I kind of agree about turning off the flow not the HPU but, for reasons of "safety" (and to pamper to the ill-informed and the risk-averse members of the board), some organisations do require the HPU to be turned off completely before connecting and disconnecting the QRC's. It's not nice for the pump or for the electric motor to be turned on and off too often. For the electric motor in particular there are real issues regarding the maximum permissible number of "starts per hour" (might be as low as 6 but it depends on if its a DOL, star-delta or soft start). At the end of the day though, the pump and the electric motor do not have a God given right to be treated with respect. If the end user has some sort of operational policy which shortens the service life of the equipment then that's just fine ... because it is his equipment to do with as he pleases.
I suppose the supplier of the HPU might want to say to his customer something along the lines of "If you are going to treat the equipment that badly then don't expect any warranty cover" but it's a tough economic climate and the supplier might just have to say nothing and hope for the best.
There's not many bells and whistles on this HPU, for example, I can't see a cooler, a dual level float switch, any over temperature thermostats, a filling QRC, a tank drain valve, a limit switch on the suction isolator etc. Would it be fair to say that the HPU has been built to a price because of budget constraints? That might be why there is no DCV.
Don't get me wrong, "Horses for courses" is what I say, but, speaking personally, I often have to shrug my shoulders and hand over a system design which is more "Flybe economy class" than "Singapore Airlines business class". Is it possible, however, that this system is just too basic and that it will cost the end user much more in the long run?
In the link below (if it works) is a scheme I came across once to give a simple co-ordination between the unloading relief valve and the pilot operated DCV. I'm not suggesting it for the solution here because it is more suitable for a fixed displacement pump (but, pob786, if you like it then feel free to adopt it). I've drawn the DCV symbol out with all it's detail because that allows you to see the unusual choice of pilot valve. Also note that the pilot supply to the DCV (port X) comes from the vent port of the relief valve.
When you energise either one of the solenoids just the bypass pressure is applied to the DCV main stage spool. That's not enough to get it to shift but the vent line is no longer connected to tank and the system pressure will now begin to rise. The spool shifts a bit (but not too fast because the pilot line flow is limited to a 3 bar drop across a 0.8 mm jet). As the spool shifts the pressure begins to rise, and as the pressure rises the spool shifts more, and so on ... until the DCV spool is fully shifted; and only then can the supply pressure rise to maximum.
When both the solenoids are off the T port of the DCV connects to tank with absolutely no flow going through that line. If you had QRC's on the connections to the actuator then, when the DCV is in neutral, there will be no pressure on either of the couplings, not even any back pressure from a flow to tank. This would allow the QRC's to be connected or disconnected easily and with minimal leakage. You might even be able to convince a Luddite that it would be quite safe to operate the system like this.
DOL