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
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.