A moving story about VSAs
Ever since they were invented, control valves have been on the move - and they are still moving today.As an integral and essential element of the water industry, from drinking water processing to WwTW, the issue of managing the performance of these valves has been the driving force behind the development of new generation actuators.
With integral electronic frequency converters replacing the traditional solution of motor plus reduction gear, variable speed actuators introduced in the late 1990s provided many advantages for bespoke applications.
A key area that the VSAs addressed was the issue of soft-starting or "caressing".
The reason that the valve needs caressing is best explained by drawing a parallel with the motion of a lift. To avoid impact on both the lift, and the people using it, a soft start is required. It is equally important that the lift does not stop dead in its tracks when it reaches its destination. The lift should ease gently into its journey, gradually gather momentum and slow to a stop at its selected end point.
The science is the same when applied to water: flow needs to be managed, as anyone knows if their children have pinched the end of a garden hose and released it in your direction when water is running through it at full power.
Pressure peaks associated with water hammer have been known to rupture pipes and valves. WTWs have to resist major differential pressure of this nature, which can cause cavitation and water hammer to spread across the entire plant. The damaging impact that this causes can be mitigated by controlling the operation speed of the flow restrictor with a variable speed actuator that provides the necessary soft-start technology.
And, with a modular design with components contained within the actuator, the integral VSA is a neat, robust solution. It can operate in the challenging water sector environment, which is dirty, wet and can frequently involve vibration and extremes of temperature.
The VSA's integrated frequency converter ensures that motor speed is automatically reduced in the end positions. Therefore, there are no magnification torques if the valve is blocked between the end positions. The voltage for each of the many available speed/cut-off torque combinations is pre-selected so that the cut-off torque setting corresponds to the stalling torque of the motor.
In other words, if a VSA is used, the usual massive current peaks experienced when the motor starts are eliminated, and even an unscheduled indeterminate stop does not result in torque damage. As a result, the VSA provides a solution that offers definite dimensioning advantages for UPS systems - the valve moves into the end position softly and safely at low speed, without any magnification torque (see graph below).
A further design feature of the VSA is its ability to operate the actuator at different speeds in different sections of the valve stroke - this bypasses critical positions within the process without stimulation by resonances. It also maintains the process variables, such as pressure, temperature and flow at a fixed level or a level that linearises the valve characteristics. What can only be achieved via switching on/off cycles (stepping mode) for fixed speed actuators, and with many small impulses with start up current spikes, is provided as a standard function with the variable speed actuator.
Open and shut
The VSA can also be programmed in such a way that the positioning speed of the valve in the close direction differs from the speed of the open direction. A typical application for the different speeds over travel functionality is for WwTW decanters with sequencing batch reactor procedures. The decant arm is lowered into the medium at low speed to avoid swirls in the activated sludge basin: after the water has run off, the decant arm lifts at a higher speed from the wastewater surface, returning to the initial position to allow fast filling of the basin for the next batch process.
Combining the integrated frequency converter with a control board in the VSA gave the flexibility and functionality to provide a solution to soft starting. The landmark initiative in the history of the actuator also enabled control of the valves in both an open or closed loop. It provided functional reliability, which could be monitored in the interests of efficient process operation, and ensured that a general high standard of performance and quality could be achieved and maintained. Additionally, actuator performance is extremely tolerant to power supply fluctuations. Even with a 20% voltage drop, speed and torque are not affected. For any remaining doubters, the alternative to VSAs with internal componentry is to fit an external frequency converter.
However, this route is not an aesthetically pleasing option and, more importantly, workers on site are required to programme and maintain highly complex converter software.
As with any technological breakthrough, we saw early innovators who embraced this evolution in actuation along with the more conservative customer, who chose to wait and review the benefits of the solution. Today, even the most cautious user has been persuaded that variable speed valves benefit from VSAs that meet the necessary technical requirements in an integral solution that, although the result of complex research and development, is both cost-effective and intuitive for the end user.
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