In our case the flow rate and the orafice size are fixed
H-G
The flow isn't fixed - it depends on engine rpm.
The main issue is that the engines we are dealing with originally have all the pressurised oil flowing through drillings in a metal casting - so no problems there, the design is sound.
But we are directing it out of the metal and down several feet of rubber hose.
When folk build bigger engines that benefit more from fuil flow filtering they often fit larger than stock oil pumps, or at least blueprint a stock size pump. (higher flow rates)
Those tuned engines also often have the capability to rev higher than stock. (Higher flow rates).
On a cold winter's morning the oil is like treacle and therefore if you give it a few too many revs, as many folk have found out to their cost - the contents of the sump can be blasted out of a blown hose or filter all over your driveway on a frosty morning.
I have been witness to 1 hose blowing off it's barbed fitting, one hose splitting under pressure, and one oil filter rubber gasket being forced out. (at least 2 of those engines had 30mm pumps - not sure about the other) - and I have heard of several other cases.
So it does happen in practice. Many folk will get away with it though because -
1 - Many of our cars are now sunny day only drivers and don't go out in the freezing cold as often as they used to. (especially the tuned ones)
2 - If you can afford it then using aircraft spec hose with proper swaged ends and special high pressure filter cans will help to keep the oil in.
3 - Most drivers, especially those who have had a hand in their own engine build will know not to give it too many beans until the engine has warmed up a bit.
However, your foot can sometimes slip or you may fluff a gearchange.
Most respected engine builders (not parts salesmen) will tell you that a 26mm pump is plenty big enough for most engines, but the ads in the magazines will tell you that any high power motor
must have a 30mm pump.
I know who I trust.
The stock pressure valve (s) in the engine case are there to protect the stock oil cooler from being blown by the high pressure from cold oil, and they do a good job, but they don't protect what goes before - i.e. an aftermarket external oil filter (and maybe cooler and 'stat as well)
Many (but not all) oil filter cans have an internal bypass in case the filter becomes blocked.
Personally, I would NOT have a pressure relief valve on the pump cover as this could, at worst, effectively stop any flow leaving the pump and at best reduce the amount of oil entering the oil-system (and therefore starving the engine of any oil) - I would prefer everything that the pump has to offer to enter the oil system (remember flow is one of the variables) and for it to be controlled within the system - this is why it's important to match the oil pump outputs to the engine.
In any well designed engine that has vulnerable components such as oil filters or oil coolers within it - you need some sort of pressure control. This is normally a spring loaded plunger (the stock VW ones) or sprung ball valve (as in a pump pressure cover plate). If you get any bits of crap in the oil it is indeed possible that they may jam the valves open resulting in a loss of pressure.
This is why it is important to keep the filter mesh screen in place on the pickup pipe, and also to keep an eye on the oil pressure gauge so you know if an abnormal situation occurs.
The most important place for the oil pressure to be maintained is at the crank bearings. If any pressure control valves fail then, yes your main bearings will get less oil than they should. This will be the case whether it happens to the stock valves or the one on the pump cover. All the valves come prior to the oil having to do any lubricating duties. - so it's an unavoidable (if pretty minor) risk anyway.
Dave.