Today's highly automated industrial cleaning equipment relies heavily on valves remotely actuated either by an operator or by an automated control system. Although performing basically the same function as manually operated valves, automated valves operated by electricity, air, hydraulics or other means are considerably different than manual valves not only in their construction but their mode of operation as well. Specifying an automated valve for a particular application often challenges the systems engineer to take several things into consideration that wouldn't be of consequence if he were specifying a manual valve. Previous blogs have discussed the construction and operational details of various kinds of manually operated valves at some length (search "valves"). It would seem simple to automate any of the manual valves discussed in previous blogs. All one would need to do is add a motor or some other type of actuator to replace the manual operator. In fact, this was probably how most early automated valves were constructed. It's easy to imagine a motor with an appropriate speed reduction device operating the shaft of a globe valve. There are, however, complications like somehow knowing when sufficient rotation has been achieved to either open or close the valve which could be a very complex controls issue. A ball valve, requiring only 1/4 turn might be a bit simpler but changing the ball valve to a piston valve requiring only a simple linear motion is a very attractive option and some automated valves are designed in just that way. Solenoids are simple and do a nice job of providing linear motion. A solenoid consists of a coil of wire wrapped around a form or bobbin within which a metal slug is able to move in one direction or the other on a linear axis as the current in the coil is energized or de-energized. In most cases, the return of the moving slug of metal to the starting position is accomplished by a spring although if the moving slug is a magnet, supplying a DC current to the coil with one polarity will move the slug in a pre-determined direction while energizing the coil with the reverse polarity will move the slug in the opposite direction. Most solenoid operated valves use the former of the two systems with the actuation providing motion in one direction and some other force supplying the return motion. The plot thickens when one realizes that although the motion of a solenoid can provide a considerable force in one direction, the force of the return stroke is at the mercy of the spring or whatever force is present to encourage it. This fact is heavily considered in the design of automated valves. The simple illustration that follows will hopefully provide an "Ahhhhhhhh" moment.
Automated valves often use the incoming pressure to hold the valve closed.In most cases, automated valves rely on the pressure of the liquid or gas arriving at the inlet side of the valve to hold them securely closed. This, of course, requires no effort supplied by the solenoid - convenient since the majority of valves are normally closed anyway. When flow is required, the solenoid moves the plunger away from the orifice to allow flow which continues as long as the plunger is held away from the seat. To stop the flow, the solenoid is de-energized allowing the plunger (with a little help from a spring) to re-seat itself and stop flow through the valve. It's immediately obvious that the arrangement described above is the reverse of the situation in the typical manual globe valve where pressure from the actuator (a screw) prevents flow. In the case of the globe valve, the pressure must be constantly supplied until flow is desired. The preceding is a pretty simple introduction to the world of automated valves. Upcoming blogs will discuss the impact that some of the seemingly simple differences between manual and automated valves have on specifying an automated valve for a specific application.
- FJF -