Method for controlling a pump arrangement, and pump arrangement

The present invention relates to a method for controlling a pump arrangement which is used, for example, in cryotechnical plants. The invention further relates to an arrangement comprising one or more pumps for providing pressurized cryogenic liquid, the arrangement being suitable, for example, for use in an air liquefaction plant.

Open-loop and closed-loop control of pump drives to predetermined target values such as a desired output pressure, as a function of various measurable values, for example a pressure difference between pump inlet and pump outlet, volume flows or mass flow rates, a rotational speed of the pump drive or similar values, is necessary particularly in large-scale fluid-processing plants. The use of three-phase asynchronous motors as pump drives is common in this context. Cryogenic pumps, i.e. pumps which deliver cryogenic liquids at temperatures lower than −170° C., are operated by corresponding three-phase asynchronous machines. Especially in cryogenic applications, such as air separation units, a cryogenic liquid or liquefied air is brought to a predetermined operating pressure by cryogenic pumps and subsequently supplied to other plant components, e.g. to a heat exchanger.

In order to keep the pressure in the cryogenic liquid as reliable and uniform as possible, redundant pumps are mostly operated in parallel, in order to maintain the necessary pressure in the low-temperature system in case of failure of one of the pumps. For example, pairs of redundant pumps may be provided, with one working pump being continuously in operation and, in case of failure, a substitute pump starting and replacing the performance of the failed pump. So-called “slow-roll” operating modes are known for such substitute pumps, with the drive motor being active, the pump performs, however, only minimal delivery work.

In order to prevent the pressure in the high-pressure range of the corresponding system from decreasing excessively in case of failure of the working pump, it is necessary to set the redundant substitute pump as quickly as possible to an operating condition which corresponds to the original operating condition of the operating pump. This means that generally the rotational speed of the substitute pump must reach the rotational speed of the failed pump as quickly as possible. Usually, the rotational speed of the pump performing delivery is determined by operation specifications of the plant in question and is adjusted in a closed-control loop. The rotational speed of an asynchronous motor is essentially predetermined by the three-phase frequency at which it is operated. For this reason, in conventional closed-loop control systems, a frequency converter is used, which provides the three-phase frequency for the motor driving the pump. A corresponding closed-loop control unit determines the three-phase frequency for the pumps or asynchronous motors depending on the product present on the output side of the pump.

If one pump fails, a substitute pump must be accelerated to the required rotational speed as quickly as possible. However, especially during the power-up phase, a corresponding pump drive does not always develop its peak torque, which delays the substitute pump\'s achieving its desired delivery result. Thus, undesired fluctuations of pressure and flow rate may occur in the delivered product at the pump output.

It is therefore an object of the present invention to provide an improved procedure of ramping up a pump arrangement.

Accordingly, a method for controlling a pump arrangement comprising a fluid-delivering pump having a pump drive, and a bypass line having a bypass valve, is provided. The bypass line is to return fluid to a reservoir provided at the pump inlet. During the ramping-up procedure of the pump drive to a predetermined target rotational speed, the bypass valve is controlled so that the volume flow through the pump at a corresponding delivery height lies between a cavitational volume flow and the cavitational volume flow increased by a predetermined maximum deviation in the volume flow.

For example, a control line as close as possible to the cavitation boundary can be defined for the operating point of the pump, which results in a favourable reduction of the volume flow which, in turn, rules out the possibility of cavitation occurring.

Especially when cryogenic pumps are in operation, it is necessary to prevent cavitation, which can be achieved by adjusting the volume flow rate to be higher than the cavitational volume flow. Moreover, the proposed closed-loop control of the bypass valve causes the volume flow to be reduced at least partially during ramping-up, and is close to a lower cavitation boundary curve. The bypass valve is preferably controlled in such a way that the volume flow at a corresponding delivery height is situated in a volume flow range between a lower limit volume flow and the cavitational volume flow increased by the predetermined maximum deviation of the volume flow.

It is possible, for example, to define a control curve for the volume flow which runs essentially parallel to the lower cavitation boundary. The lower limit volume flow then lies between the control curve and the cavitation boundary, for example, and an upper limit volume flow lies to the right of the control curve within a corresponding delivery height/volume flow diagram. The range is specified so that the cavitational volume flow is never undercut, not even in the case of overshootings in the closed-loop control.

Reducing the volume flow leads to a lower hydraulic braking torque, which facilitates acceleration to the target rotational speed. This indirectly minimizes the hydraulic braking torque during the power-up phase.

For example, an implementation of the method as a cavitation limiting controller appears appropriate to control the bypass valve on the basis of the difference between a pump inlet pressure and an outlet pressure and the current volume flow. By implementing the method in accordance with the invention, advantage can be taken of the fact that the ramping-up procedure of the pump drive runs essentially along a cavitation boundary. This ensures a particularly low volume flow. Preferably, for example, a current volume flow is calculated as a function of the pressure difference between an inlet and an outlet side of the pump and/or the current rotational speed of the pump drive.

In a variant of the method described above, at least one of the following method steps is performed:

Providing an opened bypass valve or opening the bypass valve. The operating situation is frequently such that the bypass valve is opened 100% at the time of failure of one pump. A pump provided as a substitute pump in slow-roll or stand-by mode normally features an open bypass. Based on this situation, the volume flow is then minimized in order to bring the corresponding substitute pump to the predetermined rotational speed as quickly as possible.

Determining a target rotational speed for a pump drive as a function of a predetermined outlet pressure. A higher-level pressure controller provides, for example, a rotational speed for the pump depending on the queried product in the output line. The use of several pumps in parallel arrangement which supply a common high-pressure fluid line is also conceivable. A corresponding pressure controller then provides a target rotational speed for these pumps.

Reducing the flow rate through the bypass valve in order to increase the delivery height of the pump, and reducing the volume flow while keeping the rotational speed of the pump drive within a predetermined maximum range during an initial power-up phase. For example, a maximum rotational speed change of 10%, preferably 5%, may be desired during the initial power-up phase. The bypass valve should be closed so quickly as to ensure that the rotational speed is not increased significantly. For example, if the entire ramp-up time of the corresponding pump amounts to 10 seconds, closing can be effected within one second. By reducing the flow rate while increasing the delivery height, the operating point of the pump approaches the cavitation boundary, i.e. it moves in the direction of the lowest possible volume flow without inducing cavitation. Preferably, an operating point of the pump along a delivery height-volume flow characteristic at a constant rotational speed is used during the initial power-up phase. For example, the bypass valve can be ramped, i.e., its orifice varied by a predetermined value in a predetermined time. It is also possible to set a target value for the controller acting on the bypass valve, with the valve position varying correspondingly over time during this initial power-up phase.

Increasing the rotational speed of the pump drive during a second power-up phase by controlling via the bypass valve so that in case of an increase in the delivery height, the volume flow exceeds the cavitational volume flow. In this context, the cavitational volume flow corresponds to a minimum volume flow required to avoid cavitation at a corresponding delivery height. During the second phase, a minimum acceptable volume flow rate is thus ensured without inducing cavitation. In the second power-up phase, the pump is thereby preferably operated at an operating point which runs essentially in parallel to a cavitation boundary of a set of delivery height-volume flow characteristic lines of the pump.

Closing the bypass valve once a predetermined delivery height or a predetermined outlet pressure has been reached during a third power-up phase. For example, as soon as the pressure required by a pressure controller has been reached on the outlet side, the volume flow can also be increased again, which is effected by closing the bypass valve. On principle, this process can be repeated until a pressure controller acting on the bypass valve records a maximum pressure. Then the bypass valve would have to be opened.

On principle, other operational situations may also necessitate the closing of the bypass valve. If, for example, fluid is withdrawn on the outlet side, the closed-loop control can induce a reduction of the bypass valve orifice.

The method is suited for use with an asynchronous motor as a pump drive with a three-phase frequency which corresponds to the predetermined target rotational speed. The current rotational speed can also be approached approximately by taking the synchronous rotational speed into account. The resulting slip can be neglected.

Moreover, a pump arrangement comprising at least one pump, one reservoir and one control device is provided. Here, the pump has a pump drive, and the reservoir is coupled to the pump on the pump outlet side via a bypass line. The bypass line is equipped with a bypass valve. The reservoir also supplies a fluid to be delivered to the pump inlet side. The control device is designed in such a way that a method described above is performed.