Steam Hammering when bringing on tempered water

[MarkkraM]

I was wondering if someone could help me with the problem of steam hammering that occurs when we bring on tempered water to cool certain zones on our extruder.

Background:

While the extruder is not running, we introduce steam to these area's in order to keep them hot enough so residual plastic stays melted. Once started the heat generated by the extruder needs to be removed so we isolate the steam, depressurise to about atmospheric, isolate the depressurisation vent, then bring in tempered water. Once flooded, the barrel is at about 500kPag, the surface temperature of the extruder is about 215degC, and the tempered water inlet is about 40degC.


Questions:

1. When re-introducing the tempered water onto the hot surface of the extruder barrels, would introducing the water as quickly as possible serve to reduce or worsen the steam hammer?

2. I am unsure why we don't leave the the vent to atmosphere
open until the barrel is flooded? Does this sound strange that we close it before hand?

 

[quark]

Just check whether the 5 barg pressure is pump shutoff head or shutoff head+forming steam pressure.

1. Increasing the water flow rate reduces the increase in water temperature(also turbulence increases and water temperature will be uniform and no steaming at the boundary). So chances of hammering will be less, in my view.

2. You should better keep the vent open till the barrel is completely filled up.

Sweating on mold surface may be a problem. Perhaps you don't want to cool it too much.

Regards,


Believe it or not : There is a three dimensional figure but only with one face, called Mobius Strip.

 

[Joesteam]

The hammer may be formed by the water hitting the hot surface and immediately vaporizing. If this is the case the vent should be open to allow this expanded water to dissapate.
However, it may be the case where some of the water vaporizes and then this vapor must bubble through the tempered water. This would then lead to water hammer where these steam bubbles collapse in the cooler water. If this is the case, a slower input of the water with venting will stop the hammer.

 

[MarkkraM]

Thanks for the reply quark.
5 bar refers to pressure just u/s of the barrels when tempered water flow has been established.

Regards,
MarkkraM
P.S. Yes have previously heard of Mobius Strip

http://scidiv.bcc.ctc.edu/Math/Mobius.html

 

[Latexman]

Would a tempered water system that can span the cooling and heating requirements of your extruder be possible? It would eliminate the water hammer in the extruder.



Good luck,
Latexman

 

[tkdhwjd]

My hypothesis... Hope this helps.

If the system was filled with steam and then depressurized to atmosphere, then the amount of water vapor molecules in the system prior to introducing subcooled water was very small. Therefore, as soon as the system saw subcooled water, vapor molecules condensed immediately (or collapse if you will) and low pressure pocket (vacuum) was formed. This low pressure probably pulled and accelerated water which either collided with liquid downstream of the low pressure pocket or phyiscal barrier such as pipe elbow, bends, etc. The momentum transfer during the collision was the main cause of water hammer, I think.

In your case, the effect of water hammer was worse since the system started out with low pressure at which density difference between water vapor and liquid water is huge.

To answer your questions...
1. I believe commissioning tempered water slowly is the key to avoid significant water hammer. You should perhaps install small bypass at the inlet valve if the inlet valve control is not so good due to its size.

2. If you leave the vent to atmosphere, it will definitely pull air into the system. If your system is not sensitive to air, opening or closing the vent will not make any difference. If you really want to get rid of condensation induced water hammer, consider installing a line to steam system that operates at similar pressure of tempered water. Then, even if steam collapses, the line will pull steam from the steam system and equalizes the pressure. Vapor is less dense that liquid and therefore, it will be pulled first before liquid water when low pressure pocket is formed.

 

[abdullah]

hammering caused usually if water contact with steam.
try to find the steam soure. use steam traps if necessary.