Winterizing

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Walter Duffee

Post by Walter Duffee »

After reading the comments regarding using air to purge the water lines, I had a couple of thoughts to pass on that hopefully will be useful. While most water systems operate at higher pressures, it is usually reduced at your property line with a pressure reducing valve to about 50 psi. Most home water systems work at about that pressure or less and I imagine the pipe and fixtures in a RV are about the same. There is a factor or safety added to the design working pressure of the components which is probably about 2 to 1.

50 lbs of water pressure is NOT the same as 50 lbs of air pressure since air can be compressed and water cannot. I don't know a formula to compare the 2 pressures but have always heard that it is about 10 to 1. (10 lbs water pressure equal about 1 lb of air pressure). If that is approximately correct, then subjecting the water system to 40 lbs of air would put an awful lot of strain on the water system and could cause damage. I do realize that when you have faucets open you are reducing the static pressure somewhat by letting it escape. Just be very careful using air pressure on your water system.

Next winter I may try air to clear my lines but do not think I will use over 10 lbs of pressure.
Fourwinds
Posts: 61
Joined: Mon Feb 27, 2006 12:29 am

Winterizing

Post by Fourwinds »

I find that very interesting, Walter. That ratio is most interesting.

Thanks for the info.

Jess
Jess & Bonnie Dixon
2002 Born Free, 26 ft rsb
tomdclark
Posts: 49
Joined: Wed Oct 05, 2005 7:50 pm

Post by tomdclark »

An aging physicist can't let this go by uncorrected.

PRESSURE IS PRESSURE IS PRESSURE.

It is the force per unit area exerted by the media on the surrounding container. It doesn't make any difference whether the media is water, air, or peanut butter. It may be that a given pressure will compress air ten times (or whatever the factor is) more than water, but this does NOT say that water exerts more pressure than air.

Sorry about the flaming, but sometimes urban myths have to be stopped before they become common lore.

Tom, Ph.D (physics)
Dave&JanPotter

Post by Dave&JanPotter »

Walter Duffee wrote:50 lbs of water pressure is NOT the same as 50 lbs of air pressure since air can be compressed and water cannot. I don't know a formula to compare the 2 pressures but have always heard that it is about 10 to 1. (10 lbs water pressure equal about 1 lb of air pressure)
I'm sorry Walter, but you are not correct. Pressure is the same regardless of the medium it is exerted upon/thru....water, air or hydraulic fluid. What I think you were trying to express is that since water is for all practical purposes incompressible, it will not store energy in that manner. However since air can be compressed, it will store a lot of energy. This is particularly true for things like our hot water heaters. You won't get much energy stored by pressurizing the water lines in our RV's as they don't hold much volume but you will store quite a bit by using air when pressurizing the 6 gallon hot water tank to 30 or 40 psi or less. When that energy is released all at once, as in the tank breaking a fitting or rupturing a seam, the released energy can do a lot of damage. This is why the hot water tank should always be bypassed when blowing out the water lines with compressed air.
I was trying to express this thought in an earlier post on this thread when Jesse indicated he thought we might be talking about pressurizing the fresh water tank when blowing out the lines. The fresh water tank is not designed to hold much above static pressure which in this case is only caused by a couple feet of water which equates to just a few pounds per square inch of pressure. If someone attempts to pressurize the fresh water tank with much more than that they risk serious damage and possibly injury when it explodes. As an aside, the fresh water tank is separately vented to prevent such pressurization in normal usage/circumstances. The hot water tank will hold more pressure but it can also cause a serious problem/explosion. My compressed air tank in my workshop is only 6 gallons, the same size as our hot water tanks and I wouldn't want to be around if it exploded. The hot water tank does have a safety valve and if it works, it should prevent over pressurization. I say if it works because a lot of those valves don't get exercised regularly and can corrode to the extent that they don't work at their designed pressure. I used to work with steel tanks and pressure vessels on large ships, some of them holding hundreds of thousands of gallons. It doesn't take much over pressurization to damage a storage tank.....I've seen a lot of it.
Walter Duffee

Winterizing

Post by Walter Duffee »

Thanks for the clarification on my post. From a scientific point of view, you are absolutely correct that "pressure is.....etc". I should have worded this differently. The reply from Davis did a much better job of explaining about stored energy coming into play. This stored energy from compressed air being released suddenly is where the damage can come from if a rupture occurs

The whole intent of my posting is to caution everyone not to put a lot of air pressure on water lines and damage their RV.
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