How to control hydronic pump vfd with vertical riser reverse return

[tlsps]

We replaced across the line hydronic pump starters with VFD's. The pumps are on the 6th floor of a 6 story office building, pumps down the riser with a return running to the basement and a reverse return runs back to the pump inlet on 6th. We installed a DP sensor at 2nd floor ceiling (2/3 down vertically from pump) and connected between the supply line and the common return. The pressure differential was at 12 psi at 100%, and went down to 10psi at 80%. As we reduced the VFD below 80% little happened to the pressure difference. We tried connecting to the reverse return and same thing. We tried a combination of opening and closing in various percentages of each of the return lines (we hooked up with a tee and valves so we could try all combinations). The reverse return has self balancing characteristics so may be the problem. We really wanted to control down to at least 15%, but no luck.

Does anyone know how we can control the VFD with this 3 pipe vertical riser system with the pump at the top of the 6 story riser?

 

[317069]

Could you please give more explaination about situation
"As we reduced the VFD below 80% little happened to the pressure difference"
" We really wanted to control down to at least 15%"

 

[quark]

Interesting problem, though I feel I am missing something.

Since, you have user equipment at every floor (I presume), tapping for DP at 2nd floor of the return may make the DP sensor ineffective of the flows below second floor. The pressure sensor may not experience the change in return line pressure for all the floors below it. Same may be the case with supply line tap for the floors above the tap (I am imagining that flow from floors above second floor directly goes to return line via the user equipment and supply tap of DP sensor doesn't catch this).

Since, this a loop and only dynamic losses come into picture, try installing the DP at the sixth floor between supply and reversed return.

Reversed return arrangement is basically used to prevent short cycling by creating equal resistance path to all parallel users. This will not effect your system in a negative way.

Is there any reason why you preferred reversed return instead of individual floor balancing valves?

 

[walkes]

tlsps,

At first glance it looks like you are playing with the VFD speed to see the effect on the DP. So yes 12 PSI at 100% and 10 PSI at 80% makes sense, assuming all of the control valves are still fully open.
As the system two-way control valves close the system DP should increase on a constant speed system riding the pump curve, with the variable speed system the VFD should slow down to match the design DP that matches the system curve.

 

[quark]

If the flow reduction is done by reducing the pump speed and not by throttling, then 10PSI at 80% is not correct (if all valves are open). Since the static head is negligible, the reduction in head should be 12PSI * (0.8)² = 7.68 PSI

 

[ChrisConley]

The reverse return has self balancing characteristics so may be the problem

Shouldn't cause a problem with the VFD, other than it sounds like you're having some problems identifying the 2/3 location hydraulically in the system.

Firstly, I'd verify the components. Make sure the VFD is actually slowing down the pump (rather than the DDC telling you it is). Next I'd verify the DP sensor, possibly with a manual reading if possible. I've seen many bad DP sensors.

Another path to consider is an integrated pumping solution. Armstrong pumps (and others) offer an integrated sensor-less pump solution, which might work well in your situation, or in the future.

http://www.armstrongpumps.com/prese...sectionID=01&classID=09&modelID=005&groupid=0

 

[Drazen]

Initial question confuses me so much that it is almost relaxing on friday night

First of all, I cannot imagine why would you complain for not being able to lover differential pressure at lower loads. Is it not a dream of most hydronic system, to maintain constant differenctial pressure at differing loads?

As Chris mantioned, there are integrated pump controls. Some of these achieve proportional characteristic, where dp is lowered at lower loads, but are used mostly for domestic system, where they simplify overall performance.

In commercial systems, lower dp can mean that some of your consumers will not receive enough dp, though it may need it. Different consumers have different demands, and it can happen that your the most distant consumer needs 100% capacity while others don't. Constant dp is ideal in that respect.

Second, you did not mention how your terminal units are controlled, so we don't know some of basic things. Thermostatic valves or semething else? What do you mean under 80% capacity, 80% of nominal flow?

Third, as far as I can imagine, dp above your place of setting would be unaffected by controls, means everything above second floor, as differential pressure controllers are allways placed before any consumer-circles. Not to mix with bypass valves, that have two variants of placing! So, sensors should be at sixth floor for sure.

Finally, I have long and elaborated case study that tries to prove how reverse return adversely affect balancing in all variable flow systems. It is not in english, and I really cannot summarize it as it involves detailed simulation.

 

[walkes]

Quark, it would then appear that there is a reduction in flow partly by pump speed and partly by throttling, correct? (not all valves are fully open)

 

[tlsps]

Thanks to all for some excellent feedback. I've turned all of these over to my Controls Tech and he is sorting through so I can give good technical responses to these for clearly describing the situation and hope to have by Monday. This is a retrofit so we had no control over the way it's piped/controlled but inherited it. All AHU coils have 2-way valves and are present on all 6 floors.

We checked the DP at the pumps on 6th floor from the gages in the system and showed 10psi as well as 10psi on 2nd floor. That's why we figured moving the DP to 6th wouldn't help since same reading.

I'll have good answers to the questions posed by all Monday. Thanks again for great responses.

 

[quark]

Walkes,

Yes, that is a possibility. If we introduce pressure drop by valve throttling then things go unpredictable for variable flow pumping.

Drazen,

We discussed in length, in the past, about whether individual balancing valves are required or not if we already have two way or three way valves, but whether or not a reversed return header was never discussed in this forum.

I myself is not a favorite of this concept since the inception of 2way/ 3way valves. Nevertheless, I was pretty satisfied when AHU chilled water control was by manual valves.

I would appreciate if you mention excerpts from the paper you mentioned.

tlsps,

We have many installations of the DP sensor at the end user (i,e at the end of the headers), when the user equipment are installed in one floor and these things worked perfectly. But in your case, I still imagine, both taps of the DP sensor should experience compounded flow rate of the headers. My logic for this is still vague. I will try for some concrete ideas.

Secondly, check the pump performance curve. This problem may happen if the pump curve is flat.

 

[Drazen]

I don't have english translation, and report is mostly based on measured observation, where much larger imbalance is found in reverse return systems which operate in variable flow regime. There is even no attempt of theoretical explanation, just lot of data for several scenarios, reverse return allways being worse in comparison.

I think TA Hydronics has some reports, somewhere within their case studies.

 

[Ziggypump]

Check with you pump manufacturer about the minimum speed that the pump can be reduced to while still operating properly. I've always been told 30% of maximum flow is the rule of thumb, but have known systems to get down to about 20-25%. If the pump is cut too small, the flow will be off the curve, and the pump will not work properly.

 

[quark]

Safe minimum flow of 30% BEP flow is a recommendation for fixed speed pumps and this is to avoid heating of the fluid and thus the pump.

When the speed reduces, the safe minimum flow should also reduce and we had a lengthy discussion to that effect in one of the forums (Pump forum, if I remember correctly).

With variable flow systems, there is no single pump curve to force the pump to perform. Infact, it is a combination of many pump curves and the control happens on system curve rahter than pump curve.

 

[cry22]

Few things:

1. Yes, I too, have read something in TA about too much volume being pumped in reverse return systems making VFD's less efficient than direct return.

2. Can you tell us how large a pump (HP) this pump is? 3-pipe vertical risers on 6 floors, are you at 10HP or lower? if so, Not worthy of a VFD to begin with. There is no decent pay back for such small HP. The use of a VFD as a retofit should've been questioned to begin with.

3. If this is heating water and you have temperature reset at boiler or heat exchanger, then your valves are most likley open all the time since water temperature follows your OA temperature. not much reduction in load. defeats the VFD purpose even more.

4. 3 risers? should not be a big problem to convert it to a direct return at not much of a cost. Just tap all run-outs to the reverse return and disconnect the first leg of RR.

5. I wonder if you can apply a head pressure reset, reading all the valve positions and having the DP maintain a pressure to satisfy the most open valve, this would make the DP sensor location irrelevent, but you would need DDC controls, do you? I'D LOVE TO READ YOUR PITCH ON THIS GUYS.