System Compensating for Pressure Fluctuations in a Paper Machine Headbox

FIG. 1 is an elevational view, of a portion of a paper stock supply system illustrating a stock pulse attenuator system constructed and operating in accordance with the principles of the invention in operating location;

FIG. 2 is a horizontal sectional view taken substantially along II--II of FIG. 1;

FIG. 3 is an enlarged fragmentary sectional view taken substantially along line III--III of FIG. 1;

FIG. 4 is an enlarged fragmentary view taken substantially along line IV--IV of FIG. 2;

FIG. 5 is a plan view taken substantially along line V--V of FIG. 1;

FIG. 6 is a fragmentary sectional view taken substantially along line VI--VI of FIG. 5;

FIG. 7 is a somewhat schematic side elevation illustrating a paper machine stock supply system;

FIG. 8 is a sectional view showing a tapered stock distributor embodying the features and principles of the present invention;

FIG. 9 is a sectional view taken substantially along line IX--IX of FIG. 8;

FIG. 10 is a more detailed sectional view taken substantially along line X--X of FIG. 9;

FIG. 11 is an enlarged sectional view taken substantially along line XI--XI of FIG. 8;

FIG. 12 is an enlarged sectional view taken substantially along line XII--XII of FIG. 11; and

FIG. 13 is a sectional view taken substantially along line XIII--XIII of FIG. 10.

As illustrated in FIG. 1, a stock supply conduit system is provided for conducting stock from a pressure stock supply 10, such as a fan pump, to a headbox 12. The conduit arrangement is shown at 11 and usually will be of circular piping with the stock flowing through the interior at 20. Circular pipes 11a and 11b are fitted with intervening conversion piping sections 13 and 14 which convert the circular piping to a rectangular for fitting the attenuator housing 16 in place, and convert the flow back to circular through the fitting 14.

The pulse attenuator housing 16, as illustrated in FIGS. 1, 3 and 4, defines a chamber 17 therein. Because the housing may be formed of various structural framing pieces suitable for attachment to a rectangular section of piping, the details of the housing construction need not be described, and variations will be apparent to those versed in the art from the teachings contained herein.

The attenuator is arranged so that a flexible diaphragm 18 forms one wall of a rectangular chamber 21 through which the stock flows. The diaphragm 18 is rectangular in shape and forms the entire wall facing the stock flow. The diaphragm is backed over its entire surface by air under pressure and pulsations or surges in stock pressure are absorbed by the air backed diaphragm. The air is compressible, and forms a resilient overall support. Generally, fluctuations in pressure in the stock as it flows through the rectangular chamber 21 will be absorbed by the diaphragm 18, and if the stock pressure decreases, the diaphragm 18 will move toward the stock flow, and if the stock pressure increases, the diaphragm will move away from the stock flow to attenuate these pressure changes. For improving the attenuation function and avoiding fluttering or pulsation of the diaphragm, the air chamber 17 is divided into a first diaphragm support compartment 17a and a second supply compartment 17b. These compartments are separated by an attenuator wall 19 which is formed of an attenuator felt which prevents surges of air between the two compartments. The attenuator wall 19 can take different forms to permit passage of air between the compartments 17a and 17b, but prevent rapid surges therebetween. In the preferred form, the wall 19 is in the form of a felt 19a supported between perforate plates or screens 19b and 19c. The screens are clamped between plates 23 and 24 which have circular holes therein, such as 19d, as illustrated in FIGS. 2, 3 and 4.

Control of the attenuation of the flow of air between the compartments 17a and 17b may be obtained by selecting plates with holes of a different size or by changing the character of the felt. However, a change in the attenuation effect during operation may be provided by furnishing plugs 24a which can fit into the holes, and the number of plugs which will be inserted will determine the overall open area between the compartments.

The diaphragm 18 is formed of a relatively heavy or rugged material such as rubber, suitably clamped at its edges to prevent leakage of air into the stock stream and leakage of stock into the air compartment. A clamping arrangement is shown at 25 at one edge in FIG. 3, and another form of clamping arrangement is shown at 26 at another edge in FIG. 4.

Air is supplied to the air chamber 17 through a fitting at 29 to maintain a continuous supply at a predetermined reference pressure. Normally a continual flow of air will be maintained in the chamber at a reference pressure, and excess air will be bled out from the chamber through an air release valve 30, FIG. 5. Additional vents 27 and 28, FIG. 5, may be provided having valves which may be quickly opened for quick discharge of the system should this become necessary.

The air release valve 30 is operated by the diaphragm, and for this purpose has an opening 32 which is normally just barely maintained in a closed position by the diaphragm resting against the opening 32. A duplicate valve 31 is provided in the air chamber, also controlled by the diaphragm in order to have a second valve available for better control, and for a backup.

In operation air will be introduced continually into the chamber 17 through the supply 29 up to where the pressure in the air chamber equals the pressure in the stock distribution chamber 20 on the other side of the diaphragm. At that point, the diaphragm is barely touching the valve head 30 to barely maintain the valve port 32 in a closed position. Any unintended excess pressure in the air chamber will tend to move the diaphragm away from the valve opening 32 to bleed the air out through the relief valve 30. The stem of the relief valve leads to atmosphere, passing through the wall of the housing 16. Thus, equilibrium conditions and an equilibrium position of the diaphragm 18 is maintained during operation.

This permits the diaphragm to perform its intended purpose of reducing the air pressure rapidly within the chamber 17 when the fluid pressure in the stock distributor chamber 20 suddenly drops. This maintains the flow rate and the pressure in the stock supply as constant as possible. Similarly, any surges and increases in pressure in the stock supply chamber 20 will distend the diaphragm forcing it toward the air chamber compressing the air therein so that the pressure surge is taken up by the diaphragm to maintain the stock supply pressure and stock flow rate as uniform as possible. Circular support members 35 of foam material are positioned beneath the diaphragm on the plate 23.

The air supply to the inlet 29 is provided by suitable mechanism, not shown, which is provided with a controllable pressure regulator so that a constant supply of air is available which may be admitted only intermittently, but does maintain a reference pressure based on the desired stock fluid flow pressure. The air supply is constant only up to the reference pressure.

The arrangement described herein is a preferred construction wherein the attenuation of the stock flow pressure is maintained in a supply line leading from the fan pump 10 to the headbox 12, but the principles of the concept are also employed wherein the diaphragm support is utilized at other points in the delivery system, such as the headbox itself. In the arrangement disclosed herein wherein the attenuator is positioned in a section of the stock inlet pipe, restrictions are avoided which are imposed by having the attenuator integral with the headbox distributor. In that arrangement, the attenuator extended for the width of the machine. In the present arrangement the attenuator diaphragm extends for the full width of the flow, forming one complete wall of the rectangular flow chamber. An attenuator diaphragm on the order of 6' long may be used and will give the diaphragm sufficient exposure to the stock flow and provide a size of air chamber large enough to provide a desired amount of yield or give in the diaphragm. The quicker and easier the diaphragm reacts to surge pressure pulsations, the more effective it will be. Yet, with the division of the air chamber into compartments and an attenuator arrangement between compartments so as to provide a relatively more shallow diaphragm support compartment, effective control of surges is attained without danger of introducing fluctuations due to air surges or flutter or unstable conditions.

Another form of the invention is illustrated in FIGS. 7 through 13. As illustrated in FIG. 7, a headbox 10' is shown in a stock supply system for distributing stock onto a moving forming wire 13'. The stock is delivered from the headbox through a slice chamber 11' to a slice opening 12'. The requirement for the system is to obtain continual uniform dispersion of the fibers throughout the stock and to obtain uniform flow of the stock in a cross-machine direction and a uniform rate of flow in a machine direction. To accomplish this, the stock must reach the headbox without fluctuation in pressure which is accomplished by the attenuation structure and rearrangement of the present invention.

The stock is delivered to the headbox chamber 10' to a plurality of tubes 14' which are uniformly distributed across the width of the machine. To obtain uniform velocity of flow through the tubes 14', stock is delivered to the tubes through a tapered distributor pressure stock chamber 15' which is supplied with stock through a supply line 16' from a fan pump. In known arrangements circulation is maintained through the pressure distributor chamber 15' by flow out of the small tapered end through a discharge line 17', as illustrated in FIG. 8. In this figure, the number of tubes 14' are not shown in their entirety, but a substantial number exists, and it is essential for uniform delivery of these tubes that pressure fluctuations caused by operation of the fan pump and other equipment be dampened. This is accomplished by connecting an air pressure chamber 18' directly to the pressurized stock distribution chamber 15'. In a preferred arrangement, a flexible diaphragm 21' forms a common wall between the air chamber and the pressure stock distribution chamber 15' with the diaphragm transmitting the pressure surges and fluctuations to the air chamber which functions to attenuate the surges. The air chamber 18' is pressurized with an initial pressure through a supply line 19', FIG. 8, dependent upon the pressure of the stock which will be delivered to the pressure distribution chamber 15'. In accordance with the invention, the air chamber is pressurized to a pressure slightly greater than the pressure in the stock chamber 15' or at least as great as the pressure in the stock chamber. A valve 20' is constructed and arranged so that it will bleed pressure out of the air chamber as a function of movement of the diaphragm until the diaphragm reaches the operative position whereupon it will close the bleed valve 20'. With any drop in pressure in the stock chamber 15', movement of the diaphragm will again open the valve 20' to provide additional air to bleed out.

Preferably, the supply line 19' will maintain a continuous supply to the air chamber 18' at a predetermined reference pressure which will be equal to the pressure of the stock in the stock chamber. Normally, a continual flow of air will be maintained into the air chamber similar to the operation of the fitting at 29' described in connection with the structure and operation of the unit in FIGS. 7 through 13.

The distance between the diaphragm and the valve is controlled by a valve positioning jack 22' which moves the valve 20' toward or away from the diaphragm 21', and this operation will be understood more clearly in connection with the detailed showing in FIG. 11. The valve 20' leads down through an air bleed pipe which opens at 23', FIG. 8, to release air from the air chamber 18'.

The support for the diaphragm 21' is arranged so that it is clamped at its edges by angle clamps 27', FIG. 10. The surface of the diaphragm is thus directly exposed to the stock pressure chamber 15', and preferably the mechanism is operated so that the diaphragm 21' bulges slightly toward the stock pressure chamber 15.

The diaphragm is supported underneath by a resilient material such as sponge 27' which absorbs vibration of the diaphragm and provides a yieldable support. The diaphragm is preferably made of material such as neoprene, and it will be noted that a smooth flow face is presented to the stock pressure chamber which does not interfere with the flow so that the distribution chamber 15' can take its normal optimum design.

Beneath the diaphragm 21', the air chamber is divided into a plurality of circular compartments 25'. Thus, the main air chamber is, in essence, divided into two compartments, a first compartment 25' adjacent the diaphragm, and a second larger compartment 26', and a dampening wall 29' extends between the compartments. This arrangement aids in stabilization of the pressure dampening and prevents flutter of the diaphragm and aids in the dampening or attenuation function. The dampening wall 29' is clamped between annular clamping rings 29a' and 29b' as shown in FIGS. 9 and 10. As illustrated in FIG. 13, the air flow dampening wall 29' is constructed by a perforated plate 30' having perforations 31' therethrough and supporting a felt 32'. The felt slows or dampens the flow of air from the small compartments 25' to the main compartment 26' of the air chamber 18' so as to further aid in the attenuation of the pressure surges in the stock chamber.

FIGS. 11 and 12 show the valve 20' in further detail. The head of the valve 33' coacts with the diaphragm to relieve air from the air pressure chamber 18'. Passageways 36' communicate with the air pressure chamber, and a central passageway 37' through the center of the valve leads to atmosphere. Flow of air from the pressure chamber from the radial passages 36' to the center passage 37' is prevented by the diaphragm resting on the top of the annular land 35'. Outside of the land is an annular channel 34' with which the radial passages 36' communicate. Whenever the diaphragm tends to move away from the valve head, air escapes over the top of the land 35' from the pressure chamber, and as the pressure in the pressure chamber drops, the entire diaphragm, of course, will again seat on the land 35'. By maintaining the top of the land relatively narrow, a good response will be attained.

Thus, the diaphragm provides an attenuation surface which is exposed substantially uniform across the entire width of the headbox to the distribution pressure chamber. While in the preferred arrangement the air attenuation chamber is directly connected to the stock pressure chamber, and advantages are obtained by this structure, the attenuation system could be connected to other parts of the stock system either upstream or downstream of the tapered pressurized distribution chamber.

With the present arrangements, a broad range of frequencies are attenuated, and these broad ranges are encountered due to inherent requirements of the design of the paper machine stock supply pressure system. A low mass large area diaphragm accomplishes the attenuation and coacting with the dampening of the flow of air between the compartments of the air chamber, it is wholly effective. The arrangement handles expeditiously the surges caused by rotational frequency of a constant speed centrifugal pump which has heretofore been a problem. Also, the only moving part, which is the attenuating diaphragm, is smooth and forms a portion of the stock chamber wall so that no projections or recesses are formed which would tend to adversely affect the stock flow and cause fibers to gather. The operating valve parts are exposed only to air and thus, do not need servicing or cleaning. It has been found that the structure meets the objectives and advantages above set forth and solves problems encountered in existing and newly designed paper machine stock distribution systems. The surges and fluctuations in pressure which are in part in the form of compressional waves have been a problem in reduction of quality of the web formed. With the elimination of these compressional waves, more uniform basis weight formation can be achieved and improved fiber distribution and reduction of flocculation result. Elimination of compressional waves eliminates variations in velocities, and therefore, insures a more uniform product.