Pumps – Condition responsive control of pump drive motor – Responsive to accumulation of pumped liquid in receiver
Reexamination Certificate
1997-12-02
2001-03-20
Thorpe, Timothy S. (Department: 3746)
Pumps
Condition responsive control of pump drive motor
Responsive to accumulation of pumped liquid in receiver
C417S053000, C417S007000
Reexamination Certificate
active
06203280
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to systems for determining the volume of liquid pumped by a set of pumps associated with a sewage wet well or other liquid vessel.
BACKGROUND OF THE INVENTION
Such a pumped volume totalizer provides information that may be used for many purposes, including measuring and controlling the inflow to a sewage plant or measuring and reporting discharge of effluent. If the level in the vessel is measured by a continuous level measurement system, such as an ultrasonic level measuring device, then it is a simple matter to calculate the volume represented by changes in level as long as the shape of the vessel, and therefore the volume contained in the vessel at any particular liquid level is known. One simple technique is to measure a start level when a pump cycle begins as a pump control system turns on one or more pumps to pump liquid out of (or into) the vessel. When the level in the vessel has dropped (or risen) to a predetermined value, the pump control system turns the pumps off, at which point the level is again recorded, providing a stop level. The volume pumped is the difference between the calculated volume contained in the vessel at the start level and the calculated volume contained in the vessel at the stop level. This volume can then be made available for display or adding to a running total of volume pumped, or to control an external totalizing counter.
Such a system makes no allowance for any liquid flowing into or out of the vessel during the pump cycle other than through the pumps. The calculation assumes that any change in volume that occurs between the start and end of the pump cycle is entirely due to the action of the pumps. In a typical application, liquid may be flowing into (or out of) the vessel during the pump cycle; indeed, it will often be such a liquid flow which causes a change in level which initiates the pump cycle. Thus the actual amount pumped in such circumstances will be the amount calculated as above, plus the amount that has flowed into the vessel during the pump cycle.
In order to overcome this problem, it is known to measure the rate of change of level in the vessel that is occurring just prior to the start of the pump cycle. It is then assumed that the inflow during the pump cycle was continuous and at the same rate as the value recorded just prior to the start of the pump cycle. The calculated pumped volume is then adjusted by adding to it the assumed total volume that flowed into the vessel during the pump cycle.
Although widely used, the assumption upon which a system is based, i.e. that the rate of inflow remains constant during the pump cycle, is often unjustified. For example, in a system handling storm water, the rate of inflow may vary widely during the course of a storm in a manner such that the average rate of inflow during the time that the pumps were running differed widely from that existing when the pumps cycle started, leaving two substantial errors.
BRIEF SUMMARY OF THE INVENTION
The present invention seeks to address this problem by providing an alternative method of totalizing pumped volume. In the preferred embodiment, the present invention encompasses a method for totalizing volume of liquid pumped to or from a vessel by a set of pumps during a pumping cycle. The method comprises establishing a pumping rate for each pump in the set during the pumping cycle, determining a running time for which each pump was operated during the pumping cycle, determining for each pump a product of the pumping rate and the running time, and summing the products to provide a total for the volume of liquid pumped.
The present invention also encompasses a control system for controlling the pumping of liquid to or from a vessel with one or more pumps, said control system comprising: (a) a level sensor for determining the level of the liquid in the vessel and generating a liquid-level reading; (b) an actuator coupled to each of said pumps for turning the pump on and off, and said actuator having a control interface responsive to control signals; (c) a controller coupled to said level sensor and each of said actuators, and said controller having pump control means for controlling said pumps by issuing control signals to said control interfaces in response to said liquid level readings; (d) said controller having totalizing means for totalizing the volume of liquid pumped in said vessel, said totalizing means comprising: (i) means for recording when each of said pumps is turned on and off and calculating a running time for each of said pumps; (ii) means for determining a first rate of change of volume when each of said pumps is turned on and a second rate of change of volume a predetermined period after each of said pumps is turned on; (iii) means for determining a draw down rate for each of said pumps and means for calculating the volume pumped by each of said pumps based on the running time and draw down rate for each of said pumps; (iv) means for summing the volume pumped by each of said pumps to produce a total volume pumped for said vessel over the duration said pumps operated.
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Evora Robert Z.
Milltronics Ltd.
Ridout & Maybee
Thorpe Timothy S.
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