Pressure differential in ball valves

[engrom]

Friends, I need ur expertise to understand how pressure differential can cause erosion in valves. Came across severe erosion problem on ball valve connected to header on an oil well flowline. Ball valve which suffered failure is connected to manifold end and severe erosion found on connecting/ball valve flanges. No erosion noted on opposite flange end. Suspect there may be dispersion of sand in manifold piping from other wells. What should be normal condition of header valve open position, whether full or partial? What could be the effect if the valve is not fully open in circumstance where it should be? What is the erosional effects if there is a pressure differential/gradient across ball valve? Any website which can give more info on profiles of erosion in such cases?
Thanks in advance for ur expert comments...

 

[dcasto]

If the valve is opened and close, sand will accumulate at the end of the header, near the valve when closed and then accelerate when the valve is opened and cause errosion. Solutions, have a flow loop so that there are no dead leg or put in a low spot accumlator or Vcone bottom tank with a special Belly stop valve.

 

[BigInch]

And I'd think somebody is trying to do some throttling with it.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[JimCasey]

I agree with BigInch. Throttling with sand suspended in the fluid causes high velocities in the venaContracta-the narrowest portion of the flow just after it goes thru a restriction. In this case the restriction is the sliver between the partially open ball and the seat. Damage in this case would be a rounding of one side of the ball opening, scouring of half of the inside of the body, Sharp-edged wear of the downstream edge of the the ball port, assymetical wear of the valve seats, and one-sided wear of the valve tailpiece.

The answer to reducing the damage is to use harder materials (316 instead of Carbon steel, or stellite instead of 316, ot Tungsten carbide instead of stellite) , reduce the valve size so it is run farther open or(even better) fully open. Install the valve with the shaft hotizontal, and the bonnet on the left side when looking downstream. When the ball is closed you get a layer of sand on the bottom of the pipe. Opening a properly installed valve pushes the sand up and away from the seat until flow is established, than it flushes thru. A valve with a vertical spindle just gets a bucket of sand in the bottom that grinds away at it whenever the ball is rotated.

 

[cliff6361]

Are you certain it is erosion? Have you checked the flow conditions and assured yourself that there is no cavitation taking place that is causing the damage?

 

[JimCasey]

I must clarify an earlier post. I wrote: Throttling with sand suspended in the fluid causes high velocities in the venaContracta-the narrowest portion of the flow just after it goes thru a restriction.

More accurately: Throttling causes high velocities in the vena contracta. Sand at high velocities is erosive. Throttling with sand will cause accelerated valve damage.

Cliff6361: Erosion vs cavitation is easy to evaluate. Erosion damage is usually smooth, almost polished. Cavitation damage is fractured and pockmarked-looks like the surface of a cinderblock.