Block Valve Sizes for Control Valve Stations 1

[MEDK]

In general, are there guidelines for block valve sizing on the outlet of control valve stations? Specifically we have a desuperheating control valve station to take superheat out of a low pressure propane recycle line feeding a refrigeration compressor. The inlet to the control valve station is a 3" liquid propane line. The control valve is proposed to be a 2" valve. And the outlet is a 10". My contention is the outlet from the control valve should increase to a 10" out of the valve and the downstream block valve would be a 10". My boss believes there is a rule of thumb that the downstream block valve can be two sizes larger than the control valve size. Therefore the block valve can be a 4". I think there can be erosion and/or noise issues even though the block valve will share some of the pressure drop with the control valve. So is there any validity to the rule of thumb for the block valve size? It will be more economical if the size can be smaller as there are several of these applications. I've been around the block a few times, but I am not aware of this sizing guideline. I guess I am just curious if my method is off base.

Thanks for any comments.

 

[BigInch]

A control valve station is not the place to be using RO Thumbs. And No, this valve should not share the pressure drop with the control valve. It should be a valve that can be fully opened and should always be in either fully opened OR fully closed position, but you can determine the size of the valve as you would any other valve. Take the Cv's of each possible diameter choice at the fully open position, apply the usual erosion-velocity limitations for the type valve you want to use and choose the appropriate diameter.

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[JLSeagull]

Decades ago ISA issued a control valve manifold design standard. That standard was withdrawn. Rules-of-thumb may be acceptable for ordinary systems where the control valve takes 10-25% of the total system drop. Desuperheaters are not ordinary.

 

[RossABQ]

I've never heard that rule of thumb, and it is contrary to my experience with steam control stations; the downstream block valve is always line size.

 

[MEDK]

Thanks to each one of you for taking the time to respond.