Cause of water hammer in a trap header? 4

[Tremolo]

I have a trap header that receives condensate from several different steam traps (220 to 240 F condensate temperature). Fluid from the header goes through a vertical rise of 8 feet and eventually discharges to a deaerator tank. The problem is that when condensate (200 F) is pumped from a flash tank into the trap header, strong water hammer events are observed.

Any ideas as to why the waterhammer events are occurring and what can be done to eliminate or reduce the water hammer?

Thanks for your help

TREMOLO.

 

[sailoday28]

What is the mechanism that maintains the pressure in the header? Is it the deaerator pressure?

 

[Tremolo]

Yes. The deaerator pressure is regulated at 15 psig and it is maintaining the header pressure.

 

[Gibby53]

Water hammer is typically caused by one of four reasons;
1. Rapid valve closure. Valve closes quickly and the fluid momentum is changed. Quick changes in fluid momentum result in large forces applied at the location of the momentum change, and a high energy shock wave which must dissipate its energy. Obviously valves should be closed slowly.
2. Introducing flow to empty piping. The empty pipe offers very little resistance to flow through it. Therefore, the flow accelerates to high velocities very quickly. As soon as the flow encounters an obstrucyion to flow, such as an pipe bend, etc. a large momentum change occurs in a short period of time and water hammer occurs. Piping systems should be filled and vented.
3. Introducing low temperature water into a piping system containing steam. The steam volume collapses quickly, and the water accelerates into this voided area. When the water runs into itself or a pipe wall , a large fluid momentum change occurs in a very short period of time.... water hammer occurs. Water flow systems should be vented to prevent steam pockets from forming before water flow is introduced.
4. Introducing steam flow into piping partially filled with water. When steam comes in contact with the cooler water, the steam will rapidly condense. The steam volume collapses quickly, and the water accelerates into the void area as in above. Steam flow piping should be drained before flow of steam is introduced.

If I understand your configuration correctly, it may be that the 8 foot rise pipe is partially full of cooler water sitting at ambient temperature. Just a guess. Or is the header pipe empty.

Hope this is helpful.

 

[25362]

Depending on the steam consumers' operating pressures, the condensate released by the steam traps partly flashes inside the 15+ psig (*) condensate header to a 250+[sup]o[/sup]F equilibrium temperature. When colder 200 [sup]o[/sup]F condensate enters this line, water hammering may be induced by the process (item 3) indicated by Gibby53.

(*) I say 15+ psig to account for the friction drop across the condensate collecting line with the lowest pressure at the deaerator. The 8 ft vertical line would probably carry a two-phase V/L stream with an average density similar to that of steam.


Please comment.

 

[Tremolo]

Thank you, everyone, for your contributions thus far.

I think the most likely situation is as Gibby53 and 25362 have said: a steam pocket forms somewhere in the pipe and is collapsed when the flash tank pump operates. The flash tank pump operates intermittently.

I believe that the trap header normally receives very low flow such that pockets of fluid at different temperatures can form due to the condensate flows from the several different steam traps feeding the trap header. The flow is significantly increased when the flash tank pump turns on. When one of the hot water pockets flows through the 8 foot riser, it may flash. In this case the geometry would be water above and below the steam pocket in the riser. The steam pocket cannot support the water above it and therefore collapses causing a water hammer event

A second situation that may be present, as mentioned by Gibby53 and 25362, is that the trap header may be only partially full of water such that saturated steam forms over a cooler layer of water in the lower portion of the pipe (i.e., a stratified configuration). Since the system is relatively stagnant, a layer of condensate can build up between the cooler water and the saturated steam. The condensate layer would be at the same temperature as the steam, therefore the configuration of saturated steam over cooler water could be maintained for a long period of time. Even a very small amount of flow could then cause a disturbance that breaks the insulating layer of condensate resulting in rapid condensation of the steam, thereby causing a water hammer event (similar to situation 4 of Gibby53). Thus, when the flash tank pump kicks in, even if the flow is throttled, a water hammer event will occur. The system engineer has tried to throttle the pump flow to reduce the water hammer event, but this has not had any positive effect on the situation.

So, right now, it seems fairly certain that a two-phase configuration is developing, but what is uncertain is exactly how and where the steam is forming.

Could faulty steam traps be causing the two-phase flow condition? After passing through the 8 ft riser, there is a horizontal run of pipe that connects to the deaerator. Could it be that the horizontal run of pipe slowly drains over time? Are trap headers susceptible to water hammer in general?

It is possible to instrument the header piping to determine P’s and T’s. This might help explain the fluid state and geometry in the pipe.

Any other ideas or experience anyone may have is welcome.

TREMOLO.

 

[s0eebuch]

Make sure you have condensate collection at low points, and especially after a Pressure Reducing Valve (PRV) or an area where you make a significant area change (i.e. 6" pipe reduces down to 2").

 

[sailoday28]

Do I have the correct picture?
Assuming no flash tank flow:
1-The run of piping upstream of the riser is subjected to about 30psia DA tank pressure.
2. AND no hammer has occured in any of the piping.

If the energy of input drain flows approximates that of a saturated liquid, then unless there are steep downward slopes, the piping upsteam of the riser should have subcooled water as a result of the 30+ psia of the DA tank. AND as a trap opens, the flow is pushed up the riser into the DA tank.

It is stated that hammer occurs with flash tank flow.
Where does injection of Blowdown tank water enter the system? Upstream of the riser? My thought, for other than hammer caused my transient flow-- if the injection is upstream, then hammer(bubble collapse) shouln't be in the upstream piping.
But possibly in riser, because now subcooled water is rising in the somewhat vapor atmosphere of the 30+ psia.


CLEARLY, I HAVE MADE MANY ASSUMPTIONS AND POSTULATIONS!!.
It seems to me that the piping geometry/orientation should be reviewed.
Fill me/us in.

 

[25362]

In stratified biphase systems as condensate return lines usually are, and especially upon the injection of pumped cooler condensate that floods the line, steam may become trapped in pools of condensate. When the steam collapses, water rushes in from all directions creating hammering. This is sometimes called thermal shock or cavitation.

Cavitation may sometimes combine with what is called differential shock. This is created as a result of flash steam flowing at velocities much (~10 times) higher than the condensate forming ripples and waves on the liquid surface. The added pumped condensate creates more turbulence amplifying the steam velocity. At some point waves transform into water slugs which become accelerated by the rapid steam impacting with force pipe elbows, etc., inducing hammering.

If those scenarios are correct, the design of the condensate collecting line should be reviewed. Alternatively the injection of pumped condensate could be done through a parallel line towards the final discharge point.

 

[katmar]

Tremolo,

Two phase flow can be problematic, but in my experience two phase flow in horizontal pipes can almost be guaranteed to introduce problems. With the bias of my previous experiences I would think that your problem is in the horizontal section from the top of the riser to the deaerator vessel.

One thing you can be sure of is that the condensate from the traps is not flashing. If the deaerator is controlled at 15 PSIG, the pressure in the header can only be higher than that. The saturation temperature at 15 PSIG is 250 F. If your condensate is entering at 220 to 240 F it will not flash.

Depending on the diameter of the horizontal section after the riser (you say the flow is generally "low") this section may cope with the flowrate with the pipe being only partly full. If this is so, it will be possible for steam to enter this horizontal pipe from the deaerator side and you may have a significant vapor layer above the liquid in the pipe. With the condensate being marginally below the steam temperature, and if the vapor space is large, you may be getting steam condensing at a low rate and not causing problems.

However, when the cold condensate from the flash tank gets to the horizontal section after the riser the liquid flowrate will be higher and there will be less vapor space. Also, the lower liquid temperature will increase the driving force condensing the steam. The smaller vapor space and more rapid condensation may be just the right combination to cause slugging and hammer.

If this is the cause it is easily fixed. All you need to do is to prevent the formation of the vapor space above the liquid in this horizontal section. Either run the horizontal section of the pipe at the lower level (i.e. at the level of the trap header) and then put the riser as close to the deaerator as possible, or leave the riser where it is but cut it shorter so that the "horizontal" section slopes upwards and stays flooded.

If the problem is not in this horizontal section the hammer can only be coming from the deaerator itself because with the temperatures you have given there cannot be steam anywhere else in the system. If this is the case, all you can do is route the cold condensate from the flash tank to a more appropriate place.

regards
Katmar

 

[25362]

Katmar is right. If the condensate released by the traps is subcooled no flashing would be expected and flash steam couldn't be blamed for the hammering. If the reported temperatures are right, the hammering is caused by another factor.

Tremolo, before starting to think about other factors, tell us, please:

1. whether the reported temperatures of the condensate released by the traps are correct;

2. whether the injected condensate has been flashed in an open drum and may contain dissolved air in the amount of ~0.24 mass %.

 

[Tremolo]

Thank you for the additinal ideas and questions. I will respond to the messages Friday and provide additional deatails at that time.

TREMOLO.

 

[kirsner]

Tremolo--i wrote an article which addresses your water hammer situation--that is water hammer upon start-up of a condensate return pump when there's been an opportunity for flash steam to accumulate in the line. You can reference a copy on the Articles page of my website at

http://www.kirsner.org.

It's the top article on the page. This type of water hammer is known as Column Closure waterhammer.

wayne kirsner

 

[Tremolo]

Wayne,

Thank you for the article.

Although our system is a pumped condensate return system, the geometry is most like the boiler feedwater system you depict in Figures 4a through 4c of your article "Bangin' in the Power Plant."

The following crude schematic shows the critical parts of our pipe system.
[tt]
I
C |----------|
|----------| | |
| | | J| ------
| |D |H |---| |
| | E | K |ABD |
| |---------| | |Tank|
| F| | ------
B | |----|
| G
A |
O-----------| [/tt]
Flash
Tank
Pump


I have labeled each pipe segment A through K. Segment E is the steam trap header. Several steam traps discharge to this pipe segment (210 F to 240 F). During shutdown conditions, the pump at the beginning of segment A periodically starts to pump condensate from a flash tank (200 F). Therefore, segments A through D contain cool water and segments E through K contain warm water.

The system is essentially stagnant unless the pump is operating. Ultimately, the condensate is discharged to the Aux boiler deaerator (ABD) tank at the end of pipe segment K. Discharge occurs through a sparger inside of the ABD tank. The ABD tank is maintained at 15 psig. There is also a flow control valve (not shown) in segment D and a check valve (not shown) at the pump discharge in segment A.

Elevations for the pipe segments are:

A - 587'-3"
C - 617'-0"
E - 609'-6"
G - 604'-8"
I - 617'-8"
K - 614'-10"

I suspect, as your article also points out, that condensate may be slowly draining from pipe segment I, leaving a stratified configuration in that pipe segment. This would set up a condensation induced event following pump restart. Alternatively, condensate may flash as it rises up segment H and may collect near the junction of H-I. Following pump restart, this would result in a column closure event. However, whenever the pump is started, several water hammer events occur; i.e., the waterhammer events are periodic. This leads me to believe that segment I is stratified and the condensation induced water hammer results in a water wave moving up and down segment I.

TREMOLO.

 

[katmar]

If the hammer is periodic then it has nothing to do with any pre-existing conditions (i.e. before the flash tank pump starts). The steam must be arising on a continuous basis somewhere in the system.

Given the piping geometry, pressures and temperatures there cannot be any flash steam being formed. So the steam is either coming from a leaking steam trap (in which case the given temperatures are wrong) or there is steam coming back from the deaerator via the sparger, and this can only happen if the sparger is not submerged.

Can you check these two possible sources of steam and let us know please?

Your "sketch" was very helpful. Can you please also give the lengths and diameters of sections I, J, and K and the expected flowrate from the flash tank pump.

 

[kirsner]

Katmar's questions and analysis are right on. I suspect the sparger is not submerged and the Froud # in Section I is what Katmar is after to see if the pipe is running full. if this is the case, then the hammer will be occurring in section I with the shock being transferred back thru the condensate filled pipe to section E where it seems to be originating from. On the other hand, if the shock is clearly originating from section E, I like the blowing trap theory with column closure, not condensation induced, waterhammer.

wayne kirsner (kirsner@kirsner.org)

 

[25362]

It has already been said. When some gas is present in the liquid, the celerity of the pressure wave, due water hammer, may drop from 2000-5000 fps down to 10-100 fps.

 

[Tremolo]

To all:

Here’s an update on our situation.

Unfortunately, during the last outage, the tests we had planned could not be performed. We were going to take temperature measurements along the pipe to see if there were any hot areas which could correspond to saturated fluid conditions. However, this would have required the removal of small sections of pipe insulation and there were some asbestos concerns. So, the test was scrapped. As it turned out, during the 3 week outage, there was only 1 waterhammer event and it occurred near the end of the outage.

The big question is: how does steam get into the line? The normal condensate temperatures from various steam traps and other sources are too low to lead to any flashing in the system. So, something unexpected is occurring. I am leaning toward the theory that water is draining from the piping high point (34’ pipe length) down through the sparger in the aux boiler deaerator. My co-worker and boss doesn’t think this is happening. He believes the waterhammer event is originating in the trap header.

Another possible source of steam is through leaking steam traps. I have heard some talk of leaky steam traps, but I thought the leaking trap would just allow condensate to flow back up into the steam pipe creating a waterhammer event in that pipe. Can you explain the behavior of the leaking steam trap and how it could allow steam to enter the trap header? Also, is there a way to check the steam traps to determine if they are working properly?

Thank you.

Tremolo

 

[25362]

Hammering at traps' upstream occurs when the equipment, or piping, becomes flooded (intentionally or not) with subcooled condensate contacting live steam.

As for traps leaking steam and passing non-condensables visit any website on the subject, as, for example,

http://www.maintenanceresources.com/ReferenceLibrary/Steamtraps/Inspect.htm

http://www.pnl.gov/fta/15_steamtrap/15_steamtrap.htm

http://www.armstrong-intl.com/products/traps/st.php3

 

[Tremolo]

25362,

Thank you for the web site links. They have been very helpful.

Tremolo