HEAD PUMP - MAXIMUM OPERATING PRESSURE

[BACN_mechanical]

If you have a chilled water system with only primary pumps serving a tall building. Also, the expansion tank is located at the top of the building.

How would the maximum working pressure be affected in the following arrangements?:
a. Locating the pumps on the first floor
b. Locating the pumps on an intermediate floor.
c. Locating the pumps on the top floor.

I would believe that the head pumps shouldn't change any of the above arrangements. However, should the working pressure change? Considering that this maximum operating pressure would be mainly affected by the static height.

Also, what other parameters should be considered when changing the location of the pumps?

NOTE: I understand that I don't give you pressure values ​​as in the expansion tank or the head pump, nor heights. But I would only like to know how the system would behave in the three scenarios qualitatively, that is, if it remains the same, decreases, increases, etc.

I'll be grateful for any help you can provide, and success in your activities.

 

[EnergyProfessional]

at any time at any height the pressure needs to be above atmosphere (or the de-aerators will aerate...). Pumps off, or at full speed.

Pressure on discharge of the pump will be static pressure plus the dead head pressure of the pump. Or static pressure only when the pump is off. This could apply to the entire system UP TO the closed pump inlet isolation valve (in case that is causing dead-heading)

Inlet of the pump always will be static pressure only (point of no pressure change)

Static pressure depends on height and fluid density.

At any location the equipment or pipe should be rated to be above that specific maximum pressure when pump operates.

 

[LittleInch]

EP is totally correct.

The differential pressure of a closed loop system where the highest point is above atmospheric pressure is the same regardless of position of you pump.

The "working pressure" though is, as you say, dependant on static height.

In the absence of other issues (space, connection to power etc) the higher the pump is located, the lower its working pressure and the cheaper it will be.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BronYrAur]

This got me thinking. I'm trying to work through the inlet pressure on the pump if the expansion tank is way out in the middle of the system. I always put the expansion tank on the suction side, so that the pump adds positive pressure all the way around the loop. I know that putting it on the discharge side will drop the entire system below fill pressure. But what happens if the expansion tank is way out in the system? Will the pump provide positive pressure in the system until you reach the tank and then negative (below fill) pressure all the way back to the pump suction?

 

[LittleInch]

"I know that putting it on the discharge side will drop the entire system below fill pressure" Errr, can you explain that comment as it doesn't work for me.

Normally you put the expansion tank on the inlet so that is doesn't have to be higher pressure than it needs, plus you actually end up storing more water in it and just generally works better.

"ut what happens if the expansion tank is way out in the system? Will the pump provide positive pressure in the system until you reach the tank and then negative (below fill) pressure all the way back to the pump suction?"
No. The expansion tank is simply a static thing which sits at at a pretty constant pressure, but variable volume, but has no flow. I'm afraid your understanding of such systems does not appear to be correct (IMHO) and it's difficult to explain without some diagrams and typical pressures etc.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EnergyProfessional]

Expansion tank and fill-station should be as close to pump suction (point of no pressure change) as possible.

a lot of funny things happen if you put it elsewhere. among others, the membrane may wear out due to pressure changes of pump operation.

Read this: [URL unfurl="true"]https://www.amazon.com/Modern-Hydronic-Heating-Residential-Commercial/dp/1428335153[/url]

 

[GBTorpenhow]

what happens if the expansion tank is way out in the system? Will the pump provide positive pressure in the system until you reach the tank and then negative (below fill) pressure all the way back to the pump suction?

The pump doesn't 'provide negative pressure' but yes you are pretty close. The pump will generate a certain DP for a given system resistance no matter what. Moving the expansion tank just changes the location in the closed loop where the pressure is held (relatively) constant by the expansion tank.

As a simplified example with no elevation change, if the pump DP is 10 psi and the expansion tank holds a constant 10 psi ("fill pressure"), then locating the expansion tank at the suction of the pump gives 10 psi suction pressure and 20 psi discharge pressure. Moving the expansion tank to the pump discharge gives a discharge pressure of 10 psi and suction pressure of 0 psi. With the expansion tank connection somewhere in between, with say 5 psi of pressure drop between the expansion tank and the pump suction, the discharge pressure will be 15 psi and the suction pressure will be 5 psi.

 

[Compositepro]

There are reasons to have exceptions to any rule, but generally the expansion tank should be placed at the lowest pressure location in the loop. This maybe near the pump suction or near the highest point in the loop. Where ever you place the tank it will have the most constant pressure in the loop. You almost always want to avoid negative pressure anywhere in the loop. Other considerations are to avoid freezing in winter, and ease of access and maintenance.

 

[EnergyProfessional]

The expansion tank pressure will be set to whatever the static pressure is at the suction side of the pump (which will depend on the pump elevation in the system). I cannot think of a reason to not have it near the suction side of the pump.

You typically have an isolation valve and a strainer upstream of the pump, but this is all the pressure drop you should have between pump and expansion tank. Don't connect it somewhere else regardless of height.

[URL unfurl="true"]https://www.industrialcontrolsonline.com/training/online/how-avoid-problems-your-hydronic-system-pumps#:~:text=The%20point%20of%20no%20pressure,a%20change%20in%20air%20volume.[/url]

 

[BronYrAur]

@GBTorpenhow, everything you said is exactly what I thought. Other don't seem to be in complete agreement. I think I confused the issue with my use of the word "negative." I just meant below fill pressure. But what you said above is exactly what I always thought. The OP said that the expansion tank was located at the top of the building which suggested to me that it was in the middle of the loop.

 

[Compositepro]

Air and gasses tend to dissolve more at higher pressure and come out of solution at lower pressure. If you have an expansion tank on a loop in a 60 foot tall building you can have zero gauge pressure at the roof and 30 psig at ground level. You also usually need an air vent in such a loop, and there are benefits to having the air vent on the expansion tank. The lower the pressure in the expansion tank, the lower the pressure variation as the level changes. City water will often be saturated with dissolved air.

 

[EnergyProfessional]

You install auto vents at the highest point regardless of expansion tank location. You also install a separate air/dirt separator. You also set the pressure so that at the highest location it is above atmospheric pressure. If you have 0 psi at the highest level, you did not design/set up the system properly. You can't fix that error by locating the expansion tank. Just fix the error and raise pressure.

another advantage of locating the expansion tank by the pumps is, there you have access and all maintenance can be done in one room. A somewhat remote expansion tank sounds like it will be neglected. But that is outside the inherent physics why it should be at the point of no pressure change.

 

[LittleInch]

This has gone a bit astray here, but remember GT's example was without elevation change. Add elevation into the mix and it starts getting more complex...

Bron - The OP said his expansion tank was at the top and he/she was asking where the pump should be and what was the impact of different locations.

A key risk of placing the expansion tank anywhere other than close to the pump inlet is that the pump pressure will result in water entering the expansion vessel as the internal bladder compresses. This loss of water volume can result in the pump inlet pressure dropping to below atmospheric pressure unless the fill lie is permanently connected and can limit the pressure. Problem then is that when the pump stops, the water in the expansion tank comes back out and the overall system pressure increases. If this results in loss of water due to pressure relief valves then the cycle happens each time the pump starts and stops. Not good news. If not then the pressure in the system end sup being higher than it needs to be causing leaks, damage and more expensive equipment

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EnergyProfessional]

I think discussing all the wrong options (expansion tank not at point of no pressure change) is moot. If the ET is at the wrong location, things will be wrong. Expansion tank should be at the correct location in relation to the pump. Period.

Maybe review the book and the website I posted above.

Justifying the wrong ET location by giving examples of otherwise badly designed systems isn't helpful. Obviously the entire system should be correct. That includes correct pressures and correct location of all devices. The system has to work with the pump off, and with the pump on.

 

[GBTorpenhow]

At the risk of belaboring the side conversation here, I'm having a hard time aligning some of these comments with my understanding of how expansion tanks work. Happy to de disabused of any misunderstanding on my part, but here is my argument:

In a closed loop system, filled with an incompressible liquid, equipped with a bladder type expansion tank, with a constant fluid temperature the system liquid volume is constant, therefore the expansion tank acceptance volume is constant, therefore the air bladder volume is constant, therefore the air bladder pressure is constant, therefore the pressure at the location of the expansion tank connection to the system must be constant.

why it should be at the point of no pressure change

It is usually desirable to connect the expansion tank near the pump suction to force the pump suction point to be the location of little to no pressure change because of the bad things that happen at low suction pressure, but the expansion tank bladder is the mechanism that causes this to be true, not the pump.

the membrane may wear out due to pressure changes of pump operation.

A key risk of placing the expansion tank anywhere other than close to the pump inlet is that the pump pressure will result in water entering the expansion vessel as the internal bladder compresses. This loss of water volume can result in the pump inlet pressure dropping to below atmospheric pressure

The assertion that the expansion tank bladder is compressed by the pump turning on cannot be true in a closed system with no temperature change as argued above. With an expansion tank located anywhere except for directly at the pump suction, the pump suction pressure drops when the pump starts because the moving fluid generates pressure drop between the constant pressure "reference" point (the expansion tank connection) and the pump suction. There cannot be "loss of water volume" without a density change, i.e. thermal expansion/contraction due to temperature changes.

 

[EnergyProfessional]

The expansion tank is there to accept expanded volume (from thermal expansion). The point of no pressure change also works without ET if you don't heat (or chill) the water. If it wasn't for a boiler or other means of heating the water, you didn't need an expansion tank, the pump and hydraulic gradient still works the same. Pump creates pressure, it does NOT create negative pressure on the suction side. If the water is at a constant, the ET will do exactly nothing if it is at point of no pressure change.

An ET has an air chamber and the water side. If water (from thermal expansion, or positive pressure from the pump) enters the ET, the bladder expands into the air chamber and will compress the air. This will increase the air pressure. The same will happen when you have the ET at discharge side. Higher pressure will make water flow into the ET. this will decrease system pressure at suction side. Then water will be missing in the system and the fill-station will add more water (assuming fill station is at suction side) and over time you add more system pressure. You don't have a truly closed system if you are connected to a fill station. You also have de-aerator, which can turn into aerators if they are at below atmospheric pressure.

The link I posted above explains that very well. Also the book.

 

[LittleInch]

GBT,

Your second para is correct, but only when there is n change of temperature and no flow or steady state flow. ETs work on pressure to move fluid in and out.

If there is more pressure in the connecting node than in the air bladder fluid (water) flows in until the two pressures ( water at the outlet of the ET and the air pressure equalise. There is normally not a FIXED pressure in the ET / air bladder, only a pressure which equals the water pressure at the outlet.

Your last para is the one I would dispute because of the information in the response above.
So at rest the pressure in the ET outlet at some position other than the pump inlet is equal to the air bladder pressure. Then the pump starts. To create flow the pump has a differential pressure. At the ET outlet the pressure rises in order to force water to flow to the pump inlet. This rise in pressure causes water to flow into the ET, thus reducing the water volume in the piping loop. After some time the system will get to a steady state, but there will normally be a lower inlet pressure than you would get if the ET was at the pump inlet unless the feed water is connected and starts letting more water in to compensate for the water volume which has gone into the E/T. I think.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[GBTorpenhow]

The expansion tank is there to accept expanded volume (from thermal expansion).

Yes but it is also there to set the "reference" or "point of no pressure change" of your system. The pressure at the expansion tank connection changes due to temperature/density change and nothing else.

The point of no pressure change also works without ET if you don't heat (or chill) the water.

There might be a point in the system that doesn't change pressure, but what point is it? Lets say a closed loop with no elevation change is filled exactly to 0 psig with no ET and the pump curve is such that it will generate 10 psi at whatever flow rate against the system resistance. There is nothing that forces the suction pressure to magically stay at 0 psig when the pump turns on. What keeps the suction pressure from going to -5 psig and the discharge pressure from going to 5 psig?

Pump creates pressure, it does NOT create negative pressure on the suction side.

A pump creates flow, not pressure. A pump at runout generates no pressure whatsoever, for example. To create flow in a closed system with resistance to such flow, a differential pressure must consequently be generated across the pump. The pump doesn't know or care whether the suction pressure is negative or positive. If installed improperly a pump will absolutely create a negative pressure on the suction side.

If the water is at a constant, the ET will do exactly nothing if it is at point of no pressure change.

This statement is nonsensical. There is no "point of no pressure change" other than the one created by the designer when the designer decides where to attach the ET. Wherever the ET is connected is the point of no pressure change because the expansion tank makes it so. So the ET is not "doing exactly nothing". The link you provide actually does state this quite clearly(emphasis mine):

By specifying a point of no pressure change, system pressures can be governed when the pump is on. The point of no pressure change is where the expansion tank connects into the system. This is because the air in the compression tank must follow gas laws: a change in air pressure must be followed by a change in air volume. A change in air volume results in a change in water volume in the tank. A change in water volume in the tank must cause a change of water volume in the system. Pump operation cannot increase or decrease system water volume, since water is incompressible. Therefore, pump operation cannot change tank pressure. Since tank pressure cannot change because of pump operation, the junction of the tank with the system must be a point of no pressure change.

An ET has an air chamber and the water side. If water (from thermal expansion, or positive pressure from the pump) enters the ET, the bladder expands into the air chamber and will compress the air.

As clearly explained by your link, the bold above is simply not true.

The same will happen when you have the ET at discharge side. Higher pressure will make water flow into the ET. this will decrease system pressure at suction side. Then water will be missing in the system...

Also not true. At constant temperature the pressure at the ET connection will remain fixed, so the discharge pressure will remain constant and the suction pressure will drop due to the system losses such that the DP across the pump satisfies the pump curve.

 

[EnergyProfessional]

You lost me slightly before "A pump creates flow, not pressure". I like to see the system that allows flow at no pressure differential.

The point of no pressure change refers to no change in pressure due to pump operation. Pressure will change with temperature and height, but this is different from pressure due to pump operation. At a given temperature and elevation, the pressure won't change. Yes, you can move the ET and that point to a different location. but this is the wrong location. I don't want to discuss system operation where things are wrong. I assume the OP wanted to know how to do it right.

The pump creates positive pressure at discharge compared to intake.

Please show us a valid design guide or other resource that recommends installing the ET not near the suction side of a pump.

You can install the ET the way I (and the design guides) recommend. Or wherever you want. Makes no difference to me.

 

[GBTorpenhow]

If there is more pressure in the connecting node than in the air bladder fluid (water) flows in until the two pressures ( water at the outlet of the ET and the air pressure equalise

This is something of a semantic argument, but I would argue that the ET works on volume, not on pressure. There are never different pressures across the membrane, its more accurate to say rather that the liquid volume moving into the ET pushes the air into occupying a smaller space and as a consequence/side effect the air/water pressure increases. Consider a 'perfect' expansion tank with an infinitely large vessel/air volume, where the air pressure would never change with changing acceptance volume. this fictional design that clearly does not work on pressure would function just fine as an ET.

thus reducing the water volume in the piping loop

The crux of it is, in a constant temperature closed loop filled with incompressible liquid, it is not possible to reduce the liquid volume in the loop. It follows that the ET pressure is constant with constant temperature, pump running or not. Yes the ET pressure will rise when the system temperature rises and more liquid expands into the bladder, but this is completely unrelated to whether the pump is running or not.

The concept of a fill connection is just clouding the issue IMO, plenty of closed loop systems have no fill/makeup provided or required.