Water Supply Question (municipal water pressure)

[TeeDubya 3362]

I've got a FLO water valve by Moen installed on my water main coming into the house.  (Irrigation system branches out BEFORE the valve).  

This graph is three days of my water pressure.  I'm trying to understand the bimodal peaks in it.  The vertical axes are NOT scaled to any particular time (yes, that drives me crazy).

The big drop every morning is the valve closing to do an leak check with air at around 4am.  

Peak is usually early evening, and I regularly get an alert on most warm days that it is over recommended pressure of 80psi.

Any idea what causes a morning pressure peak, and a second early evening peak?  (presuming heat on the early evening?)

Edited July 7, 2022 by TeeDubya

[TwiceHorn 31457]

I'm going to guess that pressure drops correspond to high local water usage and the peaks correspond to low local water usage.  The more taps open on your circuit, the lower the pressure, when more are closed, it builds.  But then you'd think middle of the night would be the pressure peak.  ETA:  are you sure the pressure peaks are early evening?  Judging by those graphs, it looks like it occurs right before the valve opens at 4AM, that is, the expected middle of the night.  And you have a secondary peak at what looks to be about 4-5PM, before people start getting home and watering and doing dishes and laundry and whatnot, which might be expected as well.

Water has a pretty small coefficient of thermal expansion in the liquid phase.  But a jumbo volume like a water supply, in a closed system, when it's hot af, could contribute some pressure.

It might also be temperature induced error in your pressure tranducer.

Edited July 7, 2022 by TwiceHorn

[TeeDubya 3362]

26 minutes ago, TwiceHorn said:

I'm going to guess that pressure drops correspond to high local water usage and the peaks correspond to low local water usage.  The more taps open on your circuit, the lower the pressure, when more are closed, it builds.  But then you'd think middle of the night would be the pressure peak.  ETA:  are you sure the pressure peaks are early evening?  Judging by those graphs, it looks like it occurs right before the valve opens at 4AM, that is, the expected middle of the night.  And you have a secondary peak at what looks to be about 4-5PM, before people start getting home and watering and doing dishes and laundry and whatnot, which might be expected as well.

Water has a pretty small coefficient of thermal expansion in the liquid phase.  But a jumbo volume like a water supply, in a closed system, when it's hot af, could contribute some pressure.

It might also be temperature induced error in your pressure tranducer.

Thanks Twice,

I haven't thought about temperature having an affect on pressure measurement.  Closed system, like a thermometer.  (Ideal gas law).  Letting the pressure out with it's leak test would re-calibrate the system at nearly the coolest part of the day.  Since it's hot as Eff until September, I'll have to see what happens in Winter.

I've gone back and looked at the timestamps on the low pressure alerts (all around daybreak) and the high pressure alerts (all around 6pm or so).  It's odd.

Edited July 7, 2022 by TeeDubya

[MonkeyDoughnut 10234]

I'm assuming it's more a function of how many people are using water at the time. Morning everyone getting up taking showers, watering lawns it would be lowest...evenings some take showers but less than morning and possibly laundry use so would decrease but be higher than morning pressures, etc.

Off peak times would assume to have increasing pressure and system "fills" back up.

Anything under 100psi and you should be ok, though if gets near there I would possible look at getting a water pressure regulator installed to lower it before it has change to do any damage.

[TwiceHorn 31457]

I don't think temperature has all that much to do with it.  I interpreted your reference to warm days as possibly having some impact.  It might, but maybe not on water pressure per se.  Something in the valve assembly could be getting hot and expanding and applying extra pressure on the sensor/transducer, or causing an error in the sensor itself.

The pressure alert timing seems belied by the graph/plot.  Although you cant use those vertical dividers for anything, the interval between 4am pressure drops is consistent.  So midway between drops is 4pm, and that seems to roughly coincide with the "secondary peak," while the primary peak, the highest pressure, occurs just before the 4am drop. 

ETA:

It occurs to me that the pressure alerts may correspond to relatively rapid changes in pressure according to the graph and not to the actual pressure peaks.  That might make a certain amount of sense, because pressure transients may be more likely to cause leaks than constant static pressure.  Then again, the low pressure warning seems to correspond to a rising transient and the high pressure warning to a dropping transient, and that seems odd.

Also, I see that the purpose of this valve is as an automatic cut-off in case something springs a leak in the house.  Is this just "general protection" or do you have some particular vulnerability to water leaks?

Also, I think @Hefeweizen and @Scheiss Meister are our local municipal water experts, maybe they have something to add.

Edited July 7, 2022 by TwiceHorn

[Scheiss Meister 8407]

@TeeDubya, your graph corresponds nicely with the water usage in our water distribution system in my city.  The pressure drops overnight because most irrigation systems are programed to run at some point between midnight and 5:00 am.  This is when evaporation is lowest, so irrigating efficiency is highest.  It is also when irrigation will not interfere with daily routines.  As @TwiceHorn said, when the system usage is highest, system pressure is lowest.

[Hefeweizen 5320]

Oh hai.  Water pressure is interesting because except in very special circumstances temperature is just about irrelevant.  It can affect what are called hydro pneumatic systems because the air water interface actually affects the absorption of oxygen and can interfere with the operation of the pressure maintenance (using compressed air).  
 

Most large systems are pressurized using gravity from tanks, with smaller ones relying on either variable frequency drive pumps or hydro systems.

 

In any case, you can expect about a 10 psi maximum variation in pressure.  Anything above 80 is bad for fixtures and anything under 35 will be noticeably weaker.  If you get less than 20 you are boiling  that water.  
 

Unless the pressure is causing you concern about a fixture I wouldn’t worry about it.

[Hefeweizen 5320]

Forgot to add this: plumbing code states that you should have a customer side PRV with that service pressure.  So it could be that it’s either not working or not present.  We don’t like systems with 100 psi static pressures because you can easily start seeing leaks develop at weak points in older connections.  
 

On a new main 100 psi is no big deal though, it all depends on the location.

[bernorange 8361]

OP reminded me of this Trivial Pursuit factoid.

https://www.upi.com/Archives/1983/03/14/The-last-episode-of-M-A-S-H-was-a-royal-flush/3459416466000/

[TeeDubya 3362]

5 hours ago, TwiceHorn said:

 

Also, I see that the purpose of this valve is as an automatic cut-off in case something springs a leak in the house.  Is this just "general protection" or do you have some particular vulnerability to water leaks?

Also, I think @Hefeweizen and @Scheiss Meister are our local municipal water experts, maybe they have something to add.

@TwiceHorn, It is connected to water detectors under my lavs, WCs and appliances which use water.  If one of those detects a leak, it can be enabled to notify, or turn off the main valve (the valve can also turn off water remotely).  The main valve turns off once per night, pressurizes with air briefly to check for small leaks (drips and whatnot).  It's basically for peace of mind and a modest homeowners insurance premium discount.

[TeeDubya 3362]

Thanks everyone.  Great information!