Pressure loss steam through contraction

[Martin56]

Hi Guys,

Interesting forum . I have question regarding pressure drop calculation for a inlet line to a PRV. Medium is steam 4 barg. In this inlet line we reduce (contraction) from DN65 (inside 62 mm) to DN 32 (inside 32 mm). The pressure drop accros this contraction is really huge! I have to reduce one way or another to adjust to the prv inlet connection (inlet DN32 outlet DN50). I calculated it with the standard HL=K*(V2^2/2G). PRV certified massflow is 1775 kg/h, I have to size my inlet and outlet according to this, but with this contraction it gives problems relating to alowable pressure loss.

My feeling is that this formule is not applicable for steam anymore, is there any 2 phase formulas needed?

 

[Latexman]

I estimate an H or J nozzle would be needed for that. That is usually a DN50 x DN75, so your DN32 x DN50 PRV may be a little small.

Your velocities are too high.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[georgeverghese]

If you are stuck with a given nozzle and PSV with high inlet pressure loss, another approach would be to find another nozzle or piping branch where you could install a supplementary PSV to split / share this total load of 1775 kg/hr. The PSV settings should be staggered in this case - see API 520 for guidelines on how to do this.

 

[Martin56]

We are in Europe and because of regulations we use the EN-4126. I have no doubts my PRV calculaton is ok. Case is 1058 kg/h saturated steam, set pressure 4 barg, relieve pressure 5,14 bara, gives a orifice of 22,7 mm (4,06 cm2). This PRV selected is certified for 1775 kg/h massflow and for this I calculated pressure loss inlet / outlet piping.

My question is about the contraption pressure loss for steam is the formula HL=K*(V2^2/2G) still applicable?

 

[Latexman]

More data; better insight. Yes, the PRV looks fine.

Since the inlet steam does go through a physical contraction, the contraction pressure drop does apply. So far, I have not heard a reason why two phase flow might exist in a significant amount in this case. Why do you think two phase correlations apply?

Can you give more details on the contraction and your calculations? Is it a welded bushing reducer, a screwed bushing reducer, a butt welded commercial reducer? What K value are you using? For a butt welded commercial piping reducer I use a K = 0.1 (based on smaller end/higher velocity) as a first pass.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[georgeverghese]

For a piping reducer, you've got the wrong formula. For this case,

K = 0.5 x (1 - A2/A1) = 0.5 x (1 - (D2/D1)^2)) = 0.367

The total dynamic loss should also include the pressure loss term due to kinetic energy gain across this reducer - see example 5 on page 6-16 of Perry Chem Engg Handbook (7th edn)

Obviously the total inlet loss for this PSV should also include all other fittings, vessel nozzle losses(if this PSV is connected to a vessel) etc.

The formula you've listed is for an exit - contraction loss across a vessel nozzle where V1 is negligible.

 

[georgeverghese]

Rough estimates indicate a frictional pressure drop of 32kpa and a reversible pressure drop of 60kpa through this reducer, and this total of 92kpa for this fitting alone clearly makes this configuration unacceptable. V2 of 211m/sec at this reducer exit is also very high.

 

[georgeverghese]

The ASME standard H orifice RV can be obtained with a 1.5in or a 2in inlet - I'd take a closer look at the 2inch inlet, given the results for the 1.5inch inlet.