Excessively High Static Head Reading 5

[elcid0M6]

I am working on a pump station that has everybody scratching their heads. This was an existing pump station with a forcemain that trended downward to the discharge. Our clients decided to send the flow to a different portion of the system, so we tapped the existing 16" FM and routed a new FM to the other portion of the system. This new force main is shorter but trends upward. The discharge is to a gravity sewer manhole and is the highest point along the alignment. We have 5 combo valves at the various hills along the alignment to vent any entrained air, but again the highest point on the FM is the discharge.

The pumps should operate at 1250gpm @ 59'TDH based on calculations, but we performed several drawdown tests over the past couple of months(taking an inflow first to account for it) and keep getting readings in the 800-850gpm range. The latest drawdown was 838gpm @ 77'TDH, and this point falls directly on the pump curve. We performed a shut-off head test on each of the 4 pumps and get right at what they should be.

We then re-ran the calculations (to make sure there wasn't a bust) and came up with the 1250gpm @ 59'TDH value again assuming a C-value of 120. The discharge elevation is 42' and the gauge elevation is 20.5', thus the static head reading on the gauge should be 21.5'. The issue is that the gauge is reading approx. 42' of head. We have changed out gauges and had 2 gauges operating off the same line and they are all reading the same values. We have had our surveyors out to check the elevations 3 different times and they come back correct as shown. We have physically watched the guys manually bleed the lines to ensure there is no air in the line, and as soon as they turn the valve you get liquid.

What could possibly cause the static head to be off by approx. 20'? Why would it also carry through when the pumps are running?

Any help would be greatly appreciated.

 

[elcid0M6]

bimr:

The gauge elevation is 20.5' and the average water elevation during the drawdown was -0.875', thus the static head the pumps themselves were seeing was 63.375' (42' + 21.375'). Then subtracting that from the 77' of TDH, that the pumps were seeing, the friction losses were approx. 13.6' at 840gpm.

The pumps since start-up have been in the 750-850gpm range. We have never gotten anything close to the 1250gpm design flow. The pumps were first tested in January of this year so 7 months of running sub-2fps would definetely create some build-up, but due to the fact they have never hit the design flow in the first place I don't really think I can blame it on that.

The line was pigged after installation, but then the line was taken down and refilled later. That would have cleared any blockages, but would still allow air to be trapped between ARVs.

This 16" FM discharges to a manhole on a 30" gravity line.

There is one 16" gate valve in the new FM to allow for the ability to switch flow from the old FM to new FM and vice versa. There is also one 8" plug valve in each pump discharge prior to the 16" header.

The check valves are inside the valve vault and were not changed as part of this project, so I honestly don't know what brand they are.

I have physically seen each combo valve vault and they are all clean and dry.

 

[bimr]

There are a few more parameters that you can check;

Have an electrician measure the current draw while the pump is operating. You can check the hp draw vs pump capacity.

Check the pump data sheets to confirm that you have the correct pump installed and that the operating parameters such as voltage and rpm are correct.

How are you measuring flow? Do you have a flow meter?

Is the pump installed correctly? Recently we had a situation where the submersible pump was installed too close ton the bottom of the wetwell choking the pump inlet.

 

[elcid0M6]

Now this project has taken another wierd turn.

The client sent their operations guys out to take pressure readings at the ARVs along the pipeline, and the attached graph is the information obtained.

I have no idea how to explain what is going on. There is 850gpm coming out of the pumps, so there is no way there is a complete blockage in the line. With the ARVs at the end of the line being right on, and all the readings were taken with the same gauge (they took one gauge from one ARV to the next), I am at a loss.

 

[cvg]

I would go back and re-read bimr's post -

I think what you have is the forcemain is full of solids and this fouling is causing the backpressure to be held when the pump is shut off ... it takes 3.5 ft/sec velocity to resuspend the solids once the forcemain stops and the solids settle out.

You may have to have the forcemain cleaned to remove the solids.

your velocity is less than 1.5 ft/sec and generally 2 ft/sec is a minimum design value for force mains. On top of that, you have a pronounced low point in your line which acts as a natural collecton point for solids.

 

[elcid0M6]

That explanation works for the additional static head, it just doesn't seem to explain the static head being 20ft below what it should be at ARV #2. If there is a backpressure then I could see the static head being significantly higher, but would it not be flat as well (look like a stair step down to theoretical) not go from 15' above to 20' below theoretical in 1,500ft?

As a clarification - The line shown for the top of pipe elevations is just from the elevations at the ARVs, not the entire FM. ARV #2 is just the lowest ARV, but there are sags in the pipe before and after ARV #2.

 

[cvg]

I think the issue to focus on is that your pumps are operating at just 70% of design capacity. That is likely the result of both air and solids blocking the line.

 

[SteveWag]

Why is ARV#2 at a low point? I think #2 is at 22 or 23 ft. The problem is defined in the first post, the abnormally high static head. If the pressure when the pump off is not as expected then all bets are off under dynamic conditions. The static head must be addressed!! Solids deposition, motor current pump mounting, flow meter calibration motor rotation, etc have NO influence on static head. With a waste pump, additional head can be devastating.
Steve

 

[elcid0M6]

On the pump side of ARV #2 there is a low point of (-)1.65'ELV(top of pipe) and on the discharge side there is a low point of (-)2.0'ELV. ARV #2, 7.3'ELV, is in a high spot between 2 swampy areas.

 

[bimr]

How many pumps do you have operating at one time? And what type of pumps are they?

 

[elcid0M6]

There are 4 pumps in the stations, typically only 1 is required, sometimes 2. The pumps are Myers 4VC 10"-impeller 30-hp 1750rpm. One pump should be operating in the 1200gpm range, based on design numbers and that is conservative.

The pump reps have been on-site and checked the amp draws, shut-off head, and motor information. They were all where they should be, so we know they are the correct pumps and are operating as they should.

 

[bimr]

Your pumps should have been sized for the 16" forcemain. That would equate to a minimum flow rate of around 2200 gpm. What is the purpose of having 4 pumps? Can more than one pump run at a time?

How do you come up with the 1250 gpm @ 59' TDH? My progrqam shows 9.9 feet of head plus a static of 26.1 feet for a total head of 36 feet when pumping at 1250 gpm.

Are these pumps in a wet well or dry well?

Is it possible that you are pumping air?

Are you sure that the pumps are operating on the pump curve?

Are you sure that the pumps are installed correctly. I has a Contractor sit the submersibles on the floor which caused the pump intake to be starved. Later, the Contractor had to install a spacer under the pump to restore the capacity.

 

[elcid0M6]

bimr:

The pumps were sized for the 16" FM based on state regulations which states a minimum of 2fps in a FM. That is where the ~1250gpm number comes from. The pumps are set-up as a series of lead-lag floats. #1 ON, then #1 and #2 ON, etc. State regulations say that a station's "firm capacity", wich is the amount of flow you can send to that station, is the flow the station can output with the largest pump out of service. So essentially this pump station is a three pump stations with a redundant fourth pump in it.

The bottom/invert elevation of the wetwell is -7ft and the discharge elevation is +41.8ft, so being conservative, there is a maximum static of ~49ft. Then you add the dynamic head of ~10ft to get to the 59ft of TDH. You might have confused the gauge elevation as the pump elevation, and I apologize if I didn't make that clear.

The pumps are in a submersible set-up, so they are in the wet well.

It is possible that the pumps are pumping/entraining air into the FM. There is a free-fall of 5.5ft at the final pump off elevation, which makes for a turbulent wet well. That is why there were combo ARVs installed on the new FM. In the same respect, if there were high points installed that didn't have an ARV on them they would likely collect that entrained air.

Based on the latest draw-down test, the pumps are operating on the curve.

There were HOMA pumps originally in the PS, and there were adaptors to attach to the Myers pumps to the HOMA base which is supposed to be a clean swap. Two of the bases were entirely replaced with Myers bases due to deterioration of the HOMA base. The two Myers and two HOMA bases all have pumps that operate similarly.

_______________

These issues would seem to effect the dynamic HGL, but I am not seeing the correlation to the static HGL. If we can't make the static head make sense then the dynamic is never going to make sense. I am sure that solids have settled out since the pumps were started up that have increased the friction losses, but the station has never operated at the point it should. You can't really claim the pumps not operating as they should is due to the solids that have settled when that is a consequence of the pumps not operating as they should from the start.

 

[bimr]

Weird phenomenon may occur when air is trapped in a pipeline:

1. The higher pumping headloss during normal pumping is most likely caused by air trapped in the pipeline, effectively narrowing of the flow path, most likely in the initial downward sloping segment.

2. The velocity of 2 ft/sec is too low to force the air to move down the pipe.

3. The high static reading of 21.5’ is probably caused by a combination of air trapped in the line and fouling with solids over the first 1000 feet or so. 21.5’ is not enough head to force a blockage down the pipeline. It does not take much fouling to cause 21.5' of static head. This segment is pressurized during pumping and the downstream fouling in the pipeline is causing the pressure to be maintained.

4. One would suspect that the static head reading at the 1st ARV is incorrect because of fouling.

It would be interesting if you could purge the 1st ARV and then check the pressure reading. That would be relatively inexpensive.

You may also consider pumping off the front end of the pipeline at the lift station to see the effect on the static pressure.

As to the velocity, you do not seem to understand that the minimum flow rate of 2 ft/sec is inadequate. That velocity is for a pipeline that operates continuously and the pipeline in question does not. At the minimum flow, the solids and grit content of wastewater is lowest and it is the grit that may settle out.

An initial velocity of 3.5 ft/sec is desirable to ensure that deposited solids are resuspended. Refer to Metcalf & Eddy: Collection and Pumping of Wastewater.

You are going to have to find some way to increase the velocity.

You might consider renting a pump to prove to yourself that this concept is correct.

You may also be able to operate multiple pumps in order to increase the flow rate. However, the pumps must be designed with higher HP in order to be able to operate in parallel.

If you do nothing, one would expect the pumping rate to decline.

 

[elcid0M6]

I have updated information on this project....

We let the force main sit unused for a week or so and took static readings at all of the ARVs again. ARV #s 2-5 all where right on the correct HGL, while ARV #1 was 7.8ft higher. The first round of readings showed ARV #1 as 7.9ft higher than the "correct" HGL. That says it is a consistent/repeatable issue between ARVs #1 and #2. We looked at the profile again and pegged a place that could possibly be an issue. We had the line located and probed for depth, and there were interesting revelations. The line was designed at 11' off EOP along this portion of line, and it turns out it was installed 20' off EOP. This put the ground profile approx. 2' higher than shown on plans. Then the pipe was only 3' deep instead of 4' as shown on plans. Therefore, the pipe was a total of 3' higher than it should have been, creating a significant high point and an air-trap. We are planning on tapping the line and installing an ARV at this high point to expel the existing air and any future air.

As an aside, SteveWag's theory of the uneven water columns adding static head has been a point of contention since this thread started. I explained this theory to some of client's representatives; I had a couple of them with me and a couple who completely disagreed with me. One of the gentlemen was intrigued by the theory so much, that he actually created a physical model of the scenario. It worked out exactly as SteveWag explained, where the difference between the water column elevations adds up to a “false” static head. I have attached a photo of the model, and in this case, a picture is worth a thousand words.

 

[SteveWag]

eicid0M6, you must follow up with the results of the proposed field work. And, if you see the gentleman who made the “test”, give him my best regards.
Steve

 

[bimr]

Not so fast. Your use of the picture colloquialism reminded me of another colloquialism. Like a mule with blinders on.

In tubes smaller than 15mm diameter, surface tension effects are appreciable. I don't think the demonstration is meaningful or proves anything.

After all, you referenced 20 ft of head.

 

[rconner]

I will apologize upfront as my question is probably "stupider". What are we supposed to see in the picture?

 

[elcid0M6]

bimr:

The pipe is 1/2 inches in diameter. I just did the calculations on the capillary rise, and at worst case (wetting angle of 0 degrees) the rise is 0.0460 inches. I believe the gradations are in inches, and thus there is no way 5 inches of difference in water columns can be attributed to surface tension. Besides that, what are the odds that the difference between the water columns in the two intermediate high points would add up to the difference between the outside water columns, exactly as described in the theory and be attributed to water tension?

I have come to the realization that no matter what is said or demonstrated, you will not agree with this theory. I will post results on the static head after installing the ARV and you can make your own conclusions based on that (even though I think you mind is already made up).

rconner:

If you read back through the post, the theory at hand is that if there are uneven water columns with air trapped between them, then the difference between the water columns creates a "false" static head. The picture shows that the difference between the two exterior columns (static head) is exactly the difference between the two intermediate water columns, thus the "false" static head.

 

[Spartan5]

This document will help to shed light on the situation illustrated in your photograph and how air trapped in the pipe contributes to abnormal static head readings:

http://aplv.org/files/docs/AirInPipesManual.pdf

 

[SteveWag]

So they are called “socks”. And in most sewage pumping plants we are unable to reach Qs. That also explains why I have to re-prime my siphon!
Steve