Why Three Hole Orifice Plate

[sshep]

Friends,

An 8" steam condensate return line coming from a gum column reboiler (high temp) located several hundred meters from the steam flash drum contains an a three hole restriction orifice plate. Presumably the plate is to hold backpressure on the return line to reduce flashing and two phase flow problems. Downstream of the orifice we have extreme erosion- this is how I found out about it.

A peer asked me what was the reason behind a 3 hole design and I can only speculate (noise reduction, vibration, better able to pass 2-phase flow). Does anyone know the reason for such a design?

best wishes,
Sean

 

[Idontknowit]

The reason behind this is simply to reduce the volume of the flow.
This profile reduces the pressure drop of liquids flowing through the holes, provides better jetting of fluids.

Kind Regards,

 

[insult2injury]

Does anybody have useful reference data in the design of such a plate?

I2I

 

[Latexman]

Maybe when they put in a one hole orifice the flow was too low. Based on the observed flow and required flow they needed about 3X the observed flow, so they drilled two more holes. Just a guess. I've seen it happen.

Good luck,
Latexman

 

[pennpiper]

sshep

A few questions:
1 - Does the orifice plate have a "Tab" that extends from the outside diameter of the orifice plate to what would be beyond the outside diameter of the flanges?

2 - Does this "Tab" have data stamped on it?

3 - What are the diameter of the three holes?

4 - What is the location of each hole in relation to the "Tab"?

5 - What is the location of the holes in relation to the inside diameter of the pipe?

 

[dcasto]

according to miller and spinks, the equivalent area of the three holes will have the same capacity of a single hole of the sam area.

 

[adx]

the Diameter control chocked pressure P2, and the number of hole control chocked flow, and it looks like 3X.

Best Regards
ADX

 

[sshep]

Friends,

Thanks for the interest. This is a 12" line, the tab data is d=40.75mm (no indication of 3 hole so probably the single hole dia of equal area). I noted that the orifice plate is also installed backwards, but as erosion here is a historical problem, it is possibly a coincidence. Typical steam/condensate rate is 5000 to 6000kg/hr, but chest pressure gage broken so more work is required to get vfrac conditions for the stream. In any event you can see that the velocity is probably high, and erosion seems very likely because two holes are close to the pipe wall.

Here is a picture of the plate. Anyone seen a design like this before or can tell me what the advantage is?

 

[dcasto]

I think Latexman got it right. The one hole appears to be centered and the two others added later.

I was maybe hoping to see One hole at the bottom that would have been a liquid "weep" hole to allow condensed water laying on the bottom of the line to flow past the orifice.

 

[pennpiper]

dcasto
That was my thought as well. But it seems as though I was wrong.
My next suggestion for sshep is to go back to the supplier and ask them to explain what is going on here.

 

[insult2injury]

Is the upstream side of the orifice typically exposed to two-phase conditions?

I2I

 

[gerhardl]

The orifice plate is clearly not adapted or suitable for the given flow. A rough calculation, provided we assume condensate (one-phase: liquid) give at 12inch roughly a flow of 7m/s for about 6000 kg liquid an hour. With a one hole orifice at d=40mm the same flow would give 398m/s through the one hole, and 132m/s if three holes.

The real situation is of course more complex, and we lack data (temperature, pressure and real amount) to look in detail at what could be happening.

My guess is that a wrongly sized orifice plate was put into service and 'someone' tried to improve on cavitation (not normal erosion) and an extreme noise level caused by the cavitation/flashing in the pipeline immidiately after the orifice plate. The data is indicating not much success by doing this.

The two extra holes would not improve the situation in a large degree. To properly control as large a liquid flow as 6000 kg/hour one would normally use a larger total areal of the hole(s) in a single orifice plate, and rather use several orifice plates in series to give a total restriction.

Generally on restriction orifice plates: under normal circumstances an orifice plate with more than one hole would be constructed to give a concentric flow picture. The reason is to even out the flow profile across the pipeline diameter, and/or avoid to much turbulence near the wall.

An orifice plate for steam could have hole(s)at the pipeline bottom to avoid condensate building up against the plate, but the best solution here is a properly controlled steam line with condensate drained through steamtraps.

 

[JimCasey]

Noise-reduction plates have hundreds of very small holes, so that's not it. The tiny holes have a natural frequency above audible. So if you can't hear the noise, it's not there.

A plate such as the one shown causes a "blowtorch" of cavitation into the downstream piping, with the erosion you mentioned. Mount the same plate on the nozzle to a vessel, and the cavitation can expand and dissipate harmlessly befor it impacts the opposite wall. If not discharging into a vessel, at least increase the line size downstream so the cavitation bubbles have a chance to collapse in the free stream before reaching the pipe wall. Make sure it's a long, straight pipe, or you'll be buying elbows.