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Hi Frank, From my experiences, I would expect ball valves are more resistant to sand damage then you might think. I have used small brass and SS valves on my pressurized sandblasting pot, and where I used one at the end of the hose they ware out very quickly from very fast moving sand, when you open and close the valve. But the valve at the bottom of the sand pot that regulates the amount of sand that falls into the air stream last me a long time. I'm constantly adjusting that valve to get the right sand flow, and closing it completely and still get a good seal for a few years, and very large amounts of sand and copper slag flowing thru it. The delrin is pretty resistant to sand damage, it's the balls that start to erode first on my valves when they are used in a partly open state. Regards, Szybowski From: ShellyDalg@aol.com Date: Sun, 4 Jan 2009 12:56:44 -0500 Subject: Re: [PSUBS-MAILIST] Soft ballast venting valve To: personal_submersibles@psubs.org 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. |