Calculating Staticn Head For Watermains

When the pipe first fills, the pump will need to lift the water over the highest point of the land to get to the delivery point. So one needs to allow for the difference in height between the lowest inlet level and the highest point in the pipeline when selecting the pump.

Once the pipe downstream of the highest point has been flooded, and remains flooded, the water running downstream away from the high point will give the pump a helping hand - up to the maximum water column for that particular temperature, of course.

f the pipe downstream does not remain fully flooded, then it will not help the pump.

Im not to concerned about pumps, it is really only for testing of buried pipelnes which will be filled from the highest point on the line at a slow rate.

My concern is what pressure I will end up with at the lowest point on the line.

do I need to allow for the changes in gradient between the highest point and lowest point.

As seen in my amature Sketch

The head will be HP - LP. The intermediate stuff doesn't affect it.

The advise you have gotten so far is probably good, assuming you have exhausted air pockets completely from high points in the line. If you on the other hand have large air pockets trapped at known or unknown high points of the line, you may well not necessarily just get what you expect (based simply on elevation).

I agree. Air is lighten than water.

all good, but there are additional friction losses that will occur as the distance increases due to indirect route. Head will not be affected, but flow rate will.

Also, air pockets will dramatically impact both head and flow, so they must be evacuated.

As a practitioner, I will readily admit that predicted values rarely match actual values in real systems. It's very challenging.

I have carried out a hydrostatic test on a HDPE using a digital data logger to collect the information i.e. Pressure and Time,which produces a graph of pressue against time and will show the decay curve of the pipeline. (Graph from data logger attached, appoligies for the orientation)

In the spec for testing it says "plot the pressure decay against a logarithm of time" this is where i need a bit of guidance.

Many Thanks

Good Morning TrickyDicky,

First, how's Dick Nixon doing these days? LOL

Sorry, I couldn't help myself there. That, and I'm only on my first mug of "Joe" so far and therefore I'm a bit punchy, to say the last!

Is there anyway that you can rotate the data logger plot and increase the resolution so that it's readable? I cannot read the numbers.

I believe what the specification is calling for is for you to plot the time and pressure data on a semi-logarithmic graph paper. If your data logger has that feature, then by all means re-plot the test data. Hopefully you would have saved in in it's internal memory or at least transferred the data to a PC hard drive or a CD.

If you've never plotted a graph of data points on semi-log graph paper, may I suggest that you find a friend, etc., that has done it in the past and lend you a hand. It's pretty straight forward stuff and you'll end up with an exaggerated plot (the pressure data points). You can buy the semi-log graph paper at almost any office suppl store such as OfficeMax or Staples, or you can find it in a Community College Bookstore.

From what I can tell from your datalogger plot is that after the initial pumping up of the pipeline the pressure peaked and the pumping stopped, then the pressure dropped a little as the system pressure stabilized and then mainly plateaued (system mainly held pressure or leaked very little. Of course at the end of the pressure and leakage testing the pressure was relieved in the pipe system and the pressure eventually fell to zero, or nearly so.

See what you can do to re-insert that datalogger plot at a much better resolution, okay? Me thanks ya, as my eyes aren't what they used to be.....

===Signed,

CaptMoosie, PE/PhD

ps: Almost forgot! Didn't you attempt to re-pressurize the system after the test to determine the leakage present during the Hydrostatic Testing procedure? You need to do that in order to compare to the calculated (Theoretical) leakage allowance in order to determine if the pipeline construction was adequate or not. Also, once you determine that the test was successful you could have injected your disinfectant water (Sodium Hypochlorite solution) simultaneously at the time you begin the Hydrostatic test...this would save you time and labor costs, if previously approved by the Engineer of Record.

Eye Eye Captain,

Im not sure that graph is going to be much better but its the best i could do, cant put my hand to semi log graph paper to handy here in Ireland, but I was thinking maby I could draw the graph in Auto-Cad, would it just be a matter of calculating the log values for the pressure (in bars) and the value of time (in minutes) or does that just sound to easy??

I have to put a foam pig through the pipeline before I can chlorinate them, working for a local authority everything to the book here, no such thing as a short cut, and certinaly not if its going to save a contractor money.

When I made my initial reply of 2/11/2011 4:44 I was not aware you were involved with or trying to interpret some hydrostatic testing of a hdpe pipeline. I noticed Dr. Moosie had previously provided the rather straightforward testing procedure for ductile iron piping, that can involve a small "make-up water" or testing allowance. You might have on the other hand a greater challenge e.g. if you are now trying to determine whether or not you have any leakage in an installed underground polyethylene pipeline (due to defects, damages in underground installation, or oozing out of improperly made or over-strained fusion joints?) From what little I have seen and what few numbers I have crunched, the make-up water allowance promoted for installed hdpe pipelines can actually be much greater, or even several times that for ductile iron pipelines per long-standing AWWA C600 procedure. How (with all the variables that would appear to be involved) you can really be assured how much of the pressure loss or this make up water is going to some sort of (precise?) expansion of the low modulus/strength pipe with all various degrees of confinement, and not any oozing out of problem areas in the pipeline, I don't have a clue (I suspect this may be your real question). This may be however/perhaps a very good question for the pipe manufacturers, and/or the authors of the hdpe testing procedures. Good luck, and let us know what you find out.

Hi Tricky.

As said, the static pressure is the difference between the elevations, but if air locks are in the system, (the full width of the pipe is completely dry at high points) then your pressure will effectively be lower than the maximum head pressure. (If your pipe had a rise of 2m that was full of air, then your static pressure would be reduced by this amount. If you have bleed valves on rises this will not occur.

Your mains delivery pressure must also be added onto the static head pressure also.

If you can increase your flow rate to about .5m per second then this should purge all air out of the pipe.

Dissolved gasses in the water can separate when negative pressures are preset.This can occur if the lower end of the pipe would deliver more water (due to the fall) yet the friction loss or the delivery from mains allows less).

Also high temerature water will not hold as much dissolved air as cold water does, so a pipe left on the surface can tend to get air locks without any water movement.This occurs more quickly when slow flows of water, (which do not carry out the bubbles that form), still bring more disolved air into the pipe while the degassed water passes on.

If mains supply line flow rates are low, and the pipe is open at the bottom you may also encounter pipe collapse unless air is introduced at the top to relieve the suction effect to acceptable levels. I've seen 500m of 50mm poly pipe that has been sucked flat due to this.

Bear in mind also that higher than static pressures can occur when a tap at the bottom is suddenly closed and the column of moving water can then well exceed pipe manafactures workling pressures. A shock relief valve (which dumps excessively high pressure water) or water hammer arrestor can be fitted.

Sorry I have no qualifications in hydraulics, Just a farmer!

Static pressure is calculated by P = (Z1(ft)-Z0(ft))* (62.4/144) in english units

Elevation diffrence time Yw (62.4 lbs/ft^3) / (12 inch * 12inch) resulting in lbs/in^2

Z1= elevation where you are looking

Z0= Hydraulic grade line (HGL) elevation of supply.

The high points and other valves onpe come into play when you are moving water (minor losses)