Please explain "Parallel pressure reducing valves"

[DerbyDad03]

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/

[Tim Watts]

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.


--
Tim Watts Personal Blog: http://www.dionic.net/tim/

"History will be kind to me for I intend to write it."

[tra...@optonline.net]

On Nov 28, 3:04 pm, Tim Watts <tw+use...@dionic.net> wrote:
> DerbyDad03 wrote:

> > I found this athttp://www.plumbinghelp.ca/pressure_reducing_valve.php

> "History will be kind to me for I intend to write it."- Hide quoted text -
>
> - Show quoted text -

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.

[dpb]

As shown, it wouldn't do a thing useful...there's the same flow
restriction at downstream anyway.

--

[Tim Watts]

tra...@optonline.net wrote:

> 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.

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.

--
Tim Watts Personal Blog: http://www.dionic.net/tim/

"A fanatic is one who can't change his mind and won't change the subject."

[Vic Smith]

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.

[tra...@optonline.net]

On Nov 28, 5:27 pm, Vic Smith <thismailautodele...@comcast.net> wrote:
> On Wed, 28 Nov 2012 11:39:45 -0800 (PST), DerbyDad03
>
>
>
>
>

> <teamarr...@eznet.net> wrote:
> >I found this athttp://www.plumbinghelp.ca/pressure_reducing_valve.php

> All numbers pulled out of the air, but that's the concept.- Hide quoted text -

>
> - Show quoted text -

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.

[Vic Smith]

On Wed, 28 Nov 2012 15:31:56 -0800 (PST), "tra...@optonline.net"
<tra...@optonline.net> wrote:


>
>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.

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.

[gregz]

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

[nestork]

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.




--
nestork

[dpb]

On 11/28/2012 1:39 PM, DerbyDad03 wrote:

...

> 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"?

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>

--