Vent hole for carbon steel lined pipes 2

[rambabu0608]

Dear All,

According to the client specifications of Thermoplastic Line Pipes, The vent hole size for carbons steel Pipe shall not be more than 3.0 mm Max. Basically A vent in a lined pipeline system is a hole that is drilled through the steel pipe to allow passage of any permeated gases or liquids or to monitor pressure build-up in the liner annulus.

However there is no detailed information in case of vent hole size is more than the requirement i.e > 3.0 mm, If a vent hole size is 6.0 mm or more what will be the implications during operation / technical justification for a HDPE lined pipelines? Shall we proceed for lining for such spools / pipelines or bound to the specifications? Seeking experts suggestions in this regard. Thanks in Advance - RAM

 

[LittleInch]

I don't really know why you want to make the hole bigger. 3mm should be enough for the amount of gas you would expect, but if you think it isn't big enough then why not just drill another one close by? For venting of a liner you also need externally ridged or grooved PE liner pipe to allow the very low quantities of gas to migrate towards your vent hole. Most vents though are not left open to vent, but manually or automatically vented down at intervals.

My guess is that 3mm is just a cautious approach to the issue of potential shear action on a cylinder of PE100 at your vent point. In reality a much larger hole would probably be ok, but too big and the liner could, over time, extrude into the hole or shear a plug off. How big and what the impact would depends on the internal pressure, thickness of the PE liner (often quite low), temperature of the fluid and the long term creep strength. That's one of the things with PE that is quite different to steel is that over long periods of stress which are lower than the SMYS, the material creeps very slowly.

So you could make the vent bigger, but I struggle to see the advantage of doing it and having to do all that work instead of using what is probably a tried and tested vent size.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[rambabu0608]

Dear Littleinch,

Thank you for the information and i missed in my question to mention that the vent hole is drilled by Pipeline contractor where as our's is lining activity. Sometimes It happens due to oversight by them or not giving importance on requirements. Your technical justification is similar to my thoughts considering long term effects on liner. It's always good to have second opinion from experts like you..

 

[moltenmetal]

I don't know anything about pipelines, but have used lined pipe in corrosive service process applications many times.

Vent holes in process pipe are in fact usually left open to vent, or sometimes the vents are captured to a vent system. In some services the vents will constantly release a small but significant quantity of gases which permeate through the liner even when it is 100% intact. In services where the permeating gases are corrosive when they contact the moist atmosphere, a half coupling is welded to the pipe OD around each weep hole to permit capture of the permeate or to permit the use of a sacrificial nipple to prevent the pipe shell itself from being corroded by the permeate/atmospheric moisture mixture.

If the vent hole is too large, the plastic liner has to span the distance of the hole unsupported. If the pipe is under a significant internal pressure and especially if the temperature is elevated, the liner will eventually extrude into a weep hole which is too large in diameter and this can lead to a liner failure. If this were my piping system and you drilled the weep holes too large, I would reject your pipe and make you re-do it to my specification. If the P/T combination is very modest, perhaps you can get away with it- but then, why didn't they just use thermoplastic pipe? Why bother lining carbon steel?

 

[LittleInch]

I would get them to tap it and insert a plug or just weld it up and drill another one.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

Lined piping generally come from the factory with the holes already installed. Lined piping products generally include suitable venting within the metal structure of the item. Typically one or more 3mm diameter holes in spools and PTFE lined fittings, and the injection boss of PFA moulded fittings.

Don't think it would be advisable to put a larger hole without the concurrence of the lined piping manufacturer.

When Vent Extensions are required, a 10mm high 1/4” BSPT internally threaded boss is welded to pipes pools. For moulded fittings the injection boss is drilled and tapped with an appropriate BSPT female thread. A 65mm long vent extension is then supplied to fit to this, to provide a standard 75mm vent extension.

 

[moltenmetal]

If the housings have already been lined, neither suggested repair (welding or tapping to plug the oversized holes) is feasible without risk of damage to the liner. You would need to strip the liner and re-line, and next time drill the holes per the specification and not larger!

 

[ash9144]

The holes on the exterior pipe in lined pipe is a tell tale hole. That is so when the liner fails you will know immediately avoiding a catastrophic failure of the external pipe. The external pipe is just for support if the process material is compatible with carbon steel pipe you would just use carbon steel pipe and not line it. Don't plug it or route it somewhere else unless you have some means to detect a leak.