chiller pump problem

[HARRY 7]

I have a problem with a water chiller.
The chiller is in the roof of the building
and the water pump as well.
When the pump is closed i have standing pressure 2 bar.
When the pump starts working i have:

suction water pressure 0 bar
disharge water pressure 3.2 bar

Why the suction pressure of the pump goes so down?
I have problem with air coming in the circuit.

[sedgy]

hi harry7 ,
I take it you are monitering the water just before the pump + just after the pump? < are the gauges tested ? can you change them around ?<

[HARRY 7]

Hi sedgy.Thanks for the reply.
The manometers are new!Before and after the pump.

[Sandro Baptista]

Harry 7,

But the pump guarantees the circulation of water or not? Is it being a problem or just a curiosity?

The problem (if there is any problem) could be the fact that with all water system off an equilibrium of the pressures is reached. When the water flows there is water pressure loss which will give to a decrease of pressure on the suction and an increase on the discharge. It is this difference caused by the pump who mantains the water in circulation. If you present any problems of cavitation try to pressurize your elastic expansion vessel if you have it. Check also if you don't have any excessive pressure loss on any accessory (dirty filters, pipe smashed, valve only partial open, etc.)...just ideas...

[HARRY 7]

Hi sandro baptista.The problem is that from the suction of the pump is getting air in the system through the mechanism[i do not know the word] which is getting the air out of the system!!!

[sedgy]

hi harry7,
I am stuck on the word mechanism, ? flow switch?
but running the c-water is important before running the chiller are there binda points you can moniter the water from ? are there air releafe valves that wil help you get the air out of the system ?

[HARRY 7]

Sedgy the mechanism i was refering is the air relief valve.
{I couldn't remember the word in english}
There are relief valves before an after the pump.
When the pump starts working air is getting in the
system through the air relief valve in suction side
of the pump[0 bar}.
When pump stops some air is getting out from the same relief valve.{2 bar}

[Sandro Baptista]

yeah...exactly you mean air purger, right?
Do you have expansion vessel of membrane type? I guess probably yes on some place...So pressurizing it a little more so you can get about 0,5 bar above the atmospheric pressure, so you will have a good tolerance I guess so it will never enters air again.

[HARRY 7]

Yes sandro it is in the basement of the building.
The pump is in the roof of building.
There are 6 floors in the building.
I have 2 bars in the pump when it is not working.
Why should be fault in the expansion vesel?

[ref717]

hi Harry, referring from your querries, it seems you don't have enough return pressure on the suction side of the pump that's why the pressure becomes slightly negative or vacuum. Have you tried adding more water in the circulating water system?.,if yes and still you have vacuum pressure in the return line then maybe you have a clogged return pipe line or a design issue.
Regarding the 2 bars suction pressure when you stop the pump, i think your discharge check valve is not closing that's why the pressure is returning into the suction side of the pump and then increases to 2 bar.
Some air relief valves have a built-in check valve to prevent air from coming into the mechanism and into the suction pipeline. In your case, you can temporarily replace the suction air relief valve with a good quality ball valve to prevent cavitation in your pump.
hope this helps.:)

[HARRY 7]

Hi ref!Thanks for the reply.I temporaly closed the ball valve under the releif valve.But the pump is pressing downwards and the releif valve is 1 meter higher from it.The pipe is about 10 cm thick.Could it be clogged?
I increased the pressure from 1.5 bar to 2 bar in the roof from the water pressuse adjustment in the basement.

[nike123]

I have a problem with a water chiller.
The chiller is in the roof of the building
and the water pump as well.
When the pump is closed i have standing pressure 2 bar.
When the pump starts working i have:

suction water pressure 0 bar
disharge water pressure 3.2 bar

Why the suction pressure of the pump goes so down?
I have problem with air coming in the circuit.

Exactly where on water circuit did you measured that 2 bar standing pressure?
What is pump make and model#?

[HARRY 7]

Hi nike thanks for the reply to.
I measured the pressure before and after the pump.
I do not know the model of the pump.
It is covered with insulation.It begins with star delta.
If that helps.

[nike123]

3,2 bar difference is to high for ordinary circular pump (canned rotor) usually used in such applications.
That is head of 32 meter. We need to know what type of pump is in that chiller. Is it factory fitted or on place fitted pump.


I increased the pressure from 1.5 bar to 2 bar in the roof from the water pressuse adjustment in the basement. If on the roof you have 2 bar than on lowest point in system you have 2bar + weight of water column (0,1 bar/m height diference) Your standing pressure at that point when water temperature is lowest possible must be pump pressure differential + height difference + cca. 1 bar to prevent air ingress and cavitation. Your expansion vessel pressure (before water pressurization) should be 0,1bar lower than that.

Or follow this:
http://i35.tinypic.com/23h6j3c.jpg

[Sandro Baptista]

Yes sandro it is in the basement of the building.
The pump is in the roof of building.
There are 6 floors in the building.
I have 2 bars in the pump when it is not working.
Why should be fault in the expansion vesel?

If your circuit if full of water as it should be so you should have an expansion vessel because:

- the water is cold the circuit is full » when water warm it will have an increase of specific volume which can burst the circuit

- Or if the circuit was full of warm water when the water cools the opposite happens and it will happens a depressurization and the highest points will be at very low pressure and if you open the purge valve on this points air will enter. Also cavitation due to extremly low pressures if the water is not cold enough can result on cavitation that wears the pump and loses head.

[HARRY 7]

The pump is not factory fitted.
There is an expansion vessel but it is in the basement of the building.Could it be that the problem because the pump is in the roof?But i have two bars in suction side before it starts working.

[Sandro Baptista]

There is an expansion vessel but it is in the basement of the building.Could it be that the problem because the pump is in the roof?

No, no problem once you have that in count when you pressurize the expansion vessel.

[Sandro Baptista]

But i have two bars in suction side before it starts working.

I already give my opinion on a previous email. But another question: Does your expansion vessel is pressurized? When the pump start the decrease of the suction pressure is fast or takes times as the water temperature drop?

[HARRY 7]

As they told me from the phone the pressure drops very fast.
I do not know if the vessel is pressurized.

[nike123]

Expansion vessel should be as close as possible to pump suction to maintain positive pressures throughout system.

http://i35.tinypic.com/314rtat.jpg

[HARRY 7]

So i have a problem nike?

[Sandro Baptista]

Of course it would be better to mount the expansion vessel at the suction of the pump to protect her better, but if the expansion vessel is in another place it keep to do its job allowing density flutuaction on the circuit.

Look Harry if you want my advice: stop the pump and pressurize the expansion vessel membrane until you get 3,0 bar. It will be run good. Then send me feedback as soon as possible. You will be grateful. Can you do that? Also check the filter if it is clean.

[HARRY 7]

Thanks sandro i will inform you about the results.

[Sandro Baptista]

Can you do it still in 1 hour so I can go to sleep after? :)

[HARRY 7]

I am at home ready to sleep.:)
So good night!

[Sandro Baptista]

OKAY Good night. Please don't forget to get out all the air that could exist on the system that will be concentrated on the highest points » where is the pump ready to sucked!!

Good night

[Sandro Baptista]

Hi Harry.

Any news?

[HARRY 7]

Look what i found.The pump still works
the same suction 0 bar,discharge 3.2 bar.
The expansion vessel which is in the basement of
the building has 5 bar pressure and it is in the
discharge part of the system.{discharge collecter}
The pressure regulator is there to.{5 bar}
Could it be this a problem for the system?
The pump is a WILO.No other characteristics available.

[HARRY 7]

Hi sandro!How are you?

[Sandro Baptista]

Harry,

What's the pressure now when the pump is stopped? It's very weird if also is the same. Something is wrong.

I presume that the gauges are okay...and the stop valve that communicate the expansion vessel with the circuit is open, right? :)

Do you "pumped" the expansion vessel membrane with more air as we talk about it?

Also I don't see that the problem could be of the pump because the pump is doing its job circulating water, right?
About the pressure regulator I can't also see right now the influence...but sure would be better if you can send us a water diagram.

[nike123]

So i have a problem nike?
Of course that you have, air is entering your system.

[nike123]

Look what i found.The pump still works
the same suction 0 bar,discharge 3.2 bar.
The expansion vessel which is in the basement of
the building has 5 bar pressure and it is in the
discharge part of the system.{discharge collecter}
The pressure regulator is there to.{5 bar}
Could it be this a problem for the system?
The pump is a WILO.No other characteristics available.

3,2 bar at pump discharge and 18 m height difference (about 6 floors) gives you exactly 5 bar pressure in basement. If other components of system could bare, raise pressure ( read in basement) to 6 bar (in expansion vessel air side and in instalation) and than check suction pressure at pump. If everything is same, than that pressure should be 1 bar.

Although, it is strange to me that 32m head is at pump. If pump is capable of such high head than flow is minimal (highest head of pump is when there is no flow).

Wilo BAC pumpshttp://i35.tinypic.com/mwbakk.jpg (usually used in such application) have highest head of 25m (2,5 bar at 10-30m^3/h).
http://i37.tinypic.com/2j2cfwi.jpg

[HARRY 7]

Sandro,

the pressure is two bar when the pump is stopped.
the gauges are new,i did not pressure the vessel because i found 5 bar pressure in it.But i found it is on the discharge side of the pump,but in the basement.Is this a mistake?It is my first time on the circuit.I do not know how it worked last year.The pressure vessel communicates with the system.

nike,

i beleive i have minimal flow.but i have standing pressure 2 bar in the pump.why should i raise the pressure again?
1 and a half bar before.
maybe a restriction somewhere??

[Sandro Baptista]

Harry,

If pressure the vessel as we talked you should have an increase of pressure on the pump discharge but also on its suction. The important is to run way from the zero bar at the suction of the pump. Don't forget after pressurize to purge the air on the highest point (p.e. on the suction)

[joeparera]

May be chiller pump is undersized for the connected load.Youmay check it out proper condition.

[sedgy]

hi harry
you ask why there is 5 bar on the discharge side on the ground floor when the pump is off, this is the HEAD pressure. keep the info comin we are all interested on the outcome , sedgy,

[goshen]

Hi :
we might be dealing with a water pump ,that has a negative npsh!!!
or a clogged filter.you should use a negative manometer to messure this.
regarding air relief valves ,we found that using one way relief valves, that close under vaccume, solves this problem,excpecially if the piping has crap..... insulation!!
good luck

[Sandro Baptista]

Hi Harry.

Any news?

[HARRY 7]

Hi Sandro.I have not gone to the field again.
I will inform you!

[Sandro Baptista]

okay Harry. Have a nice weekend.

[HARRY 7]

Have a nice weekend sandro.
Good night from athens.

[Sandro Baptista]

good night from Portugal. here still are 22:30

[exxu]

If the pump is working with 0 bar or negative pressure on the suction side it could cavitate and get damaged.
I had the same situation on a 5 floors building and the only way to solve the problem was to rise the water pressure until, with the pump working, i read 0.5 bar on the suction side. The pressure in the basement was about 5.6 bars. Fortunately all the components were built to withstand that pressure.

[Sandro Baptista]

...Or we can the pump at lower level...

[HARRY 7]

I went to the field today.There is no water filter in
the system.Nothing has changed in the pressures.
It is a 30 years old water system.Could it be the system
clogged somewhere?This is what i beleive.What is the next step?

[Sandro Baptista]

Have pressurize the expansion vessel membrane as I ask you to in a previous email?

Stop the pump and you reach in both sides 2 bar (as you told me at you first post). Pressurize until you get 3,0 bar and see what you get at the suction when you run the pump.

[HARRY 7]

Hi sandro!How are you?
No i have not pressurise the vessel.
So you tell me to pressurise the system to be 3 bar on the top or the vessel??

[Sandro Baptista]

Pressurize the expansion vessel membrane, please and the system will increase the pressure. Make sure the air is purged on the highest points and the system is full of glycol then the expansion vessel will done its job of compensate the volumetric thermal expansion.

[Sandro Baptista]

Hi Harry how is the pump working now?

Have you already rise the suction pressure of the pump?

[HARRY 7]

Hi sandro.I have not gone to the field again.
I will go next week.I will inform you!

[HARRY 7]

This is the drawing of the circuit:

[HARRY 7]

this is the drawing of the system:

[HARRY 7]

I believe i did it now:D

[nike123]

Relocate your expansion vessel to roof near pumps inlet and your problems will disappear.
Pump your expansion vessel (air side) to 1,5 bar and ensure that pressure when system is inoperative and cold at lowest temperature is about 1,6 bar at pump inlet (water side).
Than operate system and check that at suction of pump pressure is about 1,5-1,6 bar. If not, add water.
Than measure pressure at point where you refiling system and adjust refiling regulation valve to that pressure.
Purge system of air and you are done.
Check that pressure doesn't exceed admissible pressure of weakest components.

[HARRY 7]

So you beleive the expansion vessel does the
problem in the system??

[nike123]

So you beleive the expansion vessel does the
problem in the system??
Yep!
...........

[HARRY 7]

Can you expain me technically the reason?

[nike123]

http://www.refrigeration-engineer.com/forums/showpost.php?p=200347&postcount=20

[HARRY 7]

I will try it nike and i will let you know.

[Sandro Baptista]

Hi Harry. How it's going?

[HARRY 7]

Hi Sandro:).I am going to put
the vessel and the valve in the roof.
When i finish i will inform you!

[49ershvac]

sound to me there is a leak somewhere.do you have a water make -up tank

[HARRY 7]

There is no leak somewhere!the client has left the system works as it is!

[DTLarca]

Can you expain me technically the reason?

The pump creates static pressure sufficient to overcome the frictional pressure drop of the water circulating the system.

Water is incompressible. Also we do not expect the water to expand - there should not be room in the piping for water to expand. So we cannot expect the pump to drop the pressure of the water at its suction side.

Lets imagine a chilled water system where the water flows around a circuit which at all its points is at the same elevation. This means, say, that when the pump is not running the pressure right around the system is 1 bar.

Then when the required water flow rate is happening we might have an evaporator pressure drop of 0.5 bar, a piping pressure drop of 0.2 bar to the fan coil then a pressure through the fan coil of 0.5 bar and a pipe pressure drop back to the pump of 0.2 bar.

The total system frictional pressure drop is 1.4 bar.

When the pump starts it sucks water in from behind it. But the water can't expand to drop its pressure there - if it wanted to expand it would have to fight against the rigidity of the pipework. So in fact the pressure at the suction of the pump should be 1 bar both when it is off and when it is running.

Pressure at pump suction when off is 1 bar and when on is 1 bar.
Pressure at evaporator inlet when pump off is 1 bar and when on is 2.4 bar.
Pressure after evaporator when pump is off is 1 bar and when on is 1.9 bar.
Pressure before fan coil when pump is off is 1 bar and when pump is on is 1.7 bar.
And so on.
But at the inlet of the pump the pressure should always be 1 bar - for this hypothetical project.

Because the pressure should always be the same at the pump inlet we should connect the pressurization unit to that point so that it's pressure sensors and settings can have a nice neat narrow dead band.

But why connect the expansion vessel at the pump inlet?

Well, the vessel is sized to accept the thermal expansion of the water in the system. If it was, for example, located on the discharge of the pump then when the pump starts and so the water pressure there increases it will flow into the vessel compressing the bladder thus removing some water from the piping circuit and thus it would cause the pump suction pressure to drop when normally it doesn't change. So then the pressurization unit kicks in to raise pump suction point pressure. Now the bladder is half compressed meaning before the system's water has risen in temperature we have already lost half of our expansion space which means that later when we do need the expansion space we end up instead blowing the relief valves.

[DTLarca]

When I say we do not expect the water to expand I mean we do not expect the water at the suction of the pump to expand in the same way we expect air to expand on entering a fan or like helium would when being released into a deflated balloon. I'm talking about when the pump starts - not when the water temperature changes.

[DTLarca]

There is another reason why we connect the vessel to the suction and not the discharge and this reason is similar to the reason why you need larger compressed nitrogen vessels for heating circuits as you do for chilled water circuits. In fact in most designers minds this other reason is more important that the first reason I gave above - anyone know what it is.

thinking about that would lead onto considering how you select a vessel size wise - the formula unfortunately involves an iterative process but understanding the theory still takes a person a very long way down the road of understanding chilled water system commissioning and service works.

[Gary]

I increased the pressure from 1.5 bar to 2 bar in the roof from the water pressuse adjustment in the basement.

Keep adjusting the water pressure in the basement until there is .5 bar suction pressure when the pump is running.

And yes... the expansion vessel should be connected to the suction side of the pump.

[HARRY 7]

Thanks for the explanation DTLarca!
Gary i have 5 bar in the basement when the system is working,you tell me to increase more?

[Gary]

The basic problem is that the feed water and expansion tank should be on the return (suction) side, not the discharge side.

I'm wondering if the entire problem could be solved by reversing the direction of the pumps, pumping out of the chiller instead of into the chiller?

[HARRY 7]

We made an offer to the client to remove the feed water and the expansion tank to suction side.He has not accepted it yet!

[Gary]

In your drawing, the feedwater and expansion tank are teed together and then connected to the discharge. If that tee was instead connected to the return, the problem could be resolved without moving anything. This might be the simplest solution.