Pumps – With condition responsive pumped fluid control – Driven pump part speed responsive
Reexamination Certificate
2000-12-11
2003-07-15
Tyler, Cheryl J. (Department: 3746)
Pumps
With condition responsive pumped fluid control
Driven pump part speed responsive
C417S042000, C417S053000, C162S252000
Reexamination Certificate
active
06592340
ABSTRACT:
The invention relates to a control system for a vacuum pump used for removing liquid and a method of controlling said pump. The vacuum pump arrangement utilized in the invention may be used e.g. in drying machines for paper, paperboard and pulp as well as e.g. in various suction filters and suction washers in a pulp mill. In general, the pump control arrangement according to the invention may be used in all applications where gas, liquid or their mixture is withdrawn through a perforated or wire-like surface.
The use of various vacuum systems as means for facilitating filtration and precipitation is generally known in addition to the paper and wood processing industry, also in e.g. the mining and fertilizing industry. Vacuum systems are utilized in connection with both various wire filters and wire precipitators, into which category the drying machines for paper, paperboard and pulp can be defined, and various vacuum disc or vacuum drum filters. A well-known vacuum arrangement is the use of a barometric leg, by which method the weight of the filtrate flowing into a so-called barometric leg, an essentially vertical tube, creates the necessary vacuum. Because it is practically impossible to regulate the vacuum created by the barometric leg, various vacuum pumps have been employed, the most common of which is the so-called water ring pump. The water ring pump creates typically a constant-volume flow. The suction capacity of the water ring pump may be regulated for example by introducing additional air into the pump through a regulated valve, whereby the amount of gas being withdrawn from the suction object of the pump may be changed.
U.S. Pat. No. 4,329,201 (Albany International Corp.) relates to dewatering the web on the wire. According to said publication, e.g. a liquid ring pump or a centrifugal exhauster may be utilized for water removal. An essential feature of the idea described in the publication is the attempt of using constant vacuum for dewatering. According to the publication, this is achieved by employing in the dewatering system two water removal pipes, over which the wire passes. In the case of a new wire, only one water removal pipe is employed, but when the vacuum in said pipe increases a second pipe is opened and the suction from the vacuum means is applied to the wire also via the second pipe. The solution according to the publication utilizes also a control valve, which, together with the vacuum control, keeps the vacuum applied to the suction means constant. The publication further mentions, how the dwell time of the wire at each suction pipe may be increased, if necessary, by either increasing the width of the suction slot in the suction pipe in the moving direction of the wire or by decreasing the travelling speed of the wire.
Like the previous publication, U.S. Pat. No. 4,466,873 (Albany International Corp.) is also related to dewatering the fourdrinier section of a paper machine. The apparatus of this patent comprises an adjustable-speed vacuum pump, which may be either a conventional vacuum pump or a vacuum centrifugal pump, i.e., centrifugal exhauster. The purpose of the apparatus is to control the dewatering in two phases so that during the first phase the rotational speed of the pump is kept constant, whereby the pump creates a vacuum sufficient for dewatering the web. As the wire gradually clogs, the vacuum in the suction boxes increases. When the vacuum in the last suction box has reached a predetermined value, the control unit enforces the vacuum pump to decrease its rotational speed so that the vacuum remains in said maximum level thereafter.
The newest alternative for suction means in the market is a so-called high-speed vacuum pump, which is, in fact, a centrifugal pump applied for pumping gas. The use of said pump in dewatering in paper production is described in Fl patent 97244. Said publication concerns, e.g., a problem arising with the fabric, felt or wire, of the paper machine gradually tending to be clogged. In other words, the permeability of the fabric changes with the passing of service life. In said publication, the problem is solved so that when the amount of liquid to be removed by applying suction to the object, e.g. a suction box or a group of suction boxes, is known, the amount of liquid removed from said object is measured and compared with the predetermined target value and the operation of the suction system is adjusted accordingly. The easiest method of doing this is to adjust the rotational speed of the vacuum pump, because for example an increase in the rotational speed increases both the volume flow and the vacuum level of the vacuum pump. As a second alternative adjustment, the adjustment of both the rotational speed and the capacity of the vacuum pump, i.e. volume flow, is described.
A problem relating to the solution described in said publication is that the system comprises a mechanical device, flow meter, on which the controlling of the system is based. In addition to being a costly additional investment, said device is sensitive to damages, at least in long-term use, whereby the accuracy of the whole paper production or a corresponding process is endangered. Furthermore, the use of a flow meter, as such, brings no additional value for the system, but other interesting points, such as e.g. monitoring the condition of the suction object, such as the felt or the like, determining the need to change the felt etc. are still existing problems in the described prior art methods.
In the following, the use of a vacuum pump arrangement is described more precisely, as an example in connection with the vacuum system of a paper machine. The applications for the use of both various prior art vacuum pump systems and the method and vacuum pump arrangement of the present invention are, in principle, divided into two categories.
In the first category, suction is effected both to the fabric and the pulp web. In these cases, when the same type of pulp is being treated, the flow resistance remains almost constant regardless of the resistance of the felt, because the resistance of the pulp web is decisive. The flow resistance changes remarkably when the paper grade is changed. The conventional operating method is to determine a constant vacuum level for each paper grade, with the aim to monitor the level during the operation. Typical objects whereto vacuum is applied are, e.g., suction boxes of the web formation section of paper, paperboard and pulp drying machines, various suction rolls in the formation and press sections and various vacuum filters in the causticizing plant of a pulp mill and e.g. in the fertilizing and basic metal industry.
In the second category, vacuum is applied to the fabric only. In this case, the flow resistance increases with the aging of the fabric, due to fouling and compressing of the fabric. Typical suction objects are, e.g., the suction boxes of the felt-conditioning section of a paper machine.
With these suction boxes, placed at the press section of a paper machine, the fabric is dewatered by withdrawing air through it. The three basic functions of felt conditioning are:
1) water removal (part of the water in the fabric has been transferred from the paper web to the fabric on the press section),
2) cleaning (part of the water has come from cleaning jets used for cleaning the fabric) and
3) maintaining a level moisture profile in the fabric in the cross direction of the machine.
Traditionally, a vacuum pump has been used in connection with this kind of applications so that by the vacuum pump a constant-volume flow is withdrawn through the fabric, whereby the vacuum level increases with the aging of the fabric. A running method of this kind is achieved automatically by a displacement-type vacuum pump, e.g. a liquid ring pump.
However, the objective of felt conditioning is not to maximize water removal from the fabric, because an excessively dry fabric does not perform its function properly in the dewatering taking place in the press nip. The amount of air withdrawn through the fabric by means of a suction box
Horo Kaarlo
Kupiainen Seppo
Nixon & Vanderhye P.C.
Sulzer Pumpen AG
Tyler Cheryl J.
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