Originally Posted by Farnsworth
Originally Posted by ZeeOSix
Originally Posted by Farnsworth
The smaller orifice requires more psid for the same flow rate than the large. Psid changes to maintain flow. Psid therefore is not the same for different size valves. Flow is the constant.
The PSID across the oil filter will be determined by the restriction of the media, the oil viscosity and the flow rate going through the media. The PSID you get across the filter and bypass valve is the same, and is based on those factors. Higher flow means higher PSID if other factors are held constant. Higher PSID with no change in flow or viscosity means more restriction. The PSID that the bypass valve sees will be the same on the bypass valve area, regardless of what size the valve is.
At what PSID the valve opens, and how far the valve opens with a given PSID is determined by the valve spring pre-load, spring stiffness (the spring constant) and the valve area that the PSID acts upon. The oil volume amount that flows through the bypass valve depends on how far it opens with a given PSID (covered by factors above), and how much exposed flow area there is when the valve moves the distance it does with the given PSID that exists across the filter.
Of course the psi on the valves is the same if psid is the same. The last factor is the total flow rate, not just through the media rate as you state. Say the valves are fully open, you mean to tell the larger opening flows the same as the smaller? The larger valve will relieve more pressure and thus the psid is lower in that filter than the filter with a smaller valve. A larger bypass will result in lower psid across it's whole range, and above it's range. Once the two valves start to open the psid on the systems start to diverge. A large valve may flow at 10 psid where a small needs 11 to equal the flow or something of that nature.
I never said a larger opening flows the same as a smaller one. The flow through the bypass valve is dependent on its area and how far the valve opens as a function of PSID (ie, the spring pre-load and stiffness). A smaller sized valve could flow just as much or even more than a larger valve depending how the springs are matched up to the valve size. Spring pre-load and spring constant is what will determine initial opening pressure and how far the valve moves with increasing PSID across it. You can't make any real general conclusions on how a bypass valve is going to flow by just looking at it. They would have to be mathematically modeled, or ran through an actual PSID test procedure to verify opening pressure and flow through the valve as a function of PSID across it.
Bottom line is if 10 differently sized relief valves were all correctly designed to open at 12 PSI, then they will all start to bypass at 12 PSI. But as the PSID continues to go up, some valves may not open as far due to spring stiffness. So depending on the combination of the linear movement vs PSID curve and the size of the valve, it's entirely possible that a smaller sized valve can open farther and flow the same or more with the same increase in PSID than a larger valve having a much stiffer spring.
Anyway, I need to get off this and get ready to watch some of those old 70s shows ... "goodtimes" re-runs are always fun to watch.
