Water Hammer in Pipe System 2

[Vesselsntanks]

I have a question regarding water hammer/hydraulic shock calculations in regards to a chemical piping system.

General Information: 14 inch pipe, specific gravity of 1.3

This specific situation:

Consider a control valve that closes in 1 second. The upstream piping consists of this 14 inch piping that has approximately 4 bends that goes vertically and horizontally in a total run of approximately 60 feet before it gets to a check valve.

I am just trying to get a feel for the appropriate formula or approach to calculate an approximately pressure generation from this event. This is for a project that has been cancelled, but if I ever have to perform such a calculation (or get an expert to help me), I would like to know the best approach. Obviously if I was serious about performing the actual calculation, I would measure the exact distance between all relevant components (valves, bends, etc).

So, is the formula for hydraulic shock reasonable here? P = 60VS/t? Or is dp = 0.070 dv l / t.

 

[BigInch]

The formula you are looking for is the Joukowsky equation.
Use it to predict possible waterhammer problems.

http://www.win.tue.nl/analysis/reports/rana06-08.pdf

If you are in the problematic range you should use more detailed methods,

http://hydraulics.unibs.it/hydrauli...eview_of_water_hammer_theory_and_practice.pdf

 

[DSB123]

As BI says for a conservative approach use the Joukowsky Equation.

 

[BigInch]

Although usually conservative in predicting pressure increases, it is not necessarily so in all cases, such as where column separation occurs, vapor pressure void spaces open up and then collapse. Different pressure generation mechanisms are in effect during that scenairo. It also only predicts positive pressure increases upstream of a valve as the fluid comes to a stop against the upstream face of the valve, whereas the designer may also need to consider vacuums, including column separation, that might be generated on the downstream side of the valve as well.

 

[LittleInch]

The only reason to get distance is to see whether the pressure wave gets reflected back from some other point - pump, non return valve, other valve etc.

At 1 second closure you've got about 500m. If the pressure source or other reflective component is more than that then you're looking at instantaneous closure, but you need to work out your particular speed of sound. You also need to have no other offtakes flowing in your line.

What you don't mention is velocity, which is key, alongside valve closure time.

As a ROT, anything below 2.5 to 3 m/second shouldn't cause you too much problem unless you really do slam shut a valve in a second or less - actually quite difficult on a 14" system to do that and if you assume 1 second that will give you very high pressures which might be ultraconservative.

The only way to see what is happening is a transient analysis and then you will need full details of your system.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[davefitz]

Water hammer is a timely topic for the newly proposed long distance pipelines . A review of the 1999 Olympic pipeline accident is instructive, as it shows the risk of pipeline failure due to waterhammer is enhanced when the control system for the system's vales is DCS based. See this link for the NTSB report
<

http://www.ntsb.gov/investigations/AccidentReports/Reports/PAR0202.pdf>

For the Olympic failure, the 100 mile long pipe carried gasoline from Bellingham to Seattle , failed due to a valve fail-closed event caused when the DCS switched to a backup server, which consequently output a fail closed signal to a stop valve. The requisite safety valve did not lift, and the pipe failed at a section that was damaged by prior backhoe excavation work. The change in momentum for a 100 mile long slug of gasoline traveling at ( 10 fps) stopping in ( ?? sec) was sufficient to fail the pipe. 3 persons killed . $112 million usd settlement. And a lot of landowners were surprised to then find that the gasoline pipe was routed under their properties 50 yrs ago, and it was not explicitly disclosed on later real estate closings. Many school playgrounds were found to be built atop the pipeline, surprising parents.
So some are now building 1000 mile long oil pipelines. Multiply that change in momentum by a factor of 10.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[LittleInch]

Dave, If you read carefully - page 57 applies and is copied below. This horrible incident was not primarily caused by liquid surge, but by damage suffered by the pipeline.

"Hydraulic modeling performed after the accident indicates that the pressure in the
pipeline at the time and location of rupture reached approximately 1,433 psig. This was
below the maximum operating pressure of 1,440 psig (revised to 1,456 psig after the
accident) for this segment of the pipeline. At 100 percent of yield, the pipeline should
have held 2,028 psig and, just after its installation, the pipe had been hydrostatically tested
to 1,820 psig. Because laboratory testing of the tensile and yield strength for the pipe
material showed that it met original specifications, the Safety Board concludes that had
the accident pipeline not been weakened by external damage, it likely would have been
able to withstand the pressure that occurred on the day of the rupture, and the accident
would not have happened."

The longer length of a pipeline normally works in your favour as the pressure wave gradually attenuates before it hits something (pump or valve) and bounces back. So the longer (and slower) the better.

All major pipeline systems are operated by DCS and remote control rooms. The one in the Olympic system may have been operated incorrectly with software changes being undertaken on the live system, but valve closures can occur at any time and the systems are designed to handle this. It was a fairly strong spike in pressure and if it hadn't happened the pipeline would have survived a bit longer until some other event which it should have been able to handle caused it to fail.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

Yes the root cause was the pipe damage, however you can also see, given that the damage was present and apparently holding tight at the normal operating pressure of the day, it was actually the instrument failure causing the transient pressure that did the deed.

 

[LittleInch]

DCS failure, poor operating practices and failure to find the damage on the ILI report

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[davefitz]

2 lessons from Olympic:
a) call before you dig
b) it is a fact that when a backhoe operator hits a UG pipe, he may, or may not , report it. I know as a fact that some marginal contractors will not disclose the event.So, there are damaged pipes out there.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick