Please explain "Parallel pressure reducing valves"

I found this at http://www.plumbinghelp.ca/pressure_reducing_valve.php
There's an image showing parallel pressure reducing valves along with this text:
"Installing pressure reducing valves in parallel offers the ability to service a valve while continuing to provide water to the occupants. Parallel PRV's are also recommended for buildings with a wide variance of water demand as each PRV can operate at a different pressure setting to provide better performance at peak flow."
OK, how would that work in the case of "a wide variance of water demand"? With 2 different settings, what would be the final output pressure and how would that compensate for "peak flow"?
I don't think they expect manual switching of PRV's/
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DerbyDad03 wrote:

I suspect the 2nd PRV is set at a lower pressure - if demand is so high that the pressure in the final pipework (after the PRVs) drops below the setting of the 2nd PRV - then the 2nd PRV will also pass water in addition to the 1st PRV.
When I fitted a 22mm PRV to drop my supply (7.5 bar) to a more reasonable 4 bar, I did some measurements with a short stub of 22mm copper pipe directly on the PRV output.
The PRV is fed by 2m of 22mm (ish) bore MDPE joined to 1/2" alkathene which is about 11m length back to the main pipe in the road.
No matter where I set the PRV pressure from about 2 bar to 6 bar, the max flow remained at about 55 litres/minute (which is considered good in England[1])
[1] compared to an 80 year old scaled up bit of squashed lead pipe.
Obviously, the pressure setting will affect the flow through subsequant restricted pipework - but the PRV itself did not seem to impede flow significantly.
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But it still leaves the question as to why you'd want the valves to operate at different pressures? If all they are doing is providing more capacity, then why not set them both to the same pressure? And why not just install the right single valve that can handle the flow?
I buy the maintenance issue, but don't see what they are talking about here. Also note that citation is not from a manufacturer of valves, it's from some internet plumbing website.
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snipped-for-privacy@optonline.net wrote:

Oh, I agree with you 100%. I was following though trying to justify their position, and whilst it might have some tenuous merit, it does seem to come down to "why?".
My story of my own PRV suggested that a decent quality unit did not impede flow significantly anyway - so I cannot really see what the original text is getting at.
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Derby Dad:
PRV's are often a major bottle neck because not only does the water have to travel a tortuous path through the PRV, and the PRV will typically have a brass or stainless filtration screen (that can gradually clog), but the water has to lift a spring loaded diaphragm in order to get through the PRV, and that's the key here. So, there's a lot of flow resistance through a PRV.
Now, imagine you own a hotel where between 7:30 and 8:30 in the morning, all of the conventioneers staying in the 400 rooms of your hotel are all wanting to take showers and baths and flush toilets at much the same time before they head downstairs for breakfast. In that case, for 23 hours out of a 24 hour day, a single PRV is sufficient to supply the demand. But, it's during that one stinking hour every morning that you need more flow, and a single PRV won't do. So, one way to solve that problem would be to install a second PRV which would allow more flow because of the extra path. Same would apply to a car wash where you can have anywhere from 0 to 50 people (say) that are each gonna want full water pressure in their wand all at the same time. 25 days out of the month a single PRV is sufficient. But, the day after a rain, then everyone and their dog wants to wash their car.
That statement in the web site you linked to saying:
"Parallel PRV's are also recommended for buildings with a wide variance of water demand as each PRV can operate at a different pressure setting to provide better performance at peak flow."
doesn't make any sense to me.
A PRV works by allowing water pressure and the force of a spring to act on a diaphragm. For the PRV to close, the water pressure acting on the upstream side of the diaphragm has to equal the combined water pressure plus the force of the spring acting on the other side of the diaphragm. You adjust the downstream pressure at which the PRV closes by changing the spring pressure being applied. The lower the applied spring pressure, the higher the downstream pressure at which the PRV closes, or, the lower the applied spring pressure, the higher the downstream pressure being maintained will be.
To get greater flow through the PRV, you need the diaphragm to open widest, and that means having the least spring pressure being applied, which means that the PRV will be maintaining the highest downstream pressure.
But, that also means that the pressure in your building's water supply piping is going to be maintained at that higher pressure by the second PRV, and the pressure setting of the first PRV is irrelevant.
I can't help but suspect that whomever wrote that explanation was groping for an explanation himself and really didn't expect that anyone out there would bother to think through the one he came up with. But, I have to admit that I don't know enough about PRV's to argue with him or prove him wrong. It just don't make no sense to my way of thinking.
If it wuz me, I would assume both PRV's would be set to the same pressure, and that the second PRV simply provides additional flow capacity by providing an additional path through which water can flow, and leave it at that.
--
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On 11/28/2012 1:39 PM, DerbyDad03 wrote:

As shown, it wouldn't do a thing useful...there's the same flow restriction at downstream anyway.
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On Wed, 28 Nov 2012 11:39:45 -0800 (PST), DerbyDad03

These valves have design limits on pressure depending on flow rates. Under "normal" (design limit) conditions the "controlling" valve provides its set pressure. If flow is too high for the controlling valve, pressure drops. The second valve is set al at a lower pressure. It then opens to provide addition flow/pressure. So the final pressure will be lower. But let's say both valves can handle 50 gpm at 40 psi output pressure.. Demand goes to 75 gpm occasionally. With one valve pressure drops to 20 psi. With a second valve set to open at 38 psi, 38 psi is maintained. All numbers pulled out of the air, but that's the concept.
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Why wouldn't you just set both valves to the same pressure? That's the goal, isn't it? To maintain pressure at say 40 psi. Then just set them both to 40.
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On Wed, 28 Nov 2012 15:31:56 -0800 (PST), " snipped-for-privacy@optonline.net"

Something about how they operate when too close in output setting - maybe valve chatter. I forgot most of what I knew as a boilerman, and never was a reducing valve expert anyway.
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That might work, or it might cause an oscillation between the units if they can't decide who is the master? I thought about using two valves, two lines, but I would need two hot water tanks.
Greg
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On 11/28/2012 1:39 PM, DerbyDad03 wrote: ...

OK, here's a link that actually does make some sense--NB that the individual valves/lines are smaller capacity than the main line unlike that shown in the previous. This _can_ make some difference and be of use; imo the other has no useful purpose/can't work to any advantage other than the redundancy.
<http://www.watts.com/pages/learnabout/reducingvalves.asp#parallel
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