Operating Point Not Coinciding With Desired Design Performance Point 2

[zazapon]

I was hoping that someone here could clarify something for me regarding pump operating point. I have a project where I need to select a pump that will discharge wastewater to LPP field. I want the pump to pump 80 gpm of wastewater to the LPP field. Based on this desired flow rate of 80 gpm I have calculated my TDH to be 34.79 feet based on headloss, friction loss, etc. In addition, I have calculated TDH at different flows, other than 80 gpm, in order to graph my system curve. I then took this system curve and overlayed it with the available Goulds pump curves (see attached mark-up). I wanted to oversize the pump just in case the calculated TDH was on the low end to account for any uncertainties and therefore I selected WE1512HH pump. As you can see the calculated system curve and pump curve for WE1512HH pump intersect at 92.5 gpm and TDH of 42', making this the operating point. The state review agency commented that my desired pumping rate of 80 pgm at 34.79' TDH does not coincide with the operating point of 92.5 gpm at 42' so here are my questions.

1) The state claims that the pump will actually be pumping 92.5 gpm at 42' TDH even though the pumping rate is set to 80 gpm. Is this right? Based on a similar approved project that I have reviewed at this agency's office, a different engineer did exactly what I did and it got his project approved so they either missed it or we're both wrong. Basically, he selected a pump that would pump 82.5 gpm at 28 TDH while he showed his operating point (intersection of calculated system curve and pump curve) to be 107 gpm at 32 TDH so I am confused as to why they are questioning it since the pump has more capacity than required.

2) I think it will be next to impossible for me to get a pump curve that will intersect system curve at 80 gpm and 34.79 TDH exactly (ie. operating point = desired pumping rate of 80 gpm and calculated TDH of 34.79') unless I use variable speed pump. Based on specs, WE1512HH doesn't look to be variable speed pump.

3) I could use WE1512H pump curve instead, which would bring the operating point closer to my desired pumping rate of 80 gpm at 34.79' but it would not be exactly dead on either because operating point would be around 80 gpm at 36' TDH. In order for me to get my set pumping rate and operating point to coincide exactly I think I would have to lower my pumping rate a little bit to something like 79.something and do iterations until I could get selected pumping rate and operating point to coincide exactly but that seems like a huge waste of time when you consider uncertainties regarding losses, friction coefficients, etc.

These questions might seem very basic to some of you because this has been the first project of this kind that I have done so I want to make sure where I went wrong. Can someone help?

Thanks

 

[LittleInch]

Ok,

I'm assuming here that you've got the 1512HH and 1512 H pumps the wrong way around according to your attachment as the HH (I assume this stands for high Head) seems to intersect your system curve at 80gpm "WE1512HH pump intersect at 92.5 gpm and TDH of 42'"??

Your questions
1) I don't know and I don't think you'll find out either. However it depends on what the primary purpose / control point is. If you only want to move a certain volume of fluid in a maximum amount of time, then pumping a bit faster than you intended is not a problem.

however if, as I think a Low Pressure Pipe system is, control of the maximum flowrate is important to avoid flooding?

2) It is virtually impossible to get any simple pump and pipe system to work exactly as anticipated. There are so many variables and factors that in general if you get to within 5% of your intended flow or pressure that's very good and +/- 10% is common. It is normal for all the calculations to be conservative and hence more flow or pressure is the most common outcome. however as you can see from your curve, the steeper the two systems interact the less the flow changes with small (+/- 10%) changes in system head for the same flow. Pump heads as supplied, especially for basic units like this are often +5%.

you can also use a throttling / control valve to regulate flow and this is the most common way of modifying flow for such systems

3) As I said above, the level of accuracy you seem to be aiming for is not practically possible and if, for some reason, you want to get exactly 80 gpm and not 79 or 81, then you will need to add in a flow control loop whereby a control system takes flow readings and then adjust either a VFD controller or a control valve to modulate the flow.

For the duty you're looking at (waste water disposal) I'm very surprised that "the state" is being so apparently strict unless they don't really understand the operation. My brief understanding is that this type of system pumps water for a short period then stops, lets it drain away then starts again. Hence the difference in flow rate for 80 to 92 gpm is simply a small reduction in the time you pump for at any one time??

for more sophisticated pumps you basically set a duty point and the vendor machines the impellor to suit. I think these pumps are much more basic.

what's wrong with a WE10H which seems to cross the line at exactly where you want it
to? In reality, this pump will probably do between 75 to 90 gpm (all simulations are normally conservative)

hope this helps.

BTW your design and thinking appear to be spot on and I can't see any obvious errors.

LI



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

 

[mk3223]

I think what the State Reviewer said is that the wrong pump type was specified per pump P-Q curve on the review you submitted for review. You specified the pump WE1512HH which can only be 80gpm @ 34.79ft TDH.
IMO, your design approach to have the certain margin on the pump hydraulic is fine and typical for selecting the pump. Either WE1512HH or WE1512H can be a good fit for your project if the costs are not off too much.
Good luck to your project.

 

[LittleInch]

zazapon,

did this clarify your point / concerns?

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

 

[zazapon]

LittleInch,

Yes, it did. Thank you for your prompt reply. I have used some of your points in my reply to the state reviewer and added a bunch of others. Still waiting to hear back from them. I basically said that there are other ways to calculate losses such as using Darcy Weibach method (I used Hazen Williams method since that's what they require) and that the loss coefficients that they have published differ somewhat from other sources. Therefore, the calculated head of 34.79 ft will probably be different in the field since there are multiple ways to calculate TDH so going with a slightly oversized pump (WE1512H) instead of WE1512HH would give me some cushion.

The force main that runs to the LPP fields will also have some minor changes in the profile due to pipe and joint deflections (I did account for bends from 22.5 degrees to 90 degrees fittings) so theoretically there should be some additional unaccounted friction losses. Therefore, if they insist on making the operating point match up exactly with the selected 80 gpm rate I will just pick WE1512HH pump and add an extra 1.21 feet in losses to TDH and list that line item for losses as "safety". I have seen some other engineers arbitrarily list 1 or 2 feet of head loss in TDH calculations before. This way the system curve and WE1512HH curve will intersect exactly at Q = 80 gpm and TDH = 36' so it should be satisfactory to them.

 

[Artisi]

The field TDH will be known once the plant is commissioned, running and settled down to the normal operating condition, prior to this it's all theory.
I haven't followed the story all that closely as you were getting more than enough good advise - but my suggestion is that the pump is selected to be slightly oversized from the theoretical calculation, if it turns out to be "too" big, you could investigate an impeller trim, if cutting to fine with a margin and it turns out to be undersized running with a full diameter impeller you have a problem - you either have to reduce the operating total head or change the pump.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)