RV for viscous polymer

[jmgengr]

I need to size a relief valve &/or rupture disc for Polymer service:
viscosity = 35,000 centipoise
flow = 44 gpm at 80 psig & 115 F max
set point = 100 psig
line size = 2" sch 40
I have contacted various RV mfrs w/o results due to the high viscosity. Please help if you have this experience.

Thanks in advance

jmgengr

 

[pleckner]

Absolutely, positively NO RELIEF VALVE. Use a rupture disk ONLY! Can't emphasize this enough.

You must size the rupture disk yourself, no vendor will do it for you. However, contact Fike Corporation and get their catalogue. It shows you how to do it.

In a nutshell, you treat the rupture disk as any other piping component. The trick is to know which flow equation to use and the rupture disk's contribution to the overall system pressure drop. Talk to the vendor for the type of disk to use for your system and they will then be able to tell you the Kr value to use. Kr is similar to a K value you would use for a pipe fitting, e.g. tee connection, elbow, etc. Your unknown is what flow equation to use? Use whatever equation or algorithm you use in your plant when analyzing flow problems.

The size will depend on the total pressure drop in the system. You say you have a 2" line but you do not give enough information about your system. Is this for vessel protection or pipe protection? The point is, 2" may not be large enough to prevent your source from overpressure. For a vessel, assuming a non-fire case, you cannot exceed 10% greater than MAWP or design pressure. Therefore, your size will depend on the pressure you obtain during relief from your vessel to your discharge point. Don't forget to de-rate the flow through the rupture disk (per ASME). By this I mean, whatever your calculated relieving rate is, multiply it by 1.11 (divide by 0.9) before applying the flow equation.

Also, you say your "set" point is 100 psig. For a rupture disk, this is the bursting pressure but additional factors must be taken into account, e.g. the manufacturer's range. I wish I finished and published my paper on this very subject for you to read (soon to be found at

http://www.cheresources.com)

but in the meantime, talk to Fike. They can help you with the particulars. You just have to do the calculations.

 

[chicopee]

I agree w/ Pleckner about using rupture discs and I was lucky to get from Fike corporation a CD about their rupture discs. Unfortunately I have not had a chance to review it, however get hold of them for a copy.
It appears to me that you are dealing w/ PVC or other type of plastic mix such as an organosols or plastisols.
Since I was involved with fired and unfired pressure vessels for a few years the subject of rupture discs was not as prevelant as safety and pressure/temp. relief valves. Nonetheless, discharge rates thru any anyone of these safety devices including rupture discs in one criteria (others being pressure,temperature and medium) about sizing them and it appears that such devices would best work in an accumulation tank than on piping unless that piping can be isolated by valves from the receiving source than you would have no choice about installing such devices on the pipe.
Dicharge rate (or mass rate lbm/hr, kg/hr etc...)is based on your manufacturing capability and you must know that information in order to keep the pressure from exceeding the MAWP + 10% . You may also calculate max flow rate thru the piping as long as you knew the pressure drop thru a given length. You already know the bulk viscosity assuming isothermal condition. Check Perry Chemical Eng HDBK for the formulae.
I did something similar several years back with foam frothing equipment handling organosols but the scope of the problem was different than yours.
You may have to install more than one rupture disc to achieve that level and their rupture points can also be spread out just has you have with power boiler requiring several safety valves.
Check w/ the ASME/NB/ANSI piping code and w/ ASME unfired pressure vessel code for further current information. These codes s/b on line and may cost you a few hundred dollars but the cost is worth it for the safety of it.

 

[ChemE001]

Boy you got a tough one. This is another job for DIERS(Design Institute of Emergency Relief Systems) methodology. Go to

http://www.diers.net

and select software from the navigation bar. Choose the Free Mach II Reader and Regression Tool and download the DIERS safety valve and pipe benchmarks. You will find several safety valve and pipe benchmarks for single and two-phase flow involving high viscosity, higer viscosity, and highest viscosity. Download the User Manual and look at the two-phase design methodology for laminr flow. You can also play with the Free Mach II regression tool to plot your viscosity data using polynomial or exponential regression.

Enjoy