Water Hammer in a blowoff line

[invent11]

In a power plant a blowoff line transfers water from boiler through a long line of 4"->6"->3" into a water channel and 10' under water. Line experience water hammer downstream b/o vlv as well as at 3" Tee before line entering the channel. Per P&ID Press.& Temp. downstream b/o valve are 300 psisg & 417 deg F. Appreciate your ans. to my following questions:
1-What measures do you suggest to eliminate or mitigate the water hammer? Is reducing the b/o vlv open/close speed can be any help?
2-Exiting 1/2" check vlv used as vacuum breaker on 3" line breaks frequently and seems inadequate. Any suggestion for a better design?
3-Finally what is the press. at the tail of b/o line before channel: 300 psig - press loss for the length of line OR atm. press.- 10' water head.

 

[BigInch]

Slowing down the valve operation and increasing pipe diameters should help. A larger and stronger check valve design might be attempted, such as a 6".

The exit pressure is dependent on the velocity in the line.
You have up to 315 psia - (15 + 4) psia = 296 psia available to move your fluid. What is not consumed by frictional pressure drop, is available to increase velocity head.

If initial flowrate is low, not all of that available pressure will be consumed by friction drop and the excess differential pressure will tend to accelerate the flowrate in the downstream segment and increase the discharge velocity until steady state flow is achieved.

If the flowrate is exactly right for the available pressure drop, you will have steady state flow and a discharge head composed of (19 psia + v^2/2g).

If you attempt to flow at a high flowrate, pressure drop will not be sufficient, and the flowrate and discharge velocity will slow down until you reach steady state.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

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[katmar]

Is this a normal procedure for handling blowdown? In my (very limited) boiler experience the blowdown goes through a very short line to a blowdown tank where the liquid and vapor are separated, and then each is piped away separately.

Blowdown valves should open rapidly to encourage high velocity in the area around the outlet so that the solids can be entrained and carried out of the boiler. Opening the valve slowly may solve your water hammer problems, but may reduce the effectiveness of the blowdown.

If you cannot use a blowdown vessel then I guess large diameter piping with a minimum of bends and no low point pockets would be the way to go.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[BigInch]

Nobody can see the nasties going into the stream that way.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[eadwine]

What happens to condensate & water when it is dropped from a higher pressure zone into a lower pressure zone? It flashes to steam and there is a significant volume expansion which drives the remaining water/condensate at much higher velocities (sometimes sonic)causing the hammer you're experiencing. In some cases this has been known to rupture lines and blowoff fittings, a form of catastrophic failure.

Solutions include, installing a "Flash Tank" or Steam Vent lines with exhaust heads to atm. on the blowdown line.

 

[BigInch]

Good point! I was thinking it has already condensed, but now see the T&P if quite high and it surely will flash somwhere in that outlet pipe.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[msquared48]

Increase the 3" to 6" and the problem will go away. The surge from the blowoff system is seeing the 3" as a sudden restriction, initiating back pressures and water hammer.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask