Reduced Bore Line Sizing Local to Control Valve

[ChemEngSquirrel]

I have a 6" line with a 3" control valve installed on it. Pump min flow recycle with continuous recycle flow. Crude oil service.

When i do line sizing, 6" line velocity is below the erosional velocity limit, but the 3" line is well above it. The 3" line is only the redcers for the control valve i.e. 6" line, reducer, 3" control valve, reducer, 6" line. Do i need to consider erosional velocity in the short 3" section of piping?

Thanks.

 

[EmmanuelTop]

It depends for which flow you are calculating the velocity in 3" line. That line does not normally see full flow.
Blocked discharge is something that normally lasts a few seconds - minutes at best - so there is no logic to compare the 3" line with the 6" discharge line, in terms of full flow.

Dejan IVANOVIC
Process Engineer, MSChE

 

[LittleInch]

If you're using API 14E for the erosional limit then you can normally ignore it.

I've never understood how this equation continues to be used to justify pipe sizes.

If your connection is 3" then what can you do about it - nothing.

unless you have a lot of grit in the fluid then I wouldn't bother. If you have then your control valve will wear out a lot faster than the bit of straight piping....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[shvet]

You can enlarge DN of control valve:

1. You can add additional flow resistant downstream of control valve. Required valve's Cv will increase so control valve DN increases too. But price too... It can be a simple manual globe valve with cavitation seat or flow resriction orifice.

2. You can ask a vendor to supply another control valve with larger DN but the same seat (and the same Cv). But again price...

 

[moltenmetal]

Erosion isn't a concern for linesizing most of the time. If you were pumping a high-solids slurry or sulphuric acid, then you'd have to pay particular attention to it.

If you're concerned about erosion of the reducers upstream and downstream of what is no doubt a properly-sized control valve, just how long do you think the TRIM in that control valve, or the body of the valve where the jets exiting the trim impinge, are going to last? And what about the pump's impeller or outlet nozzle?

No, you don't need to increase the linesize, or to find a larger valve body with the same size trim in it. That would be 100% a waste of money.

 

[georgeverghese]

Straight answer is NO, no need to consider erosional velocity exceedence in the 3inch reducer section. This is industry practice. I cannot explain why though.

 

[ChemEngSquirrel]

Thanks for the replies.

If i can ignore the erosional velocity limit, what criterion should i be using to determine max allowed velocity in the 3" line?

Note that this pump constantly recycles as the the min flo wset point is well above the forward flow rate (over sized pump for current low oil rates)

 

[shvet]

According to ISA Guide "Control Valves, Practical Guide for Measurement and Control" the valve trim exit velocity head should be no more 70 psi for continuous service and 150 psi for intermittent service. These values are found in some local codes for valve body exit:
- one phase fluid <70 psi
- 2 phase fluid or cavitation <40 psi
- intermittent service (no more 5% of life time) <150 psi
- if vibration shall be avoided <10 psi

Valve body DN shall be in limits:
0.25*DNpipe < DNvavle < 1*DNpipe

Vh=(p*(v^2))/(2*g)
where
p - valve trim exit fluid density
v - valve trim exit fluid velocity
g - gravity constant

If head exceeds this limit discrete pressure drop stages or several valves should be used. Pump spillback control valves are in severe service because of cavitation damage. Because of this sometimes multistage angle valves or restriction orifice downstream of spillback control valve are used.

 

[poli60]

A lot of thougths can be done. Better to restrict the options by providing some data: flows, pressure drops....
good luck!

 

[mk3223]

As mentioned above, the piping layout of the 6" pipe to 3" CV with a reducer is a typical configuration of a control valve station design. The vendor of the control valve may help you to check you design for any erosion or noise issues, and suggest a change if needed, such as piping and/or control valve size. Smaller valve size is to help the process control design and lower cost needed.