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.

 

[bimr]

Not sure how useful this document is. The subject seems to be how does one get water streams to flow down a mountain side through 1” pipes. Unless you reside in a rural mountainous area, the information is of no use.

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.

 

[drebelx]

This is an interesting problem.

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?

 

[bccwwes]

I believe the condition described can only occur as a static head increrase if the check valve is holding the liquids as shown. If you release the check valve and re-measure the static head do you get a different reading?