Method for determining a centrifugal pump operating state without using traditional measurement sensors

• US 6,776,584 B2
• Filed: 01/09/2002
• Issued: 08/17/2004
• Est. Priority Date: 01/09/2002
• Status: Active Grant

• Current Assignee: ITT Manufacturing Enterprises, Inc. (ITT, Inc.)
• Original Assignee: ITT Manufacturing Enterprises, Inc. (ITT, Inc.)
• Inventors: Erickson, Barry, Sabini, Eugene P., Lorenc, Jerome A.
• Primary Examiner(s): Tyler, Cheryl J.
• Assistant Examiner(s): Belena, John F
• Application Number:
  US10/042,877
• Publication Number:
  US 20030129062A1
• Time in Patent Office: 951 Days
• Field of Search: 417/22, 417/42, 417/53, 417/63, 417/423.1, 415/1, 415/17, 415/13-50, 415/118, 731/68, 702/113, 702/114
• US Class Current: 417/22
• CPC Class Codes: F04D 15/0088 Testing machines

Abstract

A method for determining whether a centrifugal pump is operating within its normal flow operating range includes the steps: of determining a motor torque/TDH relationship over a range of speeds for a minimum flow rate in order to obtain a minimum flow operating range for the centrifugal pump; determining a motor torque/TDH relationship over a range of speeds for a maximum flow rate in order to obtain a maximum flow operating range for the centrifugal pump; determining the actual operating motor torque and TDH of the centrifugal pump at a given operating point; and determining whether the actual operating motor torque and TDH of the centrifugal pump fall within the minimum flow and maximum flow operating ranges of the centrifugal pump.

Claims

1. A method of determining the operating point of a centrifugal pump having a given hydraulic performance characteristic comprising the steps of:

measuring at least two independent variables capable of determining the operating point of the pump by,

measuring the two independent variables at least at a first pump rotational speed;

measuring the two independent variables at least at a second different pump rotational speed; and

processing the measured independent variables as measured at each speed to provide two sets of data indicative of the operating point of said pump,

wherein the given hydraulic performance characteristic is obtained at both minimum and maximum continuous flow for a given impeller diameter of the centrifugal pump, and the flow (Q), total dynamic head (TDH) and pump efficiency (n) are used to compute the Brake Horsepower (BHP) by computing;

$\mathrm{BHP}=\frac{Q*\mathrm{TDH}}{K_1*n}$

where Q is the flow in gpm,

TDH is the total dynamic head in feet,

n is the pump efficiency, and,

K₁ is a constant depending on the unit conversion.

2. The method according to claim 1 wherein the two independent variables are pump motor torque (T) and pump motor speed (N).

3. The method according to claim 2 wherein the pump motor torque is computed at a first pump motor speed N₁ and at a second pump motor speed N₂ by computing:

$T=\frac{\mathrm{BHP}*K_2}{N}$

where T is the torque in foot pounds, and,

K₂ is a constant depending on the unit conversion.

4. The method according to claim 3 wherein the step of processing includes computing:

$\frac{\left(N_1\right)}{\left(N_2\right)}=\frac{\left(Q_1\right)}{\left(Q_2\right)}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{and}<br><br>\text{\hspace{1em}<br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{\left(N_1\right)^2}{\left(N_2\right)^2}=\frac{{\mathrm{TDH}}_1}{{\mathrm{TDH}}_2}$

where

N1 is the first pump motor speed;

N2 is the second pump motor speed;

Q1 is a flow at said first pump motor speed;

Q2 is a flow at said second pump motor speed;

TDH1 is a Total Dynamic Head at the first pump motor speed; and

,

TDH2 is a Total Dynamic Head at the second pump motor speed.

5. Apparatus for determining the operating point of a centrifugal pump, comprising:

a centrifugal pump having a rotating impeller coupled to a drive shaft for pumping fluid by centrifugal force;

a motor coupled to the drive shaft operative to rotate the drive shaft and therefore the impeller at a selected rotational speed;

a variable speed drive circuit coupled to the motor and operative to vary the selected speed to operate the motor at any one of a plurality of selected speeds;

the motor producing different values for motor variables at each selected speed; and

a processor responsive to each speed to process the selected motor values provided at each speed to provide an output indicative of the operational point of the motor by computing at least two variables at the different speeds using the pump Affinity Laws wherein the at least two variables are Motor Torque (T) and total dynamic head (TDH).

6. The apparatus according to claim 5 wherein the processor has stored therein Brake Horsepower (BHP) of the centrifugal pump.

7. The apparatus according to claim 6 wherein the processor operates to compute the Torque (T) and total dynamic head (TDH) by solving the following equations:

$\frac{\left(N_1\right)}{\left(N_2\right)}=\frac{\left(Q_1\right)}{\left(Q_2\right)}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{and}<br><br>\text{\hspace{1em}<br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{\left(N_1\right)^2}{\left(N_2\right)^2}=\frac{\left({\mathrm{TDH}}_1\right)}{\left({\mathrm{TDH}}_2\right)}$

where

N₁=a first pump motor speed;

N₂=a second pump motor speed;

Q₁=flow at said first pump motor speed;

Q₂=flow at said second pump motor speed;

TDH₁=total dynamic head at said first pump motor speed; and

,

TDH₂=total dynamic head at said second pump motor speed.

8. Apparatus for determining the operating point of a centrifugal pump, comprising:

a centrifugal pump having a rotating impeller coupled to a drive shaft for pumping fluid by centrifugal force;

a motor coupled to the drive shaft operative to rotate the drive shaft and therefore the impeller at a selected rotational speed;

a variable speed drive circuit coupled to the motor and operative to vary the selected speed to operate the motor at any one of a plurality of selected speeds;

the motor producing different values for motor variables at each selected speed; and

a processor responsive to each speed to process the selected motor values provided at each speed to provide an output indicative of the operational point of the motor by computing at least two variables at the different speeds using the pump Affinity Laws, wherein the selected speeds are measured at minimum and maximum flow points.

9. A method for determining whether a centrifugal pump is operating within a normal flow operating range comprising the steps of:

determining the actual operating point of the centrifugal pump based on motor torque and motor speed; and

comparing the actual operating point of the centrifugal pump to minimum and maximum flow operating ranges of the centrifugal pump to determine whether the centrifugal pump is operating within minimum flow and maximum flow operating ranges.

10. A method for determining whether a centrifugal pump is operating in a normal flow operating range comprising the steps of:

determining a hydraulic performance curve for a minimum continuous flow rate in order to obtain a minimum flow operating range for the centrifugal pump;

determining a hydraulic performance curve for a maximum flow rate in order to obtain a maximum flow operating range for the centrifugal pump;

computing a torque for the maximum operating range a torque for the minimum operating range;

computing the torque and TDH at different operating speeds; and

determining the pump operating point by processing the computed values.

11. A method of determining the operating point of a centrifugal pump, the method comprising the steps of:

(a) obtaining from the centrifugal pump'"'"'s hydraulic performance curve, a first total dynamic head, a flow and a pump efficiency, at minimum and maximum continuous flow for a first pump motor speed and a given impeller diameter of the centrifugal pump;

(b) determining, for each of the minimum and maximum continuous flow, a first pump motor torque at the first pump motor speed;

(c) determining, for each of the minimum and maximum continuous flow, at least a second pump motor torque and at least a second total dynamic head at least at a second pump motor speed;

(d) plotting, for each of the minimum and maximum continuous flow, the first and at least a second total dynamic head and the first and at least a second pump motor torque on a X-Y graph having an X-axis representing pump motor torque and a Y-axis representing total dynamic head, wherein first and second data plots are generated, the first data plot representing the minimum continuous flow and the second data plot representing the maximum continuous flow, an area defined by the Y-axis and the first data plot is a low-flow operating area, an area defined by the X-axis and the second data plot is a high-flow operating area, and an area defined between the two data plots is a normal-flow operating range; and

(e) determining an actual operating point of the centrifugal pump in relation to the first and second data plots based on a measured pump motor torque and a measured pump motor speed.

12. The method according to claim 11 wherein the actual operating point is determined according to maximum and minimum flow at any pump motor speed.

13. The method of claim 11, wherein the step of determining an actual operating point of the centrifugal pump comprising:

obtaining the measured pump motor torque and measured pump motor speed of the centrifugal pump from a variable speed drive of the centrifugal pump while the pump is operating; and

determining the actual operating point of the centrifugal pump by locating a point on the X-Y graph which represents the measured pump motor speed and the measured pump motor torque, to determine whether the centrifugal pump is operating in the low-flow operating area, high-flow operating area, or the normal-flow operating range.

14. The method of claim 11, wherein the step of determining, for each of the minimum and maximum continuous flow, a first pump motor torque at the first pump motor speed comprising:

computing Brake Horsepower by computing;

$\mathrm{BHP}=\frac{Q*\mathrm{TDH}}{K_1*n}$

where Q is the flow in gpm,

TDH is the total dynamic head in feet,

n is the pump efficiency,and

K₁ is a constant depending on the unit conversion; and

computing pump motor torque at the first pump motor speed by computing;

$T=\frac{\mathrm{BHP}*K_2}{N}$

where T is the torque in foot pounds,

K₂ is a constant depending on the unit conversion, and

N is the first pump motor speed.

15. The method according to claim 11, wherein the step of determining, for each of the minimum and maximum continuous flow, at least a second pump motor torque and at least a second total dynamic head at least at a second pump motor speed comprising:

solving the following equations;

$\frac{\left(N_1\right)}{\left(N_2\right)}=\frac{\left(Q_1\right)}{\left(Q_2\right)}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{and}<br><br>\text{\hspace{1em}<br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\frac{\left(N_1\right)^2}{\left(N_2\right)^2}=\frac{\left({\mathrm{TDH}}_1\right)}{\left({\mathrm{TDH}}_2\right)}$

where N₁=the first pump motor speed,

N₂=the second pump motor speed,

Q₁=flow at the first pump motor speed,

Q₂=flow at the second pump motor speed,

TDH₁=total dynamic head at the first pump motor speed, and

TDH₂=total dynamic head at the second pump motor speed.

16. A pump apparatus comprising:

a centrifugal pump having an impeller and a motor which drives the impeller at a selected pump speed;

a variable speed drive circuit for varying the selected pump speed at which the motor drives the impeller;

a processor;

a memory associated with the processor, the memory storing normal-flow operating range of the centrifugal pump generated by;

obtaining from the centrifugal pump'"'"'s hydraulic performance curve, a first total dynamic head, a flow and a pump efficiency, at minimum and maximum continuous flow for a first pump motor speed and a given impeller diameter of the centrifugal pump;

determining, for each of the minimum and maximum continuous flow, a first pump motor torque at the first pump motor speed;

determining, for each of the minimum and maximum continuous flow, at least a second pump motor torque and at least a second total dynamic head at least at a second pump motor speed;

plotting, for each of the minimum and maximum continuous flow, the first and at least a second total dynamic head and the first and at least a second pump motor torque on a X-Y graph having an X-axis representing pump motor torque and a Y-axis representing total dynamic head, wherein two data plots are generated, first data plot representing the minimum continuous flow and second data plot representing the maximum continuous flow, the first and second data plots defining the normal-flow operating range of the centrifugal pump; and

wherein the processor compares the actual operating point of the centrifugal pump based on a measured pump motor torque and a measured pump motor speed to the normal-flow operating range of the centrifugal pump stored in the memory to determine if the actual operating point of the centrifugal pump is within the normal flow operating range.

17. The apparatus according to claim 16 wherein the pump further includes a flow sensor and a pump differential transducer.