Over the last 25 years, I’ve sat through dozens of training presentations on solenoid valves. Every time the topic turns to minimum pressure differential, the presenter starts drawing hieroglyphics on the chalkboard with squeaky chalk. If you don’t know what a chalkboard is, just Google it. Then, pointing at his third grade level “drawing”, he starts explaining about diaphragms, pilot holes and all other non-sensicle (I know non-sensicle is not a word) stuff. The room responds with blank stares and the occasional snore. Topic covered and time to move on…
There are basically two types of designs in solenoid valves. Direct Acting and Internally Piloted.
Direct Acting
The simplest to understand is Direct Acting. When a Direct Acting valve operates, the coil directly acts upon the core of the valve and directly opens or closes the orifice in the valve body. This valve does not need a minimum differential pressure across the valve body in order to operate. In other words, the upstream line pressure and the downstream line pressure can be the same, and the valve will work – Zero Minimum Differential Pressure. As the orifice size increases, and the line pressure increases, the amount of force to move the valve core also increases. Eventually the coil size and the power requirements cause you to look for another solution, the Internally Piloted valve.
Internally Piloted
There’s a little more to the Internally Piloted valve, but it’s not rocket science. Internally Piloted valves use line pressure and a diaphragm to assist the operation of the valve. By using a small bleed hole in the diaphragm, the valve core acts upon the small bleed hole, and uses the line pressure to operate the valve. These valves typically require a Minimum Pressure Differential of 5 psi. In other words, the upstream line pressure has to be at least 5 psi greater than the downstream line pressure for this valve to operate. Don’t know what your line pressures are? Install some pressure gauges. We have them!
A word of caution about Internally Piloted Valves: They work great on clean fluids. It’s a good practice to always install strainers in from of them to keep the pilot bleed hole from getting plugged. For dirty fluids, our piston valve would be a better solution.
Being a visual learner myself, I found a nice explanation on how a washing machine solenoid works, at least in the UK. Check it out http://www.youtube.com/watch?v=e5434dDB-7w