Hi Omid
I don't know enough about your system to tell you what I think is the best way, however there are a few general points which I can offer. First, 800 cum is by no means large for a surge tank; whether it is "large" or not depends on the size of the system you are protecting. I recently rejected a design which used a 3000cum surge tank; it was not necessary.
Second, a single surge tank may not protect the entire line if there are a series of high and low points. Multiple smaller surge tanks spaced along the line may offer a higher level of protection if you decide to stay with surge tanks. It pays to remember that after a pump trip, the surge tanks will be empty, so multiple trips (a real-world thing) can result in air in the system anyway unless special precautions are taken.
Third, I am not sure how you sized your surge tank; presumably you have
a surge control valve at the pump discharge which dumps water in en
event. The surge valve then closes gradually, commencing after the
pressure wave return time. If the line is long, the dump volume can be
large.The surge valve should be sized as small as possible to minimise
the amount of dumped water and to reduce the reverse line velocity when
the valve starts to close. I often size on a 20% pressure rise;
sometimes higher.
This is not always the best way; sometimes it is better to go with a
fast closing discharge (check) valve, rate the system components to
handle the pressure rise and add some vacuum breakers to guard against
local vacuum. Some systems use air vessels and limit the pressure rise
by adding compressibility to the system. Some use flywheels on the pumps
to limit the rate of pressure decrease when the pumps are tripped. If
your design is producing some difficult outcomes, then it may be worth
looking at another way of overcoming the surge problem.
Fourth, I would not worry too much about getting air into the pipeline unless there are unusual circumstances. The biggest area of concern is to protect the line during priming/filling. The pump discharge should be throttled to provide a low priming velocity; typically around 0.5m/s, to prevent bashing the pipe around too much, and also to protect the pump against overamping if the line is only partly full.
Civil engineers often go for a surge tank as a first resort; mechanical
engineers go for it as a last resort. I'm not sure why, but feel it may
be that civil engineers know how to design concrete tanks and mechanical
engineers do not.
One of the major causes of difficulty in surge design is when the
pipeline wall thickness has been selected (and the pipe has been
purchased) without providing any allowance for surge. The surge
suppression design can then become quite complex and the solution
expensive.
Norm has forgotten more about this subject than I know, so hopefully we have prodded him into providing a more authoritative response.
Cheers
Steve
-----Original Message-----
From: Omid Hamlehdar [mailto:omid_7_ae@yahoo.com]
Sent: Tuesday, December 07, 2004 11:04 PM
To: PipingDesign@yahoogroups.com
Subject: RE: [PipingDesign] Surge Tank
Hi Steve
Thank you for your mail, sorry if my description is
confusing, I meant completely same as yours about one
way surge tank operation, I found that I need 800 cu
meter water in one way surge tank to prevent entering
air to pipeline, this is very huge amount of water
which seems impractical, should I put a smaller one?
In this case air will enter to system, I am afraid
about this amount of air in priming.
Would you please let me know your opinion?
Best regards
Omid
> Hi Omid
>
> I am a little confused by your description of a one
> way surge tank. The
> ones I am used to have a check valve which lets
> water into the pipeline
> when the pressure drops below the surge tank static
> head, as described
> in Karassik's Pump Handbook. They are normally
> located below the
> hydraulic grade line to stop them running out of
> water when the system
> is off. The check valve prevents the tank from
> overflowing during normal
> operation. Recharge is normally by a branch off the
> main line and a
> float valve. In many circumstances I have found them specified where
> reliable vacuum breakers (or some times a standpipe)
> would work at far
> lower cost. Water hammer software allows for the
> cushioning effect of
> the air, although the exact details are seldom
> forthcoming from the
> software vendors.
> The main problems with vacuum breakers are
> reliability assurance (double
> breakers sometimes used), avoiding hammer in the
> empty pipes on startup
> (prime lines under throttled conditions) and what to
> do with the trapped
> air in the system (two stage air valves or separator
> at discharge if the
> hydraulic gradient permits).
> From what you describe it sounds like your check
> valve lets water into
> the tank but not out. Obviously I am missing
> something here, as such a
> system would not help on a downsurge.
>
> Could you clarify your arrangement please?
>
> Cheers
>
> Steve
>
> -----Original Message-----
> From: Omid Hamlehdar [mailto:omid_7_ae@yahoo.com]
> Sent: Tuesday, December 07, 2004 6:06 PM
> To: PipingDesign@yahoogroups.com
> Subject: RE: [PipingDesign] Surge Tank
>
>
>
> Dear Erik
>
> Thank you for your reply,
>
> As I have mentioned it is a one way surge tank and
> so,
> I have modeled a swing check valve to prevent tank
> from draining back when pump is off.
>
> As you mentioned Surge tank prevents a fluid column separation and
> rejoining but I have modeled a one way
> surge tank which is different as you know.
>
> My concern is about this column separation, I
> suppose
> to locate an air valve (twin air valve from Erhard)
> to
> allow this air will be released, but I still want
> to
> know your opinion, do you think it is ok if air will
> enter to system after water has been finished in one
> way surge tank? Actually I have found that I need a
> huge amount of 800 cu meter to prevent air entering
> to
> system but it is impractical, I have decided to
> allow
> air enter to system after water has been finished in
> a
> smaller one way surge tank and locate an air valve
> to
> allow air released in re-starting.
>
> Best regards
>
> Omid
>
> --- Erik Scheir <erik.scheir@pandora.be> wrote:
>
> > What are the boundery conditions ?
> > Is there a receiver at a height over 33ft above
> the
> > level of the tank from
> > which the water is drawn?
> > Is there a valve a the end of the pipeline who
> > closes to prevent the tank
> > from draining back when the pump is off?
> >
> > A surge tank prevents a fluid column separation
> and rejoining.
> >
> > When air enters the pipe and the pump is
> restarted,
> > the column of water will
> > be accelerated into a partial empty pipe. Given a
> > long anough void, the
> > column of water can accelerate to its terminal
> > velocity of perhaps 5 to 10
> > fps before striking the valve face.
> > Upon collision with the valve, the kinetic energy
> of
> > motion is converted
> > into potential energy by compressing the liquid.
> >
> > There should always be water in the surgetank.
> >
> >
> >
> >
> >
> > _____
> >
> > Van: Omid Hamlehdar [mailto:omid_7_ae@yahoo.com]
> > Verzonden: maandag 6 december 2004 11:51
> > Aan: PipingDesign@yahoogroups.com
> > Onderwerp: [PipingDesign] Surge Tank
> >
> >
> > Hi everybody
> >
> > I have a question and will be pleasure to have
> your
> > opinion.
> >
> > After analysis a pipe line for water transfer
> > purpose,
> > I found that I have extreme down surge, to
> suppress
> > this surge I assumed a one way surge tank, my
> > question:
> >
> > 1. Is it ok if air will enter in pipe line
> > after water
> > has finished in one way surge tank?
> > 2. In terms of restarting (priming) the line,
> > if air
> > has entered to system, is there any consideration
> > required regarding probable impact regarding
> column separation in
> > line?
> >
> > Your contribution is highly appreciated
> >
> > Omid
> >
> >
> >
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