Subsea Choke Erosion 6

[TrevorT]

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?

 

[BigInch]

That's not much of a dP so it doesn't look too bad.
What's the valve size?

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[TrevorT]

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?

BigInch.....appreciate the reply.

 

[BigInch]

Velocity corresponds to pressure drop just as much as volumetric flowrate. Low dP = even lower V.

Trick is we don't know what V it is without the valve size, or how much 35% open represents in flow cross-sectional area.

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[BigInch]

If its a 1" its fast, if 10", probably nothing.

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[TrevorT]

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.

 

[BigInch]

What valve are you using? Mfgr, model number, size and type trim.

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[TrevorT]

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.

 

[SJones]

What are the materials of construction?

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[TrevorT]

Trim is 17-4PH SS with tungsten carbide and stem is 17-4PH SS. Body is F22 low alloy steel with full 625 clad inlay

 

[SJones]

Any sand or solids in the fluid?

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[BigInch]

gas, oil and water hardly ever come in the same stream without sand, so lots of solids are pretty much assured. Question is, is there anything better than the trim he's using now to prevent erosion? I wouldn't think so, but its possible.

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[SJones]

I don't think so either.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[BigInch]

Right, so it looks like the only apparet option to get better results is to increase the size of the valve or opening and thereby reduce velocity.

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[terje61]

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.

Good luck,
Terje

 

[Boio]

to terje61-- Can I have more information about your formula for Erosion? Which spec. shall I read? Thanks for your answer.

 

[terje61]

Hello Boio,

Sorry for the late reply, I got a new computer and lost my access to the eng-tips forums for a while.

The equation from my previous post is derived from a Det Norske Veritas (DNV) report on erosion in valves. They performed numerous life tests on several types of valves and used this equation, including the matching K and n-values. I do not know if the report is open for public, but you can ask DNV in Oslo (Norway).

I mentioned the equation to show the relation between erosion, materials, velocity and amount of sand.

Regards,
Terje

 

[jhademenos]

The DNV report can be found at:

http://www.dnv.ru/Binaries/ErosiveWearInChokeValves_tcm83-8475.pdf