can air have big effect on pressure switch? 1

[electricpete]

Is it possible that air in the pressure switch sensing line of a houshold well pump / pressurized tank system can significantly affect the operation of the pressure switch?

My thought is no. Everything is near the same level. Even if there is air in the line, the only difference would be the displacement of a few inches height of water.

I have some experience which indicates otherwise. (long story). So I’m looking to see if there is any misconception in my comments in the previous paragraphs.


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[Artisi]

I have also spent a few weeks playing with a domestic pressure system and like you wouldn't have thought that air in the system would have any effect on the pressure switch other than delaying the shut down and start up a little because of the air being compressed and de-compressing as compared to a water only situation.
I would be interested in hearing your long story as to why you are thinking otherwise.

Naresuan University
Phitsanulok
Thailand

 

[quark]

Suppose you have a 20m long cylinder open ended on one side in inverted position. You also have pressure gauges one at top and another one at the side bottom. Now assume that top 10 meters is filled with air at at atmospheric pressure and bottom 10 meters with water. The gauge in the bottom should read approx.1kg/sq.cm(g) and the top one zero. Now take a plunger from bottom and push the water to top till it ascends 5meters. In a perfectly leakproof system, the top gauge will read 1kg/sq.cm(g) and the bottom one 2kg/sq.cm(g).

Now replace the plunger movement by the flowing liquid. The liquid column is pushed up and down by the flowing water and causes abrupt changes in the pressure read by the gauge on top though the liquid column height is same(I mean water pressure is same).

This can be observed by the violent fluctuation of pressure gauge pointer when air is trapped in the system.

There may be some minor mistakes in my analogy but I think I can make a better thought experiment liesurely. Suggestions are always welcome.

Regards,

 

[electricpete]

Artisi - since you asked, here is my experience in a nutshell:

The pump discharges through backpressure regulator (maintains upstream pressure to prevent pump runout) into tank bottom. Tank has a penetration around half way up where gage and pressure switch and schroder valve all tap off. The tapoff to pressure switch is hardpiped up first, and then flex hose down to the pressure switch.

Original pressure switch is 30 pounds on, 50 pounds off. Intalled by previous owner and worked fine for the last 5 years that I own the house.

On occasion when I drain the tank I allow it to drain completely and come to atmospheric pressure. Then start the pump and let the pump do the pressurizing.

One day the water stopped coming out of the faucet. Went outside – pump running, no pressure on tank gage. Turned off the pump breaker. Reprimed pump. Turned on pump breaker. System pumped up to 50 pounds and stopped. Slowly decrease to 30 pounds, pump start.

Intermittenly over the next few days I observed the pressure would drop past 30 pounds and pump would not turn on.

Therefore I replaced the pressure switch. New pressure switch is 20 pounds on , 40 pounds off. This procedure also has some kind of time delay where if pressure is below the setpoint for a minute or so, it will lock out the pump to prevent running with no prime.

I used same procedure to fill the tank. Now that with these new setpoints I have a larger air bubble at top of the tank than before.

I observed pump operation several times and it turned on at exactly 20 pounds on the tank pressure gage.

Two weeks later – intermittently the pump would fail to start. It happened probably 10+ times over two days I caught it in the act several times. Pressure would drop to about 18 (slightly lower than where it turned on before) and there was a click and the contacts moved but did not close. I believe this was the lockout feature.

I believe I have ruled out the pressure switch because the problem occurred on two differen tpressure switches.

I believe I have ruled out the electrical circuit because the contacts never even close.

I wondered about the changes and one thing I realized is that with the new setpoints the tap for my pressure switch is above the water level. I was able to verify that on occasion air comes out of the schraeder valve at that location when I poke it.

From discussion with two other pump owners – they both have pressure switch located on the pump discharge (downstream of backpressure regulator). Mine should be at an equivalent pressure location at the tank, but it may be sensing air vs water pressure. That's the only difference. And it may be a recent difference due to the pressure chagne.

Even though there is no scientific reason, I had a hunch that air in my pressure switch or sensing lines may have something to do with the failing to start after pressure switch replacement.

Therefore I made two changes:
1 – added a manual isolation valve on the sensing line which gave me the ability to vent at my pressure switch in a controlled manner (I never really vented it before because I didn’t want 20+ pounds water spraying into my pressure switch as I tried to vent it).
2 – I increased the setpoints to 25 pounds on, 45 pouinds off.
For whatever reason, those two changes seem to have solved the problem. Whereas I had 10 instances of failing to start in a few days before those two changes, I have had no instances in a week plus since I made those two changes.
I am cautiosly optimistic that the problem is sovled but I would like to know why.

Quark – I am intrigued by your comments but I don’t quite get it. I haven’t experienced violent fluctuation of pressure gages in this application or others. I have seen it for flow gages but dealing with a much smaller dp. Also, at the point in time when pressure switch is expected to function (turn on) the pump is off so no pulsations from there and only water flowing out of the tank towards the house at a relatively low rate.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[Artisi]

Sorry I can't throw any light on the problem - the only thing I can think is it is tied up in the air pressure on the air side of the diaphram / or in the airbag - not sure what system you are using.
where are the gauge and pressure switch's located on the air side or the wtare side of the unit?

Naresuan University
Phitsanulok
Thailand

 

[quark]

Two points are not clear to me. You said that the pressure switch tap is about in the middle of the tank. If so, is it a pressurized tank?

Secondly, do you have any elevation difference between the bottom of the tank and pressure sensor location(pressure sensor located below the bottom of tank?). If so, what is the elevational difference?

 

[hydrae]

Pete
What is the tank pressure when empty of water, known as precharge. Proper operation is the set precharge at 2 psi lower than pump cut in pressure. This value maximise pump run times and maintains pressure switch responsiveness.

Since you switched pressure switch setpoints I would assume the tank precharge is at 28 psi, lowering the cut in pressure while leaving the precharge at 28 will signficantly shorten the pump cycle, having a shorter pump cycle may kick in another protection feature, rapid cycle protection.
Are there other pump protection devices such as a 'pump saver', or 'well trol', these devices will monitor such things run times, off times, amperage, voltage, phase balance (for 3 phase).

You mentioned priming, is the pump suction lift or jet?

Hydrae

 

[Artisi]

Hydrae
This is good info for electricpete, just what he needs and was lacking to "set-up" the pump / tank system - the other info re protection features may also be helpful.
Plus you have answered a question I have had for some time as to what pressure the pre-charge should be.
Good clear information - worth a star.

Naresuan University
Phitsanulok
Thailand

 

[farooqhameed]

My experience is that air in a pipeline has an adverse effect on the working of a pressure switch. Once we finished a major overhaul of one of our Niigata diesel engines. The oil line was drained completly to replace the oil. Fresh oil was added and after necessary checks the engine was started. All operational parameters were correct when suddenly the engine tripped on low oil pressure. We had to open half of the engine to check for loss of lubrication after the low pressure sensors were found OK. Nothing was found and the engine was started after two days. The engine is working fine till date. The reason for tripping was found. It was due to trapped air in the pressure sensor line. We had not purged the line after oil filling.

 

[sprintcar]

To join the fray.... I just had my entire well system (pump, lines, piping and tank) replaced thanks to a local lightning strike. The new pump is 1.5hp @ 13.5 gpm vs the original 1hp @ 7 gpm. The reservoir tank (air bladder) went from 40 to 44 gallons. The pressure switch is relatively new, so it stayed in place. There was a check valve just out of the well head to prevent back flow, but the internal poppet was removed by the installer "because it wasn't needed".

The problem now is when the pump starts it tends to rapid cycle when it reaches full pressure. Apparently there is enough surge in the line (almost waterhammer) to fluctuate the switch. My thought was to add a large diameter standpipe to provide a bit of air cushion and then add very small orifice below the switch.

Perhaps replacing the check valve would be a better and/or additional solution?

Comments and help always appreciated!!

"If A equals success, then the formula is: A = X + Y + Z, X is work. Y is play. Z is keep your mouth shut."
-- by Albert Einstein

 

[sprintcar]

Problem solved -
They moved the pressure switch location to a section of pipe downstream of the tank. However, this still didn't work.
It seems when the installer initially checked the reservoir tank air charge (should be about 28psi empty) apparently his gauge was faulty. When I checked it with my gauge, the actual pressure was about 45 psi, so the 13 gpm pump could only add about 5psi to the system before the switch cut off the motor.
I guess the moral here is "When problem solving - Go back to basics first"!

"If A equals success, then the formula is: A = X + Y + Z, X is work. Y is play. Z is keep your mouth shut."
-- by Albert Einstein

 

[semo]

I've seen municipal systems develop air in the pressure switch line that controls the operation of the well. Just like a residential system; but, no pressure tank.

When the air accumulates, the controls do not work properly. Once the air is drained from the control line, the system works properly again.

Air is a compressible gas. Water is not. Because the water pressure is not static (ie. pump is on one minute then off the next) the air pressure in the air column will not be static. Higher water pressure compresses the air pocket increasing the air pressure which will then act like a spring. After compression, the air pocket will expand reducing the air pressure. This variable air pressure will not operate the controls properly.

There are pumps (ram pumps) which use this principle to take a large volume of water at lower pressure and force from it a small volume of water to a larger pressure without external energy sources.