High Viscosity Two-Phase Flow Through Relief Valves and Pipes

High viscosity two-phase flow occurs in many industrial scale reactors handling polymer systems. For example, a runaway reaction in a monomer tank can lead to high viscosity two phase flow. Following such a runaway reaction, polymerization and decomposition products are produced in the reactor and vented through the reactor’s emergency relief system. 

Many polymerization reactors are equipped with relief devices with discharge lines that are 50 to 100 feet long and in some cases longer. Discharge lines are typically connected to a vent containment header and/or a flare header. A large majority of relief device installations that exist today were designed using best industry practices such as API-520. Several publications have appeared in recent literature indicating that 30 to 40% of the relief devices that are in existence violate industry guidelines for inlet pressure drop and backpressure. These studies followed the OSHA PSM rule, which requires that the relief device design basis be documented and verified.

What is alarming is that these published numbers refer to relief devices that were sized for all-liquid or all-vapor flow for low viscosity systems. The design of a relief valve for a two-phase discharge introduces more complications. One now has to deal with fluid systems that have the density of a liquid and the compressibility of a gas. Moreover, the fluid can flash as it depressures, thereby limiting the flow capacity of the relief system. Several attempts have been made to bring best industry practices to a point where simple techniques can be used by operating plants to produce a best estimate of a safe design.

We will not dwell on the fact that a good relief design starts with a good design basis along with adequate reaction and physical property data.

It is somewhat ironic that many operating companies today do not have adequate in-house experienced resources for pressure relief design.

We often find that Emergency Relief Systems (frequently a last line of defense against runaway reactions) that protect the source of all product revenue for a plant are not adequately sized. There have been several incidents reported where the pressure relief system was identified are being inadequate and a contributing cause to loss of life, loss of containment and the associated monetary damage resulting from property loss, business loss, and of security of supply issues.

We will focus in this white paper on issues pertaining to high viscosity two-phase flow through relief valves.

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