Normal shocks form in pipe outlets?

[NIWEngineer]

Hello all,
How do you explain that downstream from a regulator you have a pressure of several hundred psig and further downstream (some 200 ft) the pipe is discharging to the atmosphere, hence the pressure is 0 psig.

Is there a normal shock standing at the pipe outlet to the atmosphere?

 

[CJKruger]

NIWEngineer, whether there is choked flow at the pipe outlet depends on the pipe length, diameter and fluid properties.

However, for "several hunderd" psi drop across a 200 ft pipe, you will probably have choked flow at the outlet.

 

[NIWEngineer]

CJKruger, thanks for the reply.

The pressure ratio certainly indicates that the flow is choked. However, there is an even larger pressure drop across the regulator (from 2kpsig to 150psig) which would indicate choking as well.

In other words, I have a regulator in which the pressure drops from 2kpsig to 150psig, and then (200 ft downstream) an opening to atmosphere where the pressure drops to 0 psig. So, the flow is choked in two locations?

 

[zdas04]

The flow path through a pressure regulator is designed to avoid (not prevent) choked flow within the regulator. I don't know all the details, but I've seen a bunch of regulators that more than satisfy the upstream/downstream pressure relationship for choked flow that were far too quiet to have sonic flow at the regulator outlet.

From what you describe, it sounds very much like you have choked flow at the outlet, which implies a standing wave immediately upstream of the outlet.

David

 

[NIWEngineer]

David, thanks for the reply.

The flow path through a pressure regulator is designed to avoid (not prevent) choked flow within the regulator

Do you know of any publicly available reference that states this? Maybe a manufacturer's catalog, or some instrumentation book?

Thanks,

 

[zdas04]

I know that all of the valves web pages talk about noise reduction, but those things tend to be non-technical and never use the term "choked flow". My guess is that the techniques they use are very much proprietary. I only know that good regulators are significantly quieter than a throttled ball valve.

David

 

[CJKruger]

NIWEngineer, yes it is quite common to have gas choke flow through a control valve or regulator (inside the valve, not the connected piping). And yes, you can have choke flow through the valve and then one or more choke locations in downstream piping (at expansions).

To reduce noise, vendors use things like "whisper trim" and staged letdown. I have not had good success with the whisper trim type installations, as they plug up.