25th International Cryogenic Engineering Conference and the International Cryogenic Materials Conference in 2014, ICEC 25–ICMC 2014

Development of a measurement and control system for 40L/h helium liquefier based on Siemens PLC S7-300

J. Lia,d, L. Q. Liua,, X. D. Xua, T. Liub, Q. Lia, Z. J. Hua, B. M. Wanga, L. Y. Xionga, B. Donga and T. Yanc

aKey Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China

bBeijing Sciample Technology Co., Ltd., Beijing 100190, China

cTechnical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China

dUniversity of Chinese Academy of Sciences, Beijing 100049, China

Abstract

A 40L/h Helium Liquefier is established at the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. A measurement and control system based on Siemens PLC S7-300 for this Helium Liquefier is developed. Proper sensors are selected, for example, three types of transmitters are adopted respectively according to detailed temperature measurement requirements. Siemens S7-300 PLC CPU315-2PN/DP is as master station and three sets of ET200M DP remote expand I/O is as slave station. Profibus-DP field communication is established between the master station and the slave stations. The upper computer HMI (Human Machine Interface) is compiled using Siemens configuration software WinCC V7.0. The upper computer communicates with PLC by means of industrial Ethernet. A specific control logic for this Helium Liquefier is developed. The control of the suction and discharge pressures of the compressor and the control of the turbo-expanders loop are being discussed in this paper. After several times’ commissioning, the outlet temperature of the second stage turbine has reached to 8.6K and the temperature before the throttle valve has reached to 13.1K.

© 2014 The Authors. Published by Elsevier B.V.

Peer-review under responsibility of the organizing committee of ICEC 25-ICMC 2014.

Keywords: 40L/h helium liquefier; measurement and control system; PLC; control logic

Introduction

A 40L/h helium liquefier is established at the Technical Institute of Physics and Chemistry, CAS. This liquefier is being established as a key technical verification system for the large-scale refrigerator, for example, 10kW@20K refrigerator. A measurement and control system based on Siemens PLC S7-300 for this helium liquefier are developed. Proper sensors are selected. All the collected data to be processed are transformed and transmitted to S7-300 CPU. The upper computer HMI (Human Machine Interface) is compiled using Siemens configuration software Wincc V7.0. A specific control logic for this helium liquefier is developed. The control of the suction and discharge pressures of the compressor and the control of the turbo-expanders loop are discussed in this paper. After several times’ commissioning, the outlet temperature of the second stage turbine has reached to 8.6K and the temperature before the throttle valve has reached to 13.1K.

Brief introduction to the helium liquefier

The process flow diagram of the helium liquefier is shown in Fig. 1. The helium liquefier consists of three main parts, i.e. the compressor station, the liquefaction part and the purifier. The compressor station is composed of a screw compressor, a helium buffer tank and a gas management valve panel composed of CV-1, CV-2 and CV-3. The liquefaction part contains six heat exchangers which are located in a vacuum insulated cold box, two turbo-expanders and a liquid helium dewar. The purifier contains three heat exchangers which are also located in that vacuum insulated cold box and a dirty gas helium tank. The designed liquefied rate of helium is 40litre per hour. The compressor adopted is KAESER CSD162 with a power of 90kW and a helium gas mass flow rate of 25g/s. The dirty helium gas is less than or equal to 1.5g/s@27bar.

Fig. 1. Flow scheme of the helium liquefier

Overview of sensors and actuators in the helium liquefier

Proper sensors are selected to measure such parameters as temperature, pressure and rotary speed of the turbines.

Temperature sensors and transmitters

Approximately 24 temperature sensors including redundant ones in the helium liquefier have been used at different locations. There are two types of temperature sensors, Rhodium-iron resistance thermometers and Platinum (PT-100) temperature sensors. All of these sensors are calibrated by the Center of Cryogenic Metrology. Each Rhodium-iron thermometer has been calibrated from 1.2K to 300K. The calibrated accuracy is 0.1K. The calibrated accuracy of PT-100 from 77K to 323K is ±0.1K. Each temperature sensor has a specific data table.

As J. Li et al. (2014) [1] indicate that different sensors should be used for different requirements. Six Rhodium-iron thermometers have been used for monitoring the important locations’ temperatures. As Rhodium-iron thermometers have non-linear resistance-temperature characteristics, the measured resistance is converted into temperature by performing interpolation on calibration data. The locations where these six Rhodium-iron thermometers are being used are: inlet of the first stage turbo-expander, outlet of the first stage turbo-expander, inlet of the second stage turbo-expander, outlet of the second stage turbo-expander (two sensors, have one redundancy) and outlet of the second stage heat exchanger (dirty helium gas side of the inner purifier).

The model 211S single channel temperature transmitters with 230 VAC supply from Lakeshore Inc. have been used to operate with these six Rhodium-iron thermometer. Output of the model 211S is 4~20 mA signal which is transmitted to the SIEMENS PLC analog input model SM331.

Fifteen Rhodium-iron thermometers have been used to monitor the other unimportant temperatures which are also in the liquid helium temperature range. Two model 218S eight channels temperature monitors from Lakeshore Inc. have been used to operate with these fifteen Rhodium-iron thermometers. The PLC communicates with these two 218S monitors by freeport communication protocol defined by Lakeshore Inc.

Three Platinum (PT-100) temperature sensors are used to monitor temperatures of the other locations. RTD model SM331 of the SIEMENS PLC analog input is used to operate with the Platinum (PT-100) temperature sensors. SM331 is an eight channel analog input model, through which all the temperature signals are input separately. The resolution of the SM331 RTD model is 15 bits.

The measurement for all of these temperature sensors (Rhodium-iron thermometers and PT-100) in transmitter is the four wire technique.

All the sensors are mounted on the cold surfaces using a specially designed copper housing block, glued with low temperature varnish to achieve good thermal contact with surface, and covered with multi-layer super insulation (MLI) to avoid direct radiation heat load on the sensors. Thermal anchoring of the sensor wires has been done on the surfaces to minimize error due to heat flow from the leads to the sensor.

The temperature sensors and transmitters summarized in Table 1 are distributed on the cold surfaces in the helium liquefier.

Table 1. Temperature sensors and transmitters used in the helium liquefier

Type	Quantity	Range covered	Adopted temperature transmitter
Rhodium-iron	6	1.2K to 300K	Model 211S temperature transmitter from Lakeshore
Rhodium-iron	15	1.2K to 300K	Model 218S temperature monitor from Lakeshore
PT-100	3	77K to 323K	RTD analog input model SM331 from SIEMENS PLC

Pressure transmitters and the other sensors/actuators

The pressures are measured by ColliHigh TM JYB-KO pressure transmitters with 0.5% full scale accuracy.

The rotation speed of turbo-expanders is measured by two in-house designed tachometers. The measuring range is 0~360000 RPM. The accuracy is better than ±0.01%.

Six sets of cryogenic valves from WEKA AG. are adopted as actuators.

Measurement and control system based on SIEMENS PLC S7-300

CPU315-2PN/DP is as the master station and three sets of ET200M are as the slave stations. Profibus-DP field communication is established between master station and slave stations. The upper computer communicates with PLC by means of industrial Ethernet. Centralized monitoring and distributed control is achieved. The control structure is shown in Fig. 2. The slave station of Compressor is setup by the KAESER Ltd, compressor producer. The in-house designed measurement and control system includes the upper computer, the master station and the slave station of free port protocol 218S collection system, the slave station of gas management panel and the slave station of cold box.

Fig. 2. The control structure of helium liquefier

The HMI (Human Machine Interface) is compiled using SIEMENS software WinCC V7.0. Fig. 3 shows the HMI of the cold box. The WINCC HMI system has many functions, such as graphic pages, monitoring, adjusting the process parameters, controlling, real time and historical trending, alarms and events.

Fig. 3. The HMI (Human Machine Interface) of the cold box

Control strategy and control logic

According to the different operating modes of the helium liquefier, the control strategy and control logic are developed. The PLC control program is developed according to these control strategy and control logic. The control program is mainly composed of sequential control and control loops. Most of the control loops can be implemented by means of a standard PID (Proportional, Integral and Derivative) controller.

The control logic of compressor station to control the suction and discharge pressures will be discussed. The control of the turbine rotary speed is also mentioned.

Control strategy for the compressor inlet and outlet pressures

Although nowadays there are many new control logics to control the compressor station for the large-scale refrigerator, for example, a model based multivariable controller is proposed by Francois Bonne et al. (2014) [2], the traditional control logic is still being adopted to control the suction and discharge pressures of the compressor station. As show in Fig. 4, the suction pressure of compressor Pin is controlled by bypass valve CV-1. The discharge pressure of the compressor Pout is controlled by the discharge valve CV-2 and the charge valve CV-3.

Fig. 4. Suction and discharge pressure control

Control strategy of the turbo-expander loop

Through controlling the inlet valve of the first stage turbo-expander to control these two turbo-expanders’ rotating speeds.

The startup and stop period are the key periods to control the turbines. In the startup period, the inlet valve of the first stage turbine opens linearly with a speed of 0.2% or 0.5% per second gradually. After the rotary speed reach to a value, the turbines will be allowed to stabilize for two minutes until the rotary speed reach to working rotary speed set point, then the inlet valve of the first stage turbine will be shifted to speed controlling mode (automatic mode).

Commissioning results of the helium liquefier

After several times’ commissioning, some results have been achieved.

Control results of the compressor suction and discharge pressures

The Fig. 5 shows the suction pressure and the discharge pressure of the compressor. The suction pressure is being kept steady. In the commissioning process, the discharge pressure of the compressor has been increased periodically.

Fig. 5. (a) suction pressure of the compressor; (b) discharge pressure of the compressor

Cool-down for the helium liquefier

The Fig. 6 shows the outlet temperature of the second stage turbine T1 and the temperature before J-T valve T2.

Fig. 6. (a) the outlet temperature of the second stage turbine; (b) the temperature before the throttle valve

Conclusion

A 40L/h helium liquefier is established at the Technical Institute of Physics and Chemistry, CAS. A measurement and control system based on Siemens PLC S7-300 for this Helium Liquefier is developed. This helium liquefier is in the process at the manual commissioning. The newest achievement is the outlet temperature of the second stage turbine has reached to 8.6K and the temperature before the throttle valve has reached to 13.1K. It has a long way to go.

This helium liquefier will be commissioned continuously, during this process, the measurement and control system, the control logic and control strategy will be verified and updated.

Acknowledgements

This work is supported by the Key Laboratory of Cryogenics, TIPC, CAS (CRYOQN201307).

References

[1] J. Li, L. Y. Xiong, N. Peng, B. Dong, P. Wang, and L. Q. Liu, 2014. Measurement and Control System for Cryogenic Helium

Gas Bearing Turbo-expander Experimental Platform Based on Siemens PLC S7-300, Advances in Cryogenic Engineering, 2013 Cryogenic Engineering Conference (CEC) and International Cryogenic Materials Conference (ICMC). Alaska, USA, pp. 1743

[2] Francois Bonne, Mazen Alamir, Patrick Bonnay and Benjamin Bradu, 2014. Model based multivariable controller for large scale compression stations. design and experimental validation on the LHC 18KW cryorefrigerator, Advances in Cryogenic Engineering, 2013 Cryogenic Engineering Conference (CEC) and International Cryogenic Materials Conference (ICMC). Alaska, USA, pp. 1610

* Corresponding author. Tel.:+86-10-6255 4669; fax: +86-10-6262 9548.

E-mail address: lqliu@mail.ipc.ac.cn