PRESSURE SURGE 5

[PUMPDESIGNER]

I have been analyzing PVC pipe lines for about 12 years with the formula below. I have found it to be useful and with 12 years of experience designing new lines and correcting lines with pressure surge problems I have good confidence that the results are useful and insightful.
However, the results I obtain with this formula seem to give high results, as if the formula gives worst case results scenario.
One factor that seems to be missing is the flexibility of the PVC pipe.
Question 1 - Is there a simple "add on" that I could use in this formula to take into account pipe material flex?
Question 2 - Does anyone know the history behind this formula?
Question 3 - Any other comments on the formula?

PR = [(.070)(V x L)] / T
PR = PRESSURE RISE
.070 = CONSTANT
V = VELOCITY
L = PIPE LENGTH FROM SOURCE TO STOPPAGE
T = VALVE CLOSURE TIME
Richard Neff
Irrigation Craft

 

[MortenA]

One of the most basic formula is the Joukowsky equation (ref Perry's Handbook):

h_wh=a*dV/g_c

where

h_wh= waterhammer head (m or ft)
a: sqrt(1/(den/g_c*(1/K+D/bE)

dV= velocity change
g_c: acceleration constant
den: Fluid density
K: Fluid bulk modulus of elasticity constant
D: Pipe inside diameter
b: pipewall thickness
E: pipewall modulus of elasticity constant

This represents the maximum pressure surge experienced at a sudded (instantaneous) velocity change and can be derived from Newtons 2. law (according to Perry - dont ask me ;-)

I cant directly see how it relates to your formula

Best Regards

Morten

 

[saxon]

pumpdesigner, If the formula is giving good conservative results as it stands; why complicate the issue by concerning yourself with the additional work required with taking into account wave propogation, valve closure time, and pipe elasticity and the wonders of partial differential equations. When these get pulled into the equation things start to get real confusing, real fast. If you have an interest, check the "Handbook of Hydraulics", by Horace King, Mcgraw Hill. There is about twenty pages of analysis on this phenomena. It would be nice to be able to put it all in a nutshell, but it requires the full analysis.

Hope this helps.

saxon

 

[BobPE]

Pumpdesigner:

You are right, the modulus of elasticity for the material is missing in your formula and I would assume it to be a compoinent of the constant you state. But I would also assumme that the constant is for a metal pipe and not PVC since these formulas are most likely from the time before PVC....It is this pipe elasticity that gives rise to the pressure wave since the fluid itself is not compressible. MortenA listed the right equation and as saxon said, its a complicated thing, but you must do it right, because to do it wrong would mean disaster with this phenomena.

BobPE

 

[stanier]

One thing that you should be careful about is that the Joukowsky equation does not predict the pressure spike resulting from column separation and recombining. These pressures can far outpace those predicted by the equation.

References see Wylie & Streeter, Boldy or Chaudry.

If you do a google on Boldy there are extracts of his book on the web.

Aslo check out Hytran from Accutech in Australia. The professor who developed the software has excellent notes on the net.

Take care with PVC as it has a reletively low fracture toughness and thus could be damaged by transients.

If you've been doing this for 12 years why not invest in some real software such as AFT's Impulse?

http://www.aft.com.

Its a great production tool. The time you save and the confidence given by a verified software package will pay for it in no time. It also satifies the QA, customer requirements etc whereas a formula from an unkown source such as yours is pretty hard to defend in court. A failed fitting in Sydney recently cause $2.5 million worth of damage to computers on the floor below.

 

[RWF7437]

If you'd like to try a simple water hammer program WHAM is a shareware offering, free to you.

Go to:

http://me.queenu.ca/sellen/home

Russ Faust, PE
Salem, OR USA

 

[waseem19]

RWF7437
i'v tried your link and it's not working can you update it or tell me where else i can find this software.

 

[Ag93]

The pressure transient that results from a water hammer event is directly related to the stiffness of the pipe material.

The shockwave speed (S) that results is determined by the stiffness of the pipe material.

S (ft/s) = sqrt(((144*E)*EB)/((w/g)*(144*E+(EB*D/t)))

The pressure surge is then a function of the wave speed :

Ps = (-(w/g)*S*V)/144

where

Ps - pressure change (psi)
w = liquid weight (lbs/ft^3)
g = acceleration of gravity (ft/s^"2)
S = Shock wave speed
V = liquid velocity change (ft/s)
E = Short term pipe elastic modulus lbs/in^2
EB = liquid bulk modulus (lbs/ft^2)
D = pipe inside diameter, ft
t = pipe wall thickness (ft)

For PVC E = 400,000 psi
For HDPE E = 150,000 psi

If water hammer is a problem consider using a pipe material with a lower modulus like HDPE, it tends to absorb the energy rather than transmit and is very fatigue resistant.

 

[RWF7437]

Waseem,

I wrote Rick Sellens asking if the program was still available. Here is his reply:

"No. The program was looking pretty ancient. It is essentially a copy of the
BASIC program provided by Streeter and Wiley in the eighth edition of their
fluids text."

It appears from the other posts that there are better programs availble though.

Good luck,

Russ Faust (Civil)
Salem, OR USA

 

[BobPE]

Just soem usefull insight on plastic pipe.

Most of the problem with transients is that the damage to the pipe occurs during the propogation of the negative pressure wave. Specifying plastic for all but the most minimal system transients will only cause more trouble. It seems that a vast majority of the problems with transients are identified through the use of plastic pipe, when it fails it tends to leak a lot. When it fails repeatidly, engineers are usually called in to look at it and then transients are usually identified as the culprit.

BobPE

 

[stanier]

One problem with the rubber ring jointed thermoplastic pipe is that when a negative pressure wave is created drinking water mains can be contaminated. This can be via faulty seals, cracks and other defects.

In fact through contaminated sites there is a case for dual containment.

 

[PUMPDESIGNER]

Thank you guys.
Ag93s comment about using HDPE is appropriate. I believe that material should be used more instead of PVC.

staniers comment about using industry standard methods is very appropriate.

This post and the answers confirmed me in my conviction that the irrigation industry is seriously in error by using so much PVC. I mentioned this to several designers and they questioned the ability of the industry to switch and re-train installation crews. But they did agree that HDPE should be the material used instead of PVC. Richard Neff
Irrigation Craft

 

[Ag93]

Pumpdesigner,

I happen to be in the Polyethylene pipe business (Manufacturer) and your comments are refreshing to hear. The response that you mention regarding re-training is a common one. The cost of re-training however, is insignificant to the savings that could be realized by switching to a more durable, fatigue and surge tolerant material.

Stainers comments regarding the gasketed joint is also valid and is another good reason to consider HDPE as a piping material. HDPE pipe is joined by heat fusion, there is no gasket, no joint.

There are many HDPE specialists / manuafactures out there who can provide the training. There are also many fittings manufactures out there that make fittings specifically for HDPE.

Good luck.

 

[stanier]

Hi All,

The use of PE is becoming more prevalent. As a result we are experiences more instances of failures. Traing in welding prepartion and execution is essential. Its no different to any other welding procedure. First you qualify the procedure, ie make some welds and test them mechanically using a third party inspectorate. The qualify the welders to make sure they can produce the quality of weld. Test their efforts routinely.

Another key parameter. The good old General Motors philosophy "employ good people"!!!!

You cannot NDE PE welds effectively. The hydrotest proves little. Quality of welds is best assured by repeatability of weld procedure and visual inspection of the weld bead. Dont rush the cooling of the PE weld else you will get failures.

In applications where there is a design life of 50years I always insist that virgin material is used. No regring. You're the customer and you can insist on this point. It doesnt cost any more ina competitive situation. Let them use the regrind in agricultural products.

Anothe rkey point is to get to know the difference between HDPE and MDPE. Some grades of HDPE (PE 80C) have a low design life at elevated temperatures. CHeck out the Driscopipe site.

If the PE is cheap compared to the majors then you are probably going to end up with a duff product. Make sure you use extra vigilance, check material certificates that the resing conforms to the manufacturer';s standard and visit the factory to check out manufacturing procedures, QA documentation and generally the cleanliness of the works.

 

[BobPE]

stainer:

You could just use DIP.....

I am just busting you since you were on your soap box there..LOL

BobPE

 

[stanier]

Hi BobPE,

Sorry for the oratory. Guess I get passionate about some things. Just doint like seeing a good product stuffed up by people short cutting.

Whats "DIP". Sorry I dont know the nomenclature as I am not in the USA.

 

[BobPE]

I know what you mean stanier!!! DIP is ductile iron pipe...I am a big fan of it here in the states...

BobPE

 

[stanier]

Hi BobPE,

DIP is known as DICL down here in Australia. Ductile Iron Cement Lined. We asmke to our own standards, Tyco has bought the only manufacturer. I use it on many projects. PE,ABS and PP are used for contaminated ground. ie acid sulphate soils, swamps, etc. Also in the mines where rubber ring joints arent popular and above ground installations where it resists ccorrosion internally and externally. ABS is used exclusively in many authority sewage treatment plants..

Use a lot of ABS for gravity sewers. Its not the ABS your used to in the USA . The master batch is very different. Very common in Australia and SE Asia. Good for waterhammer as the modulus is a lot lower than for metal pipe. Hence the celerity is reduced. ergo the transient spike is reduced. the reflected wave takes longer to dissipate becuase of the lower friction coefficients and hence time to close a valve with a thermoplastic ipe has to be longer for a steel or DICL pipe.

Please kick the soapbox away!

Cheers

 

[Ag93]

Thank you for your comments Stanier, very informative. Just FYI, Driscopipe and Plexco have combined to form Performance Pipe, a division of Chevron Phillips Chemical. The new website is

http://www.performancepipe.com

A lot of helpful engineering information can be found on this site about HDPE pipe, including the engineering manual, sample specs, fittings information, and the new generic fusion procedure etc.....

BobPE, I'll get off of the soapbox now . DIP is a great product and has been used successfully for many years.

 

[stanier]

Ag93

I get a badgateway when I use your link?

You need to move considert he use of the terminology of HDPE. This could end up with you using PE 80C which has a restricted life at elevated temperatures. PE 80B and PE 100 are both medium density PEs. These are the newer generation PEs that give greater resistance to rapid longitudinal cracking and performance at higher temperatures.