A large butterfly type control valve in a fuel-gas line has been installed in the wrong flow direction. Now, it will be difficult to extract the valve from the pipeline and turn it around because structures in the vicinity have to be dismantled first.
However, the pipeline is not yet 'charged' and we will do the job if it is absolutely required.
The butterfly valve is clearance type with 60 degree opening provision(the flow will never be shut completely in the running plant, so minor leakage will not be a problem).
Will you guys let me know whether the control valve will still work as installed now?

What type of butterfly valve is it - for example is it a triple offset type ?

The valve may not have anything like the intended flow characteristic and operating torque values installed backwards if it has an offset disc (likely given the preferred flow direction).  I would recommend that at a minimum you consult the manufacturer and obtain their feedback on how it will perform relative to the original specifications (e.g., the data sheet).  However, if it were me, I'd fix it.

Note: Operating torque for offset discs as a function of valve position are substantially different based on flow direction to the point where the torque can in fact be acting in a completely different direction.  Pressure drop can be different as well but this effect is probably not as significant as the operating torque issue.

1. You will either have some special constructed regulating composed disc, cage type or similar, to give support for a complicated and large pressure flow and drop.

If this is the case the answer to your question is more complicated and could be negative.

2. Or you will have,if you are at the other end of the scale, a controlled, slow and laminar flow, and a disc with more of a common flat and compact form. This type of valve is normally used as a typical on/off valve, but in this case used to regulate.

If this form is what you have, the answer is a bit simpler and could be positive.

The reason for this is that if you look at this case with incompressible fluids (water) a double offset (double eccentric ) valve will only be properly regulating as long as you have a full pipeline, a low pressure drop, and maintain the flow with a delta p over the valve  low enough not to give cavitation.

In addition to this it is commonly known that regulating will only be possible when the valve is at about 20 degrees of opening or less. In addition the regulating will be more coarse than for a proper regulating valve.

If the fluid is gaseous the operation is similar.

If this latter example is relevant, the regulating result could well be within acceptable accuracy with a flow in the 'non preferred direction, - the valve is usually both constructed (and could also be presuure tested) for this.

The problem could more be that 'the wrong direction' is perhaps not tested out for regulating purposes: -flat face against the flow and stem and stem bearings behind in stead of opposite, and it could be difficult or costly to establish flow curves and get the factory to give sufficient back-up and guarantee for the result and problem free (cavitation free) operation if regulating this way.

Be aware that a offset butterfly valve in preferred direction has a BTO (break to open) torque higher than TSO torque, the opposite applies when offset valve is in non-preferred direction (higher to seat/seal & lower to unseat/open).

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