Continuing on the subject of plumbing, I got this from one of our members:
"OK, I've done the research and read the results, and googled, but am still confused about "power beyond" and it's relevance when trying to hook up a second directional control valve."
"I've found nothing that explains exactly why it is needed and what it really does."
"I'm talking about simple three position valves - up, neutral/hold, down - which have pressure in, tank out, and working ports for a double acting cylinder."
"In an open center system, I would simply put the second valve in series with the first one - the return line of the first feeds the pressure line of the second, and the return line of the second goes to tank."
"This seems to be such a simple arrangement - and power beyond seems to be a such a very big deal - that I have obviously missed some important point."
Yes he has. Connecting the tank port of the first valve to the pressure port the second will do two things:
When the first valve is used at the same time as the second, there is no way for the return oil from the first valve to get back to tank; and when the second valve is used, the tank/return gallery in the first valve will be pressurized and blow the spool seals.
Power beyond is a facility in the valve where a plug is inserted between the pressure and return galleries.
Pump flow can then be "carried over" to another valve downstream - so when that valve is used, it pressurizes the pressure gallery in the upstream valve but NOT the return gallery.

By Brendan Casey.