Controlling Operation of a Centrifugal Pump

The invention relates to a method and apparatus for controlling the operation of a centrifugal pump pumping a liquid or a suspension, particularly a cellulosic fiber suspension having a consistency of about 8-12 percent, with gas entrained in the liquid or suspension. Gas, such as air, entrained in liquids or suspensions tends to separate from the liquid or suspension in centrifugal pumps, and creates a gas bubble. The bubble can slowly grow and eventually fill the entire interior of the pump occupied by the impeller, resulting in cavitation or other adverse consequences. One manner of avoiding this problem is disclosed in parent application Ser. No. 139,162, now U.S. Pat. No. 4,410,337; in that disclosure an apparatus and method are provided whereby gas is discharged from the pump interior. A valve is disposed in the gas discharge line, and the rate of gas discharge is controlled in response to the measured pressure difference between the liquid or suspension inlet pressure and the gas exhaust pressure.

Utilizing some of the basic techniques provided in the parent application, it has been unexpectedly found that it is possible to control the function of a centrifugal pump to affect the flow rate/pressure head curve (the Q/H curve, Q equals the pump discharge flow rate, preferably expressed in liters per second, and H equals the pressure head, preferably expressed in bars). By changing the size of the air bubble it is possible to control the Q/H curve in the same way that a speed control may be utilized to control a conventional centrifugal pump. This finds great utility especially where paper pulp, or like suspensions, having a relatively high consistency (e.g. about 8-12 percent) are being pumped, because at low speeds it is not possible to effect fluidization of the pulp, so that the pump--in order to be operative--must always operate at a relatively high speed (e.g. 3,000 rpms). By proper practice of pump control according to the invention it is possible to greatly reduce the energy consumption of a centrifugal pump for pumping a liquid or suspension at a given flow rate Q, and a number of other desirable results are also achievable.

According to one aspect of the present invention a method of controlling the operation of a centrifugal pump pumping a liquid or suspension containing gas is provided, the pump having an inlet liquid or suspension pressure, a pressure head, and a liquid or suspension discharge flow rate. The method comprises the steps of: Effecting rotation of the liquid or suspension within the pump so that a gas bubble is created in a central part of the pump. Discharging gas from the gas bubble in the pump central part, at a gas discharge pressure. Determining the differential pressure between the liquid or suspension inlet pressure and the gas discharge pressure. And, controlling the head and/or discharge flow rate of the pump by controlling the differential pressure between the liquid or suspension inlet pressure and the gas discharge pressure. The differential pressure is controlled by controlling the discharge rate of the gas, and by determining the pressure head H and varying the set point for a dP-controller in response to the pressure head, proper control of the differential pressure between the liquid or suspension inlet pressure and the gas discharge pressure is provided. Practicing the invention it is possible to maintain the pump speed and pump flow rate Q constant while substantially reducing the power consumption. Q may be maintained constant by determining the liquid or suspension discharge flow rate at a particular point in a discharge line from the pump, and responsive to that flow rate controlling a valve upstream of the sensing point to keep the flow rate constant. Alternatively, it is possible to vary the flow without utilizing a control valve in the pump discharge. A control valve in the pump discharge creates a relatively high back pressure. According to the invention once the Q/H curve is known, it is possible to control the flow utilizing only a dP-controller.

According to another aspect of the present invention, a cellulosic fiber suspension having a consistency of about 8 to 12 percent, and having gas entrained therein, is readily pumped utilizing a centrifugal pump. The impeller is rotated at a speed sufficient to effect fluidization of the suspension, and the differential pressure between the suspension just prior to fluidization and the gas being discharged is determined. The rotational speed of the pump is maintained substantially constant, and the discharge rate of the flow of fluidized suspension is maintained substantially constant. The power consumption of the pump is significantly reduced at any particular rotational speed (as long as high enough to effect fluidization) and discharge flow rate by controlling the pump head, which is accomplished by controlling the differential pressure between the suspension just prior to fluidization and the gas being discharged.

It is the primary object of the present invention to provide a method and apparatus for effecting the advantageous control of the operation of a centrifugal pump. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.

FIG. 1 is a schematic side view, partly in cross-section and partly in elevation, of an exemplary centrifugal pump, connected up to a liquid or suspension supply vessel, which may be controlled according to the present invention;

FIG. 2 is a diagramatic view illustrating components of an exemplary system for effecting advantageous control of the centrifugal pump of FIG. 1;

FIGS. 3 and 4 are graphical representations of the manner in which various parameters are controlled, and respond, in practicing the invention; and

FIG. 5 is a view like that of FIG. 2, illustrating a slightly modified system.

The centrifugal pump 20 illustrated in FIG. 1 is described in detail in the parent application Ser. No. 139,162, now U.S. Pat. No. 4,410,337. Basically it preferably comprises a housing 1 having a liquid or suspension inlet channel 2, connected to a liquid or suspension container 3 (e.g. a container housing pulp at about 8-12 percent consistency). An outlet channel 4 from the housing 1 is connected to a pump liquid or suspension discharge 18.

Rotor 5 is mounted for rotation in the housing 1, and includes a plurality of axially extending blades 6, which have an interior opening 15 therebetween. The rotor also includes pump impeller blades 8, which are radially extending and rotate in the spiral housing section 7. A gas chamber 10 is disposed immediately below the spiral section 7, and gas from the interior of the pump can pass through openings 12 and 13 into the chamber 10, which is separated by stationary partition wall 9 from the spiral housing portion 7. Blades 17 may be mounted on, and rotatable with, the shaft 14 within the air chamber 10 for purposes as described in the parent application. Extending from the air chamber 10 is a gas discharge conduit 11.

As illustrated in FIG. 2, the gas discharge conduit 11 leads to an air vessel 25, with a shutoff valve 31 disposed in the conduit 11. From air vessel 25 a conduit 24 leads to a conventional vacuum pump 26, or like suction source, with a metering structure, such as automatically controlled valve 27, disposed downstream of the vacuum pump 26 for metering the amount of gas drawn therethrough.

A water pipe 29 is preferably operatively connected through a check valve 34 to the air chamber 10. Upon opening of the valve 35 water can flow through the pipe 29 to flush out the air chamber 10, removing any accumulated undesirable materials therefrom.

In the embodiment illustrated in FIG. 2, an automatically controllable valve 22 is disposed in the discharge line 18. A flow meter 21 is disposed in the line 18 downstream of the valve 22, and a flow controller 19 adjusts the position of the valve 22 in response to the reading from flow meter 21 to maintain the flow at any desired value.

According to the present invention a conventional dP-instrument 23 is operatively connected between the discharge conduit 18 and the vessel 3. The instrument 23 thus measures the head H of the pump 20. The reading from the instrument 23 is used to control the set point for conventional dP-controller 28. The dP-controller 28 is operatively connected to the pump inlet 2 (P.sub.1) and operatively connected to the gas discharge 11 (P.sub.2) to measure the differential pressure therebetween (dP=P.sub.1 -P.sub.2). In response to this determined differential pressure, the controller 28 controls the valve 27 so that the set point determined by the head H sensed by instrument 23 is substantially maintained; i.e., so that H is maintained at a desired value.

The functionality of the system illustrated in FIG. 2 is enhanced by providing the sensing point for P.sub.2 within the air vessel 25. In this way it is possible to provide pressure and flow rate controlled air from conduit 30, flowing through flow-adjusting valve 37, to the vessel 25. This is especially useful at start up and when a rapid differential pressure change is required.

FIG. 5 illustrates a schematic of a system according to the present invention wherein the control valve 22 in the discharge conduit 18 is eliminated. In this arrangement, the control of the flow rate Q is accomplished by controlling the rate of gas withdrawal through conduit 11. In the FIG. 2 system it is the head H which is controlled by the gas withdrawal, the flow rate being maintained constant.

As examples of advantageous operational results achieveable according to the present invention, attention is directed to the following.

A pump 20 according to FIG. 1 which has a control system according to FIG. 2 pumps pulp at a consistency of about 10-12 percent from a storage tower 3 to a chlorination tower (not shown). The pump has a nominal pressure head H of 7.5 bars (the back pressure in this case is only 3.5 bars). The pressure loss at the control valve 22 is about 4 bars (7.5-3.5) and the valve 22 is open only 30.degree.. The pressure difference dP (P.sub.1 -P.sub.2) under these circumstances is about 4 mWc (meter water column) and the power consumption is about 90 kW. By lowering dP--by controlling valve 27 to about 2.5 mWC--the following comparative results can be seen for 10 percent consistency pulp:

Q/H curves according to Example 1 are shown in FIG. 3.

By controlling dP--again by controlling the valve 27--utilizing the system in FIG. 5, it is possible to vary the pump flow rate Q. For instance the following comparative results are achieved by controlling dP as indicated:

It will thus be seen that according to the present invention a method and apparatus are provided for effectively advantageously controlling the operation of a centrifugal pump. By effecting control according to the present invention it is possible to reduce pump power consumption dramatically for a predetermined pump flow rate, and to effect control of Q for relatively high consistency suspensions (e.g. about 8 to 12 percent) in much the same manner that speed control is utilized for conventional centrifugal pumps, while still insuring that the suspension is always fluidized.

While the invention has been herein shown and described on what is presently conceived to be the most practical and preferred embodiment thereof, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent structures and methods.