Spool Valve Design
Spool Valve Design
1. Does increasing spool clearance increase silt build-up due to increased by-pass?
2. What effect does increasing spool clearance have on spool shifting force requirments?
3. Are there any books that address the details of design of spool valves?
4. Do non-stick coatings reduce silting?
We were having some intermentant valve failures early on in the project and could not pinpoint the cause (in hindsight it was most likely silting). The problems stopped when we put the system into integrated testing (different HPU). The problems occured again after the complete system was delivered for acceptance testing. The valves at this point had corrosion inside the sleeves. At this point several things were blamed for the valve failures. Bad proceedures that would allow air and thus corrosion into the system. Bad proceedures that allowed high temperatures to accelerate the corrosion. Deflections in the structure and improper installation torques that caused the valves to bind. The "quick" fix was to increase the spool clearance to make it more tolerant corrosion and deflections. During this fix we had a single valve fail again and made the determination that it was time dependent. This is were we learned the important silting lesson. By "polishing" the fluid we were able to extend time between failures. The valves have performed good for several months but are begining to have some problems. One valve was examined and corrosion was seen. Also part of the "quick" fix we "tweaked" the valves for flow and pull-in. We shortened the stroke and adjusted the spring to lower the required pull-in voltage. We also centered the spool around the ports to even the by-pass in each position.
The fluid we use is FDC 400 (a water soluble synthetic). The fluid is .5% water and is hydroscopic. The water content in the fluid varies and we sometimes see it as high as 3%. The water however is not free and does not cause any corrosion problems with other internal carbon steel parts. We do however see corrosion in the sleeve of the valve. The pitting is in the pressure area near the sealing lands. One assumption is that cavitation in this area is pulling the water out of solution and causing the corrosion. The cause of the corrosion is only academic. We plan on using CRES materials and coatings to avoid this.