THIS MONTH’S PUZZLER
We’re suffering an annoying thermal expansion problem in the ethylene oxide (EO) lines that feed our reactors: pressure relief valves (PRVs) often pop from solar heating, because lines are blocked in, typically for up to ten minutes. (Our design standard assumes horizontal pipe and a maximum ambient temperature of 95°F, which doesn’t occur very often at our Chicago site: solar flux is 38.7 Btu/h-ft2 for the summer solstice.) The PRVs wouldn’t be such a problem but EO can polymerize and cause them to stick open, which is why the covered area under the reactors is Class 1, Division 1, Group B. The EO feed pipe runs at 55 psig at 60°F. The PRVs are set at 145 psig. Barometric pressure is 14.408 psia. The properties of EO at operating conditions are: density, 66.8 lb/ft3; viscosity, 0.302 cP; heat capacity, 0.476 BTU/lb-F; thermal conductivity, 0.0949 BTU/h-ft2-°R; volumetric thermal expansion coefficient, 0.001°F-1; and bulk modulus, 180,000 psi/ft3.
Our plant engineer is convinced that two inches of calcium silicate insulation in a stainless steel jacket over our 1½-in. Schedule-80 Type-316 stainless-steel pipe largely will eliminate our PRV reliability problem. What do you think?
LOOK AT THE MAKE-UP PUMP
For radial heat transfer conduction to/from a pipe, consider using the following equation: Q = 2πkL(T2-T1)/ln(r2/r1), where Q is heat transfer rate = heat flux × outside surface area of the pipe; k is thermal conductivity; L is length of the pipe; T2-T1 is the temperature difference between air and EO; r2 is pipe radius with insulation; and r1 is the radius without insulation. Consider foam glass insulation (k = ~0.3 BTU-in./hr-ft2 at the temperatures shown in the problem statement). Since EO under blocked-in conditions tends to polymerize, consider a rupture disc (with a pressure indicator) upstream of the relief valve. The problem statement indicates that (if the RV is set properly), for a temperature rise of 35°F (60°F to 95°F), pressure increase of the blocked section is 90 psig (55 psig to the RV set pressure of 145 psig). You can check by a process simulation (or maybe by an ideal gas approximation). In the field, verify the actual set point of the RV in question. The relief valve may be set too low. Alternatively, check the plant operations HMI (screens) to see if there were pressure fluctuations that led to lifting of the RVs.
GC Shah, senior advisor
Mustang Engineering, Houston
TACKLE TWO ISSSUES
There are two problems: polymerization of EO and thermal expansion of EO. Liquid EO polymerization is catalyzed, at ambient temperature, by acids, bases or, in the presence of aluminum, iron or metal oxides. So, all equipment handling liquid EO must be thoroughly cleaned. The thermal polymerization starts around 100°C (212°F). The polymerization reaction is exothermic and self-accelerating, with an explosive decomposition of vapors. With the given site and process conditions, the increase in temperature of fluid is calculated by a simple radiation heat transfer equation: Qradiation/A = hr×(Ts-Ta) + ε×∆R, where Qradiation is in W; A is in m2; hr = ε×σ×(Ts4-Ta4)/(T>sub>s-Ta); ε, emissivity, is 0.6; σ the Stefan-Boltzmann constant, is 5.68×106 W/(m2-K4); ∆R, radiation wavelength difference, is 63 W/m2; and Ta and Ts are the ambient and surface temperatures in K.
Based on this equation, the maximum surface temperature will be 130°F; the credit of fluid at lower temperature, i.e., 60°F, is not considered. In blocked condition, the fluid will attain the same temperature and self-polymerization may start at a slower rate. The exothermic polymerization reaction will further increase the temperature of blocked fluid and self-accelerating will start. As fluid is highly sensitive to increase in temperature, it is advisable to provide thermal insulation. You can consider fiberglass pipe insulation in place of costly calcium silicate insulation. The required fiberglass insulation thickness only will be 25 mm or 1 in.
Along with increase in temperature, the fluid volume also will increase. The coefficient of thermal expansion of EO at 20°C is 1.64×10-3 per °C. The relief valve will pop. To avoid this problem, consider a compensator at each end of the pipeline. Refer to “Save Pipe from Bursting with a Compensator,” by David Clucas and Jack Boteler, Chemical Engineering, Dec. 1999. [Ed.: See next response.]
Prakunj Trivedi, senior manager
Uhde India Private Ltd., Mumbai, India
CHECK A PAST ARTICLE
Your questioner may be interested in the following article from Chemical Engineering magazine: www.flowguardusa.com/Assets/pdf/applications/Article_TVC.pdf
The article addresses liquid expansion with temperature and cites an ethylene oxide example.
John Power, consulting engineer
GO WITH FLOW ORIFICES
Given that the minimum ignition energy for EO is only 0.06 millijoules (mJ), it’s easy to see a reason for concern; hydrogen is 0.02 mJ. I solved this puzzle with Excel by defining the allowable temperature rise for the liquid in the pipe, then the heat balance.
My approach involves two “goal seek” iterations. The first solves the temperature rise based on the allowable pressure rise from a mechanical balance on the pipe — the temperature rise is a function of ∆P, i.e., the difference between the PSV set point and the operating pressure: 145-55 psig or 90 psig. The second is a heat balance at the surface of the bare pipe or insulation jacket using the ∆T found.
THIS MONTH’S PUZZLER