Pigging through valve - Unequal wall thickness

[gfdoug]

All -

I'm preparing to order a through-conduit gate valve (24") to replace in a 24" line. On the downstream side the wall thickness is STD (0.375") on the upstream side of the valve the pipe is XH (0.500").

Since the bore of the line is changing, do I need to make any special considerations for the bore size of the valve?

If it makes any difference - I've researched how flexible pigs are capable of being, so normally it won't be a problem during day-to-day operations - I'm more concerned that at some points in time they will be using a more delicate smart pig for line double-isolation.

Any thoughts on this would be very much appreciated.

Thanks,

G

 

[zdas04]

Nearly any pig you run is going to be longer than the flange-to-flange distance of the valve. When a pig enters the expanded area, the tail end is still plugging the pipe. When the tail end gets into the oversize region the nose is back in pipe. This gets to be a problem with speres (which are only appropriate with minimal density difference between upstream and downstream, like in a batching operation in a LPG line, never in a gas line with the possibility of liquid on one side and gas on the other), but with all the other pig types you should be fine. Smart pigs are the least of your worries.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. ùGalileo Galilei, Italian Physicist

 

[racookpe1978]

How much margin can the pigs "play" with on pipe ID? Can they tolerate a 1/4 inch difference on diameters as you indicate caused by weld intrusion or gunk and garbage on the pipe wall?

 

[zdas04]

It really depends on the kinds of pigs you run. I've run some mandrel and smart pigs that would stick at a schedule change. Turbo and poly pigs can generally deal with reduced port valves. Uncoated foam pigs can handle any diameter change you throw at them.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. ùGalileo Galilei, Italian Physicist

 

[LittleInch]

A couple of questions.

What is the ID of the current valve? Equal to the thick pipe or the thinner? Which is the direction flow? Thick to thin or vice versa?

The issue is not so much the variance in ID. This sort of change should not cause an issue, but whether the change is a step or a smooth internal angle. A step can damage the feelers on an intelligent pig.

It is a little odd to change ID at a valve, but I would go for the larger ID in the valve and machine the inner face of the thicker flange to transition smoothly between the two wall thicknesses. A slope of 1:4 is commonly quoted.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[gfdoug]

The flow is from 0.375" to 0.500" - wall thickness gets larger (aka ID gets smaller in the diretion of flow). I haven't gotten as far with the valve - I am attempting to get in contact with the valve manufacturer to see their recomendations as well.

Their cut sheet has the nominal bore of the valve at 23-1/4"

 

[gfdoug]

LittleInch,

Where would I get the 1:4 slope value from - pig manufacturer, valve, engineering standard, rule of thumb?

Thanks

-G

 

[LittleInch]

Try figure 434.8.6-2 in ASME B 41.4. I know this is for welding, but quotes the 1:4 slope.

I would go for the larger ID of the valve and machine off the difference on the smaller ID (d/s)connecting flange which should have a final ID the same as the thicker wt pipe.

I doubt the valve manufacturer will offer much as this is a pipeline design issue, not a valve issue. If it was me I would say tell me what we want.....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way