Excessively High Static Head Reading
Excessively High Static Head Reading
(OP)
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.
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.





RE: Excessively High Static Head Reading
RE: Excessively High Static Head Reading
If the contractor inadvertently installed a high point between ARVs, could that cause a consistent false static head reading? The ground profile is at the highest point at the discharge, so there should be no way for an intermediate highpoint to be higher than the discharge. With that in mind it just doesn't make sense that an air pocket would cause a false static reading beacause there is a known maximum water column elevation, but I am starting to question everything at this point.
RE: Excessively High Static Head Reading
Now, with a water-filled main, with no air, the static head is just the difference in elevation from one end to the other, BUT, if air occupies the "other side" of a high spot some "Artificial Static Head " can be introduced. A series of these high spots can add up fast. I will attach a sketch later. Anyway, we profiled the main using locating and depth measuring radar, and began to add air valves. With each air valve installation the static head dropped.
Steve
RE: Excessively High Static Head Reading
Would we be able to narrow down where the issues might be occuring by placing pressure gauges at the existing ARVs and comparing gauge readings to calculated values? This force main is 7,500LF so a sub-surface exploration along the entire route would be very expensive.
RE: Excessively High Static Head Reading
Steve
RE: Excessively High Static Head Reading
your website gswagner dot com was blocked by our corporate firewall as being weapons related. can you re-post the image using engineering.com (the link below)
RE: Excessively High Static Head Reading
Steve
RE: Excessively High Static Head Reading
Contractors....can't live with them and can't live without them.
RE: Excessively High Static Head Reading
Have the surveyors traversed this line from end to end? Are the refernce points correct?
RE: Excessively High Static Head Reading
Air is not like water, it is a compressible fluid. In general terms, given pressure it will compress at pump start; but, once compressed, it can violently expand and cause high pressure readings as well as break things.
The air in a pipeline will also cause an artificial restriction in the pipe diameter and without enough velocity cause the pump to operate at a lower point on the curve (giving lower flow and higher head). This explains your duty point problem; but, without flow (at equilibrium) I don't see how your pressure will increase. Is there another pump connected to this line somewhere between your pump and discharge point?
RE: Excessively High Static Head Reading
Our surveyors have run re-run this route 3 different times and they come back as correct.
To semo:
This force main is dedicated solely to this pump station, so the static this station should see would just be the elevation difference.
I agree this concept seems illogical, I just can't come up with any other explanation. If there is pump operating every 5 - 10 minutes would that truly give the 7,500LF of forcemain time to come to an equilibrium if there was air in the line? When the pumps cut off the gauge bounces around a good bit, but it always settles back on 42' of head between pump operations.
RE: Excessively High Static Head Reading
Steve
RE: Excessively High Static Head Reading
If there is a pocket of air in the pipeline that is trapped by a high point it will be compressed when the pump begin pumping and increases the pressure at that particular reach of pipe. When the pump stops the air pocket will expand and push some of the water out. Because there is probably a check valve by the pump, any water that is displaced by the air pocket will be pushed towards the discharge side of the pipeline. This would mean that the water on the downstream side would maintain its level at the peak of the flowline. The water column from the discharge point is now placing a hydrostatic head on the air pocket and if the air pocket occupies an area that encompasses the entire area of that section of pipe, the pressure of the air pocket will match the hydrostatic head. That air pocket will then place that same hydrostatic head on the column of water on the pump side of the pipeline. Because there is a portion of the water column on the pump side of the air pocket that is higher than the bottom of the water column on the discharge side of the air pocket, there may be an increase in hydrostatic head.
Not sure if I explained this well or not, and I'm not sure I'm totally convinced in my explanation.
Let me know what you think.
RE: Excessively High Static Head Reading
Steve
RE: Excessively High Static Head Reading
Howver, you are stating:
"The discharge is to a gravity sewer manhole and is the highest point along the alignment."
RE: Excessively High Static Head Reading
Steve
RE: Excessively High Static Head Reading
It seems like if there is an unequal water surface around a high point with pressurized air between them, you "stack" the water columns. Using your intial sketch, at point B the air is getting the pressure from the water column on the down stream side (from point B to the discharge). The air then transfers that pressure/head to the upstream water column, causing an additive effect.
For example, assume the pump elevation was 0', the intermediate hight point was 5', the low point was 2' and the discharge was 10'. If the pipeline is full of water the static head would be 10', but if the air pocket was contained in the pipeline between the intermediate high point(elev 5') and the next low point (elev 2') then the water columns would add up, 5' - 0' = 5' and 10' - 2' = 8' thus the static head at the pump would read as 13' not 10'. That is why it only is an issue if there are unequal water surfaces.
Does that sound correct?
RE: Excessively High Static Head Reading
Steve
RE: Excessively High Static Head Reading
Then you would be left with the static head being the difference between the high point and the low point.
In addition, as the bubble travels moves to the high point, the static head should decrease.
After checking the hydraulics of this application, one would think that the flow rate is too small causing the forcemain to be fouled with solids.
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.
When you operate, you should only see 10 Ft of friction @ 1250 gpm or 4.5 Ft of friction loss @850 gpm. You are saying that you have 77 feet which indicates that the forcemain is fouled.
Note that 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.
Some additional questions:
How long has the forcemain been operating at this rate?
What size sewer does it discharge into?
Are there any valves in the forcemain besides the check valve?
What is the brand name of the check valve supplier?
Have you inspected the "combo valves" along the route and are sure that the vaults are not flooded?
RE: Excessively High Static Head Reading
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.
RE: Excessively High Static Head Reading
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.
RE: Excessively High Static Head Reading
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.
RE: Excessively High Static Head Reading
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.
RE: Excessively High Static Head Reading
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.
RE: Excessively High Static Head Reading
RE: Excessively High Static Head Reading
Steve
RE: Excessively High Static Head Reading
RE: Excessively High Static Head Reading
RE: Excessively High Static Head Reading
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.
RE: Excessively High Static Head Reading
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.
RE: Excessively High Static Head Reading
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.
RE: Excessively High Static Head Reading
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.
RE: Excessively High Static Head Reading
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.
RE: Excessively High Static Head Reading
Steve
RE: Excessively High Static Head Reading
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.
RE: Excessively High Static Head Reading
RE: Excessively High Static Head Reading
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.
RE: Excessively High Static Head Reading
http://aplv.org/files/docs/AirInPipesManual.pdf
RE: Excessively High Static Head Reading
Steve
RE: Excessively High Static Head Reading
Sewers and/or force mains in the developed world would not be installed with this piping scenario. The term that pipe designers use in the developed world for sags in the piping is the word "pocket", not socks. Perhaps the Spanish translation for pocket is sock.
There is the point of critical velocity mentioned in the paper. That is the minimum velocity that will flush out the air. The Water Pollution Control Federation Manual of Practice No. FD-4 "Design of Wastewater and Stormwater Pumping Stations" states "A minimum velocity of 1.2 m/s (4 ft/sec) is required in the pipeline to shear the bubble and keep it moving downgrade."
Your application involves a force main, not a gravity flow. Force mains should not have air trapped in the piping. The air should be removed on startup and should not be reintroduced during normal operations.
Reread some of my previous post:
"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 re-suspended. 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."
Unfortunately, you have a problem with the design of the system. Your force main was designed with too low of a velocity. You need 3.5 feet/sec velocity to re-suspend the deposited solids. You need 4 feet/sec velocity to flush the air out of the force main. You do not have this velocity.
Unless you increase the velocity, the problem with air and solids is going to reoccur. Removing the air will provide a short term fix until the air returns.
RE: Excessively High Static Head Reading
What is the diameter of piping for the pressure gauge?
I've heard of issues involving air or crud being in pressure gauge piping which can adversely effect the reading.
Has this piping been checked?
RE: Excessively High Static Head Reading