Method for controlling the vacuum generator(s) in a vacuum sewage system

Abstract

Method for controlling one or more vacuum generators (1) and thereby the vacuum in a vacuum sewage system, in particular controlling one or more liquid ring screw pumps in such system, including beyond the generator/s (1) one or more tubular collectors or suction pipelines (2) connected to the vacuum generator and one or more toilets, urinals, grey water sinks etc (3, 4), connected to the suction pipeline through branch pipelines (6). The rotational speed of the vacuum generator (1) is controlled on the basis of the set vacuum requirement.

Claims

1. Method for controlling vacuum generators which create a vacuum in a vacuum sewage system, the vacuum generators being in the form of electrically driven liquid ring screw pumps in such system, including beyond the generators one or more tubular collectors or suction pipelines connected to the vacuum generator and one or more toilets, urinals, grey water sinks connected to the one or more tubular collectors or suction pipelines through branch pipelines, characterised in that the vacuum of the system is maintained and controlled by controlling rotational speed (rpm) of the vacuum generators in relation to a preset vacuum requirement, further comprising controlling the rpm for each of the vacuum generators in the system with a programmable logic controller (PLC), the PLC being programmed to run a first of the vacuum generators until it reaches a set highest rpm and then start a second of the vacuum generators if the vacuum system requires increased pumping capacity.

2. A method for controlling according to claim 1, where the vacuum generators are each driven by an electrical motor, further comprising controlling the rpm of the vacuum generators by controlling the rpm for each of the electrical motors in the system with the programmable logic controller (PLC) through a frequency transformer on the basis of signals from a pressure transmitter.

3. A method for controlling vacuum generators and which create a vacuum in a vacuum sewage system, the vacuum generators being in the form of electrically driven liquid ring screw pumps in such system, including beyond the generators one or more tubular collectors or suction pipelines connected to the vacuum generator and one or more toilets, urinals, grey water sinks connected to the one or more tubular collectors or suction pipelines through branch pipelines, characterised in that the vacuum of the system is maintained and controlled by controlling rotational speed (rpm) of the vacuum generators in relation to a preset vacuum requirement, further comprising controlling the rpm for each of the vacuum generators in the system with a programmable logic controller (PLC), the PLC further being programmed to control the rpm for each of the generators when running such that they are run with the same rotational speed, from lower to higher rpm based on the required vacuum, but such that an additional vacuum generator of the generators that was not running is started when more capacity is required and one or more of the generators which are running are running at a maximum required rpm.

4. A method for controlling according to claim 3, where the vacuum generators are each driven by an electrical motor, further comprising controlling the rpm of the vacuum generators by controlling the rpm for each of the electrical motors in the system with the programmable logic controller (PLC) through a frequency transformer on the basis of signals from a pressure transmitter.

Description

(1) The invention will be further described in the following by way of examples and with reference to the drawings where:

(2) FIG. 1 shows, as an example, a schematic sketch of a vacuum sewage system,

(3) FIGS. 2-4 show curves related to power consumption, affectivity and capacity based on tests performed according to the invention. p FIG. 5 shows a flow diagram of an exemplary control regime for a vacuum system.

(4) FIG. 6 shows a flow diagram of an exemplary alternative control regime for a vacuum system.

(5) FIG. 1 shows, as stated above, an example of vacuum sewage system including two vacuum generators 1 in the form of liquid ring screw pumps with integrated macerators coupled in parallel, a common suction pipeline or collector tube 2 connected with the vacuum generators at one end and further connected with a number of toilets, urinals etc. 3, 4 through branch pipelines 6 at the other end. Vacuum is generated in the common suction pipe 2 and branch pipelines 6 by means of the vacuum generators 1 and, when flushing the toilets or urinals etc., successive charges of liquid and air is drawn towards the generators and discharged through an outlet 5 of the generator/s.

(6) The system as shown in FIG. 1 is, as mentioned above, commonly controlled by on/off running of the vacuum generators. Thus, when the system is in a low mode state of use, for instance in the middle of the night when just a few or no toilets are used, only one vacuum generator is running when required, i.e. when the vacuum reaches the high set level (40% vacuum). As soon as the vacuum generator again reaches the low set level (60% vacuum) the vacuum generator will stop.

(7) At high mode state of use, such as in the morning when a large number of toilets etc. are used, both vacuum generators will run simultaneously, and depending on the required vacuum during the day or night, only one vacuum generator will run intermittently, or one or both vacuum generators will run continuously and/or in combination intermittently.

(8) With the present invention is provided a method for controlling (a control regime for) the vacuum generators in a vacuum sewage system which is based on continuous running of the generators, but with control of the rotational speed of the generators based on a preset vacuum pressure and required vacuum capacity.

(9) FIG. 5 shows a flow diagram of an exemplary control regime for a vacuum system. With respect to blocks 10, 12, and 14 the vacuum generators in a vacuum sewage system are commonly powered by means of electrical motors and the rotational velocity (rpm) for each motor in the system is preferably controlled by means of programmable logic controller (PLC) through a frequency transformer on the basis of signals from a pressure transmitter. Thus, with the present invention, the desired vacuum is selected, normally 50%, and the PLC is set to control the rpm (revolution per minute) of the vacuum generator motor(s) based on the transmitted signal from the pressure transmitter in the vacuum system. In systems having two or more vacuum generators run I parallel, a preferred control regime would be, based on the required vacuum at any time, to program the PLC to run one first generator until it reaches a set highest rpm and then start the next, second generator if the vacuum system requires increased vacuum capacity, as indicated in blocks 16, 18, 20, 22 and 24. Then, when the second generator reaches its highest set rpm, and if further capacity is required, a third, fourth or more vacuum generator(s) is (are) started and run at the required rpm based on the selected vacuum of the system such that the vacuum is kept at the selected vacuum level (50%) at all times.

(10) FIG. 6 shows a flow diagram of an exemplary alternative control regime for vacuum systems having two or more vacuum generators is to program the PLC to control the rpm for each generator such that they are run with the same rotational speed, from lower to higher rpm based on the required vacuum as indicated in blocks 10, 12, 14 and 15, but such that a new vacuum generator is started when more capacity is required and the running generator(s) is (are) running at a maximum required rpm, as indicated in blocks 16, 18, 20, 22, and 24. This way of controlling the number of generators and each generator's rpm on the basis of the set vacuum and required vacuum capacity of the system may likely be as effective as the above preferred embodiment where each first, second etc. generator is run at full rpm and kept at full rpm before the next generator I started.

(11) As a precaution, the PLC is preferably programmed to trigger an alarm if all of the pumps in the system is started and run at full capacity (rpm) and the set vacuum level is not reached after a period of time. In such case the vacuum systems needs to be checked with regard to possible leakages or other deficiencies that would cause low pressure.

(12) Tests.

(13) Extensive tests by the inventors of the present invention has proved that it is possible to maintain sufficient vacuum at a preset level by running liquid ring screw pumps with reduced rotational speed (rpm) and still maintain sufficient vacuum. i.e. 40% vacuum or below.

(14) Equipment.

(15) TABLE-US-00001 Vacuum generator: Jets NT 220 liquid ring screw pump Electric motor: Lonne 14GI86-4AA11-Z 230/400 V 50 Hz - 22 kW, 1465 rpm 460 V 60 Hz - 23.3 kW, 1765 rpm Inverter - frequency control.: Mitssubishi FR-F740-00620 EC PLC control and logging: Mitsubishi Melsec FXN-16MR Pressure sensor: GE Druck PTX 1400
Test Conditions.

(16) TABLE-US-00002 Room temperature: 23 C. Water inn: Temperature 11 C. Amount: 20 litre/min. Air pressure: 993 mbar Lifting height (generator/pump) 2 m
Test Procedure.

(17) The vacuum generator used in the test was connected through its suction inlet and discharge outlet via a pipe loop to a tank containing water (not shown). Vacuum was obtained by means of throttling of a throttling valve provided on the pipe loop before the pump inlet (neither not shown). After each running of the vacuum generator for each test, the tank was aerated for 10 minutes before starting of the vacuum generator which was then run for 3 minutes before each test.

(18) The results of the tests are shown in the accompanying FIGS. 2-4, thus:

(19) FIG. 4 shows the capacity, Q (m.sup.3/h) versus % vacuum when running the liquid ring screw pump with different rotational speeds from 30-60 Hz.

(20) FIG. 2 shows the effect, P (kw) versus % vacuum for the same pump and with the same rotational speeds.

(21) FIG. 3 shows the affectivity, Q/P (M.sup.3/h/kw) versus % vacuum for the same pump and with the same rotational speeds.

(22) As can be seen from the curves in FIGS. 2-4, it is possible to maintain vacuum below 40% and at the same time also maintain sufficient capacity when reducing the rotational speed from 60-30 Hz.