9 replies to this topic

[arvind]

Hi to all,

I am working for the PTA project.
This is regarding the pressure drop across the control valve. Control valve PCV-21310 and the ROP-21309 are in conjuction to avoid the flashing across the control valve.

Data:
Fluid : crude terephthalic acis slurry content 36 % w/w solid.
Temp: 97°C
Pressure : mention below
Vapor press: 0.906 bar a
density: 1110.9 kg/m3
don't consider frictional loss.

Control valve upstream pressure is said to be 13.9 bar g and that of the downstream of the ROP it is 0.9 barg,

I have seen in many artical, The minimum pressure drop across the valve must be 10 psi or 0.7 bar.
So in this case what could be the right value across the PCV ? so as to avoid the flashing in worst condition.
I have done the calculation based on 2 bar across the PCV and rest of the across the ROP.
For more detail pls see the attachment.

IS it right or what is the right procedure to do it ?


Thanks in advance.

Arvind

Attached Files

•  Control_valve.pdf   85.42KB   153 downloads

[fallah]

I think, in operational point of view, you are not allowed using RO, especially with DP around 10 bar, in such mentioned slurry (with 36% solid) line.

[djack77494]

You have an erosive system and are taking a large pressure drop. Not a good situation. Assuming you want a reasonable (no defined) level of control, I'd expect a pretty significant range of flows through this system. Now if you half your flowrate, you will take only 1/4 of the original pressure drop across the orifice. Not very effective since your control valve must now handle changes due to flowrate changes PLUS magnifying changes due to the impact of the restriction orifice. When you add that to the problems in using an orifice in slurry service, then I'd say you should remove the orifice. When I think of trying to handle these requirements using a control valve, that also is pretty difficult. So I need to ask you to clarify just how widely you expect your flowrate to vary. Also, if you know, please comment on the experiences of others in this service. This looks like a hard one.

[arvind]

You have an erosive system and are taking a large pressure drop. Not a good situation. Assuming you want a reasonable (no defined) level of control, I'd expect a pretty significant range of flows through this system. Now if you half your flowrate, you will take only 1/4 of the original pressure drop across the orifice. Not very effective since your control valve must now handle changes due to flowrate changes PLUS magnifying changes due to the impact of the restriction orifice. When you add that to the problems in using an orifice in slurry service, then I'd say you should remove the orifice. When I think of trying to handle these requirements using a control valve, that also is pretty difficult. So I need to ask you to clarify just how widely you expect your flowrate to vary. Also, if you know, please comment on the experiences of others in this service. This looks like a hard one.

The range of operation for the control valve and ROP design for:
Control valve :

Normal = 78430 kg/hr max= 200000 kg/hr, min = 50000 kg/hr

And for orifice flow = 200000 kg/hr.

[JoeWong]

Hope these helps

http://www.controlgl...s/2007/086.html

http://www.documenta...d350004x012.pdf

[JLMONTREAL]

The range of operation for the control valve and ROP design for:
Control valve :

Normal = 78430 kg/hr max= 200000 kg/hr, min = 50000 kg/hr

And for orifice flow = 200000 kg/hr.
[/quote]

You don’t need to absorb all DP by the PCV; the angle bottom valve of the tank can also absorb certain DP. Even if there is certain pressure remained from the bottom valve to the tank, it will be a mixer to prevent the solid settlement which will be needed for your application.

I suggest using smaller size pipe (say 3” or 4”) for the downstream pipe of PCV to increase back pressure and choose a PCV that applies for slurry and cavitation duty(for worst case), keep certain pressure for the bottom valve of the tank and its downstream.

Your calculation will find the exact numbers.

[davebartran]

the process licensor should be the source of the sizing criteria as this particular process is so specialized that general criteria do not apply.

my last PTA plant was in the 70's, amazingly little change since then.

[fallah]

using smaller size pipe (say 3” or 4”) for the downstream pipe of PCV to increase back pressure

Smaller size pipe (the same flowrate)->Increase back pressure??
Also smaller size pipe->Velocity increase->Erosion due to 36% solid content

[JLMONTREAL]

using smaller size pipe (say 3” or 4”) for the downstream pipe of PCV to increase back pressure

Smaller size pipe (the same flowrate)->Increase back pressure??
Also smaller size pipe->Velocity increase->Erosion due to 36% solid content

In PTA plant, the fluid velocity in slurry pipeline is higher than that of the regular fluid, to take the pain of erosion is inevitable. 4" should not be too bad. Anyway, the licensor or BEP contractor should have the piping engineering instruction in their design package. The right principle may be found there.

[Alfreedo]

Good comments here. I spent 10 years working for a control vavle company. Also some good questions. What kind of turndown do you need? Also, another good question is, do you need tight shutoff? In the mining industry, we frequently supplied ceraminc lined angle valves. The plug head, seat and downstream venturi outlet were all cast out of zirconium or hexallog SA from carborundum. The problem with ceramics is that tight shut-off, to the best of my now outdated knowledge, is not readily achievable. These valves typically had limit closing stops to prevent trim component fracture. There are plenty of other materials that can be used in less severe environments. The valves I'm talking about were autoclave let down valves in extremely hot and corrosive environments with a couple hundred psi letdowns to atmospheric conditions. Wear plates in the bottom of the autoclave vessel were typically required to prevent holing through the vessel where the outlet of the valve impinged on it.

Good luck - although I know I'm a little late posting here. Let me know if I can help in any way.