Surge control system

Description

[0001] The present invention relates to a surge control system, in particular for use in a variable speed electric drive for a centrifugal exhauster such as that utilized in dewatering applications.

[0002] It is well known in the papermaking industries to use vacuum or suction pipe systems in dewatering. Such systems often utilize suction pipes coupled to elongated suction slots over which a felt passes causing the dewatering thereof. In systems of this type, a variety of devices are available to provide the vacuum necessary for dewatering. For example, liquid ring pumps, positive displacement pumps and centrifugal exhausters or blowers. While many circumstances and operating parameters dictate which type device is best suited for a particular application, a common desire in the selection of a vacuum pump is that it be efficiently incorporated and utilized in the system.

[0003] In this regard, generally the vacuum pumps are sized for maximum demand vacuum conditions in the suction pipe when the felt is new. The vacuum pump will normally run at its maximum speed with a new felt. As the voids in the felt fabric are filled during use, the fabric becomes less permeable and so requires a higher vacuum level for dewatering. However, with the decreased felt permeability and, since the vacuum pump is a constant volume unit, the vacuum level will automatically increase.

[0004] Many systems have been devised to take advantage of increasing vacuum conditions to effect cost and energy savings. See for example US-A-4,308,077; US-A-4,329,201; and US-A-4,398,996. For example, in the variable vacuum liquid ring pump having constant flow, described in US-A- 4,398,996, a variable speed drive motor responsive to an increase in the vacuum level in the suction pipe is provided. The motor is operative to slow down the vacuum pump as the felt permeability decreases, thereby retaining the desired level in the suction pipe. The lower pump speed results in lower drive horsepower and accordingly a saving in power, while retaining the desired vacuum level in the suction pipe. Similarly, when a centrifugal exhauster is used as the vacuum device, it has also been proposed that the speed of the exhauster should be automatically regulated as the permeability of the felt decreases.

[0005] While known arrangements have proven satisfactory in certain applications, it is desired to provide for yet further efficiency and energy savings in a dewatering system, particularly one that utilizes a centrifugal exhauster or blower as compared to a positive displacement unit.

[0006] In all exhauster arrangements, the occurrence of surge can have a serious and often damaging effect on the system. While many different type surge controls are available, it is desirable to have an automatic surge control that could be effectively employed in the situation in which a variable speed drive with a centrifugal exhauster is used in papermaking applications: in such a situation, there is no specific signal that can be used to sense surge because the surge points change with speed.

[0007] It is, therefore, an object of the invention to provide an automatic surge control system for a variable speed drive centrifugal exhauster used in dewatering applications.

[0008] It is another object of the invention to provide such a surge control which is of comparatively simple and inexpensive construction. According to the present invention a surge control for dewatering press felts on a papermaking machine comprises of a suction pipe; a variable speed drive motor; a centrifugal exhauster driven by the variable speed drive motor and connected to the suction pipe to provide vacuum to a felt passing over a slot for dewatering thereof, and whereby as the felt permeability and air flow demand decrease, the drive motor increases the speed of the centrifugal exhauster up to a predetermined maximum speed; characterized by control means coupled to the centrifugal exhauster for determining when the predetermined maximum speed occurs, said control means being coupled to said drive motor for monitoring when a predetermined load on the drive motor occurs at which the centri-exhauster when operating at maximum speed will surge, and a surge valve for admitting air to the centrifugal exhauster, said control means being coupled to said surge valve and operative in signalling said valve to admit air to the centrifugal exhauster when said centrifugal exhauster is operating at the predetermined maximum speed and the drive motor has predetermined load thereon so as to prevent the centrifugal exhauster from going into surge.

[0009] Additionally the invention provides a method of controlling surge in papermaking machines for dewatering press felts including a suction pipe, a surge valve, a variable speed drive motor, a centrifugal exhauster driven by the variable speed drive motor and connected to the suction pipe to provide vacuum to a felt passing over a slot for dewatering thereof, and whereby as the felt permeability and air flow demand decrease, the drive motor increases the speed of the centrifugal exhauster, the method including the step of limiting the speed of the exhauster to a predetermined maximum speed and being characterized by the steps of; determining when the predetermined maximum speed of the centrifugal exhauster occurs; monitoring when a predetermined load on the drive motor occurs at which the centrifugal exhauster when operating at maximum speed will surge; and opening the surge valve to admit air to the centrifugal exhauster when said centrifugal exhauster is operating at the predetermined maximum speed and the drive motor has a predetermined load thereon so as to prevent the centrifugal exhauster from going into surge.

[0010] An embodiment of the present invention will now be described with reference to the accompanying drawings in which:

[0011] Fig. 1 is a schematic view of a vacuum control system as part of a section of a papermaking machine, and

[0012] Fig. 2 is a representative performance curve for a particular variable speed centrifugal exhauster.

[0013] The depicted portion 10 is of a well known type of papermaking machine which utilizes one or more suction pipes 12 for dewatering a press felt 14 or similar fabric. The use of several suction pipes is discussed in US Patent No. 4,329,201. This is a common arrangement at the press section of the papermaking machine.

[0014] The typical suction pipes 12 include a hollow conduit 16 with a slot 18 forming an opening in its upper end over which the felt or fabric passes. An exit conduit 20 passes to a conventional type of liquid and gas separator 22. The separator 22 has a bottom exit for passage of separated liquid into a seal pit through a drop leg. The separator 22 is in turn connected by conduit 24 to a vacuum pump 26, which is a centrifugal exhauster type. Such exhauster may be of the type manufactured by Hoffman Air & Filtration Systems, a division of Clarkson Industries, Inc., PO Box 214, Eastwood Station, Syracuse, New York 13206. For general background material on exhausters see Publication CBE-378 entitled "Centrifugal Blowers and Exhausters" put out by the aforenoted company.

[0015] Typical in such systems, a relief or surge valve 28 is positioned between the separator 22 and the exhauster 26 for vacuum and surge release purposes when needed. Silencers 32 and 34 are also provided.

[0016] A conventional drive. shaft 36 interconnects a variable speed drive means 38 with the exhauster 26 to adjust and drive it at a chosen variety of speeds as is hereinafter discussed.

[0017] Note that the drive means 38 is a variable speed drive AC motor and may be of the type manufactured for example by Reliance Electric, 24703 Eucled Avenue, Cleveland, Ohio 44117 (A-C VS Drives; Duty Master-XE; AC Motors; and Max Pak plus); Toshiba Corporation 13-12 Mita 3 chrome, Minato-ku, Tokyo, Japan (MF Pack); Toshiba/Houston International Corporation, 13131 West Little York Road, Houston, Texas 77041 (ESP-130 series); Parametics, Orange, Connecticut.

[0018] The drive means 38 is coupled to a control panel 40 via connection 42 which may be electrical wiring. The control panel 40 is coupled through connection 44 to surge valve 28 which in turn is coupled to conduit 24.

[0019] As noted, air flow through a felt decreases with use. Heretofore, in a positive displacement unit, reducing the speed of the vacuum pump with decreasing felt permeabilities provided an advantageous way to save drive power since the power requirement of the pump is a function of its speed. Typical variable speed drives are normally set up to run at a maximum speed and then as the demand decreases, its speed automatically slows down.

[0020] With a centrifugal exhauster, such a method of operation is not desirable since, if the speed of the exhauster is decreased, the vacuum level will decrease (instead of remaining constant as would be the case with a positive displacement unit).

[0021] So, rather than slowing the speed of the exhauster 26 down; the variable speed motor 38 speeds up as the felt permeability decreases thereby producing a higher vacuum at the suction pipe 12. Since the air flow through the felt is less, a higher vacuum at a lower air flow is possible by speeding up the exhauster while maintaining the same torque or drive power.

[0022] The system allows the exhauster to run at a variable speed to meet the required vacuum at the suction pipe to dewater a felt as it goes from new to old rather than throttling the air flow by turning down a valve as was done heretofore with constant speed exhausters. Such a drive system also allows for the automatic adjustment of the exhauster during dewatering of multigrade webs for example light webs at slower speeds; and heavier webs at higher speeds.

[0023] When an AC motor is utilized in such a system, the speed of the motor can be varied by varying the AC frequency delivered to the motor. This is a standard feature in many of the models of AC motors mentioned previously.

[0024] The operation of the drive motor 38 may be automatically controlled using a feedback loop arrangement regulating the frequency and/or current at the desired level. The maximum speed of the motor is limited, to prevent overloading, by setting the maximum current (or current frequency) or by setting the maximum speed.

[0025] As aforenoted, the voids in a felt fabric fill and it becomes less permeable (volume per minute flow of air decreases) with use, causing the vacuum level required for dewatering to rise. With a constant flow, variable vacuum liquid ring style pump, the power increases as the vacuum level increases with use of the felt fabric. However, with the centrifugal exhauster 26 of Fig. 1, as the air flow decreases through the fabric with use, the power decreases.

[0026] With the variable speed drive arrangement 38 of Fig. 1, the speed of the exhauster 26 automatically increases as the air flow demand decreases. Fig. 2 shows the performance curve for a variable speed exhauster and shows that, as the speed of the exhauster varies, so do the surge points. Since the power (kW) at the surge point varies at different rpms, as shown, a monitoring thereof is insufficient since there would be no specific signal which would indicate that the exhauster is entering a surge region.

[0027] However, in the system shown in Fig. 1, the speed of the exhauster automatically increases as the air flow demand decreases. Therefore, the exhauster cannot go into surge until after it reaches its maximum speed, which is predetermined based upon the particular application, and use is made of this to prevent the exhauster going into surge, as described below.

[0028] More particularly, by monitoring the exhauster at its maximum speed via the control panel 40 and determining when the power, current or frequency drops below a predetermined amount, the control panel 40 signals the surge valve 28 to open and admit air to the exhauster 26 to prevent further reduction of air flow, so preventing the exhauster 26 from going into surge.

[0029] For example (referring to Fig. 2), if the exhauster 26 has a maximum speed of 4200 rpm then, at approximately 61.2 kW (82 hp) and below, the exhauster will be in its surge region. Accordingly, when the control panel 40 senses an exhauster speed of 4200 rpm and monitors a power output of the variable drive motor 38 that is less than or equal to 59.7 kW (80 hp), it signals the surge valve 28 to permit air to be admitted to the exhauster so preventing a further reduction of air flow and the exhauster from going into surge. As the power increases to a level greater than or equal to 61.2 kW (82 hp), the control panel signals the surge valve to close. The dead band control region, which in this case is the range 59.7 to 61.2 kW (80 to 82 hp), is used to prevent valve "hunting".

[0030] The maximum speed will vary upon the particular exhauster involved and the application. The performance curve for the particular maximum speed is readily available from the manufacturer. Rotational sensors for monitoring the speed of the exhausters are also available along with the monitor for the power output of the variable drive engine. Alternatively, as indicated above, the current demand of the motor or frequency can be monitored instead of power output.

[0031] Operation of the system described above can be summarized as follows. The speed of the exhauster 26 automatically increases as the air flow demand (ie, at the suction slot 18) decreases due to a change in felt permeability. If for example, the suction slot 18 was completely shut off, the exhauster 26 would automatically speed up to its maximum speed. With such an arrangement, it is determined that the exhauster 26 cannot go into surge until after it reaches its maximum speed setting. Therefore, surge need only be controlled at the maximum speed of the exhauster.

[0032] When the exhauster 26 is operating at its maximum speed, the power it demands becomes less as the air flow decreases. To prevent damage due to surging, the control panel 40 senses when the exhauster is running at maximum speed and when the power (kW), current draw (amp) or frequency drops below a predetermined amount. Thereupon, the control panel 40 causes the surge valve 28 to open to admit air to the exhauster 26 to prevent further reduction of air flow, thus preventing the exhauster from going into surge.

Claims

1. A surge control system for dewatering press felts on a papermaking machine which includes a suction pipe (12); a variable speed drive motor (38); a centrifugal exhauster (26) driven by the variable speed drive motor and connected to the suction pipe to provide vacuum to a felt (14) passing over a slot (18) for dewatering thereof, and whereby as the felt permeability and air flow demand decrease, the drive motor increases the speed of the centrifugal exhauster up to a predetermined maximum speed; characterized by control means (40) coupled to the centrifugal exhauster for determining when the predetermined maximum speed occurs, said control means being coupled to said drive motor for monitoring when a predetermined load on the drive motor occurs at which the centrifugal exhauster when operating at maximum speed will surge, and a surge valve (28) for admitting air to the centrifugal exhauster, said control means being coupled to said surge valve and operative in signalling said valve to admit air to the centrifugal exhauster when said centrifugal exhauster is operating at the predetermined maximum speed and the drive motor has predetermined load thereon so as to prevent the centrifugal exhauster from going into surge.

2. A system in accordance with claim 1 wherein said variable speed drive motor is an AC motor and said control means monitors the load thereon by monitoring current demand or frequency or power output of the motor.

3. A papermaking machine including a system in accordance with claim 1 or claim 2, the system being arranged to dewater a press felt.

4. A method of controlling surge in a papermaking machine for dewatering press felts which includes a suction pipe (12), a surge valve (28), a variable speed drive motor (38), a centrifugal exhauster (26) driven by the variable speed drive motor and connected to the suction pipe to provide vacuum to a felt (14) passing over a slot (18) for dewatering thereof, and whereby as the felt permeability and air flow demand decrease, the drive motor increases the speed of the centrifugal exhauster, the method including the step of limiting the speed of the exhauster to a predetermined maximum speed and being characterized by the steps of: determining when the predetermined maximum speed of the centrifugal exhauster occurs; monitoring when a predetermined load on the drive motor occurs at which the centrifugal exhauster when operating at maximum speed will surge; and opening the surge valve to admit air to the centrifugal exhauster when said centrifugal exhauster is operating at the predetermined maximum speed and the drive motor has a predetermined load thereon so as to prevent the centrifugal exhauster from going into surge.

5. A method in accordance with claim 4 which includes the steps of providing an AC motor as the drive motor and monitoring the load thereon by monitoring current demand or frequency or power output of the drive motor.

Ansprüche

1. Ein Pumpschwingungsüberwachungssystem für die Entwässerung von Pressfilzen auf einer Papierherstellungsmaschine, das ein Saugrohr (12), einen Antriebsmotor (38) variabler Drehzahl, einen Zentrifugalexhaustor (26) umfaßt, angetrieben durch den Antriebsmotor variabler Drehzahl und angeschlossen an das Saugrohr zum Einwirkenlassen von Vakuum auf einen Filz (14), der über einen Schlitz (18) für seine Entwässerung läuft, und wobei mit abnehmender Filzpermeabilität und Luftströmungsanforderung der Antriebsmotor die Drehzahl des Zentrifugalexhaustors bis zu einer vorbestimmten Maximaldrehzahl steigen läßt, gekennzeichnet durch Überwachungsmittel (40), angekoppelt an den Zentrifugalexhaustor für die Bestimmung, wann die vorbestimmte Maximaldrehzahl auftritt, wobei die genannten Überwachungsmittel an den genannten Antriebsmotor angekoppelt sind für die Überwachung, wann eine vorbestimmte Belastung an dem Antriebsmotor auftritt, bei welcher der Zentrifugalexhaustor, wenn er mit Maximaldrehzahl läuft, in Pumpschwingungen geraten würde, und ein Pumpschwingungsventil (28) für den Lufteinlaß zum Zentrifugalexhaustor, wobei die genannten Überwachungsmittel an das genannte Pumpschwingungsventil angekoppelt sind und ausgebildet sind, dem genannten Ventil zu signalisieren, Luft zu dem Zentrifugalexhaustor einzulassen, wenn der genannte Zentrifugalexhaustor mit der vorbestimmten Maximaldrehzahl arbeitet und der Antriebsmotor eine vorbestimmte Belastung aufweist, um so zu verhindern, daß der Zentrifugalexhaustor in Pumpschwingungen gerät.

2. Ein System nach Anspruch 1, bei dem der genannte Antriebsmotor variabler Drehzahl ein Wechselstrommotor ist und die genannten Überwachungsmittel die Belastung desselben überwachen durch Überwachen des Strombedarfs oder der Frequenz oder der Ausgangsleistung des Motors.

3. Eine Papierherstellmaschine mit einem System nach Anspruch 1 oder 2, wobei das System zur Entwässerung eines Pressfilzes angeordnet ist.

4. Ein Verfahren der Überwachung von Pumpschwingungen in einer Papierherstellungsmaschine für die Entwässerung von Pressfilzen, das ein Saugrohr (12), ein Pumpschwingungsventil (28), einen Antriebsmotor (38) variabler Drehzahl, einen Zentrifugalexhaustor (26) umfaßt, angetrieben durch den Antriebsmotor variabler Drehzahl und angeschlossen an das Saugrohr zum Einwirkenlassen von Vakuum auf einen Filz (14), der über einen Schlitz (18) für seine Entwässerung läuft und wobei mit abnehmender Filzpermeabilität und Luftströmungsanforderung der Antriebsmotor die Drehzahl des Zentrifugalexhaustors ansteigen läßt, welches Verfahren den Schritt der Begrenzung der Drehzahl des Exhaustors auf eine vorbestimmte Maximaldrehzahl umfaßt und gekennzeichnet durch die Schritte ist: Bestimmen, wann die vorbestimmte Maximaldrehzahl des Zentrifugalexhaustors auftritt, Überwachen, wann eine vorbestimmte Belastung auf dem Antriebsmotor auftritt, bei welcher der Zentrifugalexhaustor, wenn er mit Maximaldrehzahl läuft, in Pumpschwingungen geraten würde, und Öffnen des Pumpschwingungsventils für den Zulaß von Luft zu dem Zentrifugalexhaustor, wenn der genannte Zentrifugalexhaustor mit der vorbestimmten Maximaldrehzahl läuft und der Antriebsmotor eine vorbestimmte Belastung aufweist, um so zu verhindern, daß der Zentrifugalexhaustor in Pumpschwingungen gerät.

5. Ein Verfahren nach Anspruch 4, das die Schritte umfaßt der Bereitstellung eines Wechselstrommotors als Antriebsmotor und Überwachung von dessen Belastung durch Überwachen des Strombedarfs oder der Frequenz oder der Ausgangsleistung des Antriebsmotors.

Revendications

1. Un système de contrôle d'instabilité de marche lors d'une déshydratation de feutres de presse dans une machine de fabrication de papier, qui comprend un tube d'aspiration (12); un moteur d'entraînement à vitesse variable (38); un exhausteur centrifuge (26) entraîné par le moteur d'entraînement à vitesse variable et relié au tube d'aspiration de façon à appliquer un vide à un feutre (14) passant sur une fente (18) pour être déshydraté, et produisant, quand la perméabilité du feutre et la demande d'écoulement d'air diminuent, une augmentation de la vitesse de l'exhausteur centrifuge par le moteur d'entraînement jusqu'à une vitesse maximale prédéterminée; caractérisé par un moyen de contrôle (40) relié à l'exhausteur centrifuge pour déterminer quand la vitesse maximale prédéterminée se produit, ledit moyen de contrôle étant relié audit moteur d'entraînement pour contrôler lorsque ce moteur d'entraînement est soumis à une charge prédéterminée pour laquelle l'exhausteur centrifuge, lorsqu'il opère à la vitesse maximale, entre en instabilité de marche, et une vanne de contrôle d'instabilité (28) pour admettre de l'air dans l'exhausteur centrifuge, ledit moyen de contrôle étant relié à ladite vanne de contrôle d'instabilité et opérant pour commander ladite vanne de façon que de l'air soit admis dans l'exhausteur centrifuge quand cet exhausteur centrifuge fonctionne à la vitesse maximale prédéterminée et quand le moteur d'entraînement est soumis à une charge prédéterminée afin d'empêcher l'exhausteur centrifuge d'être soumis à une instabilité de marche.

2. Un système selon la revendication 1, dans lequel ledit moteur d'entraînement à vitesse variable est un moteur à courant alternatif et ledit moyen de contrôle assure le contrôle de la charge qui lui est appliquée en contrôlant la demande de courant ou bien la fréquence ou bien la puissance de sortie du moteur.

3. Une machine de fabrication de papier comprenant un système selon la revendication 1 ou la revendication 2, le système étant agencé pour déshydrater un feutre de presse.

4. Un procédé de contrôle d'une instabilité de marche dans une machine de fabrication de papier lors de la déshydratation de feutres de presse, qui comprend un tube d'aspiration (12), une vanne de contrôle d'instabilité (28), un moteur d'entraînement à vitesse variable (38), un exhausteur centrifuge (26) entraîné par le moteur d'entraînement à vitesse variable et relié au tube d'aspiration pour appliquer un vide à un feutre (14) passant sur une fente (18) afin d'assurer sa déshydratation et faisant en sorte que, lorsque la perméabilité du feutre et la demande d'écoulement d'air diminuent, le moteur d'entraînement augmente la vitesse de l'exhausteur centrifuge, le procédé comprenant l'étape de limitation de la vitesse de l'exhausteur à une vitesse maximale prédéterminée, et étant caractérisé par les étapes consistant à: déterminer quand la vitesse maximale prédéterminée de l'exhausteur centrifuge se produit; surveiller l'apparition d'une charge prédéterminée appliquée au moteur d'entraînement et pour laquelle l'exhausteur centrifuge, lorsqu'il opère à la vitesse maximale, entrera en condition de marche instable; et ouvrir la vanne de contrôle d'instabilité pour admettre de l'air dans l'exhausteur centrifuge quand cet exhausteur centrifuge opère à la vitesse maximale prédéterminée et quand le moteur d'entraînement est soumis à une charge prédéterminée afin d'empêcher l'exhausteur centrifuge de rentrer en instabilité de marche.

5. Procédé selon la revendication 4, comprenant les étapes consistant à utiliser un moteur à courant alternatif comme moteur d'entraînement et à contrôler la charge du moteur en surveillant la demande de courant, ou bien la fréquence, ou bien la puissance de sortie du moteur d'entraînement.

Drawing