing arrangement.

*Example (applies to upper part of Figure 7) - If d = 12 in., then 4d = 48 in. (Or 2 times S whichever is greatest.)

- If pump suction S = 30 in., then 2 x S = 60 in. - Use 60 in.


NOTE: NPSHa (net positive suction head available) should exceed NPSHr (net positive suction head required) by a

minimum of 5 feet (1.5 m) or 20%, whichever is greater.

There are differences of opinion as to whether the ends of the thick stock and secondary flow lines should be cut square or at an angle as shown in Fig. 7. Some engineers recommend restricting the end of the thick stock line to increase velocity and promote mixing.

Some general piping guidelines follow:

1. If a stuff box is used, the level should be a minimum of 13 feet (4 m) above the level in the silo. Minimum stuff box level is velocity dependent, i.e. the stuff box level above the silo should be increased where a higher discharge velocity ratio from the thick stock line is needed for acceptable mixing.

2. The thick stock line between the stuff box and basis weight valve should be sized for 1–4 fps (0.3–1.2 mps). The line between the basis weight valve and the suction side of the fan pump should be sized for 6.0–10.0 fps (1.8–3.0 mps). A high velocity discharge is suggested for where better mixing is required, i.e. 6-10:1 velocity ratio between these lines.

3. Locate the stuff box as close as possible to directly above the pump suction. Drop stock line vertically and then slope upward 1.5 inches per foot (125 mm/m) minimum to enter the fan pump suction as shown in Figs. 6 and 7.

4. The basis weight valve should be located well below silo level and preferably four pipe diameters above the elbow after the basis weight valve.

5. All pipelines should be sloped upward in the direction of flow at 1.5 inches per foot (125 mm/m) minimum wherever possible.

6. Secondary entrance lines on the fan pump suction, except where concentric or top entrance is used, should enter the main pipe in a lower quadrant pitched up at a slope greater than slope of the main pipe and not more than 45 degrees from the direction of the main pipe. In general however, this type of entry is discouraged.

7. High points in lines should be vented to the wire pit or seal pit and pipes sloped upward in the flow direction.

8. If horizontal piping lines are unavoidable due to existing equipment locations, minimum velocity of 9 fps (2.7 mps) should be used.

9. Drains should be installed in all low points. Drain plugs must be flush with the pipe inside diameter if located between the screen and headbox. They must also pass the cottonball test at this location.

10. Wire pit or silo outlets and stuff box outlets should be designed with transition pieces to gradually increase flow velocity. The submerged height in feet (m) should be equal to or greater than the numerical value of the velocity in feet per second (mps) of the outlet. Insufficient submergence may result in vortexing and air injection into the flow.

11. All lines entering chests, pits, or silos should be submerged. Maximum velocity from the outlet of the pipe should be 2–6 fps (0.61–1.83 mps) higher than the main flow channel velocity to ensure good mixing and prevent back flow. If a valve is used on a line outside the silo, it should be located below liquid level to prevent flow from cascading.

12. Pipe reducers or increasers used in sloped piping should be non-symmetrical conical sections. The top surface should be designed to allow free movement of air and avoid air pockets. The bottom surface should allow good drainage when washing pipelines. The side slopes of pipe increases should be limited (1:16) where possible to prevent flow separation from the side walls and formation of eddy currents. Otherwise slope should be determined by available commercial sizes no more than 1:4.


Fig. 7. Concentric piping arrangement. Note, upper part of figure refers to dimensions, lower part refers to

velocities.

The bottom part of the silo is designed as follows (refers to Figure 7 unless otherwise stated):

1. Diameter S (upper part of Fig. 7) is given with the suction flange size of the fan pump. 2. Fresh stock piping: Pipe diameter should not be increased after the flow meter or stock-regulating valve.

At minimum production rate, fresh stock velocity has to be several times higher than dilution water velocity at the position indicated with cutting plane “Y-Y” (including the flow rates of the standpipe), to ensure proper mixing between stock and dilution water. The higher the velocity ratio between fresh stock and dilution water, the better the mixing effect. In general, a velocity ratio of 6:1 to the right of the position indicated with cutting place “Y-Y” is recommended, however in some cases the required velocity ratio may be as high as 10:1.

3. Distance “E” should be approximately 0.4 times diameter at “B”. 4. Standpipe (without deaerator): All flows directed before the fan pump have to be connected to the

standpipe. Flange connections will be at different elevations but connections should be in one plane if possible. They cannot be installed across from each other. The highest connection must have an elevation right below the level of the standpipe during operating conditions. Connections from the secondary cleaner stage accepts and second stage pressure screen accepts should be installed at a maximum angle of 30-45° relative to the standpipe. The standpipe must be equipped with a shower nozzle. Calculation of standpipe level must consider pipe friction and velocity at the end of the standpipe. Downstream velocity in the standpipe should not be higher than 2.6-3.9 fps (0.8-1.2 mps). The total height of the standpipe should be 20 in. (508 mm) higher than maximum operating level. No additives should be introduced into the standpipe. Special care must be taken to prevent air/gas from entering the system. Introduction of additives can result in a reaction with stock and cause heavy deposits between the standpipe and fresh stock lines.

5. De-aeration vessel overflow piping (instead of a standpipe): Jet velocity at the end of the pipe should be 5.0-6.5 fps (1.5-2.0 mps) and at least 100% higher than the velocity of dilution whitewater at cutting plane


”Y-Y” (refers to Figure 8). Machine speed and paper grade determine how the thick stock and whitewater systems have to be designed to remove air and if a de-aeration system is needed.

6. A minimum continuous silo overflow is recommended to maintain stable water level to the fan pump suction. This also provides a constant head reference for the basis weight valve. An overflow height of 0.5-2.0 in. (13 - 50 mm) should be used to calculate overflow rate. Overflow should then be determined by a wet end flow balance model.

Another stock and whitewater mixing design is shown in Fig. 8.

Fig. 8. Silo with standpipe and central fresh stock supply. All re-entry pipes are oriented to centerline.

1) The silo diameter for this design should be determined such that the maximum downward velocity is that

allowing for air bubble removal. This has been shown to be in the range 0.02 – 0.08 m/s (0.065 – 0.26 fps) or < 0.02 mps if one requires even the smallest bubbles be removed (6) (note tighter range for down velocity in this design compared with that of Figure 6). The size of the silo may be limited depending on the available building space in which case higher downward velocities will result in less air removal based on bubble size. Typical downward velocity should be in the range 0.3-0.5 fps (0.1 - 0.15 mps). In some cases it may be better to install deaeration in the cleaner system or go to a compact wet end system with special deaeration equipment.

Whitewater trays

Whitewater flow in trays should be directed to the couch end of the machine at velocities of 1.0-1.6 fps (0.3-0.5 mps). A long open flow with dwell time of 60 seconds or greater is required to permit air release. Minimum tray velocity of 3.2 fps (1.0 mps) is required if filler content is greater than 30%. It should be noted that while an increase in velocity will help minimize filler sedimentation, it will also reduce the dwell time.


The overflow edge from one tray to the next should be designed to the required overflow volume and overflow-length. It should be at least 1.18 in. (30 mm) lower than previous level.

For high-speed machines (especially twin wire formers), large volumes of air enter the whitewater. To adequately remove air from whitewater, the first whitewater channel must be designed as long as possible. Flow barriers

Bends in open trays or channels can cause flow separation and resultant vortex formation. This can be avoided by accelerating the flow. Trials with a variety of different inserts have shown that inserting a circular displacement barrier across the direction of flow gives the best results. Equal kinetic energy above and below the barrier prevents local back flows. See Fig. 9 and 10.

Fig. 9. Flow barriers


Fig. 10. Layout of flow barriers

Headbox recirculation

For headboxes with a tapered header the headbox recirculation system should be designed to maintain uniform pressure in the distribution header over the full range of machine operating speeds. In some high machine speed applications, pressure drop across a single headbox recirculation control valve can be too great for proper control without cavitation. In cases where two valves are needed, design guidelines follow (see Fig. 11 and 12):

1. A V-port ball is recommended for Macro control. This valve should be manually controlled. The purpose of this valve is to share excess head loss with the Micro-control recirculation valve

2. The “Micro” control V-port ball valve should be remote control manually activated. This valve is used to control flow for taper header balance and/or optimizing CD sheet uniformity.

The arrangements shown in Figs. 11 and 12 are normally used for operating speeds above 3,000 fpm (915

mpm). In some cases, the headbox recirculation will be sent to a deaerator prior to the silo, as suggested in Fig. 12. A single valve should be adequate below 3,000 fpm but some papermakers prefer to use two valves at lower speeds. All valves in Figs. 11 and 12 should be located below liquid level in the silo to prevent possible cavitation, erratic pressure pulsations and cascading flow. All valves in Figs. 11 and 12 should be located low in the piping to avoid being influenced by deaerator vacuum (if the system has a deaerator) and to help prevent erratic operation (approximately 12 feet [3.7 m] below the vacuum leg is suggested). All valves should be as far as possible from the tapered header to provide maximum attenuation if erratic pressure pulsations occur.

Special valves are available that can take more head loss without cavitation e.g., Q-Trim valve. This can extend the range where a single valve can be used.


Fig. 11. High speed recirculation piping to silo

Header recirculation should not be piped directly to the fan pump suction with a hydraulic headbox on an open forming section unless the headbox has internal attenuation. The recirculation line should return to the body of the silo as shown in Fig. 11 to prevent pulsations coming up the recirculation line reaching the headbox. The discharge should be close enough to the pump suction so it flows directly into the suction to avoid random mixing in the silo. For gap or suction breast roll applications, it is acceptable to pipe to the suction of the pump. It has been determined that twin-wire formers are less sensitive to pulsations.

To control the headbox recirculation, sight tubes from the front side of the headbox to the back side are often used. The micro V-port ball valve is adjusted to ensure that there is no flow in the sight tube. Some mills utilize differential headbox pressure sensors in place of a sight tube. This method uses the differential pressure transmitter to determine the controlled position of the recirculation valve to maintain a balanced condition. This arrangement can be adapted to automatic control.


Fig. 12. High speed recirculation piping to deaerator.

Notice: Valves should be located low in the piping to avoid being influenced by the deaerator vacuum and help prevent erratic operation, e.g. 12 inches below the vacuum leg has been suggested.

Fan pump

Headbox feed pumps are designed to provide special features. Their main task is to create uniform flow and pressure to the headbox. The pump must have an impeller that splits the vanes in the center of the rotor and staggers vane position over the circumference around the rotor. The vanes may be angled (skewed) on the face of the rotor to promote smoother flow of stock through the pump to insure minimal pulsation in the downstream approach flow piping system. Fan pump considerations are listed in Table 2. Table 2. Fan pump selection considerations for low pulsation operation

a. Range of continuous operation: 75 to 105% of Best Efficiency Point (BEP) b. NPSH margin: NPSHa > 1.2 x NPSHr or 5 feet, whichever is greater. c. Reduced or minimum flow operating conditions

Evaluate range of operating conditions to ensure flow does not go below minimum flow criteria. d. Cutwater clearance: percent (i.e., 95%) of maximum diameter impeller or percent of cutwater

diameter. e. Closed, double-suction impeller with split, staggered and skewed vanes. Vane tip lengths and vane

pocket areas should be as uniform as possible. f. Rotor balance: ISO 1940 G2.5 g. Fan pump should not be oversized or have excessively high head. h. Impeller surface finish: ACI SIS 2 Grade IV or better (Average RMS finish – 350). i. Allow for variable speed operation. Variable speed drive stability should be 0.01%. j. Frequency of vane pulsation should be greater than 80 Hz.


k. Pump pulsation should not exceed 0.5% of process (peak-peak) at impeller rotational frequency or any harmonic below 50 Hz. This condition must be met for all pump speeds within the design operating range. NOTE: The supplier of the fan pump will normally provide a pulsation warranty and the above suggested values are not specific to any supplier.

l. The high points of the pump casing should be able to bleed air. _____________________________________________________________________________________________

Flows entering into the main body of the transition piece may be unequal. If there is wide disparity between flows from the recirculation lines, one side of the double suction impeller may be starved and the other side overloaded. In addition to increasing pressure pulsations at the pump discharge, such unequal loading of the impeller could cause pump operational problems. Separation leading to cavitation can result in excessive thrust imbalance causing reduced bearing life, bearing failures, or broken shafts at or near the impeller. Fan pump drive systems

Variable speed pumps provide energy efficiency, improved pressure control, and a smaller approach flow system through elimination of bypass flows and throttling control valves. Constant speed pumps are used on some machines when cleaning equipment requiring constant flow rates (forward cleaners) are located downstream of the pump.

Experience has shown that resonant system conditions often occur in stock delivery systems at, or near, normal fan pump operating speeds, i.e. when the frequency of other rotating equipment is close to that of the fan pump. It is therefore desirable to operate a pump at a different speed to move the pump frequency away from a system resonant frequency. The existence of a system resonant condition can be ascertained by varying pump speed and determining if frequency of the pressure pulsations varies directly with pump speed (pump induced), or remains at a constant frequency regardless of the pump speed (system resonant condition). When constant speed drives are used, pump frequency cannot be varied. This makes it difficult to accurately determine sources of pressure pulsations as well as avoid problems should they occur.

For this reason, serious consideration should be given to a variable speed drive arrangement. An additional important benefit of a variable speed drive system is the ability to closely match system head with the fan pump performance curve. This allows high pump efficiency to be maintained over a wide range of operating conditions. The discharge throttling valve can normally be eliminated. Variable frequency drive

Speed control of a fan pump drive has to have the same accuracy as a machine drive. An AC digital drive should have dynamic accuracy of 0.01%. Older generations of DC analog drives were accurate to 0.1%.

Required headbox pressure and/or flow rate should determine pump speed and should be the variable manipulated to achieve control of jet/wire ratio, rush-drag, or jet speed.

Speed control of the fan pump drive must include an adjustable ramp for start-up. Adjustable range should be 10 to 60 seconds up to full speed to prevent sudden pressure surges and allow the system to fill slowly and stabilize. Fan pump discharge piping

Good piping practice is critical when dealing with systems requiring low-pressure pulsations. Factors which should be considered when designing the discharge piping of a fan pump include elbows located at or near the pump discharge, throttling valves, and piping support in the vicinity of the pump. Use of throttling valves after the fan pump is not recommended with use of a variable speed pump. Short radius elbows close to the pump discharge can contribute to pressure pulsations and should be avoided. In situations without spatial constraints, a standard, long radius elbow can be used after the pump discharge. In some cases, use of three to five-diameter bends or hydraulic elbows should be used. Although these are more expensive, they can be beneficial for specific constraints and/or requirements.

A throttled valve can create pressure variations at frequencies similar to fan pump frequency. In this situation amplification of existing pressure pulsations can occur. Poorly supported discharge piping which is susceptible to rapid movement may amplify existing pressure pulsations in a stock delivery system. Throttling valves can also degrade the efficacy of polymer based retention systems when added upstream of the throttling valve (7).


Pressure screens

Pressure screens are used in a stock delivery system to remove foreign matter and improve stock uniformity. This is done by passing the stock stream through a basket type screen at low differential pressure (typically 3–7 psi) [20.7–48.3 kPa]. Materials that do not pass through the screen migrated towards a reject port. When foils pass the screen discharge, a pressure pulse is generated that help to keep the screen basket clear. Modern screen rotors installed in approach system screens have multiple foils so that a foil is always in the screen discharge area to minimize pulsations in the flow to the headbox. In addition, the foils may be on an angle to reduce pressure pulsations further.

Consideration should be given to relative operating frequencies of the pump and screens. If screen frequency is close to other rotating equipment frequencies, reinforcement of existing pressure pulsations can occur. All pump operating frequencies should be separated from screen foil frequency by at least 20%.

A single screen is recommended to provide the most stable operation. If multiple screens are used, a tapered header arrangement should be used. The path length of flow from each of the screens should be different. With the same path length, pulses from the two screens will be in phase and can amplify pulse magnitude. If multiple pressure screens are required, it is best to operate all pressure screens simultaneously. An example of a multiple approach flow pressure screen configuration is shown in Fig. 13. Isolation valves upstream and downstream of each individual pressure screen have to be used to isolate specific pressure screens. These should be gate valves that employ a captivated, through going side plate that slides across the flow, see Fig. 14. These valves are only used for open-close service and designed with a smooth bore to minimize any fiber hang-up. Pressure screens are normally installed in the paper machine basement to permit achieving minimum accept pressure of 5 psi (34.5 kPa).

Pressurized screens provide the best performance in secondary and tertiary screen positions. If an open type vibratory screen is used in the secondary or tertiary position, the accept line should be taken out of the primary flow loop to the headbox because of high potential for accepts containing lumps or strings to cause operating problems. Typically, the accept line from this screen would be redirected back to previous screen feed in order to capture and remove any strings. Vibratory screens should not be located in close proximity to the headbox support structure to avoid transmitting vibrations to the stock flow.


Fig. 13. Piping of multiple approach flow pressure screens.


Fig. 14. Isolation valve – used in an approach flow screening system, if multiple approach flow screens

are installed. Cleaners

Four pipe diameters of straight run should be used before each cleaner header assembly. Suggested piping design velocities for cleaner systems are given in Table 3. Table 3. Cleaner system velocities Location Velocity [fps] Velocity, m/s Feed piping 10 - 12 3.0–3.7 Accepts piping 10 - 12 3.0–3.7 Heavy rejects 6 1.8 Light rejects 10 - 12 3.0–3.7

Primary cleaner pumps are typically connected to the whitewater silo. Standpipes are used in cleaner system piping when there is a large amount of free air in stock solutions. Advantages of standpipes include:

• Evacuation of free air in suspension • Evacuation of air at startup • Dampen pulsations • Air removal efficiency 40%–50%

Standpipe design parameters are shown in Table 4 (8).


Table 4. Standpipe design parameters Accept and reject lines enter standpipe tangentially Downward velocity < 0.5 ft/s (0.15 m/s) (does not include dilution) Vortex breaker length equal to diameter Dilution inlet velocity at standpipe = 5 ft/s (1.5 m/s) Valves: 10 pipe diameters away from standpipe Two feet of water above accepts/rejects pipes Two feet of freeboard Cover over top for trash protection Adequate venting to allow air to escape Shower will help eliminate problems with cake or foam

Attenuators

Attenuators covering either a narrow or wide frequency band are available to deal with pressure pulsation problems, which would otherwise contribute to variability in the papers web structure, i.e. the distribution of material, additives, anisotropy etc. Design, location, and recommendations relating to attenuators are provided by headbox manufacturers. In general, installation of an attenuator is made during the initial stages of headbox system design.

One type of attenuator common on hydraulic headbox installations is the pulsation dampening (PD) tank, shown in Fig. 15. A second common type of pulsation dampening device (bladder type) is shown in Fig. 16. The tank will not compensate for inefficiently distributed retention aid, etc. It is not a mixing vessel. Before the PD-Tank, there should be a minimum of 3 pipe diameters of straight pipe, measured between the end of the pipe connector and the beginning of the pipe cone in front of the PD-Tank. Some headbox designs also include an internal pulsation dampener.

Fig. 15. Example of a pulsation dampening tank feed pipe arrangement.


Fig. 16. Example of an alternate pulsation dampening device.

Headbox inlet piping

Whenever possible, a minimum of 5 diameters of straight pipe should precede the inlet of the headbox (Fig. 17), especially if fiber orientation profile is an important consideration, although some vendors require on 4 diameters of straight pipe. While a final decision will need to be made between the user and the vendor, a longer section of straight pipe before the headbox inlet is generally better. Close-coupled elbows should never be used ahead of the headbox. It is recommended that a maximum of two long radius elbows be used in the piping from the screens to the headbox.

A hydraulic elbow as shown in Fig. 18 has been used successfully in some applications. Flow straighteners have been installed on some machines to help stabilize flow to the header, when adequate space for straight piping was not possible, as shown in Fig. 19.

Fig. 17. Straight pipe before header


Fig. 18. Hydraulic elbow

Fig. 19. Typical flow straightener

Central headbox distribution systems

Central headbox distribution systems were developed to eliminate many of the problems that occur with conventional headbox inlet piping and tapered distribution manifolds. Accepted stock from the approach system screen flows to a cylindrical shaped distribution system and on to the headbox with flexible hoses. The distributor replaces both the conventional tapered header and the PD tank. Stock is distributed uniformly across the width of the headbox from the hoses. The hoses are all the same length to provide uniform CD pressure distribution. No headbox recirculation line is required and CD basis weight profiles can be consistency controlled by adding whitewater to change stock consistency in the individual stock supply hoses. The system is illustrated in Fig. 20.


Fig. 20. Central distributor system

Compact wet end systems

Compact wet end systems were developed in the 1990’s to replace conventional approach system designs. Thick stock is blended in a well-agitated mix tank with approximately three minutes dwell time. A stock mixer replaces the blend and machine chests and is placed as close to the headbox as practical to minimize process volumes. Stock is pumped from the stock mixer to the suction of the primary cleaner or fan pump for mixing with whitewater. The wet end system is closed hydraulically and there is no whitewater silo. This reduces the volume of water in the approach system up to 90%. Some systems have deaeration pumps to remove air from the whitewater system.

The highest priority of compact wet end systems is to provide the foundation for very good paper profiles since paper quality and reliable operation are vital to the economics of papermaking. While compact wet end systems have the potential to improved MD and/or CD profiles, they are probably more suited to improving MD profiles. Reduced grade change times and system cleanliness provide additional advantages in paper machine efficiency.

Replacement of conventional blend and machine chests with the smaller stock mix tank offers operational and capital cost advantages. First, a significant number of chests and space is saved and this has a big impact on energy consumption. Secondly pumping energy adapts automatically to real needs. Thirdly, smaller process volume leads to improved consistency control. Significant reduction in investment costs can be achieved compared to conventional wet end systems.

Compact wet end systems eliminate many of the complexities of conventional headbox approach systems and therefore have the potential to reduce many of the common problems associated with conventional systems. Thick stock is pumped from the stock mixer tank to the suction of the primary cleaner or fan pump with a variable speed pump so there is no stuff box or basis weight valve. Low approach system volumes provide more stable systems, faster grade changes, and require less energy to pump stock and reduce the risk of low frequency fluctuations in the approach flow. On the other hand, compact wet end systems pose a greater risk of high frequency fluctuations. Compact wet end system suppliers have guidelines for stock line velocities and in some cases they are


different than the values included in this TIP for conventional systems. In general, some of the most important unit processes for successful implementation of a compact wet-end system include rapid and efficient deaeration and mixing of stock, additives and water. These topics are discussed below perhaps only with stricter requirements in the latter case. Mixing

The importance of mixing in the approach flow has been underestimated for a long time. Fan pumps are poor mixers. In addition, minor backflows in the approach flow system with undefined flow directions can have a big effect on the residual coefficient of variation in the paper. One system replaces the conventional mixing and machine chest. It consists of a hydraulic mixer in combination with two successive chests. Fig. 21 shows a mixer where one chest has been omitted. One task of the mixer is to ensure complete mixing of all stock components, including broke and stock coming back from the save-all/disc-filter. The other task is to eliminate consistency variations which have not been suppressed by the consistency control loops upstream.

Features of the Mixer Macromixing in mixing pipe Micro mixing in static mixer Attenuating of fluctuations

Fig. 21. Mixer

Mixing takes place in three stages. Macro-mixing is performed in a mixing pipe with feed pipes welded tangentially to the main pipe, left side of Fig. 21. In this pipe the kinetic energy of the stock is used to provide a high degree of pre-mixing. The individual number of components can vary considerably however it is known that mixing in this area is not complete.

A static mixer is then used to perform micro-mixing. The static mixer cannot however eliminate consistency fluctuations within a time span of e.g. 60 s. On the other hand, a time span of this order normally elapses until the dilution control has regained its set point following a disturbance.

Chests for attenuating these consistency fluctuations must not be sacrificed in the thick stock system. Long dwell time is not necessary because today’s consistency control loops are quite accurate. Therefore the mix chest and the following machine chest have only 3 minutes dwell time each. This allows a reduction of the total chest volumes by at least 50 %, compared with conventional mixing and machine chests.

If the damping behavior of chests is examined more closely, the result shows that mixing in a chest is far from being instantaneous. Trials have shown that mixing even in well-agitated chests needs at least 30 s to 60 s. additionally, even an ideally agitated chest, which means instantaneous mixing, is not very effective in suppressing fluctuations because the probability of some disturbances breaking through is relatively high. By applying the old papermaker’s concept of two chests, the probability of consistency fluctuations fed forward is reduced drastically. Water mixer

Experience has shown that the huge volume of a white water silo is neither necessary as a deaeration device nor for storing or buffer reasons. The white water silo has therefore been re-engineered into a water mixing design shown in Fig. 22. One task of the water mixer is to combine return flows e.g. from deaeration. It consists of a vertical collector and a jet mixing device. Since thick stock and white water can be mixed more easily than thick stock components, a jet mixer is used. Before injecting the thick stock, all backflows are fed consecutively into the main stream. The kinetic energy of the backflows is used to pre-mix the streams. Final mixing takes place at the


thick stock injection point. In the relevant literature it is reported that jet mixing requires a sufficient amount of mixing energy and pipe length downstream of the injection point. A single injection point is sufficient for complete mixing. These statements have been proven by trials. Video sequences have shown that a too small pressure drop at the thick stock injection point leads to the generation of consistency clouds in the suspension. These clouds can cause increased residual coefficients of variation in the paper. Features of the water mixer • Consecutive mixing of components • Most intensive mixing at thick stock addition

Fig. 22. Water mixer Vacuum deaeration

As mentioned above, an important point in the design of approach flow systems is to keep pulsations away from the headbox. Hydraulic decoupling between stock injection and the headbox is provided by a vacuum deaeration unit, as example of which is shown in Fig. 23. The advantage of decoupling is that upstream of these two components less attention needs to be paid to pump pulsations. Vacuum deaeration also offer freedom to operate the cleaner plant at hydraulically constant conditions. Air in the system can cause a number of problems, not only in the paper but also in the approach flow systems, such as amplifying pulsations, pump speed fluctuations and cleanliness problems. Air entrainment occurs in the former unit of the paper machine and deaeration begins in trays & flumes. Design of the white water tray is therefore a crucial factor.

For a paper machine speed above 1000 m/min (3200 ft/min) full stream deaeration is recommended. Stock is sprayed against the upper section of a cylindrical vessel which is evacuated at boiling point.

Features of the vacuum deaeration unit • Large overflow weir • Optimum hydraulic stability

Fig. 23. Vacuum deaeration unit


Wet end chemistry additives

Poor mixing and injection of chemicals in wet end systems is a common cause of MD/CD and random basis weight variation on several paper machines. Proper additive introduction enhances chemical performance and provides more uniform additive distribution. Complete mixing of additives should occur as quickly as possible. Some suppliers recommend within 3 seconds or two meters (7 ft) of injection. Chemical and equipment suppliers have used computational fluid dynamics and other techniques to help design good mixing systems. Modern systems have the following features:

• Wet end additives are injected close to the headbox with more than one chemical at a time. • Use whitewater or headbox feed stock as injection liquid. This reduces water consumption and the

need to heat make-up water to process temperature. • Fast and uniform mixing provides additive savings and improves paper quality.

An example of an additive mixing and injection system is shown in Fig. 24. Chemical additive and

equipment suppliers should be consulted to optimize design of injection systems.

Fig. 24. Additive mixing and injection system


Piping guidelines

General piping finish requirements suggested by one machine supplier are shown in Table 5 (5). Some paper companies and machine builders have similar guidelines that are more detailed and include stricter requirements. In general, cotton ball is a somewhat loosely defined specification whereas the Ra value is both specific and measurable. Ra is often a preferable specification with cotton-ball added as a secondary spec if desired. One disadvantage with Ra values is that they are not universally applied and may even be difficult to get agreement on. Individual companies typically have their own specifications which can differ by paper grade. Electro-polishing of piping and equipment generally provides smoother surfaces and reduces the potential for stock fiber build-up. The cotton ball test refers to rubbing cotton balls over piping and equipment surfaces that contact stock to ensure that there are no rough areas. Typically, metal-to-metal flanges are required beginning at the accepts flange of the primary screen to the headbox inlet flange. Pipe support comments

Piping system support design requires specialized knowledge. Design should be done by a structural/piping engineer to insure that vibrations and stresses are properly accounted in order to minimize operating problems and pulsations. General comments that should be considered as guidelines for piping support follow:

1. Avoid supporting from the machine room floor. Try to support from the basement floor for greater stability.

2. Because of thermal expansion, pipes should not be rigidly supported in all directions. In general, support the weight of the pipe while allowing it to move for expansion.

3. Use a rubber pad at contact point of pipe and support to help act as a vibration damper and allow movement.

4. If 90-degree elbows are used near the fan pump, anchors are recommended. However to reiterate, it is strongly recommended to not use 90-degree elbows near the fan pump.

5. In the case of a pivoting headbox, there must be flexibility in the immediate straight run to the headbox to allow for slight movement of the headbox for trajectory control.

6. Most headbox manufacturers recommend minimal piping loads be applied to their equipment. Table 5. Equipment, pipe and flange guidelines for approach flow systems Finish Flanges Pipe:

RMS Micro-Inch

Electro-Polish

Cotton Ball

Metal-to-

Metal

Standard

Primary cleaner pump to cleaner --- --- YES X Cleaner to Deaerator --- --- YES X Deaerator to headbox fan pump --- --- YES X Headbox fan pump to screen <125 --- YES X Screen to headbox <32 Optional YES X Headbox header recirculation dilution profiling

<35

Optional

YES YES

X

Second stage cleaners --- --- YES X Second stage screen --- --- YES X Equipment: Screens --- Optional YES X Deaerators Headbox

--- <35

--- Standard

YES YES

X

Fan pumps --- --- YES X Primary cleaner pumps --- --- YES X Cleaner canisters & headers --- --- YES X


General piping

• Thick stock (~3-4% consistency) piping should be set at a minimum incline of 1.5 in. per foot (> 7°). • If the pipe velocities are too low, stock settling and slime generation could result. • De-aeration-/ vent lines should be left slightly open at all times. • De-aeration-/ vent lines should be piped separately to a discharge point, which is open to the

atmosphere. • Sagging pipelines must be avoided. • Each pipe section should be self-draining. • Pipe bends should be made with long radius elbows, Radius = 1.5* OD.

The guiding factor is that the flow in the approach flow system should be under positive pressure at all

times, except in specifically designed areas - i.e., de-aeration tanks). Allowing regions in the approach flow system to create a natural vacuum can cause cavitation which will result in an increase in free air entrainment in the system, cause foam generation, and pulsations due to cavitation. Specialty piping Polished pipelines have to be installed between:

• Primary approach flow pressure screen and headbox • De-aeration and headbox, if no pressure screen is installed! • Dilution screen and the headbox

⇒ High Consistency, tank/ headbox feed - Ra 0.8/ m (32/ inch = N6) mechanical polishing followed by

Electro-polishing - Screen to Headbox ⇒ Low Consistency, dilution feed - Ra 0.8/ m (32/ inch = N6) mechanical polishing followed by Electro-

polishing, Screen to Headbox

In general, the pipe coming from the primary screen to the headbox should be installed with a minimum number of bends. Specific strategies for pipe connections are mill dependent; some mills use stepped flange - many still use concentric, flush metal to metal connections. Metal-to-metal flanges should be placed in such a way that all field welding can be polished. Pipe clean-outs guidelines are shown in Fig. 25. In general, the material used for specialty piping is application specific.


Fig. 25. Pipe clean-outs


Flow velocities in pipelines

For all pumps except the fan pumps, see Tables 6 and 7: Table 6. Suction Pipe flow velocities

Consistency [%] Pipe Velocity v < 1.5 1.5 m/s (4.9 ft/s)

1.5 – 4.0 1.0 m/s (3.3 ft/s) 4.0 – 5.0 0.8 m/s (2.6 ft/s)

> 5.0 0.6 m/s (2 ft/s)

Table 7. Discharge Pipe flow velocities ID 100 mm

(4 in.) 100-200 mm

(5-8 in.) >200 mm (>8 in.)

Consistency up to 1.5%

1.6-2.0 m/s (5.2-6.5 ft/s)

2.0-2.8 m/s (6.5-9.2 ft/s)

2.8-4.5 m/s (9.2-14.8 ft/s)

Consistency up to 3-4%

1.0-2.0 m/s (3.3-6.5 ft/s)

2.0-2.5 m/s (6.5-8.2 ft/s)

2.5-3.5 m/s (8.2-11.5 ft/s)

The smaller value applies to the smaller ID and higher consistency.

For inlet Pipe – Headbox (see Tables 8-10): Table 8. High Consistency Line: Item Headbox slice flow range

Inlet style – PD tank 1:3 maximum 2.2 m/s (7.2 fps) 1:2 maximum 1.8 m/s (5.9 fps)

Inlet type – Tapered Header 3.0-3.5 m/s (9.9-11.5 ft/s)

Table 9. Acceleration elbows – High Consistency Line Flow range v – inlet v - outlet 1:3 3.8 m/s (12.5 ft/s) 7.6 m/s (24.9 ft/s) 1:2 3.5 m/s (11.5 ft/s) 7.0 m/s (23 ft/s)

Table 10. Acceleration elbows – Low Consistency Line v –inlet v - outlet 4 m/s (13.1 ft/s) 8 m/s (26.2 ft/s)

Pulsation studies

Increased basis weight variations in CD and MD direction as well as residual deviations may be caused by the approach flow system. These variations can occur periodically or stochastically. A complete pulsation study may be required to identify system stability problems for an existing piping arrangement. Typical measurement locations are:

• Before fan pump (headbox feed pump) • After fan pump • Before cleaners • After cleaners • Before screens • After screens • Before headbox • In taper header front side and back side if accessible • After headbox (recirculation line)


Two locations may be measured from one tap, for example, after cleaners and before screens. Taps should be located at least two (ideally four) pipe diameters away from any elbows, pumps, screens, valves or other pipeline obstructions if possible.

Pump and screen RPM and the number of impeller vanes and screen foils will be needed during the study. The recommended tap installation methods are shown in Figs. 26 and 27 do not allow air to collect at tap

locations and provide an unobstructed “view” into the pipe. Do not locate taps on elbows. Allow 36 inches (914 mm) of clearance beyond the end of the ball valve for transducer installation.

A typical transducer, shown in Fig. 28, can be coupled to 1 in. (25 mm), ¾ in. (19 mm) or ½ in. (2.5 mm) female pipe threads with the use of different pipe bushings. The 1 in. (25 mm) female pipe thread is the preferred hook up. The ½” ball valve if used should be a full diameter ball valve. An alternate pressure tap arrangement flow piping system is shown in Fig. 29. This method allows for converting from flush plug to pressure transducer while on the run with no downtime required.

Fig. 26. Suggested tap before screens


Fig. 27. Suggested tap after screens. Note: The paper machine must be stopped to remove the flush

plug in this type of design.

Fig. 28. Typical transducer

Fig. 29. Alternate pressure tap. Note: flush plug can be removed without stopping the machine.


Transducer installation instructions

1. Loosen and remove gland nut, (Figure 29, item 1). 2. Pull sensor plug, (Figure 29, item 2), out to retracted position. 3. After Step 2 is complete, turn valve completely to off position. 4. Now loosen and remove removable holder, (Figure 29, item 3), along with sensor plug, (Figure 29, item 2). 5. To reassemble pressure sensor or sensor plug, reverse above steps carefully to prevent “nicking” the stock

surface end of either assembly. 6. Make sure either the plug or the sensor is clean before reassembling.

Note that the above information should be included with the pressure tap assembly drawing. Periodic pressure pulsation

The fan pump must have a volumetric uniformity of the impeller chambers in order to ensure that the maximum water hammer caused by one or several single chambers at rotational frequency (frequency = impeller rotational speed or a multiple thereof) does not exceed 0.5% (peak-peak). This requirement applies to a pump speed range of 80 % to 120 % of the speed at its BEP speed. At nominal production of the paper machine the pump has to be operated in this range.

Suggested pulsation amplitudes at key measuring points are shown in Table 11. Table 11. Summary of pressure measurement locations Location Max amplitude at

rotational frequency or multiple thereof and vane pass frequency

Suggested maximum frequency

Comments

Fan pump 0.5% (peak-peak) or 0.2% (rms)

150 Hz Applies to 80% to 120% of speed where the pump has its optimum efficiency. For nominal production the pump has to be operated in this range.

Dilution pump 0.2 % rms of total head of pump

150 Hz Applies to 80% to 120% of speed where the pump has its optimum efficiency. For nominal production the pump has to be operated in this range.

Pressure Screen

100 Pa (0.015 Psi) rms

150 Hz

Taper Header Depends upon pulsation attenuation in headbox and former design

150 Hz Taps may be located in taper header if it is accessible.

The pressure pulsation must be acquired using standard practices employed in modern digital signal

analyzers. This requires appropriate anti-alias filtering, an appropriate window such as the hanging window for the FFT, sufficient averaging and simultaneous sample and hold if used for multi-channel measurements. The data should be acquired during steady operating conditions. In addition the frequency response of the transducer being used must be sufficient for the analysis being performed.

The pressure spectrum for periodic stationary signals should be displayed as linear spectrum (square root of auto power spectrum). Note that there is not expected to be any frequency content above 400 Hz in pressure measurements meaning that signals sampled at 1000 Hz are self filtered. If implementing your own filters, fourth order Bessel filters using a sampling rate of 1000 Hz have been recommended for all pressure and consistency variations.


Aperiodic / stochastic pressure pulsation

Aperiodic or stochastic (non-stationary) pressure pulsations may occur, for example, due to hydraulic instability in the deaeration tank, or at the white water tank. If this disturbance already exists on the suction side of a pump, this may be amplified by the pump.

The pressure spectrum for non-stationary signals should be displayed as power spectral density spectra. Maximum low frequency pressure pulsation

The percent coefficient of variation (COV) (standard deviation related to the average value at the measuring point) of the pressure pulsation must be smaller than 0.3 % for gap formers and smaller than 0.2 % for fourdrinier or hybrid former (related to the static head in the headbox).

Measuring point: entry of high consistency and low consistency manifold (if a pressure pulsation damping (PPD) tank is installed: inlet of the PPD tank) Frequency: 0-2.5 Hz Stock consistency variation

A stock-consistency variation can be caused by hydraulic instabilities, poor mixing, deflocculation and unfavorably arranged return pipes, etc. The coefficient of variation (COV) of the stock-consistency variation must not exceed 1.0 % in the high consistency and the low consistency line.

Measuring point: high consistency and low consistency headbox distribution header (if a PPD tank is installed - inlet of the PPD tank) Frequency: 0-10 Hz Note: It is very difficult to get a representative sample of stock leaving the headbox slice with dilution headboxes since there are typically no sample points after the dilution water is injected. True headbox slice consistency must be calculated by performing a mass balance that includes the dilution flow in addition to the headbox feed consistency. Gas content

Gas content is an important parameter because of its influence on paper formation, dewatering capacity and pressure pulsation transmission speed. Therefore, it has an influence on pulsation and the hydraulic state of the system.

Definition of total Gas Content: Total gas content is the sum of entrained gas and dissolved gas. Entrained gases are found in two forms (9):

1. Bound or residual air (microscopic bubbles): The size of a bound air bubble is small enough to adhere to fiber and other solids, smaller than ~ 70-100 µm (3/1000-4/1000 in) in diameter. It can also be found inside a fiber, especially if the pulp has been dried before processing.

2. Free air: freely moving air bubbles, which are short-lived, and larger than ~ 70-100 µm (3/1000-4/1000 in) tending to rise to the surface as foam for velocities less than 2.75 m/s (9 fps).

Dissolved gases (gas molecule)

Dissolved gases are different gases that have dissolved into water through the walls of air and/ or gas bubbles. Dissolved gas is always present as molecules in the water and is mainly CO2 because of its high solubility.

Stock temperature, pH, pressure, chemicals used and total air/ gas content affect the relative portion of entrained and dissolved air. Sources of Air entrainment:

Thick-stock feed (from machine chest) • Agitators • Discharging pipes above the liquid surface • Some overflow arrangements • Leaking seals operating under vacuum


Broke system • Coated broke contains carbonate coating pigments, which easily dissipate to CO2 gas

Influence of entrained gas (air):

• Retards drainage, disturbs formation, causes pin holes • Increases formation of slime • Promotes foaming and consequently build-up of dirt • Disturbs the function of centrifugal pumps • Affects speed of sound in stock changing piping resonant frequency (can amplify pulsation in the piping) • Offers agglomeration surface for hydrophobic substances • Causes floatation, cloud forming and consistency variations in tanks and/ or chests

Allowable entrained air-content in an approach flow system measured at the recirculation line from the

headbox varies by paper grades. With increasing gas content in the suspension of the HC and LC line, the drainage capacity in the wire section deteriorates. High gas content can lead to pinholes in the paper. The formation of pressure pulsations is promoted by the presence of gas. Also the contamination is usually increased with high gas content. The capacity of pumps is reduced. The following values should not be exceeded: Table 12. Maximum gas content Free and bound air Wire-Speed Board & packaging grades Graphic grades < 1000 m/min <1.4* % <0.5 % 1000 - 1500 m/min <0.7* % <0.1 % >1500 m/min <0.1* % <0.1 %

*May vary for machines with dual dilution

Measuring point: Entry of high consistency and low consistency manifold (if a PPD tank is installed: inlet of the PPD tank)

Measuring method: Measurement according to Boadway Evidence: Determination of gas content on all grades at maximum operating speeds at a representative number of

measurements. The given values should be calculated from the high consistency and low consistency volumetric flow and the measured gas content of the high consistency and low consistency line. Keywords

Approach flow systems, Piping, Basis weight, Valves, Stuff boxes, Fan pumps, Wire Pits, Silos, Headbox, Deaeration, Attenuation, Wet ends, Velocity Additional information Effective date of issue: August 29, 2018. Working Group Members: Paul Krochak – Chairman, RISE Research Institutes of Sweden

Scott Pantaleo, International Paper Tom Rodencal, Rodencal and Associates Dale Midyette, Jacobs Engineering

Jake Zwart, Spectrum Technologies Marc Foulger, GL&V

Jouni Matula, Wet End Technologies Paul Kristopeit, PM Diagnostics Helmut Tausel, Voith


Literature cited 1. Giorges, A. T., White, D. E., and Bandhakavi, V., Concentric Mixing of Softwood Pulp and Water, TAPPI Technical Summit, Atlanta GA, May 3-5, 2004. 2. Giorges, A. T., White, D. E., and Heindel, T. J., Concentric Mixing of Hardwood Pulp and Water, TAPPI Spring Technical Conference and Exhibit, Chicago, IL, May 11-15 (2003). 3. Reed, B., and Taylor, T., “Reducing Product Variation Through Improved Thick Stock Mixing,” Tappi J 84(7), July 2001. 4. Voith Sulzer Approach Piping Guidelines, Rev. D, February 1996. 5. Beloit Corporation, “Headbox Piping Recommendations – Design Information,” November 1994. 6. Haapalaa, A., Honkanenb, M., Liimatainena, H., Stoora, T., Niinimäkia, J. “Hydrodynamic drag and rise velocity of microbubbles in papermaking process waters”, Chem. Eng. J. 162 (2010). 7. Krochak, P., Norman, B., Hermansson, H., Sundin, K. “Optimizing Retention System Performance Through Improved Mixing”, TAPPI Papercon Conf., Atlanta USA (2015). 8. GL&V – Celleco Approach Piping Guidelines 9. Haapalaa, A. “Paper machine white water treatment in channel flow”, PhD Thesis, University of Oulu Finland (2010). References Weissgerber, Carl, and Day, Michael, “Reduction of Pressure Pulsations in Fan Pumps,” TAPPI, Vol. 63, No. 4,

April 1980. Hamkins, C., and Lorenc, J. A. “Fan Pump Pressure Pulsation Field Measurement and Comparison with Factory

Test Data,” Tappi Journal, Vol. 67, No. 3, March 1984. Goulds Pumps Application Newsletter, No. 42–1, February 22, 1979. Norman, B. and Tenegren, A., “Mixing of Thin Stock and White Water”, XXIII EUCEPA Conf. Proc., Vol. 1, “The

Paper and Board Machine – Today and Tomorrow”, May 31–June 3, 1988, Harrogate, U.K. p. 51–56. Boadway, J.D. Gas in Papermaking Stock. Pulp and Paper Magazine of Canada 1956: Convention Issue, p. 185-194.

Documents

TIP 0404-54 Headbox approach piping guidelines · 2020. 1. 9. · The headbox approach piping system is defined as the network of piping and equipment including pumps, pressure screens,