1. Measurement and control system for liquid flow through hydraulic turbomachine
Waldemar Janicki, Adam Adamkowski
THE SZEWALSKI INSTITUTE OF FLUID-FLOW MACHINERY POLISH ACADEMY OF SCIENCES

2. Contents Genesis of the problem
Object of control and requirements for methods of the valve control
Measurement and control system
Results
Application of the constructed measurement and control system
Conclusions

3. Genesis of the problem
Water hammer is a pressure surge or wave in pipelines caused when a flow velocity changes rapidly. The water hammer commonly occurs when a valve closes suddenly what causes a pressure wave propagates along a pipeline. This phenomenon leads to the material stress increase of flow system elements (pipelines, hydraulic machines and devices) reducing their durability and reliability and in extreme cases causing serious failures.

4. Genesis of the problem – test stand
Water hammer is caused by the flow shut-off valve.
The task of a bypass valve is to mitigate the water hammer by appropriate control of the valve opening.
Cycle of a bypass valve operations:
• it is totally closed during the normal operation of the hydraulic machine;
• during the machine stopping the bypass valve is being rapidly opened causing the slowdown in the flow rate change in conduits;
• it is next followed by a very slow closure of the valve in order to avoid significant pressure change in the system.

5. Object of control and requirements for methods of valve control
Shut-off valve
Bypass valve
nominal diameter
65 mm
32 mm
maximum torque, experimentally determined
25 Nm
6 Nm
angle of rotation for closing element
90
estimated mass moment of inertia
 1.5∙10-4 kg∙m2
 0.5∙10-4 kg∙m2
nominal pressure
16 MPa

6. Object of control and requirements for methods of valve control
Tz – opening/closing time, To – time of wave cycle back and forth along the pipeline, Tw – time constant of a liquid inertia flowing in the pipe,
li – length of i-th section of a pipeline, ai – propagation speed of pressure wave in i-th pipe section, Ai – flow cross-sectional area in i-th pipe section, g – acceleration due to gravity,
Ho – nominal value of delivery head of centrifugal pump or the head of hydraulic turbine,
Qo - nominal flow rate of centrifugal pump or hydraulic turbine.

7. Measurement and control system
u - control voltage; umax - maximum control voltage; i - gear motor drive; φn - position of the valve at n point (in degrees); φn-1 - position of the valve at n-1 point (in degrees); nmax - maximum rotational speed of a motor (in rpm); tn - time at n point; tn-1 - time at n-1 point.
Gear motor of a shut-off valve
Gear motor of a bypass valve
Rated rotational speed of a motor
900 rpm
1380 rpm
Gear ratio 15 20
Rated torque on a gear shaft
71 Nm
18 Nm

8. Results – linear functions – shut-off valve
Wj - index of control quality;
φiz - given angular position of rotary valve element at time „i”;
φi - actual angular position of rotary valve element at time „i”1;
n - time period [ms], which starts at t = 0 and ends when the rotary valve element is being stopped. Angular position is a non-dimensional quantity related to the angle of rotation equal to 90o.

9. Results – linear functions – bypass valve
Wj - index of control quality;
φiz - given angular position of rotary valve element at time „i”;
φi - actual angular position of rotary valve element at time „i”1;
n - time period [ms], which starts at t = 0 and ends when the rotary valve element is being stopped Angular position is a non-dimensional quantity related to the angle of rotation equal to 90o.

10. Results – two-stage functions
Shut-off valve
Bypass valve
Tz = 0.4 s (060)/0.2s, (6090)/0.2s;
Tz = 1.7 s
(060)/0.2s, (6090)/1.5s;
Tz = 2 s
(060)/0.5s, (6090)/1.5s

11. Results – nonlinear functions
a = -360, b =360
a = -90, b =180
a = -22.5, b =90
a = -3.6, b =36

12. Application of the constructed measurement and control system
Example of experimental studies relating to the influence of bypass flow on the water hammer course.
Research on the water hammer effect was conducted taking into account the impact of the following factors:
a flow cross-sectional area of a bypass valve,
a start time of the bypass valve opening relative to the start of the transient stage of a machine,
bypass valve opening and closing speeds.

13. Conclusions
The control system of a test rig for investigating the influence of a bypass liquid flow on the water hammer course was designed, built and tested. The system enables to control the valve operation in accordance with the defined time-dependent functions for the change in opening degree, including linear, two-stage and nonlinear functions.
The constructed system was tested during the interval Tz between 0.2 s and 5 s. Time Tz = 0.2 s appeared to be too short to achieve the setpoint change in the positions of valves. Within the range 0.25÷5 s the drives do not lead to the deviations between the actual and predicted valve movements and therefore make the operation of these systems on the stand for water hammer experiments still possible. Hence, the minimum closing and opening time of the valves is twice smaller than the requirements for the test rig.
Constructed measurement and control system was used in experimental tests of water hammer phenomenon. The properties of the system allowed performing the studies in the whole assumed range and consequently allowed demonstrating the suitability of the bypass flow of a rotary machine to mitigate the water hammer effect.
In case of the need to use the developed control system on a real object, some adaptation aiming at replacing the open system with the closed-loop system should be made.