water Hammer 2

[WaterGIS]

Hi
Can anyone tell me how water hammer happens; I have been monitoring the pressure of a 600 mm ductile line, the log shows that the pressure varies for 10 bars at certain point. This fluctuation continuo from 8 am to 1 pm. Is this will cause water hammer ?? the measured point are between a tank and valve, I hoped that I could attached the log file.
Regards,

 

[quark]

Water hammer happens with sudden closure of a valve in water lines(condensate hammering in steam lines has some different cause).

A little description about your system is needed.

Regards,

 

[25362]

The pressure rise between 8 am and 1 pm seems to coincide with the morning sun heating a blocked pipe causing thermal expansion. Water at ambient (20 to 25 deg C) temperatures can build-up pressures on heating by about 4-5 bar/^oC !

On the other hand, hammering is caused by a sudden conversion of the velocity of a fluid into pressure, resulting in a surge of pressure inside the piping.

As quark says, water-hammer happens by a sudden change in the rate of flow or stoppage of flow in the line in a short time, shorter than 2L/c, as with a valve. Where L is the length of the pipe under consideration, and c is the celerity of the pressure wave. The number 2 represents the wave travelling distance back and forth.

When the pipe is ductile (ie, deformable without rupture) the value of c depends on the Young modulus of the pipe as well as that for water. Assuming we speak of steel, the wave celerity in water at ambient conditions would be ~960 m/s.

If the length of the blocked pipe is, say 1000 m, the time of closure to create hammering should be shorter than 2*1000/964=2.08 s. Any closing quicker than that would appear as instantaneous for the sake of this discussion.

Now, the pressure rise would depend on the fluid's velocity change, [Δ]V being stagnated. If, for example, the fluid travels at 1.1 m/s, the pressure increase would be:

(1000 kg/m³)(960 m/s)(1.1 m/s) = 1056000 Pa ~ 10.6 bar​

The resulting tensile stress should be added to the designed value to verify that it ís still safely below the elastic limit of the pipe.

What kind of operation is carried out between 8 am and 1 pm ? Can you enlarge your system's description as quark asked you ?

 

[WaterGIS]

our system is a main line that enter a tank and then after about 1000 m a valve after that it goes to a industrial area that work 24 hr daily. The line is covered at least 2m. the pressure varies fro 8 bar to 25 bar & there is a fluctuation in pressure from 25 bar to 13 bar every 30 min from 8 am to 1 pm. after the tank the pressure varies form 17 to 1 bar. The fluctuation is from 17 to 1 bar from 7pm to 11:30 pm 1 hr. the line is 600 mm ductile. Any further description.

 

[quark]

Is it a overhead tank? If so what is its height from the pipeline? There may be an end point which consumes water at every half an hour interval. When there no consumption, the pressure may be high and viceversa.

 

[quark]

25362,

Congratulations for being tipmaster of the week.

 

[25362]

To quark, thanks a lot. Kindly note that if colleagues like you grant me -so liberally- undeserved starts, no wonder about the selection.

 

[WaterGIS]

The tank is on the ground surface, the consumption is stable in that period. The flow is 24 hr it provide an industrial area that work 24 hr. and a populated area with about 10,000 populations.

 

[25362]

To WaterGIS, please answer the following questions to help us understand the problem and probably help out in finding the reasons and the cure:

1. Is the fluid water ?
2. How many and what type of pumps are used in parallel ?
3. What are the flow rates and pressures and their changes at all periods around the clock including the steady operation between 11 pm and 8 am the next morning ?
4. Describe the instruments that measure flow rates and pressures.
5. Is the tank an air vessel pressurized by a compressor, or is it a one-way surge tank; how do you monitor its liquid level ? Do you have another "flywheel" protection ?
6. Do you have a check valve (plus the gate valve) between the tank and the delivery pipeline ?
7. How long is the pipe line and what material is it made of ?
8. Do the consumers also have surge tanks and/or stand pipes ?
9. Do the suplly sections have different maximum pressures ?

Remember: pressure surges result from transient flows. Surging problems -at the supply end- can happen when:

-starting a pump
-stopping a pump
-switching pumps
-altering valve settings
-altering speed
-power failures
-inexpert operation

For a sudden change in flow rate the maximum pressure change, m, amounts to: (a/g).([Δ]c)

where:

a = speed of propagation of the disturbance (m/s)
g = acceleration of gravity (m/s²)
[Δ]c = change in fluid velocity (m/s)

The repetitive reflection times for pressures to rise above the norm, and then drop below it, is determined by multiples of 2L/a, where L is the pipeline length. Calculations are complicated by multiple piping ramifications.

The fact that after normal working hours, from 7 pm until 11:30 pm, pressure swing intervals are longer, and at lower pressure levels, seems to indicate that quark is right in assuming changes in consumption rates are probably the originators of the pressure surges.

Anyway, please try to describe the system in more detail.

Thanks.

 

[WaterGIS]

Hi
Thanks for your help
These the data you requested
1. the fluid is water
2. no pumps are used it is by gravity
3. The pressure at valve is from 25-20bar and after the tank exactly ranges from 3-18 bar.
4. we are using digital pressure logs and ultrasonic flow meters
5. the tank is normal tank, we have a normal level monitoring
6. No check valves are available, but there is an Automatic valve after the tank.
7. the distance between the tank and the valve is about 1.5 KM but it then continue to 20Km to an industrial area
8. The consumers have their own tanks and some of the big consumers have large tanks.
9. there is no large differences in pressure

Regards,

 

[HB2U]

since you imply that the pressure range follows a set schedule...."there is a fluctuation in pressure from 25 bar to 13 bar every 30 min from 8 am to 1 pm."

it seems that one of your industrial users may have a large water requirement in pulses as dictated by their process.

If they are drawing a significant amount of water, it could be causing a sudden depressurization of your system.

Just a thought.

HB

 

[c2sco]

Gravity flow can bring problems of its own - especially cavitation if you have vertical drops of barometric legs (~10m) in the pipe after control or isolation valves. Collapse of cavities can give excellent shock waves!
Can I suggest you look at

http://www.istec.com.uy/eng/products/hitrans/

(software seller for analysing water hammer)

http://www.process-notes.co.uk/Surge.html

(on my website that has information and some downloads on water hammer)
Regards,
Stuart

 

[BRIS]

WaterGis

Could you describe the type of valve that controls flow into the tank - is it a pilot operated globe valve. Also the type of vlave that controls flow out ofthe tank. I note that you say that there is an automatic valve 1.5 km downstream from the tank and then you have 20km of pipe after that. You cannot control a gravity pipeline from the upstream end which is what you appear to be trying to do. If you do then yes you will get erratic hydraulic performance.

 

[Cvguy]

WaterGIS

I sense a problem with the valve at 1000m. Is it a flow or pressure control valve? How is it actuated? If hydraulic does it have adjustable opening and closing speed controls?

Some hydraulic valves have special pilots to prevent over reacting on long pipelines. Ultimately a Surge Analysis will uncover problems and help with solutions. Here's one piece of information that might help.

http://www.cla-val.com/pdfsSF/B-59.pdf

Good luck.