Hi Brian. The stainless ball valves are the most common valve used for
this, and although they do get harder to turn after a while, they are easy and
cheap to replace. There are basically two kinds of ball valves. The more
expensive type are "repairable" and have little bolts holding them together,
while the cheap ones are simply replaced.
With the repairable ones, a little adjustment is possible by loosening
the bolts slightly as the delrin seats age and the valve gets harder to turn.
You can lubricate the ball on both types to extend the life a bit, but
replacement is usually necessary after 3 to 5 years, depending on how often
the valve is turned and how many pressure cycles it has withstood.
Depending on where you take the sub, that term can be shortened by how
clean the water is, and of course what maintenance schedule you have. Salt
water with normal particulates will reduce the life span, while sand or grit
can ruin a valve very quickly.
The plumbing system should be designed for ease of maintenance, with high
quality "unions" located in the line so replacement of individual components
is possible.
Swagelock fittings and stainless tubes are good choices for the air
system but very pricey in the larger sizes so typically the ballast dump
system is hard piped.
Another choice is the use of "butterfly" valves located directly on the
tanks, driven by electric or hydraulic actuators. You could even have a
mechanical rod drive the valves, using the "block-V" seals where the linkage
rod passes through the hull.
The number of through hull fittings is the same with both methods, two
per tank.
I'll be using the hard pipe ball valve method because it's simple and
cheap. I will flush the system after each dive with clean fresh
water. But because I'll be diving the sub near shore, I expect to
get some grit and sand in there and will probably need to replace the
valves fairly often.
A good quality stainless ball valve at 1-1/2 inches runs about $150,
while a brass valve is about $25 to $50.
The ball and seats are the same, just the body is different. Pressure
ratings typically are 600 psi. Exotic seats and high pressure ratings ( 1500
psi ) are possible but not needed in my application.
Frank D.