Low Flow Rate Problem.

[Tadders]

Hi all,

I hope someone can shed some light on this problem I am having. I have just installed a new heating system to a production line. The system is as follows. I have an open bath containing 2000 Litres (440 gallons) of clean water. A centrifugal pump capable of pumping 90 Litres (20 gallons) a minute at a maximum pressure of 2.2 bar in installed directly below the bath. The water is pumped along 36 Meters (118 feet) of 20 millimeter (3/4”) copper pipe passing through a heater contained in a cylinder before retuning to the bath. The pipework rises over a wall about 3 Meters high. I calculate that the pressure losses through the whole system including all bends, valves, strainers etc should be less than 1 bar.

Now here is my problem, I am looking to pump 12 Litres (2.6 gal) a minute but the maximum flow rate I can achieve is 7 Litres (1.5 gal) a minute. A pressure gauge on the delivery side of the pump has gone off the scale at 1.5 bar (22psi). I have automatic air vents at the highest point on the system and I have checked that there are no air locks elsewhere in the system.

Does anyone have any ideas as to what might be causing this problem? Is the fundamental design flawed?

|------------->------------------>-------------------------|
| |----------<------------------<------------| |
| | | |
| | | |
| | | |
| | 3 Mtr | |
| | | |
| | | |
| | |~~~~~~~~~| -----------------
| | | water | | heater |
| | ----------------- -----------------
| |_____| |
|---------<---@----|
pump


Many thanks in advance.

Tony.

 

[25362]

Can you please give the internal diameter of the tube or pipe ?

FWIW, the (US) Hydraulic Institute Pipe Friction Manual gives for a Sch 40, 3/4" steel pipe with an ID=0.824", and a surface rugosity [&epsilon;]/D = 0.00218, the following tabulated friction losses in ft of water per 100 ft equivalent length of pipe:

gpm friction drop
1.5 0.73
3.0 2.50
12 32.6
20 86.1

Any abnormal condition of the interior surface, or differences in diameter, or age effects not considered.

 

[quark]

20MM for a flowrate of 90lpm is too low a diameter. You can neglect the elevation if you have two legs of equal height, one for supply and one for return.

Put a bigger range pressure gauge in pump discharge (also one in pump suction) and read the corresponding flowrate from the pump curve. Your pipeline seems to be of small in size.

 

[25362]

You may visit

http://www.cda.org.uk/megab2/build/pub125/sec4.htm

 

[katmar]

The design is reasonable, and I agree that the expected pressure drop would be well below what you are experiencing. Assuming that your over-reading pressure gauge is accurate and you do have >22 psi, then either the pressure drop through the heater is excessive, or you have a blockage somewhere. Can you install a pressure gauge either just before or just after the heater?

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Tadders]

Thank you all for your help and advice.

I made a slight error on my original post ; the smallest diameter in the system is a 20mm (3/4”) Double Regulating Valve and Metering Station. The rest of the Pipework is in fact. 28mm (1”).

255362 - The internal diameter of the pipe will be around 24mm. All the Pipework is brand new so there should be now problems of corrosion etc. Thanks for the link… it will be a great help.

quark – I agree that 20mm for a flow rate of 90 lpm would be way to small but I only need to move 12 lpm. Before I found this forum I had already increased the pump size in an attempt to increase the flow. This only improved the flow rate very slightly.

katmar – I am happy to read that you think the design is reasonable. I started to wonder if I were attempting the impossible.

I am starting to pull together a plan in an attempt to find the cause of the low flow.

I intend to carry out the following:-
1. Install a higher range gauge on the delivery side of the pump.
2. Install a gauge on the suction side of the pump.
3. Install a pressure gauge just after the heater.
4. Temporarily break the pipework down in to sections at convenient points. Connect a hose and run the pump to check for blockages and flow rates.

Please let me know what you all think.

 

[Artisi]

What is the head loss thru the heater, it is an unknown factor and needs to be know so the total head a can be estimated.

Naresuan University
Phitsanulok
Thailand

 

[Tadders]

Artisi - The manufacturer of the heater has informed me that the pressure drop through the heater is very low at between 0.1 and 0.2 bar.

 

[MedicineEng]

Do you have any valve or check valve in the system?
If yes, check it to confirm that they are open/opening properly.

 

[Artisi]

you say the pump is capable of 90 l/m and the maximum head is 2.2 bar, fairly meaningless information - but what we really need to know is some flow versus head figures ie. 90 l/m at 1.1 bar, 50 @ 1.6, 30 @ 1.9, 10@ 2.0 etc. read form your pump curve, this allows us to construct a pump HQ curve which we can over-lay a system curve to see what the pump is capable of doing on the system. Anything else is guess work and doesn't give a starting point to work from if in fact there is a problem.

Naresuan University
Phitsanulok
Thailand

 

[Tadders]

Artisi - I have asked the pump manufacturer for the information we require. I will let you know as soon as I have it. Many thanks.

 

[Tadders]

I have received the pump curve from the manufacturer and have tabulated the graph below.

20 lpm 20.75 H[m] 2.03 bar
30 lpm 20.00 H[m] 1.96 bar
40 lpm 19.50 H[m] 1.91 bar
50 lpm 18.50 H[m] 1.81 bar
60 lpm 17.50 H[m] 1.71 bar
70 lpm 16.50 H[m] 1.61 bar
80 lpm 16.00 H[m] 1.56 bar
90 lpm 15.00 H[m] 1.47 bar

Thank you very much for all your help.

 

[jet1749]

Out of curiousity I drew out the performance curve for your pump and noticed it looked very flat. In which case a relatively small change your pressure can cause quite a large change in your flowrate. Also noticed that your flowrate (desired and actual)looked to bring you uncomfortably close to the shut off head of the pump. 28mm O/D copper pipe looks to be 26mm I/D and at flowrates of 7 litres gives a fluid head of 0.437 bar and at 12 litres gives 1.11 bar for 36m length of system. If the heater is not the problem, and the pipework isn't that only leaves you with your "double regulating valve and metering station" I would suggest that you check the pressure drop across that item.
best of luck
John

 

[jet1749]

Correction
26mm I/D X 36m length copper pipe
7 Litre flow 0.127m head
12 litre flow 0.323m head
regards
John

 

[Artisi]

I have to agree completely with jet1749 with the exception that the pump is enormously oversized and the primary problem with this installation.
I also drew out the pump performance curve and overlayed it with a system curve to give an idea of what is happening. I made a couple of assumptions by using 1" steel pipe for the friction loss calculation, didn't allow for any valves or the heater and you have not clearly defined the total static head on the pump so this is not included.

For a flow of 12 l/m you need less than 0.5 metre head at the pump not allowing for, the heater, valves and controller or static head etc. You are therefore throttling or trying to somewhere near 19m metres of head across the regulating valve - I would suggest that such a turn-down is not within the capabilties of the RV and is the secondary problem with the system, the first being the oversized pump.

For this application all you need is a hot water circulator ie, Grundfos or Danfoss etc capable of 12 l/m at 2 -3 meter head.

Trust this helps.

Naresuan University
Phitsanulok
Thailand

 

[katmar]

Artisi,

How can a pump being oversized result in the flowrate being too low? I do not understand how you come to this conclusion. I agree that Tony's pump is bigger than necessary, but I do not see how this is any sort of problem other than economic?

Harvey

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Artisi]

As we don't have access to all the detail on this pump unit, a number of assumptions need to be made, one of which is the stability of the pump at 12l/m, one other is the capability of the RV to control the flow at such a massive "turn-down" coupled with the pumps performance at near to shut-head.
Just because the supplier has given a flow at 20l/m doesn't mean the pump is stable at or beyond this point- anything could be happening, stall, massive recirculation -a complete collapse of H/Q.
However, I know little about RV valves but suspect that trying to control the output from the RV from 20 + meters to a couple metres head with what could be a very unstable pump output may be well beyond the control of the valve.

A well sized pump operating somewhere near to its BEP in conjunction with a RV - should be able to give a stable controlled output.



Naresuan University
Phitsanulok
Thailand

 

[Tadders]

I have been in contact with Grudfoss Technical and provided them with details of the system so that they can correctly size a pump to do the duty we require.

However, in the meantime the contractors on site have removed the double reg valve, metering station and flow switch to ensure that these are not causing any restriction in the system. There was a very slight improvement in flow. Unfortunately, with the metering station removed it is hard to state exactly how much extra flow was achieved. They estimate that the flow is in now the region of 10 Lpm. The interesting thing is that the contractors moved the pressure gauge and installed it between the heater and the water bath. The pressure on the gauge read 1.5 bar? This pressure is on an open-ended pipe. Does this indicate that there is a blockage in the pipe between the gauge and the outlet? There are only two places left where there can be a restriction between the gauge and the tank that’s the gate valve on the inlet, or the inlet itself. I have asked them to break the pipe between the valve and the inlet and check for flow without running through the inlet. Lets see what results from this.

 

[25362]

Is there a possibility that upon heating the water, dissolved air is released creating a two-phase flow condition ?

 

[Tadders]

25362 - We are having flow problems even when the heater is not switched on and the water is cold. :-(