Q. What is the difference
between direct acting valves and servo assisted valves?A: Direct acting valves
basically consist of a small piece of magnetic stainless steel with a rubber seal on the
end that moves within an electro magnet. As the seal uncovers a port (the orifice) the
valve opens. When the electro magnet is de-energised a return spring closes the valve.
With a servo valve (also called a pilot operated valve) the operation of a small
direct-acting solenoid valve controls the movement of either a flexing diaphragm or a
piston which is moved by the pressure of the liquid or gas that is flowing through the
valve. This diaphragm or piston uncovers a much larger orifice (up to 50mm in diameter)
and allows liquid or gas to flow through the valve. The use of this flexing
diaphragm (or moving piston) allows a small solenoid coil to control a much larger valve
orifice than would be possible if it were directly openeing and closing the valve.
Q. Where does an assisted lift valve fit in?
Modern servo valves are very efficient but even the best require a pressure
differential of 0.15 bar across the valve to hold the diaphragm fully open, whereas a
direct acting valve will operate with no differential pressure.
An assisted lift valve combines the two techniques by attaching a diaphragm to the
armature of the solenoid with a spring, when the solenoid armature moves within the coil
the spring lifts the diaphragm, uncovering the orifice and operating the valve, even with
zero differential pressure.
Q. What are the advantages of direct acting and servo valves?
- Direct acting valves tend to be smaller and faster. Servo valves come into their own at
larger sizes. Except for those that operate at very low pressure, valves with an orifice
size of greater than about 10 mm tend to be servo assisted. Above this size the cost and
power consumption of a direct acting solenoid coil tends to be prohibitive.
Direct acting valves tend to be less troubled by small amounts of dirt in the media,
whereas servo valves rely on the use of small openings within the valve which can be come
blocked if an adequate filter screen isn’t fitted.
Q. What does "hit and drop" mean?
- Hit and drop is a simple technique for reducing the power consumption of solenoids
valves. Because they require a larger voltage to open than to hold them open. It is
possible to reduce the applied voltage by using a relay or transistor to switch a resistor
in series with the coil.
Q. What is a shading ring?
- Most solenoid valves incorporate a return spring so that when power is removed the valve
reverts to its closed state. When a solenoid valve is operated on an AC supply the applied
voltage rises and falls 50 or 60 times a second, this would cause the valve to make a loud
buzzing noise as the return spring tries to close the valve as the voltage passes through
zero volts. Fitting a shading ring into the end of the armature tube induces a second
voltage in the coil that is out of phase with the main supply voltage. This means that the
magnetic field produced by the solenoid coil never disappears completely, the armature is
held in the open position, and the valve operates much more quietly.
Q. Can I use a solenoid valve on salt water?
- Generally standard solenoid valves are not considered suitable for use on salt water
because the iron content of the armature will corrode. Additionally most solenoid
manufacturers are very cautious about the use of servo-assisted valves because of fears
that un-desolved salt will block the small holes in the diaphragm. Probably the most
suitable type of valve would be a direct-acting plastic bodied dry-armature valve where
all the metal parts (armature and spring) are isolated from the brine.
Q. Can I use John Guest fittings with Hep2O pipe work?
- John Guest fittings can be used with any quality piping system that conforms to BS7291,
this standard defines the allowable tolerances on the outside diameter of the pipe. As the
internal diameter may vary from manufacturer to manufacturer it is important that the
appropriate pipe insert from the pipe supplier is fitted.
Q. I need to keep a normally closed solenoid valve open for long periods, but I
want to minimise power consumption. I’m already using the smallest valve I can
for the application, what else can I do?
Consider using a technique called "hit and drop" – the
solenoid is hit with a high voltage (normally its nominal voltage), and then a series
resistor is switched into the circuit reducing the current consumption of the coil. The
switching can be accomplished by a relay or by a solid-state device.(see What does
"hit and drop" mean?).
