Slant cut on Relief Valve Outlet Piping 1

[lgq]

Does anyone know why there is a slant cut on RV outlet piping to atmosphere? Is it supposed to "direct" the gas stream or is it for increased area? Is it related to wind direction or should just be pointed away from personnel/equipment?

 

[TD2K]

I suspect it's just to keep any rain water from getting into the piping and causing corrosion.

 

[lgq]

actually, the slant cut points up and would catch more rainwater. Nobody seems to know why it's done but almost every RV at the plant is designed this way.

 

[TD2K]

I've seen PSV discharge piping with a slant cut but never orientated the way you describe. Will be interesting to see what others say. Sounds odd to me.

 

[grt12]

The last plant I was involved with had no slant, just an open end pointing upwards. The bottom of the upward leg had a small weep hole to deal with rain water.

 

[CHD01]

Actually, cutting the discharge pipe at an angle is not recommended because it results in lateral forces on the discharge line that must be accounted for in the support structure for the discharge piping of a relief valve.

It is recommended that all discharge lines exhausting to atmosphere be square-cut for our plants.

This also helps to increase dispersion to the atmosphere. Condensate for non-hazaradous fluids can be removed via a small drain line at the bottom of the stack; large amounts of liquid may require a KnockOut Pot or other separation method.

Also, with the slant-cut it is more difficult to prevent rain from entering the vent pipe. Plastic caps are sometimes used on the end of a vent pipe to prevent rain from entering. The problem is that people forget they are there and do not replace them if there is a release.

With a square cut it has been reported that the discharge pressure and noise is less than with the slant-cut; but I do not have the theoretical basis for this observation.

Hope this helps, bottom line, you can slant-cut the line if you wish; but it results in nothing but negatives which you must then over-come.

The only situation I can envision where I might want to consider a slant-cut would be to help direct the discharge away from a working area where I did not have enough building steel to exhaust the discharge at a safe elevation and location. Of course, installing the needed steel may be the beter option.

Regards,

Charlie The more you learn, the less you are certain of.

 

[EGT01]

Can't remember where I saw this (wish I had kept the reference at the time) but seems I remember seeing an explanation saying the slant cut was to reduce reaction forces (lever arm force) at the point where the RV inlet nozzle connected to a vessel. Seems the explanation was relative to a RV mounted on a short, straight inlet pipe with the outlet being an elbow and short pipe directed up.

The slant cut would be a diaganol directed away from the RV (and inlet nozzle). To try and describe it graphically, consider (./\). The period is the inlet nozzle connection at vessel. The back-slash is the slant cut directed away from the RV. As CHD01 noted above this produces a lateral reaction force component and combined with the vertical component the net force (the front-slash in the graphic) being directed back to the nozzle rather than just a lever moment directed down if it were just a square cut end on the outlet.

Of course, this shouldn't be taken out of context and unfortunately I can't remember all the details and circumstances of the reference. As always, taking things out of context can lead to unintended practices. Seems like a square cut end with proper vertical support would be better but my job function is not piping design or stress analysis.

If no one at your plant can answer the question, this may be an unintended application of the practice. You may want to confirm the piping design is adequate for your installations. I know of one company's practices which prohibit slant cuts to avoid the horizontal reaction component unless piping stress analysis has been performed.

 

[ihg]

API RP520 Part 2 2.4.1 gives the formulae for calculating reaction forces - it uses a straight cut and a support under the elbow to avoid transfer of reaction force to the inlet nozzle. The slant cut (away from the elbow) was an attempt to neutralize some of the reaction force but it isnt very effective. Note that the inlet flange can be relatively small in high pressure valves, and a free discharge can generate surprisingly high forces.
Some folk use the tapped connection above the inlet nozzle (actually connects to the body outlet) for a rainwater drain as even a weephole in the outlet pipe tends to plug. I think that this was originally intended as a pilot warning connection to show the operator that the relief valve was blowing, but its use appears to be lost.

 

[lgq]

Thanks for the feedback. One of the reasons we are looking at this is that an RV discharge (3850 psig) did cause the pipe to bend opposite the direction of the slant cut as would be expected. Also, I found that the original Braun spec (circa early-70's) was very specific "scarf tail pipe ends 30 degrees..." which explains why it's everywhere in the plant. However, because we cannot find any other overriding reason to have the slant cuts, we will probably straight cut the ends, design better pipe supports and begin an evaluation of the other RVs in the plant.

 

[CHD01]

Sounds like a workable plan, the supports or lack thereof probably play a much larger role than the slant cuts in the damage you observed. Good Luck! The more you learn, the less you are certain of.

 

[saxon]

Igg, The story I got on "slant cuts" is, way back when, before there were Piping Engineering groups and CFD. Some fitters and welders got together on a job site and decided that it would be best to put slant cuts on all the RV discharge piping. You see, the ellipse has more area than than the circle does, so it will make the valve relieve better. Or so the logic goes. I guess the rest is history and tradition and who the engineers, designers, fitters & welders trained under.

One thing is known, it increses the cost of the discharge pipe. BTW, don't forget the weep hole on the bottom side of the discharge pipe.

 

[PSDesign]

An old pipefitter told me once that it was done to cut down the amount of rain that gets in the pipe. They would point the slant cut to the north, because that is the least likely direction the rain would come from.