Process water system surge .. not possible. 8

[jamesbanda]

We are reviewing the hazards of our network for a new project one of our hazard guidewords that is covered in the review is pipework surge.

For process water.. can anyone provide a reference or guidance document covering surge..? Should we consider surge in our network..

if so how its so hugh that is does not seem pratical..?

 

[BigInch]

I've had a lot of stress problems where a stress analysis was not required. Long pipelines entering stations without adequate flexibility or long pipelines entering stations and making turns with the resulting long pipelines pressure expansion effects (not even at increased temperatures) acting as lever arms on those turns and multiplying by many times the force applied to station pumps and compressor equipment are the most common examples in my CV.

The real problem with those examples were that they required shutting down the pipeline revenue streams and blowing down products, the costs of which far exceeded the already expensive and difficult required modifications to the pipe geometry, which of course had to be done after the systems were constructed.

I used to take a lotta' crap from my bosses because my projects took more engineering time. In fact I almost got fired for it, until I doug out all my projects design, construction and what operating-maintenance history they had and presented the VP with solid evidence that increased office engineering time reduced construction field modification work, reduced expensive offshore equipment/personnal standby time, fit together much better and had less modifications after construction and overall costs were only 75 to 80% (not counting modifications afterward, since I didn't have any) of those of engineers running other projects. Proving that engineering time was the highest overall most cost effective time that we could possibly spend. Isn't that what engineering time is really all about? If you can't decrease capital costs, increase system performance, safety and reduce construction and overall project lifecycle costs with engineering time spent, what are we doing here?

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[reena1957]

Kudos to Big Inch!! Spoken like a true blue blooded engineer!! I am with you Big Inch 100%
Reena

 

[bimr]

Getting back to the point:

Hazop Procedures:

Essentially the Hazops procedure involves taking a full description of a process and systematically questioning every part of it to establish how deviations from the design intent can arise. Once identified, an assessment is made as to whether such deviations and their consequences can have a negative effect upon the safe and efficient operation of the plant. If considered necessary, action is then taken to remedy the situation.

This critical analysis is applied in a structured way by the Hazop team, and it relies upon them releasing their imagination in an effort to discover credible causes of deviations. In practice, many of the causes will be fairly obvious, such as pump failure causing a loss of circulation in the cooling water facility mentioned above. However, the great advantage of the technique is that it encourages the team to consider other less obvious ways in which a deviation may occur, however unlikely they may seem at first consideration. In this way the study becomes much more than a mechanistic check-list type of review. The result is that there is a good chance that potential failures and problems will be identified which had not previously been experienced in the type of plant being studied.

http://www.lihoutech.com/hazop.htm

THe key phrase is 'deviations from the design intent'.

 

[rconner]

I'm not going to comment on any hazard procedures nor standards for any specific application that say it is not necessary to consider for higher pressures as a result of transient conditions. I will only say a great many past failures in many applications (of particularly weaker-type piping/joining systems) have been blamed on water hammer or surge (that the systems, since they failed, obviously could not handle). "Surge .. not possible" are thus quite strong words and should be very carefully considered.

 

[BigInch]

"Strong words" well said. And quite possibly "last words".

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[zerosum]

Saying, in the Hazard Review regarding cooling tower water circuit, "Surge - Possible, with excessively fast acting valves. Safequard - Check the actuation speed of each automatic valve during commissioning and snub air supply if necessary to prevent surge" is not the same level of engineering effort as providing a computer model of transient response for a plant cooling tower water system.

Obviouly, loss of cooling water to a unit or plant due to pipe rupture has the potential to cause some very serious consequences, so one should proceed prudently. But you have to focus your effort where there are real safety payoffs unless you have unlimited engineering resources.

 

[4Pipes]

One of the most realistic water hammer problems in a cooling circuit is a sticking / slow operating check valve. It does not matter what the forward flow velocity is. The reverse flow at the time of check valve closure is the killer. I was once involved in trip testing a set of 20" pumps. i.e. Tripping one pump from a pair running in parallel. The check valves were brand new and had were specified with defined closing charactistics. The head was not particularly high - I cannot remember the exact head but the pumps were single stage. I came extremely close to needing a new pair of trousers.

 

[zerosum]

So would or did computer modeling of the transient characteristics help the situation?

 

[BigInch]

It can. Its possible to see any pressure spikes accompanying large transient events. As 4Pipes says, pump trips, reverse flows from a broken check, slamming check valves, especially when a pump control gets a shutdown signal during a failed ramp up sequence, and control valve malfunction of a fail closed control quite often cause large transients. Water hammer is basically a function of initial velocity and holding velocities low will avoid many problems, but certain pipe and equipment configurations can worsen the situation when pressure waves occasionally add together at a common point. The more equipment you have trying to control things seems to increase the potential for trouble. Transient modeling will help you decide for example, if a thicker walled pipe, or if a surge valve and tank might be the best solution. Many times you can work out a ramp-up and shutdown sequence to avoid problems completely. You can also test and determine proper valve actuation times and relief valve settings as well, although just a simple confirmation of maximum pressures reached are less than pipe allowable pressure is often the only objective.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[Alph]

The Joukowsky equation can be used for preliminary estimate of waterhammer or surge.