I have a situation where the mainline block valve in a 36" natural gas pipeline is closed for a period of time. During this time, the pressures on either side of the valve drift apart. e.g. Upstream pressure = 800psig. Downstream = 600psig. If the mainline valve is now opened what is the maximum gas flow rate across the valve? What would be the best equation to use to estimate this?

Also, what is a good rule of thumb to use for maximum allowable gas velocities in the pipeline? I have read in many cases that 100ft/sec should not be exceeded, but I believe this is to avoid excessive noise. Is this true?

With 200 psi dP across the valve, you are a long way from choked flow, and the transient velocities should all be in the manageable range.  I wouldn't be concerned about that.

It seems to me that there are as many "rules of thumb" for velocity in pipelines as there are engineers.

The rule that makes the most sense to me is to minimize pressure drop because it is expensive to boost the pressure back up to recover the pressure lost.

In lower pressures (under abourt 300 psig) I usually go with a design range frm 11 to 100 ft/sec, and then calculate the expected dP/mile--for pipes under 12-inch I like to stay under 15 psi/mile.  For 20-inch and bigger the number is usually closer to 5 psi/mile.  These numbers come from the cost of compression vs. the cost of running bigger pipe.

For mainline kinds of pressure I like to stay under 5 psi/mile for any size pipe.

Upstream pressure = 800psig. Downstream = 600psig. If the mainline valve is now opened what is the maximum gas flow rate across the valve?

If there are long lengths of pipe up and downstream of the valve, then fast or slow opening of the valve will not come into play during the initial transient.

Assuming the gas composition is the same on both sides of the valve, up and down stream temperatures will affect the initial flow rate.

If the natural gas can be considered a perfect gas with constant specific heats, then a method of characteristics analysis will yield the flow.

Why would you say that plug valves are better suited to transient flow than ball valves?  When I look at a flow-percent vs. open-percent curve for either one they look quite similar and ball valves are lower maintenance.  I've found that for applications that truly need throttling it is best to avoid any of the inherently on-off valves (i.e., ball, gate, and plug).

Velocity limits depend on where you are.  Usually there is nothing wrong with high velocities over small distances in a station, its just that they add up to unmanageable pressure drops if you try to do the same thing out on a long pipeline, or if your line is dirty, you might increase sand abbrasion on the interior.  Limiting velocities to not "strip corrosion inhibitor" doesn't ring as I have always found higher velocities are good for anti-corrosion work as,
   1.they keep the line clean
   2. they keep the inhibitor well distributed on all interior surfaces of the pipe and not just running along the bottom,
  3. they keep any water and other crud moving and from collecting at pipeline low spots.

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