I've been tasked with determining whether our subsea choke valves are at risk due to erosion. The design data is:

flow of water = 4000 m3/day (SG = 0.96)
flow of oil = 2000 m3/day (SG = 0.87)
flow of NG = 1,800,000 Sm3/day (SG = 0.67)

Calculated Cv = 300
T1 = 95 deg C

dP = 610 kPa

The realtime data is: dP = 7600 Kpa. The valve is operating about 35% open. I have the plot of valve travel vs. Cv. The current flow is about 5000 m3/day total fluids.

My questions is, flow rate is currently almost at design, but at only 35% open. My guess is the U/S pressure is higher to push the same volume through a much smaller area. Obviously this would increse the velocity through the valve (plug and cage, linear). Will it increase the velocity so much as to cause erosion in the trim?

It's not the pressure drop I'm concerned about. It's the velocity through the valve that may give problems.

As stated earlier, the upstream operating pressure of the valve is spec'd at 8600 kPa (although MAWP is 10000 psi). However, the current U/S pressure is 11000 kPa resulting in a valve that doesn't need to be opened as wide to get the same flow. Getting the same flow through a smaller area means one thing.....increase in velocity through the valve.

Does anyone have experience with erosion in subsea choke valves?

I understand where you are coming from. However, the valve is currently doing 82% of its rated capacity but only 35% open. According to its curve, at 35%, the Cv is 110, about 30% of Cv when full open.

As far as size goes it doesn't matter. However I agree with your previous comment of how much does 35% open in flow cross sectional area mean. I've been after the vendor to provide this.

They said they will provide the area in 64ths. I've figure out what they meant but never heard of it termed that way until now.

It's a Cameron CC40 SSRC, Plug and Cage, Linear with Cv of 300.

How about an analogy. I'm flowing 5000 m3/day of water through an orifice of 1.5". Say the U/S pressure is 5000 kPa (all numbers are abitrary). If I decrease the orifice size to 0.5" but want the same flow, how do I get it? I bump up the U/S pressure. So now I'm getting the same flow through a smaller area, the velocity increases. Same case with this valve.

Basically, I have to wait for the info from the vendor on cross-sectional flowing area of this valve. This will ultimately allow me to calculate velocity with given flow.

Star for Steve. The question about the amount of sand is of mayor importance.

My equation regarding erosion is:

E ~ m(sand) * F(alpha) * K * u^n

E = erosion level (weight loss)
m(sand) = amount of sand
F(alpha) = impact angle of the particle
K = material constant
u = impact velocity
n = material exponent

So your erosion depends on the amount of sand, the design of the valve (impact angle), material of the trim parts and the flow velocity.

Since the velocity is one of the few items you can control this has normally the highest attention. My rule of tumb is to stay below 20 m/s outlet velocity in multi-phase flow for this type of application.

The next item is the valve material. Make sure that ALL high velocity components are made of erosion resistant material. These are e.g. the cage, piston, seat, bean. Do not worry about the inlet side, velocities are low there.
Tungsten carbide for the cage alone is not sufficient. Note that the material constant (K) for standard tungsten carbide is 80 times better than for steel. You can also use high quality tungsten carbide (up to 200 times better than steel) or ceramics (also approx 200 times better than steel but quite expensive).

Finally you can go for a different valve design with less erosional paths. An angle choke valve or a globe valve for example will always see more erosion than an axial valve due to the internal flow path.

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