The Truth About Water Pressure

[rockfordroo]

Ok, here's the air pressure side:

Solve for increase in air pressure due to temperature increase only

Assume:

Volume is constant for temperature increase

Then

Ideal gas law for this case: P1/T1=P2/T2

Assume:

Air pressure started at 15 psig at 60 deg (P1)

Solve for air pressure at 130 deg (P2)

Since T is in terms of absolute temperature:

T1 = 460 + 60 = 520 deg

T2 = 460 + 130 = 590 deg

Then P2 = P1T1/T2 = 15 x 590/520 = 17.019 psig



Now, solve for increase in pressure due to expansion of water by 1/8 inch

1/8 in = 0.125 inch = 0.010416 ft

Assume:

Initial air volume (V1) is 1 sq ft by 3/4” high

Then:

V1 = 1 sq ft x [.75 inch x 1 ft/12 inch] = 0.0625 cu ft.

Since the water level increase was 0.010416 ft, then final volume of air would be:

V2 = 0.0625 cu ft – [0.010416 ft x 1 sq ft] = 0.052084 cu ft

Then:

The ideal gas law for this case: P1V1 = P2V2

So:

P2 = P1V1/V2 = 17.019 psig x 0.0625 cu ft/0.52084 cu ft = 20.4225 psig

So pressure increase is about 20 psig just heating the water/air pocket from 60 to 130 deg F. Nothing like 150 psig. Even if you added 60 psig from your city water connection.

(I think I should have called the pressure units PSIA rather than PSIG, but it's been a few years since I've done this.)

[MR.M]

Originally Posted by nomad297

You’re probably not a plumber, so you may not notice the things a plumber will notice. I’m sure I wouldn’t notice some things that you would in your line of expertise, but I would believe what you told me.

How many beers do you have?

Bruce

I am a plumber and a rv owner for many years and i doubt very much any pressure ever gets close to 150psi . i have never noticed any build up of pressure in my unit , ever and that 3yrs full time . from Florida to Montana. the PRV would pop off constantly at those pressures not just drip . I think someone is over thinking what goes on in there H/W/H

[nomad297]

Originally Posted by rockfordroo

Ok, here's the air pressure side:

Solve for increase in air pressure due to temperature increase only

Assume:

Volume is constant for temperature increase

Then

Ideal gas law for this case: P1/T1=P2/T2

Assume:

Air pressure started at 15 psig at 60 deg (P1)

Solve for air pressure at 130 deg (P2)

Since T is in terms of absolute temperature:

T1 = 460 + 60 = 520 deg

T2 = 460 + 130 = 590 deg

Then P2 = P1T1/T2 = 15 x 590/520 = 17.019 psig



Now, solve for increase in pressure due to expansion of water by 1/8 inch

1/8 in = 0.125 inch = 0.010416 ft

Assume:

Initial air volume (V1) is 1 sq ft by 3/4” high

Then:

V1 = 1 sq ft x [.75 inch x 1 ft/12 inch] = 0.0625 cu ft.

Since the water level increase was 0.010416 ft, then final volume of air would be:

V2 = 0.0625 cu ft – [0.010416 ft x 1 sq ft] = 0.052084 cu ft

Then:

The ideal gas law for this case: P1V1 = P2V2

So:

P2 = P1V1/V2 = 17.019 psig x 0.0625 cu ft/0.52084 cu ft = 20.4225 psig

So pressure increase is about 20 psig just heating the water/air pocket from 60 to 130 deg F. Nothing like 150 psig. Even if you added 60 psig from your city water connection.

(I think I should have called the pressure units PSIA rather than PSIG, but it's been a few years since I've done this.)

What “air pocket”? Where have you even considered an absolute volume for an “air pocket”?

I will put my plumbing experience up against your flawed math any day.

Bruce

[spock123]

I brought a case of wine from my local winery, about $5.50 a bottle, Memorial Day special. I opened a bottle tonight and since I have a small brain like a chicken, I drank half a bottle and I’m corked, so y’all have a good night and I’ll think about this some more tomorrow or hopefully never

[rockfordroo]

Originally Posted by nomad297

What “air pocket”? Where have you even considered an absolute volume for an “air pocket”?

I will put my plumbing experience up against your flawed math any day.

Bruce

The 2nd half of my second post addresses the air pocket (see V1 and V2).

There is a device inside the water heater that creates the water pocket. Pic below. It is installed with the end pointing down. I believe it will give you AT least 3/4 inch.

I have ASSUMED 1 sq ft as a surface area. I think this is pretty close to the surface area in my Suburban, but even if it's only 0.75 sq ft, it isn't going to make that much difference in my calculations (you can change it and run through them yourself). I gave them 3/4 inch for the air pocket based on the pic below. If you don't like 3/4 inch, use 1/2 inch. Again it won't get you close to 150 psig. But I suspect the air pocket is greater than 3/4 inch.

Of course some of this air will dissolve into the water, allowing the air pocket to decrease in size, which is why you were told you should open the PT valve every few days.

[mjones12]

Who cares?

[nomad297]

Originally Posted by MR.M

I am a plumber and a rv owner for many years and i doubt very much any pressure ever gets close to 150psi . i have never noticed any build up of pressure in my unit , ever and that 3yrs full time . from Florida to Montana. the PRV would pop off constantly at those pressures not just drip . I think someone is over thinking what goes on in there H/W/H

What kind of plumber are you? To most plumbers a PRV is a Pressure Regulating Valve, more commonly referred to as a Pressure Reducing Valve. If you are referring to a Temperature and Pressure Relief Valve, most plumbers refer to them simply as a T&P. And I don’t know any plumbers who refer to a water heater as a H/W/H — a Hot Water Heater? No. We just refer to them as Water Heaters.

Again, what are your credentials?

Bruce

[BandJCarm]

I am not a plumber.

I am a showerer.

I want all the pressure I can get. So far, nothing has burst. I have some number of psi of pressure, I'm very sure. Sorta like blood pressure..........over zero is good.

[Cherokee39]

Just a quick check on PEX website their tubing is not good for 150 psi. Their chart shows 100 psi@110 degrees. Also that hose from the campground bib to your camper is no good for 150 psi.

[Wobbles]

I go to the shower house. I do not want to hear it from my DW about any mess I may leave behind. LOL...

[SecretSquirrel]

Papa Roo beet me to spec for PEX, which I believe is the plumbing used in most RVs these days but I'll post the expanded version for the sake of argument:

"Uponor PEX tubing carries the following hydrostatic temperature and pressure ratings.

200°F (93.3°C) at 80 psi
180°F (82.2°C) at 100 psi
120°F (48.9°C) at 130 psi (½" piping only)
73.4°F (23°C) at 160 psi

In accordance with ASTM F876 Standard Specification for Cross-linked Polyethylene (PEX) Piping, the excessive temperature and pressure capability for Uponor AquaPEX® is 210°F (99°C) at 150 psi."

Respectfully Nomad while I do agree pressure increases with temperature your 150 psi estimate is quite a bit too high. I'd like to refer you to the following website (there are many others as well). https://www.inspectapedia.com/plumbi..._Expansion.php
You'll note the following at this site which has been my general experience as well; " As a point of reference, in a residential hydronic heating boiler we actually observe an internal water pressure rise from 12 psi cold up to 28 psi hot as the boiler temperature increases from perhaps 60 °F up to 180 °F.

(This is much hotter than we should ever see in a domestic water heater tank where to avoid scalding hazards we limit water to about 100 °F. With a mixing valve or tempering valve installed we still limit water heater temperature to around 120 °F.)"

A real world increase in a RV would be about a 5 - 12 psi increase depending on the temp of the initial cold water being raised.

[dascribner]

Originally Posted by mjones12

Who cares?

Exactly

[nomad297]

Originally Posted by dascribner

Exactly

It appears that it is people who contribute to this forum quite a bit more than you do who care.

Bruce

[MR.M]

Originally Posted by nomad297

What kind of plumber are you? To most plumbers a PRV is a Pressure Regulating Valve, more commonly referred to as a Pressure Reducing Valve. If you are referring to a Temperature and Pressure Relief Valve, most plumbers refer to them simply as a T&P. And I don’t know any plumbers who refer to a water heater as a H/W/H — a Hot Water Heater? No. We just refer to them as Water Heaters.

Again, what are your credentials?

Bruce

well excuse the heck out of me . brain fart on the PRV it's been awhile . now i have no need to prove didely to you . the 150psi statement is BS in the worst way . now just because you don't understand the H/W/H which if you did not know is an abbreviation. It stands for hot water heater . with out having to spell it out . kind of like using TV or TW . but i gue3ss you got it . now back to the BS 150psi lets have some facts to back that up . show me a broken psi gauge hooked up to your unit cause that's the only way you'll get a reading like that .

[wmtire]

OK, the site team has permanently closed this thread, as it violates several guidelines that we do enforce, which include name calling and playing nice/courteous. Also there are several members "not letting go" and even baiting.


http://www.forestriverforums.com/for...ork&page=rules


We will most likely have to delete these guideline violation posts as we discuss it.