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.
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.

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.

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