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PRESSURE SURGE
5

PRESSURE SURGE

PRESSURE SURGE

(OP)
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

RE: PRESSURE SURGE

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

RE: PRESSURE SURGE

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

RE: PRESSURE SURGE

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

RE: PRESSURE SURGE

2
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? 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.

RE: PRESSURE SURGE

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

RE: PRESSURE SURGE

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.

RE: PRESSURE SURGE

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.  

RE: PRESSURE SURGE

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

RE: PRESSURE SURGE

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

RE: PRESSURE SURGE

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.

RE: PRESSURE SURGE

(OP)
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

RE: PRESSURE SURGE

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.    

RE: PRESSURE SURGE

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.

RE: PRESSURE SURGE

stainer:

You could just use DIP.....

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

BobPE

RE: PRESSURE SURGE

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.

RE: PRESSURE SURGE

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

BobPE

RE: PRESSURE SURGE

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

RE: PRESSURE SURGE

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 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.     

RE: PRESSURE SURGE

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.

RE: PRESSURE SURGE

Mr. Neff,

Reference to your post dated 29/01/2002; I read an article in "PVC News" that outlines a study conducted by Utah State University to re-evaluate the ability of PVC pipes to withstand cyclic pressure surges.

The study concluded (Professor A.P. Moser) that the correct design methodology must take into account both the average stress and the stress amplification to accurately predict cycles to failure.  Since the estimated time to failure for the tested samples was 322,000 cycles where as the pipe samples continued preform until the test was stopped at 3.5 million cycles without failure.

I hope the above helps; and kindly note that I am not advocating the use of PVC, HD/MD/LD-HDPE, RTRP, etc etc :)

Regards,

RE: PRESSURE SURGE

Tests on specimens that are "as manufactured" may well exhibit good long term results in their pristine condition. Lets see some results where the pipe has been dragged to the trench , rocked around on the back of a truck and generally mishandled.

CSIRO in Australia did some test back in the seventies and the results from that were not although complimentary for uPVC. Search for papaers by L S Burns.

RE: PRESSURE SURGE

stanier,

The question at hand is related to surge stresses and accuratly depicting failure values.  Material mis-handling in all cases may lead to diverse effects.  Wether the pipe is made of PVC or Stainless Steel.

Typically the influence of an adquate quality managment system is not undermined and Contract Speciciations or Company General Procedure regarding material handling, stacking hights, transportation/stringing/lowering/jointing procedures are all very well dveloped to obviate premature degradation towards any pipelines preformance.

Regards,
 

RE: PRESSURE SURGE

PVRV

This is all very well in downtown Washington with all the supervision from the water authority superintendent. Try getting out to the back blocks in SE Asia and see how its done. We dont live in a perfect world and since the economic rationalists have taken control quality has gone out the window.

Pipelines go in having been mishandled. Hence testing should include susceptibility to damge. Give me PE, ABS or PP over FRP,uPVC, MPVC,OPVC or nay other PVC any day.

RE: PRESSURE SURGE

(OP)
I have been enjoying this discussion very much.  You guys bring out a lot of useful insights.
Understanding pressure surge is tough because surge is dependent on enough variable system factors to make every job unique.  Therefore I conclude that perhaps it is wise to use materials less prone to damage from surge.

Now concerning the latest comments, based on my experience using PVC, HDPE, Copper, Steel, CPVC, etc., HDPE is less prone to installation error than the others.

Richard Neff
Irrigation Craft

RE: PRESSURE SURGE

Hi POUMPDESIGNER

Dont jump the gun here my son.

PE can be installed successfully by competent contractors.

Check out the website mentioned above

www.performancepipe.com.  It has some great technical information there. Get yourself a good supervisor experienced in welding PE and make sure that the contractor has certified welders.

I once had to rectify a pipeline that apssed a hydrostatic test but failed some time later. rember that a hydro test only subjects girth weld defects to half the design stress. There is no NDE I know of that can be repeatedly used in the field to establish if a buttweld is good.

So it comes down to repeatability by using good labour.

Best of luck in your endeavours.

RE: PRESSURE SURGE

(OP)
I agree completely with your post Stanier.
I had written more to qualify myself and then erased it, big mistake when sharp guys are afoot.
It is extremely important to have good, calm, competent men with good procedures.
I know that a fool can screw anything up like the old proverb says:
"A man would rather meet a bear robbed of here babies than a fool in his folly".  (I had written that but erased it).

My post was unbalanced and left the possibility open that unqualified people could get away with sloppiness.

Richard Neff
Irrigation Craft

RE: PRESSURE SURGE

stanier,

If quality control cannot be maintained during transportation and installation.

QUOTE
"since the economic rationalists have taken control quality has gone out the window." stanier

How do you propose to :

a) Justify cost nvolved in testing susceptibility to damage.

b) Cost involved in utilizing a superior pipe material.

Prudency would be compelling in taking extra care during handling than choosing options a) or b).  Unless HDPE is cheaper than PVC :) Wonder what the cost ratio between PVC to PP is?

At any rate, your insights are appreciated and I am not located in North America.


RE: PRESSURE SURGE

Stanier and Pumpdesigner,

There is a fusion equipment manufacuterer that makes a product called the DataLogger. This allows the user to record the critical parameters during each Polyethylene pipe fusion.  Have heard from many that this is a great tool.  Here's the link :  http://www.mcelroymfg.com/fusion/machines/datalog.htm

RE: PRESSURE SURGE

Pumpdesigner, I think your formula is the same as the other formula (Jakwsky equation from the Perry Handbook)
It has been Rearranged. Iam not sure whether its in SI units so I will substutute K for the constant 0.07 in your formula.

Lets take a= wave celerity(celocity as defined by MortenA
Time to close valve should be at least equal to:
T=2L/a (From basic Physics)otherwise closure will be    classified as instantaneous.This is used to determine whether water hammer will occur. If Time of closure greater than T, then you are safe for water hammer.

Jakowsky's Formula
Surge Pressure (H)=a*V/g
  V=change in velocity which is equal to actual velocity
  a=is defined above and can be rearranged to:

     a=(2L/T) or (L/T)=a/2
Your Formula is: H=KVL/T=(K/2)Va=k'va
k'=new constant incorporating g.
This will work when you deal with one material and probably a specific diameter(celerity changes with diameter and amterial thichness and strength).Thats why it sometimes gives conservative estimates. You are likely to underestimate as well as lond as your case has a D/t and E above those assumed in your formular( The higher the D/t, the higher the celerity, the higher the hammer).Thans why a steel pipe will generate about twice the surge pressure developed by an equivalent PVC dia PVC pipe. My advise- stick to the equation given by MartenA.
    

RE: PRESSURE SURGE

(OP)
Thank you chip1998.

PUMPDESIGNER

RE: PRESSURE SURGE

The Joukowsky equaation has limitations in that it doesnt represent the scenario of column separation and the two columns recobining. Streeter & Wylie caution engineers that the surge pressure can be many times higher in such cases. Also with thermoplastic pipe one has to consider the on set of full vacuum particularly in a buried pipeline scenario. If any ovality exists the eresistance to buckling i increased.

Suggested reading :-

Streeter & Wylie
Thorley
Boldy
Zhou, Hicks and Steffler
Molin
Janssen

Sharing knowledge is a way to immortality

RE: PRESSURE SURGE

Stainer

Its however far from every surge scenario where column separation is a risk. Could you give a better reference than "Streeter & Wylie" (i maen: where does the artickle apper, when, etc)? I have understood that Joukowsky is a conservative estimation.

BestRegards

Morten

RE: PRESSURE SURGE

MortenA

Sorry in just quoting Streeter and Wylie like that, it wsnt a paper although the gentlemen in question have certainly published.

Streeter & Wylie's Book Fluid Transients was published in 1993. It is a classic in this industry so much so that AFT provided me with a copy when I first started using AFT Impulse. They had a special print run done as it was out of print. Any good technical library (university/consulting engineers/etc)should be able to produce a copy for you.

Fluid Transients by Thorley or Pressure Surges by Swaffield and Boldy are other very good practical books on the subject.  

Sharing knowledge is a way to immortality

RE: PRESSURE SURGE

PUMPDESIGNER and all
I have done a lot of research in valve design and water hammer in water mains. One solution I have found is a water valve with damping built in to it, (in opening and closing). An other option which is a little more expensive is to use a Feed back loop in conjunction with a proportional valve. They use this in some whishing machines to avoid water hammer and achive better control.

RE: PRESSURE SURGE

(OP)
mk2000 - You are correct that the valve closing is the easiest way to get help reducing the surge.  Actually, I have described the desired valve as a linear closing valve such as would be possible with a motor operated ball valve.  Solenoid / diaphragm valves are one of the chief sources of very fast non-linear closing speeds where 90% of the flow is stopped within less than one second.  The damper you speak of would be great on a valve for this.

PUMPDESIGNER

RE: PRESSURE SURGE

I have done some work on domestic slow acting valves. These valves are mainly used in house hold appliances. Depending the size of your application you might be able to adapt these valves in to your application. But I think I have seen valves in larger application with the same sort of functionality.

Also if you can do a bit of testing you can get a linear closing/proportional solenoid valve and put a big Green Cap on the terminals. This will reduce the response time of the valve. However you have to select a DC operated coil.

RE: PRESSURE SURGE

(OP)
mk2000 - Very interesting.  Green Cap signifies DC?
How would this proportioning valve work if you have a couple of minutes to explain to me.  I have been experimenting with internal shaping to force hydraulic linearity in the face of collapsing downstream pressure which causes diaphragm to lose support and collapse onto the seat.

PUMPDESIGNER

RE: PRESSURE SURGE

PumpDesigner

What we have developed is a conversion of a digital diaphragm valve in to a proportional valve. I do this by changing the spring behind the armature and that makes it proportional (this is on a DC coil). But we us this to control the temperature of the water. However you can use the same concept to change it to a slow acting valve.

As the valve opening is some function of the coil voltage Flow = (X)Voltage + B (this is the ideal situation, the function is not exactly linear but we can approximate it). By putting a large Green cap on the valve when the voltage across the valve is dropped to zero, then the green cap will start to discharge at some rate depending on the capacitance of the cap. As the cap discharges the valve closes slowly and therefore eliminating water hammer. This also works the other way. If there is a shock wave propagating through the pipes to the valve then because of the sprung system and the way diaphragm valves work it will dampen some of the shock and reduce the water hammer effect.

The approach you are taking only looks at half of the problem. Do not forget that the valve control has two sides you need to consider Dynamic effects and Static effects. Your analysis only looks at the dynamic effect only that is why you get the collapsing effect.

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