Drainage system for compressor separators

Abstract

A drainage system is for a compressor assembly that includes at least one separator with a chamber. The system includes a drain valve fluidly coupled with the separator chamber and adjustable between open and closed states. An actuator is operatively coupled with and adjusts the valve between the open and closed states or/and a sensor senses when the valve is open. A pressure sensor is configured to sense pressure within the chamber and a logic circuit is coupled with the pressure sensor and determines when the valve either has been or should have been adjusted to the open state. The logic circuit also generates an output signal or/and operates a device when the chamber pressure remains substantially constant or varies by less than a predetermined amount subsequent to the valve being opened and/or when the chamber pressure varies by at least the predetermined amount after the valve is opened.

Description

BACKGROUND OF THE INVENTION

The present invention relates to fluid machinery, and more specifically to drainage systems for compressors.

Compressors typically include one or more separators each fluidly connected with each “stage” of the compressor. These separators are provided to remove liquid from a compressed fluid, such as air, so that the fluid is substantially gaseous. As such, liquid collects in each separator, which must be periodically removed to prevent diminished performance of the separator. Typically, a drain valve is fluidly connected with the chamber of each separator, and these valves are periodically opened to evacuate liquid from the associated chamber. As a variety of faults may prevent the liquid from being evacuated, such as a failure of one or more automatically-operated valves to open as directed or an obstruction in the valve or a fluid line connecting the valve with the chamber, it is important to ensure that the liquid is actually drained from each separator.

SUMMARY OF THE INVENTION

In one aspect, the present invention is a drainage system for a compressor assembly, the compressor assembly including at least one separator with a separator chamber. The drainage system comprises a drain valve fluidly coupled with the separator chamber, the valve being adjustable between an open state and a closed state. An actuator is operatively coupled with and configured to adjust the valve between the open and closed states or/and a sensor is configured to sense when the valve is in the open state. A pressure sensor is configured to sense pressure within the chamber. Further, a logic circuit is coupled with the pressure sensor and is configured to determine when the valve either has been adjusted to the open state or should have been adjusted to the open state. The logic circuit is further configured to generate an output signal or/and to operate a device when the chamber pressure remains substantially constant or varies by less than a predetermined amount subsequent to the valve being adjusted to the open state and/or when the chamber pressure varies by at least the predetermined amount subsequent to the valve being adjusted to the open state.

In another aspect, the present invention is again a drainage system for a compressor assembly, the compressor assembly including at least one separator with a separator chamber. The drainage system comprises a drain valve fluidly coupled with the separator chamber, the valve being adjustable between an open state and a closed state, and a pressure sensor configured to sense pressure within the chamber. The drainage system further comprises monitoring means for determining when the valve has been adjusted to the open state and for generating an output signal when the chamber pressure remains substantially constant subsequent to the valve being adjusted to the open state.

In a further aspect, the present invention is a method of operating a drainage system for a compressor assembly, the compressor assembly including at least one separator with a separator chamber. The method comprising the steps of: providing a drain valve fluidly coupled with the chamber, the valve being adjustable between an open state and a closed state; sensing pressure sensor within the chamber; determining when the valve has been adjusted to the open state; and generating an output signal when the chamber pressure remains substantially constant subsequent to the valve being adjusted to the open state.

In yet another aspect, the present invention is again a drainage system for a compressor assembly, the compressor assembly including at least one separator with a chamber. The drainage system comprises a drain valve fluidly coupled with the separator chamber, the valve being adjustable between an open state and a closed state and an actuator is configured to adjust the valve between the open and closed states. A pressure sensor is configured to sense pressure within the separator chamber and a logic circuit coupled with the pressure sensor and with the actuator. The logic circuit is configured to periodically adjust the valve to the open state upon the expiration of a predetermined amount of time and to determine chamber pressure generally at the expiration of each time period. Further, the logic circuit is also configured to generate an output signal and/or operate a device when the chamber pressure either remains substantially constant or varies by less than a predetermined amount generally at the expiration of each time period or/and when the chamber pressure varies by at least the predetermined amount generally at the expiration of each time period.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the detailed description of the preferred embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings, which are diagrammatic, embodiments that are presently preferred. It should be understood, however, that the present invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:

FIG. 1 is a schematic view of a multistage compressor having a drainage system in accordance with the present invention;

FIG. 2 is another schematic view of the multistage compressor of FIG. 1;

FIG. 3 is an enlarged, schematic view of a single stage of the compressor assembly of FIG. 1;

FIG. 4 is a schematic view of an exemplary compressor unit of a single compressor stage; and

FIG. 5 is a block diagram of the basic components of the drainage system of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings in detail, wherein like numbers are used to indicate like elements throughout, there is shown in FIGS. 1-5 a drainage system 10 for a compressor assembly 1. The compressor assembly 1 includes at least one and preferably a plurality of compressor units 2, at least one motor 3 configured to operate the compressor unit(s) 2, and at least one and preferably a plurality of separators 4 each having a separator chamber C_S. The separator chambers C_S are each fluidly connected with the associated compressor unit 2 by a separate fluid line 5 and are configured to contain a quantity of liquid. The drainage system 10 basically comprises at least one and preferably a plurality of drain valves 12 each fluidly coupled with one of the separator chambers C_S, at least one and preferably a plurality of pressure sensors 14 each configured to sense pressure P_C within a separate one of the chambers C_S, and a logic circuit 16. Each drain valve 12 is adjustable between an open state and a closed state and preferably has either an actuator 18, most preferably a solenoid 19, configured to adjust the valve 12 between the open and closed states, or at least a sensor (not shown) configured to sense when the valve 12 is (or should be) in the open state (e.g., a limit switch, etc.). The drain valves 12 are each configured to drain liquid from the connected separator chamber C_S when in the open state or “open”. Further, each pressure sensor 14 preferably includes a transducer 22 configured to generate an electrical signal S_P corresponding to pressure P_C within the separator chamber C_S and to transmit the signal S_P to the logic circuit 16. Preferably, each pressure sensor 14 is configured to sense pressure within the fluid line 5 extending between the associated separator 4 and connected compressor unit 2, which corresponds with the pressure P_C within the chamber C_S of the particular separator 4, but may alternatively be configured to directly sense the pressure P_C within the separator chamber C_S or even to sense a corresponding pressure within a section of the coupled compressor unit 2.

Furthermore, the logic circuit 16 is coupled with each one of the pressure sensors 14 and is preferably coupled with each one of the valve actuators 18 or valve sensors. The logic circuit 16 is configured (i.e., programmed, hardwired, etc.) to determine when each drain valve 12 has been, or at least should have been, adjusted to the open state or/and actually directs the “opening” of each valve 12, as discussed below. Also, the logic circuit 16 is further configured to generate an output signal S_O (or to directly operate a device, as described below) either when the separator chamber pressure P_C remains substantially constant or varies less than a predetermined amount (i.e., minimal pressure change), or/and when the pressure P_C varies by at least a predetermined amount, subsequent to the one of more drain valves 12 being adjusted to the open state, as described in greater detail below.

Preferably, the drainage system 10 is constructed or configured such that the valve(s) 12 are automatically adjusted to the open state (and thus subsequently also to the closed state) periodically or repeatedly upon the expiration of a predetermined amount of time or time period during compressor operation, preferably by means of an associated actuator 18. In other words, system 10 is configured to open, and thereafter close, the valve(s) 12 in repeated time intervals, e.g., every fifteen minutes, every half hour, etc., so that the separator chamber(s) C_S are intermittently drained continuously during compressor operation. Most preferably, the logic circuit 16 is further configured to direct the periodic opening and closing of each valve 12 by means of a control signal S_C sent to the actuator 18 associated with the particular valve 12. However, the valve(s) 12 may alternatively be opened and closed by another logic circuit or controller (none shown) for directing opening of all the valves 12, or may be operated by an individual controller for each valve 12. In these alternative constructions, the logic circuit 16 would either be configured to merely determine or “track” when the valves 12 are scheduled to be opened (e.g., by a timing circuit, etc.) or may receive a feedback signal from a separately controlled valve actuator(s) 18 or a position sensor on a moveable valve element, whether or not the valve 12 has actually opened or fluid/liquid is permitted to flow through the valve 12, as discussed below.

In all the above or other cases, when the logic circuit 16 determines that the one or more valves 12 have opened or should have been opened, the logic circuit 16 uses pressure readings from the associated sensors 14 to determine if the separator chambers C_S have been evacuated, which should result in a reduction of the pressure P_C within each chamber C_S. As such, when the chamber pressure P_C remains substantially constant or reduces only by a minimal amount, the logic circuit 16 is able to determine that the separator chamber C_S has not been drained due to a malfunction of the associated actuator 18 or a component of the valve 12, a blockage in the valve 12 or a connected fluid line, etc. More specifically, the logic circuit 16 may send a control signal S_C to the actuator 18 of each drain valve 12 or may receive a feedback signal from the associated actuator 18 that the actuator 18 has attempted to operate/open the associated valve 12, but one or more of the actuators 18 may malfunction or attempt to displace a valve closing element (e.g., spindle, etc.) that is immovable or “stuck”. In other cases, the actuator(s) 18 may actually displace the valve closing element, but no flow passes therethrough because of an obstruction in a valve passage or a connected fluid line, etc.

Preferably, the logic circuit 16 is configured to at least generate a first output signal S_O1 when the chamber pressure P_C of one or more separators 4 remains substantially constant or varies by less than a predetermined amount subsequent to the connected drain valve 12 being adjusted to the open state, either actually opened or when the actuator 18 receives a control signal S_C that should have caused the valve 12 to be opened. Thus, the first output signal S_O1 is used to indicate a failure condition and/or to initiate corrective action, as discussed below. Further, the logic circuit 16 may also generate a second output signal S_O2 when the chamber pressure P_C is reduced by a predetermined amount while each drain valve 12 is in the open state. As such, the second output signal S_O2 indicates that the connected chamber C_S has been drained and the valves 12 and other drainage components (fluid lines, etc.) are functioning properly, as described below.

Thus, the logic circuit 16 is capable of determining when each particular drain valve 12 is functioning properly, i.e., the separator chamber pressure P_C “drops” or is reduced by a predetermined amount, or that one or more drain valves 12 are malfunctioning, i.e., the pressure P_C remains constant in the separator chamber C_S associated with such valves 12. As mentioned above, such malfunctions include, but are not limited to, an actuator 18 not opening a valve 12 after receiving a control signal S_C from the logic circuit 16, a blockage in a valve passage, a blockage in a fluid line between a chamber C_S and a valve 12 or between a valve 12 and a discharge port (not depicted), or any other occurrence preventing liquid from evacuating a separator chamber C_S. In any case, when the logic circuit 16 determines that a malfunction has occurred such that at least one of the separators 4 has not been drained as required, the logic circuit 16 is preferably further programmed or constructed to take an appropriate “emergency” response or corrective action.

More specifically, when the pressure P_C in any separator chamber C_S does not drop by a certain amount, the logic circuit 16 is preferably configured to transmit the first output signal S_O1 to the one or more compressor motors 3 to thereby turn off the motor(s) 3, such that the compressor assembly 1 halts operation. Alternatively or additionally, the logic circuit 16 may be configured to directly operate one or more indicator devices 28, such as a warning light, horn, speaker, etc., or to send the output signal S_O1 to a monitoring device 29 (e.g., a display and/or controller) operably coupled with the logic circuit 16 and configured to provide a warning indication and/or create an event record (i.e., within a performance audit database) upon receipt of the signal S_O1. Therefore, either the logic circuit 16 automatically shuts down the compressor assembly 1, or provides an operator with the necessary information to enable the operator to take the appropriate action(s) upon the occurrence of a malfunction. Further, the logic circuit 16 may be constructed to first provide a warning to the operator, and then shut down the compressor assembly 1 upon expiration of a predetermined time period if no operator corrective action has occurred.

Preferably, the logic circuit 16 includes a microprocessor 24 electrically coupled with each one of the valve actuators 18 (or valve sensors 20) and with each one of the pressure sensors 14, the microprocessor 24 being configured to generate the one or more output signals S_O. Most preferably, the logic circuit 16 includes a programmable logic controller or “PLC” 26 providing the microprocessor 24 and configured to receive inputs from a plurality of the pressure sensors 14 and a plurality of the valve actuators 18 (or sensors 20), as best shown in FIG. 1. Alternatively, the logic circuit 16 may be provided by a separate microprocessor incorporated into a printed circuit board, so as to constitute a specially manufactured controller, a “hard-wired” analog electronic controller, a pneumatic or hydraulic logic device, or any other type of logic device capable of performing the logic and/or control operations as generally described herein.

Referring to FIGS. 3-5, the preferred PLC 26 is preferably configured to operate or adjust each valve 12 between the open and closed states, specifically by means of a valve control signal S_C sent to the actuator 18 of the particular valve 12, as discussed above. Most preferably, the PLC 26 is programmed to adjust each one of the plurality of drain valves 12 to the valve open state, preferably periodically upon the expiration of a predetermined amount of time (e.g., every 10 minutes, one an hour, etc. during compressor operation) as discussed above, or alternatively when the PLC 26 determines that any separator chamber C_S contains about a predetermined amount of liquid (e.g., a quarter gallon). With the PLC 26 actually controlling the operation of each drain valve 12 (i.e., the opening and closing thereof), the logic circuit 16 “knows” when each valve 12 is or should have been opened, such that no input to determine when the valve(s) 12 have been adjusted to the open state (whether or not actually opened) is required. However, with a compressor assembly 1 that has a separate device(s) for controlling the opening and closing of the valves 12, for example a controller (not shown) for each valve 12 receiving input from a floater sensor (not shown) to drain the chamber C_S when a certain amount of fluid accumulates therein, or the valves 12 are manually operated, a separate valve sensor (none shown) may be necessary. For example, each drain valve 12 may have a limit switch (none shown) electrically connected with the PLC 26 that closes (and generates a signal) when a valve member 32 is moved to a valve open position, as discussed in greater detail below.

As best shown in FIG. 3, each drain valve 12 has a passage 30 fluidly connected with the associated separator chamber C_S and a moveable valve member or “closing element” 32 (e.g. a stem, spindle, plug, etc.) configured to releasably obstruct the passage 30, as is well known. As discussed above, each valve 12 preferably includes a solenoid valve actuator 19, which is configured to displace the valve closing element 32 between a first position, at which the passage 30 is substantially obstructed (i.e., no fluid flow therethrough), and a second position at which the passage 30 is substantially open. The solenoid 19 is electrically coupled with the logic circuit 16, preferably with the PLC 26, such that the circuit 16 is able to control the opening/closing of each valve 12 and thus determines when a pressure drop should be detected by each pressure sensor 14. However, the valve actuators 18 may be provided by any other appropriate device, such as an electric, hydraulic or pneumatic motor, may be operated by a device(s) other than the logic circuit 16, or one or more of the drain valves 12 may be manually operated, for example by a hand-rotatable spindle.

Having described the basic elements and operation above, these and other components of the compressor drainage system 10 of the present invention, and preferred application(s), are described in detail below.

Referring to FIG. 1, as discussed above, the drainage system 10 of the present invention is preferably used with a multi-stage compressor assembly 1 that preferably includes a single housing or casing (not shown) containing each of the plurality of compressor units 2, but may include one or more housing/casing sections each containing one or more compressor units 2 and connected by appropriate means. Each drain valve 12 is connected with the housing or casing so as to direct fluid to flow externally of the housing, i.e., the valves 12 evacuate fluid from the compressor housing. The compressor assembly 1 also has an inlet port I_P and an outlet port O_P, such that fluid enters the housing through the inlet port I_P, passes through each stage of the compressor assembly 1 and exits the housing through the outlet port O_P. Although the drainage system 10 is preferably used with a multistage compressor assembly contained in a single housing/casing, it is within the scope of the present invention to use the drainage system 10 with only a single stage compressor or a multistage compressor assembly 1 that includes compressor units 2 each contained within two or more separate housings or casings.

Referring to FIGS. 1 and 4, each compressor unit 2 includes a moveable compression member 6, preferably a reciprocable piston 7 but may alternatively be a rotatable impeller or a screw (neither shown), operatively connected with the motor(s) 3. The compressor assembly 1 preferably includes only a single motor 3 for simultaneously displacing the compression members 6 of all of the compressor units 3, for example through a common drive shaft 3a. However the compressor assembly 1 may alternatively include a plurality of motors 3, particularly if the compressor assembly 1 includes a number of distinct, but fluidly connected, compressor units 2. Further, each of the preferred reciprocal compressor units 2 preferably includes a compression chamber C_C defined within the compressor housing and having an inlet port 35 and an outlet port 36, a piston head 37 disposed within the chamber C_C, and a connecting rod 38 extending between the piston head 37 and a crank member 39 on the drive shaft 3a. However, the compressor units 2 may be of any appropriate type and have any appropriate structure, and the scope of drainage system 10 of the present invention is in no manner limited to any particular type of compressor unit 2.

Preferably, the compressor assembly 1 also includes a plurality of coolers 8 that are each either disposed about, or fluidly connected with, a portion of each fluid line 5, such that the compressed fluid is cooled prior to separation of the liquid therefrom. Each pressure sensor 14 is preferably configured to sense fluid pressure in a section 5a of the fluid line 5 extending between one cooler 8 and the associated separator 4.

As best shown in FIGS. 3 and 4, each separator 4 preferably includes a condenser member 9 having at least one surface 9a for condensing liquid from the compressed fluid flowing through the separator 4, such as for example one or more baffles. More specifically, each separator 4 preferably includes a housing 40 defining the separator chamber C_S and having an inlet 41 and an outlet 43, the associated drain valve 12 being mounted to the housing 40, or connected thereto by a fluid line (not indicated), and the preferred condenser member(s) 9 are disposed within the chamber C_S. As such, compressed fluid flows into the inlet 41, contacts the condenser member(s) 9 so that liquid condenses thereon to separate from the gaseous component of the fluid, and the remaining fluid flows out of the outlet 43. Alternatively, each separator 4 may include a rotatable member, such as a generally tubular drum (none shown), configured to direct liquid generally radially and tangentially outwardly from a central axis so as to separate liquid from a gaseous remainder of the fluid (i.e., a centrifugal separator), or any other appropriate device for separating liquid from a fluid flow.

Referring to FIGS. 1 and 2, in one preferred application of the compressor drainage system 10, the compressor assembly 1 includes five compression stages, each stage include a reciprocal compressor unit 2, a separator 4, and a cooler 8 disposed generally between each compressor unit 2 and associated separator 4. Preferably, the drain valves 12 of the first two stages have solenoids 19 that are configured to close automatically after a predetermined amount of time (e.g., twelve seconds) after the compressor assembly starts operating, and the valves 12 of the third, fourth and fifth stages have solenoids 19 that are closed by a pilot air flow A_P from the compressor second stage, as indicated in FIG. 2. These drain valves 12 of the third, fourth and fifth stages vent off the pilot air pressure upon receipt of the control signal S_O from the PLC 26, to thereby enable these valves 12 to open and drain the connected separator 4. Most preferably, each drain valve 12 includes an ASCO bulletin 8262 style, pilot-operated solenoid valve.

Referring to FIG. 3, the preferred PLC 26 is preferably provided by a commercially available control panel 50, most preferably an Allen Bradley SLC5 based control panel, located in an appropriate operator station or area. However, the PLC 26 may be provided by a separate controller unit of any appropriate construction. Preferably, any indicator or monitoring devices are preferably also incorporated in the contained within a control panel 50, but may also be provided by separate devices. Further, each pressure sensor 14 is preferably a diaphragm pressure transducer 22, and most preferably an AMETEK Style A2 pressure transmitter commercially available from AMETEK U.S. Gauge of Feasterville, Pa., exposed to fluid flow within the fluid line 5 and electrically connected with the preferred PLC 26. Although a diaphragm pressure transducer 22 is presently preferred, any other type of pressure transducer or sensor may alternatively be used, such as for example, a transducer utilizing a Bourdon tube, a capsule or bellows as the mechanical element being sensed for displacement proportional to fluid pressure.

With the basic structure as described above, the compressor drainage system 10 functions basically in the following manner. Once operation of the compressor assembly 1 is initiated, the valves 12 are generally arranged in the closed state while fluid flowing through the compressor assembly 1, particularly gaseous portions thereof, substantially passes from the compressor inlet port I_P valve and out of the compressor outlet port O_P. However, as discussed above, the drain valves 12 are periodically opened to evacuate the liquid accumulating within the associated separators 4, preferably automatically at specified time intervals by control signals S_C sent to the valve actuators 18 from the PLC 26, as described above. Upon opening each drain valve 12, the PLC 26 may generate the second output signal S_O2 when the valves 12 function properly and evacuate the liquid, or may ignore pressure signals within the desired range and take no further action when an appropriate pressure drop occurs. However, when the PLC 26 determines that the pressure P_C within one or more separator chambers C_S remains substantially constant, or has not experienced a sufficient drop, when the associated valve 12 should have opened, the PLC 26 preferably generates the first output signal S_O1 such that the compressor assembly 1 is shut down or/and an appropriate warning is provided to enable an operator to take appropriate remedial action.

It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as generally defined in the appended claims.

Claims

1. A method of draining a separator coupled to a compressor, comprising: determining a first pressure in at least one of the compressor, the separator, and a line extending between the compressor and the separator;determining when the separator contains about a predetermined amount of liquid before signaling a drain valve to open;signaling the drain valve to open to drain the separator;determining a second pressure in the at least one of the compressor, the separator, and the line after signaling the drain valve to open; andinitiating corrective action if the second pressure is less than the first pressure by less than a predetermined amount, wherein initiating corrective action comprises signaling a drain failure with an indicator device.

2. The method of claim 1, further comprising signaling the drain valve to close after signaling the drain to open, if the second pressure is less than the first pressure by greater than or equal to the predetermined amount.

3. The method of claim 1, wherein signaling the drain valve comprises sending an electrical signal from a programmable logic controller to an actuator of the drain valve.

4. The method of claim 3, further comprising receiving a position feedback signal from the drain valve with the programmable logic controller after signaling the drain valve to open, wherein the position feedback signal indicates whether the drain valve has opened.

5. The method of claim 1, wherein determining the first and second pressures comprises receiving electrical signals from a pressure transducer disposed in the at least one of the compressor, the separator, and the drain.

6. The method of claim 1, wherein initiating corrective action comprises shutting down a motor driving the compressor.

7. The method of claim 1, wherein initiating corrective action further comprises shutting down a motor driving the compressor if an operator does not take additional corrective action within a predetermined amount of time after signaling the drain failure with the indicator device.

8. A method for draining a separator fluidly coupled to a compressor, comprising: periodically sending an opening signal to a drain valve coupled to the separator to drain a condensate stored in the separator, wherein periodically sending the opening signal comprises periodically sending an electric signal to an actuator of the drain valve, wherein each electric signal is separated from a subsequent electric signal by a substantially constant time interval;periodically sending a closing signal to the drain valve to maintain pressure in the separator;periodically receiving a feedback signal from a sensor coupled to the actuator to determine a drain valve position;determining a pressure drop in the separator after sending each opening signal to the drain valve; andinitiating corrective action if the pressure drop is below a predetermined amount.

9. A method for draining a separator fluidly coupled to a compressor, comprising: periodically sending an opening signal to a drain valve coupled to the separator to drain a condensate stored in the separator;periodically sending a closing signal to the drain valve to maintain pressure in the in the separator;determining a pressure drop in the separator after sending each opening signal to the drain valve; andinitiating corrective action if the pressure drop is below a predetermined amount, wherein initiating corrective action comprises shutting down a motor driving the compressor.

10. The method of claim 9, wherein initiating corrective action comprises: signaling a drain failure with an indicator device; andshutting down the motor after signaling the drain failure if an operator does not take additional corrective action.

11. The method of claim 8, wherein determining the pressure drop comprises taking a pressure reading in a line extending between the compressor and the separator.

12. The method of claim 8, wherein initiating corrective action comprises: signaling a drain failure with an indicator device; andshutting down the motor after signaling the drain failure if an operator does not take additional corrective action.

13. The method of claim 9, wherein determining the pressure drop comprises taking a pressure reading in a line extending between the compressor and the separator.

14. The method of claim 9, wherein periodically sending the opening signal to the drain valve comprises periodically sending an electric signal to an actuator of the drain valve, wherein each electric signal is separated from a subsequent electric signal by a substantially constant time interval.

15. The method of claim 14, further comprising periodically receiving a feedback signal from a sensor coupled to the actuator to determine a drain valve position.