(159c) How to Size a Rupture Disk Vent Line for Two-Phase Gas/Liquid Flow Based on Current Engineering Practices

How
to Size a Rupture Disk Vent Line for Two-Phase Gas/Liquid Flow based on Current
Engineering Practices

Prof. Dr. Juergen
Schmidt, Karlsruhe Institute of Technology and BASF SE, Germany (Correspondence
to: juergen.schmidt@onlinehome.de)

Keywords: rupture
disk, two-phase flow, safety, sizing method, cost saving

Abstract

In the
chemical and petrochemical industry, vessels and pipes are protected against
overpressure with safety relief devices, typically a rupture disk or safety
valve. If the device opens due to an undesired deviation from the normal
process operation the depressurization lead to a two-phase discharge of the
reactor inventory. Typically, a retention system (a cyclone separator or a
catch drum) is used to separate the liquid from the two-phase stream, see
figure 1. Gases with hazardous properties are e.g. condensed in a quench or
burned in a flare. Current regulations to protect the environment led to more
complex disposal systems, i.e. larger vent pipes flowing into gathering systems
connected to separators, drums, quenches or flares.

A
conservative over-sizing of a rupture disk vent pipe may induce a two-phase
discharge of the reactor inventory, unexpected large piping forces and a
malfunction of the rendition system or flare. Large extra-costs may result and
a significant disposal of the environment cannot be excluded. The functioning
of the vent systems can only be guaranteed, if precise methods experimentally
validated are used for sizing the system. Unfortunately, no standard is
available for sizing rupture disk vent lines for flashing two-phase flow.
Experimental data are lacking.

In lack of
more precise methods, engineers usually size a rupture disk vent line under the
assumptions of a homogeneous equilibrium flow through an equivalent nozzle with
a strong (unrealistic) flow contraction. The pressure drop in the vent line is
often neglected or assumed to be of minor effect. This sizing procedure may
lead to several hundred percent deviation from the real flow process. A rupture
disk device can arbitrarily be much over- or undersized depending on the actual
flow conditions and pipe geometry. 

Figure 1: Industrial rupture disk vent line system

The
difficulty in sizing a rupture disk vent line for two-phase flashing flow is
the interaction with the whole piping system. E.g., if the mass flow rate through
the rupture disk is estimated too small, the pressure drop in the inlet and
outlet pipe will be underestimated, resulting in a wrong estimation of critical
flow areas. Overall, uncertainties in modeling the liquid level swell and, hence,
inlet conditions into the piping system, the lack of data for the effective net
flow area and flow contraction in the rupture disk as well as uncertainties of
the pressure drop estimation in pipes and piping elements may lead to very large
differences. In addition, the simulation of the mechanical non-equilibrium
(slip) and the thermodynamic non-equilibrium (e.g. boiling delay) in all pipe
element produces even more uncertainties. A comparison of these uncertainties
will be given in the presentation.

During the
last ten years a tremendous effort have been reached by considering phase slip
between gases and liquids and thermodynamic non-equilibrium effects leading to
highly enlarged mass flow rates through safety valves compared to homogeneous
equilibrium calculations. To harmonize the calculation methods for safety
valves and rupture disks, the HNEDS model for flashing gas/liquid-two-phase
flow through throttling devices, recommended in ISO 4126-10, is extended to the
flow through pipes and rupture disks. It includes the thermodynamic
non-equilibrium (boiling delay) and may give more representative results
compared the any typical homogenous equilibrium model. The method is outlined
in detail and example calculations are given to describe the sizing procedure.

The
proposed method based on a new general equation for sizing throttling devices
and pipes represents the current state of sizing rupture disk devices for
two-phase flashing flow. The method is suggested to extend the standards for
sizing safety devices.

In future,
rupture disk manufacturer need to deliver more detailed data of their rupture
disks bursted under in-service conditions. The upcoming ISO-DIS 4126-11 and
ASME PTC25 shall be extended to include the necessary test procedures. Testing under
single phase laboratory conditions with subcritical almost incompressible gases
are not representative for typical industrial applications in the chemical and
petrochemical industry. In the presentation the need for more detailed
experimental data is outlined.