Calculate BTUs of Variable Speed Heating Loop
I have a large primary secondary heating system and with five secondary loops, each having a VFD controlled pump.The secondary loops are mostly 4" lines well over 1000' long. The secondary loops are essentially primary loops further downstream, so the flow rate is consistantly relevant to pump speed. A PLC monitors the secondary loops supply and return temps and controls the VFDs to maintain a specified delta tee. The PLC logs the power consumption of the pumps but now I'm looking for an economical way to get a general idea of the BTUhours on each seconday loop. After doing mind numbing research I realized doing calculations with water is a lot more complicated than electrical calculations. The difficult part is getting the gpm, installing flow meters would run into a lot of $$$. Can pump head in a closed loop be calculated by (pump out psi  pump in psi) then take that number to the pumpcurve chart to get gpm? Say I know my gpm at 100% pump speed. Is there a formula to factor in speed to get gpm at other speeds? I don't need a very accurate figure just a general idea. I'm not afraid of complicated equations I just need to get steered in the right direction. Sorry if this got long.
Comments

fans.....
BTU's = CFM X Delta T X 1.080 
Or gpm x delta T x 500...
To @djelectric 's question. Yes and no. That is, if you know the pump curve at a specific rpm, and the pump head gain (psi out  psi in) then you can determine flow remarkably accurately  assuming the pump is in decent shape. Unfortunately, the flow does not vary directly with rpm; the whole curve shifts. However, that said... some manufacturers publish curves with output at different rpms, and it is quite safe to interpolate between curves (it isn't entirely accurate, but close enough). Lacking that, with a little experimentation with a pump, one can measure the actual flow at some particular rpm and at several different head gains, and simply shift the full rpm curve to fit those points (I would take at least four points  shutoff head, fully open flow (no head gain) and two others in between). Depending on the size of the pump, I've done this several times with two pressure gauges, a nice globe valve, some appropriate pipe, and a couple of buckiets (little pumps!) to 55 gallon drums (bigger pumps) to in one case a couple of 500 gallon tanks (pretty good size pump) and a stopwatch.
Improvise, Adapt, and Overcome...Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England2 
Thanks for the replies so far. @Jamie Hall I would have to put the loops out of service to remove the pumps for water barrel test, plus they're very heavy. After sleeping on it, I think my best bet is using the pump suction and discharge pressures, each pump already has the gauges. I also have the pump curve charts, so I think I can get a pretty reasonable figure. Now I'll create an example. there's a 15 psi variation=34.6 Feet Head. Say I have 1000' of 4" fiberglass pipe with 8 90deg elbows, and 180deg water. How do I calculate what my flow rate is?0

An accurate gauge will be needed, ideally one gauge reading both sides to eliminate gauge error.Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream2 
Simply go to the pumps curve chart, and find 34.6 in the vertical head column go across until it intersects with the curve, and go down to the gpm horizontal line, and that’s the flow rate.djelectric said:Thanks for the replies so far. @Jamie Hall I would have to put the loops out of service to remove the pumps for water barrel test, plus they're very heavy. After sleeping on it, I think my best bet is using the pump suction and discharge pressures, each pump already has the gauges. I also have the pump curve charts, so I think I can get a pretty reasonable figure. Now I'll create an example. there's a 15 psi variation=34.6 Feet Head. Say I have 1000' of 4" fiberglass pipe with 8 90deg elbows, and 180deg water. How do I calculate what my flow rate is?
The 15 psi differential pressure is the head pressure that pump is seeing at that flow rate of that loop so it’s irrelevant of the feet of pipe, and elbows etc. that’s all in the differential pressure reading.
However one gauge as Bob noted is the best way to do it. So you eliminate gauge in accuracies.
0 
Thanks @Gordy and @hot_rod , that should be easy enough. Assuming the pump curve is accurate, that should give me a good idea of the heat load. I can add all loop loads together and whatever deviation I have from the boiler output, I can put in an additional factor.. Just out of curiousity how would I calculate the pipe head loss? The way I understand with 180deg water, the HazenWilliiams formula is inaccurate.
0 
Depends on how accurate of a number you want. The tighter the number the more steps involved. If you can see and measure all the piping and fittings use the EL method.djelectric said:Thanks @Gordy and @hot_rod , that should be easy enough. Assuming the pump curve is accurate, that should give me a good idea of the heat load. I can add all loop loads together and whatever deviation I have from the boiler output, I can put in an additional factor.. Just out of curiousity how would I calculate the pipe head loss? The way I understand with 180deg water, the HazenWilliiams formula is inaccurate.
Other methods have you assume pipe length and use a factor to add for fittings, valves, etc.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 
Thanks @hot_rod , I saw your screenshots come from idronics, and coming to there website I discovered loads of information . Thanks for all the replies in clarifying the GPM process to me.hot_rod said:
Depends on how accurate of a number you want. The tighter the number the more steps involved. If you can see and measure all the piping and fittings use the EL method.
Other methods have you assume pipe length and use a factor to add for fittings, valves, etc.
1 
A lot of the new VFDs used on pumps are using sensorless technology. No differential pressure sensor. The pump curve gets programmed into the vfd the vfd senses the pump amps and can compute anything you want. Patterson Pumps uses Schneider Electric drives and & Armstrong uses Danfoss0

To me wouldn’t psi differential be more accurate other than gauge tolerance? It is what the pump is seeing for head.hot_rod said:
Depends on how accurate of a number you want. The tighter the number the more steps involved. If you can see and measure all the piping and fittings use the EL method.djelectric said:Thanks @Gordy and @hot_rod , that should be easy enough. Assuming the pump curve is accurate, that should give me a good idea of the heat load. I can add all loop loads together and whatever deviation I have from the boiler output, I can put in an additional factor.. Just out of curiousity how would I calculate the pipe head loss? The way I understand with 180deg water, the HazenWilliiams formula is inaccurate.
Other methods have you assume pipe length and use a factor to add for fittings, valves, etc.
There is a lot of variables with EL calculations mostly pipe wall condition, and fitting connections.
0 
these are the low end drives from Schneider Electric so no such featuresEBEBRATTEd said:A lot of the new VFDs used on pumps are using sensorless technology. No differential pressure sensor. The pump curve gets programmed into the vfd the vfd senses the pump amps and can compute anything you want. Patterson Pumps uses Schneider Electric drives and & Armstrong uses Danfoss
0 
probably, I meant the more steps in the manual calculation, the more accurate them number, like the type of fluid, temperature of the fluid, actual fitting count, etc. Using this calculation method in lieu of having gauges.Gordy said:
To me wouldn’t psi differential be more accurate other than gauge tolerance? It is what the pump is seeing for head.hot_rod said:
Depends on how accurate of a number you want. The tighter the number the more steps involved. If you can see and measure all the piping and fittings use the EL method.djelectric said:Thanks @Gordy and @hot_rod , that should be easy enough. Assuming the pump curve is accurate, that should give me a good idea of the heat load. I can add all loop loads together and whatever deviation I have from the boiler output, I can put in an additional factor.. Just out of curiousity how would I calculate the pipe head loss? The way I understand with 180deg water, the HazenWilliiams formula is inaccurate.
Other methods have you assume pipe length and use a factor to add for fittings, valves, etc.
There is a lot of variables with EL calculations mostly pipe wall condition, and fitting connections.
Ideally a certified, calibrated differential pressure gauge would give the most accurate pressure reading. Gauges like that can run upwards of a thousand bucks.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0
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